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26FACTORS AFFECTING THE CONCRETE STRENGTH
Rahimov E.H.
Rahimov Elbek Hasanboy o 'g 'li - Student, URBAN CONSTRUCTION AND HOUSEHOLD
DEPARTMENT, CONSTRUCTION ENGINEERING FACULTY, FERGHANA POLYTECHNIC INSTITUTE, FERGHANA, REPUBLIC OF UZBEKISTAN
Abstract: the article under discussion depicts factors affecting the concrete strength. The author of the article discusses. concrete as one of the most important materials used in constructions in the world. Massive concreting in huge civil projects like dams, power plants, bridges, and others. For decades there was a problem to compute the compressive strength of concrete in 7 or 28 days if the cubes of concrete are tested in different age. For this reason the article reveals the main factors influencing the concrete strengthening.
Keywords: concrete, strength, formation, hardening, dams, compressive, used, weak, fluid, ratio, penetrating.
Concrete is the only material used in construction which can be delivered to the job site in a plastic state. This quality makes concrete can be molded to any form or shape demanded. Concrete provides a wide latitude in surface textures and colors and can be used to construct a wide variety of structures, such as bridges, dams, large buildings, airport runways, irrigation structures, breakwaters, piers and docks, sidewalks, silos and farm buildings, homes, and even barges and ships [1, p.p.23-25].
The strength of concrete is controlled by the proportioning of cement, coarse and fine aggregates, water, and various admixtures. Many factors influence the rate at which the strength of concrete increases after mixing. Some of these are discussed below. First, though a couple of definitions may be useful: the processes of 'setting' and 'hardening' are often confused. Setting is the stiffening of the concrete after it has been placed. A concrete can be 'set' in that it is no longer fluid, but it may still be very
weak; you may not be able to walk on it, for example. Setting is due to the formation of the early-stage calcium silicate hydrate. The terms 'initial set' and 'final set' are commonly used; these are arbitrary definitions of early and later set. There are laboratory procedures for determining these using weighted needles penetrating into cement paste.
Hardening is the process of strength growth and may continue for weeks or months after the concrete has been mixed and placed. Hardening is due largely to the formation of calcium silicate hydrate as the cement continues to hydrate.
The rate at which concrete sets is independent of the rate at which it hardens [2, p.p.512-517].
The strength of concrete can be increased, keeping in mind the factors on which the strength of concrete depends. Strength of concrete depends upon the following factors:
1) Type and age of cement. With age, the strength of cement decreases due to more moisture absorption from the atmosphere. The quality of cement also depends upon the storage conditions of the cement and hence the strength of concrete is also dependent on the storage conditions.
2) Cement-Aggregate ratio. With the increase in the cement-aggregate ratio, the strength of concrete increases to some extent, provided the other factors are kept constant.
3) Water-cement ratio. With increase in water-cement ratio, the strength of concrete decreases.
4) Degree of compaction. Strength of concrete also depends on the degree of compaction as improper compaction will lead to the presence of air voids in the concrete which will certainly reduce it's strength. It has been observed that if there is 5% void, strength of concrete decreases by as much as 30%.
5) Mixing time. Strength of concrete increases with increase in mixing time up to 2 minutes, beyond which no significant increase in the strength is observed.
6) Curing. Curing is essential for optimum strength gain and durability. It is necessary for maintaining the required moisture content as well as the temperature conditions.
7) Type and size of the aggregate. Degree of packing is the main factor to be considered here. Greater the degree of packing, lesser is the void content and more is the strength. Spherical or rounded aggregates provide much more degree of compaction than the irregular shaped ones. This is the reason as to why they provide more strength.
There is a common disbelief that greater the maximum size of the aggregate, smaller will be it's surface area, lower will be the w/c ratio and consequently greater will be the strength. But this is not so, the smaller surface area will lead to smaller surface area for gel bonds, which results in less strength.
Cement-related parameters: many parameters relating to the composition of the individual cement minerals and their proportions in the cement can affect the rate of strength growth and the final strength achieved. These include:
• alite content
• alite and belite reactivity
• cement sulfate content
Since alite is the most reactive cement mineral that contributes significantly to concrete strength, more alite should give better early strengths ('early' in this context means up to about 7 days). However, this statement needs to be heavily qualified as much depends on burning conditions in the kiln. It is possible that lighter burning of a particular clinker could result in higher early strength due the formation of more reactive alite, even if there is a little less of it. For a particular cement, there will be what is called an 'optimum sulfate content,' or 'optimum gypsum content.' Sulfate in cement, both the clinker sulfate and added gypsum, retards the hydration of the aluminate phase. If there is insufficient sulfate, a flash set may occur; conversely, too much sulfate can cause false-setting.
A balance is therefore required between the ability of the main clinker minerals, particularly the aluminate phase, to react with sulfates in the early stages after mixing and the ability of the cement to supply the sulfate. The optimum sulfate content will be affected by many factors, including aluminate content, aluminate crystal size, aluminate reactivity, solubilities of the different
sources of sulfate, sulfate particle sizes and whether admixtures are used. If this were not complicated enough, the amount of sulfate necessary to optimize one property, strength for example, may not be the same as that required to optimize other properties such as drying shrinkage. Concrete and mortar may also have different optimum sulfate contents [3, p.p.54-63].
References
1. Alilou Vahid. K. and Teshnehlab Mohammed. "Prediction of 28-day compressive strength of concrete on the third day using artificial neural networks"), International Journal of Engineering (IJE), Volume (3), Issue (6). 2010. P.p. 23-25.
2. Bentz1, Dale P., Max A. Peltz1, and John Winpigler. "Early-Age Properties of Cement-Based Materials: II. Influence of Water-to-Cement Ratio", ASCE Journal of Materials in Civil Engineering, 21 (9), 2009. P.p.512-517.
3. Kausay Tibor and Tamas K. Simon. Acceptance of concrete compressive strength. Concrete Strength , Annual Journal of the Hungarian Group of fib, Budapest , Vol. 8. P.p. 54-63.
