It is very rare that a concrete floor fails structurally.

Given time and traffic, all concrete floors will undergo wear through abrasion. It is a commonly known fact that as the compressive strength increases, the resistance to abrasion also increases.

However, there are times when merely increasing the strength cannot meet the required resistance to abrasion. The hardness of the fine aggregate, more so than the coarse aggregate, plays a very important role in the overall abrasion resistance of the concrete. The reason is that abrasion takes place at the very top surface in what is known as the near surface wear zone. The near surface wear zone is that portion of the concrete just underfoot, down to a depth of about 1/8 inch. This zone is made up of cement paste and very fine aggregate.

Even though the compressive strength of a concrete mix may be high, if the fine aggregate is soft the abrasion resistance will be low. With the aggregate being many times more abrasion-resistant than the cement paste, in the final analysis it is the fine aggregate that will, by and large, determine the abrasion resistance of the concrete surface. It is the important role of the cement paste to hold the aggregate in place. If the aggregate is lost from the surface of the concrete, the softer cement paste will be worn away very quickly by abrasion.

It is a well-known fact that cement paste can be made stronger by keeping the water-to-cement ratio low. The cement paste at the surface of the concrete can also be made stronger and denser by hard trowelling. To better understand the process of hard trowelling concrete one must be aware of the fact that cement does not dissolve in water, but the small particles of cement become dispersed. Each of these particles is a discrete and complete particle of cement, needing only water for hydration.

Hard trowelling pushes these cement particles closer together. This allows the crystalline structure that is developing during the hydration process to interlock into a very dense mass. By having the cement crystals in very close proximity to each other allows the hydrated cement to hold each individual aggregate particle very tightly. The hard trowelling process increases the abrasion resistance of concrete surfaces over those which have not.

The need for, and benefits of, using a chemical hardener/densifier

Additional abrasion resistance can be achieved by treating the hard-troweled surface with a chemical hardener/densifier like L&M Seal Hard.

Hydrated cement paste contains microscopic particles of calcium hydroxide, which are by-products of the hydration process. Calcium hydroxide is a comparatively very soft material which can be eroded away very quickly by a modest amount of abrasion, leaving microscopic pits (micro-pits) in the surface of the concrete. The edges of these micro-pits are very susceptible to abrasion, in much the same way as a highway pothole. The pothole may start small but soon becomes larger as its edges wear away.

Seal Hard converts the soft calcium hydroxide particles into very hard and dense calcium silicate hydrate. This is the same crystalline structure that is formed when cement hydrates. When calcium hydroxide is transformed into calcium silicate hydrate, the cement paste becomes more uniformly hard. Upon exposure to wear the concrete surface no longer micropits but polishes to a noticeable sheen. By using Seal Hard to densify the cement paste more calcium silicate hydrate is produced, giving the cement paste greater aggregate holding power.

Heavy abrasion conditions require additional protection of a shake-on floor hardener

ASTM-C-779 Equipment
Another method of hardening a concrete slab is to apply a dry shake-on floor hardener to plastic concrete and to then float and trowel it in. Most shake-on floor hardeners are commonly applied at rates of 3/4 to 2 pounds per square foot, occasionally more. This produces up to a 1/8 inch thick, very hard and dense protective layer of concrete.

Shake-on floor hardeners are made up of portland cement, selected hard aggregates and other additives. It is the aggregate selection that primarily determines the relative abrasion resistance of the floor hardener. The degree to which a floor hardener can resist abrasion will vary widely, depending on which aggregate is used. Three of the most commonly used aggregates are quartz, iron and emery.

These aggregates are rated by hardness according to the Mohs hardness system. The Mohs hardness system is a test that was devised by Friedrich Mohs in 1812, in which he established ten different categories of hardness. He chose ten different materials, ranging from talc to quartz to corundum to diamond, to be used in a scratching test. The higher the number of the category, the harder is the material. During the Mohs testing a material is categorized in one of ten categories. An increase of one category represents a doubling of the hardness of the selected aggregate category. Therefore, a category 7 aggregate is twice as hard as a category 6, and a category 8 is four times harder than a 6.

