It is seldom possible to cast all concrete needed in a structure without interruption. From safety and durability considerations masonry and concrete structure should not be raised more than 1.5 m at a time, i.e. in one day in one lift. Hence some kind of joint has to be provided. This kind of joint is known as construction joint.
Construction joints are a source of weakness in the structure. Hence efforts should be made to obtain good bond at these joints. To avoid trouble arising from ba construction joints, it is essential to have knowledge how and where to construct a construction joint.
Broadly joints can be classified in the following categories:
Type # 1. Construction Joints:
These are temporary joints left between subsequent concreting operations. The position of construction joints should be preplanned before starting the concreting. Till such locations the concrete must be placed in one operation. The joint should be located at such places where the concrete is least vulnerable to maximum bending moment and maximum shear force.
ADVERTISEMENTS:
All care exercised in producing and compacting concrete to ensure good quality concrete is wasted if construction joints are not provided properly. The extent of trouble likely to arise from the bad construction joints depends to a great deal on the type of structure. A water retaining structure may be more seriously damaged by leakage of water through joints than earth retaining structures. The leakage of water leaches out material at the joint, resulting in widening the joint and becomes serious as the structure becomes older. The leaching salts are deposited on the joint surface making it unsightly.
Causes of Development of Defects:
The defects in construction joints may be due to the following probable reasons:
i. The joint is made either vertical or horizontal as an inclined joint is very weak and generally flakes off. In a vertical joint proper stop end board must be used. If no stop board is used the concrete near the joint will be honeycombed and form a place of weakness.
ADVERTISEMENTS:
ii. The concrete may not have been properly spread and compacted to form a smooth horizontal surface.
iii. Due to badly fixed shuttering causing a lip to form on the surface. This may happen if the shuttering is not properly clamped to the lower portion in case of a vertical structure as a wall or column. The loss of mortar also causes honeycombing near the joint.
Correct Position of Joints:
In order to avoid defects in construction joints, the general principle is that the position of construction joints should be decided in advance so that they occurs at suitable place of minimum shear and should not be left to chance. For proper transmission of stresses across the joints, it is necessary to extend the reinforcement of the old concrete into the new concrete.
ADVERTISEMENTS:
At the joints before adding new concrete, the surface of old concrete should be treated as follows:
(a) If the second layer is to be added within 4 hours after the laying of first layer, the laitance on the surface of the old concrete should be rubbed with a wire brush and cleaned with water before the new concrete is poured, but no water should be allowed to stand over the surface.
(b) If the second layer is to be added within 48 hours after the first layer, then the surface is rubbed with a wire brush and cleaned with water as above and a 1.5 cm layer of cement mortar of the same composition as that of concrete should be applied over the cleaned surface before the new concrete is placed.
(c) If the second layer is to be added after 48 hours after the first layer or on old concrete, a layer of new concrete is to be added. In this case the laitance is removed either by sand blasting or by chiseling and the surface is cleaned with water. Then slurry of neat cement should be applied on the cleaned surface and worked into interstices with broom, after which a 1.5 cm thick layer of sand and cement mortar of the same composition as that of concrete should be applied before the slurry dries out and then concrete placed immediately.
ADVERTISEMENTS:
Apart from appearance consideration of the work, the place of joint should be such that Total loads 2 tons it may not weaken the structure.
Vertical Joints:
Vertical joints reduce the “shear” strength of reinforced concrete beams and slabs considerably, but practically have rto effect on the bending strength, if made properly. This can be illustrated with the help of Fig. 27.4 and 27.5. Fig. 27.4 shows a beam loaded with a uniformly distributed load of 2 tonne. The upward reaction at each support is of one tonne as each support shares the half of the total load. Due to this upward force there is a tendency of the beam to break due to vertical shearing force as shown in Fig. 27.5.
Thus if a vertical joint is placed near the supports, the beam is likely to fail as shown in Fig. 27.5. However at the centre of the beam, there is no such tendency of shearing. This is shown in Fig. 27.6 which is a diagram of half the beam with the same distributed load.
In this case the upward force at A is fully balanced by the down ward weight acting over the half span of the beam, thus there is no tendency for the beam to slide up or down at section xx.
Bending Strength:
Bending is a condition set up in a beam by the loading and its effect is measured by the magnitude of its bending moment. B.M. is the tendency which the applied forces have for causing rotation of a section of the beam. The force of one tonne at A exerts a clock wise B.M. on the section xx (B.M. = force x lever arm). In this case lever arm is half the span.
The distributed load on the beam also exert a turning effect on the section xx, but of opposite nature. In this case the B.M. is anti-clockwise and half in magnitude of the first. The lever arm in this case being 1/4 of the span. The net B.M. tries to rotate the section xx in clock wise direction, but the reinforcement and concrete are able to resist this rotation. The stresses developed in concrete beam section under such conditions are shown in Fig. 27.7.
From Fig. 27.7 it will be seen that compression as well as tension stresses developed in the beam act at right angles to the cross section of the beam and if a vertical plane of weakness such as construction joint exists there, it will not affect the bending strength of the section appreciably.
