When 28-day cube strengths drop, are concrete strengths also expected to drop?
By: Joe Clendenen, Technical Service Engineer, Holcim Midwest Region
In the production of ASTM C150 portland cement, the standard test for strength determination is the use of ASTM C109 mortar cubes, made with a fixed combination of the cement, standard graded sand, and water. These are then cured under highly controlled laboratory conditions. As a general rule, there is some correlation between ASTM C109 mortar cube strengths and ASTM C39 concrete strengths for a constant set of materials under similarly controlled conditions (Reference 1, Figure 1). For this reason, we closely monitor ASTM C109 results for strength trends and consistency.
It is important to note that there is some amount of difficulty in tying ASTM C109 mortar cube strengths to field data due to the inherent higher variability through effects such as temperature, production, other material variability, field mix adjustments, testing conditions, testing precision, etc., when evaluating shifts in field performance, it is important to take all potential factors into account.
While ASTM C109 mortar cubes can provide some useful information on strength performance for our customers, we believe that a more useful tool is the monitoring of strengths using concrete directly (Figure 1). At Holcim, we evaluate all plant portland cements routinely using a standard concrete mix under laboratory controlled conditions. As opposed to field data, this removes the influence of field conditions, allowing us to focus on changes in performance due to the cementitious material itself, while giving us a closer representation than ASTM C109 mortar cubes alone.