Light weight concrete with Applications
Concrete is the most widely used man-made construction material. It is obtained by mixing cement, water and aggregates (and sometimes admixtures) in required proportions. The mixture when placed in forms and. allowed to cure becomes hard like stone. The hardening is caused by chemical action between water and the cement and it continues for a long time, and consequently the concrete grows stronger with age. The hardened concrete may also be considered as an artificial stone in which the voids of larger particles (coarse aggregate) are filled by the smaller particles (fine aggregate) and the voids of fine aggregates are filled with cement. In a concrete mix the cement and water form a paste called cement water paste which in addition to filling the voids of fine aggregate acts as binder on hardening, thereby cementing the particles of the aggregates together in a compact mass.
The strength, durability and other characteristics of concrete depend upon the properties of its ingredients, on the proportions of mix, the method of compaction and other controls during placing, compaction and curing. The popularity of the concrete is due to the fact that from the common ingredients, it is possible to tailor the properties of concrete to meet the demands of any particular situation. The advances in concrete technology have paved the way to make the best use of locally available materials by judicious mix proportioning and proper workmanship, so as to produce concrete satisfying performance requirements.
CLASSIFICATION OF CONCRETE
As mentioned earlier the main ingredients of concrete are cement, fine aggregate (sand) and coarse aggregate (gravel or crushed rock). It is usual to specify a particular concrete by the proportions (by weight) of these constituents and their characteristics, e.g. a 1 : 2 : 4 concrete refers to a particular concrete manufactured by” mixing cement, sand and broken stone in a 1 : 2 : 4 ratio (with a specified type of cement, water-cement ratio, maximum size of aggregate, etc.). This classification specifying the proportions of constituents and their characteristics is termed prescripitive specifications and is based on the hope that adherence to such prescripitive specifications will result in satisfactory performance.
Alternatively, the specifications specifying the requirements of the desirable properties of concrete such as strength, workability, etc. are stipulated, and these are termed performance oriented specifica tions Based on these considerations, the concrete can be classified either as nominal mix concrete or designed mix concrete, Sometimes the concrete is classified into controlled concrete and ordinary concrete, depending upon the levels of control exercised in the works and the method of proportioning concrete mixes.
Accordingly, a concrete with ingredient proportions fixed by designing the concrete mixes with -preliminary tests are called controlled concrete, whereas ordinary concrete is one where nominal mixes are adopted. In IS: 456-1978 there is nothing like uncontrolled concrete: only the degree of control varies from very good to poor or no control. In addition to mix proportioning, the quality control includes selection of appropriate concrete materials after proper tests, proper workmanship in batching, mixing, transportation, placing, compaction and curing, coupled with necessary checks and tests for quality acceptance.
PROPERTIES OF CONCRETE
Concrete making is not just a matter of mixing ingredients to produce a plastic mass, but good concrete has to satisfy performance requirements in the plastic or green state and also the hardened state. In the plastic state the concrete should be workable and free from segregation and bleeding. Segregation is the separation of coarse aggregate and bleeding is the separation of cement paste from the main mass. The segregation and bleeding results in a poor quality concrete. In its hardened state concrete should be strong, durable. and impermeable; and it should have minimum dimensional changes,
Among the various properties of concrete, its compressive strength is considered to be the most important and is taken as an index of its overall quality. Many other properties of concrete appear to be generally related to its compressive strength. These properties will be discussed in detail later in the book.
GRADES OF CONCRETE
The concrete is generally graded according to its compressive strength. The various grades of concrete as stipulated in IS: 456-1978 and IS: 1343-1980 are given in Table 2.1. In the designation of concrete mix, the letter M refers to the mix and the number to the specified characteristic strength of 150 mm work cubes at 28 days, expressed in MPa (N/mm²). The concrete of grades M5 and M7.5 is suitable for lean concrete bases and simple foundations of masonry walls. These need not be designed. The concrete of grades lower than MIS is not suitable for reinforced concrete works and grades of concrete lower than M30 are not to be used in the prestressed concrete works.
ADVANTAGES OF CONCRETE
- Concrete is economical in the long run as compared to other engineering materials. Except cement, it can be made from locally available coarse and fine aggregates.
- Concrete possesses a high compressive strength, and the corrosive and weathering effects are minimal. When properly prepared its strength is equal to that of a hard natural stone.
