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Concrete linings are widely used, with benefits justifying their
relatively high cost. They are tough, durable, relatively impermeable
and hydraulically efficient. Concrete linings are suitable for both
small and large channels and both high and low flow velocities. They
fulfil every purpose of lining.
Properly designed, constructed and maintained concrete linings should
have an average service life of over 40 years (USBR, 1975; Kraatz,
1977; Stevenson, 1999). If the deteriorating action of salts and
the development of cracks can be checked or do not occur, the linings
can last indefinitely. They are usually subject to some cracking
caused by collapsible soils, expansive clays, freeze-thaw action
and frost heave, but cracks that permit appreciable leakage can be
sealed with asphaltic compounds. Costly maintenance is seldom necessary
provided that the installation was performed correctly and the subgrade
is not allowed to deteriorate to the extent that extensive cracking,
and eventually concrete failure occurs. Maintenance of a deteriorating
concrete lining is very expensive.
A successful concrete lining must be durable and remain substantially
watertight, to give many years of low-maintenance service life. The
success of a concrete liner is highly dependent on its installation
and the materials used.
Figure 1 Success factors for concrete
lining
Source: Developed from USBR, 1988.
| Subgrade |
 |
Concrete linings should be placed on well-consolidated subgrade.
Preparation should include filling all voids with suitable material,
ensuring adequate compaction of the rest of the foundation by rolling,
tamping or vibrating, and trimming the foundation to the correct
shape (Stevenson, 1999).
Subgrade preparation should be performed far enough in advance to
avoid delay of the lining operations. At the time of placement of
the concrete the subgrade should be adequately moist. This is achieved
by sprinkling water in a manner that does not form mud or puddles.
In addition it may be important to consider some of the general issues
in relation to the channel subgrade and prism that are discussed
in Hard surface lining techniques.
| The
concrete mix |
|
Concrete used in channel linings should be mixed so that it is plastic
enough to consolidate well and be stiff enough to stay in place on
the slopes (USBR, 1988).
For hand placing and for placing with lighter machines the consistency
should be such that the concrete will barely stay on the slope. A
slump of 50-65mm is usually satisfactory. For heavier longitudinally
operating slip-form machines, best results are obtained with a slump
of about 50mm (USBR, 1988).
A close control of consistency and workability is important, as a
variation of 25mm in slump can upset the established operating adjustments
and interfere with progress and quality of work. When placing the
concrete lining with a subgrade-guided slip form, the slump has a
critical effect on slip-form operation because if the concrete is
not sufficiently plastic it is difficult to control thickness of
the lining (USBR, 1988).
Concrete aggregate should be clean, hard and durable. The concrete
aggregate for channel lining should include enough well-graded sand
to ensure a reasonably good finish with the minimum treatment specified.
Use of more sand than necessary for this purpose should be avoided.
Entrainment of from 3.5-5.5% air also helps materially in securing
a satisfactory finish. Another factor that will considerably improve
finishability of the concrete is the reduction of pea gravel (5-10mm)
content of the mix to about 5%. USBR specifications for channel lining
usually stipulate this separation where sufficient quantities are
required (USBR, 1988).
Proper consolidation is chiefly dependent on the mix, consistency
and placing procedure (USBR, 1988).
The use of hydraulic lime has been tested in combination with Portland
cement with the aim of developing a lining with less shrinkage and
able to withstand greater tensile deformation without cracking. These
tests have been successful (USBR, 1988). Other special cements may
be required when certain soluble sulphates are present in the soil
in appreciable quantities.
| Reinforcement |
|
Most concrete linings installed in older channels in the United States
were reinforced. During recent years reinforcement has been omitted
wherever possible to reduce construction costs and because it does
not materially improve effectiveness or durability if transverse
joints are provided at sufficiently frequent intervals to control
intermediate cracking (USBR, 1988). In Australia, older channels
were generally 50-75mm thick and rarely reinforced (Stevenson, 1999).
Following problems experienced with these concrete channels it has
been tempting to include reinforcement in some new channels.
