Longitudinal Reinforcement Design
See FHWA’s CRCP Design and Construction Guidelines for the references included in this page. A new, more comprehensive FHWA CRCP Design, Construction, Maintenance and Rehabilitation manual is currently under development and this page will be updated upon its release in the spring of 2016.
Reinforcement design involves selecting the proper percentage, bar size, and bar configuration for optimum CRCP performance. Reinforcement design is focused on provid- ing the minimum reinforcement necessary to develop the desired crack spacings and widths, while at the same time keeping the steel at an acceptable level of stress. States that have been designing CRCP have established standard details for longitudinal layout and bar size.
Longitudinal steel reinforcement content is defined as the ratio of the area of longitudinal steel to the area of concrete (As/Ac) across a transverse section, often expressed as a percentage. Higher amounts of steel reinforcement will result in shorter crack spacings, smaller crack widths, and lower steel stresses. An increase in the percent of longitudinal reinforcement will result in an
increase in restraint.
As the level of restraint increases, so does the number of cracks that develop, resulting in shorter crack spacings. In addition, as the amount of reinforcement increases, the average steel stresses are reduced, producing less reinforcement elongation. As previously mentioned in Section 2.1.1, crack spacings between 3.5 to 8 ft (1.1 and 2.4 m) minimize the potential for development of punchouts and spalling. However, it has been observed that crack spacings as short as 2 ft (0.6 m) have shown good performance as long as good support underneath is provided. Crack widths under 0.024 in. (0.6 mm) prevent infiltration of water and incompressibles, and ensure adequate load transfer efficiency between the cracks thus reducing load induced stresses. In addition, keeping the steel working at an acceptable stress level minimizes fracture of the steel or excessive yield that may lead to wide cracks with poor load transfer efficiency.
Longitudinal reinforcement should be designed to meet the following three criteria:
- Produce a desirable crack pattern (spacing),
- Keep transverse cracks tightly closed, and
- Keep reinforcement stresses within allowable levels.
Although cracking characteristics in CRCP largely depend on the amount of reinforcement, they are also a function of the climatic conditions during placement, materials properties, and construction factors as discussed in Section 3. When designing for longitudinal reinforcement, all these factors need to be taken into consideration.
Specifications for maximum concrete temperatures, low CTE aggregates, and proper curing procedures can help ensure that the intended performance from the reinforcement design will be achieved.
It is also important to consider the effect that excess thickness can have on CRCP performance. Concrete pavement specifications commonly allow for a pay incentive (bonus) for additional pavement thickness due to the resulting increase in structural capacity. However, for CRCP, increasing the thickness (while maintaining the same amount of reinforcement) results in a reduction of the reinforcement percentage. This, in turn, can result in larger crack spacings, wider cracks, and an increase in reinforcement stress. This effect should be considered when specifying an upper limit for thickness pay incentives. For this reason, CRCP should also not be used as a leveling layer.
As a general guideline for conventional steel deformed rebar, reinforcement percentages between 0.60% and 0.80% have been shown to provide acceptable cracking patterns. A minimum of 0.60% is sometimes recommended since lower levels of steel reinforcement may result in wide transverse cracks, large crack spacings, and high tensile stresses in the steel.
On the other hand, steel reinforcement above 0.80% may result in very short crack spacings that later progress into punchout development, particularly under poor support conditions. These recommended limits for steel percentage are based on typical materials properties, and environmental conditions found in the US – particularly the northern states that are subjected to larger temperature extremes.
It should be noted that there exists an optimum steel percentage for any specific project. It should be based on the specified materials and environmental conditions to which the pavement will be subjected. While lower percentages of reinforcement may work in milder climates with ideal materials, it should again be emphasized that lower percentages of reinforcement can still lead to longer crack spacings adjacent to short crack spacings, with the latter ones increasing the probability of punchout development.(8)
Bar Size and Spacing
Longitudinal steel is typically designed to meet a minimum spacing in order to achieve good consolidation of concrete during placement. A maximum spacing is also considered to exist in order to ensure adequate concrete bond strength and thus tight crack widths. FHWA Technical Advisory T 5080.14 provides guidelines for minimum and maximum spacing of longitudinal steel as follows:(56)
- The minimum spacing of longitudinal steel should be the greater of 4 in. (100 mm) or 2½ times the maximum aggregate size.
