Common Concrete Terminology

• Air entrainment is a modification to the concrete that dramatically improves its durability throughout freeze/thaw cycles. Air entrained concrete has billions of microscopic bubbles of air distributed through the mix that relieve internal pressure in the concrete by providing cells for water to expand into during freezing.
• Air entrained concrete is produced by adding a carefully controlled amount of air-entraining admixture to a load of concrete during batching. Typically, air entrainment is 4% to 7%, but this can be changed depending on the requirements or conditions of the job.

Note that air entrainment refers only to the microscopic air bubbles created by the admixture, and not the visible voids often seen in cast-in-place concrete after the forms are removed. Large-sized or numerous voids are a result of poor consolidation during placement, and result in reduced strength and durability of the concrete.

• When concrete is poured into a form, it will retain large air pockets or voids that significantly reduce its strength when cured. Consolidation is a process used during placement that eliminates or reduces the size and number of these voids by mechanical agitation. Consolidation is typically completed by rodding the concrete with a pole, or by vibration with a concrete vibrator.

• Curing is the process of controlling the conditions to which the concrete is exposed immediately after placement in order to maximize hydration. The curing period is considered to be the first 5 to 7 days after placement when most of the hydration reactions occur.
• Curing is one the most important factors in determining the final properties of concrete. Proper curing conditions will dramatically improve the final strength and durability of a pour by allowing the cement to fully hydrate. For optimal curing, the concrete temperature must be kept between 5°C and 30°C, and the surfaces of the concrete must be kept fully moist.

• Ready-mixed concrete is a perishable product. It has a limited lifespan after mixing before it must be poured and allowed to harden. Plastic (unhardened) concrete that is too old at the time of discharge is difficult to work with during placing and finishing, and may not achieve full properties after hardening.

• Apparent ready mixed concrete shortages are caused by:
  • Miscalculation of form volume or slab thickness when actual dimensions
    exceed the assumed dimensions by even a fraction.
  • Deflection or distortion of the formwork.
  • Irregular subgrade & its settlement.
  • Smaller quantities wasted or used in incidental works for large pours.
• To ensure sufficient supply of ready mixed concrete:
  • Measure formwork accurately & order sufficient quantity to finish the job.
  • For large pours, include an allowance of about 2% over planned dimensions to account for wastage & potential increased thicknesses.
  • Towards the end of large pours, carefully measure the remaining volume & confirm the closing quantity to the RMC supplier

• No. Masons ought to be restricted & transit mixer operators ought not to be forced to add water. Consequences of such actions could be severe. Properly designed ready mixed concrete contains optimum water. If the workability (slump) of ready mixed concrete is not as expected, the RMC supplier should be informed. If required, dosings of admixtures along with a small quantity of water would be done by the RMC supplier’s technical personnel.

Ready mixed concrete is supplied only after exhaustive laboratory & plant trials. To ensure consistent quality, incoming raw materials are regularly tested. Sampling & testing of ready mixed concrete is done everyday as codal stipulations. At RMC Readymix (India), buyers are invited to visit our laboratories to witness the process.

• Depending on the workability, RMC would be usable for up to 3 hours.

• International standards specify that RMC must be discharged from the transit mixer truck within 2 hours of the time of loading. It is also mandatory to make arrangements at site to ensure that full load of ready mixed concrete is discharged within 30 minutes of arrival on site.

• The surface of freshly placed concrete should be kept moist for at least 7 days. Contrary to popular belief, concrete should not be allowed to ‘dry out.’ If the concrete is allowed to ‘dry out,’ the ultimate strength gain will be considerably less than its designed strength. Proper curing will also minimize the potential of cracking. The simplest method of moist curing is wetting the surface of the concrete & then covering it with polyethylene.

These defects are, generally, a result of improper finishing of the concrete. The prime factor affecting concrete strength is water / cement ratio. If excess water is added to the surface of the concrete during placement & finishing, the water / cement ratio on the surface may be drastically increased. This condition greatly reduces the strength of the concrete on the surface. Unfortunately this is where the wear takes place.

• The term slump refers, simply, to the consistency of the concrete in a plastic state (prior to hardening). Slump is a measure of how wet or stiff the concrete is. Obviously, the more the water is used in producing concrete, the wetter (or higher) the slump will be. Excessive water causes a dramatic loss of strength. Applications exist where higher slump (wetter) concrete is necessary because of difficulty in placement or specification requirements. These higher slumps attained through the use of water reducing admixtures (chemical additives).

• Concrete’s most rapid period of strength gain occurs in the first 7 days. The accepted time standard for measuring concrete strength is 28 days. However, concrete will continue to gain strength for long periods of time (months, even years) as long as moisture is present to continue the chemical process of hydration.

• The setting time is the time required by concrete to harden & set itself according to the framework in which it has been placed. The amount of time taken by concrete to set is 7 to 10 hours depending on the conditions under which the concrete is placed in the framework. But this does not mean that the framework may be removed after 7 hours. The initial setting of concrete does not impart enough strength for it to withstand its own self load so the framework should be removed only after 7 to 10 days of proper curing.

• No, this is not true. In fact, the advantages of the addition of fly ash are numerous, some of which are listed below:
  • Fly ash reduces the permeability of concrete thus helping in making the concrete structures waterproof.
  • The addition of fly ash also helps in better bonding with the reinforcement.
  • It also increases the density of concrete by reducing air voids.
  • It gives a pleasing appearance to the finished concrete.

• The water cement ratio is very important with regards to the strength of the concrete. The strength of concrete is inversely proportional to the water cement ratio, i.e. with increase in water content the compressive strength of concrete decreases & vice versa.

• It is with water that cement undergoes the process of hydration to form a gel with binding property covering the aggregates uniformly on mixing. However, quantity of water for mixing has to be limited to achieve the required consistency, as excess water is a deterrent to concrete. Potable water can be used for the same.

• First, the length & distance of the area to be poured should be measured. After that, please call our offices to obtain an estimated yardage for the project.

• We recommend referencing job site permits, checking city ordinances, or checking with the contractor for strength requirements.

• No. Mesh reinforcing does nothing to make the concrete stronger. The mesh reinforcing will keep cracks from opening up once a crack has occured.

• Much of that depends on what the concrete is used for. In most cases, fiber-mesh is an excellent no-hassle solution to providing additional strength. We also suggest using rebar or wire depending on the use.

• Intermixing millions of synthetic fibers into a cubic yard of concrete is meant to reduce or absorb micro-cracking that takes place when concrete begins to ‘set.’ If micro-cracking is reduced or eliminated in the first few hours after placement, cracking may never take place.

• The possibilities are almost as limitless as one’s imagination. From an intermixing of multiple colors to matching the siding on a house, there’s sure to be a color that is desired.

• Curing is one of the most important steps in quality concrete construction & one of the most neglected. Effective curing is absolutely essential for surface durability. Fresh concrete must be kept warm & moist until the mixing water combines chemically with the cement (hydration). Without curing, the strength of the concrete is reduced in half. Curing is usually done by applying a coat of liquid curing compound to the concrete as soon as it is hard enough to walk on without making marks on the surface.