ASSIGNMENT REPORT CES10 204 (B) PAVEMENT ANALYSIS AND DESIGN

TOPIC: 4 TH MODULE QUESTIONS AND ANSWERS

SUBMITTED BY,

GROUP NO - 4

SOORAJ CHANDRA R.S ROLL NO -12

SOORIA RAJ ROLL NO - 13 SWATHI RATNA K ROLL NO -14

V.P ROSHAN AHAMED ROLL NO -15

M.TECH, STRUCTURAL

A.W.H ENGINEERING COLLEGE

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ASSIGNMENT REPORT CES10 204 (B) PAVEMENT ANALYSIS AND DESIGN

TOPIC: 4 TH MODULE QUESTIONS AND ANSWERS

SUBMITTED BY,

GROUP NO - 4

SOORAJ CHANDRA R.S- ROLL NO -12

SOORIA RAJ- ROLL NO - 13 SWATHI RATNA K- ROLL NO -14

V.P ROSHAN AHAMED- ROLL NO -15

M.TECH, STRUCTURAL

A.W.H ENGINEERING COLLEGE

2

MODULE IV

PAVEMENT STRUCTURE AND ITS EVALUATION

QUESTIONS 1) What are the factors that affect the structural stability of flexible and rigid pavements?

Explain atleast 4 factors in detail. 2) What are the general causes of pavement failures? Discuss briefly the importance of

highway maintenance.

3) Explain briefly about the typical flexible pavement failures and their causes. 4) Explain the various types of failures in cement concrete pavements and their causes. 5) Discuss the various types of pavement evaluation techniques. What are the advantages

and disadvantages of NDT methods?

6) What is the importance of pavement evaluation? Discuss the methods involved in pavement evaluation.

7) What is FWD? How FWD can be used for pavement evaluation? 8) Discuss in detail about Benkelman Beam Rebound Deflection for pavement

evaluation.

9) Explain plate load test with a suitable test set up figure. 10) How wave propagation method can be used for pavement evaluation? Explain with a

suitable figure. 11) What are the different destructive test methods for pavement evaluation? Discuss

atleast 2 of them in detail.

12) What is an overlay? Discuss the various types of overlays that are commonly used on the pavement surface.

13) What are geosynthetics? How can they be used in pavement overlays? 14) Benkelman Beam Deflection studies were carried out on 15 selected points on a

stretch of flexible pavement during summer season using a dual wheel load of 4085 Kg, 5.6Kg/cm2 pressure. The deflection values obtained in mm after making the necessary lag corrections are given below. If the present traffic consists of 750commercial vehicles/day, determine the thickness of bituminous overlay required, if pavement temperature during the test was 30C and the correction factor for subsequent increase in subgrade moisture content is 1.3. Assume annual rate of

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growth of traffic as 7.5%. Adopt IRC guidances.1.40, 1.32, 1.25, 1.35, 1.48, 1.60, 1.65, 1.55, 1.45, 1.40, 1.36, 1.46, 1.50, 1.52, 1.45mm.

15) Existing black top pavement was tested using Benkelman Beam with a test vehicle of 8170 Kg rear axle load. Observations recorded at a pavement temperature of 43C are given below. Length of test stretch = 300m. Compute the thickness of overlay of bituminous concrete, taking allowable deflection as 1.25mm, if the factor for subgrade moisture correction is 2.0.

Serial number subsection Rebound deflection (mm)

1 1.46

2 1.52

3 1.56

4 1.76

5 1.96

6 1.74

7 1.68

8 1.74

9 1.96

10 1.42

11 1.56

12 1.62

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1) What are the factors that affect the structural stability of flexible and rigid pavements? Explain atleast 5 factors in detail.

ANS: The factors that affect the structural stability of flexible and rigid pavements are:

a) Subgrade soil b) Moisture c) Pavement layers d) Temperature e) Environment f) Traffic (wheel load)

(a) Effect of Subgrade soil on structural stability of flexible & rigid pavements Subgrade soil is an integral part of pavement structure as it provides support to

pavement from beneath. Subgrade soil and its properties are important for the design of pavement structure. The desirable properties of subgrade soil are stability, incompressibility, permanency of strength, minimum volume changes, good drainage and ease of compaction.

Properties of subgrade soil decide the thickness of pavement surface. A subgrade soil with lower stability requires thicker pavements to withstand heavier

traffic loads. Poor subgrade soil causes waves, corrugations, rutting and shoving in flexible pavements, pumping, blowing and cracking in rigid pavements.

(b) Effect of Moisture on structural stability of flexible & rigid pavements

Rainfall affects the moisture conditions in subgrade soil. The variations in

moisture condition of subgrade soil are due to climatic conditions, soil type, ground water level, drainage conditions, type of pavement and shoulders. The moisture

movement in subgrade is caused by capillary action and vapour movement. Adverse moisture conditions decreases the stability of subgrade soil.

The surface water during the rains may enter the subgrade either through

pavement edges or through pavement itself, if it is porous. Moisture content of subgrade soil at centre is different from that at the edges as soil with high plasticity causes differential rise or fall of pavement edges due to swelling and shrinkage of

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subgrade soil. High moisture variations can be controlled by providing suitable surface and sub-surface drainage system.

(c) Effect of Pavement layers on structural stability of flexible & rigid pavements

The stress distribution characteristics of the pavement component layers depend on the characteristics of materials used. The fatigue behaviour of the pavement materials

and their durability under adverse weather affects the structural stability of both flexible and rigid pavements. The different pavement materials like subgrade soil, stone

aggregates, bituminous materials and cement concrete should be of superior quality and should have good structural properties.

(d) Effect of Temperature on structural stability of flexible & rigid pavements The daily and seasonal variations in temperature have great significance on the

structural stability of both the pavements. During winter, frost action causes damaging

effects on subgrade soil. Wide variation in temperature due to climatic changes causes damaging

effects in pavements. High magnitude temperature stresses are induced in cement concrete pavements due to daily variations in temperature resulting in warping.

