United States Department of Transportation
i
Evaluation of CDOT specifications for class H and HT crack resistant concrete.
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2010-06-01
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Edition:Final.
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Abstract:This study examined the performance of concrete mixtures designed to increase cracking resistance for Colorado bridge decks. The
current CDOT Class H and HT concrete mixtures and nine other mixtures were investigated to aid in the development of a more crack
resistant concrete specification. A total of eleven concrete mixtures were designed, batched, and tested for their fresh and hardened
concrete performance. Specifically, the designs differed by type of cement, w/cm, cement content, supplementary cementitious materials
(SCMs), use of chemical admixtures, and aggregate type. Compressive strength, permeability, freeze-thaw resistance, and restrained
shrinkage cracking were evaluated and documented in this report. Lower w/cm resulted in high early compressive strengths and rates of
strength and strain development. Increasing the w/cm to 0.44 and Class F fly ash replacement levels up to 30% was beneficial in
controlling strength gain. A low cement content mixture with increased w/cm and fly ash replacement proved to be beneficial. When
SCMs were not utilized, a low cement content of 6.0 bags was beneficial. When SCMs were used, increased cement content helped to
maintain the same properties. Type G, coarse-ground cement was beneficial to strain and strength at the higher w/cm of 0.42 and low
cementitious materials content. At a lower w/cm of 0.38, the mixture behaved similarly to the control mixture fabricated using Type II
cement, developing strain and strength at an average rate.
A high dosage rate of a shrinkage reducing admixture was extremely beneficial in controlling both the development rate and ultimate
strain of the mixture, while maintaining adequate development of ultimate strength at all ages. An average dosage rate of a set retarder
only retarded the initial strength development slightly. After 1 day of age, the development of strength and strain was substantially
increased. Although the concrete containing the set retarder reached higher compressive strengths more quickly than anticipated, the
concrete did not crack in the AASHTO PP34 test and was moderately durable.
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