Understand the properties of concrete and you form a solid foundation for your DIY concrete projects.Concrete is the most versatile and dynamic man-made building material in history. The properties of concrete, until manipulated, are very uncomplicated. Concrete, in its most basic form, is made up of water, cement, and sand or other aggregate like gravel. Concrete is something almost all of us come in contact with on a daily basis. Ordinary Portland Cement is the most common cement used, and it is used in mortar, stucco, and grout as well as in concrete. The origin of the designation "Portland" is not definitely known. Cement is a powder made up of limestone with silicon dioxide with a concentration of at least 80% limestone. Other raw materials in the cement powder depend on the purity of the limestone and include shale, clay, iron ore, bauxite, and fly ash. The presence and extent of these many dynamic materials play a role in the many vital properties of concrete. Basic Properties of Concrete Made with Portland CementNon-reinforced concrete weighs 145 lbs. per cubic footReinforced concrete weighs 150 lbs. per cubic foot In metric units, that is approximately 2,240 to 2,400 kg per cubic meter Compressive strength: 20 to 40 MPa, or 3,000 to 6,000 psi (very strong) Tensile strength: 2 to 5 MPa, or 300 to 700 psi (very weak) Specific heat capacity: 0.75 kJ/kg K, or 0.18 Btu/lb oF Specific gravity (how much a given volume weighs divided by how much the same volume of water weighs): 2.65 One of the most important properties of concrete is its density which essentially is a measurement of its solidness. The internal forces on concrete structures like arches, walls, columns, and vaults are mostly compressive forces, and concrete is good at handling those. The internal forces on concrete structures like floors and sidewalks are mostly tensile, and concrete isn't so good at handling those forces. Contraction JointsBecause of concrete's low tensile strength, contraction joints are often regularly spaced in concrete structures subject to high tensile stress, like driveways, sidewalks, and in some cases, kitchen countertops.Contraction joints are deliberately created grooves that do not go all the way through the concrete. The grooves in a concrete driveway or sidewalk are contraction joints, and they are created so that the concrete can flex enough not to crack under tensile stresses. Sometimes it cracks anyway, however, despite the best efforts of the engineers and concrete workers. Concrete is going to crack - all we can do is manage/minimize concrete cracking as best we can. Steel ReinforcementAnother way that engineers and construction workers keep concrete from cracking or pulling apart is reinforcing it with reinforcement bar (rebar) made of unfinished tempered steel.Reinforcement mesh is made of either steel wire or rebar welded together into a grid. The grid spaces range from 4 inches square to 8 inches square. This minimizes shrinkage cracking in the surface of the concrete as it cures. Concrete reinforcement mesh consists of pieces of wire or rebar welded together to form a grid pattern. The size of the grid pattern is typically 4-inch-by-4-inch up to 8-inch-by-8-inch. Again, this type of reinforcement is used to minimize shrinkage cracking in the surface of the concrete. Regular vs. High Strength ConcreteRegular or standard concrete is made using the mixing instructions that you see on bags of cement using sand and gravel or bags of powdered cement and aggregate mix that only needs water. A cubic foot of regular concrete uses 10 kg (22 lbs.) cement, 4.5 kg (10 lbs.) water, 19 kg (41 lbs.) dry sand, and 32 kg (70 lbs.) dry stone.This cubic foot of concrete would weigh about 65 kg (143 lbs.). High strength concrete is made by altering the properties of concrete by lowering the water to cement ratio (W/C) to 0.35 or lower. Sometimes silica fume (fine grain, low density, high surface area silica) is mixed in to prevent strength reducing calcium hydroxide crystals in the cement matrix. The problem with a high strength mix is that it is much less workable, which is problematic in high strength applications where closely spaced rebar is needed. In these cases, there are superplasticizers that can be added to make the alter the properties of concrete to make it more workable. Recycling ConcreteWith recent attention to environmental issues increasing, concrete recycling is also getting a second look. This is an increasingly common way of getting rid of disfunctional concrete structures.It used to be shipped to landfills, but there are environmental and economic benefits to recycling concrete. The concrete is put through a crushing machine along with bricks, asphalt, and rocks. If it has rebar, magnets are used to extract it and the rebar is recycled elsewhere. Large chunks from the first round of crushing may go through the crusher again while smaller pieces are used as gravel for new construction products. If you have any question(s) on the properties of concrete, please don't hesitate to email them to me. Meet-Mr-Concrete.com provides concrete construction advice at no charge and in good faith for consideration only. We do not warrant the implementation of its content and are to be held harmless when and if any advice or ideas provided are utilized. The risk from using it lies entirely with the user. 
|
