In theory, the water-to-cement ratio should be 0.35, or 1-to-3; that is, one part water to every three parts cement. That is all the water necessary to properly hydrate the concrete. Any additional water is excess, or “free,” water. This excess water eventually dissipates, leaving pores and voids in the concrete.
However, a 0.35 water-to-cement ratio is too stiff and unworkable. Consequently, a water-to-cement ratio of 0.5, or 1-to-2, is necessary. At the majority of job sites, however, the ratio is more than 0.5. The porosity of a slab dramatically changes as the water-to-cement ratio increases.
Increasing the water-to-cement ratio also directly affects the strength of the slab in terms of pounds per square inch (PSI). When the ratio is too high, the following may occur: excessive bleeding; chalky surface; low strength; cracking; crazing; and water leakage.
The floor covering installation industry is not a champion of curing compounds; their use affects our bonds. However, the impact that they have on the ability for a concrete slab to reach its desired strength in a shorter period of time is tremendous. A concrete slab free of any curing compound will achieve a 50% strength level in 28 days. Using a low-grade curing compound will allow the slab to reach the 65% level in the same period of time, while a high-grade curing compound allows the slab to reach a 100% strength level.
Be aware, however, that the 100% strength level is not the fully hydrated strength of the slab. It may take some slabs as long as 20 years to reach their final PSI. The “100% in 28 days” figure represents approximately 85% of the slab’s full final strength.
It is important to understand concrete and the problems that may arise when a slab is placed. Being familiar with the material’s characteristics gives the installer a decided advantage.
Porosity. Since most flooring work is performed on-grade, porosity, or permeability, is a significant factor in achieving a good bond. As nearly all structures are subject to water intrusion, due to absent or punctured vapor retarders, irrigation, inclement weather, etc., it is important to have a slab with a good water-to-cement ratio. Failing this, it is just as important to recognize the need for topside waterproofing.
Cracking. Shrinkage cracks in newly placed concrete continue for several years. However, it is estimated that 60-70% of shrinkage occurs 3-6 months after the pour. An average concrete slab will shrink 1/8-inch per 20 feet, i.e. a 30-by-60-foot slab will shrink 3/16-inch along the 30-foot side, and almost 3/8-inch on the 60-foot length. If the slab is not reinforced, cracks will develop.
Crazing and Map Cracking. This is a situation wherein shrinkage cracking occurs, but they do not penetrate very deep into the slab. Crazing cracks are nearly invisible, and cover areas of 1-2 inches. Map cracking is more visible, and covers a larger area. These cracks usually develop due to rapid surface drying and incorrect jobsite practices.
Strength. The PSI of the concrete slab is important to the floor covering industry. The American Concrete Institute (ACI) set a standard of 3,500 PSI. However, the 1974 publication from Housing and Urban Development (HUD) permits the use of concrete with a considerably lower strength.
Bleeding. Generally, this is the result of a high water-to-cement ratio and/or settlement of aggregate, resulting in a large amount of water on the slab surface. The water movement to the surface also creates voids, making the concrete more porous and contributing to a weaker surface.
Alkaline Deposits. Alkaline deposits are formed by the migration of water to the slab surface during the curing/drying process. In a dry state they are harmless, but if water is present and not addressed, the deposits can affect any bond. The easiest and safest way to remove the deposits is by washing them away with water.
Floor covering installation starts from the ground up. Understanding the necessity of a good concrete slab is an important part of the process.