PRACTICAL QUESTION AND ANSWERS ON WATER RETAINING STRUCTURES
1. When designing a water storage tank, should movement joints be installed?
Ans. 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.
2. What is the crack pattern induced by hydration due to
internal restraint?
Ans. 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?
Ans. 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.
4. Is the material of formwork (timber or steel) helps to
reduce thermal cracks inconcreting operation?
Ans. 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.
5. What is the importance of critical steel ratio in
calculating thermal reinforcement?
Ans. 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.
6. In selection of waterstop, shall engineers use plain
dumb-bell type or center-bulb type?
Ans. 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).
7. Why do BS8007 specify the allowable crack width of water
retaining structure as 0.2mm for severe or very severe exposure?
Ans. 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.
8. 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?
Ans. 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).
9. Shall reversible moisture movement be taken into account
in estimating movement for movement joints?
Ans. 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).
10. In the design of watermains, how to decide the usage of
double air valves and single air valves?
Ans. 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.
11. Why are two gate valves required in normal practice to
form a washout valve?
Ans. 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.
12. After the construction of watermain, prior to
hydrostatic pressure test, swabbing is carried out. What is the purpose of
swabbing?
Ans. 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.
13. 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?
Ans. 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.
14. In the design of service reservoirs, horizontal
reinforcement in walls of reservoirs is placed at the outer layer. Why?
Ans. 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 andhence with such reinforcement arrangement
advantages are taken if the reinforcement bars in the critical direction are
placed closest to concrete surface.
15. In the design of service reservoirs, how are reservoir
floors designed to prevent leakage of water due to seasonal and shrinkage
movements?
Ans. There are in general two main approaches in designing floors
of service reservoirs:
(i) 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.
(ii) 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.
16. What is the difference between air chamber and surge
tank?
Ans. 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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