Many masonry buildings suffer distress. Proper design and detailing, in most cases, could have reduced the distress to acceptable levels or eliminated it completely. One common cause of distress is stress cracking of masonry elements caused by the absence or improper spacing of movement joints. The proper use of expansion joints or crack-control joints is not well understood.
We have produced thinner and lighter masonry elements in order to reduce both costs and weight. Older masonry buildings were far less prone to cracking since the thickness of the elements provided greater strength and resistance to tensile stresses. Modern thin masonry elements, however, are subjected to much higher stress from the same load since there is simply less material to absorb the load.
Often older mortars were of lower strength and sometimes of different composition than modern high-strength mortars. These softer mortars were more forgiving to building movements.
Proper design and adequate reinforcement can relieve tension without the use of control joints. The designer and contractor must therefore understand that movement joints are intended to provide relief from:
Differential movement at the foundation level.
- Stresses resulting from thermally induced volume changes.
- Differential movement between facing materials and backup materials in veneers or composite systems, or between the building frame and the masonry façade (or another masonry element).
- Stresses resulting from dimensional changes in masonry assemblies due to changes in moisture content.
Cracking caused by foundation movement is quite common and often results in diagonal cracks that radiate from an opening or irregularity in the wall. Proper spacing of vertical joints sometimes can provide more relief for these stresses.
Foundation settlement, however, is so difficult to accurately predict that it would be virtually impossible to develop a crack control joint pattern that would eliminate masonry cracking where there is differential foundation movement.
In existing masonry walls, attempting to introduce new joints is possible but is usually not beneficial unless the foundation movement is expected to continue. Older building foundations usually stabilize after an initial period of settlement, so cutting new joints may be unnecessary.
One older structure showing distress from the failure to account for thermally induced movement is a Richardsonian Romanesque jail currently undergoing renovation. A 1904 expansion created two large courtyards and extended three wings. Neither the wings nor the courtyard walls have any vertical control joints. This corner of the building has suffered substantially more cracking and dislocation than other parts of the building. Because of the historic nature of the building and the strong desire not to change its external appearance, the masonry was repaired without any new joints.
Thermally induced movements in a wall can cause tension stresses to develop. The tension is greatest near the center of the wall length, causing a vertical crack to form, extending from the top of the wall down. Long walls can develop several parallel vertical cracks before there is adequate stress relief to prevent further crack formation. Crack control joints should be provided in structural walls, and through-wall expansion joints should be considered to relieve stresses in long walls. When determining the joint spacing for exterior walls, consider the thermal range for the building's locale and the thermal properties of the materials.
Sometimes horizontal joints are as important as vertical joints, particularly in relieving stresses arising from the differential movement between a masonry veneer and the building structure. Because of limited elastic range and the brittle nature of masonry, it takes very little relative movement to produce significant distress. These walls were rebuilt with simple caulk joints isolating the brickwork from the perpendicular concrete elements and with vertical control joints at the corners.
Each year, where no control or expansion joints have been provided in exterior masonry walls, the brick masonry has "outgrown" the concrete foundation and overhangs it. This growth often causes secondary problems as other connected elements are pushed and dislocated by the moving brickwork. This building would have no problems had joints been provided at 25- to 30-foot intervals, or had a midwall expansion joint been installed.
Masonry construction to avoid stress cracking is the result of planning, appropriate design, and, above all, good communication between design professionals and contractors.
Take time to understand the needs of the structure that will affect the masonry behavior; then plan for ways to relieve the stresses and movements that may, and probably will, develop over time. But don't live with unreasonable expectations of what masonry joints can achieve. It's unlikely that we can account for all possible cracking phenomena in every building, but the effort may mean we all spend less time reviewing and repairing past mistakes.
Article includes a resource list for information on joint planning.