Flexible pavement:
The sub-base for a flexible pavement is laid directly onto the formation level and should consist of a well compacted granular material such as a quarry overburden or crushed rocks. The actual thickness of sub-base required is determined by the cumulative number of standard axle is taken (msa) to be carried (msa = millions of standard axles, where a standard axle is taken as 8200 kg) and the CBR (California bearing ratio) of the sub grade.
The CBR is an empirical method in which the thickness of the sub base is related to the strength of the sub grade and to the amount of traffic the road is expected to carry. A fully flexible pavement, design life should be at least 20 years.
These are suitable for light vehicular traffic where violet braking and or acceleration are not anticipated such as driveways to domestic properties.
Unbound pavement consist of a 100 to 200mm thick base of clinker or hardcore laid directly onto the information level of the sub grade and this is covered with a well rolled layer of screeded gravel to pass a 40mm ring with sufficient sand to fill the small voids to form an overall consolidated thickness of 25mm. this will give a relatively cheap flexible pavement in order to have an adequate falls to prevent the ponding of water and the pavement should be treated each spring with an effective weed killer.
Construction of flexible pavement:
The sub grade is covered with a sub-base, a base course and a wearing course; the last two components are collectively known as the surfacing. The sub-base can consist of any material which remains stable in water such as crushed stone, dry lean concrete and blast furnace slag.
Compacted dry-bound macadam in a 75 to 125 mm thick layer with a 25mm thick overlay of firmer material or compacted wet mix macadam in 75 to 150mm thick layers could also be used. The materials chosen should also be unaffected by frost and be well compacted in layers giving a compacted thickness of between 100 and 150mm for each layer.
The base course of the surfacing can consist of rolled asphalt, dense tar macadam, dense bitumen macadam or open textured macadam and should be applied to a minimum thickness of 60mm. Base course are laid to the required finished road section providing any necessary gradients or cross falls, ready to receive the thinner wearing course which should be laid within three days of completing the base course. The wearing course is usually laid by machine and provides the water protection for the base layers. It should also have non-skid properties, reasonable resistance to glare, have good riding properties and have a good life expectancy. Materials which give these properties include hot-rolled asphalt, bitumen macadam, and dense tar surfacing and cold asphalt. Existing flexible road surfaces can be renovated quickly and cheaply by the application of a hot tar or cut back bitumen binder with a rolled layer of gravel, crushed stone or slag chippings applied immediately after the binder and before it sets. The above treatment ate termed bound surfaces but flexible roads or pavement with unbound surfaces can also constructed.
Rigid pavement
Introduction
This is a form of road using a concrete slab over a base layer. The preparation of the sub grade is as described above for flexible pavement and should be adequate protected against water. The base layer is laid over the sub grade and is required to form a working surface from which to case the concrete slab and enable work to proceed during wet and frosty weather without damage to the sub grade. Generally, granular materials such as crushed concrete crushed stone, crushed slag and suitably graded gravels are used to form the base layer. Joints used in rigid pavements may be either transverse or longitudinal and are including in the design to;
•Limit the size of slab
•Limit the stresses due to sub grade restraint
•Make provision for slab movement such as expansion, contraction and warping
The spacing if joints will be governed by a number of factors, namely slab thickness, presence of reinforcement, traffic intensity and the temperature at which the concrete is placed. Five types of joint are used in rigid road pavement construction is:
•Expansion joints
•Contraction joints
•Longitudinal joints
•Construction joint
•Warping joints
Road joint can require filter and or sealers; the former needs to be compressible whereas the latter should protect the joint against the entry of water and grit. Suitable materials for filter are soft knot-free timber, impregnated fireboard, chipboard, cork and cellular rubber. The common sealing compounds used are resinous compound, rubber bituminous compound and straight run bitumen compounds containing filters. The sealed surface groove use in contraction joints to predetermine the position of a crack be formed while casting the slab or be sawn into the hardened concrete using water-cooled circular saws. Although slightly dearer than the formed joint, sawn joints require less labor and generally give a better finish.
The curing of newly laid rigid roads and paving is important if the concrete strength is to be maintained and the formation of surface cracks is to avoided. Curing precautions should commence as soon as practicable after laying, preferably within 15 minutes of completion from the rapid drying effects of the sun and wind. This form covering will also prevent unsightly pitting of the surface during rain. Light covering materials such as waterproof paper and plastic film can be laid directly onto the concrete surface, ensuring that they are adequately secured at the edges. Plastic film can give rise to a smooth surface to support the covering. Heavier covering such as tarpaulin sheets will to be supported on the frames of timber of light metal work so that the covering is completely clear of the concrete surface.