Bituminous pavements become soft in hot weather and brittle in very cold weather. (e) Effect of Environment on structural stability of flexible & rigid pavements

The environmental factors such as height of embankment and its foundation details, depth of cutting, depth of sub-surface water table etc. effects the performance of pavements. The ground slope and flora also effects the structural stability of pavements mainly in the hilly terrains.

(f) Effect of Traffic on structural stability of flexible & rigid pavements The design thickness of the pavement depends on the design wheel load.

Higher wheel load requires thicker pavements. Structural stability also increases with

thickness of the pavement but thickness designed should be economical. Static and repeated traffic loads have great effect on the structural stability of

pavements. The various wheel load factors such as maximum wheel load, contact pressure, dual or multiple wheel loads, equivalent single wheel loads and repetition of loads are to be considered while designing pavements.

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2) What are the general causes of pavement failures? Discuss briefly the importance of highway maintenance.

ANS:The general causes of pavement failures are

a) Defects in the quality of materials used. b) Defects in construction method & quality control during construction. c) Inadequate surface or sub-surface drainage in locality resulting in stagnation

of water in subgrade or other pavement layers. d) Increase in magnitude of wheel loads and number of load repetitions due to

increase in traffic volume.

e) Settlement of foundation/ embankment/ fill material itself. f) Environmental factors including heavy rainfall, soil erosion, high water

table, snow fall, frost action etc.

Highway Maintenance

Road maintenance is one of the important components of entire road system. Even if the highways are well designed and constructed, they may require maintenance. The pavements are subjected to failures ranging from minor and localised failure to major and general failures. The types of failures depends on types of pavement, whether it is flexible or rigid. The maintenance operations involve the following steps:

a) Assessment of present road condition b) Diagnosis of problem c) Adopting the most appropriate maintenance steps

The highway maintenance works may be classified as:

a) Routine Maintenance b) Periodic Maintenance c) Special Repairs

Routine maintenance include filling up of pot holes and patch repairs, maintenance of shoulders and cross slope, up-keep of road side drains and clearing

choked culverts, maintenance of road signs etc. The maintenance may be carried out at regular intervals like day to day or seasonal.

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Periodic maintenance includes renewal of wearing course, preventive maintenance of various items.

Special repairs include strengthening of pavement structure or overlay

construction, reconstruction of pavement, widening of roads, repair of damages caused by floods, providing additional safety measures like islands, signs etc.

The type and extend of maintenance requirement for a road depends on following

factors:

a) Serviceability standard laid down b) Funds available c) Priorities for maintenance operations

A systematic approach is required for road maintenance, which is known as

maintenance management system. The various factors included in maintenance management system are:

a) Field surveys for evaluation of maintenance requirements. b) Subgrade soil, drainage, climate, traffic, environmental conditions etc. c) Estimation of rate of deterioration of pavement under prevailing set of

conditions.

d) Type and extend of maintenance requirements, possible alternatives and their economic evaluation.

e) Availability of funds f) Maintenance cost, availability of materials, man power and equipments.

3) Explain briefly about the typical flexible pavement failures and their causes.

ANS: A flexible pavement failure is defined by formation of pot holes, ruts, cracks, localised depressions and settlements. The failure of any one or more components of pavement structure leads to the development of waves, corrugations, longitudinal ruts,

shoving etc. When pavement unevenness is excessive, then it is also considered as a failure. The main reasons for flexible pavement failures are

a) Aging & oxidation of bituminous films

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b) Retaining of excess water in the voids which increases the detrimental action in flexible pavements

Some of the typical flexible pavement failures are:

i. Alligator cracking (map cracking) ii. Consolidation of pavement layers

iii. Shear failure& cracking

iv. Longitudinal cracking v. Frost heaving

vi. Lack of binding to lower course vii. Reflection cracking

viii. Formation of waves & corrugation

i. Alligator cracking (map cracking) Most common type of failure due to relative movement of pavement layer

materials. The main reasons for this failure are:

Repeated application of heavy wheel loads resulting in fatigue failure.

Moisture variations resulting in swelling & shrinkage of pavement layers.

Localised weakness of base course cause cracking of surface course. ii. Consolidation of pavement layers

Consolidation of one or more pavement layers results in the formation of ruts. The main reasons for this failure are:

Cumulative deformation caused due to the repeated application of loads

along the same wheel path.

Wearing along the wheel path causes shallow ruts on the surfacing course.

iii. Shear failure & cracking Shear failure results in the formation of fracture or cracking which is due to

upheaval of pavement materials. Here upheaval is followed with a depression. The main reasons for this failure are:

Inherent weakness of pavement mixtures.

Low shear resistance due to excessive loading. iv. Longitudinal cracking

Longitudinal cracks traverse through the full pavement thickness. The main

reasons for this failure are:

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Frost action & differential volume changes.

Settlement of fill.

Sliding of side slopes. v. Frost heaving

Localized up heaving occurs in pavement which depends on ground water and

climatic conditions. vi. Lack of binding with lower layers

Lack of binding with lower layers causes slipping, opening up and loss of pavement materials thus forming patches or pot holes. The main reasons for this

failure are:

Providing bituminous surfacing over existing cement concrete base course

or soil-cement base course.

Lack of prime/tack coat in between two layers. vii. Reflection cracking

Reflection cracking occurs on the bituminous overlays provided on cement concrete pavements. Cracks are formed on the surface course so it has little effect

on the structural stability of the flexible pavements. But reflection cracks allow water to seep through them thus causing mud pumping.

The main causes for flexible pavement failures are:

I. Failures in Subgrade II. Failures in Sub-base or Base Courses

III. Failures in Wearing Course I. Failures in Subgrade

Excessive deformation in subgrade soil causes waves, corrugations and depressions in flexible pavements. Shear failure of subgrade soil causes lateral

shoving near pavement edges. The main reasons for failures in subgrade are:

Inadequate stability due to inherent weakness of soil, excessive moisture

content, improper compaction, increased load repetitions.

