3. AGGREGATE CLASSIFICATION AND IT'S PROPERTIES & TESTS

2.2.0. AGGREGATES - Aggregates which occupy nearly 70 to 75 percent volume of concrete are sometimes viewed as inert ingredients in more than one sense. However. it is now well recognized that physical, chemical and thermal properties of aggregates substantially influence the properties and performance of concrete. A list of properties of concrete which are influenced by the properties and characteristics of aggregates is given in Table 5 Proper selection and use of aggregates are important considerations, both economically as well as technically. Aggregates are generally cheaper than cement and imparting greater volume stability and durability to concrete.

2.2. 1. CLASSIFICATION OF AGGREGATE

2.2. 1.1 GENERAL -

General classification of aggregates can be on the basis of their sizes, geological origin, soundness in particular environments, unit weight or on many other similar considerations as the situation demands, In so far as the sizes are concerned, aggregates range from a few centimeters or more, down to a few microns. The maximum size of aggregate used in concrete may vary, but in each case the aggregate is to be so graded that particles of different size fractions are incorporated in the mix in appropriate proportions.

As per IS : 383  fine aggregates are those, most of which pass through 4.75 mm IS sieve; and aggregates, most of which are retained on 4.75 mm IS sieve are termed as 'coarse' aggregates. Sand is, generally considered to have a lower size limit of about 0.07 mm.

Materials between 0.06 mm and 0.002 rnrn are classified as silt and still smaller particle are called clay. Sometimes combined aggregates are available in nature comprising different size fractions of the above classification, which are known as 'All-In-Aggregates'.

In such cases they need not be separated into fine and coarse fractions but adjustments often become necessary to supplement the grading by the addition of respective size fractions which may be deficient in the total mass. Such 'all-in-aggregates' are generally not found suitable for making concrete of high quality. Aggregates comprising particles falling essentially within a narrow limit of size fractions are called 'single-size' aggregates.

2.2.1.2. GEOLOGICAL CLASSIFICATION OF NATURAL AGGREGATES -

Aggregates for concrete are generally derived from natural sources which may have been naturally reduced to size (for example, gravel or shingle) or may be required to be crushed.

As long as they conform to the requirement of IS : 383 and concrete of satisfactory quality can be produced at an economical cost using them, both gravel/shingle or crushed natural aggregate can be used for general concrete construction.

Aggregates can be manufactured from industrial products also, which are used for special purposes, for example, light weight concretes, concretes requiring better thermal insulating properties, etc. From the petrological stand-point, the natural aggregates, whether crushed or naturally reduced in size, can be divided into several groups of rocks having common characteristics. Natural rocks can be classified according to their mode of formation (for example ignious, sedimentary or metamorphic origin) and under each class they may be further sub-divided into groups having certain petrological characteristics in common. Such a classification adopted in IS : 383-19702'1 is reproduced in Table 6.

Depending upon the minerals found in aggregates the mineralogical classification can also be made. However, such classifications are not very helpful in predicting the performance of the aggregates in concrete. This is so, because each rock will probably have a number of minerals present and even among the most abundant minerals in a particular aggregate it is difficult to classify one being universally desirable or otherwise.

2.2.2 PROPERTIES OF NATURAL AGGREGATE:

As pointed out earlier, the properties and performance of concrete are dependent to a large extent on the characteristics and -properties of aggregates themselves, and knowledge of  the properties of aggregates is thus important. In the cases of marginal aggregates the record of performance of concretes made with them may be the best guide. However, tests in the laboratory as well as petrographic exanimations are used in most general cases.

2.2.2. MECHANICAL PROPERTIES:

The significance of the various tests for mechanical properties are discussed below:

a) Tests on strength of aggregate – The strength of aggregates in the conventional sense may appear to be not a criterion in so far as aggregates are generally of an order of magnitude stronger than the concretes made with them and a notional feeling that stronger aggregates are better may be sufficient. However. The localized stresses in an element of concrete may be much higher than the overall strength of concrete due to stress concentrations and in case of high strength concrete the mechanical strength of aggregates may itself become critical.

