Owing to its importance in the assessment of reinforced concrete structures, it is essential to determine the chloride diffusivity of fly ash concrete. This paper presents an investigation into the diffusion characteristics of chloride ions in fly ash concrete. Through experiment, the relationship between chloride diffusivity and curing age up to 1800 days is measured and the effects of curing age, water/binder ratio, aggregate volume fraction, and fly ash content  on chloride diffusivity are evaluated. It is found that the chloride diffusivity decreases with the increase of curing age, aggregate volume fraction, and fly ash content, but increases with the increase of water/binder ratio. In analytical modeling, an equivalent aggregate model is constructed and the equivalent interfacial transition zone (ITZ) thickness is derived analytically. With the equivalent aggregate model, three-phase fly ash concrete reduces to a two-phase composite material. By extending the Maxwell method, the chloride diffusivity of fly ash concrete is formulated. Finally, the validity of the analytical method is verified by experimental results. Reinforced concrete structures may be subjected to reinforcement corrosion when they are built in a harsh environment. As a result, some harmful species, such as chloride ions and sulfate ions, penetrate into concrete under the concentration gradient and cause corrosion of reinforcing steel bars and concrete cracking. Therefore, the chloride diffusivity of concrete plays an important role in assessing the durability of reinforced concrete structures ,2.To improve the durability of reinforced concrete structures, fly ash is often added into concrete as a mineral admixture, and the chloride diffusivity can be reduced significantly owing to the pozzolanic reaction of fly ash ,4. It haMuch effort has been made to determine the chloride diffusivity of concrete experimentally and theoretically. Thomas et al.  observedThe objective of this paper is to study the diffusion characteristics of fly ash concrete. An experiment is conducted to analyze the time dependence of chloride diffusivity and to evaluate the key influential factors. An analytical method is then presented to estimate the chloride diffusivity of fly ash concrete. Finally, the validity of the analytical method is verified with experimental results.3 and 295 m2/kg for the fly ash and 3150 kg/m3 and 342 m2/kg for the cement, respectively. Crushed stone and river sand were selected for the coarse aggregate and fine aggregate, respectively. The aggregate gradation obeyed the Fuller curve and the aggregate size was divided into seven grades, i.e., 0.15–0.3, 0.3–0.6, 0.6–1.18, 1.18–2.36, 2.36–4.75, 4.75–9.5, and 9.5–16 mm. The apparent density, water content, and water absorption were 2760 kg/m3, 0.48%, and 0.72% for the crushed stone and 2600 kg/m3, 0.58%, and 0.77% for the river sand, respectively. Tap water was used for mixing fly ash concrete, and a saturated Ca(OH)2 solution with pH of about 12.5 was adopted for curing the specimens to avoid the calcium leaching of fly ash concrete.This chloride diffusion test was conducted from September 2013 to December 2018. The fly ash concrete specimens were made with P.O 42.5 cement and fly ash of grade II . Their ca was 0.55, 0.65, and 0.75 and the fly ash content mf was 0.1, 0.2, and 0.3. The mix proportions of fly ash concrete are shown in The water/binder ratio (w/b) of fly ash concrete was selected as 0.4 and 0.5. For each water/binder ratio, the aggregate volume fraction C2 solution at a curing temperature of 20 ± 2 °C. During the curing period, the saturated Ca(OH)2 solution was replaced monthly. Finally, the chloride diffusivity Dcf was measured by the electrical conductivity method   according to sieving analyses. If the cumulative distribution function for the aggregate is approximately linear piece-wise in each sub-domain, the probability density function pN(D) and the cumulative distribution function PN(D) in terms of the number of aggregates are [In modeling fly ash concrete, aggregates are considered to be spherical. An ITZ with low strength and high porosity is formed between aggregates and the cement paste matrix due to the low packing efficiency of cement particles near aggregates. It was experimentally confirmed that addition of fly ash results in a denser and stronger ITZ, possibly due to the filling effect of small fly ash particles ,31. Howeates are (1)pN(D)The number of aggregates per unit volume of aggregate, It is well known that ITZs will overlap when spherical aggregates are randomly distributed in concrete and, the larger the aggregate volume fraction is, the higher the overlap degree of ITZ will be. In order to consider the ITZ overlap and to construct a two-phase composite material model, composed of equivalent spherical aggregates and a fly ash cement paste, the following three-step scheme is adopted to determine the equivalent ITZ thickness.i without ITZ overlap effect. If the equivalent ITZ thickness is denoted as heq, the ITZ volume for a single spherical aggregate is given by1. Calculation of ITZ volume fraction fi is equal toa is the aggregate volume fraction, and N(d), respectively. For any integer k, Thus, the ITZ volume fraction fSubstitution of Equation (1) into Equation (7) yieldsi with ITZ overlap effect. Based on the geometric statistical theory of composite materials, Lu and Torquato [i is given by1, t2, and t3 are expressed in terms of fa and 2. Calculation of ITZ volume fraction fTorquato  proposedeq. From Equations (6) and (9), heq can be obtained aseq include fa, pN(D), and h.3. Determination of equivalent ITZ thickness heq known, the chloride diffusivity of an equivalent spherical aggregate, composed of a spherical aggregate and an equivalent ITZ, is given by [a and Di are the chloride diffusivities of aggregate and ITZ and Va and Vi are the volumes of the aggregate and the ITZ, respectively. It is obvious thatWith hgiven by (16)Dea=a can be assumed to be zero. Thus, Equation (16) reduces toSince the density of aggregate is much higher than those of ITZ and bulk paste, the chloride diffusivity Df, as shown in e, D1, and D2 are the chloride diffusivities of the effective medium, phase 1, and phase 2, respectively, and φ2 is the volume fraction of phase 2. By extending the Maxwell method to the fly ash concrete with N-type aggregates, the chloride diffusivity is given byj is defined asf and Dea,j being the chloride diffusivities of the fly ash cement paste and the j-th equivalent aggregate, and dea,j and φj being the diameter and volume fraction of the j-th equivalent aggregate, respectively.With the equivalent aggregate model, three-phase fly ash concrete can be reduced to a two-phase composite material, composed of equivalent aggregates of various sizes and chloride diffusivities encompassed by a fly ash cement paste of chloride diffusivity DFinally, solving Equation (21) gives the chloride diffusivity of fly ash concretef of fly ash cement paste can be estimated. To evaluate the chloride diffusivity of the equivalent aggregate from Equation (19), the ITZ chloride diffusivity Di needs to be known beforehand. Since it is still difficult to measure the ITZ chloride diffusivity directly through testing, experimental calibration is conducted to determine the ratio Di/Db. For this purpose, the experimental results for w/b = 0.5 and Ca = 0.55, 0.65, and 0.75 are selected. As for the ITZ thickness, available experimental results do not achieve a consistent conclusion. Qian et al. [i/Db at t = 28, 60, 120, 540, and 1800 days is back-calculated as 10.5, 10.8, 8.4, 5.08, and 1.8 for mf = 0.1; 11.3, 11.7, 8.1, 2.86, and 1.35 for mf = 0.2; and 12.6, 10.6, 9.47, 2.94, and 1.43 for mf = 0.3, respectively. It is seen that Di/Db is seldom influenced by the fly ash content but decreases with the increase of t, which agrees well with the results of Bentz [i/Db at mf = 0.1, 0.2, and 0.3 is equal to 11.46, 11.04, 8.65, 3.62, and 1.53 for t = 28, 60, 120, 540, and 1800 days, respectively. Through regression analysis, Di/Db can be expressed asf = 0.1, 0.2, and 0.3, the correlation coefficient between the analytical method and the experimental results is calculated as 0.997, 0.998, and 0.996 for Ca = 0.55; 0.993, 0.995, and 0.994 for Ca = 0.65; and 0.996, 0.998, and 0.998 for Ca = 0.75, respectively.Using the analytical method proposed by Zhou et al. , the chln et al.  indicaten et al. ,37,38. In et al. , h is tai/Db known, the chloride diffusivity of fly ash concrete can be evaluated for w/b = 0.4. The results are shown in f = 0.1, 0.2, and 0.3, the correlation coefficient between them is 0.986, 0.992, and 0.997 for Ca = 0.55; 0.979, 0.989, and 0.992 for Ca = 0.65; and 0.993, 0.985, and 0.982 for Ca = 0.75, respectively. Therefore, the validity of the analytical method is verified.With D−12 m2/s and Di is calibrated from the chloride diffusivity of concrete, i.e., 8.05 × 10−12 m2/s, measured at Ca = 0.4, as 1.34 × 10−12 m2/s. With these parameters, Dcf with different Ca can be calculated by the proposed analytical method; the results are shown in To further validate the analytical method, the test results of Yang and Su  were selAlthough the proposed analytical method was preliminarily verified by the experimental results, more chloride diffusion tests of fly ash concrete and microstructural experiments need to be conducted to further validate the analytical method. Especially, the effects of fly ash content and water/binder ratio on the thickness, porosity, and chloride diffusivity of ITZ need to be investigated in future studies.The chloride diffusivity of fly ash concrete has been measured through experiment and estimated based on the equivalent aggregate model and the generalized Maxwell method. The main conclusions of this study are as follows:1. From the test results, it has been found that the chloride diffusivity of fly ash concrete decreases almost linearly with an increase in aggregate volume fraction. The optimum fly ash content is 0.3. The chloride diffusivity exhibits an evidently decreasing trend for a given curing time within 540 days, but almost remains unchanged for a given curing time from 540 to 1800 days.2. An equivalent spherical aggregate model has been proposed and the equivalent ITZ thickness has been derived analytically. The main influential factors on the equivalent ITZ thickness include the aggregate gradation, the aggregate volume fraction, and the practical ITZ thickness. The chloride diffusivity of fly ash concrete has been estimated by the analytical method and verified against experimental results.


