Answers to Your Concrete Questions #2 Answers to Your Concrete Questions #2

Q. I have a garage that has one course of concrete blocks exposed. On the outside wall of the garage, the inner surface of the blocks has started to crumble. I can wire brush it off and paint with Dryloc, but my concern is what is causing it. We do have salt dripping from the car in winter and some of the salty water runs toward the blocks. Could that be the cause?

A. Assuming you have the usual one or two courses of block to keep the wood away from the damaging moisture, it sounds like the blocks are deteriorating from repeated freeze-thaw cycles and repeated cycles of wetting and drying. Both of these can cause expansion, contraction, and failure just as bridge deck cracks and crumbles. If you eliminate the water and salt from getting to the block, you have arrested the problem, but it may continue slightly since no patch is perfect and damage has been done.

It is not worth trying to replace the block at this time. Most PT wood will not work any better. Don't touch CCA for this application unless you want the arsenic that is leached out of it by the salt water to run across your floor. The ions in the salt water do a good job of freeing the arsenic.

When it gets warm, try to aggressively clean the faces of the block and remove any loose pieces. Wash and rinse the block frequently over the summer when conditions are good for getting the salt out of the concrete. Wetting and drying cycles will not be too bad and they are better than leaving the salt in the block.

Near fall, you will be able to coat the block, patch or then coat with waterproof like DryLok or Thoroseal. If you use Thoroseal, add a little Acryl 60 (latex admixture for better bond) for the best possible job. Your floor should be okay because it probably is air entrained concrete if you live in the north and the contractor knew what he was doing.

No need for a fancy washer. A hose, some water, and a brush will be enough equipment. What you want to do is wash periodically to leach the excess salt out of the concrete. There is no big hurry, so wash it every once in a while and let it dry. As it dries, some of the salt will be carried to the surface so it can be removed the next time you wash and clean. This will slowly reduce the amount of salt absorbed.

The trick is to remove as much salt as possible to reduce the absorption of water that will freeze in the winter. You paint at any time, but the more rinsing. The better. If you use Thoroseal, you will have to moisten the face of the block just before coating to get a better bond. This can be a persistent problem, but the solution is not as much of a problem as replacing the block.

Q. Does anyone have any tips on how to screed the fresh concrete of a walkway that is being poured abutting an exterior wall?

A. Snap a chalk line on the existing foundation wall at the level that you want that side of the walk to be to provide drainage away from the wall. Using construction adhesive, glue down the expansion joint sections to the wall with the top of the joint even with the chalk line. When the concrete is poured, you can set your screed board on top of the joint against the wall and on top of your perimeter/outside form and work it back to establish grade. Be careful, as most expansion joint material is rather soft and can be torn up if too much screening/pressure is used.

Q. Can someone tell me how to build a 12x16 concrete box underground?

A. If you are digging some thing like a trench 7 feet deep, you better have something good to protect yourself. Contractors cannot afford to make the bracing system, so they rent them. Look for a contractor equipment rental dealer.

If the 7 foot deep hole is 12 feet by 16 feet, you will have trouble finding something since there is not much demand. For a 12 foot by 16 foot by 7-foot hole, it is cheaper to rent a small backhoe or a contractor with one. Then you dig the hole oversize and let it have sloping sides with no support.

Q. We are almost closing on our first house and we have an approximately 4-foot concrete porch that runs the length of the back of the house onto a sloped backyard. It has detached stairs that are too short and dangerous along with no railing. We want to make it safe with a railing and new steps that will attach to the porch. We need to do it inexpensively. The top row of cinder blocks on the outer edge was laid so that the holes are facing up. Are there any suggestions to fix this?

A. You could build a compliant railing and install it in the open holes in the blocks. Check your building code for the preferred height for the railing. This would probably be the least expensive option.

Q. We're considering laying flagstone over an existing concrete slab. However, my wife recently stained the concrete with concrete stain and applied two coats of Behr Wet Look Sealer, which made the slab wonderfully slippery when wet.
1. Can the flagstone be applied to the stained and sealed slab, or do we have to remove the sealer?
2. If so, what should we use to remove the sealer?


A. Use Muriatic acid with water to etch the surface.

Q. I'm getting a stamped concrete patio put in and don't know if I should seal it, when I should seal it, or how often I should seal it.

A. The installer will seal it for you the first time, then every few years after that.

Q. Does anyone know how a concrete garage roof is constructed? Because the underside is showing a crack from one side to the other and I need to know if it's unsafe. It's a minor crack but it obviously lets in rainwater and needs attention. It was built about 1955 and appears to have been constructed by some kind of liquid concrete mould as I can see the wooden plank grains in the underside surface. I can't see any joint lines, and this is the confusing bit - it is approximately 6 meters long and 3 meters wide and I can't imagine how it was lifted into place if it's a one-piece structure. It has an Asphalt covering which is in need of attention also, but my main concern is the safety aspect.

A. The roof was cast in place. The grain of the wood is from the forms that supported the concrete when it was poured, and after the concrete dried, the wood and supports were removed. The concrete has steel running through it. Is the concrete uneven where the crack is? If not, it will probably last another 50 years. Redo the roof on top and it will be fine.

Q. I would like to raise the floor to curb height (6 inches) along with the sliding glass door that has been put in place of the garage door. The inside space above the door is 25 inches, the outside is 18 inches, and I only need to raise it 6 inches. Will this be a problem and can you help me with details so I can do it myself?

A. Concrete over polyethylene with a 1" thick Styrofoam under the poly at the unheated perimeter would be the right way to do this. You can use fiber mesh in the mix or put reinforcement wire in. If there are bad cracks in the existing floor, it would be wise to put foam across the entire floor to prevent racks from telegraphing to the new. An expansion joint needs to be installed around the perimeter.

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