Concrete Quality Control on Site: Slump, Cylinders, Curing and Cover

Structural assessments of existing buildings in Bangladesh frequently find concrete strengths well below what the drawings specified. The design may be correct, but the building is only as strong as the concrete actually placed. Good site quality control (QC) closes that gap.

Before casting: materials

• Cement — check type, brand and manufacturing date; store bags off the ground, protected from moisture.
• Coarse aggregate — stone chips should be clean, well graded and free from clay coatings.
• Sand — coarse sand of adequate fineness modulus; avoid fine, silty sand for structural concrete.
• Water — clean and free from salts and organic matter.
• Reinforcement — verify grade and size against drawings, with mill test certificates; keep bars free from loose rust, oil and mud.

Mix design and batching

Structural concrete should follow a designed mix for the specified strength, not just a nominal volume ratio. Weigh-batching (or ready-mixed concrete from a reputable plant) gives far more consistent results than volume batching with baskets. Above all, control the water–cement ratio: adding water on site to improve workability is the single most common cause of low strength.

Slump test

The slump test checks the workability and consistency of each batch. A slump much higher than specified usually means extra water. Reject batches that fall outside the agreed limits.

Test cylinders

Concrete samples are cast into standard cylinders (or cubes) at the point of placement, cured under standard conditions and crushed — typically at 7 and 28 days. These results are the formal record of the concrete's strength. Sample at the frequency your specification requires, label every specimen with the pour location and date, and use an independent laboratory where possible.

Formwork and reinforcement checks before the pour

• Formwork is rigid, correctly aligned, sealed against grout loss and properly propped.
• Reinforcement matches the drawings: bar sizes, numbers, spacing, laps and hooks.
• Cover blocks are in place to guarantee the specified concrete cover — critical for durability and fire resistance.
• Column ties are at the specified spacing, especially in seismic confinement zones near beam–column joints.

Placing and compaction

Concrete should be placed without segregation and compacted with mechanical vibrators until air is expelled. Poor compaction leaves honeycombing — voids that reduce strength and expose reinforcement to corrosion.

Curing

Concrete gains strength through hydration, which needs moisture. Keep surfaces continuously moist — by ponding, wet hessian or curing compounds — for at least the period your specification requires (commonly a minimum of seven days for ordinary Portland cement concrete, longer for blended cements). Bangladesh's heat accelerates drying, making curing even more important.

Records

A pour register with dates, locations, mix details, slump values, cylinder IDs and test results turns QC into evidence. These records are invaluable for buyers, auditors and any future assessment of the building.

Quality control is one of the four disciplines in our engineering methodology. Talk to us about QC on your project.