Here is a list of seven special types of concrete: 1. Polymer Concrete 2. Roller Compacted Concrete 3. Fibre Reinforced Concrete 4. Saw Dust Concrete 5. Preplaced Aggregate Concrete 6. Vacuum-Processed Concrete 7. Colcrete.
Type # 1. Polymer Concrete:
Polymer Concrete may be classified into the following four groups:
i. Polymer Cement Concrete (PCC).
ii. Polymer Concrete (PC).
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iii. Partially impregnated and surface coated polymer concrete.
i. Polymer Cement Concrete (PCC):
Polymer cement concrete can be prepared by mixing cement, aggregates, water and monomers as usual in ordinary concrete. The plastic mixture is cast in moulds, cured, dried and polymerized.
Following monomers may be used in Polymer Cement Concrete (PCC):
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(a) Epoxy-styrene
(b) Polyster-styrene
(c) Furans, and
(d) Vinylidene chloride.
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However polymer cement concrete produced in this way has shown a very modest improvement in strength and durability. RUSSIAN authors have reported recently that superior polymer cement concrete can be produced by using Furfuryl alcohol and Aniline hydrochloride in the wet mix. This product has been found specially dense, non-shrinking and high resistant to corrosion, low permeability, high resistant to vibrations and axial extension etc.
Whereas U.S. researchers have found that the use of epoxy resin has produced polymer cement concrete having some superior characteristics over ordinary concrete. The PCC using polymer latex has given a tensile strength of 5.8 MPa with w/c ratio of 0.25 compared with control specimen of 4.4 MPa. The increase in tensile strength is very modest.
Uses:
Polymer cement concrete can be used for flooring in deck steel over bridges, food processing and chemical industries, wear resistant floors etc. It is also useful for repair of sea defence structures due to early development of strength.
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ii. Polymer Concrete (PC):
It is a composite in which cement water matrix of cement concrete is replaced by a polymer binder. The main technique in producing polymer concrete is to minimise the volume of voids in the aggregate mass, so that minimum quantity of binder polymer is needed to bind the aggregates. This can be achieved by properly grading and mixing the aggregates to obtain the maximum density and minimum volumes of voids.
The graded aggregates are pre-packed in a form and vibrated. After this, the monomer is diffused through the aggregates and polymerisation is initiated by chemical or radiation process. To improve the bond between the monomer and aggregates an adhesive agent is added to the monomer. In case polyester resins are used then no polymerisation is needed.
Polyester resin concrete with binder content varying from 20 to 25% has shown tensile strength in the range of 9 to 10 MPa at 7 days. The compressive strength of P.C. has been found of the order of 140 MPa with a short curing period.
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Necessity for Developing the Polymer Concrete:
The most important reason for the development of polymer concrete is to overcome the short comings of the conventional concrete under the following situations:
i. On curing the alkaline port-land cement concrete forms internal voids. In these voids water can be entrapped, which on freezing expands, resulting cracks in concrete.
ii. The alkaline port-land cement concrete is easily attacked by chemically aggressive (reactive) material, which causes rapid deterioration of the concrete.
iii. The polymer concrete can be made compact with minimum voids and hydrophobic (water replant) and resistant to chemical attack.
However polymer concrete suffers from certain short comings. Its main short coming is its brittleness. It has been found that the tensile strength and toughness of polymer concrete can be increased by the dispersion of fibre reinforcement. The use of fibrous polyester concrete (FPC) in the compressive region of reinforced concrete beams provides a high strength and ductile concrete at reasonable cost.
Also the polyester concrete is visco elastic in nature and will fail under sustained compressive loading stress at 50% higher than the ultimate stress. Thus the polyester concrete can be used for structures with a high ratio of live load to dead load and for composite structures in which polymer concrete may relax during long term loading.
iii. Partially Impregnated and Surface Coated (SC) Concrete:
In situations where in addition to strength increase, the major requirement is the surface resistance to chemical and mechanical attack, the partial impregnation may be sufficient. Partial impregnation has been found quite effective in the increase of the strength of the original concrete.