Concrete structures expand and contract in response to temperature changes which can result in structural strain and cracking. However, there is a limited amount of robust field data on hybrid concrete floor structures. Shortage of such data impacts on our understanding of how concrete structures respond to thermal effects and ultimately the overall design of concrete structures. Thus, a comprehensive structural and environmental monitoring strategy was implemented by the authors during the construction of an educational building. Sensors were embedded in the precast and in situ components of a hybrid concrete lattice girder flat slab so that the thermal response of the floor during the manufacture, construction and operational stages could be investigated. Many aspects of the thermal behaviour of the floor during the construction phase were monitored using the embedded sensors. The early-age thermal effects during curing and the impact of the variation of ambient temperature  and solar radiation on the behaviour of concrete floor is explored in the paper. Values for restraint factors and the in situ restrained coefficient of thermal expansion of concrete are calculated using the data from the embedded sensors. Numerical modelling of the thermal behaviour of the hybrid concrete floor was undertaken and validated using the real-time field measurements. The data presented and analysed in this paper can be used to improve the understanding and modelling of the thermal behaviour of a hybrid concrete floor. This will assist with improved design of sustainable buildings as it allows the environmental performance of the floor to be optimised with respect to controlling the internal environment, thermal mass and energy efficiency. Most concrete structures are subjected to frequent temperature changes during their design life. In fact, the internal temperature of a reinforced concrete element and relative humidity can be considered as two of the key indicators to monitor and evaluate the condition of the material and detect structure deterioration  that monitors and controls the environmental conditions and energy consumption during the operational phase of the building life cycle. The use of embedded sensors to monitor the actual behaviour of components within the building complements the data from the BMS and allows a more holistic evaluation of the performance of the building. Linking data from embedded temperature sensors in the concrete floor slabs with the room air temperature and external weather conditions in the BMS could assist with making the most effective use of thermal mass within a building in controlling internal comfort conditions and minimising energy consumption. The additional advantage of the embedded sensors is that they can be used to study the behaviour of components at all stages of its life cycle .Ge et al.  conducteObservations and findings from embedded instrumentation in a hybrid precast concrete flat slab floor in a recently constructed educational building are presented in this paper. The embedded sensors permitted real-time monitoring of the floor during the manufacture, construction and operational phases of the building. The embedded sensors in this project are part of a structural health monitoring (SHM) strategy which has been developed in the National University of Ireland Galway  to monitor the structural and environmental performance of a number of educational buildings  on the NUI Galway campus is the new home to three existing schools: Anatomy, Physiology & Pharmacology and Therapeutics Fig. . The buiThe basement was constructed using in situ concrete and above ground the majority of the structure was constructed using precast concrete components such as twinwall system, hybrid concrete lattice girder slabs and hollowcore slabs. All precast components were designed, manufactured and installed by Oran Pre-Cast Ltd., who is located 10 km from the site.The majority of the floor structure in the HBB was constructed using two-way spanning hybrid concrete lattice girder flat slab floor system (either 400 or 250 mm thick). This floor system was selected by the contractor to reduce construction time, reduce steel fixing on site, minimise formwork, improved safety and because of its high quality finish. The lattice girder plank Fig.  consistsVibrating Wire (VW) gauges (Gage Technique model TES/5.5/T) embedded in concrete that measure strain and temperature. The range of the measured strains is greater than 3000 microstrains and the resolution is better than 1 microstrain. The measured temperature range is − 20 to + 80 °C with an accuracy of ± 1 °C.Electrical resistance (ER) strain gauges (Tokyo Sokki Kenkyujo model FLA-6-11) bonded to steel reinforcement in concrete, which measure uniaxial strain. The ER gauges have a strain limit of 5% and the gauge resistance is 120 Ω (± 0.3 Ω).IP68 rated thermistor sensors (ATC Semitec model IP 68) which are capable of measuring concrete temperature. The thermistors have a temperature range of − 50 to + 110 °C with a 1% tolerance.This project used a number of sensors Fig.  which weThe embedded sensors are located within two zones of the second floor of the HBB along a number of orthogonal grids. The location of the embedded sensors in one zone of the second floor slab is shown in Fig. To collect data from the sensors, a data acquisition system consisting of CR1000 data loggers, AM16/32B multiplexers and AVW200 vibrating wire interface from Campbell Scientific were used to collect and store real-time data from the embedded instrumentation. This system has automatically recorded data from the precast planks since manufacture and the in situ concrete topping since the concrete pour on site. During the construction phase, data was manually downloaded onto a laptop, but after commissioning of the building, data is downloaded over a local network using Campbell Scientific’s NL116 Ethernet interface and compact flash module. This allows long term monitoring of the behaviour of the floor during the operational phase of building. This data is being stored on a dedicated server and is available for students for both teaching and research applications.A comprehensive material testing programme was undertaken to measure the properties of the concrete (precast and in situ) in the floor over time. Concrete specimens  were made and cured in water (in accordance with EN 12390-2 BSI ) and in Changes in the environmental conditions surrounding any concrete element will have a significant effect on its behaviour with respect to deformation, strength, durability and service life  measure both change in strain and temperature (via thermistors) in the concrete slab. Changes in the internal temperature of the concrete slab will cause expansion and contraction of both the concrete and vibrating wire gauge that is used to measure change in strain in the concrete. Failure to correct for the thermal effects in the VW strain gauges would lead to erroneous strain readings. The difference between the coefficient of thermal expansion of the concrete and VW strain gauge will give rise to an apparent change in strain in the concrete. The coefficient of thermal expansion of the VW strain gauges is 11με/°C and for this project it was assumed that the coefficient of thermal expansion of the concrete was 9με/°C. The measured strain readings (apparent strain) from the VW gauges are corrected for thermal effects using the following equation so that only strain due to shrinkage, flexure and creep is determined:The effect of the thermal correction can be seen in Fig. In the literature, there is no agreement on the time at which the concrete stiffness has exceeded that stiffness of the strain gauge so that measured strains are consistent with the strains in the concrete (Time Zero). Prior to this time, the concrete is not sufficiently stiff to transfer strains to the embedded strain gauge or to restrain the gauge under applied thermal loads. Boulay and Paties  conducteIn this study, the zero point for strain readings is assumed to be time at which the maximum concrete temperature is reached. Prior to the concrete reaching its maximum temperature, the concrete can be assumed to be in, or near to, a zero stress state due to creep ACI . The in A one-dimensional finite difference model was developed to model the temperature profile in the hybrid concrete slab. It was assumed that a one-dimensional model through the slab thickness was applicable for the internal sections of the slab and the model is validated against embedded sensors located internally in the slab. The model is based on the numerical method developed by Ross and Bray  and uses(i)0 and T1 are the surface and internal temperature at the first internal node, and Δx is the space increment in the model (through the depth of the concrete).Conduction: (ii)0 is the concrete surface temperature, and Ta is the ambient temperature.Convection: (iii)abs is the solar absorptivity of the concrete and Qinc is the incident solar radiation .Solar radiation: (iv)0k is the concrete surface temperature (in Kelvin), and Tsky is the sky temperature.Thermal irradiation: The following equations are used in the model to describe heat transfer through the concrete slab:The difference-calculation remains numerically stable, if the following is valid for the time increment:At the commencement of the pour, the precast plank is assumed to be the same temperature as the ambient temperature (15.1 °C) and the placing temperature of the concrete is assumed to be 20 °C . The degree of external restraint will depend on the member being cast  and the construction process .The maximum temperature differential and thermal gradient within the section. This is the condition of internal restraint and occurs where one part of a concrete pour expands or contracts at a different rate to another part of the same section. It typically occurs in thick sections in which there can be a large temperature differential across the section and can lead to both surface cracking and internal cracking (which may not be observable from the surface). An internal restraint can also occur when there is a large percentage of reinforcement restraining the concrete.Predicting early-age thermal cracking is very complex as there are numerous factors which affect the potential for cracking and properties of the concrete  are rapidly changing during the early stages of curing. The use of in situ instrumentation in this project allowed the thermal behaviour of the second floor slab to be recorded and this data can be used during the design process for checking early-age thermal cracking. Two important temperature considerations for control of early-age thermal cracking are Bamforth :The tempUsually either external or internal restraint dominates; however, there are some situations in which both types of restraint need to be considered.3. It is common to blend GGBS  with CEM I cements to reduce the heat of hydration and likelihood of early-age thermal cracking. CIRIA Report C660 by Bamforth . As noted previously, at most locations there were four VW strain gauges positioned through the depth of the slab . The temperature in the concrete at one location in the slab is shown in Fig. Bamforth  providesThe end of the hydration phase is difficult to determine precisely, but after the first few days it appears that the thermal response of the slab is primarily driven by the ambient air temperature and solar gain Sect. . After tLarge temperature differentials between the surface and centre of concrete elements can lead to internal restraint, which can lead to cracking and this is of critical importance for thick concrete elements. Quantifying and controlling the temperature differential within the concrete section is one of the critical design checks when considering early-age thermal cracking. The maximum recorded temperature differential within the 335 mm thick concrete topping in the second floor slab in HBB never exceeded 3 °C. Using the CIRIA Report C660 , the preThe temperature profile through the depth of the slab  at a location in the middle of the slab is shown for the first 24 h after pouring in Fig. As noted previously, in Fig. Similar to other models, the numerical model described in this paper uses adiabatic hydration models (i.e. heat transfer is not lost to/or gained from the surrounding). Previous studies  were assumed to be constant in the model and were based on typical values from the literature. However, the thermal properties of concrete change during the hardening process . During the construction phase of the HBB, both the top and bottom of the slab are exposed and, therefore, are affected by daily and seasonal environmental changes. The thermal gradients may lead to slab curvature (curling or warping of the slab) and because typically the slab is subject to some form of external restraint, additional temperature-induced stresses are developed which may create cracks in the concrete. The curvature is upward if the thermal gradient is positive (temperature decreasing from top to bottom) and downward if it is negative. Restraint to slab curvature is the cause of most early-age thermal cracking in pavements Bamforth .Distinct differences can be observed in the daily thermal gradients in the slab in response to the changes in the ambient air temperature. The thermal