INTRODUCTION
Concrete is a product made by using a cementing medium; which is the reaction between hydraulic cement and water. Concrete is made with different types of cements that contain pozzolan, fly ash, blast furnace slag; as well as sulphur, admixtures, polymers and fibres. These concretes can be heated, pressured hydraulically and sprayed.
Concrete is made up of water, fine and coarse granular aggregates secure in cement or a binder. The cement will fill up the spaces between the aggregates and will bind them together.
Fine mineral powders are cementitious materials that make up concrete. These fine powders react chemically when mixed with water to make a strong mass that binds the aggregates.
The report below provides guidance on
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The water content, shape of the aggregate, proportions, mixing time together with the slump and temperature of the concrete, have an effect on the admixture being used.
Admixtures that will be considered for use in concrete need to meet certain standards. Trial mixes are made with the admixture and other materials at the temperature specified. It then becomes easy to observe the compatibility of the admixture with other admixtures and job materials, as well as the effects of the admixture on the properties of fresh and hardened concrete.
Fig 4.1 below shows the various liquid admixtures. Fig 4.1 Liquid admixtures: From left to right: antiwashout admixture, shrinkage reducer, water reducer, foaming agent, corrosion inhibitor and air-entraining
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These admixtures improve the resistance of concrete to the surface scaling which is caused by chemical deicers. These admixtures improve the workability of fresh concrete and reduce bleeding.
The air retrained in concrete can be obtained by using air-retaining cement or by an air-retaining admixture or sometimes by combing the two admixtures.
During manufacturing, the air-retaining admixture is a combined with Portland cement with an air-entraining addition with a clinker. This admixture is then added to concrete materials before or during the mixing process.
WATER-REDUCING ADMIXTURES
Water-reducing admixtures reduce the amount of mixing water needed to produce concrete of a required slump, to reduce the water-cement ratio and the water content. These admixtures reduce the water content by 5-10%. Adding this type of admixture to concrete will increase the slump if the water content is not reduced. Slump loss results in decreased workability and reduced time for placing the concrete. When the water-cement ratio is decreased, the strength of the admixture increases. Concretes with an equal contents of water, air content and the slump; the 28-day strength of a water-reduced concrete consisting of a water reducer can be 10-25% greater than concrete without the
they're not. Concrete is to cement as a cake is to flour. Concrete is a mixture
Besides the known inventions and renovations of the Roman Empire, one of the building materials that the Roman Empire produced was concrete. With its strength, inexpensiveness and its easiness to work, the Roman Empire left an everlasting impact. Concrete that is a mixture of aggregate, which is usually gravel, sand or small stones, binding agents, and water is used to construct buildings and infrastructure. The first one, aggregate, gives the product its mass while the second one, binding agents, is used to harden the product. In the early times of the history, limestone powder was usually used as a binding agent in the mixture. The Romans later used “pulvis puteolanus” (pozzolanic ash), which was a fine volcanic ash as a binding agent and it allowed Romans to have strong concrete that is also durable (Yegeul, n.d.). It was easy to use
At first the Romans did not realize the superior ability of concrete. They began by using it as a fill. They would also cover up the concrete with decorated stones. The concrete they used did not look pleasing because they had mixed it with any arrogate they had available. Later, once they discovered how useful concrete could be, they used it for everything they could. They built their houses, roads government offices, theaters, temples, and everything else they could out of concrete. Concrete was far superior in strength to stone and mortar, and it was easier to make than marble was to quarry and assemble.
According to major supplies, “Use of synthetic fibers for reinforcing concrete is continuously, increasing. The increase has been considerable since 1980, but slowed somewhat in 1990, a year of substantial construction cutbacks. Apparently the construction community believes there are advantages in the use of synthetic fibers in concrete.” (Schupack) Synthetic fibers are used to improve crack control in concrete. Some reports say that synthetic fiber reinforced concrete (SFRC) would replace welded wire fabric in many slab-on-grade applications. But in reality if the welded wire fabric is placed properly it controls crack width better than the synthetic fiber reinforced concrete. In a case study of the use of synthetic fibers in reinforced concrete, the following conclusions were obtained. No matter what concrete placing job is being done, there is no substitute for good concrete practices. The use of a low fiber volume mix will help control plastic shrinkage cracks and bleeding, but not give good cracking control once the crack forms. The synthetic fibers running through a crack have a poor bond providing no shear friction. Impact and toughness tests on synthetic fiber reinforced concrete imply less edge spalling will occur. The American Concrete Institute (ACI) conducted two studies on polypropylene fiber reinforced concrete, one dealing with plastic shrinkage cracking and the other on permeability characteristics. Plastic shrinkage cracking occurs when the surface water on the concrete evaporates faster than the bleed water reaches the surface of the concrete. It was determined by the plastic shrinking cracking study that polypropylene fibers helped reduce the total plastic shrinkage crack area on test panels. Also determined is that the screeding rate affects the total crack area in polypropylene reinforced concrete, while finishing operations showed no significant effects. This study also suggests the use of longer fibers (about 0.75in.) will produce less crack area. “Permeability plays an important role in long-term durability of concrete materials. Permeability of concrete generally refers to the rate at which particular aggressive substances (water, sulfates, chloride ions, etc.) can flow through the concrete.” (Soroushian) As discussed in the plastic shrinkage study that polypropylene fibers reduce cracking. Less cracking in the concrete surface that surface would be less permeable. In the permeability study, they concentrated on the effects of chloride and the permeability of the concrete. The results of this study concluded polypropylene fibers had little effects on chloride permeability of concrete. The polypropylene fibers only help reduce plastic shrinkage cracks.