The partially impregnated concrete can be produced by soaking the initially dried specimens in liquid monomer like methyl methacrylate and then sealing them by keeping under hot water at 70°C to prevent or minimise the loss due to evaporation. The polymerisation can be done by using thermal catalytic method. Benzoyal Peroxide is added as a catalyst to the monomer.
The depth of penetration of the monomer depends upon the following factors:
i. Pore structure of hardened and dried concrete
ii. The viscosity of the monomer, and
iii. The duration of soaking of specimens in the monomer.
Use of Partially Impregnated and Surface Coated Concrete:
The main use of this concrete has been done in improving the durability of concrete where the abrasive wear, freezing and thawing, spalling and corrosion of reinforcement are the main cause of deterioration as in bridge decks. Bridge deck deterioration is a major problem everywhere. Excellent penetration can be achieved by ponding the monomer on the concrete surface.
While monomer is ponded on the surface due care should be taken to prevent the evaporation of monomer. By soaking a 5 cms thick slab for 25 hours with Methyl methacrylate (MMA), the polymer was found to penetrate upto 2.5 cm depth i.e., upto 50% depth of the slab. By surface treating and partial impregnation of the concrete surface, its tensile and compressive strength, modulus of elasticity and resistance to acid attack can be increased significantly.
Application of Monomers in the Field:
The application of monomers in the field like bridge decks, impregnation is more difficult than laboratory application and poses more problems.
A typical procedure for surface treatment in the field may be adopted as follows:
i. First dry the surface for several days with-electric heating blanket.
ii. After drying the surface, remove the heating blanket and cover the slab with over dried light weight aggregate at the rate of 0.64 m3 per 100 square metre of surface.
iii. Initially apply monomer system at the rate of 2.0 to 3.0 litre per square metre of surface.
4. To retard the evaporation of monomer cover the surface with polythene.
5. To prevent the temperature rise, which may initiate the polymerisation prematurely, shade the surface. Premature polymerisation will reduce the penetration of monomer into the concrete.
6. To keep the aggregates moist for the minimum soak time of 8 hours, additional monomer should be added into the concrete.
7. Now apply heat to polymerise the monomer. For this purpose steam, hot water or heating blanket may be used.
Monomer System to be used for the above Purpose:
Following monomer system is suggested for this purpose:
(a) Methylmethacrylate (MMA), 1% Benzoyl Peroxide (BP), 10% Trimetholpropanetrimethacrylate (TMPTMA). It acts as cross linking agent which helps in polymerisation at low temperature of 52°C and BP acts as catalyst.
(b) Isodecyl metha crylate (IDMA), 1% BP. 10% TMPTMA
(c) Isobutyl metha crylate (IBMA), 1% BP, 10% TMPTMA
Type # 2. Roller Compacted Concrete:
Roller compacted concrete, abbreviated as R.C.C. is of recent origin. It is also known as lean rolled concrete. Roller compacted concrete is almost a dry concrete having no slump. This concrete is consolidated by vibratory rollers. The applications of this concrete are mainly in the construction of dams, rapid placement of single layer paving for highways and runways and also for multi-layer placement for foundations.
Mix:
The cement content of this concrete varies from 60 to 360 kg/m3 of concrete. To minimize the heat of hydration in massive concrete placements, and also to minimize cracking, the cement content should be kept as low as possible and the maximum possible size of aggregate should be used. The use of minimum quantity of cement and large size aggregate will help in controlling the segregation of the concrete.
The use of pozzolana often has been found very economical than cement alone. The use of pozzolana is also advantageous in reducing the heat of hydration. In some cases high value fly ash has been added to the extent of 60 to 80% of the volume of cementitious materials in the construction of dams.
