If the grading of fine aggregate (sand) is such that it is deficient in finer particles, in that case increase in fine/coarse aggregate ratio will not prove useful as it may lead to an excess of middle sizes resulting in harshness of the mix.
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The relation gives the proportion of the amount of fine aggregate (W) to be mixed with one part of the coarse aggregate. The minimum void method is based on the principle that the concrete with the minimum voids is the densest and most durable. The amount of fine aggregate should be such as to fill the voids.
The principle of proportioning aggregate is that the smaller particle will fill up the voids between large particles. A well proportioned concrete mix requires a minimum amount of cement per unit volume. By this method such mixes as 1:1.5:3 ( 1 part cement, 1.5 part sand and 3 part coarse aggregate). 1:2:4, 1:3:6 are arbitrarily chosen by volume.
The general expression for the proportions of cement, sand and coarse aggregate is 1 : n : 2n by volume. 1 : 1 : 2 and 1 : 1.2 : 2.4 for very high strength. 1 : 1.5 : 3 and 1 : 2 : 4 for normal works. 1 : 3 : 6 and 1 : 4 : 8 for foundations and mass concrete works.
Surface texture of aggregates itself depends on rock hardness, grain size, porosity, previous exposure and affects workability, paste demand, initial strength of concrete. Rough-textured and elongated particles require more cement paste to produce workable concrete mixtures, thus increasing the cost. This factor is of secondary importance.
Combination of fine and coarse aggregates to achieve the lowest void volume. The amount of cement paste used in concrete depends on aggregate particle packing, where the denser combination among particles with different particle sizes results in a minimum volume of voids (Vv).
Effect of coarse aggregate sizes in concrete : As the sizes of coarse aggregate increases then the compressive strength will also increase with constant water to cement ratio(w:c -0.5 and Mix proportion 1:2:4) The workability of concrete get improved after increasing larger sizes of aggregates in concrete.
The results show that, workability of concrete decreased as fines content increases. At the same level of fines content, workability increases when the W/C increased. The study again revealed that up to 4% fines content, compressive strength increased as the fines content increases.
They concluded that, in high-strength concretes, higher strength coarse aggregates typically yield higher compressive strengths, while in normal-strength concretes, coarse aggregate strength has little effect on compressive strength.
The workability of fresh concrete increases with poor gradation of the coarse aggregate. The workability of fresh concrete is higher for gap graded coarse aggregate concrete than for well graded coarse aggregate concrete. For the gap aggregates, the bigger the particle sizes removed, the higher the workability.
The bigger the size of the aggregates the lesser is the surface area hence less amount of water is required for wetting the surface. Hence less paste is required for lubricating the concrete surface to reduce internal friction, thus we can say that the bigger size of aggregates will give higher workability.
Mix design of concrete cannot be carried out without specific gravity of fine aggregate, and higher specific gravity produced stronger concrete. Finally, the presence of silt in sand would increase water demand in concrete mixture and may reduce concrete strength.
Maximum Aggregate Size As the maximum size of coarse aggregate reduces, surface area of coarse aggregate increases. Higher the surface area, greater is the water demand to coat the particles and generate workability.
The strength of concrete also increases with the increase in size of coarse aggregate for a cement content of 150 kg/m3. However, for concrete with cement content more than 150 kg/m3, strength increases with the increase in size of coarse aggregate upto 25 mm and decreases as the aggregate size increases beyond 25 mm.
Fine aggregates generally consist of natural sand or crushed stone with most particles passing through a 3/8-inch sieve. Coarse aggregates are any particles greater than 0.19 inch, but generally range between 3/8 and 1.5 inches in diameter.
Factors which affect workability of concrete are:Cement content of concrete.Water content of concrete.Mix proportions of concrete.Size of aggregates.Shape of aggregates.Grading of aggregates.Surface texture of aggregates.Use of admixtures in concrete.More items...•
Proportioning of concrete is the process of selecting quantity of cement, sand, coarse aggregate and water in concrete to obtain desired strength and quality. The proportions of coarse aggregate, cement and water should be such that the resulting concrete has the following properties:
Methods of Proportioning Cement, Sand and Aggregates in Concrete 1 When concrete is fresh, it should have enough workability so that it can be placed in the formwork economically. 2 The concrete must possess maximum density or in the other words, it should be strongest and most water-tight. 3 The cost of materials and labour required to form concrete should be minimum.
When concrete is fresh, it should have enough workability so that it can be placed in the formwork economically. The concrete must possess maximum density or in the other words, it should be strongest and most water-tight. The cost of materials and labour required to form concrete should be minimum. The determination of the proportions of cement, ...
According to the water – cement ratio law given by Abram as a result of many experiments, the strength of well compacted concrete with good workability is dependent only on the ratio.
The principle of proportioning aggregate is that the smaller particle will fill up the voids between large particles. A well proportioned concrete mix requires a minimum amount of cement per unit volume.
It is the process of determination or selection of the relative proportion of concrete ingredients that means cement, sand, coarse aggregate, and water to obtain concrete of desired quality. From the strength point of view and economy, the main objective to select the fine and coarse aggregate in such a ratio to get a dense mix with minimum voids ...
Minimum voids Method. The minimum void method is based on the principle that the concrete with the minimum voids is the densest and most durable. The amount of fine aggregate should be such as to fill the voids.
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The physical and mechanical properties of coarse aggregates were directly related to the skid resistance performance of pavement. It was required that the coarse aggregates used in the pavement had high abrasion resistance ability that could resist the impact of driving, crushing and abrasion.
They could also affect the microstructure and macrostructure of pavement. The abrasion value of aggregate was one of the main factors that affect the microstructure of pavement. It was also an important reference index in pavement design.
There were many factors that affect the skid resistance performance of asphalt pavement. It mainly included the stone properties and particle size distribution. They could also affect the microstructure and macrostructure of pavement. The abrasion value of aggregate was one of the main factors that affect the microstructure of pavement. It was also an important reference index in pavement design. The physical and mechanical properties of coarse aggregates were directly related to the skid resistance performance of pavement. It was required that the coarse aggregates used in the pavement had high abrasion resistance ability that could resist the impact of driving, crushing and abrasion. Therefore, it was necessary to carry out a detailed research work on the skid resistance performance of the aggregate, so as to better determine the real property of the aggregate.
Micro-Deval abrasion test was a method to measure the gravel durability which was proposed in the 1960s in France. It evaluated the anti-abrasion resistance and durability of aggregate by measuring the mass change in the aggregate using steel ball in water. Many aggregates could be scuffed easily in a moisturized condition than in a dry condition. Compared to other tests (such as the Los Angeles abrasion test), it could reflect the anti-abrasion performance of aggregate better and had a higher discrimination. It could also simulate the physical weathering process of rocks in moisturized condition.
A coarse aggregate is said to be graded with size number 57 if it is sieved through a set of sieves in the size range of 1 in. to No.4 (1 in., % in., 1/2 in., % in., and No.4)
Mixture proportions should be, changed to produce workable concrete if wide variations occur in the coarse-aggregate grading. The maximum size of coarse aggregate used in concrete has a bearing on economy. Usually more water and cement is required for small-size aggregates than for large sizes.
FM is an index of the fineness of an aggregate – the higher the FM, the coarser the aggregate. The FM can be looked upon as a weighted average size of a sieve on which the material is retained, the sieves being counted from the finest. Different aggregate grading may have the same FM.