down in IRC: 37-2012. The various input required for the design is computed through deflection and existing pavement layer thickness as per IRC guidelines.
“Mechanistic Design of Overlay Based on Benkelman Beam Deflection Technique” (A Case Study of Rajkot-Morbi State Highway-24) Kunal P. Bhagat1, Chetan P. Hadial1, Ujjval J. Solanki2 1
U.G. Students of Civil Eng. of GTU, College- Darshan Institute of Eng. & Technology-Rajkot. 2 Assistant Professor–Dept. of Civil Eng. Darshan Institute of Eng. & Technology.
Abstract Conventionally in India overlay of Flexible pavement design is carried out as per IRC: 81-1997 Guidelines for Strengthening of Flexible Road Pavement using Benkelman beam deflection (BBD) technique. In the BBD technique overlay design is based on computed characteristic elastic deflection at standard load. The design thickness as per BBD study is 85 mm bituminous layer. For the mechanistic design of overlay need falling weight deflectometer to evaluate structural adequacy of pavement, which is too costly equipment and need expertise to use the same so the effort made to design of overlay as per performance criterion of fatigue and rutting failure laid down in IRC: 37-2012. The various input required for the design is computed through deflection and existing pavement layer thickness as per IRC guidelines. The design is checked by software IIT-PAVE for horizontal tensile strain computed at bottom of bituminous layer and vertical strain at top of subgrade. The computed fatigue and rutting strain is0.0837 micron and 169 micron due to material which is lower than strain due to traffic so the overlay design found safe in both criteria. Keywords: Mechanistic design, Pavement Performance, Overlay design. 1. Introduction The stretch of Rajkot-Morbi state highway-24 (15.5 to 28km, total 12.5 km) is evaluated as per IRC: 811997 overlay design criteria. The structural and functional evaluation is carried out for the pavement. The functional evaluation survey is conducted on SH-24.In this phase of operation visual observations supplemented by simple measurements for Rut-depth, hairline crack, alligator crack, longitudinal crack, transverse cracks, etc. distresses using a 3 meter straight edge. Based on observations we concluded that the functional performance of pavement is good. The present serviceability index (PSI) is calculated by measuring distress on the surface of pavement. The value of PSI is between 4 and 5 so overall condition of pavement is very good (as per AASHTO). On PSI value lower effect of roughness value, as seen the cracked and patched area is lower. The average characteristic deflection calculated based on Benkelman beam deflection testing is 1.107 mm. 2. Principles of pavement design 2.1 Pavement Model: A flexible pavement is modeled as an elastic multilayer structure. Stresses and strains at critical locations as shown in Fig-1, are computed using a linear layered elastic model. The Stress analysis software IITPAVE has been used for the computation of stresses and strains in flexible pavements. Tensile strain, Єt, at the bottom of the bituminous layer and the vertical sub-grade strain, Єv, on the top of the sub-grade are conventionally considered as critical parameters for pavement design to limit cracking and rutting in the bituminous layers and non-bituminous layers respectively. The computation also indicates that tensile strain near the surface close to the edge of a wheel can be sufficiently large to initiate longitudinal surface cracking followed by transverse cracking much before the flexural cracking of the bottom layer if the mix tensile strength is not adequate at higher temperatures.
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Fig. 1. Different Layers of a Flexible Pavement and location of strain. With every load repetition, the tensile strain developed at the bottom of the bituminous layer develops micro cracks, which go on widening and expanding till the load repetitions are large enough for the cracks to propagate to the surface over an area of the surface that is unacceptable from the point of view of long term serviceability of the pavement. The phenomenon is called fatigue (or fracture) of the bituminous layer and the number of load repetitions in terms of standard axles that cause fatigue denotes the fatigue life of the pavement. As per IRC:37-2012 guidelines cracking in 20 per cent area has been considered for traffic up to 30 msa and 10 per cent for traffic beyond that. 3. Methodology The performance criteria are fatigue and rutting in pavement so fatigue and rutting is calculated as below. 