The mixture of cementitious material, aggregate and water is mixed in a conventional batch mixer or in any other suitable mixer.
Placement:
To allow complete compaction, the roller compacted concrete is placed in thin layers. The optimum layer thickness should be between 20 to 30 cms. To ensure adequate bond between the new and old layer called as cold joint, segregation of concrete must be avoided. To achieve this objective a high plasticity bedding mix must be used at the start of the placement. The concrete can also be used for continuous placement without cold joints. The compressive strength of roller compacted concrete of the order of 7 MPa to 30 MPa has been obtained.
For effective compaction, the roller compacted concrete must be dry enough to support the mass of the vibrating equipment, but wet enough to allow the cement paste to be evenly distributed throughout the mass during the mixing and consolidation processes.
The first Roller compacted concrete dam was started in Japan during 1978 and was completed in 1980. Since then many other dams were built by this technique in different parts of the world. By the end of 1985, seven Roller compacted concrete dams were completed and by the end of 1992 the number of such dams rose to 96 in 18 different countries, mainly in Japan, U.S.A. and Spain etc.
In India though no dam has been built by this techniques but the technique has been used in the road construction. The roller compacted concrete has been used as a base concrete in the construction of Delhi-Mathura Concrete road Project. Similarly RCC (Roller Compacted Concrete) has also been used as a base course concrete in Pune-Mumbai express high way construction.
In both the projects the roller compacted concrete is called as “Dry lean concrete”. The thickness of the concrete was kept 15 cm and the grade of concrete was M10. The concrete was thoroughly compacted by vibratory rollers, over which pavement quality concrete of grade M40 was placed. The thickness of pavement layer was kept as 35 cm.
Type # 3. Fibre Reinforced Concrete:
Plain concrete has a very low tensile strength, limited ductility and little resistance to cracking. Internal micro-cracks are inherently present in the concrete. The poor tensile strength of concrete is due to the development of such micro-cracks. These micro-cracks eventually lead to brittle fracture of the concrete.
Thus the purpose of reinforcing the cement based matrix with steal or fibres is to increase the tensile strength by delaying the growth of cracks and to increase the toughness by transmitting stress across a cracked section so that much larger deformation is possible beyond the peak stress than without fibre reinforcement.
Thus fibre reinforced concrete may be defined as concrete made with hydraulic cement, containing fine or coarse and fine both aggregate and discontinuous discrete fibres. The fibre reinforced concrete generally has higher cement content, and higher fine aggregate content and smaller size of coarse aggregate in comparison with conventional concrete.
In plain concrete and similar brittle materials micro cracks (structural cracks) develop even before applying load. These cracks develop due to drying shrinkage or volume changes. The initial width of these cracks is very small and seldom exceeds a few microns, but their other dimensions may be much higher.
After loading, these micro cracks propagate and open up. Due to the stress concentration, additional cracks form in place of these minor cracks. These structural cracks spread very slowly due to the resistance at inter surface of cement paste and bigger aggregate particles and changes of direction bypassing the more resistant particles in matrix. Actually the main cause of development of such micro cracks is of inelastic deformations in concrete.
Fibres Used:
The fibres can be classified into two groups as follows:
1. Natural materials as asbestos, cellulose, sisal.
2. Artificial manufactured products as steel, glass, carbon and polymer etc.
Each type of fibre has its own characteristic properties and limitations. Hence all of them cannot be used effectively and economically.
Quantity of Fibres:
The quantity of fibres used in fibre reinforced concrete is small usually 1 to 5% by volume.
Properties of Fibres:
To make the fibres effective as reinforcement, the tensile strength, modulus of elasticity and the elongation of fibres at failure should be substantially higher than the corresponding properties of the matrix. The typical properties of some fibres are shown in the following Table 24.4.
Further the creep of fibres used as reinforcement should be very low. Their Poisson’s ratio should be similar as that of the matrix.