Construction of rigid pavement
The thickness of concrete slabs used in rigid pavement construction will depend upon the condition of the sub grade, intensity of traffic and whether the slab is to be reinforced. With a normal sub grade using a base layer of 80mm thick the slab thickness would vary form 125mm for a reinforced slab carrying light traffic to 200mm for an unreinforced slab carrying a medium to heavy traffic intensity. The usual strength specification is 28 MN/ m² at 28 days with no more than 1% test cube failure rate; therefore the mix design should be based on a mean strength of the between 40 and 50 MN/ m² , depending upon the degree of quality control possible on or off site. To minimize the damage that can cause by frost and de-icing salts, the water / cement ratio should not exceed 0.5 by weight, and air entrainment to at least the top 50mm of the concrete should be specified. The air-entraining agent used should produce 3 to 6% of minute air bubbles in the hardened concrete thus preventing saturation of the slab by capillary action.
Before the concrete is laid the base layer should be covered with a slip membrane of polythene sheer which will also prevent grout loss form the concrete slab. Concrete slabs are usually laid between pressed steel road forms which are positioned and fixed to the ground with steel stakes. These sides forms are designed to provide the guide for hand tamping or to provide for a concrete train consisting of spreaders and compacting units. Curved or flexible roads forms have no top or bottom flange and are secured to the ground with an increased number of steel stakes.
Reinforcement generally in the form of a welded steel fabric complying with the recommendations of BS 4483 can be included in rigid pavement constructions to prevent the formation of cracks and to enable the number of expansion and contraction joints required to be reduced. If bar reinforcement will depend in the thickness of concrete, for slabs less than 150mm thick the minimum cover should be 50mm and for slabs over 150mm thick the minimum cover should be 60mm.
Comparison flexible pavement and rigid pavement
Flexible pavement
•Material = asphalt
•Comfortable
•More economic
•Do not used joints reinforcement
•Wearing courses are provided the water protection for the base ;layer. It should also have non- skid properties reasonable resistance to glare, have good riding properties and have a good life expectancy
•The asphalt is easy to distress
Rigid pavement
•Material = concrete.
•Not comfortable and reduce the accident.
•Cheaper than flexible pavement.
•Used joints and reinforcement.
•The concrete are provided the water protection but before the concrete is laid the base layer should be cover with a slip membrane of polythene sheet which will also prevent grout less from concrete slab.
•The concrete difficult to distress.
The sub-base for a flexible pavement is laid directly onto the formation level and should consist of a well compacted granular material such as a quarry overburden or crushed rocks. The actual thickness of sub-base required is determined by the cumulative number of standard axle is taken (msa) to be carried (msa = millions of standard axles, where a standard axle is taken as 8200 kg) and the CBR (California bearing ratio) of the sub grade.
The CBR is an empirical method in which the thickness of the sub base is related to the strength of the sub grade and to the amount of traffic the road is expected to carry. A fully flexible pavement, design life should be at least 20 years.
These are suitable for light vehicular traffic where violet braking and or acceleration are not anticipated such as driveways to domestic properties.
Unbound pavement consist of a 100 to 200mm thick base of clinker or hardcore laid directly onto the information level of the sub grade and this is covered with a well rolled layer of screeded gravel to pass a 40mm ring with sufficient sand to fill the small voids to form an overall consolidated thickness of 25mm. this will give a relatively cheap flexible pavement in order to have an adequate falls to prevent the ponding of water and the pavement should be treated each spring with an effective weed killer.
Construction of flexible pavement:
The sub grade is covered with a sub-base, a base course and a wearing course; the last two components are collectively known as the surfacing. The sub-base can consist of any material which remains stable in water such as crushed stone, dry lean concrete and blast furnace slag.
Compacted dry-bound macadam in a 75 to 125 mm thick layer with a 25mm thick overlay of firmer material or compacted wet mix macadam in 75 to 150mm thick layers could also be used. The materials chosen should also be unaffected by frost and be well compacted in layers giving a compacted thickness of between 100 and 150mm for each layer.
The base course of the surfacing can consist of rolled asphalt, dense tar macadam, dense bitumen macadam or open textured macadam and should be applied to a minimum thickness of 60mm. Base course are laid to the required finished road section providing any necessary gradients or cross falls, ready to receive the thinner wearing course which should be laid within three days of completing the base course. The wearing course is usually laid by machine and provides the water protection for the base layers. It should also have non-skid properties, reasonable resistance to glare, have good riding properties and have a good life expectancy. Materials which give these properties include hot-rolled asphalt, bitumen macadam, and dense tar surfacing and cold asphalt. Existing flexible road surfaces can be renovated quickly and cheaply by the application of a hot tar or cut back bitumen binder with a rolled layer of gravel, crushed stone or slag chippings applied immediately after the binder and before it sets. The above treatment ate termed bound surfaces but flexible roads or pavement with unbound surfaces can also constructed.