Excessive stress application due to wheel loads II. Failures in Sub-base or Base Courses

The main reasons for failures in sub-base or base course are:

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Inadequate stability due to poor mix proportioning, inadequate thickness,

use of soft stone aggregates, improper quality control during construction etc.

Loss of binding action due to repeated load application causes internal movement of aggregates in base/sub-base course resulting in the formation

of alligator cracks.

Loss of base course materials due to wearing of surface course. Aggregates

get exposed & lost thus forming pot holes. The removal of materials is called revelling.

Inadequate wearing course results in the exposure of pavements to rain, frost, traffic etc.

Use of inferior materials

Crushing of base course materials

Lack of lateral confinement for granular base course

III. Failures in Wearing Course Failures in wearing course may be due to improper mix design, improper gradation of aggregates, inadequate binder content, use of inferior binder, poor quality control during construction.

4) Explain the various types of failures in cement concrete pavements and their causes.

ANS:The cement concrete pavements develop cracks and deteriorate due to repeated loads and fatigue effects. Rigid pavements are capable of withstanding slight variations in underlying support due to inherent bending strength. Due to temperature effects, newly

constructed rigid pavements may crack even if no vehicles move on them. Failure of rigid pavements starts from joints, corners and edges of slabs.

Some typical and basic types of failures in rigid pavements are:

i. Scaling of cement concrete ii. Shrinkage cracks

iii. Spalling of joints iv. Warping cracks

v. Mud pumping vi. Structural cracks

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i. Scaling of cement concrete Scaling occurs mainly due to deficiency in mix, presence of chemical impurities,

excessive vibrations during mixing etc. Due to scaling, pavement surface becomes rough and shabby in appearance.

ii. Shrinkage cracks Shrinkage cracks occur in longitudinal as well as transverse directions. Shrinkage

cracks develop immediately curing operation of rigid pavements. iii. Spalling of joints

Spalling of joints occurs when the filler materials are dislocated and concreting is done without noticing this faulty alignment. This causes overhanging of concrete layer on top side and the joint, later shows excessive cracking and subsidence.

iv. Warping cracks

Due to excessive warping stresses, slabs develop cracking at edges in irregular pattern. Hinge joints are provided for relieving the warping stresses. Providing proper reinforcements in longitudinal as well as transverse joints reduce the structural inadequacy due to warping cracks.

v. Mud pumping Mud pumping refers to the ejection of soil slurry through joints and cracks of cement concrete pavements. Mud pumping occurs during downward movement of cement concrete slab under heavy wheel loads. The factors which cause mud

pumping are:

Extent of slab deflection

Type of subgrade soil

Amount of free water

vi. Structural cracks

Structural cracking occurs due to inadequate pavement thickness. Cracks at corners, joints or edges are due to structural inadequacy. Cracking in the interior regions are due to temperature stresses.

Failures in cement concrete pavements are mainly due to:

a) Deficiency of pavement materials b) Structural inadequacy of pavement system

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Deficiency of pavement materials is due to

Use of soft aggregates

Poor workmanship in joint construction Poor joint filler and sealer material Poor surface finish

Improper and insufficient curing

Structural inadequacy of pavement system may be due to

Inadequate pavement thickness

Inadequate subgrade support and poor subgrade soil

Incorrect spacing of joints. 5) Discuss the various types of pavement evaluation techniques. What are their

advantages and disadvantages?

ANS:Pavement evaluations are conducted to determine functional and structural conditions of a highway section which may be either for routine monitoring or planned corrective action. Functional condition is concerned with the ride quality or surface texture of a highway section. Structural condition is concerned with the structural capacity of the

pavement which is measured by deflection, layer thickness, and material properties.

At the network level, routine evaluations can be used to develop performance models and to arrange maintenance or rehabilitation efforts and funding. At the project level, evaluations are more focused on establishing the root causes of existing distress in order to determine the best rehabilitation strategies.

The various types of pavement evaluation techniques are

i. Visual condition surveys

ii. Non-destructive testing

iii. Destructive testing

i. Visual Condition Surveys

Visual condition surveys cover aspects of both functional and structural pavement condition, but generally serve as a qualitative indicator of overall condition.

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Specialized equipment is used to quantify both functional and structural properties of the pavement structure.

For any given section of highway, two or more evaluators would arrive at the same assessment of the sections current condition. For example, in visual condition

surveys, the percent of surface area affected by alligator cracking is highly dependent upon the visual acuity of the evaluator.

ii. Non-destructive Testing

In non-destructive testing, data collected in the field are generally objective in nature, but often subjectivity appears in the data analysis and interpretation. Non-destructive testing is the collective term for evaluations conducted on an existing

pavement structure that do not require subsequent maintenance work to return the pavement to its pre-testing state.

This is generally desirable to minimize disruption to traffic, and is essential as a screening tool to determine locations where selective material sampling should be

conducted to evaluate other material properties in the laboratory. Non-destructive testing methods can assess either functional or structural condition. Its focus is to

assess

In situ properties that can be used to evaluate the need for further destructive testing (i.e., coring, boring, trenching)

Location of that destructive testing

The current structural capacity of the highway as related to layer stiffness and strength.

iii. Destructive Testing

Destructive testing provides more detailed data about the pavement which are not possible to obtain through non-destructive testing. Such detailed data include:

laboratory mechanical, physical, and chemical properties (obtained through coring, Shelby tubes, and trenching), and

visual inspection of pavement layers through coring and trenching.

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Advantages of NDT methods are:

a) No Hazardous Radiation

b) Cost Effective

c) Real time positioning

d) Reduced Interruption to Traffic or Access to Facilities

e) Capable of Detecting Metallic and Non-metallic Objects

f) High Resolution with High Speed Data Acquisition

g) Super Fast Data Acquisition with Wide Coverage

h) Simple Instrumentation Setup and Portability

i) No Limitations because of Ambient Conditions

j) Structural Profiling across Depth

k) Capable of non-destructive testing (NDT) on Wide Range of Materials

l) Non-Destructive and Non-intrusive

m) High Resolution 3D Imaging

Disadvantages of NDT methods are:

a) Can only detect defects open to the surface b) Preparation, before testing, can be time consuming and costly c) The method takes time and can rarely be completed in less than 30 minutes d) The method cannot normally be applied to painted objects e) It is messy f) Interpretation of results is sometimes difficult g) There may be a problem disposing of contaminated cleaning and penetrant fluids h) Dry, clean, conditions are essential, as is careful cleaning of the surfaces to be

examined i) The method is often abused and skimped, or not fully understood j) There can be a fume exposure problem, particularly in confined spaces

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6) What is the importance of pavement evaluation? Discuss the methods involved in pavement evaluation.

