Figure 1. Wheel Load at edge region of pavement
Roller Compacted Concrete (RCC) is an innovative pavement material for the construction of low volume rural roads. RCC can easily overcome the problems usually observed in the construction of flexible bituminous pavements. RCC is the commercial name used for concrete placed with conventional hot mix bituminous paving equipment compacted with vibratory rollers.RCC pavements are highly rigid and hence eliminates the high deformation problems such as rutting and corrugations generally encountered in flexible pavements. For rural development in India, connectivity of rural roads is an important aspect; but many rural roads such as ODR (Other District Roads) and VR (Village Roads) are of poor quality, potholed, and unable to withstand the loads of heavy farm equipment. Two construction techniques are available i.e., rigid and flexible. Of these, selection of type of construction depends on the sub-grade soil types, rainfall, traffic pattern and availability of construction materials. In the present paper, the design and analysis of RCC Pavements have been considered in place of conventional Cement Concrete Pavements and Bituminous pavements. The flexural strengths of Roller compacted concrete of 4.5MPa, 5.0MPa and 5.5MPa are considered for design and analysis. Design curves for low volume roads are presented. Proposed RCC pavement is suitable for sub-grade having low modulus of reaction.
Rural roads are tertiary road system in total road network which provides the accessibility for the rural habitations to market places and other facility centers. Connectivity of rural roads is an important criterion in rural development; it promotes the access to economic and social services and thus increases agricultural income and productive employment. Low volume rural roads in India include Other District Roads (ODR) and Village Roads (VR), which cover about 80% of the total road network of the country. These roads are usually constructed as flexible bituminous pavements with thin bituminous surfacing[2].In India, there are about 600million people in 6 lakh villages all over the country. The main aim of rural roads is to provide the means to bring this rural population onto the main stream. In spite of efforts made by the central government and state governments through different programs, about 40% of habitations in the country are still not connected by all-weather roads. The extension of rural road network is important to bring the social amenities, education and health to reach the villagers and also transportation of agricultural products from villages to market places and distribution centers. Many rural roads are of poor quality, potholed, and unable to withstand the loads of heavy farm equipment. These roads are also far from all season, good quality 2-lane or 4-lane highways, making economic resource flow slow, and logistical costs between different parts of India, one of the highest in the world. Therefore, there is an urgent need to construct and maintain the rural roads in an optimal manner.[5]
India is popular for its geographical diversities with mountains, rivers, forests, wet lands, deserts and scattered habitations in remote areas. There exists a wide range of availability of sub-grade soil types, rainfall, and traffic pattern and construction materials. These natural barriers create problems for developing a standard uniform technique to serve the requirement at all the sites. Adoption of different technologies depending on site specific conditions is required. The selection of pavement type has always been considered a tricky and complicated decision as it not only involves tedious field work, laborious calculations etc., but also on the various economic and geopolitical influences on the project at the time of designing and constructing the road.
The Government of India has come forward and undertaken a dedicated programme known as 'Pradhan Mantri Gram Sadak Yojana (PMGSY)' in 2007 to provide rural connectivity to all habitats with a population of 500 persons and above, under the Ministry of Rural Development which was funded by the Central government. As of now, flexible pavements are used for connecting villages because of low initial construction cost. In this regard, guide lines of IRC: SP: 72-2007 has been used to design the flexible pavement thickness of rural low volume roads[8].The Pavement thickness chart is given in IRC 37:2001[6] for CBR value of sub grade ranging from 2% to 10% using which the thickness of different courses of flexible pavement can be found out. But due to high cost of maintenance, sensitivity to water logging and lack of institutional set up for the maintenance, the village roads get damaged within a short period of its usage. Alternate techniques such as soil stabilization, providing of overlay, repair works like patching of potholes, etc. are being practised.[3]
At places where soil strength is poor, aggregate is costly and drainage conditions are worse; and rigid pavement is best alternative to flexible pavement. IRC has brought out the manual: IRC: SP: 62-2014 “guidelines for design and construction of cement concrete pavements for low volume roads' [1] for the construction of concrete pavements in rural areas. This manual has been introduced by IRC where cement concrete roads are preferred in populated areas to meet the problems of maintenance due to poor drainage, etc. M30, M35 & M40 grades of concrete are being used in the cement concrete roads, for construction of rural roads.
Rigid pavements consist of a number of joints, which reduces stresses caused due to temperature changes and this is a principal cause for inconvenience to the road users[4] . IRC: 101-1988[9] specifies technique of continuously reinforced concrete pavement which reduces the need of expansion and contraction joints and thus improves riding quality and reduces the maintenance cost compared to Plain Cement concrete Pavement (PCP). For the construction of conventional Continuously Reinforced Concrete Pavement (CRCP), percentage of steel required is 0.7-1.0%. Provision of steel reinforcement is important to check the cracks occuring in the concrete. Compared to flexible asphalt pavements construction, the process of construction of reinforced concrete pavements is difficult and costlier and involves more manpower.