Some other significant characteristics of the fibres are as follows:
1. Aspect ratio. It is the ratio of length to mean diameter of the fibre. The stress bearing capacity of fibre depends upon its aspect ratio. Typical aspect ratio varies from 30 to 150.
2. Shape
3. Surface texture
4. Length, and
5. Structure.
It has been observed that aspect ratio influences the properties and behaviour of the composite to a great extent. The increase in aspect ratio upto 75, increases the ultimate strength of concrete linearly and relative toughness, but beyond this value of aspect ratio both ultimate strength and relative toughness decrease. Table 24.5 Shows the effect of aspect ratio on strength and toughness.
Description of Different Fibres Used:
1. Steel Fibre:
It is the most commonly used fibre. Generally round fibres of diameter ranging from 0.25 to 0.75 mm are used. The steel fibre is likely to get rusted and lose some of its strength, however rusting takes only at the surface. A significant improvement in impact, fatigue and flexural strength of concrete has been observed by the use of steel fibre. It has been used extensively in various types of structures, specially for over lays of roads, air field pavements, bridge decks etc. It has also been used successfully for the construction of thin shells and plates.
2. Asbestos:
It is a natural fibre and has proved to be the most successful of all fibres as it can be mixed with port-land cement. The tensile strength of asbastos has been found varying between 560 to 980 N/mm2.
The flexural strength of the composite product known as asbestos cement has been found considerably higher than the port-land cement paste.
3. Polypropylene and Nylon Fibres:
These fibres possess very high tensile strength. These fibres are found useful to increase the impact strength of the composite. They have low modulus of elasticity and higher elongation, which do not contribute to its flexural strength.
4. Glass Fibre:
It has been introduced recently for the production of fibre concrete. It has been found to have very high tensile strength varying from 1020 to 4080 N/mm2. Originally glass fibre used along with port-land cement was found to be affected by alkaline nature of cement. Thus an alkali resistant glass fibre has been developed. The trade name of this glass fibre is “CEMFILL”. Concrete made with alkali resistant glass fibre showed considerable improvement in its durability in comparison to the conventional E, glass fibre.
5. Carbon Fibres:
Perhaps the carbon fibres possess very high tensile strength and Young’s modulus. Their tensile strength has been observed of the order of 2110 to 2815 N/mm2. The concrete made with carbon fibre as reinforcement showed very high flexural strength and modulus of elasticity. It has been found quite durable. It can be used for the construction of protective covering panels and shells. It has bright future.
Factors Affecting the Properties of Fibre Reinforced Concrete:
The fibre reinforced concrete is a composite material consisted of fibres in cement matrix either in the orderly manner or randomly distributed manner.
Thus its properties would depend upon the efficient transfer of stress between the fibres and the matrix, which is dependent on the following factors:
1. Type of fibre and its geometry
2. Orientation and distribution of the fibre
3. Mixing and compaction of concrete
4. Amount, shape and size of aggregate.
These factors are discussed briefly below:
1. Type of Fibre and its Geometry:
For effective and efficient transfer of stress, the modulus of elasticity of the matrix must be much lower than that of fibre used. The fibres of low modulus of elasticity as polypropylene and nylon impart greater degree of toughness and resistance to impact as they have the capacity of absorption of large amount of energy, but they do not contribute to the improvement of strength. On the other hand, high modulus fibres as steel, glass and carbon contribute more to the improvement of strength and stiffness of concrete.
The interfacial bond between the matrix and the fibres also determine the effectiveness of transfer of stress from the matrix to the fibre. For improving the tensile strength of the composite a good bond is essential. The interfacial bond could be improved by providing larger area of contact, improving the degree of gripping and frictional properties by treating the steel fibres with acetone or sodium hydroxide.
Volume of Fibres:
Usually the quantity of fibres is used between 1 to 5% by volume. The strength of the composite largely depends on the quantity of fibres used in it. The effect of volume between 0 to 1.25% by volume on strength and toughness of fibre reinforced composite is shown in Fig. 24.4.
