![]() |
![]() ![]() |
U. S. Department of Transportation
Federal Highway Administration
TECHNICAL ADVISORY
T 5080. 14
June 5, 1990
Par.
(1) Longitudinal Steel
(a) A minimum of 0. 6 percent (based on the pavement cross sectional area) is recommended to aid transverse crack development in the range of 8 feet, maximum, and 3. 5 feet, minimum, between cracks. Exceptions should be made only where experience has shown that a lower percentage of steel has performed satisfactorily. In areas where periods of extreme low temperature (average minimum monthly temperatures of 10° F or less) occur, the use of a minimum of 0. 7 percent steel is recommended.
(b) Deformed steel bars that meet the requirements set out in AASHTO Specifications, Part I,AASHTO M31, M42, or M53 are recommended. The tensile requirements should conform to the American Society for Testing and Materials (ASTM) Grade 60. Recommended spacing of the longitudinal steel is not less than 4 inches or 2 1/2 times the maximum sized aggregate, whichever is greater, and not greater than 9 inches. A minimum ratio of 0. 03 square inches of steel bond area per cubic inch of concrete is recommended. See Attachment 1 for an example problem for determining the minimum longitudinal steel spacing and the minimum bond ratio. Table 1 shows the minimum and maximum bar sizes for given pavement thicknesses and reinforcement percentages. These bar sizes meet the minimum bond ratio and the minimum bar spacing criteria stated above.
(c) The recommended position of the longitudinal steel is between 1/3 and 1/2 of the depth of the pavement as measured from the surface. The minimum concrete cover should be 2-1/2 inches with 3 inches preferable. For pavements thicker than 11 inches, several States have begun to experiment with the use of two layers of longitudinal steel. Pavements constructed with two layers of steel have not been in service long enough to evaluate performance; therefore, this technique should be considered experimental.
Minimum and Maximum Bar Size | ||||||
Pavement Thickness | ||||||
% Steel | 8" | 9" | 10" | 11" | 12" | 13" |
0.60 | 4,5 | 5,6 | 5,6 | 5,6 | 5,6 | 6 |
0.62 | 5,6 | 5,6 | 5,6 | 5,6 | 5,6 | 6 |
0.64 | 5,6 | 5,6 | 5,7 | 5,7 | 6,7 | 6,7 |
0.66 | 5,6 | 5,7 | 5,7 | 5,7 | 6,7 | 6,7 |
0.68 | 5,6 | 5,7 | 5,7 | 6,7 | 6,7 | 6,7 |
Note: Bars are uncoated deformed bars.
(d) The use of epoxy coated reinforcing steel is generally not necessary for CRCP. However, in areas where corrosion is a problem because of heavy applications of deicing salts or severe salt exposure, epoxy coating of the steel may be warranted. The bond area should be increased 15 percent to increase the bond strength between the concrete and reinforcement if epoxy-coated steel reinforcement is used.
(e) When splicing longitudinal steel, the recommended minimum lap is 25 bar diameters with the splice pattern being either staggered or skewed. If a staggered splice pattern is used, not more than one-third of the bars should terminate in the same transverse plane and the minimum distance between staggers should be 4 feet. If a skewed splice pattern is used, the skew should be at least 30 degrees from perpendicular to the centerline. When using epoxy-coated steel, the lap should be increased a minimum of 15 percent to ensure sufficient bond strength.
(f) Plan details or specifications are needed to insure sufficient reinforcing at points of discontinuity as described in paragraphs 4e(3) and 4f(1) .
(2) Transverse Reinforcing and Tiebars
(a) If transverse reinforcement is included, it should be #4, #5, or #6 grade 60 deformed bars meeting the same specifications as mentioned for the longitudinal reinforcement.
(b) Although it can be omitted, transverse reinforcing reduces the risk of random longitudinal cracks opening up and thus reduces the potential of punch-outs. If transverse reinforcement is included, the following equation [SEE PRINTED COPY OF TA FOR EQUATION] can be used to determine the amount of reinforcement required (see number 5 of Attachment 2) :
Where:
Pt = transverse steel, %
Ws = total pavement width, (ft)
F = subbase friction factor
fs = allowable working stress in steel, psi, (0. 75 yield strength)
(c) The spacing between transverse reinforcing bars can be calculated using the following equation [SEE PRINTED COPY OF TA FOR EQUATION] (see numbers 1 and 5 of Attachment 2) :
Where:
Y = transverse steel spacing (in)
As = cross-sectional area of steel, (in2) per bar (#4, #5, or #6 bar)
Pt = percent transverse steel
D = slab thickness (in)Note: The transverse bar spacing should be no closer than 36 inches and no further than 60 inches.
