United States Department of Transportation
i
Evaluation of High Absorptive Materials to Improve Internal Curing of Low Permeability Concrete
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2007-03-01
[PDF-3.68 MB]
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TRIS Online Accession Number:01053619
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Edition:Final Report 1 April 2005 – 31 March 2007
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Abstract:Early age cracking of bridge decks is a national problem, and may substantially reduce service lives and increase maintenance costs. Cracking occurs when the tensile stress exceeds the tensile strength of the concrete. This is a time-dependent phenomenon, since both the stress and strength change at early ages. Moisture loss increases stress (with increasing shrinkage) and impairs strength gain. Internal curing is one method that has been suggested to reduce early age bridge deck cracking, particularly of concretes with low water to cementitious materials (w/cm) ratios. Many state highway agencies have implemented high performance concrete (HPC) for bridge decks. The low permeability of HPC is used to protect reinforcing steel and prevent corrosion. However, if the concrete cracks, then the protection may be greatly diminished. Transverse cracks due to concrete shrinkage allow water and corrosive chemicals to quickly reach the reinforcing steel causing corrosion and shortening the lifespan of the bridge deck. Reducing shrinkage cracking has been the focus of recent research into mitigation strategies. One unintended consequence of the use of HPC may be early-age cracking. Field studies have shown that, in some cases HPC bridge decks have cracked less than a year after placement. The use of internal curing to reduce autogenous shrinkage was investigated in this study. One method of internal curing was through the use of coarse aggregates with high absorption capacities. Another method discussed is the use of a partial replacement of the fine aggregate with a structural lightweight aggregate (LWA) with a very high absorption capacity. Bridge deck cracking is also affected by the nominal maximum size coarse aggregate. The effect on shrinkage with increasing size is discussed. The Ohio Department of Transportation's (ODOT's) District 12, located in Northeastern Ohio, found in an investigation of 116 HPC bridge decks placed between 1994 and 2001 that bridges with little or no cracking used coarse aggregate with an absorption > 1%, while 75% of bridges with unacceptable cracking used coarse aggregate with absorption <1%. This report discusses the laboratory investigation of the field results to determine the better ways to prevent bridge deck cracking - internal curing or paste reduction by using an aggregate blend. The laboratory investigation found that the strongest effect on cracking was due to the replacement of a small maximum size coarse aggregate with an optimized coarse aggregate gradation. Increasing the coarse aggregate absorption level from <1% to >1% had a less dramatic effect. The use of LWA for internal curing to the low absorption coarse aggregate also had a less dramatic effect. Other classes of structural and paving concrete were also discussed, in addition to the HPC. There were numerous benefits of internal curing for HPC. The cracking tendencies were reduced. Concrete mixtures that did not have the LWA cracked in less time. Specimens that contained the LWA were far superior when dealing with shrinkage. Concrete strengths were also improved with LWA.
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