This chapter is all about failures in rigid pavement and briefly discusses the maintenance of Cement Concrete Pavement. Following are the two main actors which are responsible for failures in rigid pavements or cement concrete pavements:
- Deficiency of pavement materials: The various defects of rigid pavements are due to poor workmanship, use of soft aggregates, poor surface finish, improper curing poor quality of joint filler, etc.
- Structural inadequacy: If the pavement system is structurally unstable such as inadequate pavement thickness, inadequate subgrade support, poor subgrade soil, incorrect spacing of joints, etc., the rigid pavements develop various failures in the form of widening of joints, cracking of slab corners, etc.
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what are Rigid pavement Failures?
Following are some of the typical rigid pavement failures:
(1) Mud pumping
(2) Scaling of cement concrete
(3) Shrinkage cracks
(4) Spalling of joints
(5) Structural cracks
(6) Warping cracks.
Each of the above typical rigid pavement failure will now be briefly described.
- Mud pumping:
The water infiltrates through the joints, cracks and edges of the rigid pavements and forms the soil slurry known as mud. When heavy load passes over the pavement, the mud is ejected out through the joints and cracks of the pavement and thus the mud pumping occurs.
Due to repeated occurrence of mud pumping, there is considerable loss of fine grained soil from the subgrade and it results in lack of support at these points. The continued traffic movements further develop the cracking of pavement and thus the failure of rigid pavement due to mud pumping is generally of a progressive nature.
- Scaling of cement concrete:
Due to overall deterioration of the concrete, scaling is observed in cement concrete pavement and it mainly occurs due to the deficiency of concrete mix or presence of some chemical impurities which damage the mix. The scaling of cement concrete exposes the aggregate of the mix and the pavement surface becomes rough and shabby in appearance.
- Shrinkage cracks:
The shrinkage cracks are developed during the curing operation of cement concrete pavements immediately after their construction and they may be in transverse as well as longitudinal directions.
- Spalling of joints:
The pre-formed filler materials are sometimes placed in an inclined direction during construction and this faulty alignment results in the overhang of a concrete layer on the top side. The joint subsequently shows excessive cracking and subsidence.
- Structural cracks:
If the pavement thickness is inadequate to take up the load of vehicles, the structural cracking occurs and generally the pavements are found to crack at the corners and edges, as shown in fig. 12-8. The structural cracks may occur in longitudinal as well as transverse directions. The fact the cracking has taken place due to structural inadequacy should be carefully ascertained.
- Warping cracks:
If the joints are not suitably designed to accommodate the warping of slabs at the edges, excessive warping stresses are developed and the cracks on the slab are seen in an irregular pattern. For avoiding warping cracks, the hinged joints are provided or adequate reinforcement should be placed at joints to take up the warping stresses.
Repair and Maintenance of Cement Concrete Road
The cement concrete roads, if they are well designed and properly constructed, will require very little maintenance.
Following are the four main items of maintenance in cement concrete roads:
(1) Maintenance of joints
(2) Mud jacking
(3) Patch repairs
(4) Treatment of cracks.
Maintenance of joints:
The joints are the weakest parts of the cement concrete pavement and hence, they should be checked periodically and kept in proper order during routine maintenance work.
The damaged joint sealer should preferably be replaced before the start of monsoon. The expansion and contraction joints are opened up and after cleaning with brush, they are filled up with suitable material, preferably by a rubber based asphalt sealer.
Due to passage of time, the joint filler material may get damaged or deteriorated. In such cases, the entire joint is carefully newly constructed by removing the filler and sealer materials and then filling up with new materials. The opening of the joint is the widest in winter season and hence, the replacement of a joint should preferably be done during winter season only.
The term mud jacking is used to indicate the raising of a settled cement concrete slab or filling a void beneath slab with cement grout.
The procedure for mud jacking is as follows:
- The mud jack holes of diameter varying from 40 mm to 60 mm are drilled at 1.5 m to 3 m apart in the slab.
- The holes are cleaned with compressed air.
- The grout is forced through these holes under pressure. The grout is normally either 1:3.5 cement-sand mix or bitumen. The pressure will depend on the experience of past successfully conducted similar operations.
- Due to pressure from bottom, the slab is raised and it is held in position till the grout hardens.
It is also possible to do mud jacking from the side and such a method would eliminate the necessity of drilling holes in the slab.
A trench is made along the side of the pavement and a pipe is driven under the slab at a sufficient distance to transport the grouting material to the desired location.
The patching can be done for full depth or partial depth, depending upon the condition of the damaged or deteriorated surface area of the road.
For shallow depressions, the treatment consists in scraping out all the loose material from the depressed portion and it is followed by a covering of neat cement grout over the surface. The grouted portion is then covered up with stiff concrete mix.
The use of epoxy can be made for successful patch repair work. After applying the coat of epoxy, the surface is covered with sand and then a layer of bituminous concrete is laid. The sand prevents the slippage of the surface coat. The epoxy serves not only as an adhesive but also as a barrier for moisture between the bituminous concrete and previous surface.
For deep depressions, the full depth of defective portion of pavement is broken, removed and replaced by fresh cement concrete. The surface is manually finished with floats, trowels and edging tools. The concrete is covered with curing protection and when the surface has become sufficiently hard, the traffic is allowed to pass over the repaired pavement.
If the defective area for full depth patch repair is more, the precast slabs can be used instead of cast-in-situ concrete work. After breaking and removing the damaged portion of road surface, the precast slab is suitably placed with the help of a crane and it is then adjusted with the level of the existing road surface.
Treatment of cracks:
The cracks in cement concrete roads can be divided in the following two categories:
- Structural cracks
- Temperature cracks.
The formation of structural cracks in cement concrete roads should be considered as a serious phenomena. The reasons for the development of such cracks should be properly investigated and then only the remedial measures should be applied. The maintenance expenditure incurred on the repairs of the structural cracks will go to waste, if the root cause responsible for such cracks is not attacked and removed.
These cracks are in the form of fine cracks or hair cracks in the initial stage and they widen out subsequently. The repair work of temperature cracks is carried out as follows:
(a) The cracks are thoroughly cleaned of dirt, sand and other loose particles. For this purpose, a sharp tool, a stiff brush and a pressure blower are used.
(b) For facilitating proper bonding of the sealing material, the surface of cleaned crack is coated with kerosene oil.
(c) The cracks are filled by bituminous sealing compound of suitable grade after heating it to liquid consistency.
(d) The sealer is placed upto about 3 mm above the level of the slab along the cracks,
(e) Finally a layer of sand is spread to protect the sealer temporarily.
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