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Pavement Analysis & Design

BACKGROUND

Rigid Pavement

Rigid pavements are so named because the pavement structure deflects very little under loading due to the high modulus of elasticity of their surface course.  A rigid pavement structure is typically composed of a PCC surface course built on top of either (1) the subgrade or (2) an underlying base course.  Because of its relative rigidity, the pavement structure distributes loads over a wide area with only one, or at most two, structural layers (see Figure 1-1).    

Fig 1-1: Rigid Pavement Load Distribution

A typical rigid pavement structure (see Figure 1-2) consists of the surface course and the underlying base/subbase courses.  The surface course (made of PCC) is the stiffest (as measured by resilient modulus) and provides the majority of strength.  The underlying layers are orders of magnitude less stiff but still make important contributions to pavement strength as well as drainage and frost protection. 

Fig.1-2 Typical cross section of rigid pavement

Design of joints

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Pavement Analysis & Design

The performance of concrete pavements depends to a large extent upon the satisfactory performance of the joints. Most jointed concrete pavement failures can be attributed to failures at the joint, as opposed to inadequate structural capacity. Distresses that may result from joint failure include faulting, pumping, spalling, corner breaks, blowups, and mid-panel cracking. Characteristics that contribute to satisfactory joint performance, such as adequate load transfer and proper concrete consolidation have been identified through research and field experience. The incorporation of these characteristics into the design, construction, and maintenance of concrete pavements should result in joints capable of performing satisfactorily over the life of the pavement. Regardless of the joint sealant material used, periodic resealing will be required to ensure satisfactory joint performance throughout the life of the pavement. Satisfactory joint performance also depends on appropriate pavement design standards, quality construction materials, and good construction and maintenance procedures. The most common types of pavement joints, which are defined by their function, are as follows:

1. Expansion joints2. Contraction joints3. Warping joints4. Construction joints5. Longitudinal joints

Need of joints

Concrete pavements are subjected to volumetric changes produced by temperature variations, shrinkage during setting and changes in moisture content. If a long slab is built, it is bound to cracks can only be built if it is divided into small slabs by interposing joints.These joints will then ensure that the stresses developed due to expansion, contraction and warping of slab are within reasonable limits. It can be seen that the coefficient C in determining warping stress is governed by the slab length, the longer length between joints , the greater is the warping stress and greater is needed for reinforcing steel.

Requirements of joints

The general requirements of all type of joints are as under:

The joints must permit movement of the stabs without restraint The joints should not unduly weaken the slab structurally and the load should be

transferred from one slab to another effectively The joints must be sealed to exclude water, grit and other external matter The riding quality of pavement should not be impaired The construction of joints must interfere as little at possible with laying of the concrete

Design of joints

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Pavement Analysis & Design

1. Expansion joints

Expansion joints as the name itself signifies are intended to provide space in the pavement for expansion of the slab. Expansion take places when temperature of the slab rises above the value when it its laid. It is normally transverse joint. Expansion joints also relieve stresses caused by contraction and warping.

Load Transfer

Load transfer is a term used to describe the transfer (or distribution) load across discontinuities such as joints or cracks (AASHTO, 1993).  When a wheel load is applied at a joint or crack, both the loaded slab and adjacent unloaded slab deflect.  The amount the unloaded slab deflects is directly related to joint performance.  If a joint is performing perfectly, both the loaded and unloaded slabs deflect equally.  Load transfer efficiency is defined by the following equation

where:a

=approach slab deflection

l

=leave slab deflection

This efficiency depends on several factors, including temperature (which affects joint opening), joint spacing, number and magnitude of load applications, foundation support, aggregate particle angularity, and the presence of mechanical load transfer devices.  Figure 1.3&4 illustrates the extremes in load transfer efficiency.  Most performance problems with concrete pavement are a result of poorly performing joints.  Poor load transfer creates high slab stresses, which contribute heavily to distresses such as faulting, pumping and corner breaks.  Thus, adequate load transfer is vital to rigid pavement performance.  Load transfer across transverse joints/cracks is generally accomplished using one of three basic methods: aggregate interlock, dowel bars, and reinforcing steel.