Roller Compacted Concrete can overcome the problems usually encountered with flexible asphalt pavements or conventional plain or reinforced cement concrete pavements[12]. RCC pavement is much quicker to construct than conventional concrete pavement[7]. RCC is the commercial name used for concrete placed with conventional hot mix asphalt paving equipment, which is then compacted with vibratory rollers. The strength properties of RCC are similar to that of a conventional concrete and consists of the same basic ingredients as conventional concrete; but RCC is dry mix made with lower water cement ratio and consists of mixture of dense graded aggregates, cement and water. The major difference between RCC mixtures and conventional concrete mixtures is that RCC has a higher percentage of fine aggregates, which allows for tight packing and consolidation[10]
The material is carried in dump trucks, placed into an asphalt-type paver equipped with a standard or highdensity screed and is then compacted under steel wheel rollers. Steel Drum Finish Roller with smooth rubber rear wheels and a vulcanized rubber drum is used to produce final surface texture. The time required for placing and compaction process is critical to obtain adequate density, strength and smoothness of the finished RCC pavement. The concrete is placed and compacted at given by, the fresh and workable stage i.e., usually within 60 min of delivery. Due to stiff consistency, it remains stable under vibratory rollers, yet wet enough to permit adequate mixing and distribution of paste without segregation [10]
Structural behavior of RCC is similar to that of conventional paving concrete, and so the design procedure given here follows the methodology used for concrete pavements[11]. The proposed RCC pavement has been designed for single wheel load placed on edges. Stresses due to wheel load and temperature variation are obtained on the basis of provisions of IRC: SP: 62-2014 [1]. The design steps are similar to Plain Cement Concrete Pavement (PCCP). It is observed that critical condition occurs at the edges.
Figure 1 shows the wheel load at edge region of pavement.
Figure 1. Wheel Load at edge region of pavement
For μ = 0.15, above equation is reduced to Equation
Where,
σe= load stress in the edge region, M Pa
P = Single wheel load, N
h = pavement slab thickness, mm
μ = Poisson's ratio for concrete
E = Modulus of elasticity for concrete, M Pa
K = Modulus of subgrade reaction of the pavement foundation, M Pa/m
l = radius of relative stiffness, mm given by,
a = radius of the equivalent circular area, in mm given by,
Where, α= Coefficient of thermal expansion
t=temperature difference (o C) between the top and the bottom of the slab
σte =Temperature stress in the edge region, Mpa
c =Coefficient depending upon the ratio of Length (l) or Width(b) and Radius of relative stiffness.
The design of roller compacted concrete pavement for low volume roads with lean concrete as base is done as per IRC:SP:62- 2014[1] specifications. Following is the model design procedure adopted for RCCP design.
The design parameters considered in the design are listed in Table 1.
Table 1. Design Parameters
In the following section, the test procedure of RCC pavement is given for a typical data mentioned in Table 1.
Step1: Assume a Trial thickness, h= 150mm
Step2: Assume joint spacing, L= 3.75m
Step3: Calculation of Edge stress due to wheel load, (σe)
where a = radius of the equivalent circular area in mm and is given by
l = radius of relative stiffness, mm and is given by
Edge load stress (σe)> Flexural strength (F) (5.7>4.5)
Hence design is unsafe. Assume another trial thickness of 190mm and repeat steps 2-3.
Calculation of Edge stress due to wheel load, (σe) is
Edge load stress (σe) Flexural strength (F) (4.198<4.50)
Hence the design is safe.
Step 4: Calculate warping stress at edge region,
where, C= Coefficient depending upon the ratio of Length and Radius of relative stiffness.
L= length of joint, 3750 mm
As Per IRC:SP: 62-2014 [1] Recommended Temperature Differentials for Concrete Slab is, t = 18.66oC
Total stress(σtotal)=Edge load stress (σe)+Warping stress(σte) =4.198+2.05=6.25 MPa
Total stress (σtotal ) > Flexural strength[F] (6.25 > 4.50), Hence the design is unsafe.
Revise the thickness as 230mm and repeat steps 2-4
Calculation of Edge stress due to wheel load, (σe)
Edge load stress (σe)< Flexural strength (F) (3.28 < 4.50),Hence the design is safe.
Calculate warping stress at edge region
t = 19.82; C = 0.37
Total stress(σtotal)=Edge load stress (σe)+Warping stress(σte)=3.28+1.10=4.39 MPa
Total stress (σtotal )< Flexural strength[F] (4.39 < 4.50), Hence the design is safe
Step 5:Calculate the Corner stress(σc +σtc)
Corner stress σcorner (σc + σtc ) =1.56+0.88 = 2.44 MPa;
Corner stress σ corner< Flexural strength(F) (2.44 < 4.50)
Hence the design thickness of RCC Pavement is fixed as 230 mm.
The Graphical representation of Variation of Edge load stress and Edge warping stress for soil sub grade of CBR= 5%, 10%, 15% and 20% are shown in Figures 2-5 respectively.
