Pavement Design MCQ Quiz - Objective Question with Answer for Pavement Design - Download Free PDF

Explanation:

Combination of stresses:

(i) There is no frictional stress at the corner region. Out of various wheel stresses

• Corner stress is minimum as there is a discontinuity in both direction
• Interior stress is maximum
• Edge stress is in intermediate-range

(ii) Temperature stress is critical at the edge and interior and it is minimum at corner. At the corner, resistance due to weight is minimum, hence warping stress is minimum.

(iii) In combination of wheel load and temperature, edge region is mos critical, hence designing is done using edge region stress and however checking is done for corner region.  

Critical cases of stress combination:

(i) Summer mid-day: The critical stress for edge region is given by

Scritical = Sedge + Stemperature - Sfrictional

(ii) Winter, mid-day: The critical combination of stress for the edge region is given by

Scritical = Sedge + Stemperature + Sfrictional

(iii) Mid-nights: The critical combination of stress for the corner region is given by

Scritical = Sedge + Stemperature 

Explanation:

Subgrade Soil in Highway Pavements

1. Proper drainage is essential for subgrade soil in highway pavements to prevent the accumulation of water in the soil, which can lead to moisture retention.
2. Excess moisture can significantly weaken the subgrade by reducing its shear strength and causing expansion or contraction of the soil.
3. This can lead to cracking, rutting, and deformation of the pavement over time.
4. Effective drainage ensures that water is quickly removed from the soil, maintaining the stability and integrity of the pavement.

 Additional InformationKey Properties of Subgrade Soil:

1. Strength: The ability of subgrade soil to support traffic loads without excessive deformation or failure. This is determined by its shear strength and bearing capacity.

2. Compaction: Proper compaction of the subgrade is necessary to ensure the soil is dense enough to resist settlement under traffic loads.

3. Moisture Content: The moisture content of subgrade soil greatly affects its behavior. If it becomes too wet, the soil may become weak and lose its load-bearing capacity, while if it’s too dry, it may become overly rigid and crack.

4. Plasticity: This refers to the soil's ability to deform without cracking. Soils with high plasticity are generally less stable and more prone to shrinking and swelling.

Explanation:

Dowel bar: 

• Dowel bars are provided at expansion joints and sometimes at contraction joints also.
• It guides the direction of movement of concrete expansion.
• It links the two adjacent structures by transferring loads across the joints.

Tie bars 

• They used in longitudinal joints in concrete pavement.

• These are not load transfer devices but serve as a means to tie two slabs.

• These are used for holding faces of rigid slabs in contact to maintain aggregate interlock.
• It is not a component of expansion joint.

Explanation:

The difference between rigid and flexible pavement is given by:

Reference

IRC  37  2018 (page 25)

https://www.manuneethi.in/uploads/files/books/IRC-37-2018.pdf

Stage construction is not allowed for pavements with cement treated bases and sub-bases.

Explanation:

Dowel bars:

• Dowel bar acts as a load transfer device across the transverse joint and they keep the two slabs at the same height.
• They are of mild steel round bars, bounded on one side and free on the other side.
• Normally have 25 mm to 40 mm diameter and 400 - 500 mm length.
• The stress in dowel bars is given by Bradbury analysis.

​Additional Information

Tie bars: 

• Tie bars used for holding faces of rigid slabs in contact to keep aggregate interlock. They are also used in the plain jointed concrete pavement to connect two lanes.
• They are used to reduce transverse cracking.
• Tie bars avoid separation and differential deflection in lanes.
• They are not designed to work as a load transfer device.

Explanation:

Prime coat:

The prime coat is an application of low viscous cutback bitumen to an absorbent surface like granular bases on which binder layer is placed. It provides bonding between two layers. Unlike the tack coat, prime coat penetrates into the layer below, plugs the voids, and forms a watertight surface. 

Seal Coat:

Seal Coat Seal coat is a thin surface treatment used to water-proof the surface and to provide skid resistance.

Tack coat:

Tack coat is a very light application of asphalt, usually asphalt emulsion diluted with water. It provides proper bonding between two layers of binder course and must be thin, uniformly cover the entire surface, and set very fast.

The bituminous prime coat is the first application of low viscosity liquid bituminous material over an existing porous or absorbent pavement surface like the WBM base course. 

Explanation:

Flexible pavement is modeled as an elastic multilayer structure. Failure criteria in flexible pavement are of following types:

1) Fatigue Cracking

• It is due to the build-up of horizontal tensile strain at the bottom of the asphaltic concrete layer.
• The pavement is considered to fail if 20 % of the surface has cracked.

2) Rutting failure

• It is due to the build-up of excessive compressive strain at the top of the subgrade layer.
• The pavement is considered failed if it exhibits a rut depth of 20 mm.

Note:

According to IRC 37: 2018, Theoretical calculations suggest that the tensile strain near the surface close to the edge of the wheel can be sufficiently large to initiate longitudinal surface cracking followed by transverse cracking much before the flexural cracking of the bottom layer occurs, if the mix tensile strength is not adequate at higher temperatures. 

So, Vertical sub-base strain is not a critical parameter to control cracking and rutting in flexible pavement.

Concept:

Expansion joint:

\(\frac{Δ }{2} = Lα t\)

Where, Δ = width of expansion joint (This gap is such that Δ/2 distance is always maintained after expansion i.e. filler material generally made of cork, Fibre-board is assumed to be compressed by 50%)

L = Length of the slab or spacing between transverse joint

α = Thermal coefficient of concrete per ^◦C (Generally taken 10 × 10^-6)

t = Temperature difference in ^◦C

Calculation:

Δ = 2 cm = 0.02 m

α = 10 × 10^-6

t = 50^◦C - 10^◦C = 40^◦C

\(\frac{Δ }{2} = Lα t\)

\(\frac{0.02 }{2} = L× 10 × 10^{-6} × 40\)

L = 25 m

Concept:

The warping stresses are generated in a reinforced cement concrete pavement during a summer mid-day is Tensile in bottom fibre and compressive in top fibre.

