Yes, it’s that time of year again; as business slows down in the winter months concrete producers scramble to make use of this extra time to get mix designs updated for the coming year – hence a lot of laboratory trial batches are being made at either a commercial laboratory or at the concrete producer’s facility. Generally known as the “three-point curve”, these laboratory trial batches become the cornerstone for establishing the cementitious content or w/cm to achieve an adequate strength overdesign to satisfy production performance requirements for mix designs.

Although these slower months may free up some additional labor to help in this endeavor, Mother Nature may not equally cooperate. As outdoor temperatures drop so do the temperatures inside laboratories and especially in those “make do” laboratories. Therefore, controlling the temperature of the concrete as batched is much harder and lower concrete temperatures may result. Did you know, ASTM C192 Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory  calls for conditioning all materials to a minimum temperature of 68° prior to batching?

The big question is what, if any, affect these lower temperatures may have on the concrete properties; specifically, water demand and subsequent strength test results.

To answer this question, a laboratory study was conducted to evaluate the influence of initial concrete batch temperature on the resulting water demand required for constant slump of 4 – 5 inches and the subsequent 28-day compressive strength. Three target temperature ranges were selected for the evaluation: control 70°-75°F, cold 50°-55°F, and hot 90°-95°F. All materials were conditioned to the desired temperature range prior to batching and mixing in an insulated revolving drum mixer. Initial concrete batch temperatures were measured at a point two minutes after mixing began. All batches were prepared and tested in a controlled laboratory environment at 73°F and all test specimens were prepared and subsequently cured in accordance with ASTM C31. The results are shown in Table 1.

Table 1

The initial batch temperature was found to have a significant influence on both the water demand for a constant slump and the corresponding 28-day compressive strength. On average, a 10-degree F change in the initial concrete temperature resulted in a change of 1.4 gallons/cubic yard in water demand and a change of 290 psi in compressive strength.

So for instance, if mix proportions (cementitious contents) were based on lab batches made at 55°F but then the concrete mix design is used at 91°F there would be a 1000 psi strength loss expected. Since the standard overdesign is on 1200 psi, this example clearly shows that problems are to be expected. This must be the reason why ASTM C192 requires that the materials for laboratory concrete batches be conditioned to a minumum temperature of 68°F prior to batching and mixing. This minimum 68°F temperature requirement is to assure that concrete mixture proportions derived from the tests of laboratory prepared batches will reflect the anticipated performance of production concrete under more typical ranges of temperature encountered in practice.

What if the lab cannot condition the aggregates and cement at a minimum 68°F? Simple solution – use heated batch water. However, there are practical limitations as to how hot the water can be. Domestic hot water is generally limited to a maximum of 120°F. It takes some very hot water to raise the concrete temperature to 68°F when the aggregates are cold. Table 2 shows the required batch water temperature needed as a function of various aggregate temperatures to achieve a minimum concrete temperature of 68°F. This based on a typical concrete mix with the fine aggregate at 4% moisture and coarse aggregate at 1.0% moisture.

Table 2

Although a water temperature greater than 120°F can be used, there is an increased risk for injury to lab personnel as the water temperature increases. Therefore, from a practical standpoint the laboratory facility must have the ability to have the materials conditioned at a minimum temperature of 55°F to be able to get by using water at the maximum safe level of 120°F to achieve the minimum concrete temperature required by C192.

When considering a concrete testing program, special attention must be paid to the capability of the lab facility to provide adequate and consistent temperature controls. The minimum temperature requirement in C192 is proven to be well adjusted. Achieving a realistic temperature of lab batches is critical. This will assure that mix proportions derived from the tests will have a sufficient overdesign avoiding the potential for unsatisfactory performance.

Courtesy of Robert E. Neal, FACI Technical Services Engineer with Lehigh Cement Co.