gradient in the slab is typically non-linear through the depth of the slab and approximately parabolic in shape at peak differentials. Temperature profiles through the slab are shown in Fig. The predicted temperature profiles on the 30 December 2015 and 19 July 2016 using the 1-D model are also shown in Fig. Although the 1-D finite-difference numerical model described in this paper is relatively simple, it can be used to predict the temperature profile during the early-age heat of hydration phase and at later stages when the environmental conditions are the dominant factor on the temperature in the concrete slab. During curing of the concrete slab, the model will tend to predict temperature rises which are conservative, but could be used to identify potential early-age thermal cracking. Accurate prediction of the concrete temperatures in slabs is critical if the thermal mass of the concrete is used as one of the passive design strategies to control comfort (heating and cooling) in a sustainable manner. Exploiting the potential of exposed concrete in buildings can lead to enhanced energy efficiency and carbon savings over the lifetime of the building.Thermal gradients were also recorded horizontally in the slab in addition to vertically through the depth of the slab as outlined above. The temperature distribution along gridline (GL) H . However, local deviations in the horizontal temperature distributions are noted in the slab perimeter along GL 22. The horizontal temperature distributions for the slab along GL 22 are shown in Fig. c) is a measure of the free strain (unrestrained concrete) produced by a unit change in temperature and its magnitude will control the expansion and contraction of the concrete in response to a given temperature change. In design of concrete structures, the value of αc can influence the control of early-age thermal cracking, provision of movement joints and construction tolerances for elements connected to concrete. A concrete with a low coefficient of thermal expansion is desirable when trying to control the risk of early-age thermal cracking; the type of aggregate used in the concrete is the dominant factor with respect to the coefficient of thermal expansion of concrete. The value of αc can vary from 8 to 13με/°C depending on the type of aggregate used in the mix. Eurocode 2 (BSI (British Standards Institution) c changes over time and with moisture content.The coefficient of thermal expansion of concrete (αr) to be measured for the in situ topping that is poured on top of the precast plank. The slope of the in situ strain–temperature relationship during the cooling phase of the temperature cycle can be used to determine αr at a number of locations in the slab and at different depths in the slab. In Fig. r is lower in the middle (4.7με/°C) of the concrete in comparison with the top (6.0με/°C) and bottom (5.9με/°C) and this finding was consistent at all locations in the slab.There is no standard method for determining the coefficient of thermal expansion of concrete in CEN, although there is a method provided in BS EN 1770 BSI  related r through the depth of the slab is shown in Fig. r determined in strain–temperature plot previously  using the embedded sensors in the 335 mm thick in situ topping was 6.6με/°C with a coefficient of variation (Cv) of 13% . The average αr for gauges located in the middle of the in situ topping was 5.7με/°C (Cv = 11%) and the average αr for gauges located in the top and bottom of the in situ topping was 6.9με/°C (Cv = 10%).The variation of αsly Fig. . The aver with depth of pour was found to be approximately represented by a parabolic curve and this correlates with the in situ data from the slab in the HBB even though the top of the in situ topping is exposed and the bottom of the in situ topping is partially insulated by the precast plank. The variation of αr with depth correlates with the variation in concrete temperature, the central core being subjected to the greater temperature rise and imposing movement on the cooler surface regions. The reduced value of the restrained coefficient of thermal expansion of concrete (αr) in the middle of the in situ topping in comparison with the top and bottom  are used in many design codes to quantify the degree of restraint when two concrete elements are cast against each other . Annex L of Eurocode 2 BSI  providesc is the coefficient of thermal expansion of unrestrained concrete (free strain) and αr is the in situ restrained coefficient of expansion strain which can be measured as outlined above. Assuming that αc was 9με/°C for the in situ topping in the HBB using limestone aggregates, values for the restraint factors can be determined using the in situ values for restrained coefficient of expansion strain (αr). The average restraint factor at the bottom of the in situ topping which is cast on top of the precast plank was 0.25 (Cv = 34%). A method for estimating the restraint factor at a joint (Rj) was proposed by ACI (American Concrete Institute)  pour; Ao is the cross-sectional area of the old (restraining) concrete; En is the modulus of elasticity of the new pour concrete; Eo is the modulus of elasticity of the old concrete.CIRIA Report C660 Bamforth  outlinesstitute) :8\documen/Ao) is in proportion to the relative thicknesses of the two slabs (hn/ho). Using the above equation, the estimated restraint at the interface of the in situ topping and the precast plank after 12 h is calculated as Rj = 0.24. This compares favourably with the restraint factor determined using the in situ data (R = 0.25) from the embedded instrumentation during the cooling phase after the heat of hydration even though there is a large variability. Thus, the equation developed by the ACI could be a useful method for determining restraint factors in hybrid concrete slabs.It is recommended that for a slab cast against an existing slab that the relative area .2) and this allowed the daily total solar irradiance (kJ/m2) to be determined and compared with the thermal behaviour of the precast plank and hybrid concrete lattice girder flat slab during construction. The total area of the second floor slab which was instrumented had limited exposure to solar radiation during construction because 11 days after the in situ topping was poured (31 July 2015), precast planks for the third floor were erected. The third floor precast planks located directly over the second floor provided shading for the majority of the slab, except at the perimeter which was still subject to partial solar radiation.The automatic weather station IRUSE  at the NThe 65 mm thick precast planks for the second floor were erected on the 6 July 2015 and data was recorded after the 10 July 2015. Nine VW strain gauges were embedded in one of the 65 mm thick precast planks  and potential of using concrete to manage thermal energy flows in a building.The concrete temperature in the in situ topping in the centre of the slab and the daily solar irradiance is shown in Fig. The magnitude and source of thermal induced loadings will vary during the lifetime of concrete structures. Initially, the heat generated during the heat of hydration phase can cause strains which may result in cracks which will affect the durability and performance of concrete elements. After the curing phase, interaction of the concrete with the environment (ambient temperature and solar radiation) can lead to daily and seasonal temperature changes within the structure which will cause the material to expand and contract. If movement in the structure is restrained, stresses may be induced which may contribute to cracking in the structure. In some cases, the thermal induced stresses may exceed the stresses induced by service loads (permanent and variable loading).An estimate of the thermal strains induced in the concrete slab may be determined by assuming a coefficient of thermal expansion of the 9με/°C for the concrete. As noted in Sect. Exposed concrete surfaces are subject to temperature increases due to solar radiation which can induce curvature in the slab due to the creation of temperature differentials through the concrete Sect.  and alsoA detailed account of a real-time monitoring programme is described which allows the thermal behaviour of a hybrid concrete floor during the construction phase to be investigated. The project involved embedding sensors in the precast concrete plank during manufacture and the instu topping during construction so that the response of a hybrid concrete floor to temperature variations for 1 year could be monitored. The relationship between concrete floor and external environmental conditions was investigated using data from an automatic weather station located close to the building.The data from the embedded sensors demonstrates that thermal behaviour of the concrete changes during the construction process. For the first few days after the in situ topping is poured, the heat of hydration is the dominant factor and has the potential to cause thermal cracks if the tensile strains generated in the concrete exceed the tensile strain capacity of the concrete. Although the concrete slab monitored in this project was exposed to direct solar radiation for a short period of the time, the significant effect of solar radiation on the concrete temperatures in the slab were recorded. High levels of solar radiation have the potential to cause curvature of the slab and also increase the average temperature of the concrete section. During the monitoring period described in this paper (1 year), differences in the thermal response of the slab to diurnal and seasonal temperature variations were observed and time lag between ambient temperature and concrete temperatures were noted. Variations in concrete temperatures in the concrete slab were recorded vertically through the depth of the slab and horizontally in the region of the slab which was monitored.A one-dimensional finite-difference model is described which takes account of the various heat transfer components acting on the slab and allows prediction of the temperature profile in the slab during construction and operational phase of the building. Although the model is conservative when predicting the temperature rise during the heat of hydration, it can be used to model the temperature rise and temperature differential in a hybrid concrete floor when the in situ structural topping is poured. The thermal response of the floor to the diurnal and seasonal changes in the ambient temperature is also modelled using the numerical model.The data presented in this paper can be used in conjunction with laboratory experiments to develop and validate numerical models which will allow designers to simulate the thermal behaviour of concrete components. This is particularly important in buildings in which the thermal mass of exposed concrete is used to regulate the internal environment and reduce energy consumption.Although, hybrid concrete construction is increasingly popular in concrete structures, there is little data available on the actual thermal behaviour of concrete components. The data presented in this paper can be used in conjunction with laboratory experiments to develop and validate numerical models which will allow designers to simulate the thermal behaviour of concrete components. This is particularly important in buildings in which the thermal mass of exposed concrete is used to regulate the internal environment and reduce energy consumption.The effect of solar gain on exposed concrete components can be significant in regions which have high levels of solar radiation and needs to be considered at design stage. Thermal strains induced by solar irradiance can induce additional curvature and moments in slabs. The thermal data presented in this paper is related to actual weather data and can be used for analysing the effect of solar irradiance on concrete slabs.Guidelines for designers in codes of practice in relation to restraint for suspended slabs is limited. At design stage, it is very difficult to determine the degree of restraint and the nature of restraint . The methodology described in this paper for determining the restraint factors based on the measured restrained coefficient of thermal expansion is applicable to similar hybrid precast concrete slabs.This paper demonstrates the important role which embedded sensors can play for better understanding and prediction of thermal behaviour of concrete components in real buildings. The comprehensive thermal information from the embedded sensors described in this paper have many potential applications for designers of concrete buildings:Thermal effects during the construction and operational phases of concrete, if not considered during the design process, can have a significant negative effect on the performance of concrete structures. As performance requirements of buildings continue to increase, designers must adopt a holistic approach to design and be able to demonstrate how the various components and materials work in unison to enhance the overall performance of the building. If implementing a passive design strategy, designers will require accurate numerical models validated using field data from buildings to predict the thermal behaviour of concrete floors so that it can be optimised with respect to controlling the internal environment, thermal mass and energy efficiency.


