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.
The base course directly beneath a CRCP is a very critical component. The base course must provide the following:
- Smooth construction platform on which to construct a smooth CRCP;
- Permanent support to the CRCP over its design life;
- Non-deforming and smooth surface for accurate reinforcement placement and placement of a uniform CRCP slab thickness; and
- Sufficient friction with the CRCP slab to help form adequate crack spacing.
Several types of bases have been used successfully, including unbound aggregate, cement-treated and lean concrete, sphalt-stabilized, and combinations of the above. Each of these base courses must be designed and constructed properly to avoid contributing to problems in CRCP performance.
It is also common to place a subbase layer, either an unbound granular material or treated subgrade layer, between the base and the subgrade. This subbase layer is extremely important when the subgrade is wet and soft, as it can reduce erosion of the top of the subgrade and provide a construction platform for base construction.
The width of the base course is important during construction. Ideally, it should extend beyond the CRCP slab edge by at least 3 ft (0.9 m) to provide increased edge support and to provide a stable track-line for paving operations. It may be necessary to widen the base further to accommodate some newer paving equipment.
Base thicknesses in the range of 4 to 8 in. (100 to 200 mm) are common for highways. Subbase thicknesses are often 6 to 12 in. (150 to 300 mm) or greater.
Field studies have shown that the addition of asphalt to granular materials to produce a non-erodible stabilized mix- ture for base construction can improve CRCP performance. (73) Proper (as-designed) asphalt content, density, and other mix quality parameters must be achieved during construction. The key benefits include minimizing moisture- related loss of support, providing a smooth construction platform for steel placement and improved ride quality, and supplying an adequate amount of friction with the CRCP to provide proper crack spacing, and to provide stress relief during curling and warping cycles.
Stripping of asphalt binders from aggregates in these bases has been a concern. Stripping is a direct result of moisture in the pavement system that brakes down the bond between the aggregate and asphalt binder. Consequently, the use of asphalt stabilized bases in areas with high water tables should be designed with proper additives, and its use, if any, should be used with caution.
A cement-treated base consists of crushed stone base commonly blended through a pugmill with an optimized quantity of cement (typically 5 percent) to achieve a 7 day compressive strength of 500 psi (3.5 MPa) and water at 1 to 2 percent below the optimum moisture content. A strong and non-erodible cement-treated base can be very effective in improving CRCP performance and has been successful in areas with high water tables. Some erosion and deterioration of cement stabilized base courses have occurred in the past, creating loss of support problems for CRCP. However, this can be prevented through proper materials selection and construction, particularly achieving adequate density and consistency of material.
For cement-treated base, the time between mixing, placement, and initial compaction should not exceed 1 hour. Complete bonding between a cement-treated base and concrete slab is not recommended due to potential of reflection cracking and the increase in the effective CRCP slab thickness, which results in the need to increase the amount of steel reinforcement.
An interlayer of some type should be used between the slab and the base to serve as a stress relief layer. Most often, a 1 to 2 in. (25 to 50 mm) layer of rich, dense-graded HMA is placed on top of the cement-treated base layer to minimize erosion potential while providing stress relief for curling, warping, expansion and contraction.
While not necessarily for CRCP until recently, contractors in Germany have been using a 0.2 in. (5 mm) thick non- woven geotextile as a stress relief layer between concrete pavements and cement stabilized bases. The fabric is fastened to the subgrade or base layer with a washer and nail to secure it during construction. Cores taken in a pavement built in 1981 noted no deterioration in the geotextile or cement-treated base.
Cement-treated bases should not be placed under freezing or near freezing conditions. It is recommended that the material be placed and cured when ambient air temperatures are greater than 40°F (4°C) (measured in a shaded area) until adequate strength is reached.
Cement-treated bases can be cured by covering with polyethylene sheeting for 3 to 5 days or spraying a fine water mist several times a day after placement. At the end of each day’s construction, a straight transverse construction joint should be formed by cutting back into the completed base to form a vertical face.
Lean concrete, sometimes known as “econocrete”, is made of aggregates that have been plant mixed with a sufficient quantity of cement to provide a strong and non-erodible base. Lean concrete has been used as a base course for many CRCP. It can provide a smooth uniform surface as a construction platform for steel placement and paving. It can be placed using the same equipment that will pave the concrete surface. Lean concrete bases should be cured using white-
pigmented curing compound and be left untextured to prevent bonding to the CRCP.
