J.B. Sengupta, Scientist, Dinesh Ganvir, Scientist, and Dr. Renu Mathur, Head, Rigid Pavements Division, Central Road Research Institute, New Delhi

Introduction

The revival of the cement industry in the country coupled with other factors that have boosted the prospects of constructing road projects in the country has made the transportation engineers to sit together, and re-orient their pavement options towards concrete pavements. So far India had preferred the option of flexible pavements due to certain compelling reasons. Most obvious among them were lower initial cost, amenability to stage construction and scarcity of cement. But now the country is on the verge of accomplishing highly ambitious road modernization programme which ncludes the construction of expressways and four laning projects. This calls for a re-appraisal of the past strategies. The country’s road system has got a fillip and boost due to the recently introduced National Highway Development Programme (NHDP) and Pradhan Mantri Gram Sadak Yojana (PMGSY) provoking a profound paradigmatic shift towards the construction of concrete roads. In this context, cement concrete roads have a major role to play. The know-how and the current state of knowledge is very minimal since large scale projects involving concrete road constructions are still in infancy stage in India. There are many compelling issues at stake in the design and construction aspects of rigid pavements in India. This paper addresses few of them.

Current Practice of the Design of Rigid Pavements Followed in India

The early approach to the design of rigid pavements was based on Westergaard’s analysis. Recent advances in knowledge have led to vast changes in the design methodology. In the earlier version of IRC: 58-1988, the calculation of load stresses was done as per Westergaard’s equations modified by Teller and Sutherland. The use of these equations has its own limitation because they do not take into account the configuration of the wheels. Picket & Ray’s chart can be used for stress computation in the interior portion as well as at the edge of concrete slabs. Using the fundamental concept of Westergaard and Picket & Ray’s pioneering work, a computer programme IITRIGID developed at IIT Kharagpur is used for computation of stresses for the edge load condition in the revised guideline IRC 58-20021. The stress charts for single as well as tandem axles for different magnitudes of single and tandem axle loads are given in the revised IRC guideline. Thus the salient features of the revised guideline are computation of flexural stress due to the placement of single and tandem axle loads along the edge and the introduction of the cumulative fatigue damage approach in the design. Most of the work till recently was aimed at modifications and adaptations of Westergaard’s work for matching better with actual performance and for simplifying the analysis of design.

The present philosophy in design is based on warping stresses developed in the slabs due to temperature variations and bending of the slabs due to axle load. The stress ratio concept is used to account for axle load repetitions by which actual repetitions of axle load and allowable repetitions of axle load are compared as per the recommended design procedures given in IRC: 58-20021and IRC: 15-20022.

Recently, the guidelines for design of low volume roads were formulated by IRC: SP: 62-2004 3In the design of concrete pavement for low volume roads also temperature stresses and load stresses are required to be considered in combination so that the sum of these two shall not exceed the concrete flexural strength. As the traffic at the end of design life is very low in rural roads the stress ratio concept is not considered in the design of the PQC slab of the pavement. The guidelines for construction of Roller Compacted Concrete Pavement which makes use of zero slump concrete were also reported in IRC: SP: 62-20043.

The Issue of Temperature Stresses

Research that has been conducted at IIT Kharagpur and elsewhere in the world4has indicated that Bradbury’s equations adopted in IRC: 58-20021and IRC: SP: 20-20025may lead to overestimation by over 100 percent of the actual warping stresses in concrete pavements. In the conventional method, the foundation is assumed to be made of springs attached firmly to the slab and when a slab warps up, the spring pulls it down causing warping stresses. In actual practice, the slab warps up leaving contact with ground and it is the self–weight of the slab that causes warping stresses. Using an iterative Finite Element method6, accurate values of warping stresses were computed in concrete pavements for linear temperature variation across the depth. Temperature warping stresses were analyzed by taking the self–weight of the concrete slab and also taking into account the loss of subgrade support due to linear temperature differential across the depth of the concrete slab. The thickness by this method of analysis can bring down the pavement design thickness by 10 percent4. The study clearly establishes a need for change in the temperature stresses calculation.

While comprehensive data on distribution of temperature differentials in the pavement for the annual cycle of seasons is needed to adequately and effectively account for such differentials, all that is available now is some four decade old data on maximum annual temperature differential in pavements of four different thicknesses for six zones defined in the country for this purpose, based on the work and recommendation of CRRI1, 7. Further work needed in this direction includes field studies at a few locations in the country, covering different climatic zones, for comprehensive year round temperature differential data on an hourly basis and updating of this data bank.

