HMA mixtures are routinely used to overlay PCC pavements. The following slides describe construction processes that are used.
These are the four procedures used. The first two are basically the same process except one is used for Jointed Plane Concrete Pavement (JPCP) and the other is used for Jointed Reinforced Concrete Pavement (JRCP).
The most common form of distress in HMA overlays of PCC is reflection cracking. This is a result of the concrete slabs contracting and expanding with temperature changes, which cause the joints and cracks to open and close. This opening and closing causes a crack to form in the HMA directly over the opening in the concrete. Also, if the concrete slabs are not firmly seated on the underlying material, traffic loads going over the joints will cause the joints to move up and down vertically. This movement will cause shearing in the asphalt over the joint and create a reflection crack.
The concept behind cracking and seating and breaking and seating is to shorten the slab length and therefore the amount of force transferred to the overlay from the movement of the slabs.
The goal is to break the slab through the entire pavement structure. But, load transfer still exists through the aggregate interlock that will form around the cracks.
This is a machine used for the process. It is basically a large guillotine that rises and falls and cracks the pavement slabs.
The surface of the pavement after the guillotine has gone over it.
The crack pattern from cracking and seating.
After the pavement has been cracked and seated it should be proof rolled with a large pneumatic roller.
Rubblization eliminates slab action by 1) destroying the geometry of the slab and 2) seating the concrete firmly against the underlying subbase or soil. It also breaks the bond between the concrete and any steel present in the slab, eliminating detrimental effects caused by corrosion of the steel and further reducing the ability of the concrete to act as a mass. The rubblized concrete responds as a tightly packed, high-density granular material. The rough, hard particles provide an internal friction to resist rutting while the lack of tension prevents cracking.
Fracturing the surface of the concrete to a relatively small size means that features and problems in the old concrete surface such as joints, cracks and faults cannot reappear in the HMA overlay. This prolongs the life of the overlay and helps to ensure a smooth pavement. Production rates with rubblization are up to 1 lane-mile per day, much faster than other types of PCC rehabilitation. This means lower construction costs and less inconvenience to road users, giving substance to the slogan “Get In, Get Out, Stay Out.”
The more concrete there is to expand and contract as the temperature changes, the greater the movement of the slab, and the greater the opening of joints and cracks. (Presenter: Hit enter to show progress of movement and widening of PCC joint and reflection crack.)
Rubblization reduces the size of concrete pieces so that the expansion and contraction causes a minimum of movement. The space between the fractured pieces moves less so that the cracks are not reflected through the surface. And, because the process seats the pieces of concrete, reflection cracking through shearing at the joints does not occur.
In addition to better HMA overlay performance, rubblization offers savings in terms of flexibility for construction time. So, if paving during rush hour is prohibitive due to traffic volume, construction can be moved to off-peak hours. And, because production rates of up to 1 mile/day can be achieved, the road can be rehabilitated quickly. Rubblization with an HMA overlay means economic savings to both users and agencies. Because of the speed of construction, users may avoid costly delays. Construction costs are reduced due to the speed of construction, the ability to leave the old PCC in place, and the lower cost of HMA placement.
Rubblization leaves the PCC material in place so landfill space is conserved and haul trucks do not contribute to air pollution and fuel consumption. The problems in the existing PCC associated with cracking and faulting will not appear in the new HMA surface, ensuring a smooth ride.
The construction process is straight forward. First concerns about subsurface drainage should be addressed by placing edge drains in the pavement if they are needed. If there is an existing HMA overlay, it should be milled off. Any HMA patches in the concrete should be removed and replaced with aggregate base material as needed. The point is to give uniformity to the pavement support and ensure proper drainage. The concrete is fractured using either a multiple head breaker or a resonant breaker. The rubblized material is then compacted using a combination of rollers to ensure that the concrete pieces are seated and locked together. And, finally, the required thickness of an HMA overlay is placed over the top.
The Resonant Pavement Breaker (RPB) uses a low-amplitude 2000-pound force applied at a frequency of 44 cycles per second to fracture the concrete pavement.
The multi-head breaker (MHB) has a number of hammers mounted laterally in pairs. Half of the hammers are located in a forward row and the rest are diagonally arranged in a rear row so that breakage occurs continuously from side to side. The MHB is capable of rubblizing a 13-foot lane in one pass.
Once the rubblization is complete, a 10-ton steel wheel vibratory roller is used to pack the loose rubble together initially. This is usually followed by one or two passes of a pneumatic roller. Finally, the material is rolled for one or two more passes by a vibratory roller just before the first lift of HMA.
Most specifications allow for a maximum particle size of 6 to 9 inches in the top part of the concrete slab. The majority of the concrete particles are in the range of 1 to 4 inches. Aggregate interlock is maintained in the concrete because the subsurface particles are only fractured, not reoriented. The rolling after rubblization helps to knit the surface particles together and seat the bottom of the concrete firmly against the underlying material.
Rehab & Maint. HMA Overlays 2Slab Preparation Techniques• Crack and seat (JPCP)• Break and seat (JRCP)• Rubblize (JPCP, JRCP, CRCP)• Saw and seal
Rehab & Maint. HMA Overlays 3PCCReflection Cracking• By far, the biggestproblem in HMAoverlays of PCCpavement• Caused by movementat PCC joints andcracksLoad-inducedmovementJoint ClosesHMASlabs Contract Joint OpensSlabs ExpandReflection crack
Rehab & Maint. HMA Overlays 4Cracking and Seating• Shortens effective slab length• Standard practice in many States• Not “generally” recommended for use on poorsubgrades• Design methods (overlay thickness)
Rehab & Maint. HMA Overlays 20Particle Size• PCC fractured into 9in.-minus pieces• Most pieces are 1- 4in. diameter• Aggregate interlockmaintained beneathsurface• Rolling knits togethersurface particles
Rehab & Maint. HMA Overlays 21Sawing and Sealing Joints• Concede appearance of reflection cracking• Objective: control rate of deterioration• Reduces spalling of reflection cracks• Candidates should have well-defined joints• Sawcut must be directly above the• underlying joint