However, it is noted in the literature that it is generally better
to use unreinforced concrete (Stevenson, 1999). Reinforcement cannot
be justified, except under unusual conditions such as high back pressure,
high flow velocities in the channel, where movement of the subgrade
is a possibility, or in reaches where failure would endanger life
and property adjacent to the channel. Unreinforced concrete linings
are more susceptible to damage by hydrostatic pressures, subgrade
movement or failure under the linings than reinforced concrete, but
not to the degree that the difference in cost might suggest. Unreinforced
concrete fractures more readily than reinforced concrete, thus relieving
the pressure and reducing the area of damage (Swihart and Rutenbeck,
2001).
Steel reinforcement does not prevent cracking. It should, however,
control crack width and hold the pieces of a badly cracked section
of the channel together. Unfortunately, steel reinforcement complicates
construction and poses the risk of deterioration due to corrosion
of the steel. On balance it is better to use unreinforced channel
linings, except possibly in areas of turbulence, where the maintenance
of integrity despite cracking could prevent loose pieces of lining
being lost (Stevenson, 1999).
When the lining is reinforced, consolidation of the concrete is both
difficult and uncertain unless the steel is held firmly in its proper
position in the middle of the slab and not permitted to sag during
placing operations. This is not easily accomplished and examinations
reveal that steel is often much lower than it should be. When it
sags during concrete placing, there is poor consolidation under the
steel. As a consequence steel is often exposed and corrodes.
To ensure proper positioning and prevent displacement, reinforcement
must be adequately tied and supported. Rocks or precast concrete
blocks are commonly used satisfactorily as supports if adequate in
size and spaced at proper intervals. When a general downward displacement
of the reinforcement cannot be entirely avoided, an allowance should
be made in setting the steel to compensate for the displacement.
A more recent option for reinforcement of concrete involves the use
of fibre reinforcement, where either synthetic of steel fibres (approximately
20-40mm long) are mixed with the concrete prior to construction of
the lining. Recent case studies in the United States have experimented
with this form of reinforcement in shotcrete (USBR, 1994).
| Placing
the lining |
|
Placing methods for concrete range from the hand method commonly
used on small channels or laterals to the longitudinally operating
slip-form machine designed for lining of large channels.
Manually placed concrete liners
Placing concrete liners by hand may prove economical when low-cost
labour is available or when reach of channel is too short or its
cross-section too small to be economical for mechanised placement.
Success relies on good formwork or guides, the right concrete for
the job, and sufficient labour to handle the work (Stevenson, 1999).
The simplest hand operation is placing unreinforced lining in small
laterals and farm ditches where the concrete is dumped and spread
on the sides and bottom in alternate sections. Screed guides are
laid on the subgrade and the concrete is screed up the slope to proper
thickness. Guide rails or boards must be set up carefully and fixed
securely in place, as the line and level of the whole channel depends
on them. The channel shape profiles suspended from the guides and
used as screed rails must also be accurate and robust enough to be
used repeatedly without distortion or collapse (Stevenson, 1999).
Three-metre screed planes are practicable for two-person operation.
Manually placed concrete liners are consolidated mainly in the screeding
operation. One or two passes with a long-handled steel trowel completes
the finishing. Transverse grooves are cut at 2m intervals, and the
lining is cured by use of sealing compound. Mixes for this method
should be well sanded to simplify placing and finishing. (USBR, 1988).
Larger linings constructed by hand are usually placed in alternate
panels to facilitate placing, finishing and curing operations. There
may also be some reduction in overall shrinkage cracking if enough
time elapses before placing the intervening panels. In this method,
it is best to place the bottom slab first to provide support at the
toes of the side panels. The panels are screed up the slope, and
the concrete is vibrated ahead of the screed (USBR, 1988).
If forming large rectangular channels in suitable (non-expansive)
soils, the sides can be made vertical and poured against conventional
formwork. In this case the concrete should be at least 100mm thick,
partly for robustness and partly so that it can be compacted with
an immersion vibrator (Stevenson, 1999).
Slip-form concrete liners
More efficient placement of concrete on slopes is accomplished by
use of weighted slip forms, similar to the weighted slip form depicted
in the figure below. After excavation and trimming of the subgrade,
pouring, shaping, compacting and smoothing of the concrete lining
are done with the slip form. Slip forms are used for all sizes of
channel.
Figure 2 Slip-form screed for placing
concrete on side slopes
Source: USBR, 1976.
