In this article we will discuss about:- 1. Joints in Concrete Structures 2. Guniting Process for Repairing Concrete Work 3. Formwork of Concrete 4. Quality Control of Concrete.
Joints in Concrete Structures:
Following are the two types of joints which are to be provided in concrete structures:
(1) Construction joints
(2) Expansion and contraction joints.
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(1) Construction Joints:
The construction joints are provided at locations where the construction is stopped either at the end of day or for any other reason. The provision of a construction joint becomes necessary to ensure proper bond between the old work and the new one.
If the construction activity is arranged in such a way that the work is stopped at expansion or contraction joint, the necessity of a construction joint will not arise. However it is often difficult to complete the concreting for large work in one operation. It then becomes necessary to take special measures to achieve perfect continuity between the previously laid concrete that has already set and hardened and the new concrete.
The construction joints may be horizontal (fig. 8-8) or vertical (fig. 8-9). For an inclined or curve member, the joint should be at right angle to the axis of the member.
It is necessary to determine the locations of the construction joints well in advance from the view point of structural stability.
Following points should be kept in view in case of the construction joints:
(i) The columns should be filled with concrete to a level few centimetres below its junction with the lowest soffit of the beam or bottom of the hunching, if any. The operation of concreting above the construction joint should be taken up after an interval of at least 4 hours.
(ii) The construction joints should be located along or near the planes of the least bending moment and shear forces.
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(iii) In case of T-beams or L-beams, the ribs should be filled with concrete first and then the slabs forming the flanges can be filled up to the centre of the rib, as shown in fig. 8-10. If a construction joint between slab and beam becomes unavoidable, especially as in the case of long and deep beams, the rib of the beam can be concreted upto 25 mm below the level of soffit of the slab and the construction joint should be located at that level.
(iv) For water tanks and other structures which store water, the strips of copper, aluminium, galvanized iron or other corrosion resistant material, known as the water stops or water bars, are placed in a construction joint as shown in fig. 8-11.
The function of water stop is to seal the joint against passage of water. The water stop may also be of natural and synthetic rubber or polyvinyl chloride (PVC). One-half of the water stop is inserted when the concrete is placed and the other-half is left projecting to be covered up by the next stage of concreting.
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(v) For slabs supported on two sides, the construction joints should be vertical and parallel to the main reinforcement. Alternatively, the construction joint can also Horizontal construction joint be provided at the middle of for R.C.C. wall span at right angles to the main reinforcement. For two way slabs, the construction joint can be provided near the middle of the either span.
(vi) For R.C.C. walls, the location of horizontal construction joints is governed by the convenience in placing the formwork and the ease of access for compaction of concreting. The continuity at the joint is achieved by the formation of key.
The bevelled edged planks are placed on the inner face of the wall shuttering and they are removed before the pouring of concrete above the joint. Fig. 8-12(a) shows the arrangement of planking and fig. 8-12(b) shows the shape of joint after casting.
(vii) The curing of horizontal construction joint should be suspended a few hours before new concreting is started. If the new concrete is to be laid within 48 hours, the surface of construction joint should be thoroughly cleaned with wire brush and water and treated with cement mortar of the same proportion as that of concrete.
However, if the new concrete is to be laid after 48 hours or more, the surface of the construction joint should first be roughened by chiselling. It should then be cleaned of scum, loose aggregates and other foreign matter, wetted and a coat of cement mortar of the same proportion as that of the concrete is applied before placing new concrete.
(2) Expansion and Contraction Joints:
These joints are provided in all the concrete structures of length exceeding 12 m, mainly for two purposes:
(i) To allow changes in volume of concrete due to temperature, and
(ii) To preserve the appearance and the original shape of the concrete structures.
These joints generally consist of some elastic material, known as the joint filler and dowels or keys.
‘The joint filler should be compressible, rigid, cellular and resilient. In cold weather, it should not become brittle and it should be easy to handle. The usual joint fillers are built-in strips of metal, bitumen-treated felt, cane fibre-board, cork bound with rubber or resin, dehydrated cork, natural cork, soft wood free from knots, etc.
The dowels or keys are provided in these joints to transfer the load.
The contraction joints are installed to allow for shrinkage movement in the structure. It may either be a complete contraction joint as shown in fig. 8-13 or a partial contraction joint as shown in fig. 8-14.
In the former case, there is complete discontinuity of both concrete and steel. In the latter case, there is discontinuity of concrete, but the reinforcements continue across the joint. In both the cases however no gap is provided for the joint. But only complete or partial separation of the adjacent sections is made.
Fig. 8-15 shows another– form of the contraction joint. It is also known as the dummy joint and in this case, a groove of about 3 mm width is created in the concrete member to act as a joint. The groove is filled with joint filler and its depth is about one-third to one-fifth of the total thickness of the member.
Guniting Process for Repairing Concrete Work:
The guniting is the most effective process of repairing concrete work which has been damaged due to inferior work or other reasons. It is also used for providing an impervious layer.
