Concrete and Reinforcement – a Hand in a Glove

When it comes to building materials, concrete is one of God’s gifts to mankind! There’s simply no better way to explain it. The components which make up concrete are both naturally plentiful – making it economic to produce – and environmentally friendly, due to it not having any byproducts. And did you know concrete can be recycled as well? Yes, concrete can be crushed and reused as aggregate for new concrete as long as non-concrete debris is removed such as wood, plastic and sheetrock.

About 7 and a half cubic kilometers of concrete are produced every year! – More than a cubic meter of concrete for every man, woman, child, dog, and cat on this planet! Concrete is used to make a wide variety of structures including roads, piping, foundations, bridges, parking lots, and walls, making up about 50% of all our infrastructure! Being as user-friendly as it is, with all its qualities, I’m not surprised.

Modern concrete is made out of a mixture of Portland cement, aggregate, and water. What kind of aggregate you add depends on what you will use the finished product for. Mortar for example, is just sand and Portland cement, whereas concrete will have coarser aggregate as well as sand. Despite concrete’s amazing compressive strength, it has significantly weaker tensile strength, and needs to therefore be reinforced by a material with stronger tensile properties before it can be reliable for most building projects.

Reinforced Concrete

The most common reinforcing material is steel “rebars”, short for reinforcement bars. Other methods and materials such as grids, plates or fibers are known to be used to lesser degrees, but rebar is the most common. There is also what is known as pre-stressed concrete, which is the same as normal reinforced concrete except that the rebar on the tensile face is placed in tension prior to concrete pour, and released after the concrete is cured.

On a typical concrete beam, there is an “outer face” and an “inner face”. When this beam undergoes flexure, a curvature will form. The outer or tensile face will experience tensile stress and the inner or compressive face, compressive stress. Installing pre-stressed rebar on the outer face of the beam creates a built-in compressive force on this outer face. This means that when loads are applied, the built-in compression will experience a reduction, but will not translate into a tensile force – until the load overcomes the compression in the rebar.

Drawbacks to Concrete

With all its astounding qualities, you might be wondering where the drawbacks are. Well, there aren’t all that many, but there are a few. All concrete will eventually form cracks. Sorry, but it’s true. No matter how well you followed the rules, you just can’t get away from this one. This is another major reason for reinforcement. Cracks are inevitable, but the reinforcement will hold it together and keep it from getting worse. Despite concrete’s extremely low coefficient of thermal expansion, repeated freeze/thaw cycles are one of the major contributors to concrete cracking.

Preventative measures such as the adding of expansion joints to absorb the movement from these cycles aid tremendously in preserving the life of your concrete. Other potential damages include corrosion of reinforcement material, freezing of trapped water that enter cracks causing additional damage as well as aiding in the corrosion of rebar, shrinkage, and creep. Shrinkage continues throughout the lifespan of concrete, but decreases quickly in rate and becomes all but irrelevant after about 30 years.

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