In this article we will discuss about:- 1. Meaning of Aggregates 2. Classification of Aggregates 3. Deleterious Substance 4. Soundness 5. Thermal Properties 6. Sieve Analysis 7. Fineness Modulus 8. Maximum Size 9. Handling.
Meaning of Aggregates:
Originally mineral aggregate was used as inert filler in the cement concrete. In fact the aggregate is not inert filler and its physical, thermal and chemical properties influence the performance of the concrete to a great extent. Further the aggregate is cheaper than cement and thus it is cheaper to use as much quantity of aggregate and as little of cement as possible, but economy alone is not the reason for using aggregate in the concrete. Aggregate provides better strength, stability and durability to the structure made out of the cement concrete than cement paste alone.
In any concrete, usually aggregate occupies about 70 to 75% of the total volume of the concrete mass. The selection and proportioning of aggregate should be given due attention as it not only affects the strength, but durability and structural performance of the concrete also.
While selecting aggregate for a particular concrete, following requirements should be kept in mind:
(a) Economy of the mixture,
(b) Strength of the hardened mass, and
(c) Durability of the structure.
Classification of Aggregates:
Generally aggregates may be classified as:
1. According to Source:
According to this classification aggregates may further be classified as:
(a) Natural Aggregates
(b) Artificial Aggregates
(a) Natural Aggregates:
These aggregates are formed from naturally occurring material. The natural aggregates such as gravel and sand are the product of weathering and action of running water. Natural aggregates may be derived from any rock as igneous, sedimentary or metamorphic rock. The classification of aggregates according to rock has been shown in Table 4.1.
(b) Artificial Aggregates:
These aggregates are usually produced for some special purpose as burned clay aggregates for making light weight concrete. Some artificial aggregates are the byproduct of an unrelated industrial process as blast furnace slag, cinder etc. Sometimes crushing of rock may also be adopted to produce a desired sized aggregate.
2. According to Mineralogical Composition:
According to this classification aggregates may be classified as silicious or calcareous etc.
3. According to Mode of Preparation:
According to this classification distinction is made between aggregates reduced to its present size by natural agents and crushed aggregate obtained by a deliberate fragmentation of rock.
4. According to Size of Aggregate Particle:
According to this classification the aggregate is further sub-divided as-
(a) Coarse aggregate and
(b) Find aggregate
(a) Coarse Aggregates:
It is defined as the aggregate most of which is retained on 4.75 mm I.S. sieve and containing only so much finer material as is permitted for various types described in I.S. 383- 1970 in Table 4.2.
(b) Fine Aggregates:
An aggregate most of which passes through 4.75 mm I.S. sieve is called fine aggregate. The four zones of fine aggregates are shown in Table 4.3.
Deleterious Substance in Aggregates:
Deleterious substances can be classified into the following three categories:
1. Impurities which interfere with the process of hydration of cement.
2. Coatings on aggregates which prevent the development of good bond between aggregate and the cement paste.
3. Unsound or weak particle.
1. Organic Impurities:
Natural aggregates may be sufficiently strong and wear resistant even then they may not be satisfactory for concrete making if they contain organic impurities which interfere with the hydration of cement. The organic matter found in aggregate usually is vegetable matter (mainly tannic acid and its derivatives) which appear in the form of humus or organic loam. Such materials are more likely to be present in fine aggregate (sand) than in coarse aggregate which is easily washed.
All organic matter is not found harmful and it is best to check the harmful effects by making actual test cubes. However it is time saving to ascertain first whether the amount of organic matter is sufficient to cause harmful effect to the concrete. The amount of organic matter can be determined as per I.S. 2386 part-II.
The sand is taken as delivered at site without drying 3 percent solution of sodium hydroxide (NaOH) in water is filled in a 350 cc graduated clear bottle upto 75 cc mark. Now the given sample of sand is poured in this bottle gradually till the sand reaches upto 125 cc mark. The volume in the bottle is made upto 200 cc marks by adding more solution of 3% sodium hydroxide. The bottle is stoppered and shaken vigorously and then allowed to stand for 24 hours. The organic content can be judged by the colour of the solution above the sand. It has been found that greater the organic matter darker the colour of the solution.
The colour of the solution can be compared with a freshly prepared standard solution. This solution is prepared by adding 2.5 cc of 2 percent solution of tannic acid in 10% alcohol, to 97.5 cc of 3 percent sodium hydroxide solution. This solution is filled in a 350 cc bottle, stoppered and shaken vigorously and allowed to stand for 24 hours before comparison with the above solution.
