Construction Site Reference
What happens between the drawing and the finished structure — how long to cure, when formwork can come off, where the steel goes, and which defects are telling you something went wrong upstream.
Curing
The cheapest operation in concrete work and the one most often cut short. Concrete gains strength by hydration, which stops when the water leaves.
What the code actually requires
Exposed surfaces are to be kept continuously in a damp or wet condition — by ponding, or by covering with sacking, canvas, hessian or similar material kept constantly wet — for at least seven days from the date of placing concrete made with ordinary Portland cement.
Approved curing compounds may be used instead of moist curing with the engineer-in-charge's permission, applied to all exposed surfaces as soon as possible after the concrete has set. Impermeable membranes such as polyethylene sheeting laid closely over the surface also work as a barrier against evaporation.
Many sources state that IS 456 requires 10 or 14 days of curing for concrete made with Portland pozzolana cement, Portland slag cement or mineral admixtures. The clause covering those cements says only that the period of curing "may be increased" — it specifies no number. Treat the longer periods as good practice and as something to agree in the specification, not as a code requirement you can cite.
- Curing that starts late. Evaporation begins as soon as the surface loses its sheen, well before the seven-day clock is convenient.
- Intermittent wetting. Alternate wetting and drying is worse than continuous damp — it drives surface cracking rather than preventing it.
- Counting days from casting rather than from placing the last pour in a lift.
Source: IS 456:2000 clauses 13.5.1, 13.5.2 and 13.5.3. Text verified against the standard as published at law.resource.org.
Formwork striking times
Striking early is one of the few site decisions that can cause immediate collapse. These are minimum periods, and they assume ordinary Portland cement with adequate curing and an ambient temperature not below 15 °C.
| Formwork | Minimum period before striking |
|---|---|
| Vertical formwork to columns, walls and beams | 16–24 hours |
| Soffit formwork to slabs (props refixed immediately after removal) | 3 days |
| Props to slabs — spanning up to 4.5 m | 7 days |
| Props to slabs — spanning over 4.5 m | 14 days |
| Soffit formwork to beams (props refixed immediately after removal) | 14 days |
| Props to beams and arches — spanning up to 6 m | 14 days |
| Props to beams and arches — spanning over 6 m | 21 days |
- The table is a guideline that applies where ordinary Portland cement is used and adequate curing is done. Blended cements gain strength more slowly at early ages, and cold weather slows every figure here.
- "Props refixed immediately" means the soffit form comes off but the propping goes straight back. Removing both at three days is not what the table permits.
- Where strength rather than time governs — early striking, a tight programme, unusual loading — the decision should be made on tested strength, not on the calendar.
Source: IS 456:2000 clause 11.3. Verified against the published standard.
Reinforcement on site
Cover, and how it is measured
Nominal cover is measured to the outermost surface of all reinforcement, links included — not to the centre of the main bar. It is what protects the steel from corrosion and fire, and losing it is the most common cause of premature deterioration in reinforced concrete.
| Exposure | Nominal cover, minimum |
|---|---|
| Mild | 20 mm |
| Moderate | 30 mm |
| Severe | 45 mm |
| Very severe | 50 mm |
| Extreme | 75 mm |
For main reinforcement up to 12 mm diameter in mild exposure the nominal cover may be reduced by 5 mm, and for severe and very severe exposure a 5 mm reduction is permitted where the concrete grade is M35 or above.
- Too few cover blocks, or blocks of the wrong depth. Steel sags between supports and the cover goes with it.
- Walking on top reinforcement in slabs before and during the pour, pushing top steel down into the section.
- Cover blocks made from weak mortar, which crush under the cage and leave the bar sitting on the formwork.
Source: IS 456:2000 Table 16 (clause 26.4.2) and its notes. Verified against the published standard.
Rebar weights and cutting lengths →Detailing limits worth knowing on site
| Item | Limit |
|---|---|
| Minimum tension steel in a beam | As/(bd) = 0.85/fy |
| Maximum tension steel in a beam | 0.04 bD |
| Maximum compression steel in a beam | 0.04 bD |
| Side face reinforcement — required when web depth exceeds | 750 mm |
| Side face reinforcement — total area | ≥ 0.1% of web area |
| Side face reinforcement — spacing | ≤ 300 mm or the web thickness, whichever is less |
Source: IS 456:2000 clauses 26.5.1.1, 26.5.1.2 and 26.5.1.3. Full derivations and worked examples on the formula reference.
Concreting in difficult conditions
Under water
| Requirement | Value |
|---|---|
| Water–cement ratio | not exceeding 0.6, and may need to be lower |
| Minimum cement content, 40 mm maximum aggregate | 350 kg/m³ |
| Water flow through the space being concreted | reduced to less than 3 m/min |
| De-watering by pumping | not while concrete is being placed, nor until 24 hours afterwards |
Concrete cast under water must not fall freely through the water, or it leaches and segregates. It is deposited continuously until it reaches the required height, with the tremie's end piece kept inserted well into the previously placed concrete.