A novelty in the presented research is the evaluation of the fracture toughness of concretes with two mineral additives, assessed at an early age. During the tests, the structures of all composites and the nature of macroscopic crack propagation were also assessed. A modern and useful digital image correlation (DIC) technique was used to assess macroscopic cracks. Based on the conducted research, it was found the application of SF to FA concretes contributes to a significant increase in the fracture toughness of these materials at an early age. Moreover, on the basis of the obtained test results, it was found that the values of the critical stress intensity factor of analyzed concretes were convergent qualitatively with their strength parameters. It also has been demonstrated that in the first 28 days of concrete curing, the preferred solution is to replace cement with SF in the amount of 10% or to use a cement binder substitution with a combination of additives in proportions 10% SF + 10% FA. On the other hand, the composition of mineral additives in proportions 10% SF + 20% FA has a negative effect on the fracture mechanics parameters of concretes at an early age. Based on the analysis of the results of microstructural tests and the evaluation of the propagation of macroscopic cracks, it was established that along with the substitution of the cement binder with the combination of mineral additives, the composition of the cement matrix in these composites changes, which implies a different, i.e., quasi-plastic, behavior in the process of damage and destruction of the material.This paper presents the results of the fracture toughness of concretes containing two mineral additives. During the tests, the method of loading the specimens according to Mode I fracture was used. The research included an evaluation of mechanical parameters of concrete containing noncondensed silica fume (SF) in an amount of 10% and siliceous fly ash (FA) in the following amounts: 0%, 10% and 20%. The experiments were carried out on mature specimens, i.e., after 28 days of curing and specimens at an early age, i.e., after 3 and 7 days of curing. In the course of experiments, the effect of adding SF to the value of the critical stress intensity factor— Such actions, which are undoubtedly pro-ecological, directly affect:Currently, many research centers conduct tests on the separate use of SF ,14,15,16It should be noted that several basic properties of OPC–FA–SF concrete, such as consistency, setting time, workability, sorptivity, compressive strength, durability and resistance to migration corrosive substances, were provided so far in papers ,46,47,48VP-δ), crack mouth opening displacement (VP-CMOD) and crack tip opening displacement (VP-CTOD) were also analyzed in papers [Furthermore, in papers ,48,49, tn papers ,49. BaseHowever, from previous reports, it is known that the addition of FA to concrete has a positive effect on the parameters of fracture mechanics of 28-day and older concretes and a negative effect on the properties of composites at a young age ,60,61,62Therefore, in order to supplement the previous information included in papers ,49,50,51Microcracks that may appear in the structure of concrete at an early age negatively affect the progress of the final strength of the composite and reduce its fracture toughness. Therefore, it is important to track how this important property of the material changes, i.e., fracture toughness from the beginning of the formation of the composite structure until it is cured.Moreover, they are also very important from a technical point of view. The early strength of concrete is extremely important in today’s construction industry. Due to the pace of construction and the number of investments in the area of road, service, residential and industrial infrastructure, time plays an extremely important role.Monolithic massive constructions, where the young concrete is subjected to load increases during the subsequent stages of construction of the structure as a result of which numerous technical decisions must be made depending on the instantaneous strength of the concrete or its other properties, such as time of stripping the formwork, moving the sliding formwork, allowing young concrete to be subjected to technological and assembly loads, etc.;Prefabricated reinforced concrete constructions, where it is important to know the interoperational strength, e.g., formwork stripping, transport, storage, shipping, assembly;Prestressed reinforced concrete constructions, in which it is important to know the possible short-term losses of prestressing forces or not fully cured concrete.It should be noted that the knowledge of parameters of fracture mechanics for young concrete is particularly important in the following concrete construction areas:Siliceous FA used in the amount of approx. 20% of the cement mass significantly improves the mechanical parameters of concrete, e.g., their fracture toughness after 28 days of curing. However, the disadvantage of this solution is the reduction of the early strength of concrete. The article attempts to increase the early strength of FA concretes using a second pozzolanic additive. The experimental tests were carried out in order to confirm the positive effect of the applied modifications on the early parameters of compressive and tensile strength as well as fracture toughness of concrete. The presented research results can be practically used in many areas of construction, where the pace of construction works plays a decisive role. Moreover, the obtained test results may be helpful in selecting the composition of concrete mixtures containing FA and SF to be used in concrete and reinforced concrete structures subjected to loads at an early age.Therefore, in order to assess strength parameters and parameters of fracture mechanics of young concrete, the tests were conducted after 3, 7 and 28 days following the preparation of batches of concrete.The parameters of concretes modified with FA in the amount of up to 30% have already been fully tested, e.g., ,55,56. Tfcm), splitting tensile strength (fctm) and fracture toughness of young and mature concretes containing two pozzolanic additives, i.e., FA and SF. The analyses took into account the basic parameter of concrete fracture toughness determined under Mode I fracture. The parameter of fracture toughness assessed in the tests was a critical stress intensity factor ;Loss of ignition by burning the individual materials for one hour at 1000 °C;Color—visually.The chemical composition of both additives was determined by the XRF method. A Epsilon 3X spectrometer was applied . On the other hand, analyzed physical parameters were assessed as follows:In addition, Based on the pictures shown in On the basis of the observations of SEM FA pictures shown in The observation of SF grains, shown at 3 different magnifications in Moreover, 0–2 mm pit sand as fine aggregate, 2–8 mm gravel as coarse aggregate, pipeline water and plasticizer were used to prepare concrete mixtures. The tests assessed the combined effect of the addition of both wastes, i.e., FA and SF. SF had a constant amount of the additive, i.e., 10%, the amount of FA was varying. In each series of concrete, OPC was substituted with additives by weight.Mixture without additives (FA00+SF00);Mixture without FA additive and with a 10% SF additive (FA00+SF10);Mixture with a 10% FA additive and with a 10% SF additive (FA10+SF10);Mixture with a 20% FA additive and with a 10% SF additive (FA20+SF10).Four concrete mixtures were used for testing the basic strength of materials characteristics and the fracture toughness, which are as follows:The detailed composition of the above concrete mixtures was given in .fcm, according to PN-EN12390-3 [6 cube specimens (150 mm) for compressive strength tests—N12390-3 ;fctm, according to PN-EN12390-6 [6 cube specimens (150 mm) for splitting tensile strength tests—N12390-6 ;6 beams  for fracture toughness tests at Mode I fracture—endation .For each type of concrete mixture and for each curing time, the following were made:All tests were carried out after 3, 7, and 28 days of curing.Both compression and tensile strength were tested using a compression machine  with a maximum load of 3000 kN. The specimens were loaded statically.Fracture toughness tests, under Mode I fracture, were conducted in accordance with the draft guidelines of RILEM recommendations . The resThe research results were collected thanks to the recording of the results from the press  and clip gauge axial extensometer, which was placed on the clamping test grips. The test setup apparatus with all details is presented in F)–crack mouth opening displacement (CMOD) curve was recorded. The exemplary curve of F–CMOD is shown in For each of the tested cuboidal specimens , the loaMaximum load obtained in the tests;F–CMOD relationship, highlighted in blue (Ci) in Tangent in the first phase on the F–CMOD relationship, highlighted in purple (Cu) in Tangent in the second phase on the It should be noted that the most important data obtained for each of the graphs were :Maximum Additionally, the digital image correlation (DIC) technique was used to assess the shape and trajectory of the crack paths in particular concretes. This modern and increasingly often used measurement method allows for accurate and precise tracking of the cracking processes on the surface of concrete elements during the progressive crack propagation process. Thanks to it, it is possible to distinguish subtle differences in the shape of macrocracks depending on changes in the composition of the tested concretes. Such experiments provide valuable information and are a valuable supplement to the test results of concrete mechanical parameters.The average values of the analyzed mechanical parameters for particular periods of curing (with error bars) are given in From 48% to 81% (after 3 days);From 54% to 65% (after 7 days);From 26% to 34% (after 28 days).Based on the results presented in The positive changes in the mechanical properties of concretes containing fine and chemically active mineral additives  are directly related to the structural changes occurring in the composites. It has been proven in numerous papers to date that these materials significantly modify the microstructure of concrete with a changed cement binder composition, which translates into an improvement in their mechanical parameters. This phenomenon applies to ordinary concretes, e.g., ,92,93,94Due to the chemical composition , physicaDue to the undeniable relationship