Building a garden or patio wall with concrete blocks is perhaps the easiest approach for the beginner-it also is fast. The size of the blocks alone (8 by 8 by 16) makes a wall go together much more quickly and smoothly than with bricks.
There are two basic types of concrete blocks used for build-ing walls: heavyweight and lightweight. A heavyweight block is made with the same ingredients as used in ordinary concrete; a lightweight block is composed of such aggregates as volcanic lava, clay or shale, cinders, and often pumice. The latter has a more porous texture than the ‘true concrete’ block, but offers better insulation.
Most concrete blocks come in two sizes: 8 by 8 by 16, or 4 by 8 by 16. They also come with three or two core holes.
THE USE OF DECORATIVE BLOCKS
In recent years a wide selection of decorative shapes and textures for concrete blocks has become available. You can select blocks that look like rough-cut stones or flagstone slabs. You can buy a variety called slumped block or Slumpstone; the face of each block has individual contours. A wall built of slumped rock gives the appearance of being made of weathered stone or adobe.
One of the most striking of the decorative blocks is screen block, each of which is an individual grillework. Using various combinations, you can build a highly stylized wall.
CHOICE OF FOUNDATIONS
A concrete block wall may be laid on a dry concrete foundation or a wet concrete foundation. A dry foundation is one that has been allowed to set up; a wet foundation is still in a plastic state. The latter produces a sturdier wall but is harder to work with as you must pour the footing and start laying blocks all in one working session. For the week-end handyman, it is better to rely on transit mix for the footing, then proceed to set the blocks on the base as soon as it is firm. Following are some helpful hints for both types of foundations.
Dry foundation. After the poured foundation has set up (2 to 3 days), check on the positioning of the blocks by fitting a dry first course on the foundation. Keep a space between the blocks of about ½ inch. Mark the position of each block and lay it aside.
Wet the foundation. Use a trowel to spread 2 inches of mortar over the foundation, place a damp block on the mixture, and tap it firmly with the trowel handle until it is securely embedded in mortar.
Wet foundation. Before pouring the footing, check the positioning of the blocks by laying out a test row alongside the foundation trench. Then pour the concrete footing and let it set up until it is the consistency of mortar. Set the blocks in place, pressing them into the mix until they are embedded about 2 inches deep.
Set succeeding courses by troweling mortar along the edges of the first course. Put blocks in place, tapping each with the trowel handle to give it a firm bedding.
No matter what type of foundation you use, the top course of the wall must be sealed against water penetration by filling in the hollow cores with a mixture of grout-1 part cement and 5 parts sand. (You can add coarse gravel to extend this mixture.) After the holes have been filled, cap the surface with a coping of wood, brick, or concrete block veneer attached with a smooth layer of sand-and-cement grout.
REINFORCING RODS NEEDED FOR HIGHER WALLS For a wall greater than 4 feet high, set vertical reinforcing rods solidly in the concrete foundation while it is still plastic, spacing them so they will fit through the holes of the blocks in each course. If you use a wet foundation, simply drive the rods through the block holes after laying the first course. The holes surrounding the rods in each course must be filled with a mix-ture of grout and gravel.
USE EPOXY MORTAR TO SET SCREEN BLOCK Because of their make-up and design, screen blocks lack the solidity of ordinary concrete blocks and therefore require a stronger type mortar than is normally used in construction. Epoxy mortar gives an incredibly hard joint, so hard, in fact, that it is much easier to break the block than the joint. Once the wall is completed, the joint is almost invisible.
Epoxy costs about twice as much as ordinary mortars. It con-sists of two components (available from brick or block manu-facturers) which you mix together just before using it.
With epoxy mortar, you usually do not have to reinforce a wall unless it exceeds 8 feet in height. If you need to reinforce, steel rods cannot be used because they will show with most screen block designs. Instead give horizontal reinforcing by using strips of wire mesh between each course.