Project Report – Six Months Summer TrainingPosted in Project Reports | Email This Post |
Concrete is a mixture of cement, sand, stone aggregates and water. A cage of steel rods used together with the concrete mix leads to the formation of Reinforced Cement Concrete popularly known as RCC.
Concrete has two main stages
1) Fresh Concrete
2) Hardened Concrete
Fresh Concrete should be stable and should not segregate or bleed during transportation and placing when it is subjected to forces during handling operations of limited nature. The mix should be cohesive and mobile enough to be placed in the form around the reinforcement and should be able to cast into the required shape without loosing continuity or homogeneity under the available techniques of placing the concrete at a particular job. The mix should be amenable to proper and through compaction into a dense, compact concrete with minimum voids under the existing facilities of compaction at the site. A best mix from the point of view of campactibility should achieve a 99 percent elimination of the original voids present.
The stability of a concrete mix requires that it should not segregate and bleed during the transportation and placing. Segregation can be defined as separating out of the ingredients of a concrete mix, so that the mix is no longer in a homogeneous condition. Only the stable homogeneous mix can be fully compacted
The segregation depends upon the handling and placing operations. The tendency to segregate, amount of coarse aggregate, and with the increased slump. The tendency to segregate can be minimized by:
a. Reducing the height of drop by concrete.
b. Not using the vibration as a means of spreading a heap of of concrete into a level mass over a large area.
c. Reducing the continued vibration over a longer time, as the coarse aggregate tends to settle to the bottom and the scum would rise to the surface.
d. Adding small quantity of water which improves cohesion of the mix.
Bleeding is due to the rise of water in the mix to the surface because of the inability of the solid particles in the mix to hold all the mixing water during settling of particles under the effect of compaction. The bleeding causes formation of a porous, weak and non durable concrete layer at the top of placed concrete. In case of lean mixes bleeding may create capillary channels increasing the permeability of the concrete. When the concrete is placed in different layers and each layer is compacted after allowing certain time to lapse before the next layer is laid, the bleeding may cause a plane of weakness between two layers. Any laitance formed should be removed by brushing and washing before a new layer is added. Over compacting the surface should be avoided.
One of the most important properties of the hardened concrete is its strength which represents the ability if concrete to resist forces. If the nature of the force is to produce compression, the strength is termed compressive strength. The compressive strength of hardened concrete is generally considered to be the most important property and is often taken as the index of the overall quality of concrete. The strength can indirectly give an idea of the most of the other properties of concrete which are related directly to the structure of hardened cement paste. A stronger concrete is dense, compact, impermeable and resistant to weathering and to some chemicals. However, a stronger concrete may exhibit higher drying shrinkage with consequent cracking, due to the presence of higher cement content.
Some of the other desirable properties like shear and tensile strengths, modulus of elasticity, bond, impact and durability etc. are generally related to compressive strength. As the compressive strength can be measured easily on standard sized cube or cylindrical specimens, it can be specified as a criterion for studying the effect of any variable on the quality of concrete. However, the concrete gives different values of any property under different testing conditions. Hence method of testing, size of specimen and the rate of loading etc. are stipulated while testing the concrete to minimize the variations in test results. The statistical methods are commonly used for specifying the quantitative value of any particular property of hardened concrete.
The compressive strength of concrete is defined as the load which causes the failure of specimen, per unit area of cross-section in uniaxial compression under given rate of loading. The strength of concrete is expressed as N/mm2. The compressive strength at 28 days after casting is taken as a criterion for specifying the quality of concrete. This is termed as grade of concrete. IS 456 – 2000 stipulates the use of 150 mm cubes.
The concrete has low tensile strength; it ranges from 8-12 per cent of its compressive strength. An average value of 10 per cent is generally adopted.
The concrete subjected to bending and shear stress is accompanied by tensile and compressive stresses. The shear failures are due to resulting diagonal tension. The shear strength is generally 12-13 per cent of its compressive strength.
The resistance of concrete to the slipping of reinforcing bars embedded in concrete is called bond strength. The bond strength is provided by adhesion of hardened cement paste, and by the friction of concrete and steel. It is also affected by shrinkage of concrete relative to steel. On an average bond strength is taken as 10 per cent of its compressive strength.
Facts about Cement and Concrete
1) Water required by 1 bag of cement is something in the range of 25-28 litres
2) Quality of concrete has nothing to do with its color.
3) The mortar / concrete should be consumed as early as possible after addition of water to it. The hydration of cement starts the moment water is added to it. As the hydration progresses the cement paste starts stiffening and loses its plasticity. The concrete should not be disturbed after this. Normally, this is about 45 – 50 minutes.
