Efflorescence and rising damp in a coastal brick hostel
A two-storey load-bearing brick hostel near the coast was plastered 18 months ago. On the north side, the ground-floor walls now show white, fluffy salt deposits up to about 900 mm above floor level, the plaster has begun to blister and shell off in patches, and paint on the inner face shows the same band. The foundation is random rubble in 1:6 cement mortar on sandy soil; the plinth is 450 mm above ground, but no damp-proof course was provided and the floor is a 75 mm cement concrete bed laid directly on soil with no waterproofing membrane. Bricks delivered to site had a water absorption of about 22% and were dipped in water for only a few minutes before laying; several mortar joints are 20 mm thick and are not filled. The site engineer proposes to scrape off the salt, apply two coats of cement paint and add one brick course to the plinth. The owner asks you for the source of the salts, an opinion on that proposal, and a remedy that will last.
1. Identify the source of the salt deposit and the paths by which moisture reaches that band of wall. Then say, with reasons, why the engineer's three-step proposal will not cure the problem.
Read the worked answer
The white deposit is efflorescence - soluble sulphates and chlorides (commonly sodium sulphate, loosely called saltpetre) crystallising out as water evaporates from the wall face. The wall is not making the salt; it is being carried in solution and left behind.
Where the water comes from
- Rising damp. There is no DPC at plinth level, and the floor is plain concrete on soil with no membrane, so the subsoil is in direct capillary contact with the brickwork. A 450 mm plinth does not help if the floor bed and the foundation are both open to the soil.
- Porous bricks. At 22% absorption the bricks are well above the 20% normally demanded of good burnt clay bricks; high suction pulls water up faster and higher (typically to 1 m or more).
- Bad joints. Bricks dipped for only a few minutes rob the mortar of its water, so the joints are porous and poorly bonded. Unfilled 20 mm joints are both reservoirs and easy paths.
- Coastal air supplies chlorides and wind-driven rain to the same face.
Why the proposal fails
- Scraping removes the visible deposit, not the salt already in the pores; it will re-appear in one or two monsoons.
- Cement paint is a thin decorative skin. Crystallisation pressure behind it will blister and lift it, exactly as has happened with the earlier finish.
- Adding a brick course raises the plinth line but leaves the existing wall connected to the soil through the floor slab and the rubble footing; the capillary path is untouched.
A durable repair must first cut the moisture path, then allow the wall to dry, and only then be finished.
2. Write the remedial specification you would issue - sequence of work, materials and the key construction details - for the wall and the floor.
Read the worked answer
1. Investigate and strip. Trace the full extent of damp (moisture meter or carbide test). Remove plaster to a line 300 mm above the highest salt band, rake out defective joints to about 12 mm, and remove the loose floor finish along the wall.
2. Cut the rising path - DPC. Provide a continuous horizontal DPC at plinth level by cutting slots in 1 m alternating lengths and inserting 40 mm cement concrete 1:2:4 with integral waterproofing compound (or two layers of 400-gauge polythene / lead sheet, lapped 100 mm). Where cutting is unsafe, inject a chemical DPC (silane/silicone) at 150 mm centres into drilled holes at plinth level. Ensure the DPC is continuous with the floor membrane and is not bridged by plaster or cladding.
3. Floor. Break back 600 mm of the floor bed, relay 100 mm sand, 100 mm PCC 1:3:6, a waterproof membrane turned up 150 mm at the wall, then the screed and finish.
4. External protection. Provide a 600 mm wide plinth-protection apron sloping away at about 1:20, keep the ground level at least 150 mm below the DPC, and check that no rainwater pipe discharges against the wall.
5. Replaster. Use 1:4 cement mortar with waterproofing compound, 12 mm undercoat plus 6 mm finish, curing each coat; on the external face apply two coats of a cement-based waterproof coating.
6. Dry, then decorate. Allow four to six weeks of drying before painting, and specify a breathable coating rather than a sealed cement paint. Do not accept new bricks with absorption above 20% for the coastal face.
3. Which tests would you require on the bricks before accepting fresh supplies for the repair, and what limits would you apply?
Read the worked answer
Bricks should be accepted only against the standard tests, with sampling from each consignment:
- Water absorption (24-hour cold soak) - dry the brick, weigh, immerse for 24 h, re-weigh. Limit: not more than 20% for good quality burnt clay bricks of the classes normally used in exposed walls; the 22% already recorded is grounds for rejection or for restricting those bricks to internal, protected work.
- Efflorescence test - stand the brick in shallow distilled water in a tray and allow it to dry; a deposit of salts covering more than about 10% of the face indicates unsuitability for a wall that is already salt-affected. Nil efflorescence should be insisted on here.
- Compressive strength - test five bricks, report the average; the value must meet the class specified (for example not less than 3.5 N/mm² for a class 3.5 brick, 7.5 N/mm² for class 7.5). Strength and absorption usually move together, so a 22% brick will rarely give a good result.
- Dimensional tolerance and shape - check for warping and for uniform size, since irregular bricks force the thick, unfilled joints seen in this wall.
- Soundness - a clear ringing note when two bricks are struck, and no cracking after soaking.
On site, the batching rule is equally important: soak the bricks until air bubbles stop rising (generally several hours, not a few minutes) so that they do not suck water from the mortar, and keep joints 10–12 mm, well filled and tooled.