S. R. Vijayaraghavan, Scientist
M. L. Bhatia, Ex-Scientist
M. Dinakaran, Ex-Scientist
Y. R. Phull, Deputy Director
Central Road Research Institute, New Delhi
ABSTRACT
Rural roads constructed in flood prone areas using thin bituminous surfacings over granular bases get worn off and damaged when they get covered under flood water. When the floods recede, again a new pavement is required to be constructed. For the limited traffic which ply on roads such a recurring expenditure is not justified and at the same time to allow the movement of agricultural produce the road link has to be maintained. To obviate these difficulties, a new pavement system has been evolved at the Central Road Research Institute, NEw Delhi making use of cement has been evolved at the Central Road Research Institute, NEw Delhi making use of cement concrete trackways. This consists of two separated rows of precast rectangular oncrete blocks interconnected by dowels, laid over a Lean-Cement Fly Ash Concerte base. Apart from providing a good service to normal rural road traffic including iron-tyred bullock carts, the trackways do not get damaged when immeresed under flood water and remain intact for further use for a long time.
Based on the design and construction technology provided by CRRI, Haryana PWD laid on experimental trackway of 400 metres length in the flood prone area adjacent of Jamuna river on the road from village Chauri Mundi Garhi to village Titapur in Karnal District of Panipat Division.
The paper presents the specification evolved, the methodology adopted for the manufacture of block at site, handling, transportation and placement of blocks and other construction details, as well as the uptodate performance, which is satisfactory.
Introduction
We have been having ambitious programme of providing roads in the rural and tribal areas of the country with estimated annual expenditure of the order 300 corores which is enhanced every year. While part of the expenditure is towards providing all-weather roads to thousands of villages, the balance is spent in providing atleast some fair-weather roads intended to provide connections to villages when they are free from flooding from rains or from flooding from rains or from rivers etc., across or in the basins of which they are located.
Such roads are likely to be under the flood waters of about 1-2 metres fro a period of 2-3 months during rains. While the provisions of high embankments is prohibitively costly and such high cost is not waranted from consideration of low intensity traffic in such cases it is also likely that the embankments may get washed off
due to the fast current of flood waters. Under such conditions of inundation over a period of couple of months, the specification of thin bituminous surfacings over granualr bases normally provided in such cases get worn off in no time, leading to infructuous expenditure towards their construction and maintenance.
Keeping in view the above factors a new pavement system has been evolved at the Central Road Research Institute, New Delhi making use of cement concrete trackway, which consists of two separated rows of precast rectangular concrete blocks, with adjacent block being interconnected by dowels, laid over lean cement flyash concrete or cement stabilised base. These have the advantage that while they are able to withstand the floods in general (without scour), local repairs if any needed, can be carried out by removing a few blocks and replacing them when the flood recede,
Even if the pavement levels are to be raised at a later stage, the block can be reused as surfacing after raising the subbase/subgrade levels.
The Haryana State PWD experienced the problem reffered above due to floods damaging some of their village roads frequently and CRRI was requested to suggest a suitable pavement design which could cope up with the above problem. Considering the various aspects of site conditions, costs etc. the trackway design evolved was suggested for adoption in this case. With technical assistance from CRRI a stretch of 400m length was constructed near Jamuna river in the Panipat Division.
The paper presents the details of the new pavements system evolved, its construction and performance.
Basis of Design
The design and specifications have been prepared on the basis of the following requirements:
(a) Roads are to be temporary.
(b) They will remain under flood water for about 2 to 2½ months in a year.
(c) No high embankment should be provided for these roads in view of high cost and their likelihood of being washed away.
(d) While the anticipated traffic will be very small, it will generally comprise of different categories of traffic such as bullock-carts, tractor-trailors, occasional buses/trucks and jeeps.
(e) Transportation of material to the site will be difficult and should be brought down to minimum.
Details of the New Pavement System
While it is possible to design such pavements for higher intensities of traffic also, the system indicated herein is intended for light intensity of traffic normally plying in our rural road system.
The components of the System adopted are as follows.
Precast Cement Concrete Blocks
Dimensions 90 cm x 60 cm
Thickness 10 cm
Spacing of two 1.65 m c/c and
Parallel rows 0.75 clear
Concrete Strength 28 days flexural strength of 40 kg/cm² in the field
interconnecting M. S. bars 12mm dia, 20 cm long bitumen coated - 3 nos for each block
Hotels for Dowels 3 Nos. 14 mm dia and 10 cm long.