ANS: Pavement evaluation refers to the study of various factors such as subgrade support, pavement composition and its thickness, traffic loading and environmental conditions. The main objective of pavement evaluation is to assess whether and to what extent the pavement fulfils the intended requirements so that the maintenance and

strengthening jobs could be planned in time. Pavement evaluation investigates the structural adequacy of pavements and requirements for providing safe and comfortable

traffic operations.

Pavement evaluations are conducted to determine functional and structural conditions

of a highway section which may be either for routine monitoring or planned corrective action. Functional condition is concerned with the ride quality or surface texture of a

highway section. Structural condition is concerned with the structural capacity of the pavement which is measured by deflection, layer thickness, and material properties.

At the network level, routine evaluations can be used to develop performance models and to arrange maintenance or rehabilitation efforts and funding. At the project level, evaluations are more focused on establishing the root causes of existing distress in order to determine the best rehabilitation strategies.

The methods of pavement evaluation are: i. Structural evaluation of pavements

ii. Function Evaluation of pavements (Evaluation of pavement surface condition) i. Structural evaluation of pavements

Structural capacity refers to the ability to withstand the various loads acting on the pavements without causing any damage to its surface. It is done by plate bearing

test. It can be either destructive or non-destructive evaluation. Performance of flexible pavement is related to elastic deflection under loads or

rebound deflection. Transient deflection measurements are also carried out to determine the structural ability of pavements. Benkelman beam deflection is the most commonly used method.

ii. Function Evaluation of pavements (Evaluation of pavement surface condition)

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The pavement unevenness is measured by unevenness indicator, profilograph, profilometer or roughmeter. Equipment capable of integrating the unevenness of

pavement surface to cumulative scale thus giving unevenness index of surface(cm/km) is called Bump Integrator or Unevenness Integrator. The pavement serviceability concept was introduced to compare the relative performance of various test sections during different periods. The present

serviceability rating (PSR) is the mean opinion of the members of the rating panel and this is related with physical measurements such as longitudinal and transverse

profile of pavement, degree of cracking, patching etc. Mathematical models and two equations are formulated to get present serviceability index (PSI) values for flexible and rigid pavements.

7) What is FWD? How FWD can be used for pavement evaluation?

ANS: FWD refers to Falling Weight Deflectometer. This device is essential in establishing the in situ stiffness properties of the pavement layers through analysis of the deflection data by back calculation. The falling weight deflectometer (FWD) is a trailer mounted device that places an 11.8 in. (300 mm) diameter load plate in contact with the highway at each test location. The testing interval is set at 0.1 mi. (maximum) or 30 locations per project. A load column above the load plate carries a stack of weights that are dropped to impart a load to the pavement similar to that imparted by a passing dual truck tire set. A series of seven geophones spaced away from the load plate at 12 in. increments measure the surface deflection, generating a deflection bowl. Measurements are generally acquired in the right wheel path of the outside lane.

Where a divided roadbed exists, surveys should be taken in both directions if the project will include improvements in both directions. Some care in the placement of the load plate and sensors is required by the survey crew, especially where the highway surface is rutted or cracked. The load plate should lay on a flat surface; the load plate and

all geophones should lie on the same side of any visible cracks. A human spotter should be used to ensure placement in these case.

Temperature data collected at the time of testing is necessary for all flexible pavements since the modulus of bituminous materials is temperature-dependent. The FWD

has a built-in thermocouple to measure ambient air temperature and records this temperature at each drop location. In addition, for pavements with bituminous surfaces at

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least 3.0 in. thick, in-pavement temperature measurements should be made at the beginning and end of each survey, at a minimum. Longer surveys or surveys made on days where

rapid temperature changes occur will require temperature checks at additional intermediate locations. A portable drill with masonry bit is provided to accomplish this; holes are drilled to mid-depth or 3.0 in. maximum. Alternatively, surface temperatures measured by infrared devices can be made with correlations available for mid-depth temperature

estimation.

Liberal use of comments placed in the FWD data file at the time of data collection is encouraged. Comments pertaining to proximity to reference markers, bridge abutments, patches, cracks, etc., are all important documentation for the individual evaluating the data.

Back calculation of Deflection Data

TxDOT currently uses version 6.0 of the MODULUS software for backcalculation of deflection data collected by the FWD. Version 6.0 is a Windows compatible program with many added analyses and graphing features compared to the previous version. The software and users manual in PDF document format is available from the TTI on-line pavement design training site. Also, basic operation and discussion of inputs, cautions, and

example problems is presented in the Rehabilitation Strategies for Flexible Pavements(CD/ROM). Contact the Research & Technology Implementation Office (RTI) at (512) 465-7555 to request a copy of this CD.

The raw deflection file, pavement layer thicknesses, layer Poisson ratios, probable layer

moduli ranges, and asphalt temperatures at the time of testing are all required inputs to perform back calculation. The back calculation process works on the assumption that the

pavement structure can be modelled as a linear-elastic layered system. If the parameters of layer thickness, deflection, and Poisson ratio are known, the modulus can be approximated. A likely range of probable layer moduli provided by the program user facilitates the process by forming the basis of a small internal database against which mathematically

generated deflection bowls are compared to the actual measured deflection bowl by the software. Once a reasonable match is made, the moduli that allow this match are reported

as the individual layer moduli. In addition, the program reports a depth to stiff layer or bedrock.