(d) In cases where transverse steel is omitted, tiebars should be placed in longitudinal joints in accordance with the FHWA Technical Advisory, Concrete Pavement Joints.
(1) The base design should provide a stable foundation, which is critical for CRCP construction operations and should not trap free moisture beneath the pavement. Positive drainage is recommended. Free moisture in a base or subgrade can lead to slab edge-pumping, which has been identified as one of the major contributors to causing or accelerating pavement distress. Bases that will resist erosion from high water pressures induced from pavement deflections under traffic loads, or that are free draining to prevent free moisture beneath the pavement will act to prevent pumping. Stabilized permeable bases should be considered for heavily traveled routes. Pavements constructed over stabilized or crushed stone bases have generally resulted in better performing pavements than those constructed on unstabilized gravel.
(2) The friction between the pavement and base plays a role in the development of crack spacing in CRCP. Most design methods for CRCP assume a moderate level of pavement/base friction. Polyethylene sheeting should not be used as a bond breaker unless the low pavement/base friction is considered in design. Also, States have reported rideability and construction problems when PCC was constructed on polyethylene sheeting.
(1) Longitudinal Joints. Longitudinal joints are necessary to relieve stresses caused by concrete shrinkage and temperature differentials in a controlled manner and should be included when pavement widths are greater than 14 feet. Pavements greater than 14 feet wide are susceptible to longitudinal cracking. The joint should be constructed by sawing to a depth of one-third the pavement thickness. Adjacent slabs should be tied together by tiebars or transverse steel to prevent lane separation. Tiebar design is discussed in the FHWA Technical Advisory entitled "Concrete Pavement Joints. "
(2) Terminal Joints. The most commonly used terminal treatments are the wide-flange (WF) steel beam which accommodates movement, and the lug anchor which restricts movement.
(a) The WF beam joint consists of a WF beam partially set into a reinforced concrete sleeper slab approximately 10 feet long and 10 inches thick. The top flange of the beam is flush with the pavement surface. Expansion material, sized to accommodate end movements, is placed on one side of the beam along with a bond-breaker between the pavement and the sleeper slab. In highly corrosive areas the beam should be treated with a corrosion inhibitor. Several States have reported premature failures of WF beams where the top flange separated from the beam web. Stud connectors should be welded to the top flange, as shown in Figure 1 (below), to prevent this type of failure. Table 2 and Figure 1 contain recommended design features.
Figure 1: Recommended WF Steel Beam Terminal
Joint Design
WF Beam (weight and dimensions) | |||||
CRCP thickness (in. ) | Embedment in "Sleeper" slab - in. | WF Beam Size | Flange | Web Thickness (in. ) | |
Width | Thickness | ||||
8 | 6 | 14 x 61 | 10 | 5/8 | 3/8 |
9 | 5 | ||||
10 | 6 | 16 x 58 | 8-1/2 | 5/8 | 7/16 |
11 | 5 |
(b) The lug anchor terminal treatment generally consists of three to five heavily reinforced rectangularly shaped transverse concrete lugs placedin the subgrade to a depth below frost penetration prior to the placement of the pavement. They are tied to the pavement with reinforcing steel. Since lug anchors restrict approximately 50 percent of the end movement of the pavement an expansion joint is usually needed at a bridge approach. A slight undulation of the pavement surface is sometimes induced by the torsional forces at the lug. Since this treatment relies on the passive resistance of the soil, it is not effective where cohesionless soils are encountered. Figure 2 shows a typical lug anchor terminal treatment.
(3) Transverse Construction Joints
(a) A construction joint is formed by placing a slotted headerboard across the pavement to allow the longitudinal steel to pass through the joint. The longitudinal steel through the construction joint is increased a minimum of one-third by placing 3-foot long shear bars of the same nominal size between every other pair of longitudinal bars. No longitudinal steel splice should fall within 3 feet of the stopping side nor closer than 8 feet from the starting side of a construction joint. Refer to paragraph 4b(1) (e) for recommended splicing patterns. If it becomes necessary to splice within the above limits, each splice should be reinforced with a 6-foot bar of equal size. Extra care is needed to ensure both concrete quality and consolidation at these joints. If more than 5 days elapse between concrete pours, theadjacent pavement temperature should be stabilized by placing insulation material on it for a distance of 200 feet from the free end at least 72 hours prior to placing new concrete. This procedure should reduce potentially high tensile stresses in the longitudinal steel.
