1. The importance of aggregate grading in concrete mix design
Aggregates are inert granular materials such as sand, gravel or crushed stone etc. Grading is the particle-size distribution of an aggregate as determined by a sieve analysis.
There are several reasons for specifying grading limits and normal aggregate size; they affect relative aggregate proportions as well as cement and water requirements, workability, pumpability, economy, porosity, shrinkage, and durability of concrete. Variations in grading can seriously affect the uniformity of concrete from batch to batch. Very fine sands are often uneconomical; very coarse sands and coarse aggregate can produce harsh, unworkable mixtures.
Proper gradation ensures that a sample of aggregates contains all standard fractions of aggregates in required proportion. Such a sample may contain minimum voids. Properly graded sample will require the minimum paste to fill up the voids in the aggregation and it means less amount of cement and water is needed. Aggregate gradation, which determines the relative proportions of aggregate and cement paste in a concrete, therefore dictates the shrinkage behavior of concrete and hence long-term durability of concrete
2. The use of graded, single sized and gap – graded aggregates in concrete
There are three typical range categories of aggregate grading.
a. Graded aggregates /well graded aggregate
In this case, the spaces between layer particles are filled by smaller particles to produce a well packed structure. It is characterized by the S-shaped in gradation curve.
Characteristics of well-graded aggregates
• Better Workability
• Economy in mix
• Better Compaction
• Increased Strength
• Low Soundness and shrinkage
b. Single sized aggregates
Single sized contains aggregate particles that are almost of the same size. It means that the particles pack together but leaving relatively large voids in the concrete. Single sized aggregates are used in two different main purposes, general purpose uses such as drain back fill and specific uses such as surface dressing chippings. Characteristics of single sized aggregates
• Low void content
• Low permeability
• High stability
• Difficult to compact
c. Gap graded aggregates
In gap graded aggregates voids created by higher size and filled up by immediate next lower size.
Characteristics of Gap-Graded Mix
• Increase strength and reduce creep and shrinkage
• Moderate void content
• Specific area of total aggregates will be reduced due to less sand
• Requires less cement as net volume of voids is reduced
• More economical concrete
Segregation problems can be occurred, while using gap graded aggregates in high workability mixtures. As the result, concrete mixture can be very stiff and causes to less workability. This concrete is difficult to handle and compact.
3. The practical importance of moisture content of aggregate (Both absorb moisture and free moisture)
The moisture content of an aggregate when batched in concrete mixture is an important consideration. Aggregate practices have internal pores that absorb water. aggregate stored in a stockpile or bin is typically in a wet condition, meaning it contains absorbed water and moist coating the surface, called free of surface moisture. Since the moisture content of aggregate change with weather and changes also from one part of stockpile to another part, the value of the moisture content need to be determined frequently. As the moisture content variation occurs mainly in vertical direction from a water-logged bottom of pile to its drying or dry surface, caring about lying out of the stockpile is necessary. The total moisture content of an aggregate sample is the sum of the absorbed moisture and the free surface moisture.
The moisture content of the aggregate is defined at four states:
a. Oven dry aggregate
When the aggregate has no moisture on the surface or the interior pores then the aggregate. b. Air-dry aggregate
When some of the interior pores of the aggregate are filled with water and there is no moisture on the surface.
c. Saturates surface dry aggregates (SSD)
When all the interior pores of the aggregate are filled with water but there is no moisture on the surface. d. Wet aggregate
All the interior pores of the aggregate are filled with water and there is moisture on the surface.
The properties of fresh and hardened concrete are affected by the quantity of mixing water in the concrete mixture. The mixing water includes all source of water including batch water, free moisture on aggregates, ice etc. the mixing water content used determines the water to cementitious material ratio (W/C) of a concrete mixture. For a set of material, there is a unique relationship between the W/C and the strength and durability characteristic of concrete.
As mentioned earlier, the total moisture content of the aggregate is the sum of the absorbed water and free surface water. the absorbed water remains within aggregate pores and does not affect the slump of concrete and do not included when calculating the W/C. the surface moisture makes up a portion of the mixing water, affects slump and is included when calculating W/C of the mixture.
