CONCRETE ROADS AND CONSTRUCTION
On the basis of initial cost of pavement, Concrete Roads are reported to
be around 14-15% costlier than the Bituminous Roads. However, on life
cycle cost basis Concrete Roads, due to their long life of 30-40 years
and high durability, Concrete Roads are estimated to be around 25-30%
cheaper than the Bituminous Roads.
Construction of Cement Concrete Road:
The construction of cement concrete road involves following operations :
1. Preparation of Sub-grade :
(I) Sub-grade is natural soil on which concrete slab is laid.
(II) It is cleaned, shaped and levelled.
After cleaning, it is prepared to the required grade and profile.
(III) It should be seen that sub-grade has uniform strength over its entire width.
(IV) If any local weak spots are found, they should be removed and strengthened by placing new material which is compacted.
(V) When concrete is to be directly placed on sub-grade, the surface should be saturated with water for 6 to 20 hrs in advance of placing the concrete.
This is done to ensure that sub-grade does not absorb water from the concrete.
2. Provision of Sub-base :
(I) When natural sub-grade is not very firm, a sub-base over the sub-grade is provided.
(II) Depending upon the type of soil, design load, intensity of traffic and economic consideration, the decision for providing the sub-base is taken.
3. Placing the Forms in Cement Concrete Road :
(I) The forms may be made up of steel or timber.
(II) The steel forms are of mild steel channel sections and their depth is equal to the thickness of the pavement.
(III) Forms are properly braced and fixed to the ground by means of stakes.
(IV) Forms are fixed in position by 3 stakes at back of each 3 m length.
(V) When the forms are fixed, they must be checked for their trueness.
(VI) The maximum deviations permissible in the vertical plane is 3 mm and in horizontal plane 5 mm in 3 m length of the form.
(VII) The forms are oiled before placing concrete in them.
4. Watering the Prepared Sub-grade or Sub-base :
(I) After the forms are fixed, the prepared surface to receive concrete is made moist.
(II) If the sub-grade is dry, it should be sprinkled with as much quantity of water as it can absorb.
(III) It is advisable to wet the surface at least 12 hrs in advance of placing the concrete.
(IV) When insulating layer of water-poof paper is provided, the moistening of the surface prior to placing the concrete is not required.
5. Batching of Materials and Mixing :
(I) After determining the proportions of ingredients for the Concrete mix, the fine and course aggregates are properly proportioned by weight in weight-batching plant.
(II) They are then fed into the hopper along with necessary quantity of cement which is also measured by weight.
(III) The ingredients of concrete are mixed in proper proportions in dry state. The mixing should preferably be done in a concrete mixer.
(IV) The measured quantity of water is added so that the desired water cement ratio is obtained.
6. Transporting and Placing of Concrete :
(I) After mixing, the concrete is transported to the site in wheel burrows or in pans which are manually carried.
(I) The mixed concrete is deposited rapidly on the sub-grade in layer of thickness not more than 50 mm to 80 mm or about two or three times the size of aggregates.
(III) The concrete should be placed over the entire width of bay in successive batches as a continuous operation and topmost layer is laid about 10 mm higher than the actual profile for further tamping.
(IV) The top layer should also be laid to the required camber and gradient, while placing the concrete it is roded with suitable tool to eliminate voids.
(V) Segregation of concrete is avoided during transportation and placing. When reinforcement has been specified in road slab, concrete is placed in two stages.
(VI) In first stage, concrete is placed and compacted to the depth corresponding to the level of reinforcement shown on the drawings.
(VII) Reinforcement is then placed on top of compacted concrete and remaining thickness of slab is then completed in second stage.
7. Compaction of Cement Concrete Road :
(I) After the concrete is placed in its position, it should brought in its proper position by heavy screed or tamper fitted with suitable handles.
(II) The wooden tamper is at least 75 mm wide and its underside is shaped to the finished cross-section of the slab.
(III) Its weight is about 10 kg/m.
It should have sufficient strength to retain its shape under all the working conditions
(IV) Its length is equal to length of bay plus 60 mm.
(V) The underside of tamper is provided by a metal plate of 5 mm thickness as shown in Fig. 4.14.4.
(VI) The tamper is placed on the side form and its handles are griped by the men who use the tamper.
(VII) Concrete is also compacted by means of a power driven finishing machine or by vibrating hand screed.
Upto 12.5 cm thickness of slabs screed vibrators alone can be used for compaction.
(VIII) For greater thickness, immersion vibrator is used.
8. Floating :
(I) After compaction, the entire slab surface is floated longitudinally with a wooden float board.
(II) The purpose of floating is to provide an even surface free from corrugations.
9. Belting :
(I) After floating, the surface is further finished by belting just before the concrete become hard.
(II) The purpose of belting is to make the road surface non-slippery and skid resistant. This operation is sometimes omitted.
10. Brooming :
(I)After belting, brooming is done by drawing brushes at right angles to the center line of road from edge to edge.
(II)Brooming is done just before the concrete becomes non- plastic.
(III) This operation is also sometimes omitted.
11. Edging:
After brooming, the edges of the slab are carefully finished with an edging tool before the concrete is finally set.
12. Curing Process of Cement Concrete Road :
(I) Curing consist of checking the loss of water from the concrete slab, and keeping the fresh concrete slab moist during hardening period.
(II) Initial curing is done for 24 hrs.
(III) By this time, the concrete becomes hard enough to walk upon and then wet mats are removed and final curing done for 2 to 3 weeks.
(IV) Final curing is done by any one of the following methods :
Ponding Method.
(V) By covering the slab with 4 to 8 cm thick layer of wet sand or earth.
(VI) Spraying a suitable chemical such as sodium or calcium chloride on concrete surface.
13. Fillings of Joints and Edging :
(I) After curing, the surface is cleaned and washed.
(II) The joints are then properly filled-in attains with a suitable sealing compound.
14. Opening to Traffic :
Concrete road is opened to traffic when it attains the required strength or after 28 days of curing.
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