| Concrete starts life as cement, aggregates and water. The aggregates are typically varying sizes of sand and gravel depending on the mix design. Ready Mix trucks are filled at a concrete batch plant like the basic version seen to the right. A long aggregate conveyor (seen in the back) fills overhead storage bins (seen on the right). A weigh batcher hanging below the overhead bins weighs a batch and then drops the material on to a truck charging conveyor (between the storage bins and the silo) which loads into the ready mix truck. Meanwhile the cement is being weighed in the cement batcher hanging below the cement silo (on the left) and above the discharge of the truck loading conveyor. A ready mix truck will then back up underneath the silo and receive the materials. The water can be either weighed or metered at the same time and discharged into the truck at specified times to prevent balling of the cement. A small amount of water is kept behind until after loading the truck to help wash out the discharge point of the plant and the loading point of each truck. From start to finish loading a truck takes approximately 5-10 minutes depending on the design of the ready mix plant. |  Basic Ready Mix Concrete Batch Plant designed by CMC |
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.
]]>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.
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.
Concrete:
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)]]>