5-Minute Classroom:
How to harden a concrete floor

The one characteristic that all commercial and industrial floors have in common is that they seldom fail structurally, but nearly all undergo continuing abuse from surface abrasion and impact. In the early days of industrial floor protection, well before we had the specialized toppings and treatments, the only method to protect a floor was to shield the surface using steel plates. Obviously, this was very expensive, and in all but a very few cases was over-kill. The science of floor protection has had to go back to the basics to discover how to make concrete more abrasion and impact resistant.

Four basic concepts were discovered.

1) When the surface of the concrete is densified by hard steel troweling, the abrasion resistance of the concrete is greatly increased.

Hard troweling takes place just before the concrete takes its initial set. At this time the concrete is becoming hard but has not yet started to hydrate, therefore the surface of the concrete can be worked without harming the concrete strength. The process of hard troweling forces entrapped air and moisture out of the near surface wear zone, thereby forcing the unhydrated cement grains closer together. With the cement grains being forced closer together as the cement hydrates, the crystalline structure being produced (Tobermorite Gel) is more tightly bonded. This produces a concrete that has a harder and more abrasion resistant surface.

2) A second method for increasing abrasion resistance is to increase the density of the concrete through control of the mix design.

It is a long established fact that as the water/cement ratio of concrete decreases, the compressive strength increases. As the compressive strength increases, up to about 8,000 psi, the relative abrasion resistance also increases. This second concept is closely related to the first in that low water/cement ratio concrete will, by its very nature, bring the hydrating cement grains closer together, forming a more tightly bonded, denser Tobermorite gel.

3) The third method treats concrete surfaces with a chemical hardener.

Most chemical hardeners are rich in silica in the form of soluble silicate or siliconates. These siliceous compounds, when placed on the surface of hardened concrete, chemically react with calcium hydroxide, a weak by-product of the hydration of cement, to produce a hard compound known as calcium silicate hydrate (CSH). The formation of C-S-H hardens and densifies the concrete surface. This chemical treatment is effective primarily in the near surface wear zone area, as this protection is only beneficial in that zone area.

4) Finally, the fourth method for improving the hardness and abrasion resistance of concrete uses a very hard aggregate that is placed on the surface of the concrete. The method of placement can take one of three forms. First, hard mineral aggregate can be placed on the surface of freshly place concrete and floated (troweled) into the surface of the concrete. This process is called seeding.

A second process employs a product known as a dry shake-on hardener. A dry shake-on hardener is made up of select mineral or metallic aggregates blended with portland cement and admixtures. The dry shake-on hardener is applied to the surface of freshly-placed concrete and floated (troweled) into the surface of the concrete. These methods and processes act to make concrete in the near-surface wear zone denser and more resistant to wear and abrasion. The near-surface wear zone is the point of maximum wear, approximately 1/8 of an inch thick, at the very top of the concrete.

The next category of protection includes toppings. Products of this type must not only protect the near-surface wear zone from abrasion but also protect the underlying concrete from impact and point loads. In order to protect industrial floors from impact and point loads the protective material must produce a zone of protection.

To understand how this zone of protection works we must first understand the physics by which a load travels through a material. When an impact or point load contacts the surface of a material the area over which it is dispersed is very small. As the load travels through the material the area becomes larger as the load increases its depth of travel across the surface. The pattern of travel takes on the shape of a right circular cone, with the base being the area over which the load is dispersed at a given depth into the substrate material and the upper point being the initial point of contact.

Materials that produce a zone of protection are placed over either hardened or plastic concrete as a topping. The thickness of the topping generally ranges from .75 to 2 inches, sometimes more. It should be noted that the thicker the topping the greater the area over which the load can be dispersed, and the less effect the impacting or point load will have on the concrete below. (See illustration below)

L&M manufactures and supports a number of products that improve the abrasion resistance of commercial and industrial concrete floors. Popular L&M products include SEAL HARD for chemical densifying, EMERYPLATE FF shake-on floor hardener, and EMERYTOP 400, for the most demanding of impact absorbing conditions. Please contact your local L&M representative or visit our website www.lmcc.com for more information. We look forward to helping you extend the life of your concrete floors.

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© 2003 L&M Construction Chemicals, Inc. | ConcreteNews Winter 2002/2003.

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