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Publication Number: FHWA-HRT-06-117
Date: December 2006

Chapter 5: Conclusions

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This study investigated the freeze-thaw resistance of several marginal air void mixes in the absence of deicing salts. The influence of water-cement ratio and the type of air-entraining admixture were evaluated.

The project was divided into two phases: 1) the first phase, designed to evaluate the w/c ratio influence on the freeze-thaw resistance of the mixes containing Vinsol resin air-entraining admixture; and 2) the second phase, designed to compare the performance of marginal air content mixes containing either Vinsol resin or synthetic air-entraining admixture.

In phase 1, the mixes had air contents that varied from 2.4 percent to 7.2 percent (measured according to ASTM C 457(18)) or 3.5 percent to 4.5 percent fresh air content (ASTM C 231(14)). The spacing factors ranged from 0.23 mm to 1.12 mm. The w/c ratios used were 0.40, 0.45, and 0.50. The mixes with fresh air contents of 3.5 percent or higher, except for mix 116-1, showed satisfactory freeze-thaw resistance, with DFs above 80 percent and lasting at least 300 cycles. No trend was observed in terms of the effect of w/c ratio on freeze-thaw resistance for the mixes investigated.

In phase 2, all mixes were designed to be in the range of marginal air (2.5 percent to 4.5 percent). Some had spacing factors and specific surface areas higher than the minimum recommended for a good freeze-thaw resistance. set 1 (with Vinsol resin admixture) showed a better freeze-thaw performance than set 2 (with synthetic admixture), although in most of the mixes of set 1 the air void system was much poorer, when measured by ASTM C 457(18) linear traverse, with higher spacing factors and lower specific surface areas for the same levels of air contents.

In set 1 (VR AEA), all the air-entraining mixes lasted at least 300 cycles and had a DF above 80 percent. The DF did not increase with increasing specific surface, decreasing spacing factor, or increasing air content, as expected.

In set 2 (SYN AEA), only mix 350 had a DF above 80 percent. In this set, the expected trends were confirmed, i.e., the higher the spacing factor, the lower the DF; the higher the specific surface, the lower the DF. Nevertheless, no trend was found for hardened air content and DF.

For the specific materials and mix proportions used in this project, the marginal air mixes presented an adequate freeze-thaw performance when Vinsol resin based air-entraining admixture was used. The synthetic admixture used in this study did not show the same good performance as the Vinsol resin admixture. A different behavior may occur when other Vinsol and synthetic admixtures are used and higher levels of air entraining are present. The reasons for this unexpected observation could not be explained.

There are well-established thresholds for the air void parameters that would be expected to give good concrete freeze-thaw resistance. The test data presented in this study suggest these limits may not be applicable in all cases to air entrained concrete containing synthetic admixtures.

There is insufficient data in this study to generalize these results for all the Vinsol resin and synthetic air-entraining admixtures and all levels of air content. More research is needed in order to confirm this finding.

 

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