Durability, Construction, and Early Evaluation of Low-Cracking High-Performance Concrete (LC-HPC) Bridge Decks

Abstract

Laboratory evaluations of concrete mixtures based on specifications for low-cracking high-performance concrete (LC-HPC) incorporating internal curing (IC) and supplementary cementitious materials (SCMs) are described. In addition, the development, construction, and evaluation of four IC-LC-HPC bridge decks with IC provided by pre-wetted fine lightweight aggregate (FLWA) in conjunction with a partial replacement of portland cement with slag cement are described along with the evaluation of two Control decks without IC constructed in accordance with standard high-performance concrete (HPC) specifications in Minnesota. Bridge decks containing IC provided by pre-wetted FLWA and SCMs are also evaluated, including two bridge decks in Utah with a partial replacement of portland cement with Class F fly ash and six bridge decks in Indiana, four with IC and a partial replacement of portland cement with silica fume and either slag cement or Class C fly ash constructed in accordance with Indiana HPC specifications (IN-IC-HPC), one with IC and portland cement as the only binder, and one Control without IC.

The laboratory evaluations were performed on three groups of concrete mixtures, one for each for the first three years of IC-LC-HPC bridge deck construction in Minnesota. Variations in IC-LC-HPC mixture proportions include the amount of IC water (contents ranging from 0 to 14.1% by total weight of binder), total absorbed water content (IC water from the FLWA plus water absorbed by the normalweight coarse and fine aggregates ranging from 2.9 to 17.7% by total weight of binder), water-to-cementitious material (w/cm) ratios ranging from 0.39 to 0.45, and binder compositions examining the effects of using only portland cement, a 35% Class F fly ash replacement of portland cement, 27 to 30% slag cement replacements of portland cement, and a 2% addition of silica fume of cement for the mixtures containing 27 to 28% slag cement, all by total weight of binder. Tests for scaling resistance, freeze-thaw durability, rapid chloride permeability (RCP), and surface resistivity measurements (SRMs) were completed. The scaling resistance of the IC-LC-HPC mixtures was affected most by the air content, with mixtures having an air content below 7% exhibiting more mass loss than similar mixtures with more than 7% air. Including IC and slag cement did not negatively affect scaling resistance. Freeze-thaw durability was affected most by the total absorbed water content, with increases in absorbed water leading to a decrease in freeze-thaw durability. RCP and SRM results were affected most by the binder composition (specifically, including a partial replacement of portland cement with slag cement).

Experiences and lessons learned during the construction of the first four IC-LC-HPC bridge decks along with the failed placement of one deck indicate that the primary aspects of successfully implementing IC with LC-HPC include determining the moisture content of the FLWA shortly before batching and adjusting mixture proportions to maintain the target quantity of IC water (based on the FLWA absorption). Evaluation of the IC-LC-HPC decks and IN-IC-HPC decks demonstrate that low cracking can be achieved for concrete containing IC and SCMs as long as the paste content (volume of cementitious materials and water) is kept below 26%. An overlay with a paste content of 34.3% on one of the IC-LC-HPC decks exhibited high cracking within the first two years after placement. The two IC decks in Utah and one IC deck in Indiana with paste contents of 28% and 27.6%, respectively, also had high cracking. Durability issues in the form of scaling and aggregate popouts were observed during surveys of the IN-IC-HPC decks; the decks had higher IC water contents than planned (leading to a high total absorbed water content), lower air contents than the IC-LC-HPC decks, and late-season placement dates that provided minimal time for the concrete to dry prior to being exposed to freezing conditions.