Moreover, the mechanical strength of aggregates is important from the point of view of quarrying, stability in the mixer, better resistance to abrasion or attrition during subsequent service life of concrete. Quite often among aggregates of similar geological classifications one having higher mechanical strength has been found to be sounder in chemical environments.

IS : 2386 (Part IV) prescribes the following three tests for testing the strength of aggregates: 1) Crushing strength,

2) Crushing value, and

3) Ten percent fines value.

The tests on crushing strength do not give very reproducible results but essentially measure the quality of the parent rock rather than those of the aggregates derived from it. This test may be useful for assessments of new sources of aggregates without proven records.

Among these three, crushing value test, which is performed on bulk aggregates is more popular and results are reproducible, 1S : 383 specifies limits of crushing value as 45 % for aggregates used for concrete other than for wearing surfaces and 30 % for concrete for wearing surfaces, such as runway, pavement and roads.

For weaker aggregates with a crushing value of over 25 to 30 %, crushing value, test is not so reliable in the sense that material crushed before the full load having been applied tends to get compacted thereby inhibiting crushing at a later stage and the intrinsic value may not be measured in such cases. For such situations 'ten percent fines value' test may be more reliable which measures the load required to produce 10 percent fines from 12.5 to 10 mm particles.

BS : 882 prescribes a minimum value of 10 tonnes for aggregates to be used in wearing surfaces and 5 tonnes when used in other concretes.

Another related aspect is the toughness of aggregates which is a measure of the resistance of the material to failure by impact. IS : 2386 (Part IV) prescribes a method for determining the impact value which is sometimes taken as an alternative to crushing value test. The results also in general correspond to each other and the requirements of IS : 383 are similar to those for crushing value test.

This is a convenient test which can be carried out in the site laboratory.

b) Hardness and abrasion resistance-

In addition to crushing strength and toughness resistance, abrasion resistance is an important consideration specially for concretes exposed to wearing actions. Concretes made with aggregates having good abrasion resistance are necessary for making concrete which will be subjected to abrasion and attrition during service.

More than that, the abrasion and attrition resistance of aggregates are also important to assess the likelihood of breakage during handling and stockpiling as well as during mixing in a mixer.

As per tests conducted by NRMCA32 have reported that certain fine aggregates degrading due to attrition during mixing, more so in prolonged mixing in case of ready-mix concrete, with consequent increase in the proportion of fines in the combined aggregates thereby lowering the workability.

In these tests, the sand samples were considered otherwise 'satisfactory'. Some typical results as to the effect of prolonged mixing on fineness modulus of sand and gravel are reproduced in Fig. 9.

IS : 2386 (Part IV) recommends Los-Angeles test for the hardness and abrasion resistance of aggregates in addition to scratch test essentially for the detection of soft particles.

The Los-Angeles attrition test combines test for attrition and abrasion and it is quite popular. This test is found to be more representative of the actual performance expected of the aggregates and results can also be correlated with other mechanical properties of aggregates.

IS: 383 requires that a satisfactory aggregate should have Los-Angeles abrasion value of not more than 30 percent for aggregates used for wearing surfaces and 50 percent for aggregates used for non-wearing surfaces. Table 7 indicates the type of relationship that can be expected between various tests for different rock groups.

2.2.2.2 PARTICLE SHAPE AND TEXTURE -

The external characteristics of mineral aggregates in terms of physical shape, texture and surface conditions significantly influence the mobility of the fresh concrete and the bond of aggregates with the mortar phase. Two relatively independent properties, Sphericity and Roundness define the particle shape.

Sphericity is defined as a function of the ratio of the surface area of the particle to its volume whereas roundness measures the relative sharpness or angularity of the edges and corners of a particle. To avoid lengthy descriptions of the aggregate shape, IS : 383 lists four groups of aggregates in terms of particle shape (see Table 8).

Well rounded particles require less water and less paste volume for a given workability; but crushed or uncrushed rounded gravels generally tend to have a stronger aggregate-mortar bond and result in substantially the same compressive strength for a given cement content. The unit water content could be reduced by 5 to 10 percent and sand content by 3 to 5 percent by the use of rounded gravel. Use of crushed aggregates, on the other hand, may result in 10 to 20 percent higher compressive strength.

