Trained Eye

 
 
 

Heaving Garage Floor

Question:

Dear Inspector:

I am writing to you regarding my garage floor. I live in a 35 year-old house with a single attached garage. Over the years, the garage floor has cracked and heaved. Having had a neighbour’s Cottonwood tree close to the garage, that has since been removed, didn’t helped either. It appears that the floor has sunk about 4 inches, judging by the framing around the garage door. The sinking seems to be especially bad in the corner where the garage wall meets the cement front steps.

I have not noticed any cracking of walls or ceilings in the house. I only plan to be in the house another 5 years. I am in a quandary as to which method to use to fix the floor, mudrucking or digging up the old concrete and pouring a new floor.

I have had estimates on both, and there is not that much difference between the two, cost-wise. The mudrucking would not require the old concrete to be removed but the cracks would still be there. Also, if I get new concrete, is there a specific method that should be used for the new floor?

Thank you for your time.

Answer:

Cracking and minor heaving in garage floor slabs is an all too familiar problem in our area due to our expansive clay soil. Concrete slabs poured on grade will be subject to seasonal movement with changes in the soil moisture content and temperature. Most attached garages have walls that are built on a grade beam, which is independent of the floor slab and is much less subject to movement. That is the reason that you have seen little evidence of movement in the house or garage, other than the sunken floor slab.

Normally, garage floors will heave and settle moderately and have some cracking, but will remain in reasonable condition due to the reinforcing steel installed before pouring the concrete. In this case, it appears that the slab has moved and cracked excessively, likely due to poor preparation of the original base, combined with soil drying caused by the nearby tree. Remedial action in this case is normally to break up and remove the old concrete slab and pour fresh concrete, but another alternative has been explored.

Mudjacking or Mudrucking is a process of raising sunken concrete slabs that have settled. This is done by injecting a combination of air and other material under the settled concrete, to raise it back to its original position and support it from underneath. The injections are done by drilling small holes in the concrete, which are later patched, after the slab is raised.

I have seen several sidewalks and driveways on newer homes that have been raised by mudjacking, that appear to be quite successful, but I have not seen many on older concrete. I think this may be for the reason, that there is not enough of a cost difference between replacement and repairs to justify lifting old slabs. I am somewhat surprised by her statement, as removal and replacement is a much larger job and should be substantially more costly that mudjacking, if done properly. I would suggest obtaining at least one additional estimate for replacement, to ensure proper workmanship.

The concern I have in opting for mudjacking over replacement would be with the condition of the older slab. Concern has been raised by the homeowner about cracks still being present after repairs are done, but I am wondering about other deterioration. Most older garage floors will have deterioration and spalling due to salt and moisture damage from parked vehicles. I believe that the mudjacking repairs I have seen are mainly on newer concrete, because the condition of these slabs is quite good with the exception of the settlement. Raising the old floor will remedy the settlement, but will not improve the overall condition of the concrete, which may be quite worn after 35 years.

If replacement is chosen, care in proper preparation of a base for the new concrete floor is very important. Once the old concrete is broken up and removed, a base of several inches of crushed stone should be installed and compacted prior to re-pouring. The concrete should have proper reinforcing steel mesh or re-bar installed, to strengthen the finished floor. This will minimize the cracking and settlement, seen in the old floor slab.

Careful scrutiny of the estimates for new concrete should be done with relation to the above 2 items. Lower priced estimates often do not include compaction of a substantial base or heavier reinforcing steel. Spending a little more to ensure these items are included in the repairs may prevent a repeat of the previous problem, for years to come.