Q: Are cement and concrete the same thing?
A: While the terms are often used interchangeably, concrete and cement are not the same. Cement is an ingredient of concrete. It’s the fine grey powder that, when mixed with water, sand and gravel or crushed stone, forms the rock-like mass known as concrete. Cement typically accounts for 10% to 12% of the concrete mixture. In practice, cement acts as the binding agent or glue in concrete. When combined with water, cement forms a paste that coats the sand and gravel. Through a chemical reaction called hydration, the mixture sets and hardens into concrete. When freshly mixed, concrete can take any shape or form; when hardened, it’s strong and durable. These qualities explain why one material, concrete, can build everything from skyscrapers and sidewalks to highways and dams. Although natural cement dates back to the Romans, by the late 1800s builders were turning to its manufactured counterpart, Portland cement, for higher strength and more consistent performance.
Q: What are aggregates?
A: “Aggregates” is a general term for rocks and minerals used in a variety of industries for a range of purposes. Aggregate is classified by particle size and consistency. There are two basic types: sand and gravel (sometimes called natural stone) and crushed stone. More recently a third type of aggregate has emerged: recycled concrete aggregate, which is produced by crushing concrete reclaimed from demolished highways, buildings and other structures.
Aggregates are truly foundational. They make up 94% of asphalt pavement and 80% of concrete. They are a vital component of our homes, from the concrete in the basement to the shingles on the roof. They are the bedrock of our infrastructure – the roads, airports and waterworks that fuel our lifestyle and economy.
With more than 200 aggregate production sites and distribution terminals across the U.S. and Canada, Lehigh Hanson is one of North America’s leading aggregate producers.
Q: What is ready mixed concrete?
A: Ready mixed concrete is more of a delivery system than a type of concrete. Like all concrete, it has three basic components: cement, water, and aggregates – the stone, sand, and gravel that bind together.
In the early days of concrete construction, workers mixed these components right on the project site – and still do for certain types of construction. But as concrete construction evolved and advanced, bringing concrete to the job site with truck-mounted mixers became more efficient and offered a more consistent and higher quality product.
Truck mixers have a revolving drum to mix the concrete and discharge it through a chute. Inside the mixer drum, blades or fins mix the concrete when the drum spins in one direction and discharge it when the direction is reversed.
The process begins at the ready mixed concrete plant, where the raw ingredients are loaded into the truck-mounted drum. Once loaded, the concrete ingredients can be mixed immediately by rotating the drum or mixed on the way to the construction site.
Other types of plants thoroughly mix the concrete ingredients before loading it into the truck. The drum need only revolve slowly on the way to the site to keep the mix agitated.
If the construction industry has a public face, it’s the familiar ready mixed concrete truck with the revolving drum – a daily fixture on our roads and construction sites.
High visibility stems from the near universal use of the product it delivers. Easily the world’s most widely used construction material, concrete is part of everything we build. And nearly three-quarters of it comes in the form of ready mixed concrete.
Resources and more information:
ASTM C150 Standard Specification for Portland Cement
National Ready Mix Concrete Association
Design and Control of Concrete Mixtures, Portland Cement Association’s reference on the fundamentals of concrete technology and construction.
The National Stone, Sand and Gravel Association
Portland Cement Association