Keep Concrete Moist for Maximum Strength

From Popular Mechanics

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, 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 to 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 that a film of water remains on the entire surface of the concrete throughout the curing period.

How to Deal With Ceiling Water Stain

Q: A water stain on my ceiling keeps bleeding through every time I repaint. How can I seal that stain for good?

A: First, you have to be sure that you correct the problem that causes the staining. After stopping the flow of water, apply a coat of pigmented shellac over the stain. Wait a couple of weeks before applying the finish coat to be sure the problem has been solved.


How to Work Safely While Doing Roofing

Q: I own a 2 1/2-story home that needs reshingling, and I could use some tips on working safely on a multi-gabled, steeply pitched roof. I’m also wondering about the most economical and practical way to go about the job. The roof now has three layers of cedar shakes over tar paper and boards of varying widths. Many of the boards have 1- or 2-inch spaces between them. We like the shakes, but the cost seems prohibitive compared to asphalt shingles.

A: For working safely on a roof, wear loose-fitting clothes so you can move around freely. Also wear soft-soled shoes to prevent slipping. High-top sneakers with good ankle support are recommended. Never go on a roof on a wet or windy day. Shingles can be slippery when wet.

On a steep roof, the roofing ladder should be anchored in place with a bracket or framework that extends over the ridge or roof peak. This is especially important for roofs with a 4 to 12 pitch or greater. (The first number indicates how many feet the roof rises vertically for every 12 feet of horizontal distance.) You can also use roof brackets in conjunction with a ladder.

The ladder’s rungs provide a firm footing that will keep you from sliding down as well as convenient hand holds. Position the ladder so you won’t have to reach out to the sides to work. If you do have to reach out sideways, always hold the ladder with the other hand and keep your hips between the ladder rails.

As to reshingling, regardless of whether you use asphalt shingles or shakes, you will have to remove the existing three layers first. If you use wood shingles, these can go over existing boards. However, if you want to use asphalt, you’ll have to cover the roofing boards with plywood sheathing. This is required to span the gaps between the boards so the asphalt shingles won’t sag, forming horizontal grooves in the roof.

You are concerned about the cost difference between shakes and asphalt shingles. Wood shingles are considerably more expensive than asphalt--as much as three or four times the cost--and are more time-consuming to install. A professional might charge five to six times as much for the shakes.


Asphalt shingles come in various weight classes from 210 to 400 pounds per roofing square. A roofing square is 100 square feet. The heavier the shingles, the greater their durability and the higher the cost. The cost for top-of-the-line asphalt shingles, however, compares favorably with that of ordinary wood shingles.

Covering Hole Left by Removing Heaters

Q: I recently installed a central heating system in my home. The old heaters, measuring 20-by-60 inches, are mounted back to back in the wall between rooms. Removing these units will leave a huge pass-through between the rooms.

I want to frame out the openings and repair the walls so the patch won’t be noticeable. Should I use lath and plaster, or should I try to make a flush patch with easier-to-handle wallboard?

A: If you want a perfectly smooth wall, you should cover the entire wall from corner to corner with wallboard. It’s very difficult to achieve perfection with a patch. Depending on how light strikes the wall, you will see ripple shadows at the patched joints.

But, if you intend to hang pictures on this wall or cover it with a textured paint or wallpaper, patching would be adequate. Because of the size of the opening, filling it with wallboard would be best.