What is a cement concrete road?
The cement concrete roads are in the form of monolithic slabs of cement concrete which serve two functions simultaneously, namely, as the load-carrying base and as the wearing surface. According to the structural behavior, the pavements can be classified as flexible pavement or rigid pavement. Bituminous concrete is one of the best flexible pavement layer materials. Various other types of bituminous roads are considered as flexible pavements. The cement concrete roads, on the other hand, are treated rigid pavements because of their rigidity.
Cement concrete road is a highly rigid surface and hence, for the success of such roads, the following two conditions should be satisfied:
(i) They should rest on non-rigid surface having uniform bearing capacity.
(ii) The combined thickness or depth of the concrete pavement and the non-rigid base should be sufficient to distribute the wheel load on a sufficient area of the sub-base so that the pressure on the unit area remains within the permissible safe bearing capacity of the soil.
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The cement concrete roads are becoming popular because of the fact that concrete of desired quality can be prepared by the moderm techniques of cement concrete construction.
Advantages of cement concrete roads:
Folllowings are the advantages of concrete roads:
- It can be designed more accurately for load distribution.
- It does not develop corrugations and hnce , it grants noisless surface.
- It has good visibility at night due to colour contrast.
- It has low cost of maintenance.
- It has low tractive resistance which diminishes the cost of running of vehicles.
- It has more useful life than road of any other -type of construction.
- It is dustless and has no internal attrition. (viii) It is not unduly slippery.
- It is possible to make it resistant to s_evere stresses and strains caused under unfavourable soil and temperature conditions by placing suitable reinforcements.
- It is possible to make use of old concrete road as a base or foundation for a new concrete road or for further bituminous treatment at the top.
- It is practically unaffected by weather and temperature.
- It permits the calculation of super-elevation on curves with more mathematical accuracy than other types of roads.
- It requires flat camber as the surface is impermeable.
DISADVANTAGES OF CEMENT CONCRETE ROADS
Following are the disadvantages of a cement concrete road:
(i) It causes the surface to shine in severe sunlight and the white glare reflected from surface may disturb the concentration of driver causing serious accident.
(ii) It does not permit easy access to the subsoil when trenches have to be opened to locate water mains, sewers and electric cables.
(iii) It is difficult to repair and needs expert supervision.
(iv) It is less resilient than W.B.M. or bituminous binder surfaces.
(v) It is liable to crack, warp and twist.
(vi) It is noisy under iron-tired traffic and vibrations are set up by heavy, rapid-moving, and self-propelled vehicles.
(vii) It proves to be very expensive in initial cost especially when a suitable local aggregate is not available.
(viii) It reacts. more readily because of its rigidity to stresses and strains caused by impact and may therefore crush under such conditions .
(ix) It requires long time to protect, cure and keep the surface free from the traffic which may cause delay and difficulties in business centres.
COMPARISON BETWEEN BITUMINOUS CONCRETE ROADS AND CEMENT CONCRETE ROADS
It will be interesting to summarise the two important types of ad surfaces in common use, namely, flexible pavement in the
form of bituminous concrete roads and rigid pavement in the form of cement concrete roads.
Table 1 shows the comparison of these types of roads with respect to specific items.
Table1. COMPARISON BETWEEN BITUMINOUS CONCRETE ROADS AND CEMENT CONCRETE ROADS
|No.||Item||Biuminous concrete road||Cement Concrete roads|
|1||Compaction||It is achieved by rolling with usual precautions.||It is done by the use of vibrators.|
|3||Formwork||Not necessary.||Required for edges and near joints.|
|5||Joints||Note to be provided||To be invariably provided|
|7||Method of placing||The premix is prepared in hot mix plant and it is to be placed in hot condition.||The concrete is prepared by mixing cement, aggregates and water and it is to be deposited before cement starts to set. No heating is required.|
|8||Opening of underground trenches||Can be easily opened up.||Difficult to open.|
|9||Opening to traffic||After 24 hours.||After 15 to 20 days.|
|10||Salvage value||Practically nil.||Can serve as a base or foundation for the new bituminous surface.|
|13||Utility||Less durable for heavy traffic, cost of repair an maintenance more.||More durable and reliable for heavy traffic.|
|14||Visibility at night||Bad||Good|
|15||Wear of surface||more||Less|
METHODS OF CONSTRUCTION CONCRETE ROADS
The cement concrete roads can either be constructed in a single course or two courses. In single course pavement, or two courses. In single course pavement, the entire depth of concrete is composed of homogeneous material. The concrete is laid in two courses or layers of equal or different depths with different concrete compositions in the two-course method.
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Following are the three methods of construction of cement concrete roads:
(1) Alternate bay method
(2) Continuous bay method
(3) Expansion joint and strip method.