- The green concrete can be easily handled and moulded into any shape or size according to specifications. The form work can be reused a number of times of similar jobs resulting in economy.
- It is strong in compression and has unlimited structural applications in combination with steel reinforcement. The concrete and steel have approximately equal coefficients of thermal expansion. The concrete is extensively used in the construction of foundations, walls, roads, airfields, buildings, water retaining structures, docks and harbours, dams, bridges, bunkers and silos, etc.
- Concrete can even be sprayed on and filled into fine cracks for repairs by the guniting process.
- The concrete can be pumped and hence it can be laid in the difficult positions also.
- It is durable and fire resistant and requires very little maintenance.
DISADVANTAGES OF CONCRETE
- Concrete has low tensile strength and hence cracks easily. Therefore, concrete is to be reinforced with steel bars or meshes.
- Fresh concrete shrinks on drying and hardened concrete expands on wetting. Provision for contraction joints has to be made to avoid the development of cracks due to drying shrinkage and moisture movement.
- Concrete expands and contracts with the changes in temperature. Hence expansion joints have to be provided to avoid the formation of cracks due to thermal movement.
- Concrete under sustained loading undergoes creep resulting in the reduction of prestress in the prestressed concrete construction.
- Concrete is not entirely impervious to moisture and contains soluble salts which may cause efflorescence.
- Concrete is liable to disintegrate by alkali and sulphate attack.
- The lack of ductility inherent in concrete as a material is disadvantageous with respect to earthquake resistant design.
MATERIAL OF CONCRETE
Cement is a well-known building material and has occupied an indispensable place in construction works. There is a variety of cements available in the market and each type is used under certain conditions due to its special properties. The cement commonly used is portland cement, and the fine and coarse aggregates used are those that are usually obtainable, from nearby sand, gravel or rock deposits. In order to obtain a strong, durable and economical concrete mix, it is necessary to understand the characteristics and behaviour of the ingredients.
Although all materials that go into a concrete mixture are essential, cement is by far the most important constituent because it is usually the delicate link in the chain. The function of cement is first, to bind the sand and coarse aggregates together, and second, to fill the voids in between sand and coarse aggregate particles to form a compact mass. Although cement constitutes only about 10 per cent of the volume of the concrete mix, it is the active portion of the binding medium and the only scientifically controlled ingredient of concrete.
Cement is an extremely ground material having adhesive and cohesive properties, which provide a binding medium for the discrete ingredients. It is obtained by burning together, in a definite proportion, a mixture of naturally occurring argillacious (containing alumina) and calcareous (containing calcium carbonate or lime) materials to a partial fusion at high temperature (about 1450°C). The product obtained on burning, called clinker, is cooled and ground to the required fineness to produce a material known as cement. Its inventor, Joseph Aspdin, called it portland cement because when it hardened it produced a material resembling stone from the quarries near Portland in England.
Types of Cements
i.Rapid-hardening Portland Cement
iii.Low-heat Portland Cement
v.High-strength Portland Cement
vi.Super Sulphate Cement
ix.White Portland Cement
x.Coloured Portland Cement
Aggregates are generally cheaper than cement and impart greater volume stability and durability to concrete. The aggregate is used primarily for the purpose of providing bulk to the concrete. To increase the density of the resulting mix, the aggregate is frequently used in two or more sizes. The aggregates provide about 75% of the body of the concrete and hence its influence is extremely important.
Aggregate was originally viewed as an inert, inexpensive material dispersed throughout the cement paste so as to produce a large volume of concrete. In fact, aggregate is not truly inert because it’s physical, thermal and, sometimes, chemical properties influence the performance of concrete, for example, by improving its volume stability and durability over that of the cement paste. From the economic viewpoint, it is advantageous to use a mix with as much aggregate and as little cement as possible, but the cost benefit has to be balanced against the desired properties of concrete in its fresh and hardened state.
Classification of Aggregate
1.Classification according to the Geological Origin:-
2.Classification according to size:-
3.Classification according to shape:-
iv.Flaky and elongated aggregate
4.Classification based on unit weight:-
iv.Bloated clay aggregate
Generally, cement requires about 3/10 of its weight of water for hydration. Hence the minimum water-cement ratio required is 0.35. But the concrete containing water in this proportion will be very harsh and difficult to place. Additional water is required to lubricate the mix, which makes the concrete workable. This additional water must be kept to the minimum, since too much water reduces the strength of concrete. The water-cement ratio is influenced by the grade of concrete, nature and type of aggregates, the workability and durability.
