Causes of cracks, remedial measures and repair of cracks in concrete are discussed.
Causes of Cracks in Concrete
Cracks in concrete may be plastic shrinkage cracks, settlement cracks, cracks due to bleeding, cracks due to delayed concrete casting and cracks due to construction effects are discussed in Section 8.11.
Preventive Measures for Crack in Concrete
Preventive measures for cracks in concrete are discussed in Section 8.11. Apart from them, joints in concrete structures are provided to continue a specific work or prevent cracks due to change in temperature.
The following two types of joints are usually provided in concrete structures:
- Construction joints
- Expansion and contraction joints
1. Construction Joints
This type of joints are provided at the location where the construction is stopped at the end of day’s work or for any other reason so as to bridge the old work and the new work by a proper bond. Such a situation generally occurs when large concrete work has to be executed which cannot be done within a day.
If the work is well planned such that the day’s work is to be stopped at an expansion or contraction joint, in such a case there is no need for a construction joints.
The construction joints may be vertical, horizontal or inclined depending on the type of structure. In the case of inclined or curved members the joint should be perpendicular to the axis of the structural member. The position of the construction should be well planned and constructed keeping in view the stability of the structure. Construction joints may be provided for structures which are vertical, horizontal or inclined (Fig. 30.1).
Figure 30.1 Construction joint in different positions
Depending on the type of concrete structures the following factors should be considered.
(i) Columns
Columns should be concreted to a few centimetres below its junction with the lowest soffit of the beam. Construction above joint should be at least 4 hours after the completion of the joint. Care should be taken such that the construction joint is at the location of least bending moment.
(ii) T or L Beams
The ribs of T or L beams are first concreted and then slabs forming the flanges are concreted up to the centre of the rib (Fig. 30.2). In certain situations, if a construction joint has to be provided between the slab and beam, the rib of the beam is concreted up to 25 mm below the level of soffit of slab and the joint should be located at that level.
Figure 30.2 Construction joint on T-beam
(iii) Simply Supported Slabs
In slabs supported on two sides the construction joint should be vertical and parallel to the main reinforcement. Alternatively the joint may be provided at the middle of the span perpendicular to the main reinforcement. For two-way slabs the construction joint is provided near the middle of either span.
(iv) RCC Wall
In this case the location of the joint depends on the convenience in placing the framework and the access of compaction of concrete. The continuity of the joint is made by the formation of a key (Fig. 30.3(a)). The arrangement of the formation of key and the finished position of the key is shown in Fig. 30.3(b).
Figure 30.3 Construction joint in RCC Walls
2. Expansion and Contraction Joints
These joints are provided in all concrete structures when the length exceeds 12 m. These joints are made to satisfy the following requirements:
- To accommodate for changes in volume of concrete due to temperature.
- To retain the appearance by maintaining the same shape of the concrete structures.
The joints are filled with some elastic materials like filter or dowels of keys. The quality of the filter should be in a position to withstand cold weathers. It should be compressible, cellular and not brittle. The Conventional materials used as filter are strips of metal, bitumen-treated felt, cane fibre-board, cork, soft wood, etc.
Provision of dowels or keys is to transfer the load. The movement due to shrinkage is controlled by the contraction joint. The contraction joint may be complete or partial. In the complete contraction joint there is complete discontinuity of both concrete and steel (Fig. 30.4). In the partial contraction joint, there is a discontinuity of concrete but the reinforcement bars are continued across the joint (Fig. 30.5).
Figure 30.4 Complete contraction joint
Figure 30.5 Partial contraction joint
Stitching of Cracks in Concrete Members
At times cracks do develop in concrete members due to several reasons such as design of construction deficiency, problems with concrete, disturbance at early stage, etc. If such cracks are localized in nature they can be repaired either by epoxy grouting or by stitching. The choice of either of them depends on the discretion of the concerned engineer and the intensity of the cracking (Gambhir, 2010).
Stitching consist of drilling holes on both sides of the crack, and inserting U-shape metallic strips (stitching dogs) spanning across the cracks. The bonding of the metallic strips with parent concrete can either by with non-shrink grout or epoxy-based bonding system. It is preferable if the metallic strips are variable in length and orientation with closer spacing at the ends of cracks (Fig. 30.6).
Figure 30.6 Stitching of concrete cracks
However, stitching does not close a crack, but only prevents it from further widening. Such stitches, when encased with an overlay have been reported to be performing well in several cases.
Repair of Concrete by Gunite and Shortcrete
Gunite is one of the popular methods undertaken to repair cracks and other damages in concrete members. Guniting is the process of injecting mortar with low water-cement ratio at a high pressure through a nozzle. This method has been improvised in that only concrete with small-sized coarse aggregate instead of mortar is used. This method is called shortcrete.
In both the methods, the affected concrete part should be cleaned before the operation and gunite or shortcrete is applied. Former small thickness or width of gap gunite is used whereas shortcrete is recommended for larger thickness. The materials used for gunite are cement, sand and water and for shortcrete are cement, sand and small aggregate and water. For quick setting of gunite, accelerators are also added.
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