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There are precautions and limitations to the back calculation procedure that the user must consider. In the end, engineering judgment will be needed to decide on the viability of solutions generated. The following are some pointers when using MODULUS 6.0:

The modulus for layers thinner than 3.0 in. cannot be back calculated. This situation

arises most often for thin-surfaced flexible pavements. The user must assign a reasonable modulus to this layer (minimum and maximum are input as the same value in the program) based on thickness, level of distress, temperature, etc.

The surface layer is always the layer that the load plate is in contact with, so a thickness must be entered. Where the surface is a bituminous surface treatment, it is allowable to use a nominal thickness such as 0.5 in. and assign a nominal modulus such as 250 ksi. Alternatively, the surface treatment can be combined with the underlying layer as the surface reducing the total number of layers by one.

The maximum number of layers for which the modulus can be back calculated is four (one of which is always the natural subgrade) in MODULUS 6.0. For circumstances where more layers are known to exist, the user must either consolidate or ignore layers. Consolidation is recommended for materials that are more likely to have a similar

modulus and shear strength properties (i.e., different types of HMAC or flex base). Ignoring layers may be reasonable in certain cases where the materials contribution to

the overall stiffness of the structure is minimal (i.e., foundation course, or lime treated subgrade constructed as a working platform).

There are times when a more reasonable solution is obtained modeling your pavement structure as a 3-layer system even if you know there are four layers present. This situation may develop for a number of reasons, such as variable stabilization (leaching), variable depth to bedrock, etc.

Reasonableness is related to the in-place stiffness characteristics of the layers being modeled and not necessarily to the size of the average errors reported by the software

in comparing the mathematically generated bowls to the measured bowls. While the 4-layer solution will generally give lower overall errors, the backcalculated material

moduli may be unrealistic with respect to the in-place material. When there is doubt of reasonableness, the user should perform backcalculation runs using both 3- and 4-layer solutions (employing guidelines given in bullet 3). Additional field testing such

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as with the dynamic cone penetrometer (DCP) along with engineering judgment is necessary to ensure a valid, reliable solution.

A check of the MODULUS summary table should be made to detect outliers that skew the average value reported. Outliers may be the result of full-depth patches (different pavement structure) or very weak areas.

For the purpose of using MODULUS-reported values as input to FPS-19, adjustment of the average modulus should be considered; otherwise the performance of any pavement design solution based on these inputs could be jeopardized. As a rule-of-thumb, consider removing values that exceed one standard deviation from the unadjusted average, and then re-average. This should always be done for modulus values that are much higher than values that are more typical for the section.

Consideration can be given to eliminating very low values only if the intention is to include a bid item for repair of weak areas (i.e., Item 351 Flexible Pavement Structure Repair) as part of the job.

Shallow bedrock (typically less than 60 in. deep) will almost always result in underestimation of the subgrade modulus and over-estimation of the flexible base

modulus. The recommended workaround is to fix the depth to bedrock at 120 in. or alternatively 240 in. if the solution using the program-generated depth to bedrock

produces suspect subgrade/base moduli. Another clue that the default solution is suspect would be if the ratio of the flexible base modulus (unstablized layers only) to the subgrade modulus is very high (>5).

MODULUS can perceive a shallow depth to bedrock in high water table situations

(water is incompressible) as can be the case in east Texas. It may be beneficial to override the program-generated depth-to bedrock value, by using a fixed value of 120 in.

Soft upper subgrade can also lead to high errors in the back calculation process. In these

cases, the use of a 4-layer solution where the soft portion of the subgrade is modelled as the sub base layer (fix depth at 12 in.) may provide better fit and more realistic back calculated values for the base and deep subgrade. Verification with a dynamic cone penetrometer (DCP) may be warranted.

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8) Discuss in detail about Benkelman Beam Rebound Deflection for pavement evaluation.

ANS:Standard test procedure for benkelman beam deflectionmeasurements are given by IRC 81:1997. This test procedure covers the determination of the rebound deflection of apavement under a standard wheel load and tyre pressure, with or withouttemperature measurements. Basic equipment shall consist of:

(a) A Benkelman beam to the Ministry of Works and Development patternhaving the dimensions shown on Figure 1. The beam must be fittedwith a satisfactory locking device

designed to secure the beam whenmoving to a new site and a suitable vibrator mounted at the pivot point.In sunny weather the beam may pass from shade into sunshine as the vehicle moves away. Therefore a shield similar to that described inRoad Research Unit Newsletter No 49 should be used. (b) A truck or trailer with an axle load of 8.20 0.15 tonnes equallydistributed on two dual tyred wheels operating at the inflation pressurenecessary to give a tyre contact area of 0.048

0.0002 m2.* The tyresshall preferably be 10.00 x 20, 12 ply with tubes and rib treads. (c) A tyre pressure gauge graduated in 20 Kpa divisions or smaller. (d) A thermometer with a range of 0-6C in 1C divisions. (e) A mandrel suitable for making a 100mm deep hole in the pavement for inserting the thermometer. The diameter of the hole should be 13mm. (f) A can containing either glycerol or oil for filling the thermometer hole. Deflections shall be measured as follows: (a)The test point shall be preselected and marked. (b) The tyre pressure should be checked before the first test and then at intervals not exceeding three hours. (c) The truck shall initially be positioned with the test wheel between 100 and 150mm to the rear of the test spot, ie position A.

(d) The probe of the beam shall be inserted between the dual tyres. (e) The locking device shall be released and the rear of the beam adjusted so that the plunger is in contact with the dial gauge. (f) The dial gauge shall be set to read between 9 and 11mm (the actual reading need not be recorded) and the vibrator set in operation. (g) The truck shall be moved forward at creep speed so that the test wheel passes over the test spot and continues advancing to position 8 which is 2.7 0.1 metres beyond the test spot.

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(h) The START READING, S, is the maximum dial gauge reading occurring during this movement of the truck from position A to position B, and will normally occur as the wheel

passes over the test spot. This reading shall be recorded. (I) The INTERMEDIATE READING, I, is that figure indicated by the dial gauge at the moment the truck stops with the test wheel in position B. This reading shall be recorded. (j) The truck shall be moved forward until the test wheel is in position V which is not less than 10 metres from position B. (k) The FINAL READING, F, is that figure indicated by the dial gauge when the truck has stopped in position C. This figure shall be recorded.

