Concrete Masonry Units

Home | FAQ | Finishing | Sump Pumps | Foundations

If, in the pursuit of low maintenance, you should think twice before using concrete block for belowground foundation walls, you might want to think again before using it in aboveground walls. Except in the Southwest where houses traditionally are less often built of wood, and in certain other areas where a combination of warm, mildewy weather and termites make wood risky, people tend to avoid concrete block for their homes but use it voluminously in their public and commercial buildings. Masonry’s fire safety and therefore reduced insurance rates overcome our notion that they are ugly and provide poor insulation when we are considering non-residential buildings. We can also readily see the tremendous advantage of masonry for low maintenance in, say, a supermarket wall. But in our houses, the verdict remains: too ugly, too hard to insulate.

We may be shortchanging our homes.

Today walls of concrete masonry units need be neither ugly nor poorly insulated. A visit to a large concrete block manufacturer will dispel that notion in a hurry. There are over 100 different precast masonry units made besides the standard 8-by-8-by-16-inch block, not counting all the decorative blocks used for screening walls, garden walls, and the like, nor counting the many beautiful and colorful payers for sidewalks and drive ways. You can even buy concrete blocks that come with insulation in their cores (see “Insulated Concrete Block”).

The standard block, along with a few other routinely made sizes, can be laid in at least 42 recognized patterns, from what is called ashlar to basketweave, to give even a plain old ho-hum block wall more dramatic visual impact. You can buy concrete brick the same size as clay brick at a much lower price. This unit, and larger ones, are available with rough textured slump or split faces that very nearly resemble cut stone in a wall.

Many other textures are possible. To avoid the plainness of the standard block, manufacturers now make fluted, ribbed, scored, recessed, beveled, serpentine, and taper-faced block. Some of these blocks are so cannily designed that when laid in a wall the mortar joints are not evident, and many people don't realize they are looking at a block wall.

There is even an acoustical block for extra sound absorbance. Slots on the face of this block, which also make a decorative pattern, conduct sound into the block’s hollow core, muffling the noise. Acoustical block is used around bowling alleys, noisy factory rooms and gymnasiums, but might be a possibility in a children’s playroom.

Any block can now be ordered in various colors. The aggregate used can produce various earth tones itself, but the latest advances are in mixing mineral oxide pigments with the concrete. Tan, buff, red, brown, pink, and yellow are now practical. Green and blue are expensive, and so far no method has been found to keep blue permanent. If you don’t want colored block you might choose colored mortar instead It s more practical than colored block because it's much cheaper to color than the block, and for a lot less cost the effects achieved can be nearly as dramatic. Iron oxide makes red, yellow, brown, or black mortar; chromium oxide makes green; cobalt oxide, blue; and carbon black, black or gray.

Glazed Concrete Block

Glazed concrete blocks are another choice rarely given consideration in homes—many people are not aware that they exist. Although expensive, companies with an eye to the ultimate in low maintenance and dramatic color use them on commercial buildings. Walls around institutional indoor swimming pools are often done in glazed masonry where sanitary but beautiful permanent finishes are desired. Both smooth and textured glazed faces are made in several sizes and shapes. Why they are not used more in kitchen, laundry, and bathroom walls can only be explained to me by our preference for wood houses. My only experience with glazed masonry was years ago when we used them for part of the walls in a Grade A dairy milkroom. Their use was thought to be most extravagant for a milkhouse, but we were able to secure them as seconds because of some slight defect, so they cost little more than plain concrete block. The smooth glazed faces were extremely easy to clean compared to the concrete block around them. They were a beautiful pale green color— a little pleasure to look at each day at the beginning and end of work.

Sources of Block—and Information on It

Not all the various kinds of concrete block are available from every manufacturer. Most of the outlets you will buy from don't make their own block, as in years gone by, but order from a large manufacturer. If you want something the larger manufacturer in your area does not make, the company will invariably refer you to one that does. Since shipping concrete long distances is cost prohibitive, you might not be able to get precisely what you want, except at considerable extra expense. But as with clay brick, if your order is large enough, your local manufacturer may be only too glad to custom-make it for you.