Field studies have shown that a lean concrete base of adequate strength will reduce erosion of the base and loss of support.(75,106,107,108) Some agencies specify sawcut weakened plane joints once the lean concrete base has set to prevent large cracks from forming and reflecting into the CRCP surface. Other agencies successfully placing a layer of asphalt-stabilized base or HMA (1 to 2 in. [25 to 50 mm] thick) on top of the lean concrete base layer to minimize erosion, provide stress relief, and provide a moisture barrier, similar to that recommended for a cement-treated base.
Dense-graded unbound granular materials with low plasticity have been used successfully as a base and subbase for CRCP. To minimize consolidation and settlement problems, a relative density of 95 to 100 percent as determined by AASHTO T 180 (Modified Proctor) is necessary.
Care should also be exercised during construction and fine grading to avoid segregation and minimize loss of density and uniformity. Any of these conditions can result in loss of slab support and subsequent CRCP failures. Specialized equipment is often used to place the granular materials to a uniform depth without segregation.
It should be noted that some agencies have had significant problems with pumping and loss of support with unbound bases, even on strong dry subgrades. Use of an untreated aggregate base under CRCP will result in much longer crack spacing for the same reinforcement content. This may cause serious problems in crack spacing for the same crack deterioration and punchout development. This can be accommodated by increasing the reinforcement content. The AASHTOWare Pavement ME (MEPDG) provides recommended friction values for unbound base courses calibrated from field conditions.
Because CRCP is normally used for heavily trafficked highways, most agencies utilize a stabilized base to minimize erosion and loss of support, and instead opt for a granular subbase. This combination has helped facilitate construction and provide the uniform foundation needed for long term and high quality performance.
An open-graded permeable base is a drainable layer with a typical laboratory permeability value of 1000 ft/day (300 m/day) or greater. Permeable asphalt-treated and cement-treated bases have seen limited use as open-graded drainage layers for CRCP.
The primary function of this layer is to collect water infiltrating the pavement and move it to edge drains within an acceptable time frame. However, the permeability of the base should always be balanced with stability. Stability is more critical than permeability in CRCP foundation systems.
The main problem with open-graded bases for CRCP is that concrete mortar often infiltrates the base resulting in additional bonding between the slab and base/subbase, which increases the effective CRCP slab thickness, thereby reducing the steel as a percentage of the slab cross section. These effects can change the crack spacing and lead to performance problems.
In addition, due to the relative “flexibility” of CRCP, the unbound layer just beneath the open-graded layer may pump into and infiltrate the permeable layer causing localized settlements, which has occurred with lime treated subgrades on some projects. For these reasons, permeable bases are not generally recommended for CRCP, unless strong practical measures to prevent these problems are taken, such as the use of geotextiles.
Open-graded permeable bases were very popular in the late 1980s and early 1990s, but due to a number of failures from these mechanisms, many agencies now discourage their use. If deemed a necessary component, however, measures should be taken to reduce the target permeability to 100 ft/ day (30 m/day) in order to improve stability.
Edge Drains and Impervious Moisture Barriers
Heavy traffic combined with moisture variations in susceptible materials often leads to erosion in the base and subgrade or shrinkage and swelling that leads to premature concrete pavement failures, including CRCP. Infiltration of water into the CRCP structure can be controlled with proper cross slopes, maintaining a seal that prevents moisture changes in unbound pavement layers, and construction of an edge drainage system to transport water away from the pavement structure.
Edge drains work best in granular soils. However, in cohesive soils, which do not allow water to drain freely, edge drains can accentuate shrinking and swelling by causing the soils to be drier adjacent to the drains, which causes shrinkage. Alternatively, if drains become clogged and fill with water, adjacent soils will become saturated, which can lead to swelling.
To function properly in cohesive soils, edge drains should incorporate a moisture barrier to prevent wetting or drying of adjacent soils under the pavement. In addition, base and subbase layers must be free draining and extend over the edge drain. Care should be exercised during the construction of these systems to:
- Ensure that the as-designed gradation of the drainage layer is obtained to preserve permeability,
- Provide a system that allows access and can readily flush out the system without causing any erosion,
- Ensure that the drainage layer is placed and lightly compacted without fracturing aggregate particles, which create additional fines, and
- Avoid collapsing, breaking, and clogging of the pipes and outlets.
It is also suggested to regularly conduct a video survey of the outlets and longitudinal pipes to ensure they can function.
An impervious moisture barrier can be used to effectively prevent both changes in surface grades and cracking along the pavement edges due to swelling soils. A vertical moisture barrier should be constructed as deep as the zone affected by seasonal moisture variations. Alternatively, a conservative minimum depth of 7 ft (2.1 m) can be used.
Before any drainage feature is added to a pavement system, an assessment should first be made of the cost of the system as it compares to the potential improvement to performance.