Although concrete has very good compressive strength, that is the ability to withstand static loading, it is necessary to add reinforcing steel (rebar) to compensate for its limitations on the flexural and tensile sides. Concrete people like to say there are only two types – concrete that has cracked and concrete that is going to crack. However there are ways to minimize the cracking, or control it so that the substance remains structurally sound and architecturally pleasing.

First, take into consideration the weather when placing and finishing concrete. In extremely hot and windy conditions the chemical reaction can commence and accelerate so rapidly that cracking is impossible to control and final finish is harder or almost impossible to achieve. By final finish, for people familiar with industrial warehouse property, that would be the smooth hard-troweled surface inside the warehouse; or on site-work concrete around your office building, for slip resistance the light to heavier broomed finishes on your sidewalks and parking lots. For this very reason, in summer months most of the concrete being poured at Loop 410/I-10 is done during the night.

As soon as possible after the final finishing process – when it can be walked upon without marring the surface – crack control can begin. Sometimes a liquid membrane-forming curing compound that impedes evaporation is applied to the concrete. This allows the concrete to continue to hydrate (remain moist) by locking in water while the strength (exponential for the first seven days and still rising rapidly up to 28 days) of the substance increases. Because portland cement, which is the glue that holds everything together, has a very high shrinkage factor, keeping it moist over a longer period of time can help minimize shrinkage cracking.

Totally immersing concrete in water after its initial set would work; however, flooding it is not very practical. More often curing blankets that are kept constantly moist over a period of up to 28 days are utilized. You will see this method utilized at the aforementioned I-10/410 job on the column and pier cap structures going up. Also, contractors will put sprinklers on slabs and site work concrete to accomplish the same results or sometimes clear plastic sheeting will be utilized.

The next time you are in a Costco or Sam’s Club warehouse, look at their interior slabs. You will see in them “control joints” that were purposefully saw-cut to weaken the concrete at those specific locations. What you will be looking at is a clean 1/8” – 3/16” wide groove that typically penetrates ¼ the thickness of the concrete. What you will not see is the hairline crack that extends through to the substrate. This is a designed attempt to control the inevitable and is undertaken as soon as the specialized equipment can accomplish it without marring the final surface finish, by specification usually within 24 hours.

Some words of advice about membrane-forming curing compounds: they can impact adhesion of subsequent floor coverings like glue down carpets, carpet squares and VCT. However, there are dissipating formulas that could make for the better choice if flooring specifics are known.

For crack control in concrete paving and exterior concrete flat work not only will one see control joints but also expansion joints. Expansion joints consist of a compressible material that is typically ½” or ¾” thick that extends almost the full depth of the concrete. Concrete on either side of the expansion joint is poured around horizontally-installed steel dowels that penetrate the compressible material. The shoulders of the concrete on either side of the joint remain level as a result. Dowels on one side are slip-sleeved or sometimes just greased to allow concrete to expand and contract with changes in temperature, which are known to be extreme in South Texas. Typically the top ½” of the joint is caulked after installing plastic bond breaking material on top of the compressible material. This is done to minimize the influx of water. The bond breaker assures two-sided adhesion necessary for proper joint performance. There is a concrete-to-concrete bond on either side of the expansion joint only at these caulked points. The concrete paving moves independently on either side of the expansion joint and the elastomeric sealant expands and contracts across the top of the compressible material. Spacing on expansion joints, as on control joints, is a function of concrete thickness and the relation to temperature extremes and other structural and civil parameters. Sealants have a lifespan. To minimize structural failures in your concrete paving and exterior flat work it would be best to maintain them, as water penetration is the primary reason for concrete failure.

In concrete repair, preparation is everything. It is extremely important to take the concrete’s use into consideration, and have a good understanding of the different types of bonding products available. Pedestrian traffic is obviously different than slow-moving or fast-moving vehicular traffic. In industrial applications, concrete under hand dollies or pneumatic tire will be subject to different forces than that under hard-rubber tire traffic. Whenever you apply repair mortar, you must know the depths and widths necessary to handle these different loads. It is imperative to honor dynamic joints and not bridge them with a repair. Also critical is the profile of the bond line of the old concrete as it interfaces with the repair mortar.

If you want to color concrete there are three ways to do it properly. Allow me here to differentiate between coloring and coating concrete.

1. Add a portland cement binding pigment to the rotating ready-mix truck. The recipe calls for so many pounds of pigment per sack of cement per yard. Since typical design mixes call for 5-sack or 6-sack formulation (meaning one uses 5 or 6 sacks of portland cement per yard of concrete) then if it were 3 pounds of pigment per sack color, one would add 15 pounds or 18 pounds of color respectively. The color is full depth.

2. Dust-on a color hardener and float it into the surface of the concrete at final finish. A limitation of this process is that the color might extend to just a ¼” of depth and concrete will abrade either with the passage of time and traffic or its lack of protection early-on by the trades. Tools and fixtures seem to fall in the worst places and the color is hard to match in a repair.
3. Chemically stain concrete by applying a colored-salt suspended acid solution. The salts react with the free lime in the concrete and give you a color-pigmented product. A common mistake in preparation is using an acidic cleaning solution prior to an acid stain application. You would be consuming the free lime needed for the reaction of the colored-salt suspended solution.