Sand + crushed coarse aggregate ( b ) Sand + gravel coarse aggregate
Self-consolidating concrete (SCC) mixtures designated for precast, prestressed applications should be highly workable to flow easily through restricted spacing and completely encapsulate reinforcements without any mechanical vibration. Key workability characteristics of SCC can be described in terms of filling ability, passing ability, and resistance to segregation. These properties are typically characterized by data that relate to specific testing methods. In general, these methods include the components required for evaluating simultaneously filling ability, passing ability, and resistance to segregation, since these properties are rather interrelated. In this investigation, 33 SCC mixtures made with various mixture proportioning parameters, including maximum size and type of aggregate, type and content of binder, and w/cm were evaluated. The mixtures were prepared using crushed aggregate and gravel of three different nominal sizes, w/cm of 0.33 and 0.38, and three binder compositions: Type MS cement, Type HE cement with 30% of slag replacement, and Type HE cement with 20% of Class F fly ash. Comparisons and correlations among various test results used in evaluating the workability responses obtained for these mixtures are established. This is done to highlight advantages and limitations of the various test methods that can be used to assess workability of SCC designated for prestressed applications. Appropriate combinations of test methods that can be used to assess workability of SCC at the precast plant are recommended, and ranges of acceptance of the various test methods are established.
Performance-Based Specifications of Self-Consolidating Concrete Used in Structural Applications
Proper selection of test methods and workability specifications are key concerns in the optimization and control testing of self-consolidating concrete (SCC). An experimental program was carried out to evaluate the suitability of various test methods for workability assessment and to propose performance specifications of such concrete used in structural applications. Various workability characteristics were determined for approximately 70 SCC mixtures made with water-cementitious material ratios (w/cm) of 0.35 and 0.42. Workability responses included the slump flow, J-Ring, V-funnel flow time, L-box, filling capacity, and surface settlement tests. Comparisons of various test methods indicate that the L-box blocking ratio (h 2 /h 1 ) and the J-Ring flow diameter can be related to filling capacity values determined using the caisson test. It is recommended that SCC used in structural applications should have slump flow values of 620 to 720 mm. To ensure proper filling capacity greater than 80%, such concrete should have high passing ability that corresponds to L-box blocking ratio (h 2 /h 1 ) ≥ 2 0.7, J-Ring flow of 600 to 700 mm, slump flow minus J-Ring flow diameter ≤ 50 mm, or V-funnel flow time ≤ 8 seconds. Such SCC should have a settlement rate of 0.16%/h at 30 minutes, corresponding to 0.5% maximum settlement.
Heavy metal adsorption from industrial wastewater by PAMAM/TiO2 nanohybrid: Preparation, characterization and adsorption studies
Abstract This study investigates the removal of heavy metals from industrial wastewater using TiO 2 nanoparticles modified with poly-amidoamine dendrimer (PAMAM, G 4 ). The metal ions Cu 2 + , Pb 2 + , and Cd 2 + were selected as model pollutants. FTIR, SEM, BET surface area and Zeta potential measurements, and Dispersion Observing methods have been employed for characterizing the synthetic nanohybrid and these techniques indicated that the PAMAM loaded on to TiO 2 have been favorably synthesized. The effects of several parameters including initial metal ion concentration, temperature, pH solution, the nanohybrid dosage and contact time were studied. The experimental data were analyzed using equilibrium isotherm and kinetics models. The adsorption process was studied using isotherm, kinetics, and thermodynamics models. The analysis shows that the Langmuir isotherm and pseudo-second-order kinetics are appropriate models for the adsorption process. The thermodynamic parameters illustrate that the absorption is endothermic and spontaneous. The findings suggest that the nanohybrid is a good super absorbent for the removal of heavy metals from industrial effluents.