between the microstructure of concrete composites modified with mineral additives and the mechanical parameters of these materials, i.e., the processes of formation and propagation of microcracks, e.g., ,98, an aA high quantity of the hydration products in the form of ettringite (E) and calcium silicate hydrate of form I (C–S–H (I)) in FA-00+SF-10 and FA-10+SF-10 concretes can be seen in After four weeks, the FA-00+SF-10 and FA-10+SF-10 concretes already had a compact structure with barely visible CH crystals. This is due to the pozzolanic reaction that occurred between SF and CH, which led to a reduction in CH. Furthermore, in concretes containing FA—after 28 days of curing—the FA particle of approximately 8 μm and 2 μm were observed (circled with green dashed lines) c,d. MoreThe rapid development of the C-S-H phase after only 3 days of curing in FA-00+SF-10 concrete b and FA-The above assessment of the microstructure of concretes with a modified composition leads to the conclusion that the combination of chemically active and very fine-grained pozzolanic additives in the form of FA and SF clearly affects changes in the structure of composites at a young age and in the mature period. This is mainly reflected in the nucleation of additional regions of the C-S-H phase, which changes its structure from disordered to compact as the maturation process progresses . It is pIn addition to the microstructural analysis, an in-depth evaluation of the macroscopic cracks formed in the specimens after the tests were conducted. For this purpose, diagnostics with the naked eye and with the use of up-to-date and precise digital image correlation (DIC) technique were used.The beams were usually damaged as a result of a single large crack a, similaThis type of crack development is also confirmed by the results of experiments carried out with the use of the DIC technique ,106,107.The observed increase in the plasticity of concretes containing the combination of the mineral additives could probably be influenced by changes in their structure related to the slow development of additional phases in the cement matrix. The plasticity phenomenon was the most noticeable in FA-20+SF-10 concrete b. This cAdditionally, more brittle forms of failure were observed in concretes with the shortest curing time, i.e., those tested three days after the specimens were formed. This is also confirmed by the results presented by other authors, e.g., ,109,110.This paper investigates the behavior of ternary concretes after incorporating FA and SF. The main purpose of the new tests was to determine the effect of curing time of concretes modified with two additives on their fracture toughness assessed in accordance with Mode I fracture. During the experiments, the fracture toughness of young concretes (3 and 7 days old), and mature concretes (28 days old) were assessed.The experimental results revealed that the addition of wastes, such as FA and SF in OPC, very positively affects the mechanical properties of concrete. Therefore, it is planned to continue the topic discussed in the article and to extend the tests on fracture toughness of concretes containing siliceous fly ash and noncondensed silica fume in terms of the analysis of fracture processes in these materials in complex stress states and the assessment of the microstructure of composites, after the process of their destruction.Nonetheless, based on the obtained test results, the following conclusions can be drawn:Both FA and SF grains are characterized by a homogeneous structure and a similar particle morphology. In the case of FA, the grains have regular and spherical shapes with glazed surfaces  with a uConcrete composites at an early age with siliceous fly ash and noncondensed silica fume in the cement composition obtain higher values of strength parameters (30% to over 60%) as well as fracture mechanics parameters (40% to over 80%) in comparison to the values obtained for plain concrete.3/2 after 3 days, 1.30 MN/m3/2 after 7 days and 1.42 MN/m3/2 after 28 days.In all periods of curing, the maximum fracture toughness occurs in concrete containing only noncondensed silica fume. It is 1.05 MN/mSubstitution of the cement matrix with siliceous fly ash reduces the fracture toughness of concrete, mainly at an early age. This trend is more clearly noticeable in FA20+SF10 concrete.All series of modified concretes have higher values of fracture toughness in all periods of curing compared to FA00+SF00 concrete.A clear increase in values of the critical stress intensity factor is noticeable between the 3rd and 28th day of curing, particularly in the concrete from batches FA10+SF10 and FA20+SF10 . It meanFA00+SF-10 and FA-10+SF-10 concretes had the most developed phases in the structure of the cement matrix, in which C-S-H (I) and (E) dominated at a young age, while after 28 days, mainly the compact C-S-H phase dominated. Moreover, the structure of these concretes was more compact. In contrast, the reference concrete had less developed phases, while the composite with a greater amount of FA showed a distinct delay in growth and the transformation of the C-S-H (I) phase from a fibrous to a rigid form. It was also observed that the structures of these two materials were not fully ordered.The process of crack development in the reference concrete was brittle while in modified concretes—quasi-plastic.In the first 28 days of concrete curing, the preferred solution is to replace cement with SF in the amount of 10% or to use a cement binder substitution with a combination of additives in proportions 10% SF+10% FA.The composition of mineral additives in proportions 10% SF + 20% FA has a negative effect on the fracture mechanics parameters of concretes at an early age.


Concrete is one of the most common and versatile construction materials and has been used under a wide range of environmental conditions. Temperature is one of them, which significantly affects the performance of concrete, and therefore, a careful evaluation of the effect of temperature on concrete cannot be overemphasized. In this study, an overview of the temperature effect on the compressive behavior of plain hardened concrete is experimentally provided. Concrete cylinders were prepared, cured, and stored under different temperature conditions to be tested under compression. The stress–strain curve, mode of failure, compressive strength, ultimate strain, and modulus of elasticity of concrete were evaluated between the ages of 7 and 90 days. The experimental results were used to propose constitutive models to predict the mechanical properties of concrete under the effect of temperature. Moreover, previous constitutive models were examined to capture the stress–strain relationships of concrete under the effect of temperature. Based on the experimental data and the proposed models, concrete lost 10–20% of its original compressive strength when heated to 100 °C and 30–40% at 260 °C. The previous constitutive models for stress–strain relationships of concrete at normal temperatures can be used to capture these relationships under the effect of temperature by using the compressive strength, ultimate strain, and modulus of elasticity affected by temperature. The effect of temperature on the modulus of elasticity of concrete was considered in the ACI 318-14 equation by using the compressive strength affected by temperature and the results showed good agreement with the experimental data. Concrete is considered one of the most important materials in construction. Therefore, it is vital to explore the impact of engineering environments on concrete behavior. Factors affecting the concrete strength are temperature, water/cement (W/C) ratio, coarse/fine aggregate ratio, relative humidity, concrete age, and concrete curing . Since cMost of the mechanical properties of hardened concrete are also related to its compressive strength . The uniPrevious studies were performed on the compressive behavior of concrete at different temperatures. The compressive strength of concrete with the same moisture content decreased as the heating temperature increased ,7. The cThe mechanical properties of concrete can be predicted through fitting analyses of constitutive models. Hognestad et al. developed the first polynomial constitutive model for concrete, which was also one of the earliest constitutive models . This moGood amounts of data exist on the effect of temperature on the mechanical properties and stress–strain relationships of concrete. However, there are limited data for this effect on concrete at different ages. Therefore, concrete cylinders were prepared, cured, and stored under different temperature conditions to be tested under compression at different ages. The stress–strain curve, mode of failure, compressive strength, ultimate strain, and modulus of elasticity of concrete were evaluated at ages between 7 and 90 days. The experimental results were used to propose constitutive models to predict the mechanical properties of concrete under the effect of temperature. Moreover, previous constitutive models for the stress–strain relationships of concrete at normal temperatures were examined to capture these relationships under the effect of temperature.Concrete cylinders were prepared to consider the effect of temperature conditions on the stress–strain curve, mode of failure, compressive strength, ultimate strain, and modulus of elasticity of concrete at different ages. For the relevant experiment, two concrete grades (C20 and C27) of seventy-five concrete specimens each were used in this evaluation. The specimens were prepared according to ASTM C31/C31M . To address the temperature effect alone, no admixtures were added to the concrete mix to counterbalance the alteration in workability due to temperature effects. To produce the concrete batches, an open pan mixer was used as shown in To thoroughly investigate the influence of temperature on the mechanical properties of hardened concrete, the specimens were cured and stored at five different temperatures; 0 °C (32 °F), 21 °C (70 °F), 40 °C (104 °F), 121 °C (250 °F), and 260 °C (500 °F). The concrete specimens were stored in temperature control ovens at the desired temperature until the day of testing as shown in To detect the mechanical properties of concrete, standardized compression tests were performed after the specimens had come to the thermal equilibrium of the normal laboratory temperature. These tests were carried out according to ASTM C39/C39M–20  using a The mechanical properties of the tested specimens were investigated