If the observed colour is darker than the standard solution i.e., if the colour of the solution appears brownish or brown, then aggregate has a rather high quantity of organic matter and the further tests are necessary. Concrete cubes are made using the suspected aggregate and their strength is compared with concrete of the same mix proportions but made with aggregate of known quality.
2. Coatings on Aggregates:
Clay, silt and crusher dust etc. may be present in aggregate. Silt may be present in aggregate in the form of surface coatings and may interfere with the bond between the aggregate and cement paste. To have a good strength and durability concrete, it is essential to ensure a good bond between aggregate and cement paste. The problem of silt coating is important.
Silt and crusher dust also may form similar coatings as those of clay. These materials should not be present in excessive quantity as due to their fineness and large surface area, they need more water for wetting their surface and consequently need more water for a fixed water-cement ratio, to produce a concrete of a definite workability. The quantities of these materials may be determined by sedimentation method.
As per B.S. 882-1975, the quantities of all the three materials together should not be more than the following quantities:
1. 15% by weight in crushed stone sand.
2. 3% by weight in natural or crushed gravel sand.
3. 1% by weight in coarse aggregate.
The quantities of three materials may be determined by sedimentation method. The sand sample is placed in a sodium oxalate solution (containing 8 gram sodium oxalate per lit of distilled water) in a stoppered jar and rotated with the axis of the jar horizontal for 15 minutes at approximately 10 revolutions per minute. The fine solids become dispersed and the amount of suspended material is then measured by means of Anderson pipette. The percentage of clay and fine dust is given by the relation.
Percentage of slit, clay or fine dust = (1000/w1) [(100w2/v) – 0.8]
w1 = weight in gram of the original sample
w2 = weight in gram of the dried residue
v = volume in cc or ml of the pipette
0.8 is the weight in gram of sodium oxalate in one lit, of dilute solution.
The result should be correct upto 0.1%.
Sand excavated from sea shore or from the bed of river contains salt. If salt is not removed, it will absorb moisture from the air and cause efflorescence (unsightly) white deposits on the surface of concrete) and a slight corrosion of reinforcement may also result. However the danger of corrosion in good quality concrete with adequate cover to reinforcement is not of dangerous degree. These salts may be removed by washing sand with fresh wafer.
Sea bed sands may contain large quantity of shell contents. Though shell content is not found to have adverse effect on the strength of concrete but workability of concrete is reduced if aggregate having large content of shell is used.
3. Unsound Particles:
The unsound particles of aggregate may be classified into two categories as follows:
1. Those particles which fail to maintain their integrity.
2. Those particles that lead to disruptive expansion on freezing or even exposure to water. The disruptive properties are characteristics of certain rock groups.
Shale, clay lumps, wood, coal etc. and other particles of low density are regarded as unsound. If these particles used in concrete they may lead to pitting and scaling if present more than 2 to 5% of the weight of aggregate, these particles may adversely affect the strength of concrete. Thus they should not be used in concrete which is exposed to abrasion.
Coal in addition to being soft material, is undesirable for other reasons as well. Coal swells very much, causing disruption of concrete. If present in large quantities in finely divided form, it can disturb the process of hardening of the cement paste. However discrete particles of hard coal amounting to not more than 0.5% of the weight of aggregate have no adverse effect on the strength of concrete.
The presence of coal and other low density materials can be determined by flotation in a liquid of suitable specific gravity.
The presence of mica in fine aggregate even by few percent of the weight of the aggregate affects adversely the water requirement and consequently the strength of the concrete. The presence of mica in the fine aggregate has been found to reduce considerably the durability and compressive strength of concrete. Hence its effects on the concrete should be determined. Thus before using an aggregate, its mica contents should be determined and an allowance is made for the possible reduction in the strength.
It is the most expansive material in the aggregate. This sulphide reacts with water and oxygen in the air to form ferrous sulphate which subsequently decomposes to form hydroxide, and the sulphate ions react with the calcium aluminate in the cement. Due to this chain reaction, surface staining of the concrete and disruption of the cement paste may result, particularly under warm and humid conditions.
Gypsum and other sulphates also should not be present in the aggregate. The permissible quantities of deleterious substances are shown in Table 4.14 as per I.S. 384-1970.