Issue of Design Strength of Concrete

Cement concrete continues to gain flexural strength with age, 90 days and one year flexural strengths are about 110 and 117 percent of the 28 days flexural strength4. The 28 days test results have been commonly used for thickness design of highways and streets. In a big project, a concrete pavement is often opened to traffic long after the construction and only the last part of the pavement constructed towards the end is exposed to traffic early. Otherwise also, cumulative traffic is much smaller in the first few months as compared to the cumulative design traffic, resulting in little fatigue damage. The total traffic for fatigue check may be considered in two parts–
  • Traffic during the first year–using 28-day strength, or that at the age of opening to traffic, whichever is later
  • Traffic during the first year–using one year strength
Advantage of gain in strength with age can be safely taken in thickness design to achieve economy. This is the practice followed in PCA method of design 8also.

Issue of Separation Membrane

As per the practice in India, a plastic sheet is usually provided at the interface of dry lean concrete sub base and the concrete slab to break the bond between the two layers to minimize reflective cracking from the unjointed lean concrete sub base. If the two layers are bonded together giving rise to a composite action, stresses in the pavement slab as well as in the lean concrete sub-base can be reduced significantly and ower thickness of concrete slab can be provided. In order to prevent cracking of the concrete slab due to contraction, the subbase must be of strength not exceeding 28 day compressive strength of about 5 to 7 MPa. If lean concrete of higher strength is provided, it must have contraction joints at the same spacing as that of the pavement slab so that cracks are confined to the weakened sections in both the layers.

Issue of Widened Outer Lane

If the outer lane of a concrete pavement is widened by about 0.5 to 0.6 metres while the wheel loads are confined along the longitudinal edge of the outer lane, the flexural stresses are reduced significantly resulting in a thinner pavement. The widened part forms a part of the shoulder and it can be given a rough texture to discourage vehicles from coming over the widened part during the normal operation. Only those wheels traveling tangential to the edge of the outer lane are critical for design. A very small percentage (less than 2 %) meets this condition as per the research in India9. Portland Cement Association8considers only six percent of the traffic for thickness design. IRC: 58-20021 recommends a conservative estimate of 25 % which is to be further debated. The tie bars in the longitudinal joints within the carriageway are not designed for load transfer, as such, whichever of these longitudinal joints caters to the maximum traffic will be critical for design of tie bars. This traffic will be very much higher than at the outer edge of the outer lane.

Critical Review Points Regarding Construction Aspects

Issue of Early Morning Construction during Summer

It has been observed in some of the Golden Quadrilateral projects that the pavement slabs that are constructed in the early morning time of summer period in Northern India developed shrinkage cracks (Photo 1). IRC: 15-20022prohibits concreting when the temperature is in excess of 350C. Shrinkage cracks can occur when the temperature is higher than the 350C and the pavement quality concrete is still in the green state. So proper time period should be allowed for the concrete to cure and the temperature in the early curing period time should not exceed the prescribed limit. The morning temperature in summer months, while the concrete being cured in its initial hours can bring about shrinkage cracks as observed in some of the Golden Quadrilateral projects. This problem needs to be addressed by the contractors.

When ambient temperature is not more than 350C, take care of some, but not all problems in the case of hot weather concreting. It certainly does reduce the risk of shrinkage cracking, but does not eliminate it if the temperature soon rises beyond 350C. Additional precaution pertinent to curing in hot weather (IRC: 61-197610, IRC: 84-198311) would still need to be taken in ground against shrinkage cracking. One of the advantage of early morning concreting is that the materials are relatively cooler, and it would not be necessary by cooling water with ice, or by addition of ice during mixing, which could otherwise be needed.

Issue of Expansion Joint

Expansion joints are necessary when the concrete pavement abuts with structures like bridges and culverts. In the earlier version IRC: 58-1988 recommended expansion joints at spacing ranging from 50 to 140 m depending upon the slab thickness, period of construction and slab thickness. Construction of these joints involves considerable effort and also the joint has to be constructed with a stop end. This has a bearing on the progress of work. In some of the projects in India, the expansion joints have been recommended at the end of the day’s work and not in between the day’s work. The IRC: 58-20021does not recommend expansion joints at locations other than where the slabs abut a bridge or culvert. The expansion joints are not being recommended in advanced countries any more. Yang H. Huang12 says that the expansion joints are difficult to maintain and susceptible to pumping and hence they are no longer in use except at the connection between the pavement and structure. A view of the failed expansion joint is given in Photo 2.

