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

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.