3.1 Fatigue Model [1] Fatigue model adopted as per IRC: 37-2012. Two fatigue equations were fitted, one in which the computed strains in 80 per cent of the actual data in the scatter plot were higher than the limiting strains predicted by the model (and termed as 80 per cent reliability level in these guidelines and the other corresponding to 90 per cent reliability level). The two equations for the conventional bituminous mixes designed by Marshall Method are given below: Nf = 2.21 * 10-04 x [1/εt] 3.89 * [1/MR] 0.854(80 per cent reliability) Nf = 0.711 * 10-04 x [1/εt] 3.89 * [1/MR] 0.854 (90 per cent reliability) Nf = fatigue life in number of standard axles, εt = Maximum Tensile strain at the bottom of the bituminous layer, and MR = resilient modulus of the bituminous layer. Form the above two fatigue model of 80% and 90%, 90% reliability model is used and strain calculated for projected traffic of 45 msa. 3.2 Rutting in Pavement Rutting is the permanent deformation in pavement usually occurring longitudinally along the wheel path. The rutting may partly be caused by deformation in the subgrade and other non-bituminous layers which would reflect to the overlying layers to take a deformed shape. The bituminous mixes also may undergo rutting due to secondary compaction and shear deformation under heavy traffic load and higher temperature. Excessive rutting greatly reduces the serviceability of the pavement and therefore, it has to be limited to a certain reasonable value. In these guidelines the limiting rutting is recommended as 20 mm in 20 per cent of the length for design traffic up to 30 msa and 10 per cent of the length for the design traffic beyond. 3.3 Rutting model [1] Like the fatigue model, rutting model as per IRC: 37-2012 at 80 per cent and 90 per cent reliability levels. The two equations are given below: N = 4.1656 x 10-08 [1/εv] 4.5337 … (80 per cent reliability) N = 1.41x 10-8x [1/εv] 4.5337… (90 per cent reliability) Where, N = Number of cumulative standard axles, and εv = Vertical strain in the subgrade Form the above two rutting model of 80% and 90%, 90% reliability model is used and strain calculated for projected traffic of 45 msa. 4. Computation for performance criterion 4.1 Fatigue strain calculation [1] The strain due to fatigue is calculated for 90 percent reliability and traffic data for design life of 10 year is given 45 million standard axles by R. & B. State Morbi division –Morbi. Another input required is modulus of resilient of bituminous mix that is considered for 350 C for bituminous mix as per IRC: 37-2012 is 1700 MPa.
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Nf = 0.711 * 10-04 x [1/εt] 3.89 * [1/MR] 0.854 Where, Nf= fatigue life in number of standard axles, εt= Maximum Tensile strain at the bottom of the bituminous layer, and MR= resilient modulus of the bituminous layer. 45*106 = 0.711*10-04 *[1/εt] 3.89* [1/1700] 0.854 εt = 180.7 µε (micro strain) (developed strain due to traffic) Tangential Strain developed due to traffic is 545 micron at bottom of bituminous layer which is responsible for fatigue damaging. 4.2 Rutting strain calculation [1] The strain due to rutting is calculated for 90 percent reliability and traffic data for design life of 10 year is given 45 million standard axle by R. & B. State Morbi division –Morbi. N = 1.41*10-8*[1/εv] 4.5337 Where, Nf = Number of cumulative standard axles, and εv= Vertical strain in the sub-grade 45*106 = 1.41 *10-8*[1/εv] 4.5337 εv = 380 µε (developed strain due to traffic ) Vertical Strain developed due to traffic is 380 micro strain at top of subgrade layer which is responsible for rutting damaging. 5. Determination of Modulus of Elasticity of three layers 5.1 Computation of Sub-grade Modulus [3] The subgrade modulus is determined through two approaches, one is using Dynamic cone penetrometer test and second is as per AASHTO (1993) recommends the back-calculation of moduli from a single deflection. In overlay design purpose lower value is selected among both. First approach: The subgrade modulus is found using dynamic cone penetrometer (DCP) test as per IRC: SP-72. The equipment as shown in Fig-2 (a) and (b) the 8 Kg weight is dropped from predefined height and penetration of 60° cone is measured. The test is performed on study stretch at three locations on sub-grade. DCP value mm/blow is calculated and based on DCP value, sub-grade modulus is calculated as per IRC: 115-2014“Structural Evaluation and strengthening of Flexible Road Pavement use falling weight deflectometer”.
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Fig 2. (a) & (b) Dynamic Cone penetrometer test on study stretch and sketch Esubgrade = 357.87*(DCP)-0.6445 Where, Esubgrade = Modulus of sub-grade (MPa) DCP = Dynamic Cone Penetration value (mm/blow) Table 1. Subgrade Moduli computation form DCP test Test Numbers Penetration DCP value (mm/blow) location of blow (mm) 1 20 289 14.45 2 20 264 13.20 3 20 317 15.85 Average 20 290 14.50 The subgrade moduli is calculated in Table-1
Modulus Value of Subgrade Esubgrade = 357.87*(DCP)-0.6445 64.00 67.84 60.29 63.86 Say ≅ 64
Second Approach: Sub-grade resilient modulus for Characteristics deflection 1.107 mm. (As per IRC: 81-1997 Benkelman beam deflection calculation) Sub-grade resilient modulus [4] MRsubgrade= 0.24*P/dr*r Where, MRsubgrade =sub-grade resilient modulus value, psi P = applied load, in pounds (20 kN * 449.6 pound per kN= 8992 pound) dr= surface deflection measured at a distance r from the center of the loading plate, in inches ( 1.107 mm/25.4 = 0.04358 inch) r = distance from the center of load, in inches (155 mm from center of tyre = 6 inch) =0.24*8992/0.04358*6 =8253.32 psi (145.05 psi/ MPa) MRsubgrade =56.90MPa As per DCP test modulus of subgrade is 64 MPa and as per AASHTO recommendation it is 56.90 Mpa. Consider lower modulus of elasticity from above two values that is 56.90 MPa.
