Concrete floors are one of the most troublesome items in today’s industrial buildings. Surveys over the years indicate that about six of every ten buildings have floor problems, most often because the floor is manufactured on the construction site and subjected to variances of temperature and other weather conditions, inexperienced work crews, and poor quality control during and after its manufacture. In addition, it receives greater use than any other part of the building. Typical problems reported include:

• Base slab concrete or sub grade conditions that produce floor cracks and rocking slabs.
• Surface dusting, uneven or premature wear, or a floor that never looks clean.
• Joints which break down and are difficult, and sometimes impossible, to repair.
• Misapplication of floor surfacing materials-light duty concrete vs moderate or heavy duty; non-corrosion resistant surface vs corrosion resistant.

From The Bottom Up

To start at the bottom and work up; subsoil must be firm. The load of the slab itself is great, but the dead weight factors for machinery and stored goods, as well as the dynamic factors created by materials handling vehicles, are even greater. Therefore, the soil must be permanently firm or the slab must be designed as a structural slab.

If soil conditions are perfect, but a building is not enclosed, problems can occur. A lack of levelness caused by a rainstorm, for example, can result in cracking because as the slab dries it shrinks, with resistance created by the friction set up between the bottom of the concrete slab and the top of the sub grade. When the sub grade is uneven, the resistance increases because the slab is keyed into the sub grade at both high and low points. Moreover, if the high points in the sub grade are substantial in any one linear dimension, a plane of weakness is introduced, almost guaranteeing cracks. This can usually be avoided simply by waiting a few days and correcting the rain-caused soil condition. The slab should never be poured unless the roof and walls are tight.

The mix design also will have a substantial effect on slab shrinkage. Most slabs are poured too wet and with too much sand.  These two factors make for good workability, but they cause concrete to shrink and crack more than necessary.

In theory, the solution to this problem is easy: make the mixes harsher. But, each mix has to be
looked at individually. The gradations of sand and stone used by ready mix plants- even site adjacent plants- often vary widely.

Surface Failures

Two major misunderstandings about industrial concrete floors seem to be at the root of surface failures.  Many owners feel that specifying high compressive strength for a concrete floor is all that is required. However, there is a vast difference between compressive strength and surface wear resistance. A 3000 or 4000-psi concrete, for example, means only that it will withstand that compressive force in a 6 by 12-in. concrete cylinder. The compressive strength figure relates only to an internal breaking point, not wearability.

Another misunderstanding involves aggregates. Special aggregates are often used to help toughen floor surfaces. A variety of these materials are commercially available ranging from large stone aggregates to sand or metallic aggregates. These materials do help, but they are not a panacea. Their full contribution cannot be made if other elements are not properly controlled.

The major factor in producing good Portland cement concrete floors is the water-cement ratio. The strength of a concrete mixture depends on the quantity of mixing water used in the batch, so long as the concrete is workable and the aggregates are clean and structurally sound. The strength of the concrete decreases as the water ratio
increases.

Three stipulations are given equal importance: the water-cement ratio, workability, aggregate cleanliness and type. Starting out with the strongest material, we want to include as much of it as possible that is the coarse aggregate. The minimum amount of water for Portland cement is 3.1 to 3.2 gallons per sack. Anything in excess of that, according to the water-cement ratio law, will take away some qualities of the Portland cement. Finally, workability is important because without it all the cement particles will not be saturated with water, the cement paste coating of aggregates will not be complete, voids will be trapped in the mix when it is placed, and the mix so produced will be costly because of the long mixing times required and the extreme difficulty in handling it.

Unfortunately, when we try to mix a batch of cement with only a 3.1 to 3.2 ratio, including the water that is in the aggregate- especially the sand- we find that it is not workable if it has the maximum amount of coarse aggregate.

A plethora of concrete admixtures will produce workable concrete with low water content.  However, while producing easily finished concrete, lower water contents permitted by the use of some admixtures does not result in lower shrinkage concrete, the ultimate goal.  Overuse of admixtures can result in the coarse aggregate sinking to the bottom of the slab which aggravates slab edge curling.  A shrinkage compensating concrete (not shrinkage reducing admixture concrete) is a very good choice to eliminate floor joints thereby eliminating curling (no joint = no curl) and avoiding the main location of surface distress in concrete.