Soundness of Aggregates:
The ability of aggregate to resist excessive changes in volume due to the changes in physical conditions is known as soundness of aggregate. The physical causes of permanent volume changes of aggregate are freezing and thawing, thermal changes at temperatures above freezing, and alternate wetting and drying.
An aggregate is said to be unsound when volume changes induced by the above causes, results in deterioration of concrete. This may range from local scaling to extensive surface cracking and to disintegration over considerable depth and can thus vary from impaired surface to structurally dangerous situation.
A test for soundness of aggregate can be performed as described in I.S. 2386 Part-V-1963. In this test the sample of graded aggregate is subjected alternately to immersion in the saturated solution of sodium or magnesium sulphate for not less than 16 hours and not more than 18 hours in such a manner that the solution covers the aggregate to a depth of at least 15 mm. The evaporation of solution is checked by covering the container. The temperature of the test is maintained at 27 ± 1°C.
After immersion, the aggregate is removed from the solution and allowed to be drained for 15 ± 5 minutes and placed in the oven at temperature of 105°C to 110°C. The sample is dried to constant weight at this temperature. The formation of salt crystals in the pores of aggregate tends to disrupt the aggregate particles in a similar manner as the action of ice. The reduction in size of the particles as shown by sieve analysis, after a number of cycles of exposure denotes the degree of unsoundness. No. of cycles of exposure depends upon the mutual agreement of the supplier and the purchaser.
Thermal Properties of Aggregate:
Following thermal properties of aggregate are significant in the performance of concrete:
1. Coefficient of thermal expansion.
2. Specific heat, and
Coefficient of Thermal Expansion of Aggregate:
It influences the value of the coefficient of thermal expansion of concrete containing the given aggregate. Higher the coefficient of expansion of the aggregate, higher is the coefficient of expansion of the concrete. However the coefficient of thermal expansion of concrete also depends upon the aggregate content in the mix and the mix proportions as well.
However it has been seen that changes in temperature upto 60°C have no large difference between the coefficients of aggregate and concrete. Thermal coefficients of linear expansion of some rocks are shown in Table 4.16.
Sieve Analysis of Aggregates:
The operation of dividing a sample of aggregate into fractions, each fraction consisting particles of the same size is called sieve analysis. In practice each fraction contains particles between the openings of the standard test sieves. Test sieves used for concrete aggregates have square openings and described by the size of the openings in mm for larger sizes and by the number of openings per lineal cm for sieves smaller than about 3 mm.
Coarser test sieves (4 mm and larger) are made of perforated mild steel plate with a screening area of 44 to 65 percent, whereas smaller sieves (less than 4 mm) are made of wire cloth. The screening area in this case varies between 34 to 53 percent of the gross area of the sieve. The sieves used for concrete aggregate consist of a series in which the clear opening of any sieve is approximately one half of the opening of the next larger sieve sizes.
Usually sieve sizes used for concrete aggregate as per I.S. 383-1970 are as follows:
80.0, 63.0, 40.0, 20.0, 16.0, 12.5, 10.0, 4.75, 2.36 and 1.18 mm, 600 micron, 300 micron, and 150 micron. In U.K. as per B.S. 812 Part-I, for grading aggregate following sieves are used – 75.0, 50.0, 37.5, 20.0, 10.0, 5.0, 2.36, 1.18 mm and 600, 300 and 150 micron. The standard British and American sieve sizes are shown in Table 4.17.
In order to obtain satisfactory and comparable results from sieve analysis sample of the aggregate must be representative of the lot. To reduce a large sample to the size of the test, quartering of the aggregate is advocated. Before resorting to quartering, sampling of the aggregate is essential. The lot of the aggregate should be divided into the sub-lots depending upon the volume or size of the lot as shown in Table 4.18.
After dividing the aggregate into sub-lots, the representative gross sample should be taken from each sub-lot from the full cross section and thickness of the sub-lot and should be kept separate for each sub-lot. Scoop may be used for sampling as shown in Fig. 4.11.
Its dimensions vary according to its capacity. The number of increment to be taken and weight of gross sample are shown in Table 4.19.
After drawing the sample, it should be spread on a clean surface and divided into four quarters, discard two diametrically opposite quarters and combine the remaining two quarters. The process should be repeated till the size of sample is reduced to the required quantity.
Before the sieve analysis is performed, the aggregate sample should be air dried to avoid lumps of fine particles and also to prevent clogging of the finer sieves. The weights of the reduced samples for sieving as per I.S. 2386-1963 is shown in Table 4.21.