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5.2 Computation of Granular layer Modulus [3] The granular layer moduli is calculated as per IRC: 37-2012. Using following relationship. The thickness of granular layer in existing pavement is 500 mm as per cross section detail. MRgranular= 0.2*h 0.45*MRsub-grade =0.2*5000.45*56.90 MRgranular=186.5 MPa Where, h = thickness of granular sub-base, mm 5.3 Modulus of Bituminous layer. The modulus of bituminous layer is selected from IRC: 37-2012 for BC/DBM mix using VG-30 bitumen at 350 C temp. It is given 1700 MPa, same is adopted for the analysis. 6.0 Design of Overlay The overlay design is carried out as per characteristic deflection observed from BBD study and it is checked for performance criteria using IIT-PAVE software developed by IIT-Kharagpur which is given along with IRC: 37-2012. The software screen as shown below Fig No-2. 6.1 Calculation as per IIT PAVE
Fig 3. IIT Pave Screen. The software required input as shown in below Fig-4. Elastic Modulus of five layer in MPa, bituminous layer, granular layer (WBM, GSB ...etc.) and Subgrade. Poisson’s ratio of all the five layers. Thickness in mm. The thickness of subgrade layer is not required it is considered infinite. The cross section detail of pavement and layer material property as shown in Fig. -4
Fig 4. Layer thicknesses and material property detail
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Wheel load is required to be input as standard load 40000N for single wheel assembly, for dual wheel set require 20000N same is adopted. The tyre pressure is considered 0.56 MPa. The Point of analysis has to be given to know strain developed at critical location as discussed in fatigue and rutting model, so it is given at bottom of bituminous layer at a depth from top of pavement 85mm- in new layer as adopted from BBD design approach, 125 mm, 425 mm in old layer to know fatigue strain and on top of subgrade at 825 mm from top to know the rutting strain. All the input is given in below screen of IIT PAVE as shown in Fig-5. Layer thicknesses considered are: -85, 40, 200, 500 mm Elastic Moduli used are: - 1700, 1000, 1000, 186.5 and 56.9 MPa Poisson's ratio values used in analysis are: - 0.5, 0.4, 0.4, 0.4.
Fig 5. IIT Pave Input Screen. For the above input the output available in strain value at different location. Our point of consideration is epT means tensile strain and epZ means vertical strain. Which shown as in Fig-6.
Fig 6.Output of IIT PAVE.
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As shown in output of IIT PAVE Fig-6 the safe strain developed due to material and strain developed due to traffic is tabulated in Table-2. Table-2 Strain calculation Sr. No
Layer
Layer thickness
Depth from top of pavement
Strain due to traffic in micro strain for 90 % reliability
Strain due to material in micro strain
Remarks
01 02 03 04
Bituminous layer –New Bituminous layer- old Bituminous layer –old Top of subgrade layer
85 mm 40 mm 200 mm 500 mm
85 mm 125 mm 325 mm 825 mm
180.7 203 203 380
83.7 87.5 133 169
The strain developed due to material is less than strain developed due to traffic, hence design is safe.
Conclusion:
The fatigue strain calculated as per multi layered elastic theory using software IIT-PAVE is 83.7 micro strain at bottom of provided overlay (bituminous layer) which is quite lower than 545 micro strain calculated as per performance model for 90 % reliability, so the pavement is safe in fatigue failure in its service life.
The fatigue strain also calculated at bottom of both old bituminous layer and found safe against fatigue failure.
The rutting strain calculated as per multi layered elastic theory using software IIT-PAVE is 169 micron at top of subgrade layer which is lower than 380 micron strain calculated as per performance model for 90 % reliability so the pavement is safe in rutting failure in its service life.
The technique is mechanistic and no need of costly equipment such as falling weight deflectometer and design is carried out as per new IRC: 37-2012 approach so one can adopt as a low cost techniques and test the design as per performance criterion.
Acknowledgement
The project team is thankful to R. & B –Morbi department for assigning such studies to us in our final year industry defined problem and also providing facility of truck for standard loading as used in Benkelman beam deflection study. References [1] IRC: 37-2012 “Guidelines for Designing of Flexible Pavements” IRC New Delhi. [2] IRC: 81-1997 “Guidelines for Strengthening of Flexible Road Pavement Using Benkelman Beam Deflection Technique” IRC New Delhi. [3] IRC 115-2014 “Guidelines for structural evaluation and strengthening of flexible road pavements using falling weight deflectometer (FWD) technique” IRC New Delhi. [4] R. Srinivasa Kumar, Text book of Highway Engineering by Universities press. pp.495.
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