Issue of Mix Design

The mix design method as suggested by IRC: 44-197613 is very old. At the time of formulation of this code usage of super plasticizers was not common. High strength concrete can now be produced using various new additives and high strength cement. Cements with 43 and 53 Grade are commonly available in market now a days which were not available at the time of formulation of this code. So a new mix design procedure has to be evolved in the light of the developments in cement industries and allied admixtures.

Relationship between Cube Strength and Core Strength

Most of the contract documents contain the following clause “In the case of pavement concrete, where the requirements are not met with, or the quality of concrete or its compaction is suspect, the actual strength of the concrete in the slab shall be ascertained by carrying out tests on cores cut from the hardened concrete at such locations.” The equivalent cube strength of concrete shall be obtained by multiplying the corrected cylinder strength by 5/4., where height to diameter ratio of the core is two, IS:516 -195914. As per article 17 of IS: 456-200015, the concrete shall be accepted if the average equivalent cube strength of cores is equal to at least 85 percent of the cube strength for the corresponding age and that no individual core has a strength less than 75 percent. As experienced by the contractors it is very difficult to achieve this specification. More research work is to be carried out on this aspect.

Issues related with Texturing and Re-texturing

Surface texturing is important to cement concrete pavements. Right depth of texturing can be achieved with right slump and correct timing. Use of low slump concrete and delayed texturing leads to insufficient depth of texturing and high slump concrete and early texturing shall lead to extra depth. Insufficient depth of texturing can lead to skidding of vehicles; extra depth can lead to holding of water and noise pollution. Time of texturing is critical to the surfacing of rigid pavement and should be done carefully. It is very difficult to maintain the texture depth requirement over a period of time. Retexturing is required as the road is subjected to the movement of vehicles and the surface worn out. Currently there are no specifications available for retexturing and the methodology of retexturing. A view of retextured surface is given in Photo 3.

Conclusions

The issues highlighted above throw light on the urgent needs for re-examining and formulating new guidelines/specification with regard to design and construction of concrete roads in India. The issues raised become more pertinent in the context of the large scale construction of cement concrete roads at the anvil. Feedback from the various construction agencies and from research and development fraternity can lead to meaningful contribution in solving many issues at stake in the field of design and construction of concrete roads in India as discussed in the paper.

References

  • IRC:58-2002 “Guidelines for the Design of Plain Jointed Rigid Pavements for Highways”
  • IRC:15-2002 “Standard Specifications and Code of Practice for Construction of Concrete Roads”
  • IRC-SP:62 -2004 “Guidelines for Design and Construction of Cement Concrete Pavement for Rural Roads”
  • K.Suresh, S.K.Singh and B.B. Pandey “Innovative Design of Concrete Pavements.” Seminar on design, construction and maintenance of cement concrete pavements 8-10. Oct,2004, New Delhi
  • IRC-SP:20-2002 “Rural Road Manual”
  • B.B. Pandey and Captain R.K. Bhatnagar, “Streeses in Rigid Pavement a Reassessment,” Highway Research Bulletin No:53,1995, pp-149-161
  • R.K. Ghose, R. Krishnamachari and R.Kumar “Measurement of Temperature Differential in Cement Concrete Pavements,” Thermometry, National Physical Laboratory, New Delhi 1964, pp 5.7.1-5.7.10
  • Portland Cement Association, “Thickness Design for Concrete Pavements” Chicago, USA,1966
  • L.R. Kadiyali and Bageshwar Prasad “Are India’s Concrete Roads Over Designed”? Seminar on design, construction and maintenance of cement concrete pavements 8-10. Oct,2004, New Delhi
  • IRC:61-1976 “Tentative Guidelines for the Construction of Cement Concrete Pavement in Hot Weather”
  • IRC:84-1983 “Curing of Cement Concrete Pavement–Code of practice”
  • Yang. H. Huang, “Pavement Analysis and Design,” Prentice Hall, New Jersey1993
  • IRC:44-1976 “Tentative Guidelines for Cement Concrete Mix Design for Pavement”
  • IS:516-1959 “Method of Tests for Strength of Concrete”
  • IS:456-2000 “Plain and Reinforced Concrete–Code of practice.”

NBMCW December 2008