- The spacing of longitudinal steel should be not greater than 9 in. (230 mm).
Typical bar sizes used in CRCP range from #4 (0.5 in.) to #7 (0.875 in.) [#13M (12.7 mm) to #22M (22.2 mm)]. Selection of the steel bar size (diameter) is governed by steel percentage and minimum and maximum spacing permitted.
With the required amount of reinforcement and bar size selected, the reinforcement spacing, S, may be computed as follows:
- = Reinforcement spacing (in. or mm)
- = Bar diameter (in. or mm)
- = Slab thickness (in. or mm)
- = Longitudinal reinforcement percentage (fraction)
It is recommended that the reinforcement spacing determined with the above equation be considered as the maximum to maintain the required longitudinal reinforcement percentage. If this spacing needs to be adjusted, it should be done so by rounding down to a practical spacing according to pavement geometry.
Another option commonly exercised is to space the bars near the slab edge closer, as indicated in TxDOT standard “CRCP (1) 03” (shown in Appendix A). Figure 12 provides recommended bar spacing for various slab thicknesses and bar sizes as a function of reinforcement percentage.
Other considerations should be made when selecting the bar size including evaluation of the reinforcement surface (bond) area. It has been observed that the average crack spacing decreases with an increase in ratio of reinforcement surface area to concrete volume. A possible explanation for this is that the high tensile stresses in the steel at crack locations are transferred to the concrete as a function of the reinforcement surface area and deformation characteristics of the longitudinal reinforcement.(55) On the other hand, the greater the bond area, the more restraint to movement of the concrete is imposed by the steel, and therefore, tighter cracks are expected to result.(57)
For a given reinforcement percentage, higher surface area is achieved using smaller bar sizes. Therefore, reinforcement design should also consider this. For this reason, the ratio of reinforcement surface area to concrete volume, Rb, is typically controlled to take into account the bar size effect. This ratio can be determined with the following relationship:
(sq.in./cu.in. or m2/m3)
- = Ratio of reinforcement surface area to concrete volume
- = Bar diameter (in. or mm)
- = Reinforcement spacing (in. or mm)
- = Slab thickness (in. or mm)
A minimum ratio of steel surface area to concrete volume of 0.03 sq.in./cu.in. (1.2 m2/m3) is typically recommended for summer construction and of 0.04 sq.in./cu.in. (1.6 m2/m3) for fall or winter construction.(9)
Vertical Position of Reinforcement
There are two primary considerations that should be made when selecting the vertical position of the longitudinal reinforcing steel.
On one hand, drying shrinkage and temperature fluctuations are typically more pronounced at the pavement surface, and can result in wider cracks at this location. It is believed that by positioning the reinforcement closer to the surface, narrower crack widths and higher load transfer efficiency can be achieved.
On the other hand, keeping the reinforcement closer to the surface increases the probability of exposure to chlorides from deicing salts, which may lead to corrosion. Future diamond grinding of the pavement surface would further reduce the distance of the reinforcement from the surface. Given these two considerations, it is common to position the reinforcement between one-third and one-half the slab thickness measured from the pavement surface.
To provide sufficient concrete cover, it is typically recommended to specify a reinforcement depth of at least twice the maximum aggregate size. Illinois DOT requires a minimum reinforcement depth of 3.5 inches (8.9 centimeters) from the pavement surface to the top of the longitudinal reinforcement to minimize the possibility of corrosion and to accommodate variations in construction procedures.
It is also recommended that the maximum reinforcement depth be no more than half the slab thickness measured from the surface. Placement of reinforcement in two layers has also been used. This is implemented in the TxDOT specifications for pavements thicker than 13 in. (330 mm ). The TxDOT standard CRCP(2)-03 in Appendix A shows a detail for placement of reinforcement in two layers.