(1) Alternate bay method:
In this method, if the road is of a single lane, it is divided into suitable bays of 6 m to 8 m length, and the construction work is carried out in alternate bays as shown in fig. below.
If the road is of the double lane; the construction work is carried out in odd bays of one lane and even bays of the other lane as shown in fig. 3. The construction of the next bays is commenced after the concrete laid in earlier bays dries out, i.e., nearly after one week or so.
Following are the advantages of this method:
(i) The bays which have been cured can serve as additional working space for the preparation of concrete.
(ii) The construction of joints can be easily carried out and its width can be suitably kept as desired.
The method, however, suffers from the following drawbacks:
i) As the construction activity is spread over the full width of road, the traffic will have to be completely diverted.
ii) During rainy season, the water collects on the surface of bays which are not constructed.
(iii It requires large number of transverse joints which may increase the construction cost and reduce the smooth riding quality of the surface.
iv) It requires more time to complete the work.
(2) Continuous bay method:
This is also known as strip method or full-width method. The entire width of the road is constructed continuously from one end to the other. No provision for expansion joint is made. However, a construction joint is provided where the new concrete meets the previously laid concrete.
This method is suitable for roads having width not exceeding 4.5 m and it is very simple in construction. However, it does not stand a high temperature variation as no provision is made for expansion and contraction.
It also requires the provision of a temporary diversion road since the entire road width is under construction. This method is generally not favoured.
(3) Expansion joint and strip method:
In this method, the road is divided into longitudinal strips and transverse bays by means of timber formwork as shown in fig 4. The joints are then suitably filled up with fillers like asphalt and finished so as to provide for the expansion of the concrete slab.
By this method, any width of road can be carries the traffic and it gives better alignment and finish. It also carries the traffic during construction and hence, no temporary diversion aroad is necessary . There has been considerable impovement in the technology of joints in concrte roads and hence, most of the modern concrete roads are constructed by this method.
CONSTRUCTION PROCEDURE FOR CEMENT CONCRETE ROADS
Following procedure is adopted for the construction of cement concrete roads:
- Preparation of subgrade and sub-base: The subgrade is the natural soil on which the concrete slab is laid. It must be cleaned, shaped, and leveled. It is properly and uniformly compacted with a roller. It is brought to the true grade and profile. It should be seen that the subgrade has Uniform strength over its entire width and it extends at least 300 mm on either side of the width to be concreted. If any local weak spots are found, they should be removed and strengthened by placing new material which is compacted by hand tamping. Where the subgrade consists of soft clay or at places where subsoil water trouble is anticipated, a suitable under-drainage system should be provided.
On the prepared subgrade, an insulating layer of 75 mm thick sand is provided. It is thoroughly wetted so that it may not absorb the water of concrete. It also allows easy creep of the concrete slab on the top of layer. Sometimes, special waterproof paper is used instead of sand layer as an insulating material. This paper prevents any absorption of water by the subsoil or by the subgrade.
If the subgrade has reasonable uniform bearing capacity, there rs no necessity of providing sub-base or foundation course to the concrete road. Depending upon the type of soil design load, intensity of traffic and economic consideration the decision for providing the sub-base is taken. The sub-base serves the following three purposes:
(I) It provides a capillary cut-off and thus the damage caused due to mu de pumping is prevented.
(ii) It provides a strong supporting layer.
(iii) It reduces the thickness of concrete slab and thus leads to lower cost of construction.
The sub-base may consist of W.B.M., granular material, stabilized soil or lean cement concrete of proportion (1 : 4 : 8). The W.B.M. is the most popular. type of sub-base which is generally adopted for concrete roads in our country.
The subgrade or sub-base, as the case may be, is completely checked at least two days m advance of concreting.
- Placing of forms: The forms may be of steel or timber. The steel form work are generally of mild steel channel sections and their depth is equal to the designed pavement thickness. They are usually in 3m lengths except on curves of radius less than 45 m where shorter sections are used. They are fixed in position by 3 spikes at the back of each 3 m length. When the consolidation is to be done by mechanical means, the steel forms are invariably used as the mechanical appliance can move on the top of these forms along the length of road and carry out the compaction of concrete. They are also suitable at places where deeper sections of forms are required.
The timber forms are dressed on side. They have a minimum base width of 100 mm for slab thickness upto 200 mm and a minimum base width of 150 mm for slab thickness over
200 mm. The depth of forms is equal to the thickness of the slab. These forms should rest upon stakes or pegs driven. into the ground at the back side of side forms. They should be firmly nailed to the stakes to resist the pressure of concrete. The cross forms are also fixed at the ends of bays. It. is also desirable to provide the top inside corner of the form with an angle iron to protect it during construction. They should be coated with boiled linseed oil before placing of concrete commences.