(b) Special provisions for the protection of the headerboard and adjacent rebar during construction may be necessary.
(1) Leave-outs require 50 percent more longitudinal deformed bars of the same nominal size as the regular reinforcement. The additional reinforcement should be spaced evenly between every other normal pavement reinforcing bar and should be bonded at least 3 feet into the pavement ends adjacent to the leave-outs. All regular longitudinal reinforcement should extend into the leave-out a minimum of 8 feet. Required slices should be made the same as those in normal construction.
Figure 2. Lug Anchor Treatment (please refer to source document)
(2) Leave-outs should be paved during stable weather conditions when the daily temperature cycle is small. Because of the closeness of the steel extreme care should be exercised in placing and consolidating the concrete to prevent honeycombing or voids under the reinforcement.
(3) If it becomes necessary to pave a leave-out in hot weather, the temperature of the concrete in the free ends should be stabilized by placing an adequate layer of insulating material on the surface of the pavement as described in paragraph 4e(3) (a) . The curing compound should be applied to the new concrete in a timely manner. The insulation material should remain on the adjacent pavement until the design modulus of rupture of the leave out concrete is attained.
\S\
Anthony R. Kane
Associate Administrator
for Engineering and
Program Development
Attachments
The design engineer should perform the following calculations to ensure that
the bond between the reinforcing steel and the concrete and the longitudinal
steel spacing meet the criteria in paragraph 4c. The equation to determine the
ratio of bond area to cubic inches of concrete is as follows and the equation
to determine the minimum longitudinal steel spacing follows it:
[SEE PRINTED COPY OF TA FOR EQUATION]
Where: | Ps = Perimeter of Bar(in. ) |
L = Length of slab = 1" | |
W = Width of slab (in. ) | |
t = Slab thickness (in. ) | |
n = Number of Longitudinal Bars |
Given: #6 reinforcing bars, therefore Ps = 2. 356" and Bar Area = 0. 44 in. 2
W = 12' t = 10" |
|
Assume: | 0. 6% steel |
Determine: | The required minimum area of steel and the required minimum number of bars
Area of Conc. = 10 x 144 = 1440 in. 2 |
Determine: | The minimum ratio of bond area to cubic inches of concrete. [SEE PRINTED COPY OF
TA FOR EQUATION] the minimum ratio of bond area to cubic inches of concrete is met so the minimum spacing should be checked. |
Determine: | Longitudinal steel spacing should be checked as follows: [SEE PRINTED COPY OF TA FOR EQUATION] therefore the minimum bar spacing is also met. |
REFERENCES (CRCP)
1. "AASHTO GUIDE FOR DESIGN OF PAVEMENT STRUCTURES," 1986.
2. "FHWA Pavement Rehabilitation Manual," FHWA-ED-88-025, September 1985 as supplemented.
3. Mooncheol Won, B. Frank McCullough, W. R. Hudson, Evaluation of Proposed Design Standards for CRCP, Research Report 472-1, April 1988.
4. "Techniques For Pavement Rehabilitation - A Training Course," FHWA, October 1987.
5. "Design of Continuously Reinforced Concrete for Highways," Associated Reinforcing Bar Producers - CRSI, 1981.
6. "CRCP - Design and Construction Practices of Various States," Associated Reinforcing Bar Producers - CRSI, 1981.
7. "Design, Performance, and Rehabilitation of Wide Flange Beam Terminal Joints," FHWA, Pavement Branch, February 1986.
8. Darter, Michael I., Barnett, Terry L., Morrill, David J., "Repair and Preventative Maintenance Procedures for Continuously Reinforced Concrete Pavement," FHWA/IL/UI-191, June 1981.
9. "Failure and Repair of CRCP," NCHRP, Synthesis 60, 1979.
10. Snyder, M. B., Reiter, M. J., Hall, K. T., Darter, M. I., "Rehabilitation of Concrete Pavements, Volume I - Repair Rehabilitation Techniques, Volume III - Concrete Pavement Evaluation and Rehabilitation System," FHWA-RD-88-071, July 1989.