The combination of the concrete material and placement operations directly influence the performance of a CRCP. Specifically, mixture properties and construction features during the first 72 hours after placing the concrete have a significant impact on short- and long-term performance.
Aggregates constitute about 60 to 75 percent of the concrete mixture. Aggregate properties—including the coefficient of thermal expansion (CTE), surface texture, and coarse aggregate size—affect crack spacing and width for a CRCP. As these are critical properties in CRCP performance, aggregates should be selected carefully and not be changed in the field before consulting with pavement engineers and concrete mix designers.
The following characteristics should be considered when selecting aggregates for a CRCP mixture:
- Coefficient of Thermal Expansion – The CTE of the coarse aggregate affects crack spacing and crack width in CRCP. Use of a lower CTE coarse aggregate, and thus lower CTE concrete, will reduce crack width opening for the same crack spacing.
- Size – Generally, a larger coarse aggregate results in better aggregate interlock across cracks and thus a higher Load Transfer Efficiency (LTE) of transverse cracks. The maximum size of coarse aggregates should not be less than 1.0 in (25 mm), and preferably larger, to achieve adequate crack LTE. However, the maximum aggregate size must allow for proper placement and consolidation of concrete. It is recommended that the maximum coarse
aggregate size be less than half the spacing between longi- tudinal bars. Currently, States observe this recommendation by specifying maximum coarse aggregate size to be 1.5 in. (38 mm).
Placing the Concrete
In CRCP paving, haul vehicles cannot drive on to the base due to reinforcing steel. Therefore, the concrete is generally discharged onto a high-speed belt placer from the side. This method allows rapid and efficient unloading and places the concrete in the proper location. Another less desirable option is to discharge on grade using the chutes on transit mix trucks or agitators. This practice greatly increases the possibility of displacing reinforcing steel and segregating the concrete.
Consolidating the Concrete
Concrete is consolidated to achieve required strength and durability, reduce entrapped air, and ensure bonding between the concrete and steel. Thus, adequate consolidation is a critical factor in achieving desirable long-term performance of CRCP. Like all concrete paving, concrete used in CRCP is consolidated using mechanical vibrators. Though rare, over-vibration can cause aggregate segregation, excessive bleeding, and reduction in entrained air. Furthermore, vibrator trails indicate failing vibrator equipment.
Either over-vibration or under-vibration can reduce bonding strength between steel and concrete and thus result in poor CRCP performance. It should be noted that problems associated with under-vibration of concrete appear more frequently than those associated with over-vibration.
Vibrators must not come in contact with reinforcing steel bars for extended periods of time because this can cause weakened mortar to concentrate around the steel bars. Also, contact between vibrators and joint assemblies, base material, and side forms must also be avoided for the same reason. Extra care should be taken to attain sufficient consolidation by manually vibrating at construction joints and leave outs.