Ready Mix concrete is proportioned in a concrete batch plant according to any number of set recipes. The concrete is then mixed and delivered to the desired location via truck mounted mixers. Using a good quality batch process and good quality equipment can result in a precise mixture.
A typical mix will comprise of about 60 to 75 percent aggregates (mix of sand and stone), 10 to 25 percent is cement and 10 to 20 percent is water.
Each batch of concrete can be made to order in both size and type. In Canada the typical size of a full load batch is between 7 and 9 cuM. Some trucks are capable of carrying up to 11 cuM. However many roadways have weight restrictions that limit the over all mass that can be carried per axle of the vehicle, especially during spring and fall when the ground is soft.
When choosing a concrete mix design the formula will depend on the needs of the project in terms of the required strength, the weather conditions it will be exposed to, the final appearance, the cost of various additives, building codes and local legislation.
Cement vs. Concrete:
Portland Cement is the most widely used type of cement in the world. It consists of Limestone and several other materials heated in a kiln resulting in a substance called "clinker". The clinker is then ground and a small amount of gypsum is added to create Ordinary Portland Cement.
Portland Cement is an ingredient in Concrete. Usually comprising approximately 10% of the mix by weight. The remaining ingredients are gravel, sand and water. Admixtures can be added to the mix to alter various properties.
All concrete will crack regardless of whether or not its compressive strength is high enough. In fact a mix with a high Portland cement content can actually crack more easily due to the increased amount of paste, hydration rate and from heat. As concrete hydrates (water combining with cement), the material undergoes shrinkage.
Standard concrete weighs approximately 150 lb/cuft which is 4,050 lb/cuyd and approximately 2,400 kg/cuM, depending on the aggregates and mix design.
A large factor in the ultimate strength of concrete is influenced by the water-cementitious ratio (w/cm). With all other factors being equal, concrete with a lower water-cement (cementitious) ratio makes a stronger concrete than that with a higher ratio. A ratio of 0.50 or less is required in some situations for high strength, faster curing or when the concrete is exposed to freezing and thawing. With a standard 4,050 lb/cuyd batch with a w/cm of 0.50 the mix would have approximately 405 lbs of cement and 202.5 lbs of water.
Some experimental concretes have been made with a compressive strength of up to 800 MPa (116,000 PSI). Most standard strength concrete used in industry has a compressive strength between 20 - 40 MPa (3000 - 6000 psi)