The graphical representation of the results of sieve analysis is called grading curves. By using grading curves it is possible to see at a glance whether the grading of a given sample confirms to that specified or is too coarse, or too fine or deficient or in excess in a particular size. In the grading curves usually ordinates represent cumulative percentage passing and abscissa the sieve opening plotted to a logarithmic scale. A logarithmic plot shows these openings at a constant spacing as the openings of sieves in a standard series are in the ratio of 1/2.
Requirements of Grading Curves:
The grading of the aggregate should be such that mix prepared by it could be compacted to a maximum density with a reasonable amount of work for given water cement ratio. It means that the grading should be such that the voids of the larger particles are filled by the next lower fraction and so on and the voids of fine aggregate are filled by cement.
The strength of a fully compacted concrete with a given water/cement ratio is independent of the grading of the aggregate. Grading only affects the workability of the mix and the development of strength corresponding to a given water/cement ratio requires full compaction, which can be achieved only with the sufficiently workable mix. In actual practice there is no ideal grading curve, but a compromise is aimed at.
Interpretation of Grading Curves:
From the study of the grading curve say for zone I sand, it will be seen that higher percentage of passing material represents upper curve and lesser passing percentage lower curve. These curves may be denoted by the letter U and L respectively. The upper curve represents grading of finer material while lower curves those of coarser material. The desired grading curve of a mixed aggregate can also be similarly obtained.
If the actual curve is lower than the desired curve, then it is an indication of coarser aggregate and the segregation may result. On the other hand if the actual curve is higher than the required, then it shows finer aggregate. In this condition for the same workability more water will be needed. If the actual curve is steeper than the required, then it indicates that the average particles are more.
In this case the percentage of fine aggregate will be more and its workability for a fixed water/cement ratio will not be good. In case the actual curve is more flat, it indicates the deficiency of average particles, in which condition the resulting concrete will be honey combed. Thus the grading curve should be smooth; parallel and as near as possible to the required curve.
Fineness Modulus of Aggregates:
It is only a numerical index of fineness. It gives some idea of the mean size of particles in the entire body of the aggregate. To a certain extent it is a method of standardization of the grading of the aggregates. It is obtained by adding the cumulative percentages of weights retained on the sieves of the standard series 150, 300, 600 micron and 1.18, 2.36, 4.75 or 5 mm and upto the largest sieve size to be used and dividing the sum by 100.
It should be remembered that when all particles in a sample are Coarser than say 600 micron, then the cumulative percentage retained on 300 micron should be entered as 100. Same value also would be entered for 150 micron. Coarser the aggregate, higher the value of fineness modulus. Fine aggregate having fineness modulus less than 1.0 should not be used. Higher the F.M., harsher the mix, lower F.M. gives an uneconomical mix.
Generally the fineness modulus for fine aggregates varies from 2.0 to 3.5, and for coarse aggregate between the 5.5 and 8.0 and for all in aggregate between 3.5 to 6.5. Following example of table 4.25 will illustrate the method of determining the fineness modulus of aggregate.
Find the fineness modulus of fine and coarse aggregate for which the sieve analysis is given in the following Table 4.25.
Surface Index of Aggregate:
In order to get good workability of mix, the importance of a good grading of coarse and fine aggregate already has been discussed. The quantity of water required to get a given workability of the mix depends to a large extent on the surface area of the aggregate.
Factors affecting the grading of aggregates, the concept of specific surface gives somewhat misleading picture of the workability to be expected. To overcome this difficulty Murdock suggested the use of surface index. Surface index is an empirical number related to the specific surface of the particles. More weightage has been given to the finer particles. The empirical numbers representing the surface index of aggregate particles with in a set of sieve size are shown in Table 4.26 below as suggested by Murdock.
Calculation of Surface Index:
The total surface index (fx) of a mixture of aggregate may be determined by multiplying the percentage of material retained on its sieve by the corresponding index shown in Table 4.26. The product of all particles is added. To this sum an empirical constant of 330 is added and the result is divided by 1000. The method of calculation is illustrated by the following example.
GAP Grading of Aggregate:
The aggregate particles of a given size pack, in such a way to form voids that can be penetrated only if the next smaller size of particles is sufficiently small. This means that there must be a minimum difference between the sizes of any two adjacent particle fractions. In other words sizes of aggregate differing only very little cannot be used side by side. This fact has led to the advocacy of gap grading of aggregates.
