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1 What is the difference between cement and concrete?
Although the terms cement and concrete often are used interchangeably, cement is actually an ingredient of concrete. Concrete is basically a mixture of aggregates and paste. The aggregates are sand and gravel or crushed stone; the paste is water and Portland cement. Concrete gets stronger as it gets older.
Portland cement is not a brand name, but the generic term for the type of cement used in virtually all concrete, just as stainless is a type of steel and sterling a type of silver.
Cement comprises from 10 to 15 percent of the concrete mix, by volume. Through a process called hydration, the cement and water harden and bind the aggregates into a rocklike mass. This hardening process continues for years meaning that concrete gets stronger as it gets older.
So, there is no such thing as a cement pavement, or a cement mixer; the proper terms are concrete pavement and concrete mixer.
2 How is Portland cement made?
Materials that contain appropriate amounts of calcium compounds, silica, alumina and iron oxide are crushed and screened and placed in a rotating cement kiln. Ingredients used in this process are typically materials such as limestone, marl, shale, iron ore, clay, and fly ash.
The kiln resembles a large horizontal pipe with a diameter of 3 to 4 metres and a length of 90 metres or more. One end is raised slightly. The raw mix is placed in the high end and as the kiln rotates the materials move slowly toward the lower end. Flame jets are at the lower end and all the materials in the kiln are heated to high temperatures that range between 1480 and 1650 °C. This high heat drives off, or calcines, the chemically combined water and carbon dioxide from the raw materials and forms new compounds.
For each ton of material that goes into the feed end of the kiln, two thirds of a ton comes out the discharge end, called clinker. This clinker is in the form of marble sized pellets. The clinker is very finely ground to produce Portland cement. A small amount of gypsum is added during the grinding process to control the cement's set or rate of hardening.
3 Are there different types of Portland cement?
Although all Portland cement is basically the same, eight types of cement are manufactured to meet different physical and chemical requirements for specific applications:
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Type I is a general purpose Portland cement suitable for most uses.
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Type II is used for structures in water or soil containing moderate amounts of sulfate, or when heat build-up is a concern.
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Type III cement provides high strength at an early state, usually in a week or less.
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Type IV moderates heat generated by hydration that is used for massive concrete structures such as dams.
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Type V cement resists chemical attack by soil and water high in sulfates.
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Types VI, VII and VIII are cements used to make air-entrained concrete. They have the same properties as types I, II, and III, except that they have small quantities of air-entrained materials combined with them.
4 What sized panels are available in precast concrete?
While Wilco Precast has the ability to cast panels up to 15 metres x 4 metres, in varying thicknesses, the determining factor is road transport and/or site craneage constraints. Orders are manufactured to ensure that these parameters and the design requirements are met.
See TECHNICAL - Delivery Parameters.
5 Who invented concrete?
Long before the time of Moses, mud and moist clay had been moulded into bricks or placed between forms to build the walls of homes and fortifications. Eventually it was discovered that a moist coating of thin, white, burnt limestone protected these earthen surfaces from erosion.
In the western world, concrete was probably first created in Italy, some 200 years before the birth of Christ.
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6 Can it be too hot or too cold to manufacture precast concrete panels?
Temperature extremes make it difficult to properly cure concrete. On hot days, too much water is lost by evaporation from newly placed concrete. If the temperature drops too close to freezing, hydration slows to nearly a standstill. Under these conditions, concrete ceases to gain strength and other desirable properties.
In general, the temperature of new concrete should not be allowed to fall below 10 °C during the curing period. Wilco Precast use heated moulds to ensure a year-round finished product of the highest consistency and quality.
7 What is air-entrained concrete?
Air-entrained concrete contains billions of microscopic air cells per cubic foot. These air pockets relieve internal pressure on the concrete by providing tiny chambers for water to expand into when it freezes.
Air-entrained concrete is produced through the use of air-entraining Portland cement, or by the introduction of air-entraining agents, under careful engineering supervision as the concrete is mixed on the job. The amount of entrained air is usually between 4 percent and 7 percent of the volume of the concrete, but may be varied as required by special conditions.
8 Why do concrete surfaces flake and spall?
Concrete surfaces can flake or spall for one or more of the following reasons:
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If air-entrained concrete is not used in areas of the country that are subjected to freezing and thawing there will be subsequent damage to the surface.
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The water/cement ratio should be as low as possible to improve durability of the surface; too much water in the mix will produce a weaker, less durable concrete that will contribute to early flaking and spalling of the surface.
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The finishing operations should not begin until the water sheen on the surface is gone and excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because the finishing operations are begun too soon, the concrete on the surface will have too high a water content and will be weaker and less durable.
9 What does 28-day strength mean?
Concrete hardens and gains strength as it hydrates. The hydration process continues over a long period of time. It happens rapidly at first and slows down as time goes by.
To measure the ultimate strength of concrete would require a wait of several years. This would be impractical, so a time period of 28 days was selected by specification writing authorities as the age that all concrete should be tested. At this age, a substantial percentage of the hydration has taken place.
10 How do you control the strength of concrete?
The easiest way to add strength is to add cement. The factor that most predominantly influences concrete strength is the ratio of water to cement in the cement paste that binds the aggregates together. The higher this ratio is, the weaker the concrete will be and vice versa. Every desirable physical property that you can measure will be adversely affected by adding more water.
11 Why are plastic fibres used in concrete?
Fibres are typically polypropylene fibres, introduced into the mix during the batching process and serve as a secondary reinforcement in the concrete. There are two types: fibrous (soft), ranging in lengths from 6mm to 19mm; rigid, 40mm long. In most cases fibres can replace welded wire fabric as a means of secondary reinforcement.
12 Why are steel fibres used in concrete?
Steel fibres are used to significantly increase the load-bearing capacity of concrete slabs, particularly floor slabs in heavy industrial applications. It eliminates the need for placement of reinforcing steel and provides for reinforcement throughout the entire concrete section.
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