The screed may be pulled up the slope by equipment on the berm or
by air hoists mounted on the slip form. Concrete should be vibrated
internally just ahead of the slip form. Under proper conditions of
operation the surface made by the slip form requires no further screeding
and very little finishing. The slip form itself should not be vibrated,
as this procedure causes a swell in the concrete at the lower edge.
This excess concrete is not only laborious to remove but it also
emphasises sags that tend to form at longitudinal bars.
For many years, concrete channel linings have been built in the United
States with longitudinally operated lining slip forms. This type
of channel lining is only cost-effective where there are long stretches
of channel with uniform cross-section that require lining. It is
for this reason that longitudinal slipforms are rare in Australia.
The longitudinal slip form is a steel plate, curved at the leading
edge, extending across the bottom and up the slopes of the channel
and shaped to conform to the finished surface of the lining. A distributor
plate when used is fastened to the leading edge of the slip form
and extends upward on a steep incline to the working platform.
On some machines a continuous row of hoppers in the working platform
feeds into drop chutes, each supplying one compartment of the trough
below. As the concrete is distributed through the bottom of the trough
and under the slip form, a vibrating tube parallel to and a few centimetres
ahead of the leading edge of the form consolidates the concrete.
The trailing edge of the slip form is usually adjustable to positions
somewhat lower than that of the leading edge. This provision improves
consolidation and moulds the concrete more closely to the subgrade.
Modern longitudinal slip forms commonly used in the United States
today on large channel lining operations can be described as travelling
channel-lining machines. These machines frequently exceed their design
rate of lining of approximately 500m/day. Many improvements have
been made in the longitudinally operating slip-form machines for
lining channels of all sizes. Large hydraulically operated lining
machines have been developed, with some electrically controlled to
line and grade. Preformed longitudinal plastic joints can be extruded
in the lining. Similar transverse joints are placed during finishing
operations.
Efficient channel construction requires careful coordination of the
successive operations. The trimming machine should be closely followed
by the lining machine and by separate grooving and curing jumbos
just behind the lining machine. If it is to proceed smoothly and
effectively, this technique depends on an abundant supply of concrete
with consistent properties. Such a supply is best furnished by a
local premixed concrete batch plant and delivered in agitator trucks
(Stevenson, 1999).
Precast concrete slab or block linings
Precast concrete has been used in lining works throughout the world
for channels of all sizes, but the trend is declining in countries
with expensive labour. Precast slabs are usually made 50-70mm thick
and are factory cast, ideally using steel moulds and high-strength
concrete. They are generally cast in sections that can easily be
handled by two workers. They should be cured and stored for some
months so that shrinkage occurs before use, minimising shrinkage
in the channel (Stevenson, 1999). Joints provided on the block are
sealed with mortar or bituminous mastic. Precast concrete slab linings
in which all joints are sealed by a sealant such as bituminous mastic
are flexible enough to absorb minor movements of the subgrade without
damage. Precast concrete slabs are subject to deterioration on sharp
curves, which should therefore be lined using another method.
Precast concrete linings of this type appear most promising for use
by small maintenance crews in lining or repairing short sections
of channel or laterals. No particular skill and very little equipment
is required. However, the large amount of manual labour required
in placing the blocks and sealing the joints makes this type of lining
slow and too costly for extensive use (USBR, 1976).
Some water authorities have had success with the use of precast channel
sections for smaller U-shaped channel profiles. These are made in
a precasting yard and incorporate externally placed waterstops at
each end (Stevenson, 1999).
The precast sections are placed as alternate sections on the channel
foundations and the missing sections between them are then formed
in-situ, using the precast sections as screed guides (Stevenson,
1999).
| Concrete
thickness |
|
There is no general rule for the thickness of concrete linings. For
small canals and ditches in locations where severe frost action is
likely to occur, unreinforced linings of about 40mm thickness have
been satisfactory. In most countries with mild climates, concrete
linings are 50-75mm thick for small channels and 75-100mm thick for
larger channels. Under more severe climatic conditions or in channels
with frequent changes in level and or unfavourable subgrades, thickness
is increased and may exceed 150mm for large channels.
Concrete linings need to be fairly robust, yet as economical as possible.
A lining thickness of 75-100mm seems to be the optimum for economic
construction with good service life, depending on channel size (Stevenson,
1999).
| Finishing
and curing |
|
Proper curing greatly improves the durability, wear resistance and
watertightness of concrete. A smooth hand-trowelled surface finish
increases the carrying capacity of a channel and is justified where
labour is inexpensive.
