Reinforced and Unreinforced brickwork
Most brickwork is built without reinforcement. Historically, mortar was of low strength, and the bonding of the masonry units provided strength to the panel. Thick walls had to be bonded through the thickness as well as along the length of the wall, and old construction textbooks show in detail the ways of bonding headers with stretchers in alternate courses to avoid any continuous vertical joints. Nevertheless, the bending strength of a panel of brickwork is limited because the bricks themselves are much weaker in tension than in compression.
Modern brickwork is laid in a mortar whose strength is matched to that of the brick units themselves. Walls are mostly laid in half-brick thickness (110mm for standard metric bricks), in stretcher bond. Engaged piers or thicker sections of walls can be joined by using headers in a normal bonding pattern, but for simplicity of laying they are often joined using metal cavity ties between the stretcher-bond leaves.
Entire panels of brickwork can be reinforced to increase their strength to resist wind loads, or to enable a panel of prefabricated brickwork to be handled and transported. Reinforcement can also be used locally to resist potential cracking at the corners of openings, or to restrain freestanding parapets, particularly in seismic areas, where unreinforced brickwork would be too unstable.
Reinforcement can also be used over openings, to form a lintel in the brickwork itself. This has the advantage of showing a clean brick profile without a separate steel lintel, and also has better fire resistance. Special provisions have to be made to support the lowest course of bricks, which are hung below the reinforcement, usually with wire loops in the perpends.
The simplest form of reinforcement for brick walls is to incorporate wire reinforcement into the bed joints. This is commonly done above and below openings, since the corners of openings create a stress concentration and they are common sites for the commencement of a crack.
There are practical difficulties in placing reinforcement in the wet mortar of a bed joint. The common form of reinforcement is a ladder of wires (from 3mm to 6mm diameter longitudinally, with thinner wires welded across them), supplied flat. The reinforcement must be totally enclosed by the mortar. Galvanised steel is likely to be satisfactory in locations of low corrosion hazard, while stainless steel is preferred in maritime or polluted environments.
Horizontal joint reinforcement will provide a slight improvement in resistance to wind loads, if the panel is supported at both ends. It is of little assistance if the panel is supported top and bottom. It does not contribute to vertical loadbearing capacity.
Vertical reinforcement cannot be incorporated into the joints of a single-leaf stretcher bond panel, because there are no continuous vertical joints. In a one-brick (230mm) wall, it would be possible to place small-diameter bars in the 10mm central joint, but they would have to be located very accurately, and placed as the wall is laid. The practical ways of incorporating vertical reinforcement involve creating continuous voids in the bond pattern, or using purpose-made bricks with holes in the right place.
A one-and-a-half brick wall (340mm thick) laid in Flemish bond has small continuous vertical spaces in the central thickness, normally filled with cut bricks. These can be used as cores into which vertical bars can be inserted, and grouted in after several courses have been laid. This detail has been used for heavy work such as air-raid shelters and blast walls, but walls of this thickness are seldom used in normal construction.
Bricks with purpose-made core holes are the most practical solution for accommodating vertical reinforcement. The core holes must be located in the bricks so that they will align in normal stretcher bond. They have to be large enough to allow some tolerance in laying, and also large enough to allow bars of the required diameter to be inserted, lapped where necessary, and grouted with mortar or fine-aggregate concrete.
It is unreliable to insert a storey-height reinforcing bar down the holes after the wall is laid, because any mortar droppings in the holes will be impossible to remove. It is also impossible to put storey-height bars in place first and thread the bricks over them. Therefore the use of this system for laid-in-place walls requires some pre-planning and discussion with the bricklayers involved. It is more easily adapted to prefabricated wall panels in which the bricks are laid out dry on a horizontal table and the joints are then filled with grout.
On the other hand, if the weather resistance provided by a cavity wall is not an issue, the cavity itself could be reinforced and filled with a fine concrete grout. It is still necessary to use cavity ties to hold the leaves together, and to fill the cavity in reasonably small lifts to avoid excessive hydrostatic pressure from the wet concrete.