He will also have in his office brochures and pictorials displaying some of the latest ideas in masonry, published by the National Concrete Masonry Association. (Their address is 2302 Horse Pen Road, Herndon, VA 22070. No one with an address like Horse Pen Road could possibly steer you wrong, right?)

Also, if you are thinking seriously of building or remodeling with low- maintenance masonry construction, by all means get a copy of the Concrete Masonry Handbook for Architects, Engineers, Builders, by Frank A. Randall, Jr., and William C. Panarese (Portland Cement Association, 5420 Old Orchard Road, Skokie, IL 60077). Even though written for professionals, this guide is easy to understand, with many precise drawings of how various masonry walls should be built. And even if you are not doing the work yourself, a copy is handy to have around. Your builder’s back ground will no doubt be in carpentry, and I have a notion there are timely instructions in this guide even he may not be aware of.

Insulated Concrete Block

There is a new concrete block available that will considerably increase a load-bearing concrete block wall’s R-value-—from less than 1 to 5 or more depending on block density. Called Insul Block, it's a concrete block whose hollow core is filled with an expanded polystyrene liner. The block is almost entirely divided by this insulative barrier, which not only increases thermal resistance but virtually eliminates condensation within a wall, sometimes a problem in concrete block walls (see the section entitled “Leaking”). Though the block looks weak because the two sides are connected only by a narrow bridge at the bottom, it's code-approved for any load-bearing wall situation that an ordinary block can be used for.

Split and scored face block are available, and integral colors can be added as in any concrete block. Used with ½ inch foil-backed gyp sum board on furring strips, a standard 8-by-8- by-16-inch Insul Block wall has an average R value of about 9, depending on block density. With 1 inch of expanded polystyrene and ½ inch of gypsum board, R-values range between 10 and 13.4, again depending on block density. A 2-inch cavity wall with 2-inch polystyrene board insulation in the cavity and an outside brick veneer or stone exterior would give higher R-values yet. Ten- and 12-inch-wide Insul Blocks with these additions have appropriately higher R-values, the highest being 12 to 15 depending on the density of the block.

Insul Block is entirely maintenance-free unless you paint it and should at some point want to change the color or need to add a fresh coat of paint. It is manufactured by Insul Block Corporation (55 Circuit Avenue, West Springfield, MA 01089).

Insulating a Masonry Wall

Insulating a masonry wall can be as simple as backing it with a stud wall and proceeding conventionally, but new developments are making all-masonry load-bearing walls just as easy to insulate as frame walls. To appreciate these developments, one must first realize that insulating a house for energy savings is not yet as cut-and-dried as the R-value maniacs, as I call them, have led us to believe.

The R-value maniacs leapt into instant prominence back when oil prices skyrocketed by saying that if an R-value of 12 in a wall conserved energy and saved money, then an R-value of 24 would make one rich and teach “them damned Arabs a thing or two.” So we all dutifully buttoned up our homes until we found that they were so tight you could barely close a door without popping an eardrum, and inside environments were be coming polluted because there was so little ventilation. Having spent more money than we would have spent trying to insulate the damned Arabs out of our lives, we then had to spend even more money getting the house (and us) to breathe healthfully again while still trying to preserve the supposed savings of all that extra insulation.

What we have learned now is two things: First, heat loss through walls is only a minor part of total heat loss—only 10 to 26 percent of the whole in single family residences. Roofs, floors, windows, and doors (the latter often left open for 5 minutes at a time by children trying to make up their minds whether to go back outside or come in) are more critical areas. (See The Concrete Approach to Energy Conservation from the Portland Cement Association, 5420 Old Orchard Road, Skokie, IL 60077.)

And second, once R-values of about 12 are reached in walls “adding insulation beyond that level would be cost-effective only in very cold climates,” say the editors of Rodale’s Practical Homeowner magazine.

Furthermore, with new electronic gadgetry, architects are learning that the conventional way of figuring heat and cold transfer in a building does not necessarily apply for heavy masonry construction. Because masonry has a much greater heat storage capacity than lighter (wood) construction, it does not respond to temperature fluctuations as fast as wood. Heat transfer is calculated upon the assumption that there is a constant difference between extreme outdoor and indoor air temperatures— “steady state response” in architectural terms. But because heavy masonry walls act as heat or cold sinks or reservoirs, they temper the actual difference between indoor and outdoor temperatures, which in fact are fluctuating all the time. This is called, and can now be measured as, “dynamic thermal response.”