It is believed that placement of the required reinforcement in this way not only helps to maintain the optimum reinforcement bond area to concrete volume ratio, but also ensures proper spacing while at the same time allowing reinforcement to be positioned closer to the CRCP surface where shrinkage strains tend to produce larger crack widths.
Based on long-term field testing in Illinois and on other projects in Belgium and elsewhere, the depth of reinforcement was a major effect on crack width. The higher the position of the reinforcement, the tighter the transverse cracks. Illinois sections with mid-depth steel had much more full depth repair than those with reinforcement above mid-depth over a 20 year period. Illinois recommends a 3.5 in. (89 mm) covering over the reinforcement. The highly successful CRCP pavements in Belgium also place reinforcement above mid-depth.
Adequate design of the reinforcement lap splices is necessary to maintain reinforcement continuity. Forces induced in the reinforcement by thermal and shrinkage movements are transferred through the lapped splice from one bar to the other by the surrounding concrete bonded to both bars. A minimum lap length of the splices is therefore necessary to ensure sufficient load transfer. Inadequate design and/or construction of the lap splices can result in failure of the reinforcement and poor CRCP performance, ultimately requiring expensive repairs.(58)
The effectiveness of the splice relies on achieving sufficient bond development length between the concrete and the reinforcement. Special consideration should be given to ensure that the concrete achieves adequate bond strength during the critical early-age period. This is particularly important during cold weather construction when the concrete gains strength at a lower rate.
Guidelines on splicing length among the different States vary from 25 to 33 bar diameters.(32) Some State specifications require the use of a fixed length of lap splicing varying between 16 and 20 in (406 and 508 mm). An experimental study looking at the bond development length for CRCP reported that lap splices of 33 bar diameters provide good performance, and may be the basis for the larger splice length specified.(59)
FHWA Technical Advisory T 5080.14 recommends a minimum splice length of 25 bar diameters if the splicing is performed in a staggered or skewed pattern. For a staggered splice pattern, no more than one third of the bars should terminate in the same transverse plane. In addition, the minimum distance between staggers should be 4 ft (1.2 m).
For the skewed splice pattern, the skew angle should be at least 30 degrees from perpendicular to centerline. In prac- tice, an approximate skew configuration may be achieved by skewing the reinforcement by half the pavement width. In any case, it is recommended that a minimum lap splicing of no less than 16 in (406 mm) be provided.
Some states require epoxy-coated rebar in CRCP to prevent corrosion of the reinforcement, especially in urban areas where maintenance and rehabilitation activities are strongly discouraged. This step may also be justified in environments with high exposure to chlorides from deicing salts, especially where corrosion has been previously identified as a problem.
The use of solid stainless steel, stainless steel clad, and other proprietary reinforcement non corrosive materials, such as Glass Fiber Reinforced Polymer (GFRP), may be consid- ered in areas with high chloride, with heavy deicer applications, or where long life (50 years or greater) is desired. Corrosion of reinforcing steel in CRCP has been rarely reported though, and is usually attributed to inadequate reinforcement design resulting in wide transverse cracks.(60)
The designer should strive to provide sufficient reinforcement to maintain narrow crack widths and sufficient reinforcement depth. These measures will help to minimize the probability of reinforcement exposure to chlorides from deicing salts. Additionally, increased steel percentages to account for potential corrosion may be also considered during design.
In the case where epoxy-coated bars are employed, the effect of the epoxy coating on the reinforcing steel bond develop- ment length should be accounted for. The FHWA Technical Advisory TA 5080.14 recommends an increase of 15 percent in the bond area when epoxy coated rebar is used.(56) Although, some studies have found no significant difference in cracking patterns between the use of uncoated and epoxy- coated reinforcement. It is believed that additional research is needed in order to better understand the effect of epoxy coatings on CRCP behavior and performance.