5  Rigid Pavement Basics

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 2.19).    

Figure 2.19: Rigid Pavement Load Distribution

This section describes the typical rigid pavement structure consisting of:

• Surface course.  This is the top layer, which consists of the PCC slab.  
• Base course.  This is the layer directly below the PCC layer and generally consists of aggregate or stabilized subgrade.
• Subbase course.  This is the layer (or layers) under the base layer.  A subbase is not always needed and therefore may often be omitted.

5.1  Basic Structural Elements

A typical rigid pavement structure (see Figure 2.20) consists of the surface course and the underlying base and subbase courses (if used).  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. 

Figure 2.20: Basic Rigid Pavement Structure

5.1.1  Surface Course

The surface course is the layer in contact with traffic loads and is made of PCC.  It provides characteristics such as friction (see Figure 2.21), smoothness, noise control and drainage.  In addition, it serves as a waterproofing layer to the underlying base, subbase and subgrade.  The surface course can vary in thickness but is usually between 150 mm (6 inches) (for light loading) and 300 mm (12 inches) (for heavy loads and high traffic).  Figure 2.22 shows a 300 mm (12 inch) surface course.

Figure 2.21: PCC Surface	Figure 2.22: Rigid Pavement Slab (Surface Course) Thickness

5.1.2 Base Course

The base course is immediately beneath the surface course.  It provides (1) additional load distribution, (2) contributes to drainage and frost resistance, (3) uniform support to the pavement and (4) a stable platform for construction equipment (ACPA, 2001).  Bases also help prevent subgrade soil movement due to slab pumping.  Base courses are usually constructed out of:

1. Aggregate base.  A simple base course of crushed aggregate has been a common option since the early 1900s and is still appropriate in many situations today.
2. Stabilized aggregate or soil (see Figure 2.23).  Stabilizing agents are used to bind otherwise loose particles to one another, providing strength and cohesion.  Cement treated bases (CTBs) can be built to as much as 20 - 25 percent of the surface course strength (FHWA, 1999).  However, cement treated bases (CTBs) used in the 1950s and early 1960s had a tendency to lose excessive amounts of material leading to panel cracking and settling. 
3. Dense-graded HMA.  In situations where high base stiffness is desired base courses can be constructed using a dense-graded HMA layer.
4. Permeable HMA.  In certain situations where high base stiffness and excellent drainage is desired, base courses can be constructed using an open graded HMA.  Recent research may indicate some significant problems with ATPB use.
5. Lean concrete (see Figure 2.24).  Contains less portland cement paste than a typical PCC and is stronger than a stabilized aggregate.  Lean concrete bases (LCBs) can be built to as much as 25 - 50 percent of the surface course strength (FHWA, 1999).  A lean concrete base functions much like a regular PCC surface course and therefore, it requires construction joints and will crack over time.  These joints and cracks can potentially cause reflection cracking in the surface course if they are not carefully matched.

Figure 2.23: Completed CTB with Curing Seal	Figure 2.24: Lean Concrete Base Material

5.1.3  Subbase Course

The subbase course is the portion of the pavement structure between the base course and the subgrade.  It functions primarily as structural support but it can also:

1. Minimize the intrusion of fines from the subgrade into the pavement structure.
2. Improve drainage.
3. Minimize frost action damage.
4. Provide a working platform for construction. 

The subbase generally consists of lower quality materials than the base course but better than the subgrade soils.  Appropriate materials are aggregate and high quality structural fill.  A subbase course is not always needed or used.   

5.2  Joints

Joints are purposefully placed discontinuities in a rigid pavement surface course.  The most common types of pavement joints, defined by their function, are (AASHTO, 1993): contraction, expansion, isolation and construction.

5.2.1  Contraction Joints

A contraction joint is a sawed, formed, or tooled groove in a concrete slab that creates a weakened vertical plane.  It regulates the location of the cracking caused by dimensional changes in the slab.  Unregulated cracks can grow and result in an unacceptably rough surface as well as water infiltration into the base, subbase and subgrade, which can enable other types of pavement distress.  Contraction joints are the most common type of joint in concrete pavements, thus the generic term "joint" generally refers to a contraction joint.

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 2.25).  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 2.27).  Skewed joints are cut at obtuse angles to the direction of traffic flow to help with load transfer.  If the joint is properly skewed, the left wheel of each axle will cross onto the leave slab first and only one wheel will cross the joint at a time, which results in lower load transfer stresses (see Figure 2.28).

Figure 2.25: Rigid Pavement Showing Contraction Joints	Figure 2.26: Missing Contraction Joint (The middle lane contraction joint was not sawed resulting in a transverse slab crack.  The outer lanes have proper contraction joints and therefore, no cracking)

Figure 2.27: Skewed Contraction Joint
(The Tining is Perpendicular to the Direction of Travel While the Contraction Joint is Skewed)

5.2.2  Expansion Joints

An expansion joint is placed at a specific location to allow the pavement to expand without damaging adjacent structures or the pavement itself.  Up until the 1950s, it was common practice in the U.S. to use plain, jointed slabs with both contraction and expansion joints (Sutherland, 1956).  However, expansion joint are not typically used today because their progressive closure tends to cause contraction joints to progressively open (Sutherland, 1956).   Progressive or even large seasonal contraction joint openings cause a loss of load transfer — particularly so for joints without dowel bars.