For water-cement ratios below 0.4, the use of crushed rock aggregate has resulted in strengths up to 38 percent higher than when gravel is used

Elongated and flaky particles, having a high ratio of surface area to volume, lower the workabilijy of the mix and can also affect adversely the durability of concrete since they tend to be oriented in one plane with water and air voids underneath.

A flakiness index not greater than 25 percent is' suggested for coarse aggregates. Surface texture is the measure of polish or dullness, smoothness or roughness and the type of roughness of the aggregates.

IS : 383 classifies surface characteristics of the aggregate into five headings or groups (see Table 9).

The grouping is broad and it does not purport to be a precise petrographical classification, but is based upon a visual examination of hand specimens.

Rough porous texture is preferred to a smooth surface; the former can increase bond of cement paste by 1.7 times and leads to 20 percent more flexural and compressive strength in concrete.

The shape and texture of fine aggregate significantly affect the water requirement of the mix. As a typical example, Fig. 10 shows the influence of void content (indirect expression of shape and texture of fine aggregate) of sand in a loose condition on the mixing water requirement of concrete.

2.2.2.3 POROSITY AND ABSORPTION -

Porosity, permeability and absorption of aggregate influence the bond between aggregate and cement paste, the durability of concrete with regard to the aggressive chemical agencies, resistance to abrasion of concrete, and freezing and thawing; out of these, resistance to freezing and thawing is not an important consideration in the conditions prevailing in India, unlike the colder climates of western countries. The porosity of some common rocks is given in Table 10.

The water absorption properties of aggregates are important in the sense that depending upon the condition in which the aggregates are used that is, saturated, surface dry, dry or bone dry.

Porous may become reservoir of free moisture inside the aggregates. Afterwards this moisture may be available for hydration or may actually extract some water used for mixing and the entire water may not be available for perfect workability and subsequent hydration of cement.

The absorption of water by aggregate is determined by measuring the increase in weight of an oven-dried sample after immersion for 24 hours. However, the absorption of water from the mix by dry aggregates is somewhat proved as the paste or mortar paste surrounds the surface and indeed the absorption may not proceed to the full and may come to an end within first 10 or 20 minutes. Under such circumstances the water absorption of aggregate in the first 10 or 20 minutes is sometimes more meaningful than the full absorption capacity determined in 24 hours. Some typical ranges of values of different rocks are given in Table l1.

Since the outer layer of the gravel particles can be more porous and absorbent due to weathering, gravel generally absorbs more water than crushed rock of the same petrolollcal character.

2.2.2.4 DELETERIOUS CONSTITUENTS -

A number of materials may be considered undesirable as constituents in aggregates because of their intrinsic weakness, softness, fineness and other physical characteristics, the presence of which may affect the strength, workability and long-term performance of concrete.

By way of their actions, these can be classified as falling into anyone of the following categories: -

a) Which are present as coating around the aggregates and may interfere with the bond characteristics.

b) Which are essentially fine particles and increase the total specific surface area of the aggregate thereby affecting the workabililty.

c) Which are themselves soft and friable, and can be considered as a weak inclusion in the composite, being potential side of stress concentration.

d) Which can affect the chemical reactions of hydration of cement.

IS : 383 identifies iron pyrites, coal, mica, shale or similar laminated material, clay, alkali, soft fragments, sea shells, organic impurities, etc, as such undesirable ingredients in aggregates. It has to be remembered that each of these will have actions falling in one or more-of the above categories; for example clay can be present as coating around the aggregate thereby reducing the bond. They can be present as fine particles which increase the water demand for a particular workability and they themselves are soft enclosures. It is difficult to precisely tell what proportion of each will definitely pose adverse effects on the properties of concrete as it depends upon the particle size and shape, the size of the concrete members. The distribution of such impurities in the aggregate and above all the exposure conditions. Nevertheless many codes of practices including IS : 383 have set limits about the presence of deleterious constituents and have prescribed relevant methods of tests to determine the amount thereof. The related Table from IS : 383 as to the limit of deleterious materials in aggregates is reproduced in Table 12.

