Low-Cracking High-Performance Concrete (LC-HPC) Bridge Decks: Shrinkage-Reducing Admixtures, Internal Curing, and Cracking Performance

Abstract

The development, construction, and evaluation of low-cracking high performance concrete (LC-HPC) bridge decks is described based on laboratory tests of mixtures containing shrinkage-reducing admixtures and mineral admixtures in conjunction with internal curing and experiences gained during the construction of decks bid in accordance with LC-HPC specifications and control decks constructed in accordance with standard specifications in Kansas. The laboratory portion of the study involves the 53 concrete mixtures evaluated based on free shrinkage, freeze-thaw durability, scaling resistance, compressive strength, and air-void system stability. The study includes mixtures containing different dosages of two shrinkage-reducing admixtures (SRAs) in combination with surfactant-based and polymer-based air-entraining admixtures (AEAs) and air contents ranging from 3.5 to 9 percent. Mixtures containing different combinations of pre-wetted lightweight aggregate (LWA), Grade 100 slag cement, and silica fume are also evaluated. The majority of shrinkage occurs at early ages. Higher dosages of SRA reduce both early-age and long-term shrinkage, with these reductions in shrinkage concentrated within the first 90 days. Higher SRA dosages contribute to larger air-void spacing factors and greater losses in air content from plastic to hardened concrete, leading to decreased freeze-thaw durability and scaling resistance. The detrimental effects on freeze-thaw durability and scaling resistance caused by SRAs can be mitigated by the use of air contents of 7 percent or more. When used with an SRA, mixtures containing the polymer-based AEA exhibit significantly lower freeze-thaw durability and scaling resistance than mixtures containing the surfactant-based AEA. This lower durability is likely due to the larger air-void spacing factors that are observed in the mixtures containing the polymerbased AEA. The replacement of a portion of total aggregate with an equal volume of pre-wetted LWA reduces both early-age and long-term shrinkage. Shrinkage is reduced additionally as slag cement is used as a partial replacement (30 percent by volume) for portland cement in conjunction with LWA, and again as silica fume is used a partial replacement (nominally 3 percent by volume) for portland cement in conjunction with LWA and slag cement. The additions of slag and silica fume contribute to reduced shrinkage primarily within the first 30 days of drying. The use of LWA, slag, or silica fume do not significantly affect freeze-thaw durability, scaling resistance, or strength; slag and silica fume, however, were observed to decrease scaling resistance to a degree. The second portion of the study involves the construction and evaluation of 16 LC-HPC and 11 control bridge decks, the latter constructed in accordance with standard specifications for state bridge construction, in Kansas, as well as another deck bid under but not constructed in accordance with the LC-HPC specifications. Experiences and lessons learned during construction are described, as is the cracking performance of each deck. The results indicate that the degree of compliance with LC-HPC specifications corresponds to the degree of reduction in cracking. The LCHPC decks exhibit lower early-age cracking and a slower increase in cracking over time than do the other decks, with LC-HPC decks exhibiting approximately one-third of the cracking of the control decks at similar ages. Factors observed to increase cracking include the use of overlays, increased paste content, slump, compressive strength, and air temperature range on the day of construction, increases in concrete temperature relative to air temperature on the day of construction, and decreased air content. Techniques used by individual contractors also influence cracking.