How Is It Made?
After the aggregates, water, and the cement are combined, the mixture remains in a fluid condition for about four to six hours which permits transporting, placing and finishing in its final location, then the mixture starts to harden. All portland cements are hydraulic cements that set and harden through a chemical reaction with water. During this reaction, called hydration, crystals radiate outwards from cement grains and mesh with other adjacent crystals or adheres to adjacent aggregates. The building up process results in progressive stiffening, hardening, and strength development. Once the concrete is thoroughly mixed and workable it should be placed in forms before the mixture becomes to stiff. During placement, the concrete is consolidated to compact it within the forms and to eliminate potential flaws, such as honeycomb and air voids.
The proportioning of a concrete mix design should result in an economical and practical combination of materials to produce concrete with the properties desired for its intended use, such as workability, strength, durability and appearance.
The ready mixed concrete producer may independently select the material proportions to provide the performance you need or may receive instructions through the job specifications, such as minimum cement content, air content, slump, maximum size of aggregate, strength, and others. The RMC producer is the expert in selecting the proportions based on previously developed guidelines and experience.
Regardless of the source of instructions, there are established methods for selecting the proportions for concrete for each batch. The Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete (ACI 211.1-91) published by the American Concrete Institute Committee 211 is often referenced for selecting concrete proportions.
Here are the basics of a good concrete mix:
- Cement and water combine chemically to bind the sand and aggregate together. Fly ash or other cementitious materials, which enhance concrete properties may supplement some of the cement. The key to quality concrete is to use the least amount of water that can result in a mixture that can be easily placed, consolidated and finished.
- Fine and coarse aggregates make up about 70% of the concrete volume and impart volume stability to the concrete. Concrete aggregates are required to meet appropriate specifications and in general should be clean, strong and durable.
- Admixtures, are generally products used in relatively small quantities to improve the properties of fresh and hardened concrete. They are used to modify the rate of setting and strength development of concrete, especially during hot and cold weather. The most common is an air-entraining agent that develops millions of tiny air bubbles in concrete, which imparts durability to concrete in freezing and thawing exposure. Water reducing admixtures enable concrete to be placed at the required consistency while minimizing the water used in the mixture, thereby increasing strength and improving durability. A variety of fibers are incorporated in concrete to control cracking or improve abrasion and impact resistance.