The salient points about 'deleterious materials are discussed below:

a) Clay lumps, clay and silt – Clay lumps could be of two types:

1) Those which will get broken during the mixing operations; and

2) Those which survive mixing operation and will be present in the concrete.

The amount of clay lumps which can be handpicked is required to be limited to one percent of the clay. The proportion of clay which is likely to be broken down in mixing is included in the test for 'material passing 75 microns IS sieve' which also include the other fines in the aggregate like silt and fine dust.

Another test prescribed in IS: 2386 (Part 11) to determine specifically the proportion of clay, silt and fine dust is by sedimentation method. While IS : 383 prescribes a limit of 3% for fine aggregates and in crushed coarse aggregates or natural coarse aggregates and, 1% for crushed rocks for the material passing 75 micron IS sieve, there is no corresponding limitation in the deleterious constituents detected by the sedimentation test. The deleterious constituents determined by these two test methods mayor may not be the same. However, the British practice limits the proportion of clay silt and fine dust content in aggregates (as determined by the sedimentation method) to 3% by weight in sand and one percent by weight of crushed coarse aggregates.

The effect of such fine particles on the workability and strength of concrete may be appreciated by the fact that for everyone percent clay in the fine aggregate, the compressive strength of concrete can decrease by 5 percent (see Fig. 11).

b) Lightweight and soft fragments IS  : 383 sets different limits for lightweight pieces and soft fragments in the aggregates (see Table 12) and gives two different test methods to determine the same; however in practice a deleterious constituent may indeed be falling under both categories.

Lightweight pieces are essentially coal and lignite and the method of determination is by gravity separation in a liquid of specific gravity 2.0. Coal and lignite may cause localised pitting and staining in concrete surfaces. In addition, some particles specially of softer variety may swell when come in contact with moisture and thereby weaken the concrete. Because of the staining where the appearance of the finished surface is a criterion, the permissible amount of coal and lignite is limited to 0.5 percent. It may be noted that all the particles having specific gravity less than 2.0 may not be coal and lignite alone, and may include other type of particles which may not have such deleterious effects on surface appearance.

That is why some specifications (for example, ASTM C33 36 and IS : 2386 (Part 11) imply that particles having specific gravity less than 2.0 and of black and brownish black colour are considered as coal and lignite.

The test (scratch-hardness) for soft fragments as prescribed in IS: 2386 (Part I1) is essentially to detect materials which are so poorly bonded that the separate particles in the piece are easily detached from the mass. Soft inclusions, such as clay lumps, shale, wood and coal are included in the test for lightweight pieces if the specific gravity is lower than 2.0.

Such soft small fragments when present in larger quantities (2 to 5%) may lower the compressive strength of concrete by being a weak spot in the composite and giving rise to stress concentration as discussed earlier. IS: 383 specifies that soft fragments shall not be more than 3 percent in natural coarse aggregates whereas the shale content in natural fine aggregates is limited to one percent.

c) Organic impurities - Organic impurities like those resulting from products of decay of vegetable matter interfere with the chemical reactions of hydration of cement. Such organic impurities are more likely to be present in fine aggregates; those in coarse aggregates can be removed during washing. Not all the organic matter present in aggregates may be harmful

and in most cases it is desirable to test the strength properties made with such aggregates (mostly fine aggregates) and compare with those made with aggregates known to be free from organic impurities.

This test is, however, resorted to only when the organic impurities are detected by a colorimetric test as prescribed in IS : 2386(Part 11). In this test the change in the colour of a 3% sodium hydroxide solution, in contact with the sample for 24 hours indicates the presence of organic matter; darker the colour, more the organic content. The colour of the solution is compared with a standard solution.

This test is mainly a negative test which means that if there is no colour change, no organic matter is present but if the colour changes, the presence of organic matter is indicated.

