Determining the Water-to-Cement Ratio of Freshly Mixed Concrete: Applying the Modified Coarse Aggregate Correction Factor to the AASHTO T 318 Method

Abstract

The water-to-cement ratio (w/c) is of great importance for concrete quality control. The microwave oven drying measurement method given in the AASHTO standard, Standard Method of Test for Water Content of Freshly Mixed Concrete Using Microwave Oven Drying (AASHTO T 318), is one of the most widely accepted standard w/c determination methods. The National Ready Mixed Concrete Association (NRMCA) proposed applying a coarse aggregate correction factor (CF) to adjust the AASHTO T 318 calculation to increase the accuracy of the test. In this work, the CF is further modified by introducing the coarse aggregate grading factor (GF), and a modified coarse aggregate correction factor (CF′) is proposed that aims to address errors caused by fine particles in coarse aggregates and ensure precise w/c measurements. A total of 130 mixtures were prepared and tested using the microwave oven drying method. The results show that terminating the drying test when the sample mass change is less than 0.5 g yields more accurate w/c values than does a 1.0 g threshold. The CF effectively corrects for sampling errors in mixtures without fine particles, whereas the CF′ is essential for accurate results in mixtures with fine particles. Statistical analysis confirms that w/c values determined using CF′ show no significant difference from design values, with mean differences near zero. This study also examines the effects of aggregate moisture content and testing delays. Aggregate moisture does not significantly impact the results when CF′ is applied. Testing within 60 min of mixing ensures that w/c values remain within an acceptable error range (i.e., ±0.02). This research comprehensively evaluates the microwave oven drying method, emphasizing the importance of CF′ for accurate w/c determination in concrete mixtures with fine particles. The findings contribute to improved quality control in concrete production. The findings from this study offer a significant advancement in the accurate determination of the water-to-cement ratio (w/c) in freshly mixed concrete, which is critical for ensuring the desired durability and strength of concrete structures. The modified coarse aggregate correction factor (CF′) introduced in this research allows for more precise w/c measurements by accounting for variations in coarse aggregate grading. This method is particularly useful in field applications for which sample sizes are small, and traditional methods may lead to inaccuracies due to sampling errors or the presence of fine particles in coarse aggregates. By employing the CF′, engineers can achieve more reliable w/c determinations, thereby improving quality control in concrete production. Additionally, the findings demonstrate that the aggregate moisture content does not affect the accuracy of w/c measurements when using the CF′, providing further confidence in the method’s application in varying field conditions. This enhanced approach to w/c determination supports the development of stronger, more durable concrete, ultimately contributing to the longevity and safety of concrete infrastructure.