Interlocking bricks (ISSB): how they are made and used

What interlocking bricks are

Interlocking bricks, also called interlocking stabilised soil blocks (ISSB) or compressed stabilised earth blocks (CSEB), are made on or near the building site. Damp soil mixed with a stabiliser is compressed in a hand or powered press, and the mould gives each block raised keys and recesses, so a course sits locked onto the one below.

UN-Habitat’s 2009 manual, Interlocking Stabilised Soil Blocks: Appropriate earth technologies in Uganda, presents them as an alternative to fired brick. It notes that fired bricks are often uneven and can need up to 3 cm of cement mortar between them, while the interlocking shape means little cement is needed in the joints.

What they are made of

The manual names cement as the most common stabiliser, but warns that it is not recommended for soils with a high clay or salt content; lime works better for clayey soils. It recommends murram soil, which lies below the topsoil, so the topsoil is removed first.

The amount of cement is set by testing the soil, not by habit. Two simple tests are described:

• Sedimentation test: fill a bottle a quarter with soil and three quarters with water, shake it hard and wait 30 minutes. Gravel settles first, then sand, silt and clay, which shows the proportions.
• Shrinkage test: pack moist, sieved soil into a greased wooden box 60 × 4 × 4 cm, leave it a week in the shade, and measure how much it shrinks. The result tells you how much stabiliser to add.

How interlocking bricks are made

Following the UN-Habitat manual, production runs in this order:

1. Remove topsoil and dig out the murram.
2. Sieve the soil and let it dry, for a compact, smooth block.
3. Mix dry soil with cement, measured as wheelbarrows of soil per bag of cement, then add water.
4. Fill the press and compress until the machine’s stop shows full compression.
5. Check each block as it comes out; a poor one goes back into the mix.
6. Stack blocks in layers of five and cover them with grass or polythene.
7. Leave them to cure. Full curing takes 28 days, but blocks can be used before that.

As a quality check, the manual suggests putting a fully cured block in a bucket of water for a day and seeing how it holds up. It adds that training a team takes one to two weeks and that the press it describes weighs 140 kg, light enough to move between sites.

Interlocking block sizes

There is no single size; each press makes its own. The Auroville Earth Institute in India lists these interlocking blocks on its page on hollow interlocking blocks:

Interlocking block sizes

Block	Size (L × W × H)	Note from the institute
245	245 × 245 × 95 mm	laid with 5 mm cement-sand mortar
295	295 × 145 × 95 mm	single-storey buildings only
D 300	299 × 150 × 100 mm	30 × 15 × 10 cm module with mortar
D 250 / Innobloc 250	250 × 250 × 100 mm	25 × 25 × 10 cm module with mortar

Ask the block maker for the size their press produces before you draw the walls, since door and window openings should fall on whole blocks.

Where interlocking bricks work and where they need reinforcement

The interlock helps alignment, not strength against sideways forces. The Auroville institute states plainly that interlocking blocks do not have a higher shear strength than ordinary compressed earth blocks. For resistance to earthquakes, cyclones and floods, it uses hollow interlocking blocks reinforced at regular intervals with reinforced concrete cast into the holes, and sets strict limits: block height consistent to 1 mm, and a 5 mm cement-soil-sand mortar mixed 1:1:3.

The institute limits its 295 block to single-storey buildings; anything taller or heavier needs an engineer’s design with reinforcement.

Water tanks and below-ground walls

UN-Habitat describes curved double interlocking blocks used for water tanks, pit latrine linings and septic tanks, with above-ground tanks of up to 30,000 litres and below-ground tanks of up to 200,000 litres; the round shape and the interlock resist the water pressure. Walls that hold water or soil also need a waterproof face. JUYONS lists basements and pools among the uses of its K11 Polymer Cement Waterproof Coating, a brush-on cement coating that Kitovu holds in stock in Zanzibar.

What to check when you buy

1. The block size and the number of blocks per square metre of wall.
2. The stabiliser and its share: cement or lime, and how many wheelbarrows of soil per bag.
3. How long the blocks have cured; full curing is 28 days.
4. That blocks are even in height, since thin mortar joints depend on it.
5. Whether hollow blocks are available where the walls must be reinforced.

Interlocking bricks in Tanzania

The UN-Habitat manual is written from Ugandan projects. When buying interlocking bricks in Tanzania, ask the block maker which soil, stabiliser and press they use, and for the soaked-block test described above. For conventional walling, compare concrete blocks; for ventilation openings in a block wall, see louver blocks; for paths and yards, see paving blocks.