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Water Retaining Structure and Waterworks

  1. When designing a water storage tank, should movement joints be installed?

 

In designing water storage tanks, movement joints can be installed in parallel with steel reinforcement. To control the movement of concrete due to seasonal variation of temperature, hydration temperature drop and shrinkage etc. two principal methods in design are used: to design closely spaced steel reinforcement to shorten the spacing of cracks, thereby reducing the crack width of cracks; or to introduce movement joints to allow a portion of movement to occur in the joints.

 

Let’s take an example to illustrate this. For 30m long tanks wall, for a seasonal variation of 35 degree plus the hydration temperature of 30oC, the amount of cracking is about 8.8mm. It can either be reduced to 0.3mm with close spacing or can be absorbed by movement joints. Anyway, the thermal movement associated with the seasonal variation of 35oC is commonly accounted for by movement joints.

 

For water-retaining structure like pumping stations, the crack width requirement is even more stringent in which 0.2mm for severe and very severe exposure is specified in BS8007. It turns out to a difficult problem to designers who may choose to design a heavy reinforced structure. Obviously, a better choice other than provision of bulky reinforcement is to allow contraction movement by using the method of movement joints together with sufficient amount of reinforcement. For instance, service reservoirs in Water Supplies Department comprise grids of movement joints like expansion joints and contraction joints.

 

  1. What is the crack pattern induced by hydration due to internal restraint?

 

Let’s take a circular column as an example to illustrate this.

 

When the temperature is rising, the inner concrete’s temperature is higher than outer concrete’s temperature and the inner concrete is expanding. This induces pressure to the outside and the induced compressive stress will result in formation of radial cracks near the surface of concrete.

 

When the temperature drops, the concrete at the outside drops to surrounding temperature while the concrete at the central region continues to cool down. The contraction associated with inner concrete induces tensile strains and forms cracks tangential to the circular radius.

 

3.What is the purpose of adding cooling pipes or even using cold water for concrete in concreting operation?

 

All these measures aim at reducing the placing temperature and reducing thermal cracks induced during concreting of massive pours. Since the final concreting temperature should be the ambient temperature, reducing the initial placing temperature will also lower the peak hydration temperature. Therefore, the temperature difference between the hydration peak and the ambient temperature is reduced accordingly and subsequently the thermal effect to concrete structure can be reduced by controlling the placing temperature.

 

  1. Is the material of formwork (timber or steel) helps to reduce thermal cracks inconcreting operation?

 

To answer this question, one must fully understand the effect of formwork on the temperature of concreting structure. Without doubt, with better insulation of structure by timber formwork, the overall rise of temperature and hence the peak hydration temperature is also increased. However, for a well-insulated structure, the temperature gradient across concrete element is reduced. Therefore, the use of well-insulated formwork (like timber) increases the maximum temperature and reduces the temperature gradient across the structure at the same time. Hence, whether steel or timber formwork should be used to control thermal cracking is dependent on the restraints and the size of section.

 

If the section under consideration is thick and internal restraint is the likely cause to thermal cracking, then timber formwork should be used. On the other hand, if external restraint is the main concern for thermal cracking, then steel formwork should be used instead.

 

  1. 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.

 

  1. In selection of waterstop, shall engineers use plain dumb-bell type or center-bulb type?

 

The plain dumb-bell type is used for joint location where small movements are anticipated. Therefore, construction joints are desirable locations of this type of waterstop. On the other hand, center-bulb type waterstop is suitable for expansion joints or locations where lateral and shear movements occur due to settlement or deflection. Reference is made to W. L. Monks (1972).

 

  1. 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.

 

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.

 

  1. In designing reservoirs, the indirect tensile strength of the concrete mix is specified to be less than a specific value (e.g. 2.8N/mm2) for potable water. Why should engineers put an upper limit of indirect tensile strength?

 

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. However, if the strength of reinforcement is lower than immature concrete, then the subsequent yielding of reinforcement will produce isolated and wide cracks which is undesirable for water-retaining structures. Therefore, in order to control the formation of such wide crack widths, the concrete mix is specified to have an indirect tensile strength at 7 days not exceeding a certain value (e.g. 2.8N/mm2 for potable water). Reference is made to R. D. Anchor, A.W. Hill and B. P. Hughes (1979).

 

  1. Shall reversible moisture movement be taken into account in estimating movement for movement joints?

 

The size of concrete is affected by changes in atmospheric humidity: moisture causes expansion while drying causes shrinkage. Such moisture movement is reversible. This is totally different from drying shrinkage in which concrete slowly loses moisture during hardening, thus causing irreversible shrinkage.

 

In fact, the variation of humidity and the estimated reversible moisture movement is not significant (about 30%) and therefore, its contribution to movement does not justify for movement joints as suggested by MN Bussell & R Cather (1995).

 

  1. In the design of watermains, how to decide the usage of double air valves and single air valves?

 

Single air valves allow squeezing air out of the pipeline in automatic mode in high-pressure condition and are normally designed in high points of watermain in which air voids are present. Double air valves basically serve the same purpose except that it has another important function: it can get air into/out of the pipeline during low-pressure condition.

