The Concrete Chronicles
When you consider the amount used at airport runways alone, it isn’t surprising that concrete is the most widely used material in the world- recent estimates put concrete usage at over 20 billion tons per year.
Although concrete is commonly thought to be a modern material, its use dates back over 2,000 years, to the Egyptians who used a form of concrete to build some of the pyramids. However, use of concrete as a singular building material was developed by the Romans in the second century A.D. Testimony to concrete’s durability can be found in the Parthenon in Rome, built around 120 A.D. It remains the largest unsupported concrete structure in the world.
Although the terms “concrete” and “cement” are sometimes used interchangeably, concrete nerds are quick to point out they are not the same materials. The difference is a bit like that between “hops” and “beer”. Hops is but a single ingredient found in beer, which is the reason, no one says, “Pour me a hops”, or “Make it a hops light”.
Cement is the “glue” that holds concrete together. In addition to cement, concrete also contains water and an aggregate (sand, rock, gravel or a combination thereof). The typical percentages are 8% air, 7%-15% cement, 14%-21% water and 60%-75% aggregate. The water/cement mix forms a paste that attaches to and surrounds the aggregate, and holds it in place. Concrete reaches approximately 90% of ultimate strength within four weeks of being poured. However, it continues to strengthen over time as calcium hydroxide converts to calcium carbonate.
Whether it’s in raw mixture form or completely cured, all concrete looks pretty much the same, assuming it hasn’t been dyed or modified for decorative purposes. However, appearance doesn’t tell an accurate story. Depending on its composition, concrete may vary in compressive strength from 2,500 psi, typical for residential use, to 4,000-5,000 psi for commercial structures. To meet certain requirements, a compressive strength of up to 10,000 psi may be specified.
Concrete’s base compressive strength can be increased by using higher grade cement, adding certain minerals to the mix, using a low water-to-cement ratio and incorporating consistently sized angular aggregates. Regardless of composition, a common misunderstanding is that it requires 28 days for concrete to “cure”, or reach maximum compressive strength. That’s a result of industry-standard specifications for concrete that include a minimum compressive strength measured at a certain age. The most common age specified for testing to take place is 28 days after pouring.
The 28-day age specification was chosen to establish a consistent industry-wide testing regimen, not because concrete magically attains its maximum compressive strength during that time period. Actual curing times vary considerably depending on mixture composition, and knowledgeable estimates state that in seven days, most concrete reaches 75% of the compressive strength it reaches in 28 days. This is relevant to the use of concrete in construction projects because some contractors, inspectors and project managers mistakenly believe that newly poured concrete is unusable until it has reached the ripe old age of 28 days.
Compressive strength is but one aspect of concrete, and even the highest strength slabs or panels are prone to fracturing, (cracking) an inherent trait of concrete. Depending on a variety of factors, the question isn’t if concrete will crack, but when and where. Expansion joints are incorporated in many slabs to mitigate cracking and control location, but in some structures, expansion joints may not be an option, or may not provide sufficient control.
Rebar and wire mesh are commonly used to control fracturing, yet are not always as effective as desired. It is precisely for these reasons that Tendon Systems has provided post-tensioned cast-in-place structures for over 30 years. Post-tensioning has proven to be the most effective means of minimizing or eliminating fracturing.
As with most materials, concrete is continually evolving as new technologies are developed to improve performance. Compared to the ancient concrete used in structures that are thousands of years old, the collapse of some modern buildings, in less than 50 years, may seem to indicate that modern concrete is inferior. Nothing could be further from the truth. Modern concrete enables the construction of structures that would be all but unbuildable, or unaffordable, with the concrete of ancient Rome.
Concrete is an incredible building material. Even with the best materials and construction practices concrete faces foes which will take its toll on a structure. Those foes time, temperature extremes, ice, salt and other natural events. In our next blog we will discuss how to restore concrete to ensure a more durable building.