If too much water is added to concrete, the excess water along with cement comes to the surface by capillary action and this cement-water mixture forms a scum or thin layer of chalky material known as laitance. This laitance prevents bond formation between the successive layers of concrete and forms a plane of weakness. The excess water may also leak through the joints of the formwork and make the concrete honeycombed. As a rule, the smaller the percentage of water, the stronger is the concrete subject to the condition that the required workability is allowed for.
Effect of impurities in water on properties of concrete:-
2.Miscellaneous inorganic salts
3.Salts in sea water
4.Acids and alkalies
BS 2787: 1956 ‘Glossary of term for concrete and reinforced concrete’ gives the following definition for the term ‘admixture’, with ‘additive’ given as an alternative term with the same definition:
‘A material other than coarse or fine aggregate, cement of water added in small quantities during the mixing of concrete to produce some desired modification in one or more of its properties’.
Admixtures are the materials other than the basic ingredients of concrete, cement, water, and aggregates. The use of admixture should offer an improvement not economically attainable by adjusting the proportions of cement and aggregates, and should not adversely affect any property of the concrete. Admixtures are no substitute for good concreting practice. An admixture should be employed only after an appropriate evaluation of its effects on the particular concrete under the conditions in which the concrete is intended to be used. It is often necessary to conduct tests on the representative samples of the materials for a particular job under simulated job conditions in order to obtain reliable information on the properties of concrete containing admixtures.
The admixtures ranging from addition of chemicals to waste materials have been used to modify certain properties of concrete. The properties commonly modified are that rate of hydration or setting time, workability, dispersion and air-entrainment. The admixture is generally added in a relatively small quantity.
FUNCTIONS OF ADMIXTURES
1.To accelerate the initial set of concrete, i.e. to speed up the rate of development of strength at early ages,
2.To retard the initial set,
3.To increase the strength of concrete,
4.To improve the workability,
5.To reduce the heat of evolution,
6.To increase the durability of concrete, i.e. its resistance to special conditions of exposure, like repeated freezing and thawing cycles,
7.To control the alkali-aggregate expansion, to decrease the capillary flow of water through concrete and to increase its impermeability to liquids,
8.To improve the penetration and pumpability of concrete,
9.To reduce the segregation in grout mixtures,
10.To increase the bond between old and new concrete surfaces,
11.To increase the bond of concrete to the steel reinforcement,
12.To inhibit the corrosion of concrete,
13.To increase the resistance to chemical attack,
14.To produce cellular concrete,
15.To produce coloured concrete or mortar for coloured surfaces,
16.To produce concrete of fungicidal, germicidal and insecticidal properties,
17.To produce nonskid surfaces, and
18.To decrease the weight of concrete per cubic metre.
SPECIAL CONCRETE AND CONCRETING TECHNIQUES
Notwithstanding its versatility, cement concrete suffers from several drawbacks, such as low tensile strength, permeability to liquids and consequent corrosion of reinforcement, susceptibility to chemical attack, and low durability. Modifications have been made from time to time to overcome the deficiencies of cement concrete yet retaining the other desirable characteristics. Recent developments in the material and construction technology have led to significant changes resulting in improved performance, wider and more economical use.
The improvements in performance can be grouped as:
i.Better mechanical properties than that of conventional concrete, such as compressive strength, tensile strength, impact toughness, etc.
ii.Better durability attained by means of increased chemical and freeze-thaw resistances,
iii.Improvements in selected properties of interest, such as impermeability, adhesion, thermal insulation, lightness, abrasion and skid resistance, etc.
4.Waste material based concrete
6.Shotcrete or guniting
9.Polymer concrete composites (PCCs)
10.Sulphur concrete and Sulphur-infiltrated concrete
11.Jet (Ultra-rapid hardening) cement concrete
Unfortunately, there is no acceptable test which will measure directly the workability as defined earlier. The following methods give a measure of workability which is applicable only with reference to the particular method. However, these methods have found universal acceptance and their merit is chiefly that of simplicity of operation with an ability to detect variations in the uniformity of a mix of given nominal proportions.
