Constructing Crossovers

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.

CRCP design procedures and specifications are developed around the concept of continuity in load transfer and thermal stress resistance in a monolithic pavement slab. Thus, temporary gaps in CRCP should be avoided as much as possible. Giving proper consideration to the paving schedule can minimize the necessity for these gaps. However, temporary gaps are necessary in some paving situations, such as providing a haul road crossing or an intersection where cross traffic must continue to flow.

These gaps are referred to as leave-ins or leave-outs. If paving in the gap area precedes mainline construction, the gap pavement is referred to as a leave-in; if it follows mainline construction, the gap is referred to as a leave-out.

Leave-Ins

If the concrete is placed in the gap before paving the mainline, the crossover is referred to as leave-in. Leave-ins are preferred to leave-outs because movement of the hardened concrete of the short pavement section is not likely to have a damaging effect when abutting freshly placed concrete of the mainline section. Crack spacing in the leave-in may be greater than what develops in the mainline due to free movement of the ends. However, additional cracks will develop over time after it is connected to the mainline pavement.

Normal lap staggering and construction joint procedures can be used in leave-ins. Extra care should be taken to provide high quality concrete, effective consolidation, proper reinforcement placement, and a smooth surface. If the leave-in is in the form of an intersection, the two sides of the intersection can be constructed separately, or the entire intersection can be constructed at once.

Leave-Outs

If the concrete is placed in the gap after paving the mainline, the crossover is referred to as leave-out. A major con- cern with leave-outs is that the movement of the mainline hardened concrete can overstress the fresh concrete in the gap, causing cracking, crushing, and permanent loss of bond between concrete and steel in the leave-out area. Thus, this type of crossover should not be used unless it is the only practical method of gap construction. In fact, some agencies do not permit the use of leave-outs.

In the event a leave-out does become necessary, the following precautions should be taken to reduce distress in the leave-out concrete:(85,89)

  • Leave-out should be at least 100 ft (30 m) in length.
  • 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 regular longitudinal reinforcing bar and should be bonded at least 3 ft (1 m) into the pavement ends adjacent to the leave-outs. All regular longitudinal reinforcement should extend into the leave-out a minimum of 8 ft (2.5 m). Required splices should be made the same as those in normal construction.
  • Leave-outs should be paved during stable weather conditions when the daily temperature range is small. This condition is likely to exist when the sky is cloudy and the humidity is high.
  • 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 to minimize movement. Curing compound should be applied to new concrete in a timely manner. Insulation material should remain on adjacent pavement until the design modulus of rupture of the leave-out concrete is attained.
  • Because of the shortness of the steel spacing, extreme care should be exercised in placing and consolidating concrete to prevent honeycombing or voids under reinforcement.
  • Place terminal treatments at each end of the leave-out.

Temporary Crossovers

This type of crossover is sometimes needed to accommodate truck movement across the grade after reinforcing steel is in place. These crossovers can be installed by placing wooden mats over the steel after temporary removal of bar supports. The wooden mats can be designed so that cleats underneath are spaced to fit between longitudinal and transverse reinforcing members.