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(Click on the questions below as this will take you directly to the answer, or scroll down to see all questions and answers)
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In what forms can concrete be manufactured?
Concrete is produced in three basic forms:
In-situ concrete which is manufactured either by the contractor or a ready-mix company on the site of the project (also referred to as site-mix). The operation is completely under the contractor’s control, and a high degree of flexibility in site management is possible, e.g. small quantities can be made at short notice.
Ready-mixed concrete, which in South Africa accounts for almost 50% of all concrete, is batched at local plants by specialist manufacturers for delivery in the familiar trucks with revolving drums. This allows more space to be made available on site (important in many urban projects); the supplier takes responsibility for quality control of the concrete and also has the resources and technical expertise to provide a wide range of mixes.
Precast concrete products are cast in a factory setting. These products benefit from tight quality control achievable at a production plant. Precast products range from concrete bricks and paving stones to bridge girders, structural components, and panels for cladding.
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What is Hydration?
Soon after the aggregates, water, and the cement are combined, the mixture starts to harden. All portland cements are hydraulic cements that set and harden through a chemical reaction with water. During this reaction -called hydration - a node forms on the surface of each cement particle. The node grows and expands until it links up with nodes from other cement particles or adheres to adjacent aggregates.
The building up process results in progressive stiffening, hardening, and strength development. Once the concrete is thoroughly mixed and workable it should be placed in forms before the mixture becomes too stiff. This hardening process continues for years meaning that concrete gets stronger as it gets older.
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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.
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What are recommended mix proportions for good concrete?
Good concrete can be obtained by using a wide variety of mix proportions if proper mix design procedures are used.
The key to achieving a strong, durable concrete rests in the careful proportioning and mixing of the ingredients. A concrete mixture that does not have enough paste to fill all the voids between the aggregates will be difficult to place and will produce rough, honeycombed surfaces and porous concrete. A mixture with an excess of cement paste will be easy to place and will produce a smooth surface; however, the resulting concrete is likely to shrink more and be uneconomical.
Cement and water form a paste that coats each particle of stone and sand. Through a chemical reaction called hydration, the cement paste hardens and gains strength. The character of the concrete is determined mainly by quality of the paste. The strength of the paste, in turn, depends on the ratio of water to cement. The water-cement ratio is the weight of the mixing water divided by the weight of the cement. High-quality concrete is produced by lowering the water-cement ratio as much as possible without sacrificing the workability of fresh concrete.
Generally, using less water produces a higher quality concrete provided the concrete is properly placed, consolidated, and cured. The following chart provides a range of trial mixes for a given strength of concrete at 28 days.
Cements with higher extender contents (e.g. CEMII/B or CEM III) may develop strength more slowly and will require particular care with curing.
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Can it be too hot or too cold to place new concrete?
Temperature extremes make it difficult to properly cure concrete. On hot days, too much water is lost by evaporation from newly placed concrete, unless sufficient measures are in place to prevent this. 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 5oC during the curing period.
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Can I use any water for mixing concrete?
Almost any natural water that is drinkable and has no pronounced taste or odour may be used as mixing water for concrete. However, some waters that are not fit for drinking may be suitable for concrete.
Excessive impurities in mixing water not only may affect setting time and concrete strength, but also may cause efflorescence, staining, corrosion of reinforcement, volume instability, and reduced durability. Specifications usually set limits on chlorides, sulphates, alkalis, and solids in mixing water unless tests can be performed to determine the effect the impurity has on various properties.
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Why is it so important to "cure" concrete?
Curing is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength and durability. Concrete hardens as a result of hydration: the chemical reaction between cement and water. However, hydration occurs only if water is available and if the concrete's temperature stays within a suitable range. During the curing period - from five to seven days after placement for conventional concrete - the concrete surface needs to be kept moist to permit the hydration process. New concrete can be wetted with soaking hoses, sprinklers or covered with wet burlap, or can be coated with commercially available curing compounds, which seal in moisture.
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What is air-entrained concrete?
Air-entrained concrete contains billions of microscopic air cells per cubic metre. These air pockets relieve internal pressure on the concrete by providing tiny chambers for into which water can expand 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 % and 7% of the volume of the concrete, but may be varied as required by special conditions.
Air-entraining agents are used to produce a number of effects in the concrete mix:
· To improve cohesion and reduce bleeding
· To improve compaction of low workability concrete
· To provide stability to extruded concrete
· To give improved handling properties, stability and cohesion to bedding mortar
· To improve freeze/thaw resistance of hardened concrete (not a major problem in South Africa)
When designing air-entrained concrete it should be remembered that the compressive strength is reduced, compared to non-air-entrained concrete.
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Why does concrete crack only a short time after placing?