9) Explain plate load test with a suitable test set up figure.

ANS: The plate load test is a semi-direct method to measure the allowable pressure of soil to induce a given amount of settlement. Plates, round or square, varying in sizes, from 30 to 60 cm and thickness of about 2.5 cm are employed for the test.

The load on the plate is applied by making use of a hydraulic jack. The reaction of the jack load is taken by a cross beam or a steel truss anchored suitably at both the ends. The settlement of the plate is measured by a set of three dial gauges of sensitivity 0.02mm placed at 120 apart. The dial gauges are fixed to independent supports which do not get

disturbed during the test. Fig shows the arrangement for the plate load test.

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Procedure: The method of performing the test is essentially as follows:

Excavate a pit of size not less than 5 times the size of the plate. The bottom of the pit coincides with level of the foundation.

If water table is above the level of foundation, pump out the water carefully and it should be kept just at the level of the foundation.

A suitable size of the plate is selected for the test. Normally a plate of size 30cm is used in sandy soils and bigger size in clay soils. The ground should be leveled and the plate is

seated over the ground.

A seating load of about is first placed and released after sometime. A higher load is next placed on the plate and settlements are recorded by means of the dial gauges.

Observations on every load increment shall be taken until the rate of settlement is less than 0.25mm per hour. Load increments shall be approximately one-fifth of the estimated safe bearing capacity of the soil. The average of the settlements recorded by 2 or 3 dial gauges taken as the settlements of the plate for each of the load increment.

The test should continue until a total settlement of 2.5cm or the settlement at which the soil fails, whichever is earlier, is obtained. After the load is increased, the elastic rebound of the

soil should be recorded.

Limitations

Since a plate load test is of short duration, consolidation settlements cannot be predicted. The test gives the value of immediate settlements only. If the underlying soil is sandy innature immediate settlement can be taken as total settlement. If the soil is of clayey type, the

immediate settlement is only a part of the total settlement. Load tests, therefore do not have much significance in clayey soils to determine allowable pressure on the basis of settlement

criterion.

Plate load test results should be used with caution and the present practice is not to rely too much on this test. If the soil is not homogenous to a great depth, plate load tests give very

misleading results.

Plate load tests is not at all recommended in soils which are not homogenous at least to a depth

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equal to 1.5 to 2 times the width of the prototype foundation.

Plate load tests should not be relied on to determine the ultimate bearing capacity of sandy

soils as the scale effects give misleading results. However, when the tests are carried on clay soils, the ultimate bearing capacity as determined by the test may be taken as equal to that of the foundation since the bearing capacity of clay is essentially independent of the footing size.

The plate load test is possibly the only way of determining the allowable bearing pressures in gravelly soil deposits. For tests on such soil deposits the size of the plate should be bigger to eliminate the effect of grain size.

Plate Load Test setup

10) How wave propagation method can be used for pavement evaluation? Explain with a suitable figure.

ANS: A seismic method for the in-situ measurement of elastic modulus profiles of pavement has been developed at the Center for Transportation Research at the University of Texas at Austin TX. This method is called the Spectral-Analysisof SurfaceWaves (SASW) method. The method is based on generation and detection of elastic stress waves, known as surface waves. There are three different kinds of waves that can be generated. The types of waves are; Surface - or Raleigh waves, shear waves and pressure waves. The key points in SASW testing are generation and measurement of surface

waves, which makes up more than 67% of the response. Surface waves have a unique

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characteristic that, if the wavelength is varied, the velocity of propagation of the surface waves also varies. This phenomenon is called dispersion. The dispersion characteristic of

surface waves can be utilized to determine the layering and modulus profiles of pavement systems quite accurately.

The method is non-destructive. Measurements are made at strains below .001 % where the elastic properties of the materials are independent of the strain amplitude.

The method is such that both the source and the receivers are located on the ground surface, see figure. The source is simply a transient vertical impact, which generates a group

of surface waves of various frequencies that the medium transmits. Two vertical receivers, located on the surface, monitor the propagation of surface wave energy past them. By

analysis of the phase information of the cross power spectrum for each frequency determined between the two receivers, phase velocity, shear wave velocity and finally elastic moduli are determined.

The methodology used was to measure a number of spacing distances between

receivers, starting at 50mm apart up to 600mm apart. The equipment used could only measure from 50 mm upwards. For the very close distances a small ball bearing was used as source and at the greater distances a small hammer was used. The temperature of the asphalt in the HVS section was about 250 Celsius, due to the shade from the HVS machine. The testing was done according to the mid-point concept. At each site a central line was

established as the mid-point. The receivers were placed equidistant from the mid-point at a

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spacing of 50mm increasing in steps of 50mm up to 600mm. Per spacing a total of five impulses were generated.

11) What are the different destructive test methods for pavement evaluation? Discuss atleast 2 of them in detail.

ANS: Destructive Evaluation of Pavement Structural Properties includes the following test methods:

Trenching and Coring

Trenching and coring have been used in forensic and routine evaluation to determine the source of the problematic layer or layers. For example, there was severe rutting on US 281 and the district expressed a need to determine the source of the rutting. Although FWD and

GPR tests were performed, evaluation of the data could not differentiate from which layer(s) the rutting came. Trenching provides a viable option. For example, Figure 4-17 illustrates the pavement section profiles on US 281. Chalk and stringiness were used to differentiate different pavement layers, the rutting was found to be from the surface AC layer.

Trenching Procedure

A trenching procedure has been developed to run tests and collect samples while minimizing disturbance to the base material. The saw cut pattern is shown in Figure 4-19. The size of each trench is 3 ft. 12 ft. Each saw cut is deep enough to penetrate all the way through the AC layer(s). A strip about 6 in. wide is cut and removed by hand from one end of the trench, as shown in Figure 4-20. This creates a slot for the backhoe to reach into and lift the AC block without major disturbance to the base layer as shown in Figure 4-21. A saw cut is also made across the center of the trench so the AC blocks can be removed without breaking.After the asphalt layer is removed and samples are collected, tests can be

run, and samples collected on the base layer.