The effect on the toughness is more than on strength. From Fig. 24.5 it can be seen that the increase in tensile strength and toughness of the composite is liner with the increase in volume of fibres upto 1.5%. The use of higher percentage of fibre is likely to cause segregation and harshness of concrete and mortar.
2. Orientation of Fibres:
It has been observed that the orientation of the fibre relative to the plane of crack in concrete influences the reinforcing capacity of the fibre. The maximum benefit occurs when the fibre is unidirectional and parallel to the tensile stress. Experiments have shown that the fibres aligned parallel to the applied load offered more tensile strength and toughness than randomly or perpendicular distributed fibres.
3. Workability and Compaction of Concrete:
The addition of fibres decreases the workability of the concrete considerably. The workability of fibre reinforced concrete decreases with the increase in fibre content and its aspect ratio. The decrease in workability effects the consolidation of concrete to a great extent. Even prolonged external vibration has no effect at this low workability. The fibre volume at which this situation arises depends upon the aspect ratio of the fibre i.e. on its length and diameter.
Another effect of low workability is non-uniform distribution of fibres. The workability can be increased by increasing the water-cement ratio or by the use of some water reducing admixtures.
4. Size of the Coarse Aggregate:
In order to avoid appreciable reduction in the strength of the composite, many researchers have recommended the maximum size of coarse aggregate as 10 mm. In fact fibres also act as aggregate. Though, the geometry (shape) of fibroses simple, but their influence on the properties of the fresh concrete is complex. The inner particle friction between fibres and between aggregates and fibres controls the orientation and distribution of the fibres and thus the properties of the composite. The use of friction reducing admixtures and admixtures which can improve the cohesiveness of the mix will be useful.
Proportion of Fibre Reinforced Concrete:
A typical mix proportion of fibre reinforced concrete is shown below:
Mixing of the Mix:
To avoid balling (bundling) of fibres, segregation and difficulty in mixing the materials uniformly careful handling is necessary. Increase in the aspect ratio, volume percentage of fibres and size and quantity of coarse aggregate all intensify the balling tendency and difficulties. Steel fibre content more than 2% by volume and an aspect ratio more than 100% is difficult to mix. It is important to note that the fibres are dispersed uniformly throughout the mix.
The uniform dispersement of fibres can be achieved by the addition of fibres before the water is added. While mixing in laboratory mixer, the addition of fibres through a wire mesh basket will help the even distribution of fibres. For the use in the field some other suitable method may be adopted,
Methods of Determining Workability:
For determining the workability of fibre concrete usual methods are slump and Vee Bee methods, but slump test is not always a good indicator of workability. For this reason a new method known as inverted slump test has been developed to find the workability of fibrous mixes. The time taken for loosely filled inverted standard cone into a standard container by internal vibration is the measure of the fluidity of the mix.
Some researchers have suggested cone penetration test for measuring the workability of fibre reinforced concrete. They have reported that the depth of penetration in mm of a metallic cone with an apex angle of 30° and weight of 40N has been found to give representative workability for normal range mixes. The depth of penetration for normal range of mixes has been found to vary from 30 mm to 50 mm.
Factors Affecting Workability:
Aspect ratio and fibre volume concentration predominantly affect the workability of fibre reinforced concrete. A minimum fibre volume concentration is known as critical concentration and is needed to increase its strength. The critical concentration is inversely proportional to the aspect ratio. For aspect ratio l/d = 100, a volume concentration of 0.5% for flexural strengthening and 1.7% for tensile strengthening is required.
Properties of Fibrous Concrete:
Following properties of fibrous concrete have been observed:
1. It has increased static and dynamic strength.
2. It has higher energy absorbing characteristics.
3. It has better fatigue strength.
4. It has better isotropic properties than normal reinforced concrete due to uniform dispersion of fibres throughout the concrete.
