In concrete, fine aggregates and coarse aggregates make up about 75% of the total concrete materials. It is, therefore, significantly important to acquire the right type and quality of aggregates at the site.
The aggregates from the main matrix of the concrete/mortar. The aggregate particles are glued together by the cement & water paste. There are two types of aggregates the coarse aggregates which form the main matrix and the fine aggregates which form the filler matrix between the coarse aggregates. With cement & water, the entire matrix binds together into a solid mass called concrete.
Aggregates influence the properties of concrete or mortar such as water requirement, cohesiveness, and workability of the concrete in the plastic stage, while they influence strength, density, durability, and permeability, surface finish, and color in the hardened stage.
Types of Aggregates
The aggregates are generally classified into two types – natural and manufactured or processed. Natural aggregates are either dredged from the river or creek or dug from a pit. Manufactured or processed crushed rock aggregates are obtained by quarrying solid rock and then crushing it to a suitable size and grading it.
Aggregates dredged from the river or obtain from the pits, creek, or sea are most often not clean enough or well-graded to suit the quality requirement. They, therefore, require sieving and washing before they can be utilized in concrete.
Natural aggregates may be water-worn resulting in rounded shape or maybe crushed from massive boulders and hence angular in shape. The density of the aggregates differs depending on the porosity and type of parent material. They may be igneous or sedimentary rock.
Geologically aggregates may be from basalt, granite, limestone, quartzite, gabbro, or schist, etc. but generally, they are all suitable for concrete depending on their degree of weathering, density, and shape.
Choice of Aggregates
The aggregate choice may depend on several factors. The first and the most significant factor is availability. Often the site engineers have to produce concrete from the aggregates generally available close to their site.
However, if availability is not a problem, the choice depends on the following factors that influence the performance in concrete.
|Sr. No.||Factors||Influence on Concrete Property|
|3||Surface texture||Bond grip|
|4||Shape||Water demand (strength)|
|5||Particle size distribution or Gradation||Water demand (strength) cohesion, bleeding and segregation|
|6||Maximum size||Strength and water demand|
|7||Deleterious materials||Water demand, bond, cohesion & durability|
Clay, dust silt, or mud in aggregates because of insufficient washing at the aggregate source will produce lower strength concrete.
Crushed aggregates will generally not contain clay silt or mud but may contain a percentage of dust or grit. This dust if present as a coating around the larger aggregate will result in a drop in the strength of the concrete. Similarly, grit and dust portions of the aggregate will cause and an increase in water demand and a subsequent drop in concrete strengths. It is, therefore, important that concrete mix design should make allowance for stone dust and grit and thereby prevent an unexpected drop in concrete strengths.
There are large variations in the amount of dust between different deliveries or large variations within the stockpile for different loads it is likely that water demand will vary thereby varying the strengths of concrete.
The dust-coated crushed aggregates may tend to assemble towards the bottom of the heap particularly during the rainy season. The rainwater along with dust forms a coating on the aggregates mostly at the bottom of the heap. These dust-coated aggregates if used in concrete can cause a considerable reduction in the concrete strengths. It is thus, suggested that the bottom-most layers in an aggregate stockpile should not be utilized unless it is cleaned & washed thoroughly.
Coarse gravel aggregates sometimes contain clay and silt owing to inadequate washing and improper dredging. Clay lumps or clay coating will result in low concrete strengths. Clay in the form of a coating on the aggregate particles needs to be carefully washed as even a small proportion, even within the Bureau of Indian Standard limits, will reduce the concrete strengths considerably.
The maximum quantity of deleterious materials in the coarse and fine aggregates shall not exceed the limits specified below.
|Sr. No. (1)||Deleterious substances (2)||Method of test (3)||Uncrushed fine aggregate percentage by weight, maximum (4)||Crushed fine aggregate percentage by weight, maximum (5)||Uncrushed coarse aggregate percentage by weight, maximum (6)||Crushed coarse aggregate percentage by weight, maximum (7)|
|(I)||Coal & lignite||IS: 2386 (Part II)- 1963||1.00||1.00||1.00||1.00|
|(II)||Clay lumps||IS: 2386 (Part II)- 1963||1.00||1.00||1.00||1.00|
|(III)||Materials finer than 75-microns IS sieve||IS: 2386 (Part I)- 1963)||3.00||15.00||3.00||3.00|
|(IV)||Soft fragments||IS: 2386 (Part II)- 1963||—||—||3.00||—|
|(V)||Shale||IS: 2386 (Part II)- 1963||1.00||—||—||—|
|(VI)||Total of percentages of all deleterious materials including Sr. No. I to V for column 4, 6 and 7 and Sr. No I and II for column 5 only||—||5.00||2.00||5.00||5.00|
However, the site-in-charge depending on the requirements can relax some limits after further testing or observing the performance of the aggregates. At the same time, even though deleterious material in the aggregates may be within the requirements the site-in-charge may recommend changes in the concrete mix design to avoid strength or durability problems.
