Concrete Basics | LYCON INC.

Education Concrete Basics

Concrete is a building material composed of four main ingredients: cement, water, sand (fine aggregate) and stone (coarse aggregate). The cementitious material in concrete is usually composed of the classic ‘Portland cement,’ but supplemental cementitous materials—such as Flyash and Slag—can also be used to change the physical properties of the concrete to fit a wide variety of job-specific applications.

A variety of additional products, called admixtures, can be added to a concrete mix to achieve job specific needs such as a reduced set time, increased strength, and improved workability of the material. For decorative applications, we also offer liquid integral color.

Air Entrainment is an admixture added to a load of concrete to relieve pressure on the material caused by freeze-thaw cycles. These microscopic air voids allow the water that naturally resides in concrete a place to expand and contract as it freezes and thaws. Typically the amount of entrained air is between four and eight percent of the volume of concrete, but can vary depending on job specifications. It is generally suggested to order full air entrainment for material that is subjected to freeze-thaw cycles.

Slump is a measure of the consistency and workability of concrete. A higher slump will mean the concrete is more flowable. To achieve a higher slump, water is added to the mix unless the slump is changed chemically through water-reducing admixtures. Increasing the slump using water alone increases the water/cement ratio and the result is lower strength and durability. Because of this, water-reducing admixtures should be used if increasing the slump beyond the common 4” in order to maintain the quality of the concrete.

The water/cement ratio is the weight of water compared to the weight of cementitious materials used in a concrete mix. This measurement is imperative to the overall quality of concrete. A lower water/cement ratio results in increased strength, reduced shrinkage cracking, lower permeability and increases the resistance to weather and wear. A specified water/cement ratio often ranges between .40 and .60, with the lower often used on high-strength jobs.

Fiber reinforcement is a common addition to concrete because it can help to reduce cracks, increase the impact resistance, and can increase the concrete strength in certain applications. A variety of fiber types and dosages are available in order to accommodate different needs both in commercial and residential use. Some common types are micro-synthetic, macro-synthetic and steel fibers. These are added at the batch plant and can save time, energy and labor cost compared to other common reinforcement options.

When added to concrete, the special packaging dissolves allowing the fibers to mix in with the concrete easily. Synthetic fibers must be mixed thoroughly prior to discharge to ensure equal distribution throughout the mix.

In more technical terms, synthetic fibers provide a secondary reinforcement to both strengthen concrete and increase its durability. It does this by controlling internal cracks caused by gravity and drying conditions. If untreated, these cracks increase the permeability of the concrete allowing salts and other harmful chemicals to take hold. That spells disaster for concrete in the long run.

The first 24 hours after placement of concrete is generally when these cracks occur due to moisture loss, the cement hydration process, and temperature changes. Because most concrete is restrained, volume changes resulting from moisture loss creates stress on the concrete and can causes cracking. Restraint is caused by the difference in shrinkage between the surface and the underlying concrete area.

Synthetic fibers reduce the amount of cracking of concrete in three ways. One, it provides more strength to resist volume changes. Two, it bonds with the fresh concrete and distributes stress more evenly. Three, it reduces evaporation of water by reducing the bleed rate.

Synthetic fibers are easy to use compared to welded wire fabric due to placement issues. (If placed on the bottom of the slab, wire provides no benefit.) Each help control cracking. Wire fabric helps control cracks after they are formed, while fibers help prevent cracks from forming.

Note: Synthetic fibers are not recommended to increase joint spacing or as a substitute for any reinforcement required by building codes and standards. Adding fibers does not justify changing the mix designs in most situations.

Product information provided by the manufactures of synthetic fibers.

An accelerator is a chemical added to a load of concrete which reduces the time it takes for the concrete to set. They are often added during cold weather to reduce the setting times which can lower the chance of the concrete being damaged by freezing conditions. Accelerators are often ordered as a percentage of cementitous materials.