Curing involves keeping the concrete moist for at least a week after
it has set, in order for it to achieve its maximum potential strength
and impermeability. Good curing can be achieved by covering the work
with hessian and then keeping the hessian wet (with soaker hoses
or similar), by covering the concrete with plastic sheeting, or by
spraying a film-forming curing membrane over the concrete surface
(Stevenson, 1999).
Filling the channel with water would also work, but it is unlikely
to be a practical option, since it would have to be emptied again
for joint sealing. Early filling can also cause cracking in green
concrete if there is any ‘give’ in the supporting soils
(Stevenson, 1999).
Modern construction using slip forms, as described above, requires
minimum or no finishing and curing of the concrete.
| Joining |
|
Cracks result in concrete-lined channels due to contraction caused
by drying, shrinking and temperature changes. To prevent uncontrolled
cracking in the channel prism, cracks are confined to select locations
by creating weakened planes or joints. Cracks are initiated at these
locations, where they are easily sealed, and random cracking is minimised,
which reduces leakage and consequent damage such as loss of foundation
soils (Stevenson, 1999; Swihart and Rutenbeck, 2001).
Both transverse and longitudinal contraction joints are recommended
in channels having a lined perimeter of 9m or more, particularly
those that are unreinforced. Even smaller channels may require two-way
crack control if the sub-base material warrants it. Transverse joints
are normally spaced every 3-4.5m depending on the material, slab
thickness and reinforcement (USBR, 1988).
Previously, contraction grooves were generally provided by cutting
or forming grooves in the upper surface of the slab while the concrete
was still plastic. In this method, transverse grooves are cut either
by hand along a straightedge or by a mechanical knife or cutter impressed
and vibrated into concrete. Longitudinal grooves are cut by stationary
or revolving cutters attached to the rear of the slip form. Shrinkage
cracks are then largely confined to the location of the grooves where
the thickness of the lining had been reduced. Once cracks formed
at these locations, the cracks are often left open, except where
a high degree of watertightness is required. This policy was dictated
partly for reasons of economy but also because asphalt-based sealers
were virtually the only materials obtainable, and these were subject
to rapid deterioration from weathering.
Extensive field and laboratory studies have resulted in the development
of two basic systems for sealing contraction joints and one for sealing
random cracks. For contraction joint sealing one of the following
methods is recommended:
- A polyvinyl chloride (PVC) strip waterstop inserted
into fresh concrete.
- An elastometric sealant extruded into joints in
cured concrete.
For random crack sealing, the moulded-in-place
cap strip formed from an elastometric joint sealant shows the
most promise of long-term service (Swihart and Rutenbeck, 2001).
Figure 3 Cap seal for random cracks
Source: Swihart and Rutenbeck,
2001.
Contraction joint-forming waterstops
Transverse contraction joints are provided in channel linings by
inserting polyvinyl chloride (PVC) plastic contraction joint-forming
waterstop. This consists of a plane-weakening vertical member added
to a miniature waterstop, which is normally referred to as a PVC
strip (as shown in the figure below). Extruded from PVC and inserted
into the concrete during lining placement, this strip controls
cracking effectively.
Figure 4 PVC strip waterstop
Source: Swihart and Rutenbeck,
2001.
Experience proves that the installation must be correctly made:
- The top of the strip must not be more that 13mm
below the concrete surface, or the contraction crack might not
develop at the desired location.
- The strip must not be tilted sharply, or the crack
might lead to a sealing bulb, destroying the waterstop effect.
- Installation must be made using plastic concrete
and the strip is usually fed into the concrete and kept in tension
to ensure proper depth and orientation.
- Sufficient vibration is required to produce thorough
consolidation of the concrete around the strip and to provide
continuous contact between the concrete and all surfaces of the
strip. With T-shaped water stops the vibration of the concrete
around the water stop is critical (Stevenson, 1999).
The advantages of the PVC strip are as follows:
- It forms a weakened plane in the lining, producing
excellent crack control when properly installed.
- It seals by waterstop action, so the seal is not
dependent on bonding to the concrete.
- It is buried, so weathering is virtually eliminated.
- It is chemically inert and unlikely to be affected
by extended water immersion.
