We are a cast in place concrete contractor. Our clients like our work because the concrete we cast comes out tight in appearance, but we are always trying to do better. How should we reduce our surface voids or "bug holes?
There are many means and methods your crews can become expert in doing, careful placing of concrete in controlled lifts, minimizing the "free fall" of the concrete and of course, expert consolidation of concrete between the forms. The next thing you can do for your crews is provide them with a high performance chemical form release agent. The high performance form release agents are totally fuel or in layman's language "diesel fuel" free. The form release agents your crews need are very low odor and function by reacting with the lime water released from the concrete.The released water, out of the concrete, is part of the "bug hole" forming problem. A premium form release agent allows the water droplets to rise up and out of the formed face region, fuel form oils do not.
We suggest your company try L&M™ Debond®and continue your quality approach to cast concrete.
We are always fighting the same fight at the end of every concrete floor project. Dirt, mud stains, rubber tire marks and sometimes rust stains sticking to our once clean concrete floor. Our crew pours the concrete and gives it a smooth and clean finish, as soon as we leave the other workers dirty the floor so bad, we have to clean the floor at our expense before receiving our final check. What can we put on the floor to minimize this problem?
The first thing to do is develop a "understanding" with the general contractor's onsite supervision. This understanding should give knowledge or meaning to your reoccurring situation. You want to promote a sense of caring, if even in a small way, for the cleanliness of the new concrete floor. We know that sounds futile, but you have to communicate your concern to the leadership on site. Once you have done that, we suggest examining the contract and if the specification language calls out a cure and seal product, use it.
The choice of the product may come from a suggested or approved list in the specifications language, or it may be a performance specification. The product you want should produce a "skin" or membrane on top of the concrete. Your plan is to lock out as much dirt and stains as possible, without going overboard. Many of these membrane cures are rated in moisture retention ability, and that is good, you want moisture retention and you want film thickness, too. Luck would have it, these two properties usually coincide.
Look for a cure or a cure sealer meeting the new ASTM C 1315, this requires 25% solids as a minimum and a moisture retention ability greater than low solids formulas from the past.
The idea is to bond a tough membrane to the surface of your clean, smooth concrete and minimize the mess you have to deal with from the other trades at the end of your contract. We suggest using L&M DRESS & SEAL WB 30™ and always check to see if the specifications' language permits these membrane-producing products.
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?
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:
- A variation in the temperature of the concrete at the time of placement
- 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 phone call from LATICRETE 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, we 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!