Optimum concrete temperature

Concrete Construction, April, 2003

Q. Almost all manuals say that 70[degrees]F is the optimum concrete temperature for strength. Do you find this to be true? The higher the concrete temperature, the more water needed to maintain a specified slump, therefore lowering the compressive strength. We have been keeping strength data on a given mix design for many seasons and one of the dramatic findings for us in the Midwest is that you start to see a strength drop-off when the ambient air temperature goes over 70[degrees]F. We see as much as a 500-psi drop from spring to summer, mostly due to temperature (that is, water demand and poor cylinder curing).

A. First, we must define the age in which we are interested. Our "standards" are somewhat arbitrary. Is it 28-day, 1-day, 7-day, or 182-day strength we need? This is an important distinction. For standard ready-mixed concrete, higher temperatures result in lower 28-day strengths and you are correct in relating this to summer temperatures and w-c ratio, but there is more to it than that. Strength will increase substantially (regardless of what cement type is used) in the early stages of "maturity" if the temperature is increased, but you sacrifice the later stages of strength development (28-day). Cylinders submerged in a water bath at 72[degrees]F will usually develop a higher 28-day strength than "air dried" cylinders because the cement is subject to moisture at all times.

On the other hand, anything that slows the hydration process at the early stages will result in higher 28-day strengths. This can be achieved with admixtures, such as retarders, or temperature. Concrete (in cylinders) kept at cool temperatures for the first few days before standard curing will develop higher strengths at 28 days than the same concrete cured continuously at 72 [degrees] F. Every winter there is a panic when people see low 7-day strengths. Unfortunately, they forget (or refuse to consider) that the 28-day strengths are routinely much higher at that time of year, so that we have to wage the same battle when the next low 7-day strength occurs. This is typically due to the nearly universal failure to comply with standard ASTM curing practices for cylinders. Going into the summer, we do typically see about a 500-psi drop. Ready-mix producers that do not utilize their admixtures well may see more than that.

So, as you can see, you can "combat" much of the high-temperature strength loss simply through proper testing. You can prevent strength loss with admixture technology, and you'll be surprised how inexpensive it can be. Preventing strength loss at higher concrete temperatures is simple, inexpensive, and will probably save you some money to boot. A little ingenuity, a solid QC background, reasoning, and a drop of chemistry will go a long way with "far less cement" than you realize.