All too often, a contractor or engineer will call a testing lab to ask about the compressive strength of a three-, four-, or seven-day lab-cured cylinder. Usually this is because the contractor or engineer wants to know the strength of the concrete in the structure to decide if they can remove forms or shoring. During times of the year when ambient day and night temperature averages are close to the temperatures in our moist curing room (73.4° F ±3° F), the in-place concrete in the structure might be about the same strength as the lab-cured cylinders. But during the winter in cold climates, using lab-cured cylinder strength to make this decision is risky, and field-cured cylinders could cure more slowly than the in-place concrete, resulting in unnecessary delays.
Strength gain in concrete is dependent on several factors, including temperature, age of concrete, water-cement ratio, and type of cement.1 Many studies, however, have shown that the temperature at an early age is the most important factor in the rate of strength gain. Below 14° F, cement hydration and strength gain stops.2 Between 14° F and 32° F, concrete gains strength slowly—much slower than the rate at which lab-cured cylinders are gaining strength. Using these cylinders for decisions on stripping or form removal is dangerous.
According to ASTM C31, "Standard Practice for Making and Curing Concrete Test Specimens in the Field," field-cast and lab-cured specimens are called standard cured. Standard-cured cylinder results are used for:
- acceptance testing for specified strength
- checking adequacy of mixture proportions for strength
- quality control
If only three or four cylinders are made from a batch of concrete, the ready-mix supplier will want them to be standard cured prior to testing, in order to demonstrate that the concrete was delivered at the specified strength. If the specimens are field cured, the resulting strength test data, according to ASTM C31, can be used for:
- determining if a structure is capable of being put into service
- comparing the field-test results with test results of standard-cured specimens or with test results from various in-place methods
- indicating the adequacy of curing and protection of concrete in the structure
- determining form or shoring removal time
To obtain results with field-cured cylinders that closely compare with the in-place strength of the structural element, the cylinder must be kept at the same temperature and humidity as the in-place concrete. But even that may not be enough. Concrete generates heat during hardening as a result of the chemical process by which cement reacts with water to form a hard, stable paste. This heat is called heat of hydration.
Say a 2-foot square column is poured in a form in 20° F weather, then wrapped in an insulating blanket. The mass of the concrete in the column means that more heat is generated in the column concrete than in the cylinder that contains only about 0.2 cubic feet of concrete. Although using the strength of these cylinders will give a conservative estimate of when the forms can be removed, there is a delay because the field-cured cylinder strength at any given time is lower than the actual in-place concrete strength.
In an attempt to align these values, we have seen a column wrapped in an insulating blanket after concrete placement, with the cylinders also placed inside the blanket at the base of the column. The cylinders were frozen in the first few hours and never gained proper strength. The owner was frustrated that they had no cylinders to indicate the in-place strength of the concrete. Later testing and examination showed that the column concrete did not freeze prior to its initial hardening and gained adequate strength.
A few years ago, my company, Giles Engineering Associates, Waukesha, Wis., was on a hospital project in the middle of a brutally cold winter. The structural engineer for the project wrote the project specifications, including those for testing. For each testing interval, a set of four cylinders were made and standard cured, and another four cylinders from the same batch were field cured. All cylinders were placed in a cure box immediately after casting and kept there for 24 hours. Then the standard-cured cylinders were taken to the lab, and the field-cured cylinders were placed under the general contractor’s trailer, in a space open to the elements. After a week of single-digit and below-zero temperatures, the structural engineer demanded to know the strength of the concrete in the structure. The field-cured cylinders, of course, had very low strength results because they had hardly gained any strength after the first 24 hours in the cure box, while the lab-cured cylinders were gaining strength as expected. When we told the engineer that we really did not know the in-place strength given the information we had, he became irate. No amount of information on field-cured and lab-cured cylinders calmed him down and he didn't want to spend any money on other in-place test methods, which was the only way to really know the in-place strength.
In next month’s column, in-place strength-test methods, including maturity testing of concrete, will be discussed.
1 Naik, T.R., “Maturity of Concrete: Its Applications and Limitations,” Advances in Concrete Technology, CANMET, 1992, Canada.
2 Design and Control of Concrete Mixtures, Portland Cement Association, 14th Edition, pg. 240.