Another test prescribed in IS: 2386 (Part VI) for mortar making properties of fine aggregates is comparative test on the basis of strengths of mortar made with suspected sand and good sand as enumerated earlier.

d) Mica in aggregates - Mica which is often considered to be harmful for concrete may be present in almost all river sands although the proportion may vary from negligible to substantial amount. The mica being flaky and laminated in structure affects the strength and workability of concrete the way other flaky and laminated particles do.

The effect on durability will mostly result from the unsatisfactory workability of the fresh concrete and leads to increased permeability; in addition that in the presence of active chemical agents produced during the hydration of cement, alteration of mica to other forms may result.

Mica is generally found in two varieties: Muscovite, KAl2 (Si3Al) O10 (OH2), is potassium aluminium silicate which is colourless or has a silvery or pearly lustre.

The second variety of mica biotite which is black brown or dark green in colour, is a complex silicate of potassium, magnesium, iron and aluminium K2 (MgFe)16 - (SiAl)8  O20 (OH4) of these two, the muscovite variety is now believed to be more harmful for concrete.

To give an idea of their related influence if 1 to 2 % muscovite mica brings down the strength of concrete- by 15% the same reduction may be expected in the presence of 10 to 15% of biotite type mica in concrete sand. In most situations both the varieties are present together and if the muscovite variety is' more prominent it poses more problems.

The amount of mica present in some of the Indian river sands is given in (Table 13).

It may be seen that in many cases the mica content could be as high as 12 % and the problem is often aggravated by the fact that the sand is of finer variety, the fineness modulus being as low as 0.6 or so in the case of sand from river Ganges.

IS : 383 does not specify upper limit of mica in fine aggregates; however, a cautionary note is added stating that the presence of mica in the fine aggregate has been found to reduce considerably the compressive strength of concrete and further investigations are underway to determine the extent of the deleterious effect of mica. It is advisable, therefore, to investigate the mica content of fine aggregate and make suitable allowances for the possible reduction in the strength of concrete or mortar.

IS : 383, . however, specifies tolerable limits 'of mica in terms of deleterious contents of alkali mica and coated grains taken together and these substances are limited to 2% by weight in sand. In the investigations done so far on the extent of deleterious effect of mica it has been found that under such situation mica content as low as even one percent has resulted in adverse effects on the properties of concrete and mortar, specially of muscovite variety. It has generally been seen that the workability is adversely affected when the mica content is of the order of 5% percent or more; above 8% the mortar mixes may indeed become unworkable or the water content may become too high.

The effect on workability and strength is generally more the leaner the mix; in one case with 2% mica content the compressive strength has been reported to go down by 11 to 19%  at the age of 7 days and. 24 to 30% at 28 days. With 12 percent mica content, the respective reductions were reported to be 40 to 50 percent at 7 days and 47 to 55% at 28 days.

The tests on durability have been on the basis of abrasion resistance, percentage loss of weight of mortar specimens exposed to 10 alternate cycles of wetting in concentrated sodium sulphate solution and drying and tests on coefficient of permeability. The results indeed are of relative nature. Nevertheless it can be concluded that mica content in excess of 6% may be considered to be deleterious for good abrasion resisting concrete.

The coefficients of permeability for typical concrete mixes were found to increase about 10 to 20 times as the mica content increased from 2.5% to 8.7%.

e) Salt contamination in sea dredged aggregates - Aggregates dredged from sea are also used for making concrete, though to a very limited extent. Properties peculiar to sea-dredged aggregates are that the salt primarily sodium chloride-(NaC1) and sea shell are present more frequently. Sea dredged aggregates may also contain organic impurities due to sea-weed, dead fish, coal, oil and disposal at sea.

The salt content in the sea-dredged aggregates is directly proportional to the moisture content at the time of dredging but the final salt content depends upon the amount of washing and the type of water used for washing.

Approval for the use of sea-dredged aggregates is not normally granted in the following situations":

1) Where calcium chloride is also used;

2) Where the cement to be used is other than ordinary Portland cement or rapid-hardening Portland cement or sulphate resisting Portland cement.

3) Where the concrete is to be pre-stressed or steam cured.