Figure 2. Edge & Warping Stress Vs Thickness of pavement CBR=5%
Figure 3. Edge & Warping Stress Vs Thickness of pavement CBR=10%
Figure 4. Edge & Warping Stress Vs Thickness of pavement CBR=15%
Figure 5. Edge & Warping Stress Vs Thickness of pavement CBR=20%
The Thickness Design Charts for different Moduli of Subgrade reactions ( Edge load Stress) is shown in Figure 6.
Figure 6. Variation of Edge load stress with k
The Thickness Design Charts for different Moduli of Subgrade reactions ( Warping Stress) is shown in Figure 7.
Figure 7. Variation of Warping stress with k
The Thickness Design Charts for different Moduli of Subgrade reactions ( Sum of Edge load Stress and Warping Stress)shown in Figure 8.
Figure 8. Variation of Sum of Edge & Warping Stresses with k
The stress values of both edge load stress and edge warping stress were represented in Figures 2-5. From the graphs it is seen that, as the thickness of RCC Pavement increases the stress values are decreased for different moduli of sub grade reactions for different sub grade conditions of CBR= 5%, 10%, 15% and 20 % respectively.
For the design of RCC pavement for low volume rural roads, a program is developed in MS-Excel. The values of the total flexural stresses in the rigid pavement with a joint spacing of 3.75m and varying sub-grade modulus are obtained. Design curves for determining thickness of Roller Compacted Concrete pavements are shown in Figures 6 to 8, for different moduli of sub-grade reactions in Zone-III, in order to facilitate the road designers in designing RCC pavement for low volume rural roads.
Figure 6 shows the variation of Edge load stress with different Moduli of Sub grade reactions for different thicknesses of RCC Pavement. The variation is logarithmic and it can be given by the following equations.
For h = 150mm, σe = -1.11ln(k) + 9.801
For h = 170mm, σe = -0.97ln(k) + 8.432
For h = 200mm, σe = -0.80ln(k) + 6.908
For h = 220mm, σe = -0.70ln(k) + 6.098
For h = 250mm, σe = -0.61ln(k) + 5.211
Where σe= Edge load stress in M Pa and k = Modulus of Sub Grade Reaction in M Pa/m
Figure 7 shows the variation of Edge warping stress with different Moduli of Sub grade reactions for different thicknesses of RCC Pavement. The variation is logarithmic and it can be given by the following equations.
For h = 150mm, σte= 0.586 ln(k) -0.204
For h = 170mm, σte= 0.720ln(k) -0.947
For h = 200mm, σte = 0.859ln(k) -1.821
For h = 220mm, σte = 0.893ln(k) -2.172
For h = 250mm, σte = 0.862ln(k) -2.313
Where σte = Edge warping stress in M Pa and k = Modulus of Sub Grade Reaction in M Pa/m.
Figure 8 shows the variation of Total stress due to load and warping with different Moduli of Sub grade reactions for different thicknesses of RCC Pavement. The variation is logarithmic and it can be given by following equations.
For h = 150mm, σtotal = -0.53 ln(k) + 9.635
For h = 170mm, σtotal = -0.24 ln(k) + 7.457
For h = 200mm, σtotal = -0.056 ln(k) + 5.080
For h = 220mm, σtotal= -0.186 ln(k) + 3.918
For h = 250mm, σtotal= -0.249 ln(k) + 2.912
Where σtotal = Total edge stress due to Load and Warping in M Pa and k= Modulus of Sub Grade Reaction in M Pa/m
Using the specifications mentioned in ACI 211.3R-02 [13] mix proportions of RCC can be designed by considering different trial mixes using soil compaction method to obtain the required flexural strength. In soil compaction method, the proportioning of aggregates is fixed depending on the recommendations of RCC pavement combined aggregate grading limits (Table.A3.2: ACI 211.3R-18) [13] For the fixed or adjusted aggregate proportion, a number of concrete mixtures varying in their cement contents (i.e., 13, 14, 15, and 16%) are prepared using the procedure for compaction of soils, using a compaction hammer with modified proctor energy [14] . Water content ratio for the mixes is then obtained by using a plot between dry densities vs. moisture content (%).Beam specimens of size 500x100x100mm were cast to find the flexural strength of different RCC mixes [15] . A graph is drawn representing the % cement content and flexural strength of different RCC mixes. From this graph, % of cement content is obtained against any flexural strength in N/mm2 . Table 2 presents the mix proportion for different flexural strength levels proposed on the basis of soil compaction method of mix proportioning [13] .
Table 2. Mix proportions of RCCP for different Flexural Strength Levels per one cubic meter of concrete
Roller Compacted Concrete pavement is designed for low volume rural roads in India for different flexural strength levels. An attempt is made to provide an appropriate solution to the existing problem of rural roads construction and maintenance. The design of Roller Compacted Concrete pavements for rural roads especially in weak soil and sub base conditions has been done. A detailed study of calculation of stresses developed in RCC pavement and subsequent design have been done as per IRC: SP: 62- 2014. A study of thickness of RCCP has been done and three levels of flexural strength of RCCP have been considered. It has been found that Roller Compacted Concrete Pavement (RCCP) of thicknesses between 150mm to 250mm are appropriate for Low volume roads where the sub grade conditions are varying from CBR values of 5% to 20%.