Fig 1-3

Design of joints

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Pavement Analysis & Design

Figure 1-4:  Load Transfer Efficiency across a PCC Surface Course Joint

A suitable design for an expansion joint is shown in the fig.1-5. Its features are:

1) A space for expansion joint which is generally 20mm2) A joint-filling compressible material interposed in the above space.3) A joint sealing arrangement4) A dowel bar for load transfer5) Thin coating of bitumen in the expanding portion of the dowel bar to break bond with

concrete and permit expansion6) A card board or metal cap at the expanding end of the dowel bar filled with cotton waste

Fig.1-5 expansion joint

Spacing of Expansion joints

Design of joints

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Pavement Analysis & Design

The width or gap in expansion joint depends upon the length of slab. Greater the distance between the expansion joints, the greater is he width required of a gap for expansion. The use of wide expansion joint space should be avoided as it would be difficult to keep them properly filled in when the gap widens during winter season. The dowel would develop high bending and bearing stresses with wider openings. It is recommended not to have a gap more than 2.5cm in any case. The IRC has recommended that the maximum spacing between expansion joints should not exceed 140m for rough interface layer.

It (δ’) is the maximum expansion in a slab of length Le with temperature rise from T1 to T2,

Then:

δ’= LeC(T2-T1) where C is the thermal expansion of concrete per degree rise in temperature.

The joint filler may be assumed to be compressed up to 50 percent of its thickness and therefore, the expansion joint gap should be twice the allowable expansion in concrete i.e.2 δ’. From the relation given above if δ’ is the halt the joint width, then the spacing of expansion joint Le is given by the equation:

Slab contraction and friction resistance

2. Contraction joints

Design of joints

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Pavement Analysis & Design

When the temperature of concrete falls below the laying temperature of the slab concrete If a long length of slab is laid the contraction induces tensile stresses and the slab cracks. If the joints are provided at suitable intervals transversely the appearance of cracks take places other than the joints can be eliminated. Contraction joints also relieve warping stresses to some extent. A groove joints also called a dummy joint is a popular form of contraction joint and is illustrated in the fig.1-6

Fig.1-6 Dummy contraction joint

The primary purpose of transverse contraction joints is to control the cracking that results from the tensile and bending stresses in concrete slabs caused by the cement hydration process, traffic loadings, and the environment. Because these joints are so numerous, their performance significantly impacts pavement performance. A distressed joint typically exhibits faulting and/or spalling. Poor joint performance frequently leads to further distresses such as corner breaks, blowups, and mid-panel cracks. Such cracks may themselves begin to function as joints and develop similar distresses. Contraction joints are chiefly defined by their spacing and their method of load transfer.  They are generally between 1/4 - 1/3 the depth of the slab and typically spaced every 3.1 - 15 m (12 - 50 ft.) with thinner slabs having shorter spacing (see Figure 1-7).  Some states use a semi-random joint spacing pattern to minimize their resonant effect on vehicles.  These patterns typically use a repeating sequence of joint spacing (for example: 2.7 m (9 ft.) then 3.0 m (10 ft.) then 4.3 m (14 ft.) then 4.0 m (13 ft.)).  Transverse contraction joints can be cut at right angles to the direction of traffic flow or at an angle (called a "skewed joint", see Figure 1-8). 

Fig.1-7 Skewed contraction joint

Design of joints

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Pavement Analysis & Design

Fig.1-8 Rigid Pavement Showing Contraction Joints

The contraction joint spacing design is governed by the anticipated frictional resistance and allowable tensile stress in concrete during the initial curing period or the amount of reinforcement if any.

Features of Contraction joints

1) A surface groove formed by driving a flat metal plate when the concrete is green. It is not less than 6mm wide and has a depth equal to one-fourth the depth of the pavement.

2) A sealing compound to prevent ingress of external material3) A dowel bar arrangement to adequately transfer the load across the joint. This is

dispensed with if it is considered that the aggregate interlock is able to transfer the load

Spacing of contraction joints

The slab contracts due to the fall in slab temperature below the construction temperature. Also the initial curing period, shrinkage occurs in cement concrete.

This movement is resisted by the subgrade drag or friction between the bottom fiber of the slab and the subgrade. Fig 7,26

If Lc is the length in meter the maximum stress happens at half the length.

Total friction resistance upto distance Lc/2 = W*b*(Lc/2)*(h/100)*f

Allowable tension in cement concrete = Sc*h*b*100

Equating the above two values:

Design of joints

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Pavement Analysis & Design

Where;

Lc = slab length or spacing between contraction joints, m

h = slab thickness, cm

f = coefficient of friction (maximum value is about 1.5)

W = unit weight of cement concrete, kg/m3 (2400kg/m3)

Sc = allowable stress in tension in cement concrete, kg/cm2 ( 0.8kg/cm2)

Since the contraction or shrinkage cracks develop mainly during initial period of curing a very low value of Sc is considered in the design. The permissible stress is generally kept as low as about 0.8kg/cm2.