Application or Uses of Fibre Concrete:
There are many uses of fibrous concrete as follows:
1. The glass fibre reinforced concrete is used for precast, flat or shaped decorative panels and facings for architectural and outer coverings known as cladding.
2. The asbestos cement is cheaper and is used for the production of flat and corrugated sheets. The asbestos cement is fire resistant also. Thus it has been widely used for covering the cheap houses, garages, fire resistant panels and pipes etc.
3. Polypropylene fibre concrete. The modulus of elasticity of polypropylene fibres is low under normal rate of loading but it increases subsequently under impact loading. Thus this concrete is used for making casing for conventionally driven reinforced concrete piles.
4. Steel and glass fibre concrete. As the flexural, fatigue and impact strength of these fibres is good, they are used as the overlays (bases) to concrete pavements and airfield pavements. Steel fibre can also be used in shot crete. However the steel fibres may get corroded especially near or at the surface exposed to weather.
Other general uses of fibre reinforced concrete products are permanent and reusable form works and protection and strengthening of the skin of concrete members. In general it can be said that fibre reinforced concrete can be used for all types of works as road pavements, floorings, bridge decks, canal lining, beams, boats, roof and wall panels, manhole covers etc. It is resistant to cavitation or erosion effect, hence used in hydraulic structures.
Type # 4. Saw Dust Concrete:
Sometimes in partition walls and some types of roofs, nailing properties of the concrete are essential. Nailing is that property of the material by virtue of which nails can be driven in it and are firmly held. This type of concrete can be obtained by using saw dust as aggregate.
Saw dust concrete consists roughly equal parts by volume of port-land cement, sand and saw dust with water sufficient to give a slump of 25 to 50 mm i.e. cement to saw dust may be 1:2 to 1:3 by vol. Such a concrete bonds well with ordinary concrete and is a good insulator. The saw dust should be clean and without a large amount of bark.
In order to avoid adverse effect on setting, hydration, and rotting of saw dust and to reduce moisture movement, chemical treatment of saw dust is advisable. To offset the delay in setting and hardening addition of about 5% calcium hydroxide has been found effective. The density of saw dust concrete varies from 652 to 1600 kg/m3.
As the properties of this concrete depend on the quality of saw dust, use of trial mix is recommended. The size of saw dust may vary from 1.18 mm to 6.3 mm. Saw dust concrete can be used in the manufacture of precast concrete products, joint less flooring and roofing tiles etc.
Type # 5. Preplaced Aggregate Concrete:
This type of concrete is produced in two operations. In the first operation, coarse aggregate is placed in the form and compacted by form vibrators. In the second operation the voids from 30 to 35% of the overall volume to be concreted are filled with cement mortar. Preplaced aggregate concrete is also known as pre-packed concrete or intrusion concrete or grouted concrete.
Aggregate used. In this concrete, usually gap graded aggregate is used. Typical grading’s of coarse and fine aggregates are shown in Table 24.6 and 24.7 respectively. The coarse aggregate must be free from dust and dirt as these will impair the bond between cement paste and aggregate. The coarse aggregate should also be thoroughly wetted or inundated before the mortar is intruded, but water should not be allowed to stand for long.
Mortar:
Mortar may consist two parts of port-land cement, one part of very finely divided and highly active pozzolana, and three to four parts of fine sand with sufficient water to form fluid mixture. Pozzolana is added to improve fluidity of the mortar. It also reduces bleeding and segregation of the mortar. An intrusion aid is also added to improve the fluidity of the mortar and to hold the solid constituents in suspension and delaying the stiffening of the mortar. The mortar is made of a thick cream consistency.
The mortar is pumped under pressure through about 35 mm diameter perforated or slotted pipe spaced at 2.0 m apart starting from the bottom of the mass. These pipes are gradually withdrawn after the grouting is complete.