The gunite is a mixture of cement and sand, the usual proportion being 1:3. A cement gun is used to deposit this mixture on the concrete surface under a pressure of about 20 to 30 N/cm2.
The cement is mixed with slightly moist sand and then necessary water is added as the mixture comes out from the cement gun. A regulating valve is provided to regulate the quantity of water.
The surface to be treated is cleaned and washed. The nozzle of gun is generally kept at a distance of about 750 mm to 850 mm from the surface to be treated and the velocity of nozzle varies from 120 to 160 m/sec.
Following are the advantages of guniting:
(i) The high compressive strength is obtained. Strength of about 56 to 70 N/mm2 at 28 days is generally obtained.
(ii) The high impermeability is achieved.
(iii) The repairs are carried out in any situation in a short time.
Formwork of Concrete:
The concrete is contained in a timber or steel casing for a certain period after its placing. This casing is known as the shuttering, centering, form-work or moulds and it is to be removed when concrete has hardened sufficiently to support its own weight.
Following precautions should be taken for formwork of concrete:
(i) The formwork should be designed in such a way that it can be easily removed and used again.
(ii) The formwork should be fixed in such a way that the least hammering is required for its removal. Otherwise it may injure the concrete.
(iii) The inside surface of formwork should be coated with crude oil or soft soap solution. This will make removal of formwork easy.
(iv) The formwork should be sufficiently strong to bear the dead load of wet concrete as well as the impact of ramming or vibrating the concrete. The over-estimation of loads results in expensive formwork and the underestimation of loads results in the failure of formwork.
(v) It is desirable to bring down the cost of formwork to a minimum consistent with safety. The various steps such as reduction in number of irregular shapes of forms, use of component parts of commercial sizes, putting the formwork in use again as early as possible, etc. may be taken to effect economy in the formwork.
(vi) The formwork should be so arranged that there is minimum of leakage through the joints. This is achieved by providing tight joints between adjacent sections of the formwork.
Quality Control of Concrete:
The term quality control in civil engineering parlance is used to mean that the work is done according to the specifications provided in a contract document.
The concrete structures like multi-storeyed buildings, skyscrapers or dams are built to last many decades and hence the quality control of concrete during the construction of such important structures is a must and it has to be attended to with intelligent and scientific approach. The specifications of work should be framed with extreme care so as to serve effectively as a guide to get the job done well. As a matter of fact, the specifications are as important as the design of the project.
The concrete as such is a heterogeneous material and hence it would assume a wide range of properties, if produced without exercising any control.
For preparing a high quality concrete, the field organisation may broadly be divided into the following divisions:
(i) The engineering division which provides lines and grades and makes the initial inspection of all forms, reinforcement and installation of all embedded parts.
(ii) The manufacturing division which exercises control over concrete materials, batching and mixing.
(iii) The placing division which is concerned with the control of concrete placing and of related operations prior and subsequent thereto.
The concrete produced at site should be strongest, densest, most workable and most economical for the job for which it is prepared. The amount of cement should be low and that of aggregates should be high.
The most economical concrete with the highest possible density is obtained by observing the following general requirements:
(i) The air bubbles should be eliminated from the body of the concrete.
(ii) The cement particles should be of the smallest size.
(iii) The concrete would be compacted fully so as to remove voids.
(iv) The concrete should be cured sufficiently and adequately, say for 28 days.
(v) The cubical particles of the aggregates should be used so that good interlocking is gained.
(vi) The water-cement ratio should be kept low.
The quality control of concrete demands a high degree of awareness among the personnel connected with the production of concrete, namely, supervisors, labourers, etc. regarding the factors which influence the quality of concrete such as adding of water, cleanliness of various ingredients, adequate curing, etc.
It therefore becomes necessary to train such personnel for getting concrete of high quality. In a similar way, the engineer in charge of the work should have sufficient knowledge about proper working and maintenance of equipment used in producing concrete such as concrete mixers, vibrators, batching plant, etc.
The quality control aspect of concrete is a detailed topic by itself and extreme care is to be exercised at every stage of concrete production so as to obtain the desired results.
The quality control, if properly achieved, grants the following advantages:
(i) It helps in improved utilization of scarce resources and in extended utilization of low-grade materials.
(ii) It helps to minimize failures.
(iii) It results into lower costs of construction as higher stresses can be assumed.
(iv) The structure becomes durable with lower costs of maintenance.
It should be remembered that the road to achieve quality is not easy and hence, if a uniform product is required, all the ingredients and procedures which make up this conglomerate must also be uniform within permissible limits, not once in a while, but every day until the structure is completed.
Hence, all suitable precautions must be taken to insure proper inspection of the ingredients, batching, mixing, transporting and placing and if found necessary, the standards applicable to all these processes must be improved so as to deliver to the owner a project of acceptable quality.