The backhoe then digs out the base layer and a few inches of subgrade. One of the

transverse trench walls (at base and subgrade levels) is then smoothed using shovels, chisels, and brooms. Once a clean trench wall is achieved, the asphalt layers are

highlighted with chalk and the base/subgrade layers are highlighted with string lines held with small nails. The thickness of each layer is then measured at regular intervals to determine its contribution to rutting.

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Coring Procedure

To extract a core sample, the coring rig is placed at the desired location and the barrel-cooling water is turned on. Note that dry ice can be used to cool the core barrel if samples at in situ moisture content are desired.The barrel is spun at about 500 rpm and gradually lowered through the asphalt layer. If the base is stabilized, the operator may want to cut through it as well to obtain an intact sample.

After the barrel has cut to the desired depth, it is retracted while still spinning. Then the core barrel is stopped and the location of the core is observed. If the core has twisted off

and is now lodged in the barrel, the barrel is struck lightly with a mallet to loosen the core. If the core is still in its original position, thin pieces of metal (or screwdrivers) are used to rock the core back and forth until it breaks free from the base or subgrade. Then the core can be lifted out with bent welding rods or loops of thin wire.Normally, the core diameter is either 4 in. or 6 in.

Shelby Tube

Shelby tube samples have been used to determine the in situ density, moisture content, plasticity index (PI), potential vertical rise (PVR), sulfate content, optimum soil stabilizer, and modulus of the subgrade soil.

The Shelby tube is a sharpened pipe that is pushed into soil by a hydraulic ram on a truck-

mounted boom. An auger can be used to remove the HMAC and base layers and collect Shelby tube samples without introducing water in the process. After the HMAC and base layers are removed, the Shelby tube is positioned on the subgrade and pushed roughly 2 ft. (0.6 m) into the soil, as shown in Figure 4-24. The pipe is then pulled out and placed on a rack. A hydraulic ram then pushes the soil out of the pipe in (hopefully) one cylindrical piece. Aluminium foil and cardboard tubes are used to protect the Shelby tube samples.

The tube is labelled with information including the location, orientation and depth of the sample.

If deeper samples are desired, the Shelby tube can be extended and pushed into the bottom of the same hole. That sample is then labelled as coming from a lower depth at the same

location as the first. In this way, several samples can be collected at any location, then pieced together later for a complete soil profile.

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12) What is an overlay? Discuss the various types of overlays that are commonly used on the pavement surface.

ANS: Strengthening of existing pavements may be done by providing additional thickness of pavement of one or more layers over existing pavement which is called overlay. In partially damaged pavement sections, patch repair works are carried out before constructing the overlay.

The overlay combinations are divided into 4 categories based on the type of existing pavement and overlay as:

a) Flexible overlay over flexible pavement b) Flexible overlay over rigid pavement c) Rigid overlay over flexible pavement d) Rigid overlay over rigid pavement

13) What are geosynthetics? How can they be used in pavement overlays?

ANS: A geosynthetic, according to ASTM (1994) is a planar product manufactured from a polymeric material used with soil, rock, earth, or other geotechnical-related material as an integral part of a civil engineering project, structure, or system. Since ASTM provides this definition, the department has had a number of applications in materials other than the traditional geo- applications.

For the purposes of this manual, we will define a geosynthetic as A manmade material that consists of one or more products used to provide added benefit to the infrastructure.

In this manual, we intend to address only those roadway applications. Applications of geosynthetics for use in other structures are addressed in the Bridge

DivisionsGeotechnical Manual.

There have been relatively few studies that have offered concrete evidence regarding

geosynthetic material performance in the roadway. Although the department has conducted or is a participant in a number of studies, there is plenty of room for improvement in

characterizing all geosynthetic materials to quantify their benefit top pavement performance.

Although improper usage or installation has contributed to early failure in some cases, department geosynthetic usage has increased in recent years with good success. Applications have been in asphalt concrete overlays of existing asphalt concrete and hydraulic (Portland) cement concrete surfaces, unbound (flexible) base, soft subgrade,

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drainage, and encapsulation. Shared terminology exists between these applications; clarification of the terminology will hopefully alleviate the confusion.

This section begins with a description of materials for applications that are most frequently used in the department. Following this, a discussion of the materials frequently used in

each application and the expected behavior of the material is presented.

Description of Materials and Applications

Several materials are available for incorporation into pavement structures. The two geosynthetics primarily used are geotextiles and geogrids. Recently, manufacturers have combined the two materials creating a type of geocomposite (combination of a geosynthetic and another product).

Applications

Applications of geosynthetics are typically used by the department within pavement

structures. Primarily, there are four applications:

Materials

The term geosynthetic is broad and encompasses numerous materials. A geotextile is a permeable geosynthetic made of textile materials; this material has uses in all applications.

Numerous other materials may be used in combination with geotextiles to create a geocompositeincluding grids, nets, meshes, and webs; these are most frequently used in

HMAC but do have applications with soil moisture barriers and even soil erosion blankets.

Geogrids are primarily used for reinforcement and are formed from integrally connected

and attached elements creating apertures in which adjoining material embed and are of sufficient size to interlock with it. The reference to geogrid is primarily reserved for the

application in unbound bases and subgrades; although, reinforcing grids are available for use in HMAC. Geomembranes are low-permeability geosynthetics used as moisture barriers.

Geosynthetics for Surface Layer Reinforcement

Geosynthetics used for HMAC applications have been used by the department since the mid-1980s. Since then, there have been numerous products manufactured for various purposes to incorporate in the pavement surfaces. These products are used to:

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reduce reflective cracking from:

o existing layers of HMAC or

o cracks and joints in rigid pavements and resist moisture intrusion into lower pavement layers. Fabric underseal has worked well

in some parts of the state when placed on CRCP and overlaid with HMAC.