Stresses in rigid pavements are divided into 2 categories:

i) Wheel load stress

ii) Temperature stresses

Temperature stresses are classified into 2 categories:

i) Warping stresses: Due to daily variation of temperature

During day time the top surface of the pavement is at a higher temperature as it is directly exposed to the sun than the bottom surface, therefore there occurs a temperature difference between the top and bottom layer. Which causes warping of the pavement.

As at top slab tries to expand to resist that internal compression stress develops on top.

At the bottom, the slab tries to contract but to resist that internal tensile stress develops at the bottom.

During night time the top surface of the pavement is at a lower temperature than the bottom surface, therefore there occurs a temperature difference between the top and bottom layer. Which causes warping of the pavement.

As top slab tries to contract but to resist it internal tensile stresses develop at the top

And compressive stress at the bottom.

ii) Frictional stresses: Due to seasonal variation of temperature.

Change in seasonal temperature induces frictional tension or frictional compression due to expansion and contraction.

In an analysis, frictional stresses are assumed as constant throughout the length lx but actually, frictional stresses are zero as corners and maximum at center.

In winter the slab will try to contract but the frictional resistance between the slab and the soil will resist the slab to contract. Frictional force will develop from both side.

During summer compressive force is induced at top and bottom.

iii) The worst combination of stresses

Concrete is weak in tension and therefore a combination of tensile stresses are worst condition.

Concept:

CBR Test:

• The California bearing ratio test is penetration test meant for the evaluation of subgrade strength of road and pavements. The results obtained by these tests are used with the empirical curves to determine the thickness of pavement and its component layers.
• This is the most widely used method for the design of flexible pavement.
• Generally CBR value is calculated at 2.5 mm and 5 mm penetration.
• The CBR value at 2.5 mm penetration will be greater than the CBR value at 5 mm penetration and in such a case the former will be taken as the CBR value.
• If CBR for 5 mm exceeds that for 2.5 mm, the test should be repeated. If identical result follows, the CBR for 5 mm penetration should be taken for design.

Concept:

Vehicle Damage Factor:

• The vehicle damage factor (VDF) is a multiplier to convert the number of commercial vehicles of different axle loads and axle configurations to the number of standard axle load repetitions.
• It is defined as an equivalent number of standard axles per commercial vehicle.

For Single Axle Load:

EALF = \(({axle\: load\over 8160})^4\)

For Tandem Axle Load:

EALF = \(({axle \:load\over 14968})^4\)

Where EALF = Equivalent Axle load Factor

Concept:

Flexible pavements: 

• Flexible pavements are so named because the total pavement structure deflects, or flexes, under loading.
• Flexible pavement may be constructed in a number of layers and the top layer has to be of the best quality to sustain maximum compressive stress, in addition, to wear and tear. 
• The lower layers will experience a lesser magnitude of stress and low-quality material can be used.
• The thickness of the base of a flexible pavement is 10 - 30 cm.

Explanation:

Fog seal:

(i) Fog seal is used to repair the defects.

(ii) It is a spray of slow setting emulsion diluted with equal amount of water at a rate 0.5 - 1 lit/m².

(iii) Traffic is allowed after the seal sets in

(iv) It is used to increase the binder content of bituminous surface.

(v) It can also be used as an emergency treatment measure for hungry surface. 

Additional Information

Prime coat:  

(i) Bituminous prime coat is the first application of a low viscosity liquid bituminous material over an existing porous or absorbent pavements surface like the WBM coarse.

(ii) The main objective of priming is to plug in the capillary voids of the porous surface and to bond the loose mineral particles on the existing surface using a binder of low viscosity which can penetrate into the voids.

Tack coat:

(i) Bituminous tack coat is the application of bituminous material over an existing pavement which is relatively impervious like an existing bituminous surface or cement concrete pavement or a pervious surface like the WBM which has already been treated by a prime coat.

Concept:

Group Index method:

• It is an empirical method that is based on the physical properties of the soil sub-grade.
• In this method, the thickness of the pavement is decided based on the group index value which is given by -

G.I = 0.2a + 0.005ac + 0.01bd

where,

a - percentage of soil passing through 75μ sieve in excess of 35%, not exceeding 75%

b - percentage soil passing through 75μ sieve in excess of 15%, not exceeding 55%

c - liquid limit in percent in excess of 40% ≯20%

d - plasticity index in excess of 10%≯ 20%

Important Points

• The GI value of soil varies in the range of 0 to 20.
• The following table shows the suitability of soil for subgrade -

Concepts:

Modulus of Subgrade Reaction:

It is defined as the pressure per unit deformation of the subgrade at specific pressure or deformation. It is expressed as:

k = p/Δ

Here,

'k' is the modulus or coefficient of subgrade reaction,

'p' is the pressure, and 'Δ ' is the deformation of soil settlement.  

The relation between modulus of sub-grade reaction (k)and plate diameter (d) is  given as:

k × d = constant

The standard size plate has dia = 75 cm

Calculation:

Given data;

k1 = 20 kg/cu.cm , d1= 30 cm, and d2 = 75 cm

k₂ = the corrected modulus of subgrade reaction for the standard diameter plate

The relation between modulus of sub-grade reaction (k)and plate diameter (d) is  given as:

k × d = constant

So

k1 × d1= k2 × d2

20 × 30 = K2 × 75

∴ k2 = 8 kg/cu.cm