by monitoring the stress–strain relationship, mode of failure, compressive strength, ultimate strain, and modulus of elasticity as following. The stress–strain relationships of concrete are often used as input data in mathematical models. Therefore, it is crucial to evaluate the temperature effect on these relationships to explore the thermal resistance of concrete structural members. The stress–strain curve of concrete is mainly constructed of an ascending part, which ends at the compressive strength and the corresponding ultimate strain, and a descending part . The behavior of the stress–strain relationships of concrete under compression is inevitably changed due to the damage caused by temperature . These cThe increasing rate of slopes of the linear portions of these relationships decreased through the concrete age as the temperature increased. This behavior indicated reductions in the stiffness of concrete. However, dramatic reductions were monitored in the case of freezing. These reductions are attributed to the alteration in the physical form of the hardened cement paste when the hydration proceeds at lower and faster rates in the case of freezing and high temperature, respectively ,6,27. StThe exposure to different temperature conditions altered the mode of failure of the tested specimens as shown in The average compressive strength of each tested group is summarized in The specimens exposed to the moderate temperature of 21 °C achieved only 75% of their 28-day compressive strength at the early age of concrete and further strength was achieved over the concrete age see . The devPrevious constitutive models from the literature to predict the compressive strength of concrete under the effect of temperature are summarized in 2) equal to 0.94. The experimental results and the proposed model indicated a non-linear relationship between the compressive strength and temperature. However, the previous models gave more conservative values and showed linear relationships. More experimental tests are required at a variety of temperatures lower than 21 °C to develop another constitutive model to consider the effect of low temperatures and freezing on the compressive strength of concrete.Based on the experimental data and the proposed model, concrete lost 10–20% of its original compressive strength when heated to 100 °C and 30–40% at 260 °C. A previous study confirmed 10–20% loss of the original compressive strength when heated to 300 °C and 60–75% at 600 °C . Moreove2 equal to 0.91. The ultimate strain corresponding to the maximum stress increased as the temperature increased as demonstrated in 2 equal to 0.97.The slope of the linear portion of the stress–strain relationship for concrete is defined as the initial tangent modulus of elasticity which was considered in this study. The modulus of elasticity is dependent mainly on the W/C ratio in the mixture, the age of concrete, and temperature . The varcf’) using the following Equation (4):The modulus of elasticity of concrete can be predicted using empirical methods provided in the current standards. These methods depend on the compressive strength of concrete. According to ACI 318-14 , the modTo consider the effect of temperature in the ACI 318-14 equation, the compressive strength affected by temperature and predicted by Equation (1) was used. Comparisons were conducted between the modulus of elasticity experimentally measured and analytically predicted as presented in Previous constitutive models were proposed to trace the compressive stress–strain relationship of concrete at normal temperatures. The constitutive models proposed by Shah et al.  and Kentcf is the stress value at any strain cε, c′f and coε are the maximum stress and the corresponding ultimate strain obtained from the proposed models (Equations (1) and (2)), respectively. The parameters A and K are used to determine the shape of the curve in the ascending and descending parts, respectively. The modulus of elasticity (cE) of concrete is obtained based on the proposed model for concrete under the effect of temperature (Equation (3)). 50uε is the strain corresponding to 50% of the maximum concrete strength.Where In this study, an overview of the temperature effect on concrete is experimentally provided. Concrete cylinders were prepared, cured, and stored under different temperature conditions to be tested under compression. The stress–strain curve, mode of failure, compressive strength, ultimate strain, and modulus of elasticity of concrete were evaluated at ages between 7 and 90 days. The experimental results were used to validate previous constitutive models and to develop new models to predict the mechanical properties of concrete under the effect of temperature.The previous constitutive models for stress–strain relationships of concrete at normal temperatures can be used to capture these relationships under the effect of temperature by using the compressive strength, ultimate strain, and modulus of elasticity affected by temperature and developed in this study.The effect of temperature on the modulus of elasticity of concrete can be considered in the ACI 318-14 equation by using the compressive strength affected by temperature.The increasing rate of slopes of the linear portions of the stress–strain relationships of concrete decreased through the concrete age as the temperature increased. This behavior indicated a reduction in the stiffness of concrete. However, dramatic reductions were monitored in the case of freezing.The exposure to different temperature conditions altered the mode of failure of the tested specimens. A well-formed cone without vertical splitting was the mode of failure observed for the concrete cylinders at 21 °C and 40 °C. Columnar vertical cracking was the mode of failure observed at a temperature of 121 °C. Side fractures at the top or bottom of the concrete cylinders were the mode of failure noticed for the specimens at the highest temperature (260 °C).Based on the experimental data and the newly proposed model, concrete lost 10–20% of its original compressive strength when heated to 100 °C and 30–40% at 260 °C.The compressive strength of the frozen specimens remained at low levels over their ages. No significant strength was earned as the concrete aged. Keeping the concrete cylinders frozen at an early age could be very harmful and caused more damage to the concrete strength. Therefore, it is particularly important to protect concrete from freezing at an early age if possible, to complete the hydration process and gain most of the strength.Most of the compressive strength was gained at the early age of concrete due to the exposure to elevated temperature. However, reductions in the strength occurred at later ages.The specimens exposed to the moderate temperature of 21 °C achieved only 75% of their 28-day compressive strength at the early age of concrete and further strength was achieved as the concrete age increased.


Pseudomonas aeruginosa (PA) persisters, including viable but non-culturable (VBNC) forms, subpopulations of tolerant cells that can survive high antibiotic doses, is the main reason for PA lung infections failed eradication and recurrence in Cystic Fibrosis (CF) patients, subjected to life-long, cyclic antibiotic treatments. In this paper, we investigated the role of subinhibitory concentrations of different anti-pseudomonas antibiotics in the maintenance of persistent (including VBNC) PA cells in in vitro biofilms. Persisters were firstly selected by exposure to high doses of antibiotics and their abundance over time evaluated, using a combination of cultural, qPCR and flow cytometry assays. Two engineered GFP-producing PA strains were used. The obtained results demonstrated a major involvement of tobramycin and bacterial cell wall-targeting antibiotics in the resilience to starvation of VBNC forms, while the presence of ciprofloxacin and ceftazidime/avibactam lead to their complete loss. Moreover, a positive correlation between tobramycin exposure, biofilm production and c-di-GMP levels was observed. The presented data could allow a deeper understanding of bacterial population dynamics during the treatment of recurrent PA infections and provide a reliable evaluation of the real efficacy of the antibiotic treatments against the bacterial population within the CF lung.The occurrence of  Pseudomonas aeruginosa lung infections represent the main cause of morbidity and mortality for Cystic Fibrosis (CF) patients, who suffer from recurrent pulmonary exacerbations and decreased lung function [P. aeruginosa strain is isolated again in sputum bacterial culture [function . Patientfunction , which f culture . Such re culture . Moreove culture ,6 report culture . Both th culture ) or indu culture  as eithe culture . An inte culture ,11, acco culture .Staphylococcus aureus and P. aeruginosa VBNC cells [P. aeruginosa in sputum samples of intermittent and/or chronic CF patients [P. aeruginosa VBNC subpopulation in an in vitro biofilm model, exposed to both starvation and low antibiotic concentrations. The use of flow cytometry, after live/dead staining, allowed us to confirm the viability of the non-culturable subpopulation [P. aeruginosa strain producing the Green Fluorescent Protein (GFP), and exposed them to stress conditions, mimicking those found in the CF lung environment between antibiotic treatments, i.e., starvation and low drug concentrations. The development/maintenance of culturable persisters and VBNC cells was also tested by a novel flow cytometry protocol and the behavior of tobramycin, ciprofloxacin and six further antibiotics and/or their combinations was evaluated.We have long investigated the role of different antibiotics and antibiotic concentrations in the development of NC cells ,11,13. Mpatients , and havP. aeruginosa PAEG1 was obtained by transforming P. aeruginosa PAO1-N with the recombinant shuttle vector pHERD30T encoding the modified thermostable GFP(ASV) (The strain GFP(ASV) .gfp(ASV) gene was cloned under the influence of the araBAD promoter, the ability of the recombinant strain to produce the GFP protein was assessed by fluorescence microscopy and flow cytometry, after the exposure for 3 h to 0.2% arabinose.As the P. aeruginosa cells over time, as shown in The microscopy assays evidenced an increasing fluorescence emission by The observed data were then confirmed in flow cytometry assays, which reported the same increase of fluorescence intensity in the 3 h-experiment, as evidenced in P. aeruginosa by flow cytometry in planktonic and sessile cultures of