The forms should be set to the exact grade and alignment at least 30m in advance of the point where concrete is placed. It is essential that the forms are checked before the concrete is placed because on them depends the alignment of concrete slab.
Watering the prepared subgrade or sub base: After the forms are fixed, the prepared surface to receive concrete is made moist. But it should be seen that pools are not formed. If the subgrade is dry, it should be sprinkled with as much quantity of
water as it can absorb. It is advisable to wet the surface at least 12 hours in advance of placing the concrete. When insulating layer of waterproof paper is provided, the moistening of the surface prior to placing of concrete is not required.
(4) Mixing and placing of concrete: The ingredients of concrete are mixed in proper proportions in a dry state. The mixing should generally be done in a concrete mixer. A measured quantity of water is added to achieve the desired water-cement ratio is obtained.
The mixed concrete is deposited rapidly on the subgrade in layer of thickness not more than 50 mm to 80 mm or about two or three times the size of aggregates. The concrete should be placed over the entire width of bay in successive batches as a continuous operation and the topmost layer is laid about 1 O mm higher than the actual profile for further tamping. The top layer should also be laid to the required camber and gradient.
All the operations from the time when water is added in the mixer to the surface finishing should be completed within the setting time of the cement. While the concrete is being placed, it should be vigorously spliced to avoid the formation of air pockets which give honeycomb concrete. It should also be seen that the concrete is not poured from a height, but it is gently lowered and placed on the surface.
When necessary, the steel mesh reinforcement should be placed as indicated on the drawings before the placing of concrete commences. The reinforcement should be securely supported in its correct position and care should be taken to see that it is not displaced during the concreting operations.
(5) Compaction and finishing: After the concrete is placed in its position, it should be brought in its proper position by a heavy screed or tamper fitted with suitable handles. The wooden tamper is at least 75 mm wide and its underside is shaped to the finished cross-section of the slab.
Its weight is about 10 kg/m and it should have sufficient strength to retain its shape under all the working conditions. Its length is equal to length of bay plus 600 mm. The underside of tamper is provided by a metal plate of 5 mm thickness. Fig. 6-4 shows a typical wooden tamper.
The tamper is placed on the side forms and its handles are gripped by the men who use the tamper while standing. The tamper is used across the bay. It is moved forward in a saw-rn motion in combination with a series of lifts and drops of 25 mm to 50 mm with a forward shift of 25 mm.
At the transverse joints, the tamper is stopped within one metre of the joint, lifted up and lased on the joint and drawn backward therefrom. The surplus concrete is removed by means of the shovel and thrown ahead of the joints. The surface is immediately inspected after this operation for high and low spots and defects, if any, are rectified.
The entire concrete surface is then floated longitudinally with a wooden float board 20 mm wide, 50 mm thick and 5 m long. This board is provided with convenient handles at its ends. While using it, the two men sit on portable timber bridges that span the slab’s width. The board’s lower surface is straight and it is placed with its longer side parallel to the longitudinal axis of the road slab. It is then drawn back and forth in strokes of 600 mm and advanced slowly from one side of the slab to the other.
The purpose of this operation is to give a uniform and even surface free from ransverse corrugations. When the entire width of slab is floated, the two bridges are moved forward so that the next section to be floated overlaps the previous one by 1 m to 1.20 m.
The trueness and straightness of the finished surface in longitudinal direction is checked by a straight wooden edge 75 mm deep, 37.50 mm wide and 3 m long after the excess water has disappeared. The straight edge is swung from handles and irregularities in the surface, if any, are rectified.
Similarly, the trueness of surface in the cross direction is checked and the defects, if any, are removed. The straight edging and re-floating are continued until the entire surface is true to camber and grade.
. (6) Belting, brooming and edging: If necessary, the surface 1s further finished by a rubber or canvas belt 150 mm to 300 mm wide and of sufficient length longer than the width of the road. It is fitted with handles at both ends. It is moved in a saw-in motion in both the transverse and longitudinal directions.
If it is desired to have a rough surface,. the brooming is carried by a fibre broom brush after belting. The broom is came ouII d in a direction perpendicular to the centre-lne of the gentIY put in such a manner that a uniform roughness is obtained.
The edges of the slab are carefully finishe.d with an edging tool before the concrete becomes hard and stiff. The expansion joints are also rounded off at the corners.
(7) Curing: After 12 hours or so, the finished surface is covered with wet gunny bags for a duration of 24 hours. The bags are then removed and 40 mm to 50 mm layer of sand is spread on the surface. The sand layer is kept moist for 14 days. As an alternative, the curing can also be carried out by ponding. The surface s divided into a number of bays by forming small 5 mm high earthen ridges and the bays are filled with 40 mm depth of water and the surface is kept thoroughly wet for 14 days. The surface is cleaned and washed after curing.
