27 Nov Even More on Concrete
There is certainly a lot that I can continue to point out about concrete, but I think that I will leave on a couple of high notes and mention concrete strength, specifically high-strength concrete.
Concrete strength is determined by the force required to crush it, measured in pounds per square inch. Strength can be affected by many variables including moisture and temperature. 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.
The primary difference between high-strength and normal-strength concrete relates to the compressive strength. The American Concrete Institute defines high-strength concrete as one with a compressive strength greater than 6,000 psi. Likewise, ultra-high performance concrete, has greater compressive strength than high-strength concrete and other superior properties.
Pozzolans, such as fly ash and silica fume, are the most commonly used mineral admixtures in high-strength concrete. These materials impart additional strength by reacting with portland cement hydration products to create additional C-S-H gel, the part of the paste responsible for concrete strength.
It would be difficult to produce high-strength concrete mixtures without using chemical admixtures. A common practice is to use a superplasticizer in combination with a water-reducing retarder. The superplasticizer gives the concrete adequate workability at low water-cement ratios, leading to concrete with greater strength. The water-reducing retarder slows the hydration of the cement and allows workers more time to place the concrete.
High-strength concrete is specified where reduced weight is important or where architectural considerations call for small support elements. By carrying loads more efficiently than normal-strength concrete, high-strength concrete also reduces the total amount of material placed and lowers the overall cost of the structure.
Applications of High-Strength Concrete:
High-strength concrete is required in engineering projects that have concrete components that must resist high compressive loads. It is typically used in the erection of high-rise structures and has been used in components such as columns (especially on lower floors where the loads will be greatest), shear walls, and foundations. High strengths are also occasionally used in bridge applications as well. The most common use of high-strength concrete is for construction of high-rise buildings. At 969 feet, Chicago’s 311 South Wacker Drive uses concrete with compressive strengths up to 12,000 psi and is one of the tallest concrete buildings in the US.
In high-rise structures, high-strength concrete has been successfully used in many U.S. cities. A high-rise structure suitable for high-strength concrete use is considered to be a structure over 30 stories. Not only has special concrete made such projects feasible due to load capacity, it has also allowed for the reduction of column and beam dimensions. Lower dead loads result, reducing the loads associated with foundation design. Also, owners benefit economically since the amount of rentable floor space, primarily on the lower floors, increases as the space occupied by the columns decreases. It is estimated that a 50-story structure with 4-foot diameter columns using 4000 psi concrete can reduce column diameters by approximately 33% by using 8000 psi concrete.
High –strength concrete is occasionally used in the construction of highway bridges. High-strength concrete permits reinforced or prestressed concrete girders to span greater lengths than normal strength concrete girders. Also, the greater individual girder capacities may enable a decrease in the number of girders required. Thus, an economical advantage is created for concrete producers in that concrete is promoted for use in a particular bridge project as opposed to steel.
The following table gives high-strength mixes that are commercially available:
a. Maximum aggregate size: mixes 1-5,at ½ in. (12.7mm);mix 6, 1 in. (25mm)
b. Weight of total water in mix including water in admixture.
c. Tests on 6 x 12 in. cylinders.
Manufacturing high-strength concrete involves making optimal use of the basic ingredients that constitute normal-strength concrete so producers know what factors affect compressive strength and how to manipulate those factors to achieve the required strength. In addition to selecting a high-quality portland cement, producers optimize aggregates, then optimize the combination of materials by varying the proportions of cement, water, aggregates, and admixtures.
When selecting aggregates for high-strength concrete, producers consider the strength of the aggregate, the optimum size of the aggregate, the bond between the cement paste and the aggregate, and the surface characteristics of the aggregate. Any of these properties could limit the ultimate strength of high-strength concrete.
Though all portland cement is basically the same, eight types of cement are manufactured to meet different physical and chemical requirements for specific applications:
- Type I is a general purpose portland cement suitable for most uses.
- Type II is used for structures in water or soil containing moderate amounts of sulfate, or when heat build-up is a concern.
- Type III cement provides high strength at an early state, usually in a week or less.
- Type IV moderates heat generated by hydration that is used for massive concrete structures such as dams.
- Type V cement resists chemical attack by soil and water high in sulfates.
- Types IA, IIA and IIIA 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.
White portland cement is made from raw materials containing little or no iron or manganese, the substances that give conventional cement its gray color.
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