Concrete is often made of cement, sand, varying amounts of coarse aggregate (gravel), water, and additives.
The concrete mixture is then poured into a form, allowed to set and cure, and the finished product can then be polished or treated in a number of ways to achieve a variety of surface finishes.
Cement paste or mortar may be used as the finish coating. Still, it has been found that this is not always enough when smoothness and durability are required in exterior or interior surfaces where wear from foot traffic is common. To give added protection against erosion or wear, it is customary to add a thin overlay of polish repellent over the cured hardened concrete surface.
To most people, polishing concrete seems like an unnecessary step in the production phase and would be considered just another process that adds time and cost to the project. This could not be further than the truth. Cured concrete is a porous material and, as such, it will allow water to penetrate its surface. Over time, this penetration will cause the surface of the concrete to corrode and deteriorate. This deterioration can be prevented by polishing or honing (if necessary) curing concrete surfaces after they have been placed.In cement and concrete basics, the raw materials typical in modern concrete mixes are: water, cement, aggregate or crushed stone, sand or gravel, fly ash, water reducing admixture/plasticiser, superplasticiser, air entraining agent, and colouring agents. Other agents may also be added depending on the application. Additives such as reinforcing fibres to help improve strength, fireproofing agents to carry a higher temperature load without damage, or retardants that prevent damage from freeze-thaw cycles may also be found in certain mixes.
The main part of most concretes is made of fine aggregate and or coarse aggregates (sand). The function of the aggregate is twofold; firstly, they act as "fillers" taking up space that might otherwise be taken by the cement paste (and hence lower the strength of the concrete). Secondly, they greatly increase the toughness of the finished product. The sand used may be either natural sand or sand produced by crushing stone or gravel.
The cementing material for most concrete mixes is portland cement. This type of cement sets through a chemical process between dry calcium-silicate crystals with hydrated magnesium-silicate crystals formed when mixed with water. When portland cement is properly combined with water, its minerals begin to chemically react, producing a binding effect that forms these strong interlocking crystalline structures. These interlocking crystalline structures in the cured concrete provide it with much of its strength, rigidity, and durability. Concrete can also be made using other hydraulic cement like fly ash, slag cement, masonry cement, and polymer concretes. The desired type of concrete will vary depending on the application. For example; A blend of portland cement with some of these hydraulic cements can be used to make high quality concrete that is extremely hard. Alternatively, portland cement can be used to make high permeability porous concrete in projects where ground infiltration is paramount. However, it would not be ideal for producing a polished surface.Concrete is made of cement, sand, water, and additives. Cement is one of the ingredients necessary for concrete to work. One of the most widely used cement is portland cement. It sets the concrete solid by allowing calcium-silicate crystals to form between dry powdered chemicals with water through a chemical process. This creates strong interlocking crystalline structures in the cured concrete, providing its strength, rigidity, and durability.
Cement is a fine powder that comes from a mechanical process of grinding clinker. Its main ingredient is calcium silicate, with small quantities of aluminium and iron oxide, magnesium oxide, and various admixtures. "Clinker" is the product obtained by heating together with a mixture of certain commonly occurring minerals - mainly clay or shale, limestone, and/or seashells. A calcining kiln provides the heat required for this reaction to take place. The type of material used and the temperature at which it is calcined determines the composition of the clinker. In mining operations where rock with high amounts of clay or shale are being mined as the primary resource, clinkers are often found as a secondary item in those mines as well as other locations. In processing, the clinker is ground into very small bits inside a ball mill, becoming cement when mixed with water.
When concrete is mixed, the cement sets. This chemical process between dry calcium-silicate crystals with hydrated magnesium-silicate crystals that are formed when mixed with water. When properly combined with water, portland cement's minerals begin to chemically react with one another, producing a binding effect that forms these strong interlocking crystalline structures. It is these interlocking crystalline structures in the cured concrete that are responsible for the concrete's strength, rigidity, and durability. To accelerate the concrete setting time, calcium chloride or aluminium silicate is added to the cement to slow down the shrinkage process, thus avoid concrete cracking.
Typically, pre-mixed concrete will be more expensive than poured concrete, prepared before pouring at the ready mix plant. Poured concrete is frequently made from a blend of dry cement and aggregate that requires only the addition of the appropriate water to cement ratio to create a ready mix. Pre-mixes typically contain powdered chemical additives mixed with the cement and fine aggregates, requiring only the addition of water before it can be installed.
Additionally, certain types of poured concrete may require placement in steel forms rather than wooden forms, which are generally required for pre-mix installations. The placement in steel forms is mandatory for prestressed concrete, which combines the compressive properties of concrete with the high tensile strength of steel. Concrete pumping has a raft of safety concerns, most of which is outlined by Safe Work Australia.Concrete cracks can be a result of a number of reasons:
The concrete mixture on its own is one of the weakest elements to carry vertical loads, especially one that has no reinforcement. This cracking problem is solvable by the use of reinforced concrete in combination with steel bars, which help the resulting concrete to withstand tensile stresses.
The most common reasons for cracking are:
A lack of steel reinforcement in concrete structures to hold vertical loads, causing them to crack under stress.
Poor quality control where too much mixing water is used and/or cement and sand ratios changed during batching and placing the concrete, which causes it to be weak and less durable than normal.
Concrete placed in sections versus being poured all at once to reduce temperature differentials between the top and bottom of the slab, which also causes it to be weak and less durable than normal.
Driving on fresh wet concrete while it is too soft pushes up on the surface, causing cracking under stress over time.
Placing new asphalt or other hard surfaces over existing concrete that has not had the chance to cure completely before covering, which prevents self-healing from occurring, causing early deterioration of the surface below.
The design itself where flat slabs (other than driveways) that lack steel reinforcement and control joints like expansion joints normally require a thicker layer of concrete that is more prone to large cracks unless special care is taken during installation.
Fluctuations in moisture content due to groundwater movement underneath or on top of the concrete, causing it to crack over time. Water that seeps into the surface can also cause cracking by freezing and thawing with temperature changes, especially if no control joints are cut into the slab ahead of time, which is common for driveways.
Concrete installed on the ground that is not prepared properly where water can collect under it, causing it to be weak and less durable than normal until the soil dries out completely without any puddles remaining for more than a few days before placing it.
Placing concrete directly on new dry soil instead of compacting the soil first before pouring wet concrete, which is not recommended.
Not watering down or misting fresh concrete to keep it damp, causing it to be weak and less durable than normal. The water in the concrete evaporates completely over time which usually takes at least a few days depending on the weather conditions. Wind and temperature changes affect how slowly or quickly concrete dries. Extremely hot weather and dryness cause rapid shrinkage that can lead to normal concrete cracking.