Concrete Basics: What is Concrete?

Beautiful Sydney Opera House Is Made of Cocnrete

Concrete is a synthetic rock primarily made of Portland cement, coarse aggregate (washed 3/4in stone), fine aggregate (sand) and water. It is a strong, inexpensive and versatile building material used to build foundations, bridges, roads, driveways, parking garages, dams and more!

Part of what makes concrete such a great and inexpensive building material is the broad availability of the ingredients, workability, moldability and it’s very good compressive strength. Compressive strength is its capacity to withstand loads to reduce its size, or compress it. While concrete has excellent compressive strength, it has fairly poor tensile strength of about 10 percent of its compressive strength. Tensile strength is its capacity to withstand loads wanting to pull it apart. Steel reinforcing is commonly used to improve the tensile strength of concrete. Unlike many other building materials, concrete doesn’t burn, rot or mould. The structural integrity of concrete provides added protection against earthquakes, hurricanes, tornados and other severe weather.

A common mistake people often make is to use the word cement to describe concrete. Cement is an ingredient in concrete; concrete is the finished product. Calling cement concrete is like calling a cake flower because flower is an ingredient. And those funny looking trucks are called concrete mixers not cement mixers!

Portland cement and water react to crete a cement paste which binds the fine and coarse aggregates together to make concrete. Supplementary Cementing Materials (SCM) and admixtures are often added for performance and to reduce the carbon footprint of concrete. It takes little water to start the hydration (reaction with cement) process. In fact, it takes so little water that contractors would not be able to place and finish it because it would be too stiff. Additional water along with synthetic water reducers, admixtures and SCMs are often added to concrete to make it more workable so it can easily be placed and finished. That being said, concrete should be placed, not poured. In general, if concrete is poured, there is too much water which will weaken the concrete. Adding one (1) gallon (3.78 litres) of water to one (1) cubic yard (.76 cubic metres) of concrete beyond what the mix design calls for will decrease the compressive strength of the concrete by about 250psi (1.75MPa) and will increase plastic shrinkage by as much as 10%!

Cement paste gains strength over a period of time, taking 28 days to reach it’s designed strength, although it never stops curing! Hydration and hardening of the concrete over the first 3-7 days is critical to concrete’s final strength and plays a significant role in minimizing plastic shrinkage cracks leads to increased strength and lower permeability. The slower moisture escapes from the placed concrete, the better. Flooding the concrete or covering it with burlap and/or polyethylene sheeting and chemical curing compounds are common methods for curing concrete. Improper curing can cause scaling, reduced strength, poor abrasion and chemical resistance and cracking. Care must also be taken to avoid freezing or overheating due.

Concrete cracks. Its a fact. Concrete cracks due to internal and external stresses acting on it. Plastic shrinkage of the concrete can cause cracking as can improper sub base preparation and compaction, excessive loading and lack of reinforcing. A properly prepared adequate base for along with the proper amount, size and spacing of reinforcing steel for your area will go a long way to minimizing cracking. The next thing is to introduce control joints into the concrete as soon as possible. Typically within 12-24 hours of placing the concrete. A control joint is a man-made joint that is either tooled or cut into the concrete to a depth of 1/4 of the concrete thickness which creates a weak spot in the concrete which controls where the concrete cracks when it shrinks or moves. Proper curing as noted above is also critical to minimizing cracking.

In addition to proper mix designs, hydration and control joints, another consideration for long-lasting, durable concrete is proper finishing. Concrete finishing is part art, but mostly science. Over working the concrete or adding water to the surface to get a pretty finish will weaken the concrete and eventually lead to dusting, surface spalling, scaling, pitting and crazing.

When one understands concrete, what it consists of and what makes it durable and then puts that knowledge into practice, a concrete structure will be durable and resistant to chlorides, freeze-thaw and abrasion for decades and beyond. Concrete was used extensively by the ancient Romans in building of the aqueducts and other structures.