With few exceptions, standard-cured cylinder strengths are greater than field-cured cylinder strengths because standard curing temperatures create faster rates of hydration and strength gain than typical field curing temperatures. For this reason, always use standard-cured cylinder strengths for concrete strength acceptance. More important, use only field-cured cylinder strengths to make construction decisions such as determining when to remove formwork and shoring, begin post-tensioning or determine when to place a structure into service. Never use standard-cured test cylinders in lieu of field-cured test cylinders. Failing to properly estimate in-place concrete strengths can jeopardize worker safety and may damage the structure.
The maturity method (ASTM C1074) is more accurate, reliable and economical for estimating in-place concrete strengths than field-cured test cylinders. It is based on the concept that concrete temperature and curing time is directly related to concrete strength. Specifically, this method uses a pre-established temperature-time-strength relationship for a given concrete mixture to estimate in-place concrete strengths.
Steps to estimate in-place concrete strengths using the maturity method include:
1. Prepare at least 15 laboratory test cylinders and embed temperature probes into at least two of the cylinders for a given concrete mixture, cure at room temperature and compute maturity factors M(t) for various elapsed times corresponding to strength tests using the following equation:
M(t) = SUM(Ta minus To)Δt
M(t) = maturity factor at age (t), degree–hours, ° F–h
Δt = time interval, hrs
Ta = average concrete temperature during time interval (Δt), ° F
To = temperature below which no strength increase occurs, ° F (14° F to 32° F)
Then establish a smooth strength-versus-maturity curve by plotting computed maturity factors M(t) versus corresponding concrete strengths.
2. Measure the temperature-time history of the in-place concrete by embedding temperature probes at critical locations as defined by the severity of concrete exposure and loading conditions.
3. Read the temperature-time data and calculate the maturity factor for the elapsed time of the in-place concrete using the maturity factor M(t) equation. Modern maturity equipment automatically computes and records maturity factors.
4. Estimate the in-place concrete strength by entering the pre-established strength-versus-maturity curve with the computed M(t) for the in-place concrete and read the estimated strength as shown in Figure 1. Again, this step is typically performed automatically with today’s modern maturity equipment and software.
Due to an approaching cold front, a contractor installed temperature probes into a wall placed at 9:00 a.m. September 1. The concrete supplier provided a maturity-versus-strength curve for the concrete being used as shown in Figure 1. Specifications for the project required a minimum concrete strength of 3,000 psi before placing and compacting backfill against the wall.
As shown in Table 1, elapsed times and in-place concrete temperatures were recorded in Columns 2 and 3 for the dates shown in Column 1. Using Column 3, average in-place concrete temperatures were computed and recorded into Column 4. Next, the contractor subtracted 23° F, or the temperature where strength gain essentially stops from the average temperatures shown in Column 4, and entered the adjusted temperatures into Column 5. Elapsed times in hours from Column 2 were computed and entered into Column 6. The contractor then multiplied the temperatures in Column 5 times the elapsed times in Column 6 and entered the values (° F-h) in Column 7. For Column 8, the cumulative maturity factors were computed and entered for the different elapsed times.
Finally, the contractor entered the pre-established strength-versus-maturity curve (Figure 1) provided by the concrete supplier with the in-place cumulative maturity factors from Column 8 and read the corresponding in-place concrete strengths. The estimated in-place concrete strengths were entered into Column 9 (e.g. for an in-place 5,070 ° F-h maturity factor, the corresponding concrete strength was found to be 3,100 psi from Figure 1).
Since the specifications required a minimum 3,000 psi concrete strength to ensure adequate wall strength for installing the backfill, the contractor must wait until the concrete has achieved at least a 5,000 ° F-h maturity factor. To shorten the cure period, the contractor can use hot water for batching, add a chemical accelerating admixture to the concrete or add additional insulation blankets so more heat can be generated and maintained.