Preplaced concrete is economical in cement as about 120 kg/m3 to 150 kg/m3 of cement is used, but the strength obtained is low of the order of 200 kg/cm2 only due to high water/cement ratio.
Advantages:
Following are the advantages and disadvantages of preplaced concrete:
1. Preplaced concrete can be placed in locations which are not easily accessible for ordinary concreting.
2. It can be placed in sections containing a large number of embedded items such as in nuclear shields.
3. Segregation of heavy and coarse aggregate is eliminated.
4. Pumping over long distances is possible.
5. Under water construction is also possible by its use.
6. Its drying shrinkage is lower than that of ordinary concrete.
7. Its use is preferable in mass concrete construction where temperature has to be controlled.
8. It is also used to provide an exposed aggregate finish.
Disadvantages:
1. It develops shrinkage cracks.
2. It needs special skill and experience in application, hence costly.
Type # 6. Vacuum-Processed Concrete:
The problem of obtaining high workability with a minimum water/cement ratio is solved by the use of vacuum processing of freshly placed concrete. In this method a part of water mixed with concrete for suitable workability is sucked back by the pump after the concrete is laid in position. This concrete is known as vacuum concrete.
Procedure:
A mix with suitable workability is placed in the forms in the usual manner as fresh concrete contains a continuous system of water filled channels, the application of a vacuum to the surface of the concrete results in a large amount of water being extracted from a certain depth of concrete. Thus the final water/cement ratio before setting is reduced, which controls the strength. The magnitude of the decrease in the water cement ratio is shown in Table 24.8.
Effect of Vaccum Processing:
It has following effects:
1. The final strength of concrete increases by about 25% and 28 days strength is reached in 8 to 9 days.
2. The permeability of concrete decreases sufficiently.
3. It gives high density concrete.
4. The increase in bond strength of concrete is found about 20%. It’s modulus of elasticity is always higher.
5. The overall durability of concrete is increased.
6. Coarser grading of aggregate yields more water than finer grading i.e. more water is extracted.
However, some of the water extracted leaves behind voids, so that full theoretical advantage of water removal may not be achieved in practice. The increase in strength on vacuum treatment is proportional to the amount of water removed upto a critical value, beyond which there is no significant increase.
Thus prolonged vacuum treatment is not useful. The critical value depends on the thickness of concrete and on the mix proportions. It has been observed that the strength of vacuum processed concrete broadly follows the usual dependence on the final water/cement ratio as shown in Fig.24.6.
Procedure of Applying Vacuum:
The vacuum is applied through porous mats connected to a vacuum pump. The mats consist of an air tight cover, usually made of plywood, with a vacuum chamber formed by expanded metal. This is faced with a fine wire gauge covered by muslin which prevents the removal of cement particles along with water. Fig. 24.7 shows a cross section of a vacuum mat. The mat can be placed on the top of the concrete surface immediately after finishing it and also be incorporated inside the faces of the forms.
Vacuum is created by a vacuum pump, the capacity of which is governed by the perimeter of the mat and not on its area. The magnitude of pressure usually ranges between 40 cms to 65 cms of mercury. This vacuum reduces the water content upto 20% over a depth of 15 cms to 30 cms. The reduction is greater nearer to the mat and usually the suction is assumed to be effective upto 15 cms to 20 cms depth only. Thus for a concrete section mat about 30 cms thick vacuum should be applied from two opposite sides.
The withdrawal of water produces settlement of the concrete to the extent of about 3% of the depth over which the suction acts. The rate of withdrawal of water falls with time. Duration of processing of 15 to 25 minutes has been found most economical and effective. Beyond 30 minutes very little reduction in water content has been observed.
The formation of voids can be checked, if in addition to vacuum processing, intermittent vibration is also applied. By the application of vibration higher degree of consolidation is achieved and the amount of water withdrawn is almost double.
