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CHAPTER 3 PAVEMENT DESIGN AND
Aric A. Morse, P.E. Pavement Design Engineer
Ohio Department of Transportation Columbus, Ohio
Roger L. Green, P.E. Pavement Research Engineer
Ohio Department of Transportation Columbus, Ohio
The movement of people and goods throughout the world is primarily dependent upon a transportation network consisting of roadways. Most, if not all, business economies, personal economies, and public economies are the result of this transportation system. Considering the high initial and annual costs of roadways, and since each roadway serves many users, the only prudent owner of roadways is the public sector. Thus it is the discipline of civil engineering that manages the vast network of roadways.
The surface of these roadways, the pavement, must have sufficient smoothness to allow a reasonable speed of travel, as well as ensure the safety of people and cargo. Additionally, once the pavement is in service, the economies that depend upon it will be financially burdened if the pavement is taken out of service for repair or maintenance. Thus, pavements should be designed to be long lasting with few maintenance needs.
The accomplishment of a successful pavement design depends upon several variables. The practice of pavement design is based on both engineering principles and experience. Pavements were built long before computers, calculators, and even slide rules. Prior to more modern times, pavements were designed by trial-and-error and common sense methods, rather than the more complicated methods being used currently. Even more modern methods require a certain amount of experience and common sense. The most widely used methods today are based on experiments with full-scale, in-service pavements that were built and monitored to failure. Empirical information derived from these road tests is the most common basis for current pavement design methods. More recently, with the ever-expanding power of personal computers, more mathematically based pavement design methods such as finite element analysis and refined elastic layer theory have been introduced. These methods require extensive training to use and are not developed for the inexperienced.
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Types of pavements can be broadly categorized as rigid, flexible, or composite. The characteristics of these types are reviewed in the following articles.
3.1 RIGID PAVEMENT
Rigid pavement can be constructed with contraction joints, expansion joints, dowelled joints, no joints, temperature steel, continuous reinforcing steel, or no steel. Most generally, the construction requirements concerning these options are carefully chosen by the owner or the public entity that will be responsible for future maintenance of the pave- ment. The types of joints and the amount of steel used are chosen in concert as a strategy to control cracking in the concrete pavement. Often, the owner specifies the construction requirements but requires the designer to take care of other details such as intersection jointing details and the like. It is imperative that a designer understand all of these design options and the role each of these plays in concrete pavement performance.
The category of rigid pavements can be further broken down into those with joints and those without. Jointed reinforced concrete pavement (JRCP) and jointed plain con- crete pavement (JPCP) are the two basic types of jointed concrete pavement. Continuously reinforced concrete pavement (CRCP) has no joints. JRCP is designed for maximum joint spacing permitting cracking between joints and requires temperature steel. JPCP is designed for no cracking between joints; thus, joint spacing is mini- mized and temperature steel is eliminated. Historically, many jointed pavements were constructed without dowelled joints. Past performance of undowelled jointed pavements— with the exception of warm, dry climates or low-volume roadways—has been poor. Where there are more than a few trucks per day, dowels should be considered at con- traction joints. However, low-volume roadways that do not carry significant trucks, such as residential streets, may perform satisfactorily without dowelled joints.
3.1.1 Jointed Rigid Pavement
Jointed rigid pavements tend to crack at 13 to 25 ft (4 to 8 m) lengths because of (1) initial shrinkage after placement as excess water evaporates, (2) temperature-induced expansion and contraction resisted by friction with the subgrade, (3) curling and warping caused by temperature and moisture differences between the top and bottom of the slab, and (4) load- induced stresses.
As slabs contract as a result of seasonal temperature changes, cracks form and widen, or formed joints widen, allowing incompressible materials into the cracks or joints. Subsequently, expansion is hindered and pressure is built up in the pavement. This pressure can result in pressure spalling or even blowups. To control this, partial depth saw cuts are made at regular intervals which induce concrete to crack at these locations. The timing and depth of these saw cuts are critical to ensure that the pavement cracks at the controlled location. Saw cuts should be made as soon as the pave- ment can support the weight of the saw and operator. The saw cuts should be made at a depth of one-third of the slab thickness for longitudinal joints, and one-fourth of the slab thickness for transverse joints. These saw cuts are then sealed with some type of joint sealer to prevent intrusion of incompressibles. If the saw cut interval (joint spacing) is short enough, intermediate cracks are eliminated. If longer intervals are used, interme- diate cracks will form.
Load transfer is the critical element at joints and cracks. In undowelled, unrein- forced pavements, any load transfer must be provided by aggregate interlock.
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PAVEMENT DESIGN AND REHABILITATION 225
FIGURE 3.1 Typical contraction joint in rigid pavement with dowel for positive load transfer. Conversion: 1 in � 25.4 mm.
Top of pavement
Joint seal reservoir
Dowel1/4'' ± 1/16'' T/3'' min.
Reinforced concrete or plain concrete
FIGURE 3.2 Typical contraction joint in rigid pavement without dowel for load transfer. Conversion: 1 in � 25.4 mm.
Top of pavement CL
Joint seal reservoir
1/4'' ± 1/16'' T/3 min.
Reinforced concrete or plain concrete pavement
Aggregate interlock is lost when slabs contract and the joints or cracks open up. Also, interlock is slowly destroyed by the movement of the concrete as traffic passes over. Given large temperature variations and heavy trucks, aggregate interlock is ineffectual, and faulting is the primary result.
To provide load transfer at the joints, dowels are used which allow for expansion and contraction. Figure 3.1 illustrates a typical doweled joint with saw cut and joint seal. Figure 3.2 shows a similar joint without the dowel to provide load transfer.
Where a long joint spacing is used and intermediate cracks are expected, steel rein- forcement is added to hold the cracks tightly closed (JRCP). This allows the load transfer to be accomplished through aggregate interlock without the associated problems described above. Contraction joints do not provide for expansion of the pavement unless the same amount of contraction has already taken place. This contraction will initially be from shrinkage due to concrete curing. Later changes in the pavement length are due to temperature changes.
Where fixed objects such as structures are placed in the pavement, the use of an expansion joint is warranted. Expansion joints should be used sparingly. The pave- ment will be allowed to creep toward the expansion joint, thus opening the adjacent contraction joints. This can cause movement in the adjacent contraction joints in excess of their design capabilities and result in premature failures. Figure 3.3 shows a detail for a typical expansion joint.
The design of reinforcing steel is a function of seasonal temperature change, subbase friction, and the weight of the slab. Inadequate reinforcing will not be able to hold the
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FIGURE 3.3 Typical expansion joint in rigid pavement with ASTM D6690 type II joint seal. Conversion: 1 in � 25.4 mm.
Top of concrete pavement or base
Preformed expansion joint filler
cracks together, and faulting will result. The amount of reinforcing needed to hold cracks together is traditionally calculated using a relationship based on the friction between the subgrade and the bottom of the slab. This relationship assumes that for a crack to open enough to fail the aggregate interlock, the slab will have to slide along the subbase. The current AASHTO recommendation is based on this traditional approach (Guide for Design of Pavement Structures, American Association of State Highway and Transportation Officials, 1993).
The relationship can be expressed as follows:
As � (3.1)
where As � area of steel required, in 2/ft of width (mm2/mm)
W � weight of slab, lb/ft2 (MPa) F � coefficient of resistance between slab and subgrade (1.5 unless otherwise
known) L � length of slab, ft (mm) fs � allowable stress in steel, lb/in
Although these relationships are accepted by most leading author