Where such approval is granted, the sodium chloride content of the fine and coarse aggregate shall not exceed, respectively, 0.10% and 0.03% by weight of dry aggregate. If either aggregate exceeds the limits, the total sodium chloride concentration from the aggregate shall not exceed 0.32% by weight of the cement in the mix.

In any case, the total amount of chloride and sulphate ions from all sources that is, aggregates, cement, water and admixtures, should not exceed the value specified in IS : 456  and IS : 1343.

Shell that is calcium carbonate, in sands has no harmful effect and quite good concrete can be produced if the shell is present in quantities up to 20%. But the hollow or large flat shells have detrimental effect as regards to durability of concrete as they adversely affect the quality and permeability of the concrete.

The shell content of the aggregates shall not exceed 2, 5, I5% respectively, for 40, 20, 10 mm nominal sizes of coarse aggregate and 30% for fine aggregate.

2.2.2.5 SOUNDNESS OF AGGREGATES -

Aggregate is said to be unsound when it produces excessive volume changes resulting in tile deterioration of concrete under certain physical conditions, such as freezing and thawing, thermal changes at temperatures above freezing and alternate wetting and drying.

Frost damage to concrete is distinct from the expansion as a result of chemical reactions between the aggregate particle and the alkalis in cement. Certain aggregates such as porous cherts, shales, some limestones particularly laminated limestones and some sandstones, are known to be susceptible to this frost damage. High absorption is a common characteristics of these rocks with a poor service-record, though many durable rocks may also exhibit high

absorption. Thus, critical conditions for frost damage are water content and lack of drainage. These characteristics of high absorption and lack of drainage depend upon the pore characteristics of aggregate. It has been reported in the case of some aggregates that porosities in the region of  8 micron (as determined by mercury-intrusion porosity test) appear to separate the high from the low durability aggregates.

IS : 2386 (Part V) specifies a test for determining soundness of aggregates.

This test popularly known as 'sulphate test' consists of subjecting a graded and weighed sample of aggregate to alternatively immersion in saturated solution of sodium sulphate or magnesium sulphate and oven drying and thus determining the weight loss after specified cycles of immersion and drying.

The overall mechanism of this sulphate test is yet not fully understood but probably this test involves a combination of actions that is, pressure of crystal growth, effects of heating and cooling, wetting and drying and pressure development due to migration of solution through pores but this test in no way is considered to simulate exposure of concrete to freezing and thawing due to complexity of field exposure and does not provide a reliable indication of field performance.

Consideration should always be given to the service record of the aggregate and this sulphate test serves only as a guide to the selection of aggregate. In a general sense, most aggregates with high soundness losses tend to have poor durability but there are numerous exceptions. It has been reported that few high soundness loss aggregates had excellent durability and soundness test failed to detect several aggregates of low durability". Thus reliance should always be based on the actual performance from the service-record. Dolar-Mantuani is of the opinion that the test does indicate weaknesses in aggregates. If the specification limits are used carefully together with sound engineering judgement, the test is certainly useful because of the relatively short time needed to perform it.

IS: 383 as a general guide restricts the average loss of weight after 5 cycles to 12 % when tested with sodium sulphate and 18% when tested with magnesium sulphate.

2.2.2.6 ALKALI-AGGREGATE REACTION -

This reaction takes place between the alkalis in the cement and the active siliceous constituents or carbonates of aggregates. Under most conditions, this reaction causes excessive expansion and cracking of concrete.

These deleterious reactions have been encountered in many parts of the World and in all climatic zones. The reactions are:

a) Alkali-silica reaction, and

b) Alkali-carbonate reaction.

A typical example of the effects of alkali silica reaction has been provided by the concrete of a military jetty in Cyprus, constructed in 1966. By 1972, widespread cracking and spalling of concrete were noticed and parts of the surface concrete crumbled and became friable, in some places to a depth up to 15 cm. Damage due to alkali-silica reaction had also been noticed in Tuscaloosa Lock and Dam, USA in 1952.

Dry dock in South Carolina also showed alkali-silica reaction cracking with quartz gravel aggregate in 1969.