- It is a manufactured item receptive to high-quality
control standards.
- It withstands high hydrostatic pressures.
- It enables lining and sealing a channel in a single
operation.
- It tends to unitise a channel lining by tying
the individual slabs together, although this effect is reduced
by stress relaxation
Elastomeric sealants
Elastomeric sealants are used in much the same manner as the previously
used asphaltic mastics but with markedly better results. A groove
is cut in the concrete while it is still plastic, similar to installing
contraction grooves. Once the groove is formed, the elastomeric
sealant is injected into the groove as presented in the figure
below.
Figure 5 Channel contraction joint with
elastomeric sealant
Source: Swihart and Rutenbeck,
2001.
A recent elastomeric sealant in use is a coal-tar extended polysulfide
sealer that was modified from an airport runway sealer. It weathers
well and resists hydrostatic pressure. It bonds well to concrete
and remains bonded even after long periods of immersion in water.
Being a two-component, quick curing material, it requires specialised
equipment for successful application. Similar but slower setting
polysulfide sealants are also available for hand mixing and placement
on small jobs.
| Performance |
|
A properly installed and maintained concrete lining
provides satisfactory seepage control. The estimated effectiveness
in seepage reduction is 90% with sealed joints, and the estimated
lifespan is 40-60 years (Sinclair Knight Merz, 1998; Swihart and
Haynes, 1999).
Concrete is more resistant to erosion than most other lining materials.
It is therefore preferred for high water velocities. It is recommended
that velocities in reinforced concrete channels should not exceed
2.5m/s (Swihart and Rutenbeck, 2001). Higher velocities are permissible
in well-constructed reinforced concrete channels (USBR, 1988).
In fact, properly designed and constructed reinforced concrete
linings will withstand velocities of any magnitude considered feasible
for channels (Swihart and Rutenbeck, 2001).
Linings of concrete eliminate most weed growth, with resulting
improvement in flow characteristics and reduction in maintenance
costs. Burrowing animals that cause breaks in other lining materials
cannot penetrate concrete (Swihart and Rutenbeck, 2001).
| Cost |
|
As the available field data is predominantly from
the United States, all costs are converted from US to Australian
dollars (1998). Costs are found to be similar to geomembranes because
of the need to import material, although the side slopes can be
less expensive. While properly constructed concrete linings have
many advantages, concrete is sometimes not used due to its high
cost (Swihart and Rutenbeck, 2001).
The capital costs for installing concrete lining was recently estimated
to be approximately $47.5-52.5/m2 (Sinclair Knight Merz,
1998).
| Failures |
|
Cement concrete linings have failed due to the following
(USBR, 1976):
- Adverse subgrade conditions, such as loss of support
through piping action and bulging of expansive clays
- Excessive hydrostatic pressures beneath the lining
- Frost heaving
- Surface damage from freezing and thawing
- Poor quality concrete
- Faulty design or construction methods
- Combinations of these and similar factors.
Concrete is susceptible to damage from collapsible
soils, expansive clays, alkali water, alternate freezing and thawing
action, and from frost heave. Consideration of locality and proper
design and construction can help reduce these problems (Swihart
and Rutenbeck, 2001).
It is generally assumed that concrete linings are used to prevent
seepage and that the subgrade is usually relatively free draining
and above the groundwater level. Unfortunately this is not always
the case, and serious damage has occurred to concrete channel linings
on some projects due to uplift from high groundwater levels. Proper
drainage is the only means of correcting the problem. However,
it may be costly and complicated (Swihart and Rutenbeck, 2001).
If gross soil movement is a problem in the vicinity of the channel
then concrete lining may not be the best solution, unless it is
equipped with an external membrane to control leakage (Stevenson,
1999).
Concrete lining of channels has been used extensively for seepage
control in irrigation schemes. However, it has been found to be
cost-effective under Australian conditions only for high-value
irrigated crops, and low-pressure pipelining is generally now preferred
(Sinclair Knight Merz, 1998).
|
Repair
|
|
A comprehensive description of repairs to concrete-lined
channels is provided in Stevenson, ‘Repair/replacement
options for concrete lined irrigation channels’ (1999).
| Related
pages |
|
Hard
surface lining techniques
Shotcrete
Grouted fabric mats
Soil-cement lining
Flumes and pipes
Tiles and bricks
Asphalt |
