Measurement and Modeling of the Ability of Crack Fillers to Prevent Chloride Ingress into Mortar.
A common repair procedures applied to damaged concrete is to fill cracks with an organic polymer. This operation is performed to increase the service life of the concrete by removing a preferential pathway for the ingress of water, chlorides, and other deleterious species. To effectively fulfill its mission of preventing chloride ingress, the polymer must not only fully fill the macro-crack, but must also intrude the damage zone surrounding the crack perimeter. Here, the performance of two commonly employed crack fillers, one epoxy, and one methacrylate, are investigated using a combined experimental and computer modeling approach. Neutron tomography and microbeam X-ray fluorescence spectrometry (μXRF) measurements are employed on pre-cracked and chloride-exposed specimens to quantify the crack filling and chloride ingress limiting abilities, respectively, of the two polymers. A two-dimensional model of chloride transport is derived from a mass balance and solved by the finite element method. Crack images provided by μXRF are used to generate the input microstructure for the simulations. When chloride binding and a time-dependent mortar diffusivity are both included in the computer model, good agreement with the experimental results is obtained. Both crack fillers significantly reduce chloride ingress during the 21 d period of the present experiments; however, the epoxy itself contains approximately 4 % by mass chlorine. Leaching studies were performed assess the epoxy as a source of deleterious ions for initiating corrosion of the steel reinforcement in concrete structures.
Reducing Cracks in Concrete Bridge Decks Using Shrinkage Reducing Admixture
Restrained shrinkage cracking of concrete bridge decks creates a significant durability problem. Major admixture suppliers in the United States have introduced a new category of chemical admixtures called shrinkage reducing admixtures (SRAs). SRAs work by reducing the surface tension of pore water and thereby decreasing the capillary stress and shrinkage induced by drying. Several studies have reported that using SRAs in concrete mixtures is one of the most effective ways of reducing shrinkage cracking. The purpose of this study was to investigate the effectiveness of SRAs in reducing drying shrinkage in Virginia Department of Transportation (VDOT) concrete mixtures and thus reducing cracks in bridge decks. Nine bridges located in VDOT’s Northern Virginia, Staunton, and Fredericksburg districts were selected for study. Three different SRA products were used. With the exception of one mixture, the maximum cementitious content was limited to 600 lb/yd³ . Fresh and hardened concrete properties were determined for each mixture, and field placement details were documented. The results showed that low cementitious concrete with SRA was effective in minimizing bridge deck cracking. The study showed that bridges with fewer and narrower cracks or no cracks can be constructed and that proper construction practices are needed to reduce bridge deck cracking. The study recommends the use of SRA with a lower cementitious content in VDOT bridge deck concrete mixtures. A VDOT special provision was developed for the future use of SRA in concrete mixtures.
Concrete Bridge Deck Condition Assessment and Improvement Strategies
The aging and deterioration of bridges in Utah mandates increasingly cost-effective strategies for bridge maintenance, rehabilitation, and repair (MR&R). Although the substructures and superstructures of bridges in Utah are in relatively good structural condition, the bridge decks are deteriorating more rapidly due to the routine application of deicing salts, repeated freeze-thaw cycles, and other damaging effects. Therefore, the Utah Department of Transportation (UDOT) initiated this research to ultimately develop a protocol offering guidance about when and how a bridge deck should be rehabilitated or when it should be replaced. The research specifically focused on concrete bridge deck performance issues, condition assessment techniques, rehabilitation methods, and bridge management system (BMS) concepts. An extensive literature review was conducted to identify condition assessment methods used to detect concrete bridge deck deterioration, as well as to identify rehabilitation methods for deck repairs. A questionnaire survey was also conducted to identify the state of the practice for bridge deck management by state departments of transportation (DOTs) throughout the United States. The survey addressed issues such as climate and traffic information, new deck construction, winter deck maintenance, deck deterioration, deck condition assessment, and deck rehabilitation. The results of the research show that a bridge deck management system is essential in maintaining the integrity of reinforced concrete bridge decks. Developing a successful system requires routine inspection and monitoring to enable prioritization of MR&R strategies for individual bridges. Many types of technologies are available for assessing the condition of concrete bridge decks, but the survey results suggest that only five methods are frequently used to monitor and detect bridge deck deterioration. These methods are visual inspection, chaining, chloride concentration testing, coring, and half-cell potential testing. The survey identified chloride-induced corrosion, freeze-thaw cycling, and poor construction practices as the most common sources of bridge deck deterioration. Results also indicate that distress is most frequently manifested as cracking, delaminations, spalling, potholes, and scaling. Generally, the extent of deterioration that constitutes a full-deck replacement was given by survey respondents as 30 to 50 percent of the deck area. One of the major contributors to bridge deck deterioration is winter deck maintenance. To mitigate the negative effects of deicing chemicals, DOTs are employing preventative measures such as increasing concrete cover over the reinforcement, using epoxy-coated reinforcement, including appropriate admixtures in the concrete mixture, and facilitating proper curing of the concrete. Rehabilitation options listed for bridge decks include electrochemical rehabilitation, concrete removal and patching, surface treatments, and epoxy injections. UDOT should develop and implement a formal BMS with a searchable database containing information about the types of distress manifested on individual bridges, causes for the distress, values of measured test parameters, types of rehabilitation methods performed on the bridge deck, costs for rehabilitation methods, and service life extensions as a result of particular rehabilitation methods. Supporting data should be regularly collected through inspection and monitoring programs to facilitate prioritization of MR&R strategies for individual bridges and to evaluate the impact of such strategies on the overall condition of the network. Performance indices based on selected condition assessment parameters should be developed for use in BMS analyses, and mathematical deterioration models should be calibrated for forecasting network condition and predicting funding requirements for various possible MR&R strategies.