What is Concrete?
Concrete is comprised of Portland cement, fine aggregate, coarse aggregate, water, pozzolans, and air. Portland cement got its name when it was first used in the early nineteenth century in England, because its product resembled building stone from the isle of Portland off the British coast. When cement is mixed with water the resultant product is referred to as PASTE. This is the substance that binds all other ingredients together.
When cement is mixed with water the resultant product is referred to as PASTE. This is the substance that binds all other ingredients together.
Aggregates are divided into two categories and are comprised of large number of naturally occurring manufactured products. The basic distinction is as follows:
- Fine aggregate - #4 sieve to pan (1/4” to powder).
- Coarse aggregate – 3/8” to 1-1/2”
- The addition of fine aggregate to the PASTE transforms the product to a MORTAR. The subsequent addition of coarse aggregate results in CONCRETE.
Pozzolans are defined as anything other than cement, aggregate, and water that are added to the concrete mix. The more common types of pozzolans are:
- Fly ash
- Silica fume
- ASTM C-260
- Air entraining
- Latex modifiers
- Acrylic modifiers
- Chemical admixtures
Pozzolans are used for both their cost reducing and performance enhancing properties. The proper-engineered use of these materials can greatly enhance workability, setting times, finishing characteristics, density, porosity, durability and strength gain characteristics.
All of these ingredients, when mixed together harden to form a near solid mass of concrete. The properties and quality of concrete varies significantly depending on the type and quality of the above mentioned ingredients and the amount of water used in the mix design in relation to the cement content. This relationship is referred to as the water cement ratio or simply w/c, and is the fundamental basis for “Abram’s Law”, which states that there is a direct relationship between the strength characteristic of Portland cement based concrete and the amount of water used per weight of cement. The lower the w/c ratio the higher the resultant physical properties will be. Rule of thumb (less water = better concrete).
Most technical specifications and most manufacturers’ literature, calls for a minimum 28-day curing period prior to the application of polymer based coatings and toppings. The technical basis for this recommendation of the relationship between time and the cement hydration process, is directly related to compressive strength development, and is measurable. Plain concrete is proportioned to develop 80% of its design strength in 7 days and 100% of its design strength in not more that 28 days, (concrete containing fly ash is 56 days). A good rule of thumb is to cure concrete until it has reach 80% of its design strength. There are several acceptable methods of curing concrete and may be referred to in ACI 308-86, and ACI 302.1 R-89. Some of these methods are listed below:
- Ponding Water
- Soaker hoses
- Wet Burlap
- Moisture retaining sheet membrane
- Liquid membrane curing compounds
75% of all concrete floors poured in Ontario use a liquid curing membrane
Of greatest interest and concern to our industry is the use of liquid membrane curing compounds, which leave a film and or residue on the surface, which must be removed by mechanical means, i.e., sandblasting, shot blasting, scarification, etc., prior to the application of a polymer system.