# Pavement Design Questions and Answers – Highway Maintenance – Design of Overlay

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This set of Pavement Design Multiple Choice Questions & Answers (MCQs) focuses on “Highway Maintenance – Design of Overlay”.

1. The first step in designing an overlay is estimating the strength of the existing pavement. What step is to be performed after this?
a) Determining overlay thickness
b) Determining overlay design life
c) Estimation of traffic on an overlay
d) Determining overlay type

Explanation: The order of the steps to be performed during the designing of an overlay is as follows – estimating the strength of the existing pavement, determining the design life of the overlaid pavement, estimating the traffic to be carried by the overlaid pavement, determining the thickness of overlay and the type of overlay.

2. How many types of overlays are possible to provide on existing pavements?
a) Two types of asphalt overlays and single coat of cc overlay
b) Two types of cc overlays and single coat of asphalt overlay
c) Two types of asphalt overlays and two types of cc overlays
d) Single coat of asphalt overlay and single coat of cc overlay

Explanation: Four different types of overlays can be laid on an existing pavement. They are asphalt overlay on asphalt pavement, asphalt overlay on cc pavement, cc overlay on cc pavement and cc overlay on asphalt pavement.

3. What is the equation used to find the effective thickness of the existing pavement?
a) $$h_e=∑_{i=1}^n C_i$$
b) $$h_e=∑_{i=0}^n h_i C_i$$
c) $$h_e=∑_{i=1}^n h_i$$
d) $$h_e=∑_{i=1}^n h_i C_i$$

Explanation: $$h_e=∑_{i=1}^n h_i C_i$$ represents the correct formula to find the effective thickness of the existing pavement. This equation is used to find the thickness of the overlay using the effective thickness method. The overlay thickness is obtained as the difference between the thickness of new pavement and the effective thickness of the existing pavement.
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4. Find the overlay thickness if the following data is available using the effective thickness method.
Thickness of existing pavement: 260 mm GSB, 250 mm WBM, 90 mm BM and 40 mm BC
Conversion factors = 0.1 for GSB and WBM, 0.5 for BM and 0.6 for BC
Thickness of new pavement: 300 mm GSB, 250 mm WWM, 100 mm DBM and 40 mm BC
Conversion factors = 0.2 for GSB and WWM, 1 for DBM and BC
a) 230 mm
b) 250 mm
c) 120 mm
d) 130 mm

Explanation: Overlay thickness is the difference between the thickness required for new pavement and the effective thickness of the existing pavement. It is given by ho=hn -he.
Effective thickness is given by $$h_e=∑_{i=1}^n h_i C_i$$. Where hi is the thickness of each pavement layer and Ci is the corresponding conversion factor given by the Asphalt Institute.
he=(260+250)×0.1+90×0.5+40×0.6=120 mm
hn=(300+250)×0.2+(100+40)×1=250 mm
ho=hn-he=250-120=130 mm

5. What is the conversion factor for the subgrade in the effective thickness overlay design method?
a) 1
b) 0
c) 0.1
d) 1.1

Explanation: The conversion factors for various layers of the pavement are given by the Asphalt Institute. It is based on the equivalent thickness of the new pavement. Since the subgrade would remain the same, the conversion factor would be 0.

6. The design of overlay can be done using the Benkelman beam deflection studies result.
a) True
b) False

Explanation: The deflection approach to find the thickness of the overlay is based on the result obtained from the Benkelman beam deflection studies. The IRC 81:1997 has the guidelines for the design of overlay thickness using BBD studies.

7. The design curves relating ______ with the characteristic deflection obtained from BBD studies in the code is used for overlay thickness determination.
a) Resilient modulus of the surface
b) CSA over the design period
c) Thickness of existing pavement
d) Pavement condition index

Explanation: The design chart is given in the code IRC 81:1997. The chart relates the cumulative axles to be carried over the design life to the characteristic deflection. Relating these two values form the chart, the thickness of the overlay can be read.

