You have your wheelbarrow, concrete mixer and shovel all ready. But the hard part of making concrete, even with the best of tools, is getting the mixture right. Concrete has been used for centuries, but the concrete mixtures used today have come a long way from the mud mixtures that were once used. Take this quiz to see what you know about how concrete is made.
As far back as 3000 B.C., the Egyptians used mixtures of lime and gypsum, which is a very similar mixture to concrete. The Romans used a mixture known as opus caementicium.
The two components of a concrete mixture are aggregates and paste.
In primitive times straw was commonly used as an aggregate and mud was the paste. These days commonly used aggregates include sand, various sizes of gravel and crushed stone. However, items such as oyster shells and waste ash from coal-fired plants have also been turned into aggregates by concrete makers.
Aggregates are categorized by size into fine aggregates and coarse aggregates. Fine aggregates are 0.2 inches (0.5 cm) or smaller and coarse aggregates can be as large as 1.5 inches (3.8 cm).
Cement is used as the paste in modern concrete-making. Cement is made from a mixture of materials, including limestone, clay, gypsum and other chemicals and minerals. The materials are mixed in specific proportions, according to the task they are being used for. They are then burned and ground into a fine powder before being mixed with water.
Portland cement is typically used in concrete making.
Making concrete is like baking. Just as the type and quantity of ingredients used to make a cake will affect the way the cake turns out, so the type and amount of ingredients involved in making concrete will strongly influence the strength, permeability and durability of the concrete.
Admixtures add in some way to the concrete mixture This may be by adding color, affecting hydration time or shrinkage rate, or minimizing corrosion of steel beams inside the concrete.
Premixed bags contain dry concrete that simply needs to be mixed with water. It is ideal for small projects, but can get expensive for larger projects.
Mixing is often done on-site for large projects. Otherwise the concrete is mixed at the concrete plant and is transferred to the building site in a rotating drum mixer. However, the concrete should be discharged from the mixer within 90 minutes from when the water is added.
Slump loss is when the concrete becomes unworkable after a certain amount of time has elapsed since water was added to it.
Reinforced concrete contains steel mesh or rods.
Concrete drying is affected by temperature and moisture. High temperature will cause rapid curing, while low temperatures will lengthen the drying process and will produce weaker concrete.
Concrete shrinks as it dries and can ruin the slab if shrinking takes place unevenly. To prevent this from happening, the concrete surface should be kept damp while it dries. This uniformly slows the shrinkage.
Adding calcium chloride to the concrete mixture produces heat and speeds up the curing process in cold weather. In warm weather, construction workers generally work during the cooler part of the day so that the hot temperatures will not affect concrete curing.