Lec 17 Highway Engineering - Rigid Pavemen Design

Highway Eng.

Design of Rigid Pavements

14 –15

Structural Design of Rigid Pavements In this lecture; A- General B- Types of Rigid Pavements C- Joints in PCC pavements D- Design of PCC Pavements - AASHTO 1993 Method.

Information listed in this lecture is mainly taken from AASHTO Guide for Design of Pavement Structures (AASHTO, 1993), Traffic and Highway Engineering (Garber, 2009), Asphalt Pavements (Lavin, 2003), Pavement Analysis and Design (Huang, 2004), http://www.pavementinteractive.org (Accessed on 2015) and Highways (O’Flaherty, 2007).

A- General Rigid highway pavements are normally constructed of Portland cement concrete (PPC) and may or may not have a base course between the subgrade and the concrete surface. When a base course is used in rigid pavement construction, it is usually referred to as a subbase course.

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Highway Eng.

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Dr. Firas Asad

Highway Eng.

Design of Rigid Pavements

14 –15

B- Types of Rigid Pavements Rigid highway pavements can be divided into three general types: plain concrete pavements, simply reinforced concrete pavements, and continuously reinforced concrete pavements. The definition of each pavement type is related to the amount of reinforcement used. B-1 Jointed Plain Concrete Pavement (JPCP) Plain concrete pavement has no temperature steel or dowels for load transfer. However, steel tie bars often are used to provide a hinge effect at longitudinal joints and to prevent the opening of these joints. Plain concrete pavements are used mainly on low-volume highways or when cement-stabilized soils are used as subbase. Joints are placed at relatively shorter distances (10 to 20 ft) than with other types of concrete pavements to reduce the amount of cracking. B-2 Simply Reinforced Concrete Pavement Simply reinforced concrete pavements have dowels for the transfer of traffic loads across joints, with these joints spaced at larger distances, ranging from 30 to 100 ft. Temperature steel is used throughout the slab, with the amount dependent on the length of the slab. Tie bars also are used commonly at longitudinal joints. B-3 Continuously Reinforced Concrete Pavement (CRCP) Continuously reinforced concrete pavements have no transverse joints, except construction joints or expansion joints when they are necessary at specific positions, such as at bridges. These pavements have a relatively high percentage of steel, with the minimum usually at 0.6 percent of the cross section of the slab. They also contain tie bars across the longitudinal joints.

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Dr. Firas Asad

Highway Eng.

Design of Rigid Pavements

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C- Joints in Concrete Pavements Different types of joints are placed in concrete pavements to limit the stresses induced by temperature changes and to facilitate proper bonding of two adjacent sections of pavement. These joints can be divided into four basic categories:

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Dr. Firas Asad

Highway Eng.

Design of Rigid Pavements

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D- Structural Design of PCC Pavements The main objective in rigid pavement design is to determine the thickness of the concrete slab that will be adequate to carry the projected traffic load for the design period. D-1 Methods of Design Several design methods have been developed over the years, some of which are based on the results of full-scale road tests, others on theoretical development of stresses on layered systems, and others on the combination of the results of tests and theoretical development. However, two methods are used extensively: the AASHTO and American Concrete Pavement (PCA) methods. In this lecture only the 1993 AASHTO method will be adopted. 1993 AASHTO Empirical Design Method for Rigid Pavement The AASHTO design procedure provides for the determination of the pavement thickness and the amount of steel reinforcement when used, as well as the design of joints. It is suitable for plain concrete, simply reinforced concrete, and continuously reinforced concrete pavements. Design Considerations The factors considered in the AASHTO procedure for the design of rigid pavements as presented in the 1993 guide are • Pavement performance • Subgrade strength • Subbase strength • Traffic • Concrete properties Lecture 17

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Dr. Firas Asad

Highway Eng.

Design of Rigid Pavements

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• Drainage • Reliability Pavement Performance. Pavement performance is considered in the same way as for flexible pavement. The initial serviceability index (Pi) may be taken as 4.5, and the terminal serviceability index may also be selected by the designer. Subbase Strength. Subbase thickness is usually not less than 6 in. and should be extended 1 to 3 ft outside the edge of the pavement structure. Subgrade Strength. The strength of the subgrade is given in terms of the Westergaard modulus of subgrade reaction k, which is defined as the load in lb/in2 on a loaded area, divided by the deformation in inches. Values of k can be obtained by conducting a plate-bearing test in accordance with the AASHTO Test Designation T222 using a 30 in. diameter plate. Estimates of k values can also be made either from experience or by correlating with other tests. Traffic. The treatment of traffic load is similar to that presented for flexible pavements, in that the traffic load application is given in terms of the number of 18,000 lb equivalent single-axle loads (ESALs). ESAL factors depend on the slab thickness and the terminal serviceability index of the pavement. Tables below give ESAL factors for rigid pavements with a terminal serviceability index of 2.5. Since the ESAL factor depends on the thickness of the slab, it is therefore necessary to assume the thickness of the slab at the start of the computation. This assumed value is used to compute the number of accumulated ESALs, which in turn is used to compute the required thickness. If the computed thickness is significantly different from the assumed thickness, the accumulated ESAL should be recomputed. This procedure should be repeated until the assumed and computed thicknesses are approximately the same. Lecture 17

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Dr. Firas Asad

Highway Eng.

Design of Rigid Pavements

14 –15

Reinforcing Steel Steel reinforcing may be used in concrete pavements to reduce the amount of cracking that occurs, as a load transfer mechanism at joints, or as a means of tying two slabs together. Steel reinforcement used to control cracking is usually referred to as temperature steel, whereas steel rods used as load transfer mechanisms are known as dowel bars, and those used to connect two slabs together are known as tie bars. Lecture 17

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Dr. Firas Asad

Highway Eng.

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Design of Rigid Pavements

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Dr. Firas Asad

Highway Eng.

Design of Rigid Pavements

14 –15

Concrete Properties. The concrete property is given in terms of its flexural strength (modulus of rupture) at 28 days. The flexural strength at 28 days of the concrete to be used in construction should be determined by conducting a three-point loading test as specified in AASHTO Designation T97. Drainage. The drainage quality of the pavement is considered by introducing a factor (Cd) into the performance equation. This factor depends on the quality of the drainage - as described in the design of Flexible Pavement- and the percent of time the pavement structure is exposed to moisture levels approaching saturation. The Table below gives AASHTO-recommended values for Cd.

Reliability. Reliability considerations for rigid pavement are similar to those for flexible pavement as presented in the previous lecture. Reliability levels, R%, and the overall standard deviation, So, are incorporated directly in the design charts.

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Dr. Firas Asad

Highway Eng.

Design of Rigid Pavements

14 –15

The design equation above can be solved for the thickness of the pavement (D) in inches by using either a computer program or the two design charts in the next pages. The use of a computer program facilitates the iteration necessary, since D has to be assumed to determine the effective modulus of subgrade reaction and the ESAL factors used in the design.

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Dr. Firas Asad

Highway Eng.

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Design of Rigid Pavements

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Dr. Firas Asad

Highway Eng.

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Design of Rigid Pavements

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Dr. Firas Asad

Highway Eng.

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Design of Rigid Pavements

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Dr. Firas Asad

Highway Eng.

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Design of Rigid Pavements

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Dr. Firas Asad