Source: IS 456:2000 clauses 14.1.3, 14.2.1, 14.2.2 and 14.2.4. Verified against the published standard.
Construction joints
Joints are placed at accessible locations so laitance, cement slurry and unsound concrete can be cleaned out and a rough surface created. The recommended method is to brush the surface with a wire brush immediately after initial set and clean it out straight away; where the previous pour was cast against shuttering, the aggregate is exposed with a high-pressure water jet or equivalent.
The prepared surface should be clean and in a saturated surface-dry condition when fresh concrete is placed, and the new concrete thoroughly vibrated near the joint so mortar flows between the large aggregate.
Source: IS 456:2000 clause 13.4. Verified against the published standard.
Common defects and what causes them
Most concrete defects are symptoms of a decision made earlier — in the mix, the formwork or the pour sequence. The following are general site knowledge rather than code provisions.
| Defect | What it looks like | Usual cause |
|---|---|---|
| Honeycombing | Voids with exposed aggregate, usually at the bottom of pours and around congested steel | Under-compaction, mix too stiff, or reinforcement too congested for the aggregate size |
| Cold joint | A visible line where one pour set before the next arrived | Delay between batches exceeding the initial set; poor pour planning |
| Plastic shrinkage cracking | Short random cracks on a slab surface within hours of placing | Surface evaporation faster than bleed water rises — hot, dry, or windy conditions with no early protection |
| Segregation | Coarse aggregate separated from the mortar, often at the base of a drop | Free fall from height, over-vibration, or a mix too wet |
| Bleeding and laitance | A weak, dusty surface layer | Excess water, over-finishing, or working the surface while bleed water is still present |
| Spalling with rust staining | Concrete breaking away over corroded bars | Insufficient cover, or permeable concrete from a high water–cement ratio and poor curing |
| Efflorescence | White salt deposits on masonry or concrete | Soluble salts carried to the surface by moisture movement |
Adding water on site to make concrete easier to place raises the water–cement ratio, which lowers strength, raises permeability and increases bleeding and shrinkage — the root of several rows in this table at once. Workability problems are solved with admixtures or a redesigned mix, not with a hose.
Site testing
| Test | What it measures | What it does not tell you |
|---|---|---|
| Slump | Consistency and workability of fresh concrete | Nothing about strength. A high slump caused by added water means lower strength, not better concrete |
| Cube or cylinder test | Compressive strength at a stated age, usually 7 and 28 days | Nothing about the concrete actually in the structure unless the samples were properly taken, cured and identified |
| Rebound hammer | Surface hardness, correlated loosely to strength | Not a substitute for cube results; affected by surface condition, carbonation and moisture |
| Cover meter | Depth of cover and bar positions in hardened concrete | Nothing about bar diameter or condition without calibration and verification |
| Field density (sand replacement, core cutter) | Achieved compaction of fill against the laboratory maximum | Nothing about the fill outside the test location |
- A cube result only means something if the sample is traceable to a pour, a location and a batch. Unlabelled cubes prove nothing when a result comes back low.
- Test cubes must be cured under the standard conditions, not left beside the pour. Cubes cured on site will typically under-report.
- A 7-day result is an early indicator, not an acceptance criterion. Acceptance rests on the 28-day results assessed as the code requires.
Sampling frequency and acceptance criteria are set out in IS 456:2000 Section 2, clauses 15 and 16. This page does not quote their figures because the relevant pages did not extract reliably from the published document; read them in the standard rather than relying on a secondary source.
Material storage on site
| Material | Practice | Why |
|---|---|---|
| Cement | Dry, covered store; bags clear of walls and off the floor on a raised platform; stacked no more than about ten bags high; used in the order received | Cement takes up moisture from the air and from the ground, losing strength before it is ever mixed. Tall stacks cause warehouse set in the lower bags |
| Aggregate | On a hard clean base, separated by size, protected from mud and run-off | Contamination changes grading and introduces silt and clay that weaken the bond |
| Sand | Stockpiled clear of the ground, allowed to drain | Damp sand bulks by 20–30%, so volume batching from a wet stockpile under-delivers sand |
| Reinforcement | Off the ground on timber or blocks, covered where storage is long, kept clear of oil | Loose mill scale and light rust are acceptable; oil, grease and heavy flaking rust destroy the bond |
| Bricks and blocks | Stacked on level ground, not tipped from the truck | Tipping breakage routinely exceeds the wastage allowance priced into the rate |
How this page was sourced
The curing, formwork striking, cover, detailing, under-water and construction joint sections are quoted from IS 456:2000 with clause numbers, read directly from the standard as published openly by the Bureau of Indian Standards. Where a widely repeated claim is not actually in the code — the 10 or 14 day curing requirement for blended cements — this page says so.
The defects, testing and storage sections are general site practice rather than code provisions, and are labelled as such. Sampling and acceptance figures are deliberately not quoted here.
Free to quote and link. Suggested citation: QSCivilCalc (2026). Construction Site Reference. https://qscivilcalc.com/construction-resources/