In controlled tests, a heavy masonry wall subjected to an outside cyclic temperature variation of 60°F had heat flow rates calculated by the steady state method to be 32 to 69 percent higher than when calculated by the dynamic thermal response method. In other words, steady state gives an unfair comparison with wood of the same U-value. That’s why a masonry building with the same heat transfer values (U-value) as a lighter wood building will be cooler than the latter on a hot summer day. Heat gain in a wood house peaks at about 4 PM. but not until 8 PM. in a masonry house of the same U-value, when it's cool enough to open a window or step outside. What’s more, the peak is considerably higher in the wood house than in the masonry.

It is not difficult to get close to R-12 insulation value in an all- masonry wall anymore. One way is to use furring strips inside with 2-inch batt insulation between them and also to fill the hollow cores of the block with bulk insulation. Add on interior paneling and you are then in the R- 13 range. Cavity walls filled with 2 inches of insulation between two wythes of masonry result in an R-value of about 10.

Cost of Concrete Block

The cost of a concrete block can vary from place to place, and the cost of the labor to lay them can vary even more. Right now, in 1986 in Ohio, the cost of a standard 8-by-16-inch block is 734 or less, depending on how many you order. Total cost of construction, including labor and mortar is right around $1.70 per standard block.

Fluted, scored, and other fancy load-supporting block are, of course, higher priced. For example, a gray fluted 8 X 8 X 16 costs $1.47 not counting labor; a brown fluted of the same size, $1.68; and a white fluted $1.85. In local outlets in my part of the country, plain Insul Block is $1.40 each, a little higher for the same block with decorated faces. Decorative block for screening or garden walls is $1.26 each and up.

Pricing masonry units is not much help until you know who’s going to lay them. City union masons charge a good bit more than country non union. Of course, a highly paid professional mason will usually lay block fast and well, justifying his high wage.

Mortaring

Much of the quality of a masonry wall depends on the mortar work. A block is laid in place on its bed of mortar and against the preceding block’s head so that the concrete oozes just so-so to fill all the mortar space and bulge out a little. Otherwise, holes in the mortar may result in water penetration or a weak joint. The amount of mortar laid down for a bed or buttered on the block’s head before laying is important. In some instances, even the way the mortar is laid down is dictated by building codes (see the Concrete Masonry Handbook, noted in Appendix B). Globs of mortar dropped on the scaffold and allowed to dry partially should not be thrown back on the mortarboard. Mortar splattered against the wall will have to be cleaned off. Wait until it's partially dry, scrape off with a trowel, then when the spot is dry, rub it out with a broken piece of block.

In laying up a cavity wall, you need to be careful not to drip wet concrete down into the cavity where it would block water from going out weep holes and interfere with the installation of insulation boards in the cavity. (To prevent this from happening, you can lower a board that’s the width of the cavity down into the cavity. Suspend it by a wire at each end of the board and hook these wires over the top block line you are working on. Then the concrete will fall on the board, which you can later lift out.)

MASONRY EQUIPMENT

It is a good idea to have a catalog of masonry equipment even if you don’t intend to do the work yourself. Such catalogs are very educational. They display tools the neophyte never realized there was a job for. They are also excellent for mail ordering hard-to-find tools seldom available locally. The one I have, free for the asking, is from the Masonry Specialty Company ( 4430 Gibsonia Road, Gibsonia, PA 15044). Another good one I have heard about is Goldblatt Tool Company (P0. Box 2334, 511 Osage Street, Kansas City, KS 66110). Catalogs provide a choice of quality tools usually. To the beginner, a trowel is a trowel is a trowel, but a professional will tell you it's foolish to buy a cheap one.

Controlling Cracking

With masonry walls, the biggest low-maintenance worry should not be insulation but quality construction to avoid cracking. Although concrete appears to the neophyte to be a very inert material, it actually shrinks as it dries, just like wood does, only much less, of course. If the blocks have nowhere to move, then they crack along the mortar joints. In fact, large expanses of block walls are almost bound to crack, but that alone is of no significance if the cracking is controlled. You can control cracking by doing a number of things.