4) MPa is abbreviated form of mega Pascal, which is a unit of pressure. 1 MPa is equivalent to a pressure of 10Kg /cm2. The strength of concrete & cement is expressed in terms of pressure a standard cube can withstand. The Ordinary Portland Cement, commonly called OPC is available in three grades namely 33, 43 & 53 grades. Thus, for 43 grade cement standard cement & sand mortar cube would give a minimum strength of 43 MPa or 430 Kg /cm2 when tested under standard curing conditions for 28 days.
Compressive Strength of Concrete depends on following factors
(i) w/c ratio
(ii) Characteristics of cement
(iii) Characteristics of aggregates
(iv) Time of mixing
(v) Degree of compaction
(vi) Temperature and period of curing
(vii) Age of concrete
(ix) Conditions of testing
Precautions for water to be used in concrete
* It is good to use potable quality of water.
It should be free from impurities and harmful ingredients.
Seawater isn’t recommended.
The water fit for mixing is fit for curing too
Use of minimum quantity of mixing water, consistent with the degree of workability required to enable easy placing and compaction of concrete, is advisable.
Ensure that water is measured and added.
Low water to cement ratio is essential for good performance of the structure in the long run.
Common Reasons for lack of quality in concrete work
Use of too much or too little water for mixing, or water carelessly added during mixing
Incomplete mixing of aggregate with cement
Improper grading of aggregates resulting in segregation or bleeding of concrete.
Inadequate compaction of concrete
Using concrete which has already begun to set.
Placing of concrete on a dry foundation without properly wetting it with water.
Use of dirty aggregate or water containing earthy matter, clay or lime.
Too much troweling of the concrete surface.
Leaving the finished concrete surface exposed to sun and wind during the first ten days after placing without protecting it and keeping it damp by proper methods of curing.
Construction joints are the joints provided between successive pours of concrete that have been carried out after a time lag. As far as possible the construction joints should be avoided and every care should be taken to keep their numbers minimal. Since, presence of these joints creates a plane of weakness within the concrete body, these joints should be preplanned and their location should be such that they are at places where they are subjected to minimum bending moment and minimum shear force.
POURING AND CONSOLIDATION
Concrete (M20) was used for all works in column, beams and slabs. It was well consolidated by vibrating using portable mechanical vibrators. Care was taken to ensure that concrete is not over vibrated so as to cause segregation. The layers of concrete are so placed that the bottom layer does not finally set before the top layer is placed. The vibrators maintain the whole of concrete under treatment in an adequate state of agitation, such that deaeration and effective compaction is attained at a state commensurate with the supply of concrete from the mixers. The vibrator continue during the whole period occupied by placing of concrete, the vibrators being adjusted so that the centre vibrations approximate to the centre of the mass being compacted at the time of placing. Shaking of reinforcement for the purpose of compaction should be avoided. Compaction shall be completed before initial setting starts i.e. within thirty minute of addition of water to the dry mixture.
The concrete was deposited in its final position in a manner to preclude segregation of ingredients. In case of column and walls, the shuttering was so adjusted that the vertical drop of concrete is not more than 1.5 m at a time. In case of concreting of slabs and beams, the pipe from the batching plant was directly taken to the closest point.
Green concrete has all the three phases – solids, water & air. In order to make the concrete impervious & attain its maximum strength it is required to remove the entrapped air from the concrete mass when it is still in plastic state. If the air is not removed completely, the concrete loses strength considerably. It has been observed that 5% voids reduce the strength by about 30% and 10% voids reduce the strength by over 50%. Compaction eliminates air bubbles and brings enough fine material both to the surface and against the forms to produce the desired finish. One can use such hand tools as steel rods, paddling sticks, or tampers, but mechanical vibrators are best. Any compacting device must reach the bottom of the form and be small enough to pass between reinforcing bars. Since the strength of the concrete member depends on proper reinforcement location, be careful not to displace the reinforcing steel.
C Compacting reinforced concrete work is very important and is done using iron rods. In case the thickness of concrete layers should be more than 15 cm. the most satisfactory method for compacting concrete properly is to consolidate each layer separately so that its top surface become level and fairy smooth before the next layer is placed. While tamping is carried out, care should be taken that the rod should penetrate the full layer of the last layer placed and to some extent into lying to ensure proper bond between bond between them. Secondly the reinforcement and formwork should not be disturbed from their positions.
M Mechanical compaction is done by the use of vibrators. Compaction of concrete by vibration is considered essential for all important works especially in situations where reinforcements are congested or the member is required is to have exposed to concrete surface finish. When vibraters are used leaner but stiff, concrete mix should be used to obtain greater durability and highest strength, mixes which are to stiff to consolidate by hand compaction can be easily compacted by mechanical compaction, in case the concrete is compacted by vibrations ,during which the vibrator communicates rapid vibrations to the particles, increases the fluidity of concrete. Due to vibrations the particles occupy a more stable position and concrete fills all the space and present is force out to the surface, resulting in dense and durable concrete.
