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Frequently Asked Questions about Cement & Concrete What is the difference between cement and concrete?
Cement, more correctly called 'Portland cement' is a gray powder that reacts chemically with water. When cement is added to water, sand and stones this mixture is called concrete and this hardens into a rock-like material. ^ top Why does concrete sometimes crack?
As concrete hardens a small amount of shrinkage takes place. Cracks can develop when this shrinkage is restrained: for example by concrete slabs sticking to the ground or to adjacent walls. For more information on cracking please refer to CIP 4 in the 'Concrete in Practice' series. ^ top How is cement made?
Limestone is heated in a rotary kiln where it undergoes a chemical change and is then finely ground into a powder. In most cement plants several other natural ingredients are added to the limestone to adjust the chemistry, the most common of these are shale, iron ore, marl and gypsum. ^ top What does 'curing' of concrete mean?
Concrete continues to gain strength as long as it is maintained in a moist condition and very cold temperatures are avoided. 'Curing' is the maintaining of satisfactory temperature and moisture conditions for the concrete. There are a number of alternative methods for curing concrete. For more information on this topic please refer to CIP 11 in the 'Concrete in Practice' series. ^ top What ingredients go into concrete?
The main ingredients of concrete are cement, water, sand and coarse aggregate (gravel or crushed stone). One or two chemicals are normally added at a ready-mixed plant to improve the performance of the concrete. The most common admixtures used are water-reducers that improve the strength and air-entraining admixtures that make the concrete resistant to cycles of freezing and thawing. For more information on admixtures please refer to CIP 15 in the 'Concrete in Practice' series. ^ top What determines how strong concrete will become?
The most important factor influencing the strength of concrete is the amount of water to cement (W/C ratio) in the mixture. The lower this ratio is, the stronger will be the concrete. Therefore the easiest way to increase the strength of concrete is to use more cement in the mixture; the easiest way to weaken concrete is to add more water. ^ top What is 'air-entrained' concrete?
Air-entrained concrete contains billions of microscopic air bubbles that enable the concrete to resist the damaging effects of freezing and thawing. This is achieved by the addition of an air-entraining admixture at the ready-mixed plant. Air-entrained concrete should be used whenever concrete will experience freezing so is essential for driveways, patios, footpaths and roads in cold or temperate climates. Air-entrainment cannot be achieved when mixing concrete by hand.
^ top Can it be too hot or too cold to place concrete?
When concrete is placed in very hot and sunny conditions it tends to dry out. When completely dry no further strength will be achieved. Similarly when the temperature approaches freezing the hydration (hardening) slows to a standstill. Concrete can be placed under hot and cold conditions provided that appropriate curing methods are used to protect the concrete from these adverse effects. ^ top What kind of joints should be used in concrete paving (such as a driveway)?
The most common joints used in concrete paving are called 'control joints'. These are to accommodate the shrinkage that is normal in concrete. 'Isolation joints' may also be required to separate the paving from posts, drains, or adjacent walls. For more information on joints please refer to CIP 6 in the 'Concrete in Practice' series.
^ top What causes light and dark patches in some concrete?
Discolouration is a lack of uniformity in the colour or tone of the concrete. There are a number of different types of discoloration with different causes. The most common type, known as 'trowel burn' results from placing and finishing concrete after the hydration has started and the concrete is beginning to set. For more information on this topic please refer to CIP 23 in the 'Concrete in Practice' series.
^ top How thick does a concrete driveway need to be?
Throughout North America it is conventional to form residential driveways with 2"x4" lumber. This results in a pavement thickness of approximately 3.5 inches or 90mm. This thickness is adequate for cars, pick-ups and SUVs. If heavier commercial vehicles will use the driveway more frequently then a greater thickness of concrete is needed. ^ top How is the strength of concrete measured?
The strength of concrete generally refers to its compressive strength. This is measured by casting cylinders while the concrete is still plastic and crushing them after 28 days. Most concrete has a specified 28 day strength between 20 MPa (3000 psi) and 35 MPa (5000 psi). If cylinders are not made cores can be drilled from the hardened concrete at any age and tested in the same way. ^ top Who invented cement and when?
Cement was used throughout the Roman Empire using limestone as its main ingredient and volcanic ash when it was available. The cement used throughout the world today is credited to Joseph Aspdin, a brickmason of England who patented his process in 1824. He called this product 'Portland cement' because the concrete produced resembles the natural stone quarried at Portland, England.
^ top What is the tallest concrete building in the world?
The tallest building is the Burj Dubai tower in Dubia, United Arab Emirates. It is 818 metres (2659 ft) tall and has 162 floors. It is due to be completed in late 2009. It is constructed in concrete. ^ top How can stains be removed from a concrete driveway?
The most common stains affecting concrete driveways are oil stains, rust and tire marks. Almost all stains can be removed from concrete and even the most severe can be substantially improved. For detailed treatment methods for stains please visit the website of the B.C. Ready-Mixed Concrete Association.
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