Choosing cement, rebar, bricks and sand

A house is only as strong as its weakest bag of cement or length of steel. You don't need to be an engineer to catch most bad material; you need to know what to look at when it arrives.

Cement

• Types. Shops sell Ordinary Portland Cement (OPC), Portland Pozzolana Cement (PPC) and Portland Slag Cement (PSC). For the concrete in small RC houses, NBC 205 asks for OPC that meets the Nepal standard NS 572:2076, which covers OPC of grades 43 and 53. PPC and PSC are commonly used for masonry and plaster. Ask your engineer which to use where.
• NS mark. Cement is on the Nepal Bureau of Standards and Metrology's (NBSM) list of products that must meet a Nepal Standard: OPC (NS 49 and NS 572), slag cement (NS 384) and PPC (NS 385). Buy bags that carry the NS mark.
• Manufacture date. It is printed on every bag. Cement loses strength with age and damp: NBC 205 says cement stored for more than two months should be avoided or tested first. Buy in batches you will use quickly.
• At delivery. Bags should be sealed and dry, with no hard lumps when you press them. Weigh a few bags at random: each should be 50 kg.
• Storage. Stack bags on planks or a plastic sheet, off the floor and away from walls, keep them covered, and use the oldest first.

Rebar (TMT bars)

• Grade. NBC 205 asks for Fe500 TMT bars meeting the Nepal standard NS 191, preferably Fe500D. The "D" grade is more ductile: it stretches further before it breaks, which helps in an earthquake.
• Marks and papers. Rebar is also on NBSM's compulsory list. Check the NS mark and the brand and grade marked on the bars, and ask the dealer for the mill's test certificate for the batch.
• Weight check. A bar's weight per metre is roughly diameter × diameter ÷ 162 kg (with the diameter in mm). A 12 mm bar should weigh about 0.89 kg a metre and a 16 mm bar about 1.58 kg. Bars that are much lighter are under-size.
• Condition. A thin film of surface rust is normal; flaky, pitted rust is not. Don't accept bars that have been heated or bent back straight.
• Storage. Keep bars off the ground on supports, covered and sorted by size.

Bricks and blocks

• Bricks. Good bricks are well burnt and evenly coloured, with straight edges and no cracks, and ring clearly when two are knocked together. NBC 205 asks for bricks with a crushing strength of at least 3.5 N/mm², preferably in the standard size of 230 × 115 × 57 mm. Bricks are not on NBSM's compulsory list, so there is no NS mark to rely on: ask the supplier for test results or have a few tested. Soak bricks in water before laying them.
• Concrete and AAC blocks. Hollow concrete blocks and light AAC blocks are used for infill walls. Check that the sizes are even, the edges aren't crumbling, and the supplier can show strength test results. Lighter walls reduce the load an earthquake puts on the frame.

Sand and aggregate

• Sand should feel gritty, not sticky. Rub a wet handful between your palms: clay and silt leave a muddy stain. In a simple jar test (sand and water with a pinch of salt, shaken and left for a few hours), a thick layer of silt on top is a warning sign. NBC 205 limits harmful material in sand to 5% by weight.
• Aggregate (gitti) should be hard, angular, clean crushed stone, not flaky or dusty. NBC 205 asks for 20 mm and smaller for concrete 100 mm thick or more, and 12 mm and smaller for thinner sections.
• Quantity. Sand and aggregate are sold by the truckload. Measure the truck box (length × width × height) before it is unloaded.

Concrete

• NBC 205 calls for M20 concrete (about 1 : 1.5 : 3 of cement, sand and aggregate) in small RC houses. In cities, ready-mix concrete from a plant can give more consistent quality for slabs.
• Use clean water, and don't let workers add extra water to make the concrete flow: it weakens it.
• Compact it with a vibrator and keep it wet for as long as your engineer specifies. Common practice is at least a week, and longer for PPC.

Waterproofing

• Give the roof slab a slope towards the drains and keep the outlets clear.
• Apply a waterproofing coat (cement-based, polymer or a membrane) on the roof and in bathrooms, taken up the walls and sealed around pipes.
• Test before tiling: block the drains and keep water standing on the roof or bathroom floor for a day or two, then check the ceiling below for damp.

When in doubt, ask your supervising engineer to look at a delivery before it is used. Sending back a bad load costs far less than breaking out a bad column.

Next: Picking hardware and material shops and Building an earthquake-safer house.