Check the adequacy of design?
10.2 Bituminous Pavements with Cemented Base and Cemented Subbase with Crack Relief Interlayer of Aggregate
10.2 Bituminous Pavements with Cemented Base and Cemented Subbase with Crack Relief Interlayer of Aggregate
Fig. 10.2 Bituminous Surfacing, Cement Treated Base and Cement Treated Sub-Base with Aggregate Interlayer
Fig. 10.2 shows a five layer elastic structure consisting of bituminous surfacing, aggregate interlayer layer, cemented base, cemented subbase and the subgrade. Material properties such as modulus and poisson’s ratio are the input parameters apart from loads and geometry of the pavement for the IITPAVE software. For traffic > 20 msa, VG 40 bitumen is used for limiting rutting. DBM has a design air void of 3% and less than 4.5% after rolling. Cracking of cemented base is taken as the design life of a pavement. For traffic greater than 30 msa, minimum thickness of bituminous layer consisting of DBM and BC layers is taken as 100 mm (AASHTO-1993) even though the thickness requirement may be less from structural consideration. Residual life of the bituminous layer against fatigue cracking is not considered since it cracks faster after the fracture of the cemented base. In high rainfall area, upper100 mm of the cemented subbase (D) having the gradation 4,5,6 of the Table V-1 of Annexure V is porous acting as the drainage layer over lower cemented subbase (F). The gradings III and IV of Table 400.1 of MORTH with fines less than 2 % containing about 2-3% cement can also be used, In such cases where CTB is used, it is necessary to carry out fatigue damage analysis for the safety of CTB against premature fracture by the spectrum of axle loads for design traffic ≥ 10 msa.
Thickness may have to be increased. One can also assume a pavement composition considering the constructability and carry out fatigue damage analysis without considering standard axle loads. This is illustrated in Annexure II.
Critical tensile strain location
200 250 250 250 250 250 250
Illustration Traffic 150 msa
Subgrade CBR = 10percent, MR subgrade = 17.6*CBR0.64 = 77MPa,MR bituminousl layer= 3000 MPa Consider SMA/GGRB + DBM = 100 mm, Aggregate interlayer = 100 mm (MR = 450 MPa), Cemented base = 120 mm (E = 5000 MPa), cemented subbase = 225 mm(600 MPa
Va =3.5 % and Vb= 12.5% for the DBM2 laid in a single layer (PLATE NO 5 for CBR=10 ,design traffic 150 msa)
Equations 6.2, 6.5 and 6.7 are used as design criteria.
I. Allowable Horizontal Tensile Strain in Bituminous Layer is 140 x 10–6. II. Allowable Vertical Compressive Strain on Subgrade is 292 x 10–6. III. Allowable Tensile Strain in Cementitious Layer is 64.77x10–6.
From IITPAVE Software the computed strains are
IV. Horizontal Tensile Strain in Bituminous Layer is 129.5x10-6< 140 x 10–6. V. Vertical Compressive Strain on Subgrade is 201 x 10–6 <292 x 10–6
VI. Tensile Strain in Cementitious Layer is 51.0 x 10–6 <64.77x10–6
Hence the design is safe from the consideration of standard axle repetitions. The plates above give an approximate thickness quickly. However cumulative fatigue damage analysis and its safety from the construction traffic are to be examined as explained in Annex II.3.2. If 28 day bending strength is less than 1.4 MPa, the thickness of CTB will be higher.
An example of the check for the construction traffic is given below.
200 200 225 225 225 225 225
Check for Construction traffic
Crack relief layer of WMM is generally laid after seven day curing of CTB. The CTB must be safe against the construction traffic. There may be about 70 trips of dumpers equivalent to 140 repetitions of each axle of the tandem axle of the dumper for laying two km of 100mm thick WMM by paver.
Flexural strength of CTB after 7 days= about 1.00 MPa ( 20% of 5 MPa strength at 7 days).
Dumpers carry about 10 cum of WMM weighing about 210kN.
Approximate gross weight of three axle dumper=320 kN. Consider 120 kN load on a single axle load of the 240 kN tandem axle load. Neglecting interference of tandem axle for the thin CTB, Take tyre pressure=0.80 MPa. Load on each wheel= 30,000N, allowable flexural stress for 140 repetitios-0.795 MPa (Eq.6.7),
Bending stress= 0.96 MPa> 0.795 MPa, increase the thickness to 150mm
Bending stress=0.84 Mpa > 0.795 MPa hence unsafe. Increase the thickness to 165mm Bending stress=0.781 Mpa < 0.795 MPa hence safe.
Such an exercise has to be carried.
.10.3 Cemented Base and Cemented Subbase with Sami at the Interface of Cemented Base and the Bituminous Layer
Fig. 10.3 Bituminous Surfacing with Cemented Granular Base and Cemented Granular Sub-base with Stress Absorbing Membrane Interlayer (SAMI)
Fig.10.3 shows a four layer pavement consisting of bituminous surfacing, cemented base,
Cemented subbase and the subgrade. For traffic > 20 msa, VG40 bitumen is used. The DBM mix is designed for air void of 3 percent and compacted to an air void content less than 4.5% after rolling Cracking of cemented base is taken as the life of pavement. Minimum thickness of bituminous layer for major highways is recommended as 100 mm as per the AASHTO guidelines. Stress on the
Critical location for vertical strain
SAMI layer
underside of the bituminous layer over un-cracked cemented layer is compressive
In such cases where CTB is used, it is necessary to carry out fatigue damage analysis for the safety of CTB against premature fracture by the spectrum of axle loads. Thickness may have to be increased.
250 250 250 250 250 250 250
150 160 170 150 150 160 170
50 60 60 60
40 40 50 40 40 40 40
0 100 200 300 400 500 600
1 2 3 4 5 6 7
Plate 7 CBR 5%
BC/SDBC/SMA/
GGRB DBM CTB CT_GSB
5 10 20 30 50 100 150 Design traffic, msa
Pavement Thickness,mm
Illustration Traffic 150 msa
Subgrade CBR = 10%, MRsubgrade= 17.6*CBR0.64= 77 MPa, MRbituminous layer= 3000 MPa E of cemented base = 5000 MPa, E of cemented subbase = 600 MPa
From PLATE 15, SMA/GGRB + DBM =100 mm, Cemented base=160mm,Cemented subbase=
250 mm
SAMI is provided on the top of cemented base.
250 250 250 250 250 250 250
For the given traffic, 90 % Reliability is adopted i.e., Equations 6.2, 6.5 and 6.7 are used.
I. Allowable Horizontal Tensile Strain in Bituminous Layer is 140 x 10–6. II. Allowable Vertical Compressive Strain on Subgrade is 291 x 10–6. III. Allowable Tensile Strain in Cementitious Layer is 64.77 x.10–6. From IITPAVE Software the computed strains are
I. Horizontal Tensile Strain in Bituminous Layer is –4.137 x 10–6 (Compressive).
II. Vertical Compressive Strain on Subgrade is 191 x 10–6. III. T ensile Strain in Cementitious Layer is 58 x.10–6.
The Pavement composition is safe. A minimum thickness of 100 mm has been adopted for SMA/GGRB and DBM even though there is no tensile stress at the bottom. The reduction in thickness of the cemented base increases the bending stresses considerably because it is inversely proportional to the square of the thickness. It is necessary to carry out fatigue damage analysis for the safety of CTB against fracture by the spectrum of axle loads. Structural safety of the CTB should be examined for the construction traffic also as explained earlier.
10.4 Foamed Bitumen/Bitumen Emulsion Treated Rap/Aggregates Over Cemented