Technical Questions

ACI Committees, Membership, and Staff have answered common questions on a variety of concrete related topics.

How can the surface or free water carried by the aggregates be determined?

Q. How can the surface or free water carried by the aggregates be determined?


A. Free water is simply the difference between the mass of a sample of wet-saturated aggregate and the mass of the same sample in the saturated-surface-dry condition. Standard test methods for surface moisture in fine aggregate are available (ASTM C70). If the absorption capacity of the aggregate is known, a more common method for determining surface moisture is by drying the aggregate (ASTM C566). Water within the aggregate particles does not affect the mixture or the w/cm. Allowance should be made for water that will be absorbed by dry aggregate, however, and may be as much as 95% of its total absorption capacity during the mixing period. Many concrete plants have calibrated moisture meters (based on electrical resistance or microwave technology) that continuously monitor free moisture on the aggregates and have the capability to automatically adjust the added water to maintain the mixing water content within the specified limits.


References: SP-1(02); E-1(16); ACI 221R-96; ASTM C70; ASTM C566

Topics in Concrete: Aggregate; Concrete Fundamentals; Testing of Concrete

Other Sources for Answers

Search other resources on the ACI website for answers to technical questions

Please enter this 5 digit unlock code on the web page.