Checking construction materials on site: steel, cement, bricks and sand

A house is only as strong as what goes into it. The best drawings in the world won’t help if the steel is under-size, the cement has gone hard in the bag or the sand is half mud.

You don’t need to be an engineer to catch most problems. A few minutes of checking at each delivery, plus a handful of lab tests, protects the money you are putting into the structure.

In short: buy steel and cement with the NS mark, check every delivery before it is unloaded, and ask for test results for steel, concrete cubes and bricks.

The NS mark

The NS mark is the Nepal Standards certification mark issued by the Nepal Bureau of Standards and Metrology (NBSM). It shows the factory’s product has been tested against the relevant Nepal Standard and is inspected regularly.

For building materials it is not optional. NS 191, the standard for steel reinforcement bars, has been mandatory since it was published in the Nepal Gazette in 2051 BS, and the government lists both cement and iron rods among the products that must carry the NS mark.

The mark does get withdrawn. NBSM has cancelled certification for cement brands found to contain slag when the standard allows only clinker and gypsum in OPC. So check the brand, not just the logo, and buy from dealers who can show the certificate.

Steel bars (TMT)

What the code requires. Nepal’s building code guideline for houses (NBC 205:2024) asks for Fe500 TMT bars to NS 191, preferably Fe500D, with elongation above 15%. Fe415 is only acceptable for sill and lintel bands.

What to check on delivery:

• Brand and grade marks. Certified bars carry the manufacturer’s mark and grade rolled along their length. Match them to the invoice, and be wary of unmarked bars.
• Diameter. Measure across the ribs with a vernier calliper. An 8 mm bar should be 8 mm, not 7.5.
• Weight. Under-size bars are the most common cheat, and weighing catches it. A steel bar weighs about d² ÷ 162 kg per metre, where d is the diameter in mm. Weigh a full 12 m length and compare:

Bar	Approx. kg per metre	Approx. kg per 12 m bar
8 mm	0.395	4.7
10 mm	0.617	7.4
12 mm	0.888	10.7
16 mm	1.58	19.0
20 mm	2.47	29.6

A bar that is clearly lighter than this is thinner than its label.

• Bend test. Bend a short off-cut of a small bar back on itself. Good TMT steel bends without cracking on the outside of the bend.
• Condition. A light film of rust is fine. Flaking rust, pitting, mud, paint or oil is not, because the code requires bars to be clean of anything that weakens the bond with concrete.

Storage: keep bars off the ground on timber or blocks, covered, and sorted by size.

Ask for: the manufacturer’s mill test certificate for each lot. For a bigger house, send one sample of each diameter to a lab for tensile and bend tests.

Cement

What the code requires. NBC 205 asks for Ordinary Portland Cement to NS 572:2076, and recommends cement with the NS mark if you aren’t running your own tests. It also warns that cement stored for more than two months after it left the factory should be avoided, or tested before use, and hardened cement must never be used.

What to check:

• NS mark and manufacturing date printed on every bag. Old stock in a dealer’s shop is common, especially after the monsoon.
• Feel. Push into the bag. It should feel smooth and free-flowing. Hard lumps that don’t crumble between your fingers mean the cement has taken on moisture.
• Bags intact, with weight as printed. Weigh a few bags.
• OPC or PPC. Ask your engineer which type each job needs. PPC gains strength more slowly, which matters for how long formwork stays in.

Storage: in a dry room, stacked on a raised platform away from the walls, no more than about ten bags high, and used oldest first.

Bricks

What the code requires. Bricks with a crushing strength of at least 3.5 N/mm², well burnt, of standard rectangular shape. The preferred size is 230 × 115 × 57 mm, with tolerances of ±10 mm on length, ±5 mm on width and ±3 mm on thickness.

What to check:

• Size. Measure ten bricks from different parts of the load. They should be close to each other and to the standard size.
• Burning. A uniform deep colour, sharp edges and a clear ringing sound when two bricks are knocked together. Pale, under-burnt bricks sound dull and crumble.
• Soundness. A good brick dropped flat from about chest height onto hard ground shouldn’t break.
• Soaking. Bricks must be soaked before laying, or they suck the water out of the mortar.

Ask for: a crushing strength test from a lab for each new supplier.

Sand and aggregate

What the code requires. Sand must be hard and clean, and dust, clay lumps, soft particles, organic matter, mica and salts together must not exceed 5% by weight. Coarse aggregate must be hard, angular, crushed stone, free of flaky and elongated pieces, graded 20 mm downwards for concrete 100 mm thick or more, and 12 mm downwards for thinner concrete.

What to check:

• Silt jar test. Half-fill a clear jar with sand, top it up with water, shake and leave it for a few hours. Silt settles as a separate layer on top. A thick layer means dirty sand: treat it as a warning sign and get a proper lab test before using it for concrete.
• Rub test. Rub damp sand between your palms. Clean sand leaves your hands almost clean, while muddy sand leaves a stain.
• Aggregate shape. Look for cube-like, angular pieces. Rounded river pebbles and flat, flaky chips make weaker concrete.
• Size. Pick out the largest stones. For slabs and columns they should be no bigger than 20 mm.

Concrete: test what you pour

Even with good materials, the mix and the work on the day decide the strength. NBC 205 requires concrete of at least M20 (a 1 : 1.5 : 3 mix) for footings, columns, beams and slabs, which must reach 20 N/mm² at 28 days on a 150 mm test cube.

• Ask for cubes to be cast from the actual concrete on major pours (footings, each slab) and sent to a lab for 7 and 28-day tests.
• Watch the water. Extra water makes concrete easier to place and much weaker. Ask for it to be mixed to the slump the engineer specified.
• Cure it. Keep new concrete wet for at least a week. Concrete left to dry out in the sun never reaches its strength.

A simple delivery checklist

Material	Check before unloading	Paperwork to keep
Steel	Grade and brand marks, diameter, weight of one bar per size, no heavy rust	Invoice, mill test certificate
Cement	NS mark, manufacturing date, no lumps, bag weight	Invoice with batch dates
Bricks	Size, colour, sound, breakage	Invoice, supplier’s test report
Sand	Silt jar test, rub test	Invoice, lab report if in doubt
Aggregate	Shape, maximum size, cleanliness	Invoice

Keep everything in one file. It is also useful when the municipality inspects the work.

Sources

• NBC 205:2024 Ready-to-Use Detailing Guideline for Low Rise Reinforced Concrete Buildings, Department of Urban Development and Building Construction (DUDBC), section 5 “Construction Materials”: requirements for concrete, cement, aggregate, sand, bricks, mortar and steel
• Nepal Bureau of Standards and Metrology (NBSM), issuer of the NS mark
• Final Report on Detail Study of TMT Bar Manufacturing Industry in Nepal, Department of Industry: NS 191 mandatory since its Gazette notice of 2051 BS
• NBSM provides Nepal Standards certification to 50 products, Republica: cement and iron rods among the products that must carry the NS mark
• Govt scraps Nepal Standard certification of two cement brands, The Himalayan Times (2017): certification withdrawn for slag in OPC

Field checks catch obvious problems but don’t replace lab tests. When in doubt, ask your engineer to send a sample for testing.