Types of vibrators
Following are the type of vibrates usually used to compact concrete:
1. Internal vibrators
2. External vibrators
3. Surface vibrators
4. Vibrating table
Internal vibrator consists of metal road like vibrating head which is immersed in the full depth of concrete layer. It is also known as poker or needle vibrator and is consider to be most effective type of vibrator as it comes into intimate contact with concrete. External vibrators are placed against the concrete form-work and vibrating force for compaction is conveyed to the concrete through the form work. These vibrators are also called form vibrators. The vibrator is rigidly clamped to form work resting on a elastic spot, so that both the form and concrete are vibrated. Incase considerable proportion of work done is consumed in vibrating resulting in low efficiency of the system. Surface vibrators are mounted on platform and are generally used to compact and finish bridge, road slab etc. These are also external vibrators and are suitable for precast concrete work. It provides a reliable means of compaction of pre-cast concrete and has the adv of offering uniform vibration. Vibrating table is used for consolidation of pre-cast units. Surface vibrators is used there a wide horizontal surface occurs such as dams and very thick walls .large type of surface vibrators is there but pen type vibrator are used most. When concrete is placed on such tables, mechanical compaction takes place which has many advantages. Each vibrator have its own advantages and disadvantages, hence the choice between different types should be made correctly. Concrete to be compacted by vibration, should be designed properly. The consistency of concrete depends of conditions of placing, type of mix, and the efficiency of vibrator. The slum of such concrete should not be more than 5 cm in any case; otherwise segregation of concrete will take place, which should never be allowed to occur.
FINISH TO CONCRETE WORK:
a) All concrete while being poured against form work was worked with vibrator
rods & trowels as required so that good quality concrete is obtained.
b) All exposed surface of RCC lintels, beams, columns etc. were plastered to match
with adjoining plastered face of walls after suitably hacking the concrete surface.
Concrete Mixers and Batching Plant
Concrete Plant, also known as a Batch Plant, is a device that combines various ingredients to form concrete. Some of these inputs include sand, water, aggregate (rocks, gravel, etc.), fly ash, potash, cement, and other ingredients to create concrete. There are two types of concrete plants, ready mix plants and central mix plant. A concrete plant can have a variety of parts and accessories, including but not limited to: mixers (either tilt-up or horizontal (or in some cases, both), cement batchers, aggregate batchers, conveyors, radial stackers, aggregate bins, cement bins, heaters, chillers, cement silos, batch plant controls, and dust collectors (to minimize environmental pollution).
The front view of the plant from where it hauls coarse aggregate is shown below:-
The term ‘curing’ is used to include maintenance of a favorable environment for the continuation of chemical reactions, i.e. retention of moisture within, or supplying moisture to the concrete from an external source and protection against extremes of temperature
Following are the methods for curing different building parts:-
Walls – Water should be sprinkled from the top such that it covers the whole area of the wall and it should be remain wet.
Slab – Ponding should be done on the slab by constructing bunds of mortar
Beams and columns – The beams and columns can be maintained wet by tying gunny bags around the periphery and by maintaining it wet always.
Ponding, continuous sprinkling, covering with wet cloth, cotton mats or similar materials, covering with specially prepared paper, polyethylene, sealing coat applied as a liquid commonly known as ‘curing compound’ which hardens to form a thin protective membrane, are some of the methods by which concrete is cured. Curing should be started just after the surfaces begin to dry. Normally 7 to 14 days curing is considered adequate.
Admixtures are those ingredients/materials that are added to cement, water, and aggregate mixture during mixing in order to modify or improve the properties of concrete for a required application.
Broadly the following five changes can be expected by adding an admixture
(i) Air entertainment
(ii) Water reduction for better quality
(iii) Acceleration of strength development
(iv) Improving the workability
(v) Water retention
Some of the important purposes for which the admixtures could be used are
1. Acceleration of the rate of strength development at early ages
2. Retardation of the initial setting of the concrete
3. Increase in strength
4. Improvement in workability
5. Reduction in heat of evolution
6. Increase in durability or in resistance to special conditions of exposure
7. Control of alkali-aggregate expansion
8. Reduction in the capillary flow of water and increase in impermeability to liquids
9. Improvement of pumpability and reduction in segregation in grout mixtures
10. Production of coloured concrete or mortar
The best way to test the admixture is by making trial mixes with the concrete materials to be used on the job and carefully observing and measuring the change in the properties. This way the compatibility of the admixture and the materials to be used, as well the effects of the admixture on the properties of fresh and hardened concrete can be observed. The amount of admixture recommended by the manufacturer or the optimum quantity determined by laboratory tests should be used
LEAKAGE AND WATER PROOFING
There are many reasons for leakage in concrete. Due to this leakage, the concrete not only looses its strength but also cause problem to the user. Normal concrete construction should not require water proofing materials, if it is designed and constructed properly with good quality and workmanship. But still to make it safe against the ill effects of water, liquid and powder form of water proofing material is used depending upon the availability of the material. Normally the usage per kg of cement is specified by the manufacturer for example: ACC’s waterproofing compound “ACCOPROOF” is available in powder form and 1 Kg packets. For normal purposes, 1 Kg is required to be used with 50 Kg (1 bag) of cement.
