Question:

We recently installed a large flat concrete floor in a large metal building. The floor project turned out great; however, we had one major difficulty with a long lane of concrete floor along the north side of the metal building's wall. We observed that large areas of the fresh concrete were not setting for hours, while other areas set within a normal time given the conditions of the project. What caused the "spotty setting" of the concrete?

Answer:

Spotty setting, as viewed by your concrete placement crew at the job site, is a variation in the rate of stiffening during the finishing process of the concrete. The two things that most affect the rate of stiffening of concrete are uncontrolled moisture loss from the concrete and the rate of the hardening of the concrete through the hydration process.

Spotty setting of a concrete slab can be caused by a number of things, but typically the most common causes are 1) a variation in the temperature of the concrete at the time of placement and 2) the effect of chemical admixtures on the rate of hydration.

Temperature primarily affects the rate of hydration. It is a generally accepted rule that concrete having a temperature of 90°F will set twice as fast as a concrete having a temperature of 70°F. It is equally true that concrete having a temperature of 50°F will take twice as long to set as a concrete having a temperature of 70°F. The actual temperature of the concrete mix as delivered to the jobsite often varies from load to load and frequently causes a real difference in setting time.

Admixture interaction can play a major role in spotty setting of concrete. Some retarding admixtures can be very sensitive to changes in concrete temperature. A small change in concrete temperature can cause problematic changes in the set from load to load of concrete. Concrete using both a superplasticizer and normal-setting or retarding admixture can have a delayed setting time when the temperature of the concrete falls below 65°F. It is often necessary to vary the dosage rate of set-controlling admixtures in order to get the desired set from load to load as the jobsite conditions change.

Your inquiry was followed up by a telephone call from L&M to your project manager and in the dialog it was revealed that the large metal building had vinyl covered insulation overhead, placed against the ceiling. A number of places along the north wall had water trapped in the insulation that caused a sagging or bulge in the vinyl. The water-filled bulges were punctured and the water drained down upon your granular sub-grade. This produced a number of isolated wet spots in the sub-grade. There was no aggressive cross ventilation in the building and the wet spots did not receive much drying. Your concreting operation began a few days later and the spotty setting became apparent to your concrete crew, especially the finishers. The cold weather conditions outside of the building contributed in a general way, as did the low dew point inside the building.

Your crew suspected that the wet subgrade contributed to the problem. It is our experience that isolated wet spots in the subgrade do not by themselves contribute to a significant differential in setting, but can be a contributory cause, multiplying the effect of other previously mentioned causes. We believe that good practice in this situation would have prompted your crew to uniformly dampen the subgrade, to reduce the variation in dampness.

That being said, I want to commend your onsite crew for the many correct things they did: they frequently checked the temperature of the mix, they inquired about the admixture content of the mix, and they stayed off of the slower setting areas of the floor until they did begin to set, many hours later. The use of L&M E-CON, a monomolecular film, on the faster setting concrete was a stroke of genius and permitted the finishers a larger window of time to permit a homogenous finishing of your floor. Good Work!