Reinforcement Properties

Several types of reinforcement have been used in CRCP, but by far the most common is deformed steel bars. Other innovative materials include solid stainless steel, stainless steel clad, and other proprietary materials such as fiber reinforced polymer (FRP) bars. Despite a higher initial cost, these all promise to provide more corrosion resistance than steel bars.(1)Design of CRCP Using Glass Fiber Reinforced Polymer Rebars (FHWA-HRT-05-081)(2)Evaluating the Use of Fiber-Reinforced Polymer Bars in CRCP (FHWA-HIF-09-012)(3)Field Applications of FRP Reinfocement: Case Studies (ACI SP-215) Currently, however, implementation of these materials has been more targeted more for their use as dowel bars.

Deformed steel bars are the most widely accepted type of reinforcement for CRCP. The difference in volumetric changes in the steel and the concrete generates stresses in both materials. Stress transfer from the steel to concrete depends on the steel surface area and the shape of the surface deformations on the reinforcing bar (rebar). It is thus important that the rebar comply with requirements specified in AASHTO M 31, M 42, or M 53 for billet-steel, rail-steel, or axle-steel deformed bars respectively.

Alternatively, ASTM A 615 for billet steel, and ASTM A 996 for rail- and axle-steel deformed bars, may be used. Bar designations as well as requirements for deformations and steel tensile strength or steel grade are provided in both the AASHTO and ASTM specifications. The table below shows weight and dimensions of ASTM standard reinforcing steel bars.

Weight and dimensions of ASTM standard reinforcing steel bars

ASTM standard grades for reinforcing steel barsThe required yield strength of reinforcing steel for use in CRCP is typically 60,000 psi (420 MPa), designated as English Grade 60 (metric Grade 420). Other reinforcing steel grades are presented in the table to the right. Higher steel grades have been used in some European countries and in some States.(4)Continuously Reinforced Concrete Pavements (ISBN: 2-84060-006-4)(5)Analysis of Field Monitoring Data of CRCP Constructed with Grade 70 Steel (TX-99/4925-1) It is important to note that although higher steel grades may suggest the use of less steel to maintain cracks tight, this may not necessarily be true as long as the elastic modulus of the steel remains unchanged.

The use of higher quantities of carbon in steel production typically increases its strength, but often with no significant change in its elastic properties (modulus) which control crack widths. The elastic modulus of steel reinforcing bars is typically in the order of 29,000,000 psi (200,000 MPa).

Another property of interest for CRCP reinforcement design is the coefficient of thermal expansion of the steel. Depending on the difference in the steel and concrete CTE, varying restraint will result, leading to different crack patterns. The AASHTO-86/93 Guide recommends the use of a steel CTE of 5 x 10-6 in./in./ºF (9 x 10-6 m/m/ºC) in design. However, steel CTE values provided by the AASHTOWare Pavement ME (MEPDG) range from 6.1 to 6.7 x 10-6 in./in./ºF (11 to 12 x 10-6 m/m/ºC).

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