Cement and concrete quality control on site and in the lab

Concrete is made from variable natural materials, often on a busy site, and its most important property — 28-day strength — is only known four weeks after it is placed. By then the next floor may already be built on it. Good quality control catches problems early, using tests on the materials, on the fresh concrete and on hardened specimens, so that weak concrete is found before it becomes an expensive structural problem.

QC is shared between the cement and ready-mixed concrete (RMC) suppliers, the site QC team, and the testing laboratory. Each needs clear procedures, calibrated equipment and complete records.

Think of concrete QC as a chain: right materials, right mix, right placing and compaction, right curing, and honest testing. A weak link anywhere shows up as low cube strength later.

Every consignment of cement should be checked on arrival and before use. The manufacturer's test certificate tells you what the factory measured, but the site or third-party lab should verify key properties at an agreed frequency, especially for important structures.

• Bag marking: cement type and grade (e.g. OPC 43, OPC 53, PPC), BIS standard mark and licence number, manufacturer, week and year of packing, net mass.
• Manufacturer's test certificate for the lot, showing compliance with the product standard (IS 269 for OPC).
• Condition: no lumps that do not crumble between fingers, no torn or wet bags. Hard lumps indicate moisture pick-up.
• Age: use cement in order of delivery (first in, first out). Strength can fall noticeably after about three months of storage; older cement is usually retested before use.
• Storage: dry, weatherproof shed on a raised floor, bags stacked away from walls, stacks limited in height (often about 10–15 bags).
• Lab tests on representative samples: normal consistency, initial and final setting time, fineness, soundness and mortar compressive strength (IS 4031 series), plus chemical tests where required.

Values are typical of the current IS 269; always check the edition and amendments named in your contract.

A mix design chooses proportions of cement, mineral admixtures (fly ash, slag), water, fine and coarse aggregate and chemical admixtures to give the required workability, strength and durability at the lowest practical cost. In India the usual guideline is IS 10262; other countries use ACI 211 or similar methods.

Because concrete strength varies, the mix is designed for a target mean strength higher than the specified characteristic strength (fck). A common form is f'ck = fck + 1.65 × s, where s is the expected standard deviation. For M25 with an assumed s of 4 MPa, the target is 25 + 1.65 × 4 = 31.6 MPa. IS 10262 also gives a second, fixed-margin formula; the higher of the two is used.

• Water–cement ratio is the main control on strength and durability: lower w/c gives higher strength and lower permeability.
• IS 456 sets a maximum w/c ratio, minimum cement content and minimum grade for each exposure condition (mild to extreme).
• Workability is adjusted mainly with water-reducing or superplasticising admixtures, not extra water.
• Aggregate grading (IS 383) and moisture content affect water demand. Batch weights must be corrected for aggregate moisture every day, more often in rain.
• Trial mixes in the lab confirm slump and 7- and 28-day strength before a mix is approved for site use.

Tests on fresh concrete are cheap, fast and give results while you can still do something about them — such as rejecting a truck before the concrete is placed.

• Check the RMC delivery ticket for every truck: grade, mix ID, quantity, time of batching, admixture and any water added.
• Test slump at an agreed frequency (often the first truck and then regularly, or every truck for critical elements) and whenever the concrete looks different.
• Reject or hold concrete that exceeds the allowed time since batching (often around 90–120 minutes, or as specified) or that is outside the slump tolerance.

Cubes are cast from samples taken at the point of placing. A sample is a set of cubes (usually three for each test age) from one batch. IS 456 gives a minimum sampling frequency for each grade of concrete on each day of concreting. The table below shows the typical schedule; your project may require more.

• At least one sample is normally taken from each shift.
• Each sample usually gives three cubes for 28-day testing; extra cubes are cast for 7-day results, early stripping or acceptance of special elements.
• Spread sampling across the pour so that the samples represent the whole day's work, not just the first truck.
• Label every cube with a unique ID linked to the pour location, grade, truck or batch number, date and time.