Properties of Aggregate for Concrete
Texture of Aggregate
The surface texture of aggregates influences the bond between the aggregate and cement. A smooth surface such as that found on gravels will have a poor bond. Crushed aggregates have a rough texture & give a good mechanical bond with cement. If the crushing value of the aggregates is high, the compressive strength will depend on the bond created by the cement paste between the aggregates. However, gravel or rounded aggregates have been utilized for high-strength concrete without any serious problem of the poor bond. If the gravel is clean and well washed the chances of poor bond are considerably reduced.
Shape of Aggregate
The aggregates shapes can be broadly classified as follows in order of desirability:
- Irregular rounded
- Flaky angular
- Flaky elongated
The rounded shape has a minimum surface area for the same mass as other shapes and therefore, requires minimum cement paste for bonding as compared to other shapes.
Flaky and elongated shapes have larger surface areas for the same mass as compared to rounded or cubical shapes. The cement paste required coating the surface and hence the water demand is much more for the flaky and elongated aggregates.
Definition of Flaky Aggregate
Flaky aggregates are those aggregates whose least dimension is less than 3/5th of its mean dimension.
Definition of an Elongated Aggregate
Elongated aggregates are those aggregates whose length is 1.8 times its mean dimension.
The specific gravity of an aggregate generally is indicative of its quality. A low specific gravity may indicate high porosity and therefore, poor durability & low strength. It is important that the aggregates have high specific gravity. The density of concrete will greatly depend on the specific gravity. The range of specific gravity for the aggregate is generally between 2.4 and 2.9.
Surface Moisture Content, Absorption and Porosity
Normal aggregates generally contain moisture. Moisture conditions are more significant in the concrete mix design. However, absorption tendency and porosity are equally important from the point of view of both qualities of the aggregate as well as the quality of concrete in which the aggregates are used.
Generally moisture is present in following forms:
- Moist or surface wet
- Surface dry but saturated
- Air dry
- Oven dry
The moisture on the surface determines the free water which is to be considered while measuring the water to cement ratio. The absorbed water within the aggregate isn’t considered while measuring the water-cement (W/C) ratio.
Porous aggregates will absorb more moisture or water than dense aggregates. If absorption in aggregates is more, then the concrete will lose its workability at a much faster rate. As a precaution, it is also better to do concrete trails with saturated surface dry aggregates so that the free water requirement for the required workability is easily determined.
Absorption can occur in both natural as well as in crushed aggregates. It is generally observed that, except in the rainy season, the surface moisture is present on natural aggregates which are dredged from the river or creeks or dug from pits.
Grading of Aggregates
Grading of aggregates is important as they can influence various properties of concrete such as cohesion, water demand, workability, and strength. Aggregates should be well-graded and they should be consistent in their grading.
It is very important that fine aggregates are well-graded as they generally serve as void fillers between the coarse aggregates. The fine aggregates comprise various sizes of finer fines and they greatly influence the cohesiveness, water demand, workability, and permeability as their specific surface per unit volume is much greater than other aggregate sizes.
As fine aggregates content or as finer fines content increases the specific surface greatly increases, and therefore, cement paste required to coat these fines is more. Hence, the water requirement will also be more to coat the surfaces of aggregates with cement and water glue.
IS 383 specifies four zones for fine aggregate grading. The below table gives the range of percentage passing for each zone as given in the IS 383. The table also gives the BS equivalent sieve size.
|IS Sieve||Equivalent BS Sieve||Percentage passing for Zone 1||Percentage passing for Zone 2||Percentage passing for Zone 3||Percentage passing for Zone 4|
|10mm||3/8 – in||100||100||100||100|
|4.75mm||3/16 – in||90-100||90-100||90-100||95-100|
|600 micron||No. 25||15-34||35-59||60-79||80-100|
|300 micron||No. 52||5-20||8-30||12-40||15-50|
|150 micron||No. 100||0-10||0-10||0-10||0-15|
A close look at the various ranges for each sieve size shows that Zone 1 sand is the coarsest and Zone 4 sand is the finest whereas sand in Zones 2 and 3 is moderate.
If the sand is coarse, then the water requirement will be less and if the sand is fine then the water requirement will be more to achieve the same workability.