First of all, to discourage cracking and to make the wall stronger, fill the entire top row of block with concrete. Some masons also “grout” or “bond” a row in the middle this way, too. Then install reinforcing rod or mesh of various kinds in the solid concrete vertically or horizontally or both. But the main way to control cracking is to leave control joints here and there in the wall. Control joints are not mortared in the usual manner, and when the wall moves, it cracks along this unmortared or partially mortared joint where it's unnoticeable and relatively harmless. Your builder should know all about these matters, of course, but it doesn’t hurt for you to have your masonry handbook in hand. Control joints are not nearly as crucial in smaller single-family residences as in large stretches of masonry blocks, like apartments, but it's good to be aware of them.

A top bond beam should be built into every house wall—that is filling the top row of block completely with concrete (called grouting), and usually, adding reinforcing rod to interlock the bond beam with the wall beneath it. Sometimes a bond beam is added to the middle of the wall. In basement walls, about every fourth row of block ought to be bonded and reinforced. In hurricane and earthquake country, building codes require steel rod reinforcing vertically and horizontally throughout block walls.

Leaking

Moisture transmission through masonry must also be dealt with properly. The objection that concrete blocks “leak” is almost always an exaggeration, but occasionally a driving rain will penetrate some concrete blocks, usually through a flaw in the mortar joint. Stucco (see below) is one way to waterproof block. A cavity wall is also very effective—if water gets through the outer wythe, it runs down the cavity and out the weep holes. Various concrete sealers and paints on the market also take care of the problem.

Condensation of water within block walls in very cold weather can cause frost buildup. Proper insulation almost always avoids this problem, but in very cold climates in high humidity situations (like in a laundry room), frost buildup might occur in the wall. An inside vapor barrier is the antidote.

Surface Bond Masonry

A new way of building with concrete block reduces the cracking problem in concrete block walls and all but eliminates the possibility of leaking. If the method becomes popular, the Concrete Mason Handbook will have to be partially rewritten.

“Surface bond masonry,” as it's called, allows the erection of block walls without mortar in the joints. The first row of blocks are laid in mortar on the footer very level, and then the rest of the blocks are dry-stacked. Anybody who has a strong back can do it. Once the walls are dry-stacked plumb and square, reinforcing rods are inserted at intervals in the concrete block, and the hollow cores around these rods are filled with cement. Then the surfaces of the block are plastered with a special concrete mixture strong enough to make a wall the building codes approve of.

Years ago when an obscure scientist in the U.S. Department of Agriculture first announced success with this process, everyone thought they were hearing another eager researcher trying to publish before he perished, but 25 years later the method is in practice. Many builders are still wary, and block layers are uneasy about using this method.

The secret to this method is in the fiberglass shavings and acrylic bonding additives that are mixed into the portland cement. The glass fibers are about an inch long and add great tensile strength to the concrete.

Other additives increase strength, moisture resistance, and insulating effect. Despite all this strength, the surface bonding actually still has a bit of give to it, which is one reason why it's not apt to crack. A second or third coating may need to be applied, depending on which specific product is used. The brand I’m familiar with is Surewall (W. R. Bonsal Company, P.O. Box 241148, Charlotte, NC 28224). Ask your concrete block manufacturer what is available in your area.

“The method works, really works,” says Fred Smith, president of Fostona Concrete Products, Inc., in Fostoria , Ohio . “But we’ve had a hard time getting the idea accepted by builders in this area.” One minor drawback is that without the 3 mortar joints, customary measurements that have become so habitual in block construction don’t apply. An 8-foot wall, for example, “won’t lay up right,” as one builder complains. It takes 12 standard blocks to lay up an 8-foot wall. Each measures 8 inches tall only if you count the mortar joints, since a block actually measures 7 inches tall. With dry stacking, some additional odd-size block is necessary if the usual 8-foot wall is to be maintained.

Top of Page Home Prev: Brick Next: Stucco

Updated: Thursday, March 5, 2009 12:34