2.8
Expansion and Contraction in Masonry Walls
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Q1: Why do masonry walls crack when exposed to moisture?
Brick absorbs moisture and expands slightly. Without space to expand, compressive stresses develop in the wall, causing cracks. New concrete masonry units shrink as they lose excess moisture from production, inducing tensile stresses that exceed the wall's tensile strength, resulting in shrinkage cracks.
Q2: What is the difference between expansion joints and control joints?
Expansion joints are designed to open or close, allowing masonry room to expand or contract in response to moisture and temperature changes. Control joints are intentionally created weak planes that confine random shrinkage cracks, particularly during initial curing. Both are sealed with flexible materials to prevent air and water infiltration.
Q3: Where should expansion and control joints be placed in masonry walls?
Joints should be strategically placed at geometric discontinuities including corners, changes in wall height or thickness, near openings, and where walls intersect columns. These locations experience the greatest stress concentrations and movement, making them critical for preventing structural damage and maintaining wall integrity.
Q4: How do temperature and moisture affect masonry movement differently?
Thermal movement in masonry is relatively straightforward to measure and plan for. Moisture movement poses greater challenges because new clay masonry units absorb water and expand over time, while new concrete masonry units shrink as they lose excess moisture acquired during production.
Q5: What materials are used to seal expansion and control joints?
Expansion and control joints are filled with backer rods and sealed with elastic joint sealers. These flexible materials prevent moisture and air from entering the room while accommodating the opening and closing movements of the joints as the masonry expands and contracts.
Q6: How do newly constructed concrete masonry walls behave as mortar dries?
As mortar dries in newly constructed concrete masonry walls, the wall undergoes shrinkage, which induces tensile stresses. If these stresses exceed the wall's tensile strength, shrinkage cracks appear randomly throughout the structure, requiring control joints to confine and manage them effectively.
Q7: Why are vertical joints built into masonry walls during construction?
Vertical joints are built into walls when bricks are laid to accommodate expansion and contraction. These joints are filled with backer rods and sealed with elastic joint sealers to prevent moisture and air infiltration while allowing the masonry to move freely without causing structural damage.