The Deferred Topping

The best industrial concrete floor are constructed with a separate surface- a deferred topping. For this, the base slab concrete is cast about 3/4-in. below finished elevation and the separate surface is applied after the slab hardens. The thin topping allows water to be removed from the mix after workability for mixing and placing is no longer needed. The topping may be applied to the base slab the next day or later. Delayed installation is preferred because it allows most of the drying shrinkage to occur in the base slab.

Immediately before the topping is placed, the base slab is saturated so that it will not suck water from the topping mix. Also, a cement grout is thoroughly scrubbed onto the slab surface as a bonding agent. (The slab must, of course, be prepared with a bondable surface.) The topping mix is then placed, usually containing a cubic yard of coarse aggregate for every cubic yard of topping mix. Enough fine aggregate is incorporated to fill all spaces between the particles of coarse aggregate. After the topping mix has been straight-edged to finish floor grade the workability water is extracted.

A burlap blanket is placed on the mix and an absorbing material spread over the blanket. This starts pulling water from the mix below the burlap almost immediately, and in from 10 to 20 minutes all possible water removal is completed.

Obviously, this material can be taken off before maximum water removal. But, if it is left until the maximum potential of the technique is accomplished, the resulting topping mix will be so hard and stony that special vibrating float equipment is needed to “work” the surface. Troweling does three things: it smoothes the surface, densifies it, and imparts a built-in gloss.

Smoothing can be done relatively quickly, and most floors get only this amount of troweling. The resulting surface is so “soft” that the floor will probably wear quickly and unevenly. The complete benefit of troweling may not be realized until six trowelings under the most favorable weather conditions in summer, ten trowelings in unfavorable weather in winter. Troweling squeezes moisture from the surface due to blade pressure.

Each troweling pass pulls out water particles which cling to the outside of the cement particles. As part of the same action the fibers of the concrete which were disrupted by the moisture movement are consolidated into the floor surface. Troweling must be continued until the concrete sets so hard that it has no plasticity.

The last step is water curing. The floor is flooded with water and covered with paper. This water must be available to the cement once the complicated chemical process of cement hardening proceeds.

For Average Use

Many facilities do not subject their floors to the sort of use that requires the brute strength of a topping.

For these the monolithic floor can be beefed up. After the concrete has been placed, straight-edged to finish grade, and the surface smoothed, aggregate mixed with cement is applied to the surface and embedded to a depth
of about 3/4-in. This densities the surface and helps absorb the shock and impacts which would weaken, and eventually destroy, an ordinary cement paste.

The cement paste at the surface also must be strong enough to withstand its share of punishment. Therefore, the full value of the aggregate densification can only be achieved by repeated trowelings just as in the topping installation procedure. A curing compound is then applied, and water curing is not needed because there is enough water sealed in the slab by the curing compound.

Stress relieving joints are put in the floor so that the tensile stresses developed during shrinkage will not become greater than the tensile strength of the concrete. If that point is reached, the concrete will crack. With proper mix design and sub grade conditions, 50-ft square panels are satisfactory. However, if freedom from cracks is a prime design criteria, the panels can be made smaller. Contraction joints may not be needed if small cracks are permissible.

Tooled construction joints spell trouble. Tooling produces a rounded, wide opening in the floor. Small diameter wheels, and even big ones, slam into this joint opening causing joint breakdowns.  Any joint should be filled and refilled as needed to protect the shoulders against breakdown by traffic, and for sanitary reasons.

Joints are also used to isolate the floor from differential movement of other parts of the building. Columns, for example, are usually boxed because some downward movement can be expected. However, the traditional column box is not necessarily needed. In many instances all that is needed is some form of separation between the floor slab and the column.

Construction joints are to be avoided and a considerable amount of hand troweling is required to finish them properly. Construction joints must be joined in some fashion so that the weight impositions of moving loads can be transferred from one slab to the other without any movement of either. These days is accomplished by steel bars embedded into the edge of the concrete. Additionally dowels are placed in the center of the slab, with foam-sided clips to allow for horizontal and lateral shrinkage movement.