P. aeruginosa PAEG1, as shown in The detection of the GFP(ASV) protein was used to quantify P. aeruginosa cultures and flow cytometry viable counts were compared to plate counts; although causing an increase of GFP production, an arabinose concentration of 0.2% was not satisfying to obtain a reliable and reproducible count. The increase of the arabinose concentration to 0.5% allowed us to reach counts of fluorescent cells corresponding up to 37.45% of the culturable population. Flow cytometry counts of gentamicin (20× MIC) exposed P. aeruginosa biofilms were then compared with those obtained by both culture and qPCR, showing the ability of the former technique to detect up to 37.15% of the total bacterial population. The results obtained by flow cytometry were thus considered to be 30% of the real bacterial counts and the obtained results were corrected accordingly throughout the work.The first set of experiments was performed on exponential phase P. aeruginosa PAEG1 was evaluated  of nine anti-pseudomonas drugs against the engineered strain valuated .P. aeruginosa biofilms exposed to stress conditions.Piperacillin at 1000× MIC was used to select biofilm persistent cells after 24 h exposure and 1/4× MIC of each of the additional antibiotics was used in monitoring assays over time of P. aeruginosa PAEG1 biofilms were developed in Lysogenic Broth (LB) for 24 h; then, the medium was replaced with LB broth containing 4 mg/mL (1000× MIC) piperacillin for 24 h. Immediately before and after antibiotic exposure, the amounts of culturable, Total Viable Cells (TVCs) and VBNC cells were determined, as shown in P. aeruginosa population, which significantly (p < 0.05) decreased from 7.59 × 106 to 2.91 × 105 CFU/mL , while the TVCs showed only a slight (3.08 × 107 vs. 2.38 × 107 cells/mL) decrease and the VBNC cells amount was unchanged.Also, in these assays, GFP-based flow cytometry counts resulted about 30% of the qPCR-counted cells. The high dose of the antibiotic mostly affected the culturable After exposure to 1000× MIC piperacillin (T0), biofilms were washed with phosphate buffered saline (PBS) and subcultured in Non-Nutrient (NN) broth, alone or supplemented with 1/4× MIC of each tested antibiotic. Biofilms were maintained in these conditions for 45 days, assessing the abundance of CFUs and TVCs cells every 15 days  .TVCs exhibited a major decrease in the first 15 days of stress exposure, then they remained generally stable  until the end of the experiment in almost all tested conditions, except for colistin- or ciprofloxacin-exposed biofilms, where they continuously decreased in the first 30 days and then showed a sudden increase at the last time point.P. aeruginosa culturable population was the most variable between stress conditions, exhibiting a drug-specific pattern. In NN-, tobramycin- and fosfomycin-exposed biofilms, CFUs abundance showed an increase in the first 15 days, reaching a concentration of about 1–2 × 106 CFU/mL, and remained stable for the following 30 days. When exposed to colistin, ceftazidime and ceftolozane/tazobactam they resulted overall unchanged until the end of the experiment, while, upon exposure to meropenem, their amount uniformly decreased, reaching the final concentration of 2.35 × 104 CFU/mL, the lowest detected among all experimental conditions; on the contrary, ceftazidime/avibactam-exposed biofilms reported an increase of the culturable population, which finally matched the TVC amount (about 5 × 106 cells/mL) after 45 days. Finally, ciprofloxacin subinhibitory concentrations determined a great decrease of CFUs in the first month of antibiotic exposure, reaching 4.63 × 104 CFU/mL, followed by a sudden increase up to 1.20 × 106 CFU/mL, at T45, where no discrepancy with TVC amount was evidenced.The amount of the The selection of ciprofloxacin- and ceftazidime/avibactam resistant mutants was excluded by the MIC determination of several isolates grown on Cystine-Lactose-Electrolyte-Deficient (CLED) agar plates at the last time point, which resulted unchanged compared with that determined before starting the experiment .P. aeruginosa population was then calculated as the difference between TVCs and CFUs; only values ≥0.5 log were considered  was recovered until the end of the experiment. Subinhibitory concentrations of both ciprofloxacin and colistin resulted in a decrease of VBNC cells for the first 30 days of exposure; compared to the other antibiotics, ciprofloxacin-exposure resulted in a significantly (p < 0.001) lower amount of VBNC P. aeruginosa, and eventually in their loss. On the contrary, colistin-exposure was associated to a subsequent increase of non-culturable cells, which at the end were 2.81 × 106 cells/mL, an amount similar to that evidenced at the first time point (T15). In the presence of fosfomycin, meropenem, ceftazidime and ceftolozane/tazobactam, VBNC cells remained overall unchanged throughout the 45 days of antibiotic exposure, i.e., 2–4 × 106 cells/mL. Meropenem-exposed biofilms showed the highest proportion (98.4%) of VBNC cells inside the TVCs counted at the end of the experiment. Finally, it is to note that in the presence of the association ceftazidime/avibactam, while VBNC cells were detected at T15 and T30, at T45 they disappeared, suggesting a role for the β-lactamases inhibitor avibactam in contrasting P. aeruginosa VBNC cells maintenance over time.In P. aeruginosa biofilms were developed in Mueller Hinton (MH) broth alone or supplemented with 1/4× MIC of each antibiotic and the amount of biofilm  and c-di-GMP  produced in the absence/presence of the antibiotics were compared. The results are reported in Finally, we measured the influence of sub-MIC of the eight different antibiotics/antibiotic combinations on the amount of biofilm matrix and the production of the second messenger c-di-GMP. To do this, p < 0.05) enhanced in the presence of subinhibitory concentrations of ciprofloxacin, tobramycin, ceftazidime/avibactam and ceftolozane/tazobactam; on the contrary, it was significantly (p < 0.05) repressed upon exposure to colistin, fosfomycin, meropenem and ceftazidime. The c-di-GMP production was increased only in presence of tobramycin and ciprofloxacin, while it was repressed when biofilms were grown with colistin or β-lactam antibiotics.Biofilm development was significantly  antibiotic concentrations can still be found in the lung; the same thing can also happen even during antibiotic treatments in the deepest layers of microbial biofilms , where bP. aeruginosa CF lung infections [P. aeruginosa VBNC cells in an in vitro biofilm model, highlighting a major contribution of tobramycin in their long time  maintenance [Our group has recently described the involvement of sub-MIC ciprofloxacin and tobramycin  in the ntenance .P. aeruginosa live cells by flow cytometry, the GFP-producing P. aeruginosa strain PAEG1 was constructed and used to develop the biofilm models. GFP production is a reliable marker of viability that allowed us to overcome the weaknesses inherent to flow cytometry counts after live/dead staining. As a matter of fact, the two dyes (SYBR Green and propidium iodide) used could bind biofilm intrinsic and non-specific targets, like extracellular DNA [P. aeruginosa PAEG1 strain resulted a suitable study model, since the fluorescent protein was adequately expressed by the host strain and easily detected in both microscopy and flow cytometry assays. Although only about 30% of the total live P. aeruginosa population emitted a detectable fluorescence, the same rate of fluorescent cells was recovered in both planktonic and sessile cultures, suggesting a systematic error, most likely due to the flow cytometer itself. One possible explanation is that the modified GFP(ASV) showed excitation and emission wavelengths of 500 and 510 nm, respectively, while the flow cytometer laser was set by default on 488 nm for excitation and 525/30 nm for emission; such discrepancy could account for the observed low amount of GFP-expressing bacterial cells, likely due to the loss of the cells emitting a low fluorescent signal, which resulted however unaffected by the sample type, i.e., planktonic or biofilm-embedded.In this paper, we used the previously adopted experimental settings with some improvements. First of all, to better detect ular DNA , thus afular DNA . The P. P. aeruginosa biofilms, in the present work, we added a pre-treatment of P. aeruginosa biofilms with 1000× MIC to select the persistent subpopulation already present before the subculture in nutrient-depleted medium without/with sub-MIC of antibiotics. This step was added since the presence of persisters most likely mirrors what happens in the lung environment after the antibiotic treatment, i.e., with the survival of persistent P. aeruginosa cells, both culturable and VBNC [P. aeruginosa population cells from the culturable to the VBNC state after high doses of antibiotic exposure, in agreement with what reported by Deschaght [As the second modification of the previously performed assays on VBNC induction and maintenance in and VBNC ,21. Althand VBNC . The obteschaght  in CF paThe selected persistent cells, both culturable and non-culturable, were then maintained for 45 days in absence of nutrients and presence of 1/4× MIC of different antibiotics and the development of both populations was monitored over time, with a special focus on the proportion of VBNC cells, with respect to the specific drugs.P. aeruginosa subpopulation, which allowed the survival of a live and culturable subpopulation [A remarkable result was the total loss of non-culturable cells in absence of antibiotics; such outcome likely derived from the observed TVCs reduction and CFUs increase  observedpulation , withoutP. aeruginosa VBNC maintenance confirmed our previous results obtained with six months-monitored starvation- and antibiotic-exposed biofilms [P. aeruginosa isolates resulted culturable at T45, thus excluding the induction of a resistant, predominant subpopulation [The observed involvement of ciprofloxacin and tobramycin in biofilms . Ciproflbiofilms . No incrpulation . The obsP. aeruginosa VBNC subpopulation after one month of exposure till the end of the experiment, in line with what was previously observed [Tobramycin induced the development and maintenance of a observed  and confobserved .phoP and pmrA [Cell wall inhibiting antibiotics and colistin subinhibitory concentrations were also tested. As far as concern colistin, it caused a constant reduction over time of the VBNC subpopulation, while the culturable ones were unaffected by