Department research project 0-1777 investigated the use of several geosynthetics inHMAC. Researchers developed a guidance document, Geosynthetic Guidelines, available on TxDOT's Construction and Materials Tips webpage. This document discusses the advantages and disadvantages of using geosynthetics in HMA applications, guidance on the selection of materials, cost considerations, pavement design,

as well as construction considerations.

Geosynthetics for Geotechnical Reinforcement

Several research projects have been conducted in an attempt to quantify the benefits of geosynthetics in unbound pavement layers. Applications range from a passive use of materials creating a separation between pavement layers to an active role when relying on geosynthetics to take on some of the structures load.

Materials most often used in this application are geogrids and geotextiles. Many geosynthetics have multiple uses and can serve more than one function. For instance, geogrids are often used in a way to restrain base material during compaction or loading, but they also serve as a separation layer to prevent excessive migration and intermingling

of pavement layers at interfaces.

Ongoing departmental research will attempt to better describe the interaction between pavement layers and geosynthetics used in pavement layers. Additional research on the contribution of geogrid to pavement performance is being conducted in a pooled fund

study. As the findings are made available, this section will be updated.

Current usage in Texas has been for both restraint of pavement materials and for separation

of materials. The department acknowledges the benefit of geosynthetics in pavement layers; however, there has been insufficient conclusive research to develop guidance with

regard to reinforcement of unbound materials in pavement structures at this time. As

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aresult, usage of geosynthetics is limited to separation and restraint and cannot be accounted for in FPS-19W design.

Separation

Separation of layers is intuitive. This mechanism places a physical barrier between two materials to prevent them from intermixing. Mixing of fine-grained soil particles into the overlying flexible base can create a significantly finer gradation over time, increasing the

base suction properties and greatly decreasing its strength. The result of this placement is to maintain a viable structure for a longer period of time.

Another more recent application, and possibly the most quantifiable, is grid used as both a separator layer and a restraint to base movement. It has been used to prevent or reduce

reflective cracking due to differential relative movement between pavement layers. An example of this use is between subgrades (or stabilized materials) with high plasticity index(PIs) that exhibit large volumetric shrinkage when moisture is drawn out due to weather, drainage, or vegetation demands and unbound flexible base. The grid holds the unbound material in a tight matrix allowing the shrinking subgrade (or stabilized material) to move and prevent subgrade cracking from propagating to the pavement surface.

Restraint

Restraint is the mechanism of preventing or reducing the lateral movement of materials.

This application is useful when soft subgrades are present and a platform for subsequent construction is required. Both grid and geotextiles have been used in this application. Grid is more likely to be used and can be very effective. When textiles are used, note that textiles must extend much more than geogrids to provide the same level of support.

A grid may be used for:

mitigation of reflective cracking (see mechanism in separation)

creation of a working platform on soft subgrades, sacrificial layer to obtain compaction of subsequent layers or

a substitute in lieu of lime, e.g., sulfate laden soils, where lime is detrimental, or urban areas, where lime may not be tolerated or combinations of these and soft soils.

14) Benkelman Beam Deflection studies were carried out on 15 selected points on a stretch of flexible pavement during summer season using a dual wheel load of

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4085 Kg, 5.6Kg/cm2 pressure. The deflection values obtained in mm after making the necessary lag corrections are given below. If the present traffic

consists of 750commercial vehicles/day, determine the thickness of bituminous overlay required, if pavement temperature during the test was 39C and the correction factor for subsequent increase in subgrade moisture content is 1.3. Assume annual rate of growth of traffic as 7.5%. Adopt IRC guidances. 1.40, 1.32, 1.25, 1.35, 1.48, 1.60, 1.65, 1.55, 1.45, 1.40, 1.36, 1.46, 1.50, 1.52, 1.45mm.

ANS: Mean deflection, D = (D / n) = (21.74/ 15) = 1.45mm

Standard deviation = 0.1072 mm

Characteristic deflection, Dc = 1.45+0.1072 = 1.5572mm

Deflection after temperature correction = 1.5572- (39-35)0.0065= 1.531mm

Corrected deflection for subgrade moisture= 1.531 x 1.3 = 1.9903mm

Assuming the number of years after last traffic count before the construction of overlay as n= 2, design traffic A is

A = P(1+r)(n+10)

=750(1+0.075)12

=1787 commercial vehicles/day.

As per IRC Da = 1mm

Overlay thickness of granular material,

Ho = 55o log(Dc/Da)

= 550 log(1.9903/1)

= 164.41mm= 16.5cm

Assuming an equivalent factor of 2 for bituminous concrete overlay, the design thickness of bituminous concrete overlay = 0.5 x 16.5 = 8.25 cm.

15) Existing black top pavement was tested using Benkelman Beam with a test vehicle of 8170 Kg rear axle load. Observations recorded at a pavement temperature of 43C are given below. Length of test stretch = 300m. Compute

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the thickness of overlay of bituminous concrete, taking allowable deflection as 1.25mm, if the factor for subgrade moisture correction is 2.0.

Serial number subsection Rebound deflection (mm)

1 1.46

2 1.52

3 1.56

4 1.76

5 1.96

6 1.74

7 1.68

8 1.74

9 1.96

10 1.42

11 1.56

12 1.62

ANS: Mean deflection, D = (D / n) = (19.98/ 12) = 1.665mm

Standard deviation = 0.1762 mm

Characteristic deflection, Dc = 1.665+0.1762 = 1.8412mm

Deflection after temperature correction = 1.8412- (43-35)0.0065= 1.7892mm

Corrected deflection for subgrade moisture= 1.7892 x2 = 3.5784mm

As per IRC Da = 1.25mm

Overlay thickness of granular material,

Ho = 55o log(Dc/Da)

= 550 log(3.5784/1.25)

= 251.24mm= 25.124cm

Assuming an equivalent factor of 2 for bituminous concrete overlay, the design thickness of bituminous concrete overlay = 0.5 x 25.124 = 12.562 cm.