Location of Construction Joints:
1. (a) Walls and Columns:
In case of walls and columns the construction joints should be horizontal. They should be provided at floor level, soffit level of lintels and skill level of windows. They should not be provided at the corners, as it would be difficult to tie the corners. In columns the concrete should be filled to the level, preferably few cm below the junction of beam.
(b) Beam and Slab:
In case of a beam the bending stresses act at right angle to the section i.e., they act in the direction of the span and do not affect the bending strength of the section appreciably. The vertical joints reduce the shear strength of the RCC beam or slab. Thus the best location for the construction joint in beams and slabs is where the shear stress is least. In most cases of beams and slabs this condition will be satisfied if the joint is made at the extreme position of the middle third of the span. In case of short span slabs it can be made at the centre of the span.
2. If it is unavoidable to provide a construction joint at the junction of a beam and slab, then special arrangement should be made using shear reinforcement or providing a suitable key to increase the shear strength of the joint.
3. It should be located where it is supported by other members. The construction joint should be properly covered while finishing the structure. Some construction joints are shown in Fig. 27.8.
Concreting in a Long and High Walls:
When concrete is to be poured in high and long walls, provision of suitable joints is essential. If the wall is not required to be water proof, then ordinary horizontal and vertical joints may be provided. In case, walls are to be made water proof, special care has to be taken. For vertical joints it is essential to provide a proper stop end board with a water stop as shown in Fig. 27.9. Fig. 27.9 (a) shows concreting in first portion with vertical stop board, while Fig. 27.9 (b) shows concrete in second portion after the stop board is removed and water seal stretched.
Horizontal Joint:
In case of horizontal joints, keys should be provided as shown in Fig. 27.10. In this case the wooden pieces are inserted in the first layer of concrete as shown in Fig. 27.10 and they are removed before the new concrete is poured.
Joining Columns in Multi-stored Buildings:
In this case dowels must be left in the lower columns which connect them to the upper columns. The floor slab in such cases is concreted over the lower columns at least 48 hours after placing concrete in the column. If this time is not given, the column concrete may shrink leaving a gap between the floor concrete and it self.
Joint in Water Reservoir and Tank Walls:
These structures require special care and treatment as these are subjected to large heads of water causing seepage. These joints should be made water proof. To make them water proof, a copper strip is inserted across the joint as shown in Fig. 27.9. The strip is kept loose by providing a loop at its centre and thus it can provide for the movement of the wall.
The loop is enclosed by soft mastic around it so as to enable it to move freely. The loop is kept towards the water surface of the wall. It is unusual that the walls of the tank are made monolithic and in one operation with the wall base. The floor slab is laid separately and a proper joint is given between it and the wall base as shown in Fig.27.11.
While laying wall base a step is made to receive the floor base. After this concrete has hardened the floor slab is laid and a water proof paper is put on the step before concreting. The tapered stop boards as shown in Fig. 27.11 should not be taken out till the concrete in the floor slab has hardened. Now this joint is filled with a plastic material which may remain plastic at all temperatures and whether dry or wet. Fig.27.12 shows different types of water stops.
Joints in Concrete Floors and Pavements:
Generally the concrete pavements and industrial floors are constructed in alternate bays to minimize the early shrinkage of concrete. To allow the maximum possible shrinkage in the concrete, the alternate bays should be concreted after the maximum possible time interval.
In case of dimensional roof slabs and in other special conditions expansion joints should be provided to take care of the expansion and contraction of the concrete. In pavements proper joints are provided to take care of the cracks developed due to thermal expansion and contraction, due to variation in temperature and long term dry shrinkage.
Joints in Road Surfaces:
In case of concrete roads, the area is divided into panels and each panel is cast separately from the other. In the joints between these panels some plastic material as felt, bitumen or cork is filled. This allows free movement of each panel and saves it from cracking. The edges of each panel are prevented from damage by filling enough bitumen in the joints as shown in Fig. 27.13. In case of roads, having heavy traffic, the edges are further reinforced by providing iron angles built in the slab during concreting as shown in Fig. 27.14. The angles are fixed into concrete by means of 7 mm hook bolts.
Joints in Buildings:
In buildings, vertical and horizontal joints in walls can be given more conveniently at off sets, recesses, floor lines etc. The expansion joint in a roof slab must be water tight and as well capable to allow free movement to the roof. It is always given over a wall or beam. Bitumen paint is very essential between the slab and the wall or beam under it to ensure free movement of the slab. The joint can either be vertical or made in the form of a step to eliminate further possibility of water leaking through it.
Asphalt is filled in the joint. In order to prevent cracking of asphalt over the joint, a piece of hessian is placed over the joint and covered with asphalt. This reinforces the asphalt and keeps it separate from the slab, by which the flexibility is increased. This reinforces the asphalt and keeps it separate from the slab, by which the flexibility is increased.
Type # 2. Expansion Joints:
In concrete, volume change takes place due to many reasons. Thus to safe guard against this volume change certain provision such as provision of joints must be made to relieve the stresses produced in the concrete. Expansion is a function of length. Concrete is very sensitive to change of temperature and expands if the temperature rises and contracts if it falls. The expansion of concrete causes compressive stresses and contraction causes tensile stresses in the concrete.
