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Will Lengthening Curing Process Add Strength to New Concrete?


QUESTION: I've been told that new concrete should be kept moist for at least three days while curing. I'm wondering just how much stronger can it get, and how long should it cure for maximum strength?

ANSWER: Keeping concrete moist helps the curing process. Concrete hardens as a result of a chemical reaction, called hydration, between cement and water, not because it dries. The hardening, or curing, continues as long as moisture remains in the concrete. If too much water is lost from the concrete through evaporation, hydration--and the hardening process--slows down or ceases.

Concrete continues to gain strength after pouring for as long as it retains moisture, but the longer it moist-cures, the slower the rate of strength gain. Moist-curing concrete for 20 days more than doubles its strength compared with four days of moist-curing, which is considered a minimum. Although the greatest gain is in the first week or two after pouring, the curing process continues for several months.

Concrete that is not moist-cured at all dries too rapidly, and reaches less than half its potential design strength. It will also have a greater number of shrinkage cracks.

The most frequent moist-curing method is to spread moisture-retaining fabric, such as burlap, over the concrete after it has hardened enough to prevent surface damage. The fabric should be kept thoroughly soaked with a garden hose so a film of water remains on the entire surface of the concrete throughout the curing period.

Checking Water Tank Vents for Obstructions

Q: I just replaced my gas hot water tank. The tank was not leaking, but I was afraid it would leak soon. The instructions for the new hot water tank recommend that at least once a year I perform a visual inspection of the venting system to look for obstructions, damage and rust. I checked the visible portions of the vent, but most of it runs through interior walls. Is there an alternate way to check the venting system other than through visual inspection?

A: The vent system for a water heater should discharge exhaust gas harmlessly outside and not "spill" them back into the utility room. In addition to a visual inspection, you should check the area by the draft diverter (at the base of the vent pipe) for escaping exhaust gas. Put your hand close to the opening at the top of the water heater. If exhaust gas spills out of the diverter, you will feel hot gases blowing across your hand. This is a condition that must be corrected because the gas contains carbon monoxide. This colorless, odorless gas is poisonous and can cause asphyxiation.

Another way to check for faulty venting is to hold a lighted match at the draft diverter opening. If the exhaust gases are escaping into the utility room, they will blow out the flame. If the vent system is functioning properly, the flame will be sucked into the draft diverter hood.

Checking the integrity of the concealed portions of the water heater flue can be done by a heating contractor using a smoke test or a combustible gas detector. However, this is generally not necessary.

Nevertheless, as a safety precaution, I recommend that homes with fuel burning heating systems or hot water systems be equipped with one or more UL-listed carbon monoxide detectors such as the one manufactured by First Alert. These are available at home centers and hardware stores and cost from $40 to $60.

Track Down Sources of Household Drafts

Q: We are trying to tighten up our home for the winter and are tracking down and eliminating sources of drafts. The doors and windows were no problem, but we noticed a bad draft coming from our clothes dryer vent. What's the best way to handle this situation?

A: An open dryer vent leading to the outside of the house can be a significant source of drafts in winter and windy weather. If yours is an electric dryer, check with a heating expert about the possibility of connecting the dryer vent to the furnace return duct, thus saving the heat generated by the dryer, which is otherwise wasted out the vent. Do not vent the dryer directly into the laundry area, as dryer air is damp and you risk an indoor condensation problem. Gas dryers should remain vented to the outdoors.

To seal off the dryer vent permanently or for occasional use, merely disconnect the flexible dryer exhaust pipe from the wall opening and pack the opening with fiberglass insulation. Enclose the insulation in a small muslin sack if it is to be removed often. Remove the insulation and reconnect the pipe each time you use the dryer.

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