Coarse sand won’t impart cohesiveness to the mix which in turn would cause segregation and bleeding.
Fine sand will impart good cohesiveness but would need more water for workability purpose.
Coarse sand will not give a good finish and therefore, the mix must have additional sand than normally necessary. Fine sand will give a good finish if the mix is workable and compactable.
If a particular sieve size isn’t present in aggregates, they are called gap graded. Sand that has gap grading is generally not preferred to the sand which is uniformly graded. Gap grading can create a deficiency in cohesiveness, permeability, and surface finish of the concrete.
Organic matter in fine aggregates is usually found because of the presence of vegetable matter. Even a very small fraction of organic matter will prevent or delay the hardening of concrete.
Chloride is present in fine aggregates will not be harmful to concrete or mortar but will be harmful to the reinforcement or other steel embodiments in concrete or mortar.
Chlorides may also be present in cement, water & concrete additives. The total chloride content for long-span bridges and prestressed structures must be limited to 0.10% by weight of cement and for reinforced concrete structures it must be limited to 0.15% by weight of cement.
Alkali oxides in cement shouldn’t be beyond permissible limits i.e. more than 0.6%. Alkali oxides from the cement will react with the reactive form of silica present in alkali reactive aggregates. The reaction results in the formation of alkali-silica gel and involves expansive forces which in turn cause cracking and disintegration of concrete.
Inspection of Deliveries of Aggregates
Inspection of aggregates is very significant as there are good chances that each delivery will differ from the other when it arrives at the construction site. Depending on the various parameters and controls at the quarry or at the crusher, each truckload will show variations in grading, variations in fine (dust) content, and variations in shape (cubical, angular, or flaky).
If fine aggregate (sand) is available from the natural source then it is extremely important to check the cleanliness first. Sand can be collected from various depths in the truck. Rub some sand between the hands and if the sand leaves it may be too dirty to use and must be given a further test. If clay lumps are observed, it should be rejected as it is virtually not possible to remove clay lumps by any manual process.
If silt content by volume is to be examined testing can be done at a construction site. Instead of waiting for 2hrs to 3hrs for silt to settle, it is recommended that a co-relation be developed and allowable silt content after 10 minutes can be established and followed so that trucks are not detained at the site for a longer period.
For gravel and natural sand, visual inspection is necessary for the presence of clay lumps, clay coating, silt, grading, and shape, while for crushed aggregates and sand, inspection is necessary for stone dust, flaky shape, and grading. It will be difficult to check the grading of graded aggregates if delivered at the site as the visual examination may not reveal variations in grading. Large variations cannot be ruled out in such cases while designing the concrete mixes.
Aggregates need careful storing making sure that they do not get contaminated which dust, mud, and soil due to poor storage at the site. Aggregates must be stored on a hard surface so that they don’t get mixed with soft materials at the base. If a hard surface is not available a 100mm thick plain concrete layer must be spread at the base before unloading the aggregates. If the hard layer is not put at the bottom then nearly 300mm thick bottom of the aggregate base will be wasted or will have to be washed and cleaned before use in mortar or concrete.
Store differently graded aggregates in separate well-partitioned bins so that intermixing does not take place in the storage area. Do not unload any aggregate outside the hard base as it may result in contamination of the entire stock.
In the case of the star bin batching plant hard base is very necessary as it’s difficult for the dragline to avoid scraping in some mud/dust from the soft stack base. It is also important to ensure that the partitions are well designed to take full pressure from either side in case one of the sides is fully empty.
It is essential to wash and clean the dead stock in a batching plant stack occasionally so that the silt, dust, and dirt which settle at the bottom of the stack does not flow into the mixer skip while the aggregates are being loaded.
Generally, workers feel comfortable relaxing on sand stacks hence care should be taken to see that the stack is not contaminated with waste paper, cigarette butts, tea leaves, sugar, etc.
Precautions in Extreme Weather
In extreme winter (below 0oC) it is essential to cover the stockpiles with proper covers to prevent snow and ice from contaminating them. If stockpiles are kept exposed the aggregates will stick to each other and will not flow into the mixer, skipping easily from the batching plant bin.
As a matter of precaution, it’s also important that no snow or ice should be present within the concrete at the time of placing. Hence, if aggregates are kept covered it will not only help in smoother concreting operation but also prevent quality problems.
In warm weather, it is preferable to spray cold water on the stack and cool the aggregates before mixing them with other materials. In large or massive concrete pours, chilling of aggregate may also be necessary. In warm weather, it is necessary to spray water on aggregates to avoid the rapid drop in workability due to the absorption of water by hot dry aggregates.