 

In WSD practice, watermain are normally divided into sections by installation of sectional valves to facilitate maintenance. In a single isolated pipeline section bounded by two sectional valves, at least a double air valve should be installed. During normal maintenance operation like cleansing of watermain, water inside pipelines is drawn from washout valves. However, as normal watermain is subject to very high pressure like 1.5MPa and the sudden withdrawn of water will cause a transient vacuum condition and will damage the watermain. Therefore at least one double air valve should be present to allow air to squeeze in to balance the pressure and this protects the pipeline from damaging.

 

In essence, for local high points single air valves should be installed. Within a section of pipeline, at least one double air valve should be installed.

 

 

 

 

 

 

  1. Why are two gate valves required in normal practice to form a washout valve?

 

In fact, the situation is analogous to that of fire hydrants in which two gates valves are installed with a single fire hydrant. Washout valves are used for normal maintenance work of watermain like allowing flowing out of water during cleaning of watermain. At the junction where a tee-branch out to a washout point, a gate valve is installed to separate the two pipelines. However, this gate valve is open during normal operation while another gate valve further downstream is installed (closed during normal operation). If the downstream gate valve is not installed in position, then the pipe section of branched-out watermain will be left dry during normal operation and there is a high probability that damage to watermain and frequent leakage would occur. With the downstream gate valve installed, the segment of branched-out watermain contains water in normal operation. In case there is any leakage, it can be readily detected by using the two gate valves.

 

  1. After the construction of watermain, prior to hydrostatic pressure test, swabbing is carried out. What is the purpose of swabbing?

 

Pipelines should be tested before commissioning to check the strength of watermain and the absence of leak. Before carrying out hydrostatic pressure test, swabbing is conducted to clear out rubbish and dirt left inside the pipeline during construction. Swabbing is required for pipes less than 600mm diameter because for larger size of pipes, they can be inspected internally to ensure cleanliness.

 

After carrying out of hydrostatic pressure test, test for water sterilization is then conducted which involves collecting water sample from the pipeline. The purpose is to check the water quality like colour, turbidity, odor, pH value, conductivity etc. and is compared with the quality of water drawn from water supply point.

 

  1. In the design of watermain, the normal practice is to use ductile iron for pipe size less than 600mm and to use steel for pipe size more than 600mm. Why?

 

For watermain pipe size less than 600mm, ductile iron is normally used because internal welding for steel pipes below 600mm is difficult to be carried out. Moreover, it requires only simple jointing details which allow for faster rate of construction. For watermain pipe size above 600mm, steel pipes are recommended because steel pipes are lighter than ductile iron pipes for the same material strength and therefore the cost of steel pipes is less than that of ductile iron pipes. In addition, in areas of difficult access the lighter mild steel pipes pose an advantage over ductile iron pipes for easy handling.

 

  1. In the design of service reservoirs, horizontal reinforcement in walls of reservoirs is placed at the outer layer. Why?

 

Since service reservoirs are designed as water-retaining structures with stringent requirement of crack width control, the design of reinforcement of service reservoirs is under the control of serviceability limit state. For the walls of service reservoirs, contraction and expansion of concrete are more significant in the horizontal direction of walls because of their relatively long lengths when compared with heights. In this connection, in order to minimize the usage of reinforcement, horizontal bars are placed at the outmost layer so that the distance of reinforcement bars to concrete surface is reduced. Since the shorter is the distance to the point of concern, the smaller is the crack width and

 

 

 

 

hence with such reinforcement arrangement advantages are taken if the reinforcement bars in the critical direction are placed closest to concrete surface.

 

  1. In the design of service reservoirs, how are reservoir floors designed to prevent leakage of water due to seasonal and shrinkage movements?

 

There are in general two main approaches in designing floors of service reservoirs:

 

  • In the first method, movement joints are designed in each panel of reservoir floors so that they can expand and contract freely. Each panel is completely isolated from one another and a sliding layer is placed beneath them to aid in sliding.

 

  • The second method, on the contrary, does not make provision to free movement. Due to seasonal and shrinkage movements, cracks are designed to occur in the reservoir floors such that very tiny cracks are spread over the floor and these cracks are too small to initiate corrosion or leakage. However, in this case, the amount of reinforcement used is much larger than the first approach.

 

  1. What is the difference between air chamber and surge tank?

 

Air chambers and surge tanks are normally installed in watermain to ease the stress on the system when valves or pumps suddenly start up and shut down. A surge tank is a chamber containing fluid which is in direct contact with the atmosphere. For positive surge, the tank can store excess water, thus preventing the water pipes from expansion and water from compression. In case of downsurge, the surge tank could supply fluid to prevent the formation of vapour column separation. However, if the surge pressure to be relieved is very large, the height of surge tank has to be designed to be excessively large and sometimes it is not cost-effective to build such a chamber. On the contrary, a air chamber can be adopted in this case because air chamber is a enclosed chamber with pressurized gases inside. The pressure head of gas inside the air chamber is the component to combat the hydraulic transient. However, air chamber has the demerits that regular maintenance has to be carried out and proper design of pressure level of gas has to be conducted.

 


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