Spacing of contraction joint when reinforcement is provided

If it is assumed that the reinforcement takes the entire tensile force in the slab, caused by the frictional resistance of subgrade and hair cracks are allowed then:

Where

As = total area of steel, cm2 across the slab width

Lc = spacing between contraction joints, m

b = slab width, m

h = slab thickness, cm

W = unit weight of cement concrete, kg/m3 (2400)

f = coefficient of friction (1.5 maximum)

Ss = allowable tensile stress in steel, kg/cm2 (1400)

Design of joints

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Pavement Analysis & Design

Table 1-1 contraction joint spacing (based on irc: 15-2002)

Slab thickness, cm Maximum contraction joint spacing, mUnreinforced slabs

15 4.520 4.525 4.530 5.035 5.0

3. Warping joints

Warping joints also known as hinge joints are joints which are intended to relieve warping stresses. They permit hinge action but no appreciable separation of adjacent slabs. Warping joints can be longitudinal or transverse. A major difference between the warping joint and the expansion or contraction joints is that the former appreciable changes in the joint width are prevented. This is achieved by continuation of reinforcing steel through the joints or by the installation of the bars across the joint. A tongue and groove longitudinal warping joint is illustrated in fig.1-9

Fig.1-9 Tongue and groove longitudinal warping joint

Design of joints

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Pavement Analysis & Design

4. Construction joints

A construction joint becomes necessary when work has to be stopped at a point where there would be otherwise no other joint. It is advisable to plan a day`s work such that the work stops at a contraction or expansion joint. Such joints should be regular in shape, by placing a cross-form in position. The reinforcement should be continued across the joint. A groove in the joint with a sealing compound will arrest the entry of foreign matter and is desirable.

Fig.1-10 Construction joint

5. Longitudinal joints

When pavement is more than say 4.5m it is necessary to provide a longitudinal joint and construct the pavement in strips. These joints allow for warping and uneven settlement of the subgrade.The very purpose of the longitudinal joints being to reduce warping stresses and uneven settlements, it is very necessary to provide for some form of load transferring device. Load transferring is done by tie-bars (12.5mm to 25mm dia.) at 60cm centers and of a length 1m.Tie bars are fully bonded. The joint is of a butt-type fig.22.9 alternatively a tongue and groove joint may be provided fig.22.8 with suitable tie rods 12.5mm dia. 1m long and at 60-75cm centers. And the tie-bars are fully bonded.

Design of joints

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Pavement Analysis & Design

Fig.1-11 longitudinal joint

Fig.1-12 longitudinal and transverse construction joint

Design of joints

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Pavement Analysis & Design

Joint Shape and Sealant Properties

The purpose of a joint sealant is to deter the entry of water and incompressible material into the joint and the pavement structure. It is recognized that it is not possible to construct and maintain a watertight joint. However, the sealant should be capable of minimizing the amount of water that enters the pavement structure, thus reducing moisture-related distresses such as pumping and faulting. Incompressible should be kept out of the joint. These incompressible prevent the joint from closing normally during slab expansion and lead to spalling and blowups.

Sealant behavior has a significant influence on joint performance. High-type sealant materials, such as silicone and preformed compression seals, are recommended for sealing all contraction, longitudinal, and construction joints. While these materials are more expensive, they provide a better seal and a longer service life. Careful attention should be given to the manufacturer's recommended installation procedures. Joint preparation and sealant installation are very important to the successful performance of the joint. It is therefore strongly recommended that particular attention be given to both the construction of the joint and installation of the sealant material.

When using silicone sealants, a minimum shape factor (ratio of sealant depth to width) of 1:2 is recommended. The maximum shape factor should not exceed 1:1. For best results, the minimum width of the sealant should be 3/8 inch. The surface of the sealant should be recessed 1/4 to 3/8 inch below the pavement surface to prevent abrasion caused by traffic. The use of a backer rod is necessary to provide the proper shape factor and to prevent the sealant from bonding to the bottom of the joint reservoir. This backer rod should be a closed-cell polyurethane foam rod having a diameter approximately 25 percent greater than the width of the joint to ensure a tight fit.

When using preformed compression seals, the joint should be designed so that the seal will be in 20 to 50 percent compression at all times. The surface of the seal should be recessed 1/8 to 3/8 inch to protect it from traffic.

Design of joints

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Pavement Analysis & Design

Design of joints