The reactive material was metamorphic quartz or metamorphic and highly weathered quartz. In the year 1965-66, Lachswehr Bridge of northern Germany had severe damage due to this reaction.

The area of major concern in Germany is in two classes of reactive constituent in aggregates, opaline sandstone and reactive flint.

Certain dolomitic aggregates from Bahrain have been found to react deleteriously with cement paste. The reaction has been noticed to be promoted by the presence of gypsum and excess hydroxyl in the mixture and by the marked porosity of the aggregate.

In 1956-S7, expansive reactivity of concrete was noticed at Kingston, Ontario. A close look at culverts and bridges constructed only a few months earlier showed pattern of man cracking. Observations indicated that dolomitic limestone aggregate "from local quarries was an essential ingredient of the affected concrete. By prior geological exploration of existing and potential quarry sites and subsequently testing the different rocks for alkali-carbonate reactivity, it is possible to select non-reactive aggregates.

For constructing a major concrete highway, this procedure appeared to be realistic and economical considering the extra cost of using low alkali cement or bringing aggregates from outside.

Newton and Sherwood suggested the following measures to reduce expansion of concrete where it is essential to use alkali reactive carbonate aggregates, for economy:

a) If the aggregate is of a high degree of reactivity, dilution of reactive aggregates with a non-reactive one, reduction of cement alkalis or both, are necessary to eliminate cracking and reduce expansion significantly.

b) The limit of 0.60% alkali for low alkali cement does not appear to below enough to reduce the reaction with the highly reactive carbonate aggregates, even with aggregate dilution of 50%. To reduce the reaction to an acceptable level, the reduction of alkalis to 0.40 % may be necessary. If reduction of alkalis to this limit is not possible, then corresponding greater dilution of aggregate is required.

The reactive forms of silica are opal (amorphous), chalcedony (crypto crystalline fibrous) and tridymite (crystalline). The chemical composition, physical character and ·the reactive minerals" are given in Table 14.

The reaction minerals occurring in the reactive rocks are given in Table 15

Rocks' containing opal, chalcedony, chert, volcanic glass, crystobalite , tridyrnite or fused silica have shown to be reactive in many instances. As little as 0.5% of a defective aggregate is sufficient to cause considerable damage in concrete. The maximum expansion is produced when reactive aggregates make up about 4% of the total aggregates and this disadvantageous concentration is often referred to as the pessimum.

The actual reaction occurs between siliceous minerals in aggregates and the alkaline hydroxides derived from the alkalis (Na2O and K2O) in the cement. The result of reaction is alkali-silicate gel of 'unlimited swelling' type and because the gel is confined by the surrounding cement paste, internal pressure causes cracking and disruption.

The carbonate in aggregates is generally argillaceous dolomitic limestone. A wide range of carbonate rocks have been reported as being potentially reactive, ranging from pure limestone to pure dolomite.

One method of determining the potential alkali-aggregate reactivity is by 'mortar bar' test as given in IS: 2386 (Part VII)

The method of test covers the determination of reactivity by measuring the expansion developed by the cement aggregate combination in mortar bars during storage under prescribed conditions of test.

The test is more conclusive but has the disadvantage of requiring several months and also requiring that coarse aggregate be crushed rather than tested in its normal state. With larger specimens, however, uncrushed aggregate may be tested".

The second method of determining the potential reactivity of aggregates is the 'chemical method' as prescribed by IS : 2386 (Part VII).

This method of test determines the reactivity as indicated by the amount of reaction of the aggregate with a sodium hydroxide solution under controlled test conditions. The method has the advantage that it can be performed in 3 days, but for many aggregates the results are not conclusive", However, the illustration of division between innocuous and deleterious

aggregates (based on Mielenz and Witte's work) is reproduced from IS: 2386 (Part VII) in Fig- 12.

Both the test methods mentioned above do not always detect the alkali-carbonate reactivity but this may be detected by another test, in which concrete prisms made with questionable aggregates and a high alkali cement are exposed to an environment of 23°C and with 100 percent relative humidity and noting the amount of expansion.