the presence of the drug; however, at the last time point, the appearance of a VBNC subpopulation was observed. Colistin is well known to act on dormant cells , in agreand pmrA .P. aeruginosa response to different cell wall inhibiting antibiotics was not uniform: fosfomycin resulted as the least efficient in reducing the TVCs over time and induced the highest amount of VBNC cells at the end of the experiment, compared to all tested antibiotics; exposure to meropenem induced a uniform decrease of CFUs and increase of the VBNC proportion among the total viable P. aeruginosa population, which resulted in the highest at T45  and the subsequent elicited bacterial response via specific receptors. Further experiments are currently ongoing to verify this hypothesis.P. aeruginosa VBNC cells induction, biofilm and c-di-GMP production were measured upon exposure to sub-MIC antibiotic concentrations. Biofilm production was enhanced in the presence of sub-MIC of ciprofloxacin, tobramycin, avibactam and tazobactam and it was repressed in the presence of sub-MIC of colistin and all tested cell wall targeting drugs. Ciprofloxacin, tobramycin and β-lactams effects are in agreement with previous observations [P. aeruginosa biofilm matrix and bacterial motility, which is pivotal in the first stages of biofilm formation, in agreement with our results. The exposure to β-lactamases inhibitors was shown to reduce biofilm production as well, in particular the combination piperacillin/tazobactam [P. aeruginosa VBNC subpopulation.In an attempt to determine the putative effectors of rvations ,30,31. Frvations . Accordizobactam , while oWhen considering together the induction of VBNC cells, biofilm production and c-di-GMP levels, our results succeeded in demonstrating a direct correlation among these three bacterial responses only for tobramycin-exposed biofilms; for all other antibiotics VBNC cells induction related neither to biofilm production or to c-di-GMP expression.P. aeruginosa biofilms. Two of the main goals of antimicrobial research are the infectious biofilms eradication and to avoid the insurgence of antimicrobial resistance. However, the present work shows that each antibiotic can exert a specific effect on the bacterial viability and gene expression, not following a unique and defined pathway. P. aeruginosa is known to possess several redundant regulatory mechanisms [Overall, the obtained data showed different effects exerted by subinhibitory antibiotic concentrations on chanisms ,34, baseP. aeruginosa PAO1-N, its isogenic recombinant version carrying the pcdrA plasmid, encoding the GFP, and the Escherichia-Pseudomonas shuttle vector pHERD30T were kindly provided by Professor Paul Williams . The Pfu polymerase and the plasmid pGFP29, containing a mutated gfp gene copy, encoding a thermostable GFP protein (GFP(ASV)), containing an additional tail of 13 amino acids (RPAANDENYAASV) at the C-terminal, were kindly provided by Roberto Spurio . The strain P. aeruginosa ATCC 27853 belongs to the strains collection of Microbiology section of Department of Life and Environmental Sciences, Polytechnic University of Marche, Ancona, IT.The laboratory strains All bacteriological media were purchased from Oxoid , while antibiotics and arabinose from Sigma-Aldrich .Bacterial strains were grown on LB Broth, subcultured on McConkey agar plates and conserved as stock cultures in LB broth supplemented with 20% glycerol.P. aeruginosa PAO1-N recombinant strain, able to produce a stable GFP, the gfp(ASV) gene was cloned into the pHERD30T vector using the In-Fusion HD Cloning Plus kit , according to the manufacturer instructions. Briefly the gene was amplified from the pGFP29 plasmid by PCR assays using 0.1 ng/µL of plasmid, 5 U of Pfu polymerase and 0.5 µM of the primer pair GFP-F: 5′—GAGATATACATACCCATGCGTAAAGGAGAAGAACTTTTCAC—3′/ GFP-R: 5′—ATTCTTATCAGATCCGGAGCTGCATGTGTCAGAGGTTTTC—3′. The primers were designed to present 15 bp (underlined) complementary to the multiple cloning site of the pHERD30T vector. Reactions were performed using an annealing temperature of 45 °C and the amplification product (898 bp) was visualized by gel electrophoresis in 1.5% agarose gel in 1× TAE buffer. The pHERD30T vector was linearized by digestion with NcoI restriction enzyme , then 100 ng of both vector and PCR amplicon were used in the In-Fusion cloning reaction, performed at 50 °C for 15 min and maintained in ice. The recombinant vector, visualized by gel electrophoresis, was propagated in the strain E. coli stellar (provided with the kit), then extracted using the GenElute™ Plasmid Miniprep Kit (Sigma-Aldrich) and introduced in P. aeruginosa PAO1-N by electroporation. Reactions were performed using 90 µL of competent P. aeruginosa cells and 100 ng of the recombinant plasmid in a Gene PulserXcell ™ electroporator  with the following setting: 2.5 kV, 25 µFD, 200 OHMs, 5 ms. The transformed cells were diluted in 1 mL of LB broth, incubated for 1–3 h at 37 °C and then plated on LB agar containing 20 µg/mL gentamicin. The growing colonies were amplified and then used in the subsequent assays.To produce a P. aeruginosa PAEG1 strain was assessed for its ability to produce the GFP in fluorescence microscopy and flow cytometry assays. As the gfp(ASV) gene was cloned under the control of an arabinose-dependent promoter, its expression was induced by incubating P. aeruginosa PAEG1 in LB broth supplemented with 0.2% arabinose at 37 °C for 0, 1, 2 and 3 h.The obtained recombinant For fluorescence microscopy assays, 1 mL of induced cultures was filtered in polycarbonate 0.22 µm black filters  and observed in an Olympus BX51 microscope, using 100× magnification and a blue light filter (450–490 nm).g and resuspended in PBS to be read by the cytometer. Assays were performed in a Guava Millipore cytometer and analyzed by GUAVASOFT 2.2.3 software. Both side scatter and green (GRN) fluorescence were gated to discriminate bacterial cells from the background. The GFP fluorescence was excited at 488 nm, a threshold value was set in the GRN channel and the emissions were read at 525/30 nm. Signal detection was enhanced by logarithmic amplification (4 decades); a total number of 20,000 events/replicate was analyzed to increase statistical significance. The GFP mean fluorescence emission was plotted against the incubation time. Assays were run in technical duplicate and biological triplicate.In flow cytometry assays, 200 µL of induced cultures were centrifuged at 16,000× P. aeruginosa using the broth microdilution method according to the CLSI guidelines [P. aeruginosa ATCC 27853 was used as the reference strain.The MIC values of each antibiotic was determined against idelines . When teP. aeruginosa PAEG1 was always grown and maintained in media supplemented with gentamicin 20 µg/mL, to allow the maintenance of the recombinant pHERD30T vector. P. aeruginosa biofilms were grown in LB medium at 37 °C overnight in 140 mm petri dishes, each containing fifteen 24 × 24 mm cover glasses, and challenged with LB broth supplemented with 1000× MIC piperacillin for 24 h at 37 °C. Then the medium was removed, biofilms were washed with PBS and subcultured in Non-Nutrient (NN) broth without/with 1/4× MIC of each antibiotic, as previously described [Unless otherwise stated, escribed . The samBiofilms were analyzed before and after exposure to piperacillin and every 15 days after subculture in NN broth . At each time point, 3 cover glasses/condition were removed from the plates and dipped in 2.5 mL of PBS. The biofilm adherent to the glasses was removed mechanically and briefly sonicated.P. aeruginosa populations were quantified in the detached biofilms as follows:The culturable population was counted by plate count assays, performed on CLED agar plates, evaluated after 24, 48 and 72 h from the inoculum.ecfX-qPCR and flow cytometry assays; the formers were performed using a previously described protocol [P. aeruginosa amounts counted by qPCR and flow cytometry.The TVCs were determined by both protocol , demonstThe VBNC population was estimated as discrepancy between TVCs and culturable cells. The obtained values were not considered when the discrepancy was ≤0.5 log.The different P. aeruginosa PAO1-N strain, carrying a plasmid encoding the GFP protein under the influence of the cdrA promoter and responding to c-di-GMP levels. P. aeruginosa biofilms were developed in 96-well flat bottom dark plates in MH broth, with/without 1/4× MIC of each antibiotic. When testing the combinations ceftazidime/avibactam and ceftolozane/tazobactam, the β-lactamase inhibitor concentration was set at 4 µg/mL.The role of the sub-inhibitory concentrations of the tested antibiotics was verified even in biofilm and c-di-GMP production, using a P. aeruginosa biofilms. After removing the planktonic phase and washing the plate wells with water, biofilms were mechanically detached and resuspended in PBS. The plate was then read using a Synergy HT MicroPlate Reader Spectrophotometer , measuring both biofilms optical density (OD) at 550 nm and fluorescence . The fluorescent signal was normalized by calculating the Fluorescence/OD550 ratio.Biofilm production was then quantified as previously described ; for theP. aeruginosa populations  in the different stress conditions, as well as the biofilm and c-di-GMP production was assessed by Student’s t-test .The significance of the amount of P. aeruginosa persistent and VBNC cells hamper the eradication of chronic CF lung infections and antibiotic treatment seems to be crucially involved in their induction and maintenance in the lung environment. The present work highlighted the specific contribution of different drugs to P. aeruginosa persisters and VBNC cells induction, underlining the major role of protein and cell wall synthesis inhibition in their development; on the contrary, DNA replication inhibition and the use of non-β-lactam β-lactamase inhibitors seem to limit the insurgence of the VBNC phenotype, thus constituting promising strategies to counteract P. aeruginosa resilience.The P. aeruginosa adaptation to each specific stress condition. The characterization of the molecular bases of the antibiotic-induced P. aeruginosa gene expression seems thus a pivotal approach to shed light on the factors underpinning the failure of P. aeruginosa lung infection eradication and to design novel, more effective therapeutic treatments.Moreover, the variety of the bacterial response to each antibiotic seems to suggest a complex and redundant regulatory system, based on different regulatory proteins, able to mediate