Q.: We're working on a project that requires building a secondary containment structure using concrete made with Type K expansive cement. The design compressive strength is 4000 psi, which makes us concerned about the low strength-test results we've been getting. The maximum 7-day strengths are 2890 psi for lab-cured specimens and 2180 psi for field-cured specimens. Based on these results, it appears that we may not reach the required 28-day design strength. What can we do to improve the concrete strength?
A.: Type K cements contain a reactive aluminate that combines with water to form an expansive material called ettringite. Because of the expansion that's occurring, strength gain of Type K cement concretes isn't as rapid as that for normal concretes at ages up to 7 days. After expansion is completed, strength development of Type K concrete is similar to that of normal concrete under moist-curing conditions (Ref. 1). Based on the slower strength gain at early ages, a lab-cured 7-day strength of 2890 psi doesn't sound too low to produce a 28-day strength of 4000 psi. However, curing is extremely important for Type K concrete because formation of the expansive material requires water that's also needed for cement hydration and strength development. As proof of this, note how poorly your field-cured cylinders fared compared with the lab-cured cylinders. The low field-cure results may indicate that curing methods for the structure are inadequate. If you don't have enough curing water available, both strength and expansion will decrease. Make sure both lab and field cylinders are cured as completely as possible. Curing of the field concrete and cylinders should commence immediately after final finishing. You may need a fog spray or concrete covering if other curing methods are delayed, especially during hot, dry, windy weather. Make sure lab cylinders are maintained at 60° to 80° F during the first 48 hours, as required by the ASTM test method for making and curing test cylinders (Ref. 2). And check all of the cylinder molding, curing, capping and testing procedures to make sure they are in accord with ASTM test methods.
- ACI 223-93, "Standard Practice for the Use of Shrinkage-Compensating Concrete," American Concrete Institute, Farmington Hills, Mich., 1993.
- ASTM C 31-96, "Standard Practice for Making and Curing Concrete Test Specimens in the Field," ASTM, West Conshohocken, Pa., 1996.