When concrete hardens (cures), it gives off heat proportional to its curing rate. By learning how much heat is released, an accurate estimate of the concrete strength can be determined. Generally, concrete in a structure cures at a MUCH faster rate than concrete in a test cylinder. This is due to the much larger mass of the structure, and better hydration which aids curing.
When determining the early-age strength of cast-in-place concrete, reliance on test cylinders can lead to problems. For example, if test cylinders are cured at a lower temperature than the structure, the cylinders would underestimate the strength of the slab, which means that critical construction operations are delayed unnecessarily. Or conversely, if the deck is cooler than the cylinders, the cylinders would overestimate the strength, a clear safety concern. Concrete maturity testing monitors the curing of the structure and compares it to the cylinder, to more accurately track strength gain in the structure, improving both safety and construction operations.
Concrete maturity testing has been a recognized ASTM Standard practice (C1074) for over 30 years.
Applications for Concrete Maturity Testing
What can concrete maturity testing do to help improve concrete quality in non-ideal climates?
Experts agree that ideally concrete should cure for 7 days at temperatures between 65-85°F under 100% humidity. However, even the most conscientious builder knows this is simply not possible in most cases.
Because concrete maturity testing also tracks temperature history during the critical early-age curing process, it is often possible to take valuable steps to improve the cure before problems develop. For example, heat can be applied when it is shown that the concrete is getting too cold, thus ensuring completion of the hydration reaction needed for proper strength gain. Conversely, it has been proven that adding too much heat is detrimental to long-term durability and can lead to lower ultimate strengths, so taking steps to prevent overheating is also beneficial. Plus, if the contractor can avoid adding heat when it’s not necessary, the savings add up very quickly.
Cold weather applications
Do you need to accelerate the curing process in the dead of winter? Using maturity testing will give you a clear idea of just how much heat you will need to add to obtain the needed strength in the shortest time possible.
Concrete gives off heat as it cures, and often a satisfactory cure can be obtained merely by covering the slab or wall with an insulating blanket. Also, covering keeps needed moisture in – particularly helpful in cold, dry climates.
In many cases, contractors can save a tremendous amount of money simply by turning off external heaters once the concrete maturity system shows it’s not necessary. One contractor reported saving more than $5000 in propane costs over a single weekend because he knew the structure had obtained sufficient strength.
Hot weather applications
Some mixes can tolerate heat better than others. With concrete maturity testing, one can monitor the performance of concrete cured at high temperatures to ensure sufficient strength. For example, some high-early strength mixes cure very well at high temperatures (sometimes as high as 170°F!).
Curing the initial lab cylinders at high temperature (as tracked by the concrete maturity meter) will give a clear picture of the actual performance under high temperature field conditions, and can identify problems before the job starts.