Thus gap grading of aggregates can be defined as a grading in which one or more intermediate size fractions are omitted. The term continuously graded is used to describe conventional grading when it is necessary to distinguish it from gap grading. On a grading curve, gap grading is represented by a horizontal line over the range of sizes omitted as shown in Fig.4.23.
Advantages of GAP Grading:
Some researchers as Shack-lock have advocated that for a given aggregate/cement ratio and water/cement ratio, a higher workability can be obtained with a lower sand content in case of gap graded aggregate than continuously graded aggregate. However mixes, for the more workable range of gap graded aggregate showed more likelihood of segregation. For this reason gap grading is recommended for mixes of relatively low workability that are to be compacted by vibration. In this case good control and care is essential in handling to avoid segregation.
Though from time to time various claims of superior properties of concrete made with gap graded aggregate have been made, but neither compressive nor tensile strength appears to be affected by the gap grading. The shrinkage also is found unaffected by gap grading. On the other hand resistance of concrete to freezing and thawing is found lower with gap graded aggregate.
Maximum Size of Aggregates:
The larger the aggregate particle, the smaller the surface area to be wetted per unit weight of aggregate. Thus extending the grading of aggregate to a larger maximum size lowers the water requirement of the mix, such that for a specified workability and richness, the water/ cement ratio can be reduced resulting increase in the strength of the concrete. This fact has been verified by some researchers with aggregates upto 38.1 mm maximum size and assumed to extend to larger sizes as well.
Recent experimental results have shown that above 38.1 mm maximum size of aggregate the gain in strength due to the reduced water requirement is off set by the detrimental effects of lower bond area and of discontinuities introduced by the very large particles particularly in rich mixes. Concrete becomes grossly heterogeneous and results in lower strength.
The adverse effect of increase in the size of the largest aggregate particles in the mix exists throughout the ranges of sizes, but below 38.1 mm, the effect of size on the decrease in the water requirement is dominant. Bloem has compared the actual strength with expected strength on the basis of the variation in water/cement ratio from that required with 38.1 mm aggregate. Actual decrease of strength starts with 25 mm maximum size of aggregate as shown in Fig.4.24. From the study of the Fig. 4.24 it would be seen that actual decrease of strength starts with 25 mm maximum size of aggregate.
For larger sizes, the balance of two effects depends on richness of the mix. Thus the best maximum size of aggregate from the point of view of strength is a function of the richness of mix, specially in lean concretes having cement 170 kg per cubic metre of concrete, the use of 150 mm aggregate is most advantageous. However in structural concrete of usual proportions, from the strength point of view, use of greater size aggregate than 25 mm or 38 mm has not been found advantageous. There are structural limitations also on the size of the aggregate.
As per I.S. 416-1964 the maximum size should not be greater than 1/5 to 1/4 of the thickness of concrete section. Secondly the maximum size of aggregate should be at least 5 mm smaller than the spacing of reinforcement. Thirdly the maximum size should be at least 5 mm smaller than the cover given to the concrete reinforcement.
Thus looking to these limitations and Bloem experimental results, the maximum size of aggregate for structural concrete should be limited to 25 mm preferably 20 mm. However for mass concrete aggregate sizes upto 150 mm can be used. However the decision would also be influenced by the cost and availability of different sizes of fractions.
Handling of Aggregates:
Handling and stock piling of coarse aggregate can easily lead to segregation, more specially when the aggregate has to roll down a slope.
While stockpiling aggregate at site following precautions should be adopted:
1. The coarse as well as fine aggregate should be stored on a hard and dry ground. It should never be dumped on loam or grass. If aggregate is dumped on loam or grass, dirt and rubbish will be carried into the concrete. If hard surface is not available, a platform of planks, or old corrugated iron sheets, or floor of brick or a thin layer of weak concrete (1:5:10) or so should be prepared.
2. Piles of sand and coarse aggregate and piles of different sized coarse aggregate should be kept separately by means of compartment walls. These fractions should be remixed in the desired proportion at the time of feeding them into the mixer.
3. Care should be taken to avoid breakage of the aggregate.
4. The bidi ends, tea leaves or sugar etc., should not be allowed to be thrown into the aggregate piles. The tobacco of bidi or nicotine of tea leaves or sugar will slow down the setting of the concrete. Tree leaves or grass root etc. will also damage the binding properties of concrete. Hence aggregate should be kept clean.
5. While stockpiling, successive consignments should not be dropped at the same place. This will lead to segregation of aggregate.