Curing keeps concrete moist and at a suitable temperature so that cement can keep hydrating. Poor curing reduces strength and, more importantly, makes the surface layer weak and permeable, which harms durability.

• Structural concrete: IS 456 typically calls for at least 7 days of moist curing for OPC concrete, and at least 10 days when mineral admixtures or blended cements are used or in hot, dry weather.
• Methods: ponding, wet hessian, sprinkling, plastic sheeting, or a curing compound applied as soon as the surface water has gone.
• Test cubes: kept covered at about 27 ± 2 °C for the first 24 hours, then demoulded and cured in water at 27 ± 2 °C until testing. Cube curing tanks need a daily temperature record.
• Do not leave site cubes in the sun or let them dry out before they reach the lab. Field-cured cubes (cured beside the structure) are sometimes made to estimate in-place strength for formwork removal; they are not used for acceptance.

Compressive strength is measured by crushing 150 mm cubes (IS 516) or cylinders (ASTM C39). Strength = maximum load / loaded area. The 28-day result is the basis for acceptance; the 7-day result is an early warning.

For OPC concrete the 7-day strength is typically about 65–70% of the 28-day strength. Blended cements (PPC, PSC) and mixes with fly ash or slag gain strength more slowly, so their 7-day ratio is lower and some projects specify acceptance at 56 days. A low 7-day result is a reason to investigate early — check batching records, slump, cube making and curing — not an automatic rejection.

1. 1Measure and weigh each cube; note any defects.
2. 2Place the cube with cast faces against the platens and load it centrally at the specified steady rate.
3. 3Record the maximum load and the failure pattern.
4. 4Calculate each cube strength and the sample mean (three cubes).
5. 5Check that individual cubes are within ±15% of the sample mean (typical IS 456 rule).
6. 6Enter the sample result into the running acceptance calculation for that grade.

Example: three M25 cubes failing at 640, 610 and 675 kN give 28.44, 27.11 and 30.00 MPa, a mean of 28.5 MPa. All three lie within ±15% of the mean, so the sample is valid.

Because some variation is normal, concrete is not rejected because one cube is low. Acceptance is judged statistically on samples. The rules below are the typical IS 456 criteria (as amended); always check the current edition and the project specification.

• s is the established standard deviation of the plant or site, calculated from at least the number of results stated in IS 456 (commonly 30). Until enough results exist, the assumed value from IS 456 / IS 10262 is used.
• Concrete is deemed not to comply when the criteria are not met. The quantity represented is usually the concrete placed between the samples concerned.
• Non-compliance triggers an investigation: review records, test cores (IS 516 / IS 1199 parts on cores), carry out non-destructive tests, and if needed a structural assessment or load test before any decision.

Non-destructive tests (NDT) check concrete already in the structure. They are fast and do not damage the element, but they measure properties related to strength rather than strength itself, so they work best for comparing areas and for finding weak zones. They are calibrated against cores or cubes from the same concrete when strength estimates are needed.

• Higher pulse velocity generally means denser, better concrete. As a rough guide, velocities above about 4 km/s usually indicate good concrete and values below about 3 km/s indicate doubtful concrete; use the grading table in the current standard.
• Combining rebound hammer and UPV (sometimes called the SonReb approach) gives better strength estimates than either alone, once correlated with cores.

Concrete records must let anyone trace a cube result back to the exact pour, mix, materials and people involved, and forward to the decision made. Without that link, a low result cannot be investigated properly.

• Material records: cement test certificates and lab results, aggregate tests, admixture certificates, water tests.
• Mix design and trial mix reports, with approval status and any revisions.
• Pour records: date, element and grid location, grade, quantity, RMC tickets, start and finish times, weather.
• Fresh concrete test results: slump, temperature, density, with time and truck number.
• Cube register: cube ID, casting date, sample location, curing record, test date, dimensions, mass, load, strength, failure type, tester.
• Equipment records: calibration certificates for the compression testing machine, balances, thermometers, slump cone and cube mould dimension checks.
• Acceptance calculations, non-conformance reports, investigation results and corrective actions.