The details of the blocks are given in Fig. 1 and Fig. 2 shows the view of the two blocks interconnected by dowels.
3.2.2 Lean cement flyash concrete Subbase
Lean cement flyash concrete may be used for constructing subbase over which the precast concrete blocks could be laid. Addition of flyash in lean cement concrete mixes, decreases bleeding and segregation, improved plasticity and cohesiveness and permits easier lacing and finishing concrete. With suitable designed mixes of such concrete, it is possible to achieve substantial saving in cement, to the extent of 35-40 per cent, compared to conventional lean concrete mixes.
Width of Subbase
(Lean cement flyash concrete) 2.85m
Thickness of subbase 10 cm
Strength (28 days) Compressive Strength 40-60 kg/cm² at 28 days in the field
3.2.3 Subgrade
Width of compaction 6.0m
Depth of compaction 15 cm
Degree of compaction 95-100% Proctor
3.3.4 Material between the Trackways
Lean cement flyash concrete 0.75 m
Thikckness of L.C.F.C. 10 cm (compacted
Strength (28 days) 60 kg/cm²compressive strenth at 28 days in the field
3.2.5 Shoulders
Width of shoulder (at top) 1.72 m
Side slope of soulder 1:2.5
Thickness 20 cm (compacted in two layers)
Degree of compaction 95-100% Proctor
A typical cross section of the cement concrete trackway system is shown in Fig. 3 .
Location of Experiment
The experimented length of 400 metres of cement concrete precast block trackway was constructed in the flood prone area adjacent to Jamuna river in Panipat Division and is located on the road from vllage Chauri Mundi Garhi to village Titarpur in Karnal District.At the time of taking up the work in 1981 the trackway was estimated to cost about Rs. 9000/- per kilometre length.
Manufacture of Precast Blocks
Casting of Blocks
The precast blocks were got manufactured in a central casting yard in the vicinity of the proposed road with complete arrangements for batching by weight, mixing in power driven mixers, compacting by suitable pan vibrators and a curing tank of adequate capacity to ensure 28 days curing for all the blocks cast.
Mild steel moulds of appropriate size with true surfaces were used for the manufacture of the blocks. The holes for the interconnecting dowels were made by casting M.S. pipes 14mm diameter in the precast units.
These pipes should be securely fixed to the moulds so that proper location and alignment of the pipes were ensured in the blocks and no difficulty would be experienced in assembling the blocks with interconnecting dowels. Fig.4 shows the moulds being set ready for pouring concrete.
The blocks were cured in the moulds under moist gunny bags for 24 hours and then they were removed from the moulds and properly stacked in the curing tank for water curing upto 28 days. After 28 days, the blocks were removed from the curing thank and allowed to dry. The holes for dowels were raked clean and checked for proper alignment. The blocks were then stacked ready for use.
Handling of the Blocks
A simple handling arrangement consisting of hooks and two chains clung from a rod for carrying the block was designed and got fabricated. Two people could lift one block with this arrangement. About 3 or 4 such asemblies manualy handled were found to be adequate for moving the blocks to and from curing tank and for replacement in-situ. For bigger installations with mass production capacities, mechanical methods of lifting and placement could be developed to ensure speed.
Preparation of Foundation for creteways
Subgrade: The subgrade soil (sandy silt) was excavated toa depth of 15 cm and recompacted by 8-10 tonne smooth sheeled roller to atleast 95% Proctor density at optimum moisture content. In cases where the soil is predominantly sandy, compaction may be effected through the use of vibrating rollers. Alternatively the soil may be mechanically stabilised through the addition of appropriate proportion of clay fraction or flyash and compacted in the usual manner.
Subbase: A 10 cm thick compacted layer of lean cement flyash concrete was provided as subbase for the trackways. The procedure for mixing and laying was as per IRC: 74-1979. The compaction was carried out by 8-10 tonne roller. After compaction and checking the trueness of surface and rectifying wherever necessary, the compacted subbase was cured for a minimum period of 7-days by keeping it moist through out by frequent sprinkling of water. The surface should not be allowed to dry during the curing period.