8. The thickness of the overlay obtained from design chart is in terms of bituminous macadam construction. What is the conversion if thickness in terms of DBM is required?
a) 1 cm BM = 0.7 mm DBM
b) 1 cm BM = 1.5 mm DBM
c) 1 cm BM = 0.7 cm DBM
d) 1 cm BM = 1.5 cm DBM

Explanation: The equivalent thickness of dense bound macadam can be found as 0.7 cm for every 1 cm thickness of bituminous macadam. It can be taken as 1.5 cm for wet bound macadam. In this way, the thickness of overlay for varying compositions can be found out.

9. If the design traffic is 15 msa and a characteristic deflection of 3 mm, compute the thickness of overlay if DBM binder course and bituminous concrete (BC) surface course is to be adopted. What would be the thickness of the BC surface course?
a) 151 mm
b) 51 mm
c) 150 mm
d) 50 mm

Explanation: The thickness of the overlay in terms of bituminous macadam can be read from the design chart using Fig 9 in IRC 81:1997. The chart can be referred to by using the link below:
https://archive.org
For 15 msa and 3 mm characteristic deflection, the overlay thickness in terms of bituminous macadam is 215 mm. Now to find the thickness of DBM,
1 cm BM = 0.7 cm DBM, i.e.
21.5 cm BM = 0.7 X 21.5 = 15.05 cm ≈ 15.1 cm = 151 mm
Out of the 151 mm, 100 mm can be provided for DBM and 51 mm for the bituminous concrete surface course.

10. Traffic studies were conducted on a 4-lane divided flexible pavement highway and the average CVPD was found to be 3500 in one direction. The growth rate is 5%, VDF is 4.5 and the estimated time of completion of the overlay is 2 years. The BBD studies yielded the characteristic deflection to be 2.4 mm. What would be the thickness of the overlay if it to be designed for 10 years?
a) 210 mm
b) 201 mm
c) 120 mm
d) 102 mm

Explanation: The equation to find the design traffic is as below:
$$csa=\frac{365[(1+r)^n-1]NFD}{r}$$
Where r is the growth rate = 5%
n is the design life of overlay = 10 years
N is the traffic after the completion of overlay construction = A(1+r)n
A is the present traffic = 3500 CVPD and n is the time of completion of overlay construction = 2 years
Therefore, N=A(1+r)n=3500(1+0.05)2=3858.75 CVPD
F is the VDF = 4.5
D is the lane distribution factor, as per IRC 37:2012, for a four-lane two-way highway, it is taken as 0.45.
The design traffic,
$$csa=\frac{365[(1+0.05)^{10}-1]×3858.75×4.5×0.45)}{0.05}$$=35873389.61≈35.87 msa
Now, referring the design chart using the link below:
https://archive.org
For design traffic 35.87 msa and 2.4 mm characteristic deflection, the thickness of overlay is 210 mm.

11. The overlay can be directly laid over the existing surface after cleaning.
a) True
b) False

Explanation: The existing pavement surface might have potholes, sags, bumps and all sorts of unevenness. It is very necessary to correct the level of the pavement before laying the overlay. Cleaning is not sufficient, proper filling of sags and levelling of bumps is required.

12. Traffic studies were conducted on a 4-lane flexible pavement highway and the average CVPD was found to be 3000 in one direction. The growth rate is 7.5%, VDF is 4.5 and the estimated time of completion of the overlay is 2 years. The BBD studies yielded the mean rebound deflection to be 3 mm when tested at 35°C in the monsoon time and the standard deviation is found to be 0.22 mm. What would be the thickness of the overlay if it to be designed for 10 years?
a) 240 mm
b) 210 mm
c) 200 mm
d) 250 mm