Some movements are also caused by the deflection of supports as well as shrinkage of concrete. If the concrete members are not allowed free movement, then internal stress will be developed in the structure and may prove disastrous to the safety of the structure. Thus it is essential that there should be some provision for the free movement of the structure.
A long building experiences large expansion. It is estimated that a 60 m long building with a variation of temperature of 50°F or 10°C may undergo an expansion of 2.5 cms. Thus buildings, more than 45m long should be provided one or two expansion joints.
Roof is one of the building elements which are subjected to maximum temperature variations. The roof is subjected to expansion during the day and contraction during the night or from season to season. This expansion and contraction causes pushing or pulling to the supporting load bearing walls, thus serious cracks develop in the masonry walls supporting the slab. Therefore to prevent the pushing and pulling of the wall, the slab should be made to slide over the wall.
The expansion of concrete has been found to depend on water/cement ratio of the concrete. For a 30 m long 1:2:4 concrete structure the expansion for different water-cement ratio is found as shown in Table 27.1.
Spacing of Expansion Joints:
The spacing of expansion joints is kept according to the amount of expansion. For a 60 m long concrete structure for a variation of 50°F in temperature the expansion will be about 2.5cm with the value of coefficient of thermal expansion as 6.0 x 10-6 for plain concrete and 6.5 x 10-6 for reinforced concrete. According to some researchers the amount of expansion varies from 0.6 to 2.5 cm for a variation of 50°F temperature. Thus the width of the joint should not be more than 1.25 cm. On this basis the spacing of the expansion joints may be kept upto 35 m.
As per I.S. 456-1978 any structure more than 45m longer should not be constructed without one or more expansion joints. From practical consideration the spacing between expansion joints may be kept between 18 to 20 m. However from the recent studies, it has been observed that actually concrete does pot expand to the extent indicated by simple analytical calculations due to the frictional resistance offered by the sub grade. Hence spacing between expansion joints may be kept much larger than in the past.
Further joint spacing is also affected by the season of the construction. For structures constructed in summer expansion joint spacing may be kept more than for structures constructed in winter. The joint spacing should be such that total expansion for summer constructed structures should not be greater than 1.25 cm and 2.5 cm for winter constructed structures. Apart from above intervals, expansion joints also should be provided at locations where the structure changes its direction as L, T and U shape portions of the structure.
Following points should be kept in mind to make the expansion joints more effective:
1. Adjacent to the expansion joint the structures preferably should be supported on separate walls or columns, but not necessarily on separate foundations.
2. Reinforcement should not extend across the expansion joint.
3. The break between the sections should be complete.
I.S.-456-2000 has given the following recommendations on the construction of expansion joints.
As the decision on the location, spacing and nature of expansion joints depends on many factors hence the provision of expansion joint in R.C.C. structures should be left to the discretion of the designer of the R.C.C. structure. However for general guidance it has been recommended that structures longer than 45 m in length should be provided one or more than one expansion joints.
Type # 3. Contraction Joints:
Concrete contracts or shrinks due to plastic and dry shrinkage. Stresses in concrete are developed when shrinkage is restrained, resulting in developing the cracks. To avoid the formation of these cracks, contraction joints are provided. Contraction joints are also called as control joints or dummy joints. Contraction joints can be avoided by providing sufficient reinforcement in the structural element to take up the shrinkage stresses. Contraction joints generally are provided in un-reinforced concrete pavements and floors. The spacing between the contraction joints may vary from 5 to 10 metres.
Contraction joints are made at the time of placing concrete by embedding a timber batton or plate of sufficient depth and thickness. This timber is removed when the concrete has hardened. Sometimes steel plates of sufficient depth and thickness may be embedded into the concrete instead of timber plates and removed when the concrete has hardened. The recent practice of providing contraction joints is to cut a groove in the concrete of stipulated depth and width with the help of a sawing machine.
Normally sawing is done with in about 24 hours of finishing the surface. If the saw cut is done after seven days or more, the depth isolated joint of cut should be kept as 1/3 of the thickness of the slab. The minimum width of 3 to 4 mm is sufficient. Wider widths of cuts are unnecessary and un-economical as their cutting and sealing cost will be higher.
The groove or cut should be filled with suitable joint sealing compound to improve the riding quality of the pavement. It will also protect the edges of the concrete and prevent water from seeping into the base. The depth of joint should be about 1/4 the thickness of the slab.
In residential building flooring the conventional contraction joints are avoided by casting the slab in alternate bays to allow for the total plastic shrinkage and for maximum extent of drying shrinkage. To create discontinuity between the adjacent bays and to prevent the development of continuous cracks, usually glass or aluminium strip is placed in between the bays.
Type # 4. Isolation Joints:
As the name suggests, this type of joint is provided where the concrete floor meets the permanent structural elements such as walls, columns, foundations etc. The depth of the isolation joint is kept equal to full depth of the concrete floor and its width of 10 to 12 mm is sufficient.
To avoid ingress of moisture or other undesirable elements, these joints should be filled with resilient materials and topped with joint filling compounds. A typical layout on ground of joints for concrete floor.