Concrete Engineering

The presence of joint filler is essential to the proper functioning of concrete joints though some may doubt its value. For a concrete expansion joint without any joint filler, there is a high risk of rubbish and dirt intrusion into the joint in the event that the first line of defense i.e. joint sealant fails to reject the entry of these materials. In fact, the occurrence of this is not uncommon because joint sealant from time to time is found to be torn off because of poor workmanship or other reasons.

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The shape of joint sealant affects its ability to stretch with movement. For instance, for rectangular joint sealant if the depth exceeds the width it tends to resist stretching of sealant in thermal movement. Moreover, block shape, when compared with concave shape, appears to be more resistant to stretching.

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Joint sealant should be designed and constructed to allow free extension and compression during the opening and closure of joints. In case joint sealants are attached to the joint filler so that movement is prohibited, they can hardly perform their intended functions to seal the joints against water and debris entry. Polyethylene tape is commonly used as bond breaker tape.

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Most joint sealants applied in concrete joints are adhesive and the recommended joint width/depth of joint sealant is from 2:1 to 1:1 as given by BS6213 and Guide to Selection of Constructional Sealants. When joint sealant is applied on top of joint filler in concrete joints, additional primers are sometimes necessary because:

(i) Primers help to seal the surface to prevent chemical reaction with water;

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Movement accommodation factor is commonly specified by manufacturers of joint sealants for designers to design the dimension of joints. It is defined as the total movement that a joint sealant can tolerate and is usually expressed as a percentage of the minimum design joint width. Failure to comply with this requirement results in overstressing the joint sealants.

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In a typical expansion joint, it normally contains the following components:

joint sealant, joint filler, dowel bar, PVC dowel sleeve, bond breaker tape and cradle bent.

Joint sealant: It seals the joint width and prevents water and dirt from entering the joint and causing dowel bar corrosion and unexpected joint stress resulting from restrained movement.
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Consider that the concrete structure is not subject to the problem of differential settlement.

For contraction joints, it may be possible to design a concrete structure without any contraction joints. By using sufficient steel reinforcement to spread evenly the crack width over the span length of the structure, it may achieve the requirement of minimum crack width and cause no adverse impact to the aesthetics of the structure. However, it follows that the amount of reinforcement required is higher than that when with sufficient contraction joints.

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This is a very classical question raised by many graduate engineers. In fact, there are two schools of thought regarding this issue.

The first school of thought is rather straightforward: the contractor fails to comply with contractual requirements and therefore as per G. C. C. Clause 54 (2)(c) the engineer could order suspension of the Works. Under the conditions of G. C. C. Clause 54(2)(a) – (d), the contractor is not entitled to any claims of cost which is the main concern for most engineers. This is the contractual power given to the Engineer in case of any failure in tests required by the contract, even though some engineers argue that slump tests are not as important as other tests like compression test.

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There are three types of slump that may occur in a slumps test, namely, true slump, shear slump and collapse slump.

True slump refers to general drop of the concrete mass evenly all around without disintegration.

Shear slump indicates that the concrete lacks cohesion. It may undergo segregation and bleeding and thus is undesirable for the durability of concrete.

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In normal concrete structures, 28-day test results are often adopted. However, in the construction of high-rise buildings using “High strength concrete”, compressive strengths based on 56 or 91-day compression test results are commonly used instead.

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