Rigid pavement
Introduction
This is a form of road using a concrete slab over a base layer. The preparation of the sub grade is as described above for flexible pavement and should be adequate protected against water. The base layer is laid over the sub grade and is required to form a working surface from which to case the concrete slab and enable work to proceed during wet and frosty weather without damage to the sub grade. Generally, granular materials such as crushed concrete crushed stone, crushed slag and suitably graded gravels are used to form the base layer. Joints used in rigid pavements may be either transverse or longitudinal and are including in the design to;
•Limit the size of slab
•Limit the stresses due to sub grade restraint
•Make provision for slab movement such as expansion, contraction and warping
The spacing if joints will be governed by a number of factors, namely slab thickness, presence of reinforcement, traffic intensity and the temperature at which the concrete is placed. Five types of joint are used in rigid road pavement construction is:
•Expansion joints
•Contraction joints
•Longitudinal joints
•Construction joint
•Warping joints
Road joint can require filter and or sealers; the former needs to be compressible whereas the latter should protect the joint against the entry of water and grit. Suitable materials for filter are soft knot-free timber, impregnated fireboard, chipboard, cork and cellular rubber. The common sealing compounds used are resinous compound, rubber bituminous compound and straight run bitumen compounds containing filters. The sealed surface groove use in contraction joints to predetermine the position of a crack be formed while casting the slab or be sawn into the hardened concrete using water-cooled circular saws. Although slightly dearer than the formed joint, sawn joints require less labor and generally give a better finish.
The curing of newly laid rigid roads and paving is important if the concrete strength is to be maintained and the formation of surface cracks is to avoided. Curing precautions should commence as soon as practicable after laying, preferably within 15 minutes of completion from the rapid drying effects of the sun and wind. This form covering will also prevent unsightly pitting of the surface during rain. Light covering materials such as waterproof paper and plastic film can be laid directly onto the concrete surface, ensuring that they are adequately secured at the edges. Plastic film can give rise to a smooth surface to support the covering. Heavier covering such as tarpaulin sheets will to be supported on the frames of timber of light metal work so that the covering is completely clear of the concrete surface.
Construction of rigid pavement
The thickness of concrete slabs used in rigid pavement construction will depend upon the condition of the sub grade, intensity of traffic and whether the slab is to be reinforced. With a normal sub grade using a base layer of 80mm thick the slab thickness would vary form 125mm for a reinforced slab carrying light traffic to 200mm for an unreinforced slab carrying a medium to heavy traffic intensity. The usual strength specification is 28 MN/ m² at 28 days with no more than 1% test cube failure rate; therefore the mix design should be based on a mean strength of the between 40 and 50 MN/ m² , depending upon the degree of quality control possible on or off site. To minimize the damage that can cause by frost and de-icing salts, the water / cement ratio should not exceed 0.5 by weight, and air entrainment to at least the top 50mm of the concrete should be specified. The air-entraining agent used should produce 3 to 6% of minute air bubbles in the hardened concrete thus preventing saturation of the slab by capillary action.
Before the concrete is laid the base layer should be covered with a slip membrane of polythene sheer which will also prevent grout loss form the concrete slab. Concrete slabs are usually laid between pressed steel road forms which are positioned and fixed to the ground with steel stakes. These sides forms are designed to provide the guide for hand tamping or to provide for a concrete train consisting of spreaders and compacting units. Curved or flexible roads forms have no top or bottom flange and are secured to the ground with an increased number of steel stakes.
Reinforcement generally in the form of a welded steel fabric complying with the recommendations of BS 4483 can be included in rigid pavement constructions to prevent the formation of cracks and to enable the number of expansion and contraction joints required to be reduced. If bar reinforcement will depend in the thickness of concrete, for slabs less than 150mm thick the minimum cover should be 50mm and for slabs over 150mm thick the minimum cover should be 60mm.
Comparison flexible pavement and rigid pavement
Flexible pavement
•Material = asphalt
•Comfortable
•More economic
•Do not used joints reinforcement
•Wearing courses are provided the water protection for the base ;layer. It should also have non- skid properties reasonable resistance to glare, have good riding properties and have a good life expectancy
•The asphalt is easy to distress
Rigid pavement
•Material = concrete.
•Not comfortable and reduce the accident.
•Cheaper than flexible pavement.
•Used joints and reinforcement.
•The concrete are provided the water protection but before the concrete is laid the base layer should be cover with a slip membrane of polythene sheet which will also prevent grout less from concrete slab.
•The concrete difficult to distress.
