Stack Bond and Oversized Joints in a Boundary Wall
A mason is building a free-standing 3.0 m high × 4.0 m long boundary wall of 100 mm CHB in stretcher bond. The specification calls for a 1:4 cement-sand mortar with nominal 10 mm joints, blocks wetted before laying, and full head and bed joints. To save cutting at the corner, the mason lays the first 1.2 m in stack bond. Because the blocks vary in height, he thickens the bed joints to 18–20 mm to keep the courses level. A plumb bob on the face shows the top of the first 1.2 m is 8 mm out of plumb toward the street. Many head joints are only buttered at the face, leaving voids inside. The owner's representative visits after 60 blocks are laid and before the mortar has fully hardened. The mason says the wall can be straightened later with thick plaster.
1. Evaluate the use of stack bond for the first 1.2 m and its effect on the wall's structural behavior.
Read the worked answer
Stack bond is unacceptable for the first 1.2 m of a structural boundary wall. In stack bond, all vertical joints align, so there is no overlap between units; a continuous vertical crack path exists and the wall has little resistance to flexure or shear. Stretcher bond staggers the vertical joints, so each block is tied to the ones below and the load spreads. The mason's reason-avoiding cuts at the corner-is not a valid trade-off. Corners should be built with half blocks or cut units to maintain the bond. The first 1.2 m is also the most heavily stressed part, near the base. The stack-bonded section should be taken down while the mortar is still green and rebuilt in stretcher bond. If the mortar has hardened, remove and rebuild; do not rely on plaster.
2. The bed joints are 18–20 mm instead of 10 mm. Explain the effects on mortar consumption, shrinkage, and compressive strength, and state what the mason should do.
Read the worked answer
An 18–20 mm bed joint is roughly double the 10 mm nominal. Thick joints increase mortar volume and shrinkage, reduce the masonry's compressive strength because the mortar is softer and less stiff than the blocks, and make the wall more compressible and prone to settlement cracks. They also waste cement. The mason should not use thick beds to compensate for block height variation. He should sort or reject the out-of-height blocks, use a consistent 10 mm bed (typically within ±3 mm), and keep head joints full. If the blocks are too variable, the correct action is to obtain consistent units or adjust the coursing, not to thicken the joints. A straightedge and course line should be used to check each course.
3. Plumb reads 8 mm out over 1.2 m. What corrective action and quality checks should follow?
Read the worked answer
8 mm out of plumb over 1.2 m is about 6.7 mm per metre, which exceeds normal masonry tolerance (often about 5 mm per storey or 10 mm overall). Since only 60 blocks are laid and the mortar is not fully hardened, remove the top courses and reset from the established corner, checking plumb with a spirit level or plumb bob at every course. Do not straighten the wall by adding thick plaster; that adds weight and hides the defect without restoring alignment. After resetting, check line, level, plumb, joint thickness, and full head joints. If the mortar has already hardened, dismantle the affected section rather than force it. The owner's representative should be informed and the correction recorded.