Dare Concrete - Outer Banks, North Carolina
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Concrete Repair

Crack Repair

Surface Repair

Structure Repair

 

 

 

STRUCTURE REPAIR - SLABJACKING

Inspection of slab settlement

Cracked, bumpy, and uneven concrete surfaces can present both minor and major problems. Does the neighborhood roller-hockey gang complain that your jagged driveway prevents them from playing like Gretzky? Do your forklift operators ache like bronco riders after a long workday on your warehouse floor? Or are you worried about tripping hazards and liability issues because of uneven public sidewalks and entranceways? Slabjacking may be your answer. Slabjacking has many advantages over slab removal and replacement, and many good applications.

As early as 1930s, pressure injection was used to raise deteriorated slabs and roadways. J.W. Poulter is believed to have developed the first machine for what was then called mudjacking for highway work. Early grouts ranged from soils such as clay, sand, and loam, to Portland cement, fly ash, lime, and casting plaster. Though dangerous, even hot asphalt was pumped into voids under roadways to prevent water penetration. Selecting the proper pump and proportioning grout are critical to success, a lesson learned in the 50-year history of slabjacking.

Slabjacking Process:
Picture 1 - First assess the settlement of the concrete slab. Inadequate subgrade compaction isn’t only the cause for settlement of concrete slabs on ground. "While improper subgrade compaction prior to placement of a concrete slab can lead to subsidence and cracking problems, loss of soil support due to water seepage through joints is often the cause of sinking concrete. You can look for the telltale signs of water erosion after a rain at the bottom of a sloped, sunken concrete driveway. You’ll often see a sand washout."

Picture 2 - The use of sawcuts at joints (relief cutting) to relieve binding may be appropriate if the sunken slab has to be isolated from adjacent slabs or structures.

Picture 3 – Surveying equipment and grade and string lines are used to establish the correct elevations. Based on experience, a specific pattern of holes is drilled in the existing concrete slab floor using a diamond bit or a percussion drill with carbide bit. Depending on the pumping system that will be used, these access holes range from 1 inch to 2 inches or more in diameter.


Sawcutting at joints (relief cutting)

Drilling holes (guy on the left) and pumping grout (right).


Truck mounted pump and slurry mixer

Finish elevation

A grout mixture is pumped through the holes and into the voids beneath the concrete. If the subsided slab has adhered to the soil, a void must be created using a air pressure equipment. At pressures that vary from 10 to 150 psi, the pumped grout mixture fills the void, exerting pressure on the concrete and the subsurface material and lifting the slab. During the lifting operation, the pump operator monitor both volume and pressure. The amount of pressure used depends upon the grout slump (stiffness), the weight of the slab, and the area of void space.

Slabjacking must be performed with controlled and even pressure grouting, as portions of the slab may crack if subjected to significantly different subgrade pressures. Once the concrete is properly aligned and leveled, the drill holes are patched. Within 24 hours, the grout mixture solidifies and forms a stable bearing surface for the slab. Special care is taken to ensure that the slab slopes properly to drain water away from foundations (Picture 5).

A sunken slab at a gas station. Six core holes wee drilled in the slab, and a fly ash and cement grout mix was used to raise the slab the full 4 inches back to the original position.
Applications:
Residential work: driveways, sidewalks, pool decks, and slabs, that have settled out of alignment.
Commercial work: road leveling at intersections, realignment of bridge approaches, warehouse floor repair, and equipment foundation stabilization in manufacturing plants

At the Watkins Trucking Co. terminal in Cincinnati, a quick set cement and fly ash mix was used to raise 12 sunken slabs, and truck traffic resumed in 24 hours from the start of the project.


Slabjacking Advantages: The alternative to slabjacking for restoring the levelness and proper elevation of a slab is to demolish the existing concrete and construct a new slab. Slabjacking has many advantages over such drastic measures:

  1. Low Cost: Slabjacking can result in significant cost savings compared to demolition and replacement costs.
  2. Maneuverability/Access: The size of pumping equipment makes the slabjacking pump plant highly maneuverable, enabling it to access fairly small or congested areas.
  3. Little Mess: Cleanup is fast and minimal compared with the dust and debris of demolitions. Drill hole patches are less noticeable than the color difference of a newly placed slab next to older concrete.
  4. Minimal Downtime: Besides the significant cost savings, an owner’s greatest advantage is the minimal downtime. Manufacturing facilities can minimize the time their equipment and operations are offline.
  5. Aesthetics: Instead of patched drilled holes, concrete cores can be cut out and grouted back into the holes after pumped. This repair method costs more, but it is a good alternative for owners who are concerned with a more uniform appearance in the repaired slab.
  6. Landscaping: Today’s landscaping costs can be significant. Slabjacking operations do little to no damage to landscaping adjacent to the lifted slab, compared with slab replacement.
  7. Waste Disposal: With slabjacking there are no disposal costs for demolished concrete.

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