If the surface moisture content of wet aggregate is not compensated for, the slump may be too high and W/C will not conform to the mixture requirement. The batched water should be less than the mixing water required by the amount of free moisture on the aggregate. The aggregate is storage prior to batching in concrete can also be in an dry-air state. In this case, the aggregate when batched in concrete will absorb water from the batch water resulting in lower slump. This water that will be absorbed by dry aggregated needs to be accounted for by adding additional water to the mixture. As conclusion, ow moisture content will effectively reduce the amount of water availability for hydration (W/C ) or conversely, if the aggregate is very wet, it adds excess water to a cement mix (W/C ). Therefore, the aggregate moisture has to be measured and amount of batch water adjust accordingly.
4. Importance of bulk density and voids ratio and how these properties are influenced by the grading of aggregates.
The bulk density of an aggregate is the mass or weight of the aggregate required to fill a container of a specified unit volume. The volume referred to here is that occupied by both aggregates and the voids between aggregate particles.
Bulk-density depends upon how densely the aggregate is packed. It also depends upon the size, distribution and shape of the particles if the specific gravity of material is known. If the particles are of the same size, then it can be packed to a limited extent but when the smaller particles are added, the voids get filled with them and thus the bulk density increases. Bulk density also depends upon the degree of packing.
Voids ratio
Voids means the space between the individual particles in a unit volume of the aggregate mass. Bulk density indicates the percentage of voids present in the aggregate material. This percentage of voids affects the grading of the aggregates which is important in high strength concrete.
Aggregate gradation determines the void ratio within the structure of aggregate and consequently the amount of cement paste that is required to fill the void space and ensure a workable concrete. It is desirable to optimize the aggregate gradation in concrete using Portland cement, as it is the most expensive and high carbon footprint ingredient, to minimize the void ratio in the aggregate and therefore the volume of cement paste required to achieve a workable, economical and an environmentally sound concrete for a given application.
5. Important mechanical properties which can be used to access the quality of coarse aggregates a Surface texture
Surface texture of aggregates itself depends on rock hardness, grain size, porosity, previous exposure and affects workability, paste demand, initial strength of concrete. Shape and surface texture affects usually the properties of freshly mixed concrete. Rough surface texture, especially of stone particles, tends to improve the bond between aggregate and hardened cement paste and so increase the strength of concrete. But it is difficult to compact. It requires more cement paste to produce workable concrete mixtures, thus increasing the cost
Smooth surface texture will readily but it can be unstable under the load since the particles are displaced and slide against each other. A smooth surface can improve workability, yet a rougher surface generates a stronger bond between the paste and the aggregate creating a higher strength.
b. Strength and elasticity
Strength is a measure of the ability of an aggregate particle to stand up to pulling or crushing forces. Elasticity measures the "stretch" in a particle. High strength and elasticity are desirable in aggregate base and surface courses. These qualities minimize the rate of disintegration and maximize the stability of the compacted material. The best results for Portland cement concrete may be obtained by compromising between high and low strength, and elasticity. c. Density and specific gravity
The density of the aggregates is required in mixture proportioning to establish weight-volume relationships. Specific gravity is easily calculated by determining the densities by the displacement of water. All aggregates contain some porosity, and the specific gravity value depends on whether these pores are included in the measurement. Those properties affect to the weight of concrete.
a. Toughness
Resistance of the aggregates to impact is termed as toughness. This also affect strength and durability of concrete.
b. Hardness
Hardness of aggregates is the ability of the aggregate to withstand wear or load or applied pressure. The hardness of the minerals that make up the aggregate particles and the firmness with which the individual grains are cemented or interlocked control the resistance of the aggregate to abrasion and degradation. Soft aggregate particles are composed of minerals with a low degree of hardness.
c. Permeability
Property that governs the rate of flow of a fluid into a porous solid is considered as permeability. If the permeability is high, higher amount of water can move into the aggregate. Hence it will affect the W/C ratio and the workability of concrete.