This is referred to as plastic cracking and can take two forms – shrinkage and settlement. The principal cause of plastic shrinkage cracking is the rapid removal of water from the concrete. Water loss is mainly from the exposed surface of the concrete (e.g. concrete slabs). When the evaporation rate exceeds he rate of bleeding, the surface concrete loses water and decreases in volume. Tensile stresses are induced in the because of restraint by the non-shrinking inner concrete. Plastic cracking can be minimised or avoided through proper mix design and effective early curing
Plastic settlement cracking occurs after the concrete has been compacted. After compaction there is a tendency for solid particles to settle and displace some mixing water which rises to the surface. This settlement will continue until the concrete stiffens. In a section where there is no restraint (e.g. top reinforcement, changes in section, etc.), such settlement rarely causes any problems.
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What are the most common tests for fresh concrete?
Slump, air content, unit weight and compressive strength tests are the most common tests.
- Slump is a measure of consistency, or relative ability of the concrete to flow. If the concrete cannot flow because the consistency or slump is too low, there are potential problems with proper consolidation. If the concrete won't stop flowing because the slump is too high, there are potential problems with mortar loss through the formwork, excessive formwork pressures, finishing delays and segregation.
- Air content measures the total air content in a sample of fresh concrete, but does not indicate what the final in-place air content will be, because a certain amount of air is lost in transportation, consolidating, placement and finishing. Three field tests are widely specified: the pressure meter and volumetric method are ASTM standards and the Chace Indicator is an AASHTO procedure.
- Unit weight measures the weight of a known volume of fresh concrete.
- In essence, the compressive strength test consists of crushing three cubes from the same sample of concrete. The cubes are tested in a saturated condition. The strength of the concrete is defined as the average of the strengths of the three cubes. For the test to be valid, the ranges of strengths within the set of three cubes must not exceed 15% of the average.
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How do I perform a slump test?
The slump test is a relatively simple test to perform, whereby a slump cone mould is placed on a flat plate and filled with fresh concrete in three approximately equals layer. Each layer is subjected to 25 ‘blows’ from a tamping rod, the mould being firmly held down by standing on the foot pieces. The blows are evenly distributed over the whole area of the layer; for the second and third layers, the rod should just penetrate the previous layer.
The surface is struck off by rolling the tampering rod across the top edge of the mould. After careful removal of the mould, the slump of the concrete present is measured to the nearest 5 mm. The slump as measured is the distance between the top of the inverted mould and the highest point of the concrete.
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Why do concrete surfaces flake and spall?
Concrete surfaces can flake or spall for one or more of the following reasons:
- In areas of the country that are subjected to freezing and thawing the concrete should be air-entrained to resist flaking and scaling of the surface. If air-entrained concrete is not used, there will be subsequent damage to the surface.
- 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.
- 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.
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hat 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. Cement comprises 10 to 15% of the concrete mix, by volume. Through a process called hydration, the cement and water harden and bind the aggregates into a rocklike mass.
So, there is no such thing as a cement floor, or a cement mixer; the proper terms are concrete floor and concrete mixer.
How is portland cement made?
Materials that contain appropriate amounts of oxides of lime, silica, alumina and iron 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 of about 1400 and 1450 degrees Celsius. This high heat drives off, or calcines, the chemically combined water and carbon dioxide from the raw materials and forms new compounds (tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite). 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 milling process in order to to control the cement's set or rate of hardening.
Are there different types of portland cement?
Though all portland cement is basically the same, five types of cement are manufactured to meet different physical and chemical requirements for specific applications:
- CEM I portland cement with a maximum of 5% minor additional constituents
- CEM II portland cement containing varying additions of secondary materials, i.e. fly ash, pozzolana, slag, silica fume, or limestone.
- CEM III blast furnace cement
With the wide range of secondary product addition amounts allowable under the South African standard for common cements (SANS 50197-1), there are now potentially 27 products in the family of “common cements”. The Cement and Concrete Institute in South Africa supplies a downloadable leaflet on the full composition details of these different cements. Alternatively, a full copy of the specification can be obtained from the South African Bureau of Standards.
White portland cement is made from raw materials containing little or no iron or manganese, the substances that give conventional cement its grey colour. White cement is used primarily for decorative purposes.
Is there a universal international specification for portland cement?
Each country has its own standard for portland cement, so there is no universal international standard. South Africa uses the specifications prepared by the South African Bureau of Standards:
SANS 50197-1 Cement: Part 1 Composition, specifications and conformity criteria for Common Cements and this is supported by SANS 50197-2 2000 Cement: Part 2: Conformity evaluation
SANS 50413-1: 2004 Masonry cement. Part 1: Composition, specifications and conformity criteria. SNAS 50413-2 covers the test methods which apply to masonry cements only.
There are a few other countries that also have adopted their own standard. Unfortunately, many do not use the same criteria for measuring properties and defining physical characteristics so they are virtually "non-translatable." The European Cement Association located in Brussels, Belgium, publishes a book titled "Cement Standards of the World."
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