MOXIE INTERNATIONAL FLOORING SEALERS / FLOORING WATERPROOFING - HOW TO MINIMIZE CRACKING AND INCREASE STRENGTH OF SLABS ON GRADE

Reprinted from Concrete Construction--September, 1981 MINIMIZE CRACKING IMAGE-1 (2).jpg (14598 bytes)

How to minimize cracking and increase strength of slabs on grade

MIN CRACKING-PH1.JPG (38471 bytes)

Remove excess water from the bottom

By Leo P. Nicholson

Assistant General Manager

Sequoia Rock Company

The use of proper joint control to minimize cracking in slabs on grade has long been recognized and practiced, but only in the past two decades has the concrete industry dealt directly with a major cracking problem that joint control will not solve: plastic cracking and invisible plastic shrinkage that creates weakened areas which become shrinkage cracks later on. Plastic cracking, also referred to as plastic shrinkage cracking, is cracking that occurs in the surface of fresh concrete soon after it is placed and while it is still plastic.

Preparation for casting one of the full-scale test slabs, 13 feet wide and 54 feet long. At the top is the impervious polyethylene base. The pervious sand base is in the foreground.

Concrete mixes used in the test slabs described here are reported representative of field practice in southern California. They are not necessarily the mixes which would be recommended by the author or by other authorities on concrete slab construction.

Early efforts to solve this problem centered on changes in mix design, such as decreasing the slump, or external top surface treatments such as fog spray, wind breaks, and surface films. All of these help, but they have not provided a complete answer. However, I have long felt that the bottom of the concrete was the most important area in which to control cracking, and working from that base, I concluded that:

· If we can take the extra workability water from the mix out the bottom just as fast or a little faster than it goes out the top we can density the concrete while it is in the plastic static. This will allow very little tensile stress to build up during the early curing.

· Then if the concrete is cured properly and adequate joints are provided, the cracking will take place only in the control joints. This, of course, will not take care of structural cracking or the every-now-and-then crack that seems to happen for no good reason, but it will eliminate the majority of our slab-on-grade cracking problems.

	The Southern California Ready Mixed Concrete Association agreed to test this theory by casting identical slabs on different bases including impervious polyethylene and pervious sand and sand-cement mixtures. The sand was wet down a day earlier to compact it, then lightly sprinkled just before placing the concrete. The sand-cement was not compacted and was as pervious as the sand. If my theory proved correct, the impervious base would induce plastic and shrinkage cracking; the sand would moderate it. Each 13x54-foot test slab was cast on three different bases as shown in Figure 1. Finishers were allowed to add whatever water they felt they needed, according to current practice. A dry-shake topping was used on part of each slab, and curing included no treatment in some areas, spray-on compound and wax coatings in other areas. Cracks were recorded after one year, and all but two occurred in the concrete cast directly on the polyethylene. Cracks in this surface stopped abruptly at the edge of the sand base area. The two cracks above t1i~ pervious base occurred more than 18 feet from any other cracks; such cracks might normally be expected since no joints were provided.
Figure 1. Details of the test slab program, showing three different base conditions as well as four curing conditions on each of the bases, for each slab. Three different concrete mixes are described on the right

A view across the width of Slab 3 shows cracks stopping at the edge of the sand-base area. Black spots are oil from trucks which use the test slabs as a parking area.

The results were clear, graphic and dramatic. Why was there serious cracking on the impervious base and none at all on the sand? Figure 2 shows the reasons. On the impervious base, all the extra bleed water must come out the top. As the top starts to dry, the concrete wants to curl. Because it can’t take any tensile stress at this point, the slab starts to tear open on the top surface, creating immediately visible plastic shrinkage cracks as welt as weakened areas where future cracking will occur. On the sand base, however, excess water leaves fairly evenly, top and bottom, enabling the concrete to densify without creating uneven stress within the slab. This prevents curling, virtually eliminates immediate plastic cracking and materially reduces the possibility of later shrinkage cracking.

Core tests made many months after casting have indicated another important advantage of the pervious base: greater strength. The table shows differences in core strength for various types of cure on each of the three bases. All of this concrete came from the same truck load. Concrete ca~it directly polyethlxene’ and not cured had only 44 percent of the strength of concrete cast on a sand bed and spray cured. If the specified compressive strength were 2000 psi, the uncured concrete cast on the polyethylene would have failed, while the concrete cast on the sand was over strength.