5.2.3  Isolation Joints

An isolation joint (see Figure 2.29) is used to lessen compressive stresses that develop at T- and unsymmetrical intersections, ramps, bridges, building foundations, drainage inlets, manholes, and anywhere differential movement between the pavement and a structure (or another existing pavement) may take place (ACPA, 2001).  They are typically filled with a joint filler material to prevent water and dirt infiltration.

Figure 2.29: Roofing Paper Used for an Isolation Joint

5.2.4  Construction Joints

A construction joint (see Figure 2.30) is a joint between slabs that results when concrete is placed at different times.  This type of joint can be further broken down into transverse and longitudinal construction joints (see Figure 2.31).  Longitudinal construction joints also allow slab warping without appreciable separation or cracking of the slabs.

	Figure 2.30: Construction Joint Workers manually insert dowel bars into the construction joint at the end of the work day. Construction joints should be planned so that they coincide with contraction joint spacing to eliminate extra joints.
	Figure 2.31: Longitudinal and Transverse Construction Joints

5.3  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:

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 2.32 illustrates the extremes in load transfer efficiency.  Most performance problems with concrete pavement are a result of poorly performing joints (ACPA, 2001).  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.

Figure 2.32:  Load Transfer Efficiency Across a PCC Surface Course Joint

5.3.1  Aggregate Interlock

Aggregate interlock is the mechanical locking which forms between the fractured surfaces along the crack below the joint saw cut (see Figure 2.33) (ACPA, 2001).  Some low-volume and secondary road systems rely entirely on aggregate interlock to provide load transfer although it is generally not adequate to provide long-term load transfer for high traffic (and especially truck) volumes.  Generally, aggregate interlock is ineffective in cracks wider than about 0.9 mm (0.035 inches) (FHWA, 1990).  Often, dowel bars are used to provide the majority of load transfer. 

Figure 2.33: Aggregate Interlock

5.3.2  Dowel Bars

Dowel bars are short steel bars that provide a mechanical connection between slabs without restricting horizontal joint movement.  They increase load transfer efficiency by allowing the leave slab to assume some of the load before the load is actually over it.  This reduces joint deflection and stress in the approach and leave slabs.

Dowel bars are typically 32 to 38 mm (1.25 to 1.5 inches) in diameter, 460 mm (18 inches) long and spaced 305 mm (12 inches) apart.  Specific locations and numbers vary by state, however a typical arrangement might look like Figure 2.34.  In order to prevent corrosion, dowel bars are either coated with stainless steel (see Figure 2.35) or epoxy (see Figure 2.36).  Dowel bars are usually inserted at mid-slab depth and coated with a bond-breaking substance to prevent bonding to the PCC.  Thus, the dowels help transfer load but allow adjacent slabs to expand and contract independent of one another.  Figure 2.36 shows typical dowel bar locations at a transverse construction joint.

Figure 2.35: Stainless Steel-Clad Dowel Bars (Epoxy Coating on Ends Only)	Figure 2.36: Dowel Bars in Place at a Construction Joint- the Green Color is from the Epoxy Coating

5.3.3  Reinforcing Steel

Reinforcing steel can also be used to provide load transfer.  When reinforcing steel is used, transverse contraction joints are often omitted (as in CRCP).  Therefore, since there are no joints, the PCC cracks on its own and the reinforcing steel provides load transfer across these cracks.  Unlike dowel bars, reinforcing steel is bonded to the PCC on either side of the crack in order to hold the crack tightly together.

Typically, rigid pavement reinforcing steel consists of grade 60 (yield stress of 60 ksi (414 MPa) No. 5 or No. 6 bars (ERES, 2001).  The steel constitutes about 0.6 - 0.7 percent of the pavement cross-sectional area (ACPA, 2001) and is typically placed at slab mid-depth or shallower.  At least 63 mm (2.5 inches) of PCC cover should be maintained over the reinforcing steel to minimize the potential for steel corrosion by chlorides found in deicing agents (Burke, 1983).

5.4  Tie Bars

Tie bars are either deformed steel bars or connectors used to hold the faces of abutting slabs in contact (AASHTO, 1993).  Although they may provide some minimal amount of load transfer, they are not designed to act as load transfer devices and should not be used as such (AASHTO, 1993).  Tie bars are typically used at longitudinal joints (see Figure 2.37) or between an edge joint and a curb or shoulder.  Typically, tie bars are about 12.5 mm (0.5 inches) in diameter and between 0.6 and 1.0 m (24 and 40 inches long).

Figure 2.37: Tie Bars Along a Longitudinal Joint