For example, talc is a very soft mineral, and is found in category one. In contrast, a diamond is very hard, and is used as the reference point for category ten. Mild cast iron borings are between categories six and seven and therefore would be categorized as 6.5. Finally, emery is in category 9, making it the hardest, economically practical aggregate for use in a shake-on floor hardener. L&M Emeryplate FF is an emery aggregate shake-on floor hardener, making it the most abrasion resistant shake-on floor hardener on the market.

All shake-on hardeners, even our Emeryplate FF, will benefit from a Seal Hard chemical conversion of calcium hydroxide into calcium silicate hydrate, hardening the cement paste and producing a higher quality cement paste binder, and, as a result, an even harder and more abrasion resistant floor hardener. Seal Hard-treated Emeryplate FF floors benefit in the same way as hard troweled concrete. With Seal Hard, the cement paste in Emeryplate FF is hardened and densified, which allows the hydrated cement to better hold each individual aggregate particle very tightly.

All shake on floor hardeners should be placed in accordance with ACI 302. 1R-04 section 8.6. In this chapter ACI goes into great detail regarding proper placement techniques. One crucial detail here is that concrete scheduled to receive a shake-on floor hardener should not have an air content above 3% and should not be subject to freezing and thawing cycles.

Proper curing is also of great importance toward the goal of improving the durability of a concrete floor. Field experience teaches us that while water curing by ponding is generally beneficial to many concrete placements, in the case of a shake-on floor hardener installation water curing immediately after finishing can cause delamination.

The reason for this is physics. Immediately after the final troweling, concrete is trying to shrink. This phenomenon forces entrapped air and water vapor to the surface of the slab. When a shake-on hardener is being cured by ponding with water, the capillaries' pores become filled with water. This prevents the entrapped air and water vapor from leaving the slab. The entrapped air and water vapor then may become trapped at the interface between the concrete substrate and the shake-on hardener. This causes uplift pressure to be produced and can cause hardener delamination by lifting the shake-on floor hardener off the base concrete.

If water curing is to be used, if should be started only after the concrete is 18 hours old. This will give the concrete time to gain strength and allow the capillary pores to lose some of their water, thereby reducing the likelihood of delamination. In the meantime, simply cover the concrete placement with sheet curing materials. When the concrete is not being water cured, apply a high solids acrylic curing compound.

Before starting a shake-on hardener project, have a pre-construction meeting. The meeting should be held a few days before the placement of the concrete. The owner, the concrete producer, a member of the concrete placement crew, and a representative of the shake-on floor hardener manufacturer should all be present. The topics of discussion should include the concrete mix design, techniques for the placement of the shake-on floor hardener and the method of curing the concrete. If the placement crews prepare for and follow the procedures outlined in ACI 302, the placement of the floor hardener can be a walk in the park. If shortcuts are taken, many problems can—and probably will—arise.

Severe abrasion floor requirements Some concrete slabs will be exposed to very high abrasion and impact. This condition requires a high strength concrete topping that is thicker than a shake-on hardener. Most of these toppings are placed at thicknesses of of an inch to 2 inches in depth. Toppings of this type can be installed over plastic concrete as well as over hardened concrete and, therefore, can be used to restore life to old, worn-out floors. In addition to abrasion protection, these toppings can provide protection from impacting loads.

To date, aside from a sacrificial, pea gravel concrete topping, there are two types of high performance toppings. One is made with ferrous metallic aggregate, and the other with emery aggregate. Studies indicate that the product made with the emery aggregate has approximately twice the abrasion resistance of the one made with ferrous metallic aggregate. It has also been found that both products exhibit the same impact resistance.

L&M Emerytop 400 uses emery as its aggregate base, an aggregate that is non-rusting. Therefore, it can be freely used in areas subject to water exposurea limitation of ferrous metallic aggregate toppings in some conditions.

Conclusion When it comes to abrasion protection, there are a number of choices. Some require action while the concrete is in the plastic state. Others allow you work with concrete after it has hardened. When it comes to selecting one of these systems, one should be mindful of not only the intended use of the floor but also what its future use might be.

Back to ConcreteNews

© 2006 L&M Construction Chemicals, Inc. | ConcreteNews Winter 2005/2006.

Subscribe to ConcreteNews