Assembly of Precast Block at Site: The two parallel rows of trackways 0.90m wide were marked on the prepared subbase at a distance of 0.375 m on either side of the centre-line of the alignment. Any minor depressions in the subbase was filled with a leveling course of sand to ensure full seating of the precast blocks. The precast blocks were transported from casting yard to construction site by bullock carts (Fig. 5). The first precast block was placed in position to proper level and camber and the inter-connecting dowels precoated with bitumen was inserted into the holes in the block. The second block was placed next to the first so that the dowel ends projecting from the first block just enter the holes in the second block and then the second block was gradually slided forward to join first block (Fig. 6). Further assembly was made in a similar manner. The dowels were slightly smaller than the holes and therefore could permit slight changes of curvature in alignment or transverse or vertical displacement. At curves, however, the dowels should be omitted.
Provision of Lean Cement Flyash Concrete between the Trackways: When the tracways were assembled for a sufficient length, the 0.75 m width between them was brought to level with the trackway surface with compacted lean cement flyash concrete following the procedure as mentioned in para 6.2. Fig. 7 shows view of the completed trackway.
Shoulders : On the outer side of trackways 1.735 m wide shoulders with side slopes of 1:25 was provided with compacted soil/mechanically stabilised soil; compacted at optimum moisture content to 95-100% proctor density, in two layers each of 10cm compacted thickness.
Load Tests
Plate bearing tests, using 30cm dia. Plate were carried out at different locations on the test track both on the right and left rows of concrete blocks as well as on the lean cement-flyash concrete surface. On different concrete slabs, under the load of 4000 kg on the plate, the settlement recorded varied from 0.101 cm to 0.132 cm. The recovery in removal of load was 82.3%. The 'K' value (reaction modulus) on the block varied from 47.5 kg/cm³. Results also showed load was transferred upto blocks next to the loaded block. However the subsequent blocks on either side did not indicate any deflection indicating no load transfer beyond the first. When the load tests were performed on the central (lean cement flyash concrete) portion the deflection profile extended to the full width of the pavement.
Condition Survey and Performance
8.1 The condition survey of the above constructed experimental length of 400 metres of cement concrete precast block trackway was carried out periodically. The traffic mainly consisted of bullock carts, tractor with trailors, jeeps, cars and some occasional trucks. The following observations were made at the site:
Out of the total number of blocks of about 1350 laid, 7 blocks were broken in transit during cartage. At few locations where the blocks could not be seated properly and observed to be shaky at the time of movement of traffic, the blocks (about 10%) were found to be cracked. But they were not disturbed from their original position in spite of the traffic. After 3½ years of traffic, the abrasion of the precast blocks was observed to be insignificant and brush marks on the blocks were quite visible. The central portion of lean cement-flyash concrete existing between two parallel rows of blocks were observed to be slightly abraded at some locations. Using a straight edge periodic observations at selected locations to monitor the longitudinal and transverse differential settlements were also made and no significant changes were noticed (Fig. 8).
Concluding Remarks
The construction of the pavement length with this precast concrete trackway system did not pose any difficulty or problems at site. After three and half years of traffic the performance has been very satisfactory. The abrasion of the precast blocks was insignificant as could be seen from the brush marks on the blocks which were almost intact. No dislocation of the blocks was observed with respect to its longitudinal alignment. Only at few places the blocks were observed to be shaky at the time of movement of load, due to levelling of the bottom surface of the blocks with sand and proper seating not having been done adequately. The load tests indicated that even the loose fitting dowels of 10mm dia could transfer some load, though they were intended only to prevent blocks from moving sideways. If the continuity of blocks and increased load transfer was required, the same could be achieved with the use of higher dia. road.
Acknowledgements
The authors are thankful to Dr. M. P. Dhir, Director Central Road Research Institute, New Delhi for his keen interest in the study and his kind permission to publish this paper. The guidance and encouragement given by late Dr. R. K. Ghosh, Deputy Director is gratefully acknowledged. Acknowledgements are due to Shri V. P. Arora, Scientist for assistance during load tests, and Shri O.P. Kapoor, the then S.E. (Karnal Circle), Shri Chikkara, E.E., Shri S.C. Singla, A.E., and Shri Chikkara, E.E., Shri S.C. Singla, A.E., and Shri Maha SIngh, J.E. of the Haryana PWD (Panipat Division) for their association with the experimental construction.
References
1. Central Road Research Institute, New Delhi "Development of Special Purpose Paving Blocks in India" IInd International Conference on Concrete Block Paving, Delft, Holland, April, 1984.
