Quality control of concrete at site

Quality control (QC) is the set of checks that ensure the concrete meets the specified strength and durability; quality assurance (QA) is the planned system (documents, responsibilities, audits) that gives confidence that QC is done. IS 456 (Cl. 15 and 16) sets the sampling and acceptance rules.

1. Planning and quality assurance

• A QA plan with job specifications, drawings, method statements, responsible persons and test records.
• Trained staff, calibrated equipment, a site laboratory, and an approved mix design (IS 10262) with trial mixes.

2. Materials

• Cement: check manufacturer's test certificate, date of manufacture, storage on a dry raised platform (IS 456, Cl. 5.7), use in the order received; test for setting time, fineness and strength as needed.
• Aggregates: sieve analysis, silt and organic content, moisture correction, flakiness; stored separately on clean hard floor (IS 383).
• Water: potable, pH not below 6; limits on chloride and sulphate (IS 456 Cl. 5.4).
• Steel and admixtures: approved grade, no loose rust, test certificates.

3. Batching and mixing

• Batch by weight (by volume only for small works; IS 456, Cl. 10), with an accuracy of $\pm2\%$ for cement and water and $\pm3\%$ for aggregates.
• Mix in a mechanical mixer for the minimum time (usually 2 minutes); correct for moisture in aggregate.

4. Fresh concrete control

• Slump or compacting factor on every batch, and the temperature (IS 456 Cl. 7).
• Check the w/c ratio; no extra water at site.

5. Transport, placing and compaction

• Prevent segregation, limit free fall to 1.5 m, and place before initial set (30 min to 2 hours).
• Compact with needle vibrators; clean formwork, proper cover blocks, check bar spacing and cover before concreting.

6. Curing and protection

• Cure properly (IS 456 Cl. 13.5) and keep records; follow hot/cold-weather rules (Cl. 14).

7. Sampling, testing and acceptance

• Sample at the point of placing: 1 sample for 1 to 5 m$^3$, 2 for 6 to 15, 3 for 16 to 30, 4 for 31 to 50 and 1 extra for each additional 50 m$^3$ (IS 456 Cl. 15.2).
• Three 150 mm cubes per sample, tested at 28 days; the sample strength is the mean of three (Cl. 15.4).
• Accept using Table 11: the mean of 4 consecutive results must be at least the larger of $f_{ck}+0.825s$ and $f_{ck}+4$ (M20 and above; $f_{ck}+3$ for M15), and every result must be at least $f_{ck}-3$ N/mm$^2$.
• If the cubes fail, carry out NDT or core tests and, if needed, load tests (Cl. 17).

8. Records and statistical control

Daily records of mix, tests and weather; control charts of cube strength; the standard deviation is monitored and the mix is adjusted. Non-conformities are reported and corrected.