Advantages:
1. Vacuum processed concrete stiffens very rapidly hence the form work can be removed after 30 minutes of casting even on columns about 4.5 m high. It is of considerable economic value as the form work can be reused at frequent interval.
2. The surface of vacuum processed concrete is entirely free from pitting and the uppermost I mm surface is highly resistant to abrasion. This property is very useful in the construction of concrete work where the concrete surface is in constant contact with flowing water at a high velocity.
3. Vacuum proceed concrete bonds well with the old concrete hence it can be used for resurfacing road slabs and for other repair work.
Disadvantages:
Its main disadvantage is its high initial cost.
Type # 7. Colcrete:
It is grouted concrete obtained by penetrating or injecting the colgrout into the voids of preplaced coarse aggregates. Colgrount is the colloidal mix of cement, sand and water in requisite proportions produced by high speed mixing in a mixer known as colcrete mixer. Colcrete mixer is a double drum concrete mixer. The proportioning of the ingredients can be done either by weight or by volume.
The cement and water is mixed in the first drum of the mixer and the water/cement slurry produced in this drum is transferred into the second drum and sand is added to produce the cement, sand and water grout. The mixing cycle in each drum may be kept 1.5 minutes or more till a suitable grout is obtained.
Advantages of Colcrete:
In ordinary 1:2:4 mix, the surface area of coarse aggregate per cubic metre of concrete is found of the order of 68800 square metres. To wet and activate this area with a relatively small amount of water of a particular water cement ratio concrete in a short mixing period presents great difficulty. Cement particles tend to adhere closely which entrapped air, thus mixing of conventional concrete remains imperfect. To overcome this difficulty the constituents of the concrete are passed through a colloidal mill, where each solid particle surface is brought in contact with the liquid.
Colcrete Process:
After cleaning the surface and form work, a layer of big size boulders is placed in the form section and the voids of holders are packed by graded stones of diminishing sizes, the minimum size being 40 mm. The minimum size of 40 mm is chosen to enable the concrete grout to penetrate into the voids. Again a layer of big size boulders is spread and voids filled with graded diminishing size stone pieces. The height of each layer may vary from 22.5 cms to 30 cms. The process is repeated till the height of the lift is reached. Voids in the packed aggregate may vary from 33% to 40%.
Mixing of Colgrout:
The colgrout is mixed in double drum colcrete mixers to form colloidal grout of suitable consistency. The cement and water is mixed in the first drum of the mixer and water cement slurry produced is transferred to the second drum where sand is added to produce cement-sand grout. The mixing time in each drum of 1.5 minute is found sufficient.
Grout pipes of 7.5 cms to 10 cms in diameter can be used they may be inserted either at the time of packing boulders or inserted after the packing into the holes left for this purpose. The pipes are spaced at about 2 metres centre to centre and the grout is injected at a velocity of about 30 cms per second. The pipes are taken from the bottom of the lift and are raised by about 5 to 10 cms before grout to clear the outlets.
The grouting should be done continuously and its fluidity should be maintained during the period of injection in the grout pipe. Grout should not be allowed to form air locks by flowing along surface of low penetration resistance, as along flat surfaces. After the grout has come to the surface, grout pipes are removed and surface leveled off by gravity grouting of the surface.
Characteristics of Colgrout:
1. The cement is thoroughly mixed with other constituents of the mix as each particle of the cement in the mix is completely wetted by the high speed shearing action of the mixer.
2. The colgrout has colloidal characteristics, resulting in the maximum gel formation of the cement. This prevents aggregation of sand resulting in the reduction of bleeding to a minimum.
3 As colgrout is quite fluid and stable, it can be pumped to considerable distances to the point of placement. It does not need any admixture, however for special purposes these may be used by high speed mixing.
4. The breaking up of aggregations and separations of the smaller particles of cement is achieved by high speed mixing, which results in greater fluidity.
5. Colgrout normally is composed of same ingredients of cement and sand in the proportion of 1 part of cement and upto 4 part of sand.