How to Estimate In-place Strength of Concrete

Concrete must gain adequate strength to carry its weight and construction loads before removing formwork, reshoring or backshoring. Engineers frequently specify minimum in-place concrete strengths before contractors can post-tension, backfill against walls, open pavements to traffic or terminate protection during cold weather. For these reasons, contractors must know how to properly estimate in-place concrete strengths for recently placed concrete, especially during cold weather. Otherwise, worker safety and the quality of the structure may be jeopardized.

Field-cured test cylinders and maturity factors are often used for estimating in-place concrete strengths. However, testing field-cured cylinders is the default procedure specified by the building code. Other methods — including maturity factors and cast-in-place cylinders for slabs, penetration resistance, and pullout strength — require approval by the architect/engineer and may require approval by the building official.

Temperature and time

Strength gain of concrete depends on a combination of curing temperature and time. The rate of hydration, or the chemical reaction between cement and water, is a function of the concrete temperature. As the concrete temperature increases, the rate of hydration and resulting strength gain increases. Conversely, the rate of strength gain decreases as the concrete temperature decreases. For this reason, delayed concrete strengths are common during cold weather unless contractors implement precautions. Of course, concrete strength increases with time if there are adequate curing conditions to promote hydration.

Field-cured test cylinders

Standard and field curing are different procedures specified by ASTM C31 for curing concrete test cylinders. Standard-cured test cylinders, sometimes referred to as lab-cured cylinders, represent ideal or nominal concrete strengths. Temperature ranges for standard curing are 60° F to 80° F for a period up to 48 hours (initial curing) and 73.5 ± 3.5° F for the balance of the 28-day cure period (final curing) for concretes with specified strengths up to 6,000 psi. Concrete with specified strengths of 6,000 psi or greater must follow the tighter temperature range of 68° F to 78° F for initial curing. For standard curing, temperatures and times are standardized to produce uniform curing conditions. That’s why strengths derived from standard-cured test cylinders are used for concrete strength acceptance.

Field curing is different than standard curing. It consists of storing test cylinders as near as possible to the in-place concrete and protecting cylinders from the elements in the same way as the in-place concrete. Curing conditions for the test cylinders should be the same as the curing conditions of the in-place concrete. By subjecting the test cylinders to the same temperature-time history as the in-place concrete, it is presumed the strength of the test cylinders represents the in-place concrete strength.

Field-cured test cylinders typically underestimate the true in-place concrete strength because of the thermal mass of a test cylinder (4 in. x 8 in. or 6 in. x 12 in.) as compared to the significantly larger thermal mass of the concrete element represented. Typically, curing temperatures for the test cylinders are less than the actual in-place concrete temperatures, even when test cylinders are tucked under the curing blanket and stored adjacent to the concrete represented.

Strengths obtained from field-cured test cylinders are typically conservative. However, field-cured cylinders may grossly overestimate the in-place concrete strength if the fielded-cured cylinders are stored and cured in the job trailer.