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).
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.
A typical rigid pavement structure (see Figure 2) 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.
The surface course is the layer in contact with traffic loads and is made of PCC. It provides characteristics such as friction (see Figure 3), 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 4 shows a 300 mm (12 inch) surface 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:
- 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.
- Stabilized aggregate or soil (see Figure 5). 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.
- Dense-graded HMA. In situations where high base stiffness is desired base courses can be constructed using a dense-graded HMA layer.
- 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.
- Lean concrete (see Figure 6). 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.
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:
- Minimize the intrusion of fines from the subgrade into the pavement structure.
- Improve drainage.
- Minimize frost action damage.
- 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.
Almost all rigid pavement is made with Portland Cement Concrete (PCC). Rigid pavements are differentiated into three major categories by their means of crack control:
- This is the most common type of rigid pavement. JPCP controls cracks by dividing the pavement up into individual slabs separated by contraction joints. Slabs are typically one lane wide and between 3.7 m (12 ft.) and 6.1 m (20 ft.) long. JPCP does not use any reinforcing steel but does use dowel bars and tie bars.
- As with JPCP, JRCP controls cracks by dividing the pavement up into individual slabs separated by contraction joints. However, these slabs are much longer (as long as 15 m (50 ft.)) than JPCP slabs, so JRCP uses reinforcing steel within each slab to control within-slab cracking. This pavement type is no longer constructed in the U.S. due to some long-term performance problems.
- This type of rigid pavement uses reinforcing steel rather than contraction joints for crack control. Cracks typically appear ever 1.1 – 2.4 m (3.5 – 8 ft.) are held tightly together by the underlying reinforcing steel. FHWA Tech Brief.