The mould for the slump test is a frustum of a cone, 305 mm (12 in.) high. The base of 203 mm (8 in.) diameter is placed on a smooth surface with the smaller opening of 102 mm (4 in.) diameter at the top, and the container is filled with concrete in three layers. Each layer is tamped 25 times with a standard 16 mm diameter steel rod, rounded at the end, and the top surface is struck off by means of a screeding and rolling motion of the tamping rod. The mould must be firmly held against its base during the entire operation; this is facilitated by handles or foot-rests brazed to the mould.
Immediately after filling, the cone is slowly lifted, and the unsupported concrete will now slump – hence the name of the test. The decrease in the height of the centre! of the slumped concrete is called slump, and is measured to the nearest 5 mm.
·COMPACTING FACTOR TEST
The degree of compaction, called the compacting factor, is measured by the density ratio, i.e. the ratio of the density actually achieved in the test to the density of the same concrete fully compacted.
The upper hopper is filled with concrete, this being placed gently so that, at this stage, no work is done on the concrete to produce compaction. The bottom door of the hopper is then released and the concrete falls into the lower hopper. This hopper is smaller than the upper one and is, therefore, filled to overflowing and thus always contains approximately the same amount of concrete in a standard state; this reduces the influence of the personal factor in filling the top hopper. The bottom door of the lower hopper is released and the concrete falls into the cylinder. Excess concrete is cut by two floats slid across the top of the mould, and the net mass of concrete in the known volume of the cylinder is determined.
The name Vebe is derived from the initials of V. Bahrner of Sweden who developed the test. The test is covered by BS 1881: Part 104: 1983 and is referred to also in ACI Standard 211.3-75 (revised 1980). The slump cone is filled in the standard manner, removed, and a disc-shaped rider (weighing 2.75 kg (6Ib)) is placed on top of the concrete. Compaction is achieved using a vibrating table with an eccentric weight rotating at 50 Hz so that the vertical amplitude of the table with the empty cylinder is approximately ±0.35 mm (±0.014 in.).
Compaction is assumed to be complete when the transparent rider is totally covered with concrete and all cavities in the surface of the concrete have disappeared. This is judged visually, and the difficulty of establishing the end point of the test may be a source of error. To overcome it an automatically operated device for recording the movement of the plate against time may be fitted, but this is not a standard procedure.
FLOW TABLE TEST
The apparatus consists essentially of a wooden board covered by a steel plate with a total mass of 16 kg (about 35 lb). This board is hinged along one side to a base board, each board being a 700 mm (27.6 in.) square. The upper board can be lifted up to a stop so that the free edge rises 40 mm (1.6 in.). Appropriate markings indicate the location of the concrete to be deposited on the table.
The table top is moistened and a frustum of a cone of concrete, lightly tamped by a wooden tamper in a prescribed manner, is placed using a mould 200 mm (8 in.) high with a bottom diameter of 200 mm (8 in.) and a top diameter of 130 mm (about 5 in.). Before lifting the mould, excess concrete is removed, the surrounding table top is cleaned, and after an interval of 30 sec. the mould is slowly removed. The table top is lifted and allowed to drop, avoiding a significant force against the stop, 15 times, each cycle taking approximately 4 sec.
In consequence, the concrete spreads and the maximum spread parallel to the two edges of the table is measured. The average of these two values, given to the nearest millimetre, represents the flow. A value of 400 indicates a medium workability and 500 a high workability. Concrete should at this stage appear uniform and cohesive or else the test is considered inappropriate for the given mix. Thus the test offers an indication of the cohesiveness of the mix.
BALL PENETRATION TEST
This is a simple field test consisting of the determination of the depth to which a 152 mm (6 in.) diameter metal hemisphere, weighing 14 kg (30 lb), will sink under its own weight into fresh concrete. A sketch of the apparatus, devised by J. W. Kelly and known as the Kelly ball.
The use of this test is similar to that of the slump test, that is for routine checking of consistence for control purposes. The test is covered by ASTM Standard C 360-82 and is rarely used in the UK. It is, however, worth considering the Kelly ball test as an alternative to the slump test, over which it has some advantages. In particular, the ball test is simpler and quicker to perform and, what is more important, it can be applied to concrete in a wheelbarrow or actually in the form. In order to avoid boundary effect, the depth of the concrete being tested should be not less than 200mm (8 in), and the least lateral dimension 460mm