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What is the importance of critical steel ratio in calculating thermal reinforcement?

The fulfillment of critical steel ratio means that in construction joints or planes of weakness of concrete structure, steel reinforcement will not yield and concrete fails in tension first. This is important in ensuring formation of more cracks by failure of concrete in tension, otherwise failure in steel reinforcement would produce a few wide cracks which is undesirable.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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What is the purpose of setting indirect tensile strength in water-retaining structures?

The crack width formation is dependent on the early tensile strength of concrete. The principle of critical steel ratio also applies in this situation. The amount of reinforcement required to control early thermal and shrinkage movement is determined by the capability of reinforcement to induce cracks on concrete structures. If an upper limit is set on the early tensile strength of immature concrete, then a range of tiny cracks would be formed by failing in concrete tension [4]. However, if the trength of reinforcement is lower than immature concrete, then the subsequent yielding of reinforcement will produce isolated and wide cracks which are undesirable for water-retaining structures. Therefore, in order to control the formation of such wide crack widths, the concrete mix is specified to have a tensile strength (normally measured by Brazilian test) at 7 days not exceeding a certain value (e.g. 2.8N/mm2 for potable water).

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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Why do BS8007 specify the allowable crack width of water retaining structure as 0.2mm for severe or very severe exposure?

For crack width less than 0.2mm, it is assumed that the mechanism of autogenous healing will take place in which the crack will automatically seal up and this would not cause the problem of leakage and reinforcement corrosion in water retaining structure.

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When the cracks are in inactive state where no movement takes places, autogenous healing occurs in the presence of water. However, when there is a continuous flow of water through these cracks, autogenous healing would not take place because the flow removes the lime. One of the mechanisms of autogenous healing is that calcium hydroxide (generated from the hydration of tricalcium silicate and dicalcium silicate) in concrete cement reacts with carbon dioxide in the atmosphere, resulting in the formation of calcium carbonate crystals. Gradually these crystals accumulate and grow in these tiny cracks and form bonding so that the cracks are sealed. Since the first documented discovery of autogenous healing by the French Academy of Science in 1836, there have been numerous previous proofs that cracks are sealed up naturally by autogenous healing. Because of its self-sealing property, designers normally limit crack width to 0.2mm for water retaining structures.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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What is the purpose of providing service reservoirs?

Service reservoirs, other than normal reservoirs, are provided because of the following reasons:

(i) In case of the breakdown of pumping stations and water treatment plants, it provides a temporary storage of water in emergency situation like fire fighting.

(ii) Since the demand of water supply from customers varies with time, the provision of service reservoirs aims to balance the fluctuation rate of water demand.

(iii) It provides a constant head of water to the distribution system under the design pressure.

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(iv) In the event of the occurrence of water hammer or surge during the rapid closure and opening of pumping stations, the reservoir acts to attenuate the surge and performs like a surge tank.

(v) It leads to a reduction of the size of pumps and trunk mains connecting to the distribution system as the pumps are not required to directly cope with the peak rates of water demand by the introduction of service reservoirs. As such, there is substantial cost savings arising from the use of smaller pumping pipelines and smaller pumps.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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Should pipelines be completely embedded into thrust blocks?

For unreinforced concrete thrust blocks in bends and tees for pressurized pipelines, it is recommended that the contact surface between the pipelines and concrete thrust blocks should not exceed 45o from either side of the pipe in the direction of thrust force through the center of pipelines. The reason is to prevent the occurrence of potential cracking arising from the deformation of pipelines under loading condition. If it is necessary to embed the whole section of pipelines into concrete, it is suggested to coat the pipe with a flexible material

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Filed under Hydraulics | 0 Comments