PCC Properties to Support W/C Determination for Durability
AbstractThe fresh concrete water-cement ratio (w/c) determination tool is urgently needed for use in the QC/QA process at the job site. Various techniques have been used in the past to determine this parameter. However, many of these techniques can be complicated and time consuming. Furthermore, extensive calibration is often needed to correlate the properties measured by these techniques with w/c. During the course of the present study, the method for the use of the unit weight for the determination of w/c of fresh concrete has been developed and evaluated on both laboratory and field concretes. Additionally, the accuracy of using microwave oven technique for w/c determination reported by previous research was confirmed. Finally, the accuracies of unit weight and microwave oven techniques for the determination of w/c were compared.
The unit weights required for this method have been determined either by using a “zero-air” procedure (ZAP) developed as a part of this study or by using conventional (following AASHTO specifications) methods. The ZAP technique was used to verify the w/c of 58 different laboratory concrete mixes. These verification efforts revealed that the minimum, maximum, standard error, and 95th percentile of the differences (∆w/c) between batched and determined w/c were, respectively, 0.000, 0.042, 0.017, and 0.030. The AASHTO determined unit weight (which also required measurements of the actual air content of concrete) was used to verify the w/c values of an additional set of 57 laboratory mixes. When using the AASHTO unit weights (and air contents) the minimum, maximum, standard error, and 95th percentile of ∆w/c were, respectively, 0.000, 0.075, 0.030, and 0.054. In addition, the AASHTO unit weight method was also used to verify the w/c values of 22 different field mixtures. For this case, the differences (∆w/c) between the design and unit weight-calculated values of w/c were in the range ±0.030 for all but one mixture. Finally, direct comparison of the results from the proposed method with the results obtained from the microwave oven method revealed that the former is faster but slightly less accurate. Specifically, when used on five separate concrete samples, the accuracy of the microwave oven method was 0.010, much smaller than the previously mentioned values of 0.030 (for the ZAP) and 0.054 (for the AASHTO) unit weight methods