Explanation: The equation to find the design traffic is as below:
$$csa=\frac{365[(1+r)^n-1]NFD}{r}$$
Where r is the growth rate = 7.5%
n is the design life of overlay = 10 years
N is the traffic after the completion of overlay construction = A(1+r)n
A is the present traffic = 3000 CVPD and n is the time of completion of overlay construction = 2 years
Therefore, N=A(1+r)n=3000(1+0.075)2=3466.88 CVPD
F is the VDF = 4.5
D is the lane distribution factor, as per IRC 37:2012 for four-lane single lane highway, it is taken as 0.40.
The design traffic,
$$csa=\frac{365[(1+0.075)^{10}-1]×3466.88×4.5×0.40}{0.075}$$=32223389.34≈32.22 msa
Characteristic deflection for 3000 CVPD, Dc=Dm+2s
Dc=3+2×0.22=3.44 mm
Now, referring the design chart using the link below:
https://archive.org
For design traffic 32.22 msa and 3.44 mm characteristic deflection, the thickness of the overlay is 250 mm.

13. The design traffic for a minor highway is found to be 10.55 msa. The BBD studies conducted at 30°C on sandy soil with 8% moisture content and an annual rainfall of 1300 mm gave the following results:
Rebound deflection values recorded at various points = 1.40, 1.38, 1.49, 1.50, 1.55, 1.44, 1.58, 1.43, 1.33, 1.37 mm
a) 150 mm
b) 140 mm
c) 100 mm
d) 160 mm

Explanation: The characteristic deflection is to be found out using the equation Dc=Dm+s for a minor highway. The mean deflection is found as below:
Dm= $$\frac{1.40+1.38+1.49+1.50+1.55+1.44+1.58+1.43+1.33+1.37}{10}$$=1.45 mm
The equation to find the standard deviation is given by S.D=$$\sqrt{\frac{\sum(D_m-D_x)^2)}{n-1}}$$
Sum = (1.45-1.40)2+(1.45-1.38)2+(1.45-1.49)2+(1.45-1.50)2+(1.45-1.55)2+(1.45-1.44)2+(1.45-1.58)2+(1.45-1.43)2+(1.45-1.33)2+(1.45-1.37)2
= 0.06mm2
So, S.D=$$\sqrt{\frac{0.06}{10-1}}$$=0.08 mm
Dc=Dm+s=1.45+0.08=1.53 mm
Now corrections for Dc has to be done. Temperature correction is done as
Corrected Dc=Dc+(Standard temperature-field temperature)×0.01
Corrected Dc=1.53+(35-30)×0.01=1.58 mm
The correction for moisture content has to be done by referring to the graphs provided in IRC 81:1997. The code can be accessed from the below link:
https://archive.org
The graph labelled in the code can be used for sandy soils with annual rainfall ≤ 1300 mm. From the graph, the correction factor is obtained as 1.03.
Corrected Dc=Dc×correction factor
Corrected Dc=1.58×1.03=1.63 mm
Now, referring the design chart using the link below:
https://archive.org
For design traffic 10.55 msa and 1.63 mm characteristic deflection, the thickness of the overlay is 140 mm.

14. What is the recommended thickness of bituminous concrete surface course in the overlay design?
a) Minimum 50 mm
b) Maximum 50 mm
c) Minimum 40 mm
d) Maximum 40 mm

Explanation: The recommended thickness of the bituminous concrete surface layer in the overlay design is a minimum of 40 mm. This can be obtained in clause 7.5 of IRC 81:1997. The minimum thickness of 50 mm is required for the bituminous macadam overlay with an additional minimum 40 mm BC or 50 mm DBM surfacing course.

15. If the design traffic is 20 msa and a characteristic deflection of 4.5 mm, what would be the thickness of bituminous macadam overlay if WBM binder course is to be laid?
a) 50 mm
b) 100 mm
c) 200 mm
d) 400 mm

Explanation: The thickness of the overlay in terms of bituminous macadam can be read from the design chart using Fig 9 in IRC 81:1997. The chart can be referred to by using the link below:
https://archive.org
For 20 msa and 4.5 mm characteristic deflection, the overlay thickness in terms of bituminous macadam is 265 mm. Now to find the thickness of WBM,
1 cm BM = 1.5 cm WBM, i.e.
26.5 cm BM = 1.5 X 26.5 = 39.75 cm ≈ 40 cm = 400 mm.

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