Leakages occur because of variety of reasons; some of which are mentioned below –
a) Accumulation of water, which start penetrating the surface.
b) Poor quality and improper proportioning of concrete constituents that make concrete permeable.
c) Poor compaction of concrete, which leave a lot of air voids.
d) Construction joints at two different works like concrete and brick works, and discontinuity in concrete casting (joint at old concrete and new concrete) leading minute cracks, which facilitate water movement.
e) Other structural cracks because of loading conditions and failure of the structure to withstand those stresses.
f) Movement of water from bottom to top because of capillary action.
Following figure shows the damp proofing material which was used at site:
Following measures may be useful to avoid leakages –
a) Provide good drainage facility with correct gradient at the places where there are chances for water to accumulate.
b) Use good quality of materials with correct proportioning in concrete. For example, use of blended cement and use of less water in concrete can reduce permeability of the structure. Similarly, proper proportioning of materials would help concrete becoming uniformly packed and dense.
c) Proper compaction of concrete with immersion vibrator to make it void less.
d) Avoid construction joints becoming a weaker point for water to travel. Some proactive and treatment measures would be useful.
e) Proper structural design and execution of members, which results no cracks for water to percolate.
f) Proper damp proofing course required to avoid movement of ground and other water from bottom to top. Some useful methods are like applying bitumen, concrete construction etc. at plinth level.
g) Use of water proofing compounds for water retaining structures.
DOORS AND WINDOWS
Wood absorbs moisture from air when the moisture content is high in the atmosphere and as a result it expands causing cracks in the wall. During dry atmosphere the wood contracts and a gap is formed between the wall and the frame of the door.
Plaster protects structure from temperature variations; external attacks of sulphates, chlorides, etc. Plaster also provides smooth & aesthetic surface on RCC & Brickwork surface. The proportion of mortar used at site for ceiling coat is 1:4 and wall coat is 1:3. A plaster of 10 mm is done at ceiling and a plaster of 12.5mm is done at wall. Various precautions to be taken while the work of plastering is going on are:-
•Preferably use cements which releases low heat of hydration.
•Use optimum water at the time of mixing.
•Do not use dry cement on the plaster surface.
•At the junction of Brickwork & RCC, chicken mesh or fiber mesh may be used.
•Wet the surface before plastering and cure the surface for at least 10 to 12 days.
The purpose of a floor is to provide a horizontal sanitary surface to support the occupants of a building, furniture and equipment. A good floor should have strength and stability, resistance to dampness, good appearance, and freedom from maintenance etc.
Following are the common floor finishes –
Cement concrete flooring– It consists of 1:1.5:3 cement concrete laid to a thickness of 3” to 4”, over a strong sub base. Top surface is smoothened with cement punning. It has got good wearing properties and can be easily cleaned and maintained. If thickness is less, the size of stone aggregates is limited to ½”.
Tile flooring– It consists of ceramic, vitrified, terrazzo and cement tiles. Tiles give very pleasant appearance to floors. Also, it can be executed fast. Vitrified and Ceramic tiles have gained popularity over mosaic tiles because it doesn’t require grinding and polishing and the appearance is good and the tiles are quite strong.
In ground floor the cement concrete floor is to be on 7.5 cm base of lime concrete or weak cement concrete as per standard specifications. If the bases consist of cement concrete it shall be allowed to set for about 7 days. In case the base is of weak cement concrete the flooring shall commence within 48 hours of laying the base. In first floor or upper floor if c.c. floor is to be laid on R.C.C. slab, the surface shall be made rough with brushes while concrete is green. Before laying the c.c. floor the surface shall be cleaned, wetted and a neat cement wash shall be applied to get a good bond.A layer of brick brats ie broken bricks are laid before the laying of mortar and there by the tiles
Separate conduits are laid for following systems:
a) Normal light, fan
b) Power points
c) AC points
d) Internet wiring
e) Fire alarm system