(8) Opening to traffic: The expansion joints are suitably finished. The edges of the road are provided with suitable shoulders of macadam, hard moorum, or bricks. The edging protects the road slab. The road is then opened to traffic after 28 days of consolidation and finishing of concrete or when the concrete attains the required strength.
Equipment for Bituminous road:
You can read here: Equipment used for Bituminous Road construction
EQUIPMENTS FOR CEMENT CONCRETE ROADS
For the construction of cement concrete roads, the plants and equipment will be required for batching, mixing, placing, finishing and curing of the concrete.
Following are the commonly used machinery (Equipment) for cement concrete roads:
(1) Batching plant
(2) Concrete mixer
(3) Vibrating screed
(4) Internal vibrator
(6) Straight edge
(8) Fibre brush
Each of the above machinery used in the construction of cement concrete roads will now be described in brief.
- Batching plant:
The batching plant is a mechanical equipment for measuring, either by weight or by volume, the quantities of different ingredients required to make up each complete charge of a concrete mixer. The types of batching plant in which the quantities of construction materials are measured by weight is called a weigh batcher.
If batching by volume is to be done, separate measuring boxes are provided for the different aggregates. Each box is provided with handles for convenient lifting and loading the mixer. The excess material after filling each box is removed by a straight edge.
- Concrete mixer:
The concrete mixers of various types and capacities are available in the market. It is either of tilting type or non-tilting type. They are generally provided with power-operated loading hoppers.
The concrete mixer consists of a drum with blades in the inside portion. The arrangement is provided to revolve or rotate the drum. The aggregates are collected in dry condition in hopper and they are then placed in the revolving drum of the mixer.
The aggregates together with water are rotated for a certain period. The mixing of the aggregates is thus carried out and the wet concrete thus discharged by the concrete mixer is consumed in 30 minutes.
Suitable arrangement is generally provided on the concrete mixer to allow only the required quantity of water to enter the mixer per batch. After use, the concrete mixer should be thoroughly washed and cleaned, If this precaution is not taken, cakes of hardened concrete will be formed inside the mixer. These cakes difficult to remove at a later stage and they considerably affect the efficiency of concrete mixer.
- Vibrating screed:
The vibrating screen is used for the compaction and finishing of concrete. It consists of wooden or mild steel screed with suitable handles. It is driven by vibrating units mounted thereon and propelled either electrically or by compressed air or by a petrol engine and made to travel on side forms.
- Internal vibrator:
This is also known as an immersion vibrator and it comprises of a vibrating head with suitable motive power either of compressed air, electricity, or of a petrol-driven engine. It is employed to ensure compaction of concrete along the forms to avoid any tendency in honeycombing at the edges of slab.
For roads exceeding 125 mm in thickness, the internal vibrators are provided at suitable spacings for compacting the concrete over the entire width of the slab in addition to the vibrating screed.
(5) Float: The longitudinal float is made of hard wood and it is provided with a handle. It is of 750 mm length and 75 mm width. It is used for smoothening the concrete.
(6) Straight edge: The straight edge is made of hard wood with mild steel plate at bottom. It is of 3 m length and 100 mm width. It is provided with two handles. It is used to check the finished pavement surface in the longitudinal direction.
(7) Belt: A canvas belt with two wooden handles at the ends is used for finishing the pavement surface before the concrete hardens. It is 250 mm in width and at least 600 mm longer than the width of the slab. For better working, a minimum of two belts should be provided.
(8) Fibre brush: A fibre brush is used to make marks across the pavement surface so that it becomes skid resistant. It is made from good quality hard fibres which project from the wooden brush of 450 mm length and 75 mm width. It is provided with a handle about 2 m long.
(9) Miscellaneous: In addition to the above main tools, various other miscellaneous tools and equipment required to finish up the work would be edging tools, spades, shovels, iron pans, water pots, rods, etc.
For large scale work, the loose concrete can be spread, compacted and finished by a travelling plant running slowly on the top of the steel side forms which are used as side shuttering for the concrete surfacing.
One plant is required for spreading the concrete in the required position and another for consolidating it and finishing its surface. The latter plant consists of a screed in its front, a vibrating beam in the center, and a finishing beam at its rear.
After the loose concrete is spread, the surface of the concrete is struck off by the front screed to the required thickness. The compacting unit consists of a surface vibrator and the vibrations compact the layer of freshly mixed and freshly spread concrete.
The finishing beam smoothens the surface which is compacted by the surface vibrator. If necessary, the hand float may be used for the proper finish after the mechanical finishing is over. The traveling plant permits the use of a stiff concrete mix and the consolidation of concrete is also better than when it is done by manual labor.
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