Expansion of prisms made of questionable aggregate are then compared with those obtained on companion prisms of known sound limestone.

Air entrainment is also believed to be useful in counter acting alkali-aggregate reaction. Use of reactive aggregate itself in finely divided form is also known to inhibit destructive effects of alkali-aggregate reaction.

Detailed petrographic examination and X-ray identification are being used to examine suspect aggregates. But the conclusions are unreliable and all available past evidence must be taken into account when evaluating a new aggregate.                                                        

Detailed petrographic, mineralogical and chemical data compared with similar data already available for reactive aggregates may provide the most satisfactory means of identifying potentially reactive aggregates.

2.2.3 LIGHTWEIGHT AGGREGATES-

Lightweight aggregates can be either natural like diatomite, pumice, scoria, volcanic cinders, etc, or manufactured like bloated clay, sintered fly ash or foamed blast furnace slag. Lightweight aggregates are used in structural concrete and masonry blocks for reduction of the self weight of the structure.

The other usages of lightweight aggregate are for better thermal insulation and improved fire resistance. Unlike for normal weight aggregates from natural sources, there is no Indian standard specification for lightweight aggregates.

However, IS : 9142 covers specification for artificial lightweight aggregates for concrete masonry units, while IS: 2686 covers cinder aggregate for use in lime concrete for the manufacture of precast blocks.

The main requirement of lightweight aggregates is their low density; some specifications limit the bulk density to 1200 kg/m3 for fine aggregates and 960 kg/m3 for coarse aggregates for use in concrete. Both coarse and fine aggregates "may be lightweight. Alternatively, lightweight coarse aggregates can be used with natural sands.

 

The characteristics of lightweight aggregates which require consideration for use in structural

Concrete are as follows:

a)    Some lightweight aggregates may contain closed pores or voids in the material, apart from high water absorption of the order of 8 to 12 percent.

b)  Being artificially produced by sintering or pelletizing, most of the synthetic aggregates may have a smooth surface and rather regular shapes which may reduce the bond characteristics with the mortar and thereby result in lower compressive strength.

c)    If, during mixing the lightweight aggregates get crushed, the void structure is broken down resulting in a coarse surface texture which may lower the workability.

d)   The modulus of elasticity of concretes made with most of the lightweight aggregates is lower than the normal weight concrete, may be ½ to ¾. Creep and shrinkage of concrete are also greater (will vary from equal to about double) (Fig. 13) compared to those of normal weight concrete, having the same compressive strength".

These result in higher deflection of the structural members.

Because of high absorption, workable concrete mixes become stiff within a few minutes of mixing. Therefore, it is necessary to wet (but not saturate) the aggregates before mixing in the mixer. In the mixing operation, the required water and aggregate are usually premixed prior to addition of cement. As a rough guide, the extra water needed for lightweight aggregate concrete is about 6 kg/m3 of concrete to obtain a change in the workability of 25 mm slump. Rich mixes containing cement about 350 kg/m3 or more, are usually required to produce satisfactory strength of concrete. The concrete cover to reinforcement using light weight aggregates in concrete should be adequate.

Usually it is 25 mm more than for normal concrete. This increased cover is necessary, because of its increased permeability and also because concrete carbonates rapidly by which the protection to the steel by the alkaline lime is lost.

The sintered fly ash aggregate is rounded aggregate with a bulk density of about 1000 kg/m3. This type of aggregate is suitable for making masonry units as well as structural concrete. Concrete prepared from this aggregate has unit weight of 1200 to 1400 kg/m3.

Foamed blast furnace slag aggregate is produced with a bulk density varying between 300 and 1100 kg/m3, depending on the details of the cooling process and to a certain degree on the particle size and grading. Concrete made with this aggregate has a density of 950 to 1750 kg/m3.

Vermiculite is another artificial light weight aggregate which when heated to a temperature of 650 to 1000 deg.C expands to as many as 30 times its original volume by exfoliation of its thin plates. Thus the density of exfoliated vermiculite is only 60 to 130 kg/m3 and the concrete made with it is of low strength and exhibits high shrinkage but is used as an excellent heat insulator.