The Pantheon was a Roman concrete structure. Roman concrete is a mixture of lime and volcanic ash (pozzolana, found near modern-day town Pozzouli). Roman concrete is also called hydraulic cement-based concrete. It is known for curing relatively rapidly, even in damp condition, thus it could be used for massive construction. The most daring thing about the use of Roman concrete in the Pantheon is that Portland cement needs steel reinforcement due to tendency of cracking caused by tension forces, meanwhile the Pantheon did not. The Pantheon still stands today is because of its special structure.
ement occupies approximately 50% of the mixed concrete volume and is responsible for physiomechanical properties of concrete. Cement production is essential to infrastructure and building construction, creating demands in very large quantities. Energy resources invested in the production process and resulting greenhouse gas emissions have become problematic. The cement production process has become among the world’s largest anthropogenic sources of carbon dioxide emissions, contributing to approximately 5% of global anthropogenic CO2 emissions, (WBCSD, 2009). Increased pressure is being placed on the industry to reduce CO2 emissions, owning to awareness regarding sustainability....
3) Super workable concretes have the ability to fill the heavily reinforced sections without internal or external vibration, without segregation and without developing large sized voids
[17] BS 4550-3.4:1978. Methods of testing cement, Physical tests, Strength tests. British Standards Institution ; 2007.
Concrete is one of the world’s most popular construction materials. Some six billion tonnes of concrete is produced each year in the world, making it approximately one ton of concrete for every human being per year (Fardis, 2012, p.116). However, the lifecycle of concrete does not make it the most sustainable building material at the moment. Because of limited natural resources, concerns over green house gases, and landfill problems, concrete production is being cut-back, or at least cannot be increased to keep up with population increase. In this essay, I will look at what makes concrete an unsustainable material and possible solutions to make concrete a more sustainable material.
BACKGROUND Types of Silica Crystalline silica may be of several distinct types. Quartz, a form of silica and the most common mineral in the earth's crust, is associated with many types of rock. Other types of silica include cristobalite and tridymite. Potential for Exposure During Construction Concrete and masonry products contain silica, sand and rock containing silica. Since these products are primary materials for construction, construction workers may be easily exposed to respirable crystalline silica during activities such as the following: Chipping, hammering, and drilling of rock Crushing, loading, hauling, and dumping of rock   high dust concentrations.
Now a days HPC is expensive than conventional concrete. It require additional materials in some quantities as to meet specified performance.These additional materials are cement,silica fume,...
Concrete is a composite material used widely in the construction industry. Concrete is basically a mixture of cement, water, aggregates and admixture (sometimes). Cement is a fine gray powder that consists of oxidizes calcium, silicon and aluminum. The aggregate used is normally gravel, crushed stone or sand. Admixture is a solid or liquid substance that gives a certain characteristics of the concrete. The cement reacts with water chemically and binds the aggregates together through a process called hydration during hardening or curing of concrete. It means that water helps in the hardening of the concrete while the cement bind the aggregate and also react with water to form a solid mass.
Most people may not realise but concrete plays a vital part in our lives daily. It shapes and creates the built environment in which we are surrounded by, such as schools, bridges, roads, housing, hospitals, dams and so much mores. Concrete is the most used man made material in the world, averaging around 3 tonnes annually for each person. In comparison with other building materials such as wood, steel, plastic and aluminium, over twice as much concrete is used globally than any of these materials. It is the material choice of most purposes due to its strength, durability, thermal mass and its cost.
Cement is a finely ground compound of limestone and clay or marl (Chandigarh, 2001). It is used in hydraulic binding since it hardens and binds together aggregates and becomes waterproof thereafter. The most popular use of sand is in building and construction where it is used to bind sand and gravel to obtain concrete used in constructing houses, roads, water channels, among others. Yanbu Cement company produces different types of cement such as Portland cement types I, II, III and IV.
Opus caementicium or Roman concrete is a synthetic construction substance that’s composed of an aggregate, a binding agent, and water. In Rome’s case, as discovered by UC Berkley with the extensive analysis of a sample of Roman concrete taken from a breakwater in Italy’s Pozzuoli Bay it was developed by using lime and volcanic rock which formed a mortar, the mortar and volcanic rock were then packed into wooden forms and when seawater was added a chemical reaction occurred, bonding everything together to create concrete (History, 2013). It is uncertain when Roman concrete was developed, but it was clearly in widespread and customary use from about 150 BC; some scholars believe it was developed a century before that.