Concreting in Delhi's Winter: What IS 7861 Calls Cold Weather, How Much Slower a Slab Gains Strength, and How to Plan a December or January Pour

Concrete can be placed in a Delhi winter, and most houses under construction keep pouring through December and January. What changes is the speed. Cement hydrates more slowly as the temperature falls, so a slab cast in January needs longer in its formwork, longer on its props and longer under cover than the same slab cast in October. The Indian code for this is IS 7861 (Part 2), the recommended practice for cold weather concreting, which the Bureau of Indian Standards lists as active (published 1981, reaffirmed 2021) and which IS 456:2000 clause 14.1 names as the procedure to follow 'during hot or cold weather'. It defines cold weather concreting as 'any operation of concreting done at about 5°C atmospheric temperature or below'. The India Meteorological Department's climatological normals for Safdarjung put the mean daily minimum at 8.4°C in December and 7.5°C in January, so an ordinary winter night sits a little above the code's line and a cold-wave night falls below it. This guide sets out what the code asks for, what IS 456 says about formwork and curing times in the cold, the practical rules that follow for a house site, and how the winter GRAP restrictions fit around a pour.

How Cold Does Delhi Actually Get?

Colder at night than most owners plan for, and foggier. The IMD's climatological table for New Delhi (Safdarjung), 1991 to 2020 gives a mean daily minimum of 13.0°C in November, 8.4°C in December, 7.5°C in January and 10.6°C in February, against mean daily maximums of 28.4°C, 22.8°C, 20.1°C and 24.2°C. The same table counts an average of 19.2 fog days in December and 21.6 in January. Two things follow for a site. First, the day and the night are different seasons: a slab poured at 2 pm in January goes in at close to 20°C and spends its first night near 7°C, so the precautions are about the night, not the pour. Second, these are averages; the coldest nights of a cold wave fall below them, and a minimum-maximum thermometer hung on the site tells you more than the city forecast. The code's three categories (clause 3.3) are: temperature below 5°C but above freezing; frost at night only; and severe frost day and night. Delhi's plain sits in the first category on its coldest nights and outside the code's scope on most others, and it never needs the heated enclosures and steam curing the code describes for the second and third.

What Does Cold Do to Fresh Concrete?

It slows the chemistry, and if the concrete freezes it is ruined. Clause 3.1 lists four effects: delayed setting, so that 'the hardening period necessary before the removal of forms is thus increased'; freezing of fresh concrete, with the risk of 'irreparable loss of strength'; repeated freezing and thawing during hardening; and cracking from temperature differentials inside a member, which the code says are 'likely to occur in cold weather at the time of removal of form insulations'. Appendix A gives the scale of the slowdown: during the first 28 days, strength 'could be lowered by 40 to 60 percent, even at temperature 5 to 10°C', though properly protected concrete catches up and its long-term strength is 'not significantly lower'. Freezing is the real danger, because water expands by about 9 percent as it freezes and concrete that is still plastic when that happens is spoilt. The code therefore sets a prehardening period, the time fresh concrete must be kept above freezing until it reaches about 2 N/mm2. Table 6 gives it for ordinary Portland cement without an accelerator: 24 hours for M20 concrete at 20°C, 32 hours at 15°C, 46 hours at 10°C and 71 hours at 5°C; for M25 the figures are 22, 30, 42 and 65 hours. Delhi rarely freezes at site level, so the practical reading is this: a January slab takes two to three days to reach the strength an October slab reaches overnight, and everything that follows is timed from that.

What Does the Code Ask You to Do?

For the first category, Table 1 is short: keep the formwork in position longer or use rapid-hardening cement; cover the top of the concrete with insulating material; insulate steel formwork; and make sure the concrete reaches the point of placing at not less than 5°C. Clause 6.1 adds that after placing and compaction the concrete must be kept above 2°C 'until it has hardened thoroughly'. The code's main lever is conserving heat rather than adding it. Clause 5.1 says that 'for the winter conditions prevailing in India satisfactory concreting can be achieved mainly by conserving the heat of hydration of cement with insulations and insulated formworks', and recommends a cement content of not less than 300 kg per cubic metre. Where heating is needed, it is the mixing water and, if necessary, the sand that are warmed (clauses 4.2 and 4.3), never the cement, and the hot water must meet the aggregate before the cement is added, to avoid a flash set. Clause 8.1 forbids placing concrete on a frozen subgrade or against reinforcement or formwork covered with frost, and clause 8.2 asks for every surface to be covered 'as soon as the concrete has been placed': straw blankets, sacking, tarpaulins, expanded polystyrene and plastic sheeting all count as insulation, used with an air gap. On a Delhi house site this means hessian and a polythene sheet over the slab the same evening, and the sides of columns and beams left in their shuttering for longer.

How Long Must Formwork and Props Stay?

Longer than the periods most contractors carry in their heads, which are the warm-weather ones. IS 456:2000 clause 11.3.1 gives its familiar stripping periods only 'where ambient temperature does not fall below 15°C and where ordinary Portland cement is used and adequate curing is done': 16 to 24 hours for the vertical formwork to columns, walls and beams; 3 days for slab soffits with the props refixed; 7 days for beam soffits; 7 days for props to slabs spanning up to 4.5 m and 14 days over 4.5 m; and 14 days for props to beams up to 6 m and 21 days beyond. It then says that 'for other cements and lower temperature, the stripping time recommended above may be suitably modified'. IS 7861 (Part 2) does the modifying in its Table 5, for a member carrying only its own weight. For ordinary Portland cement concrete at an air temperature of about 3°C, against about 16°C: beam sides, walls and columns 5 days instead of 1; slabs with props left under 7 days instead of 3½; beam soffits with props left under 14 days instead of 7; removal of props to slabs 14 days instead of 7; and removal of props to beams 28 days instead of 14. Rapid-hardening cement roughly halves each cold-weather figure. Delhi's January nights are warmer than 3°C and its afternoons are near 20°C, so the right figure for a given slab lies between the two columns, and the code's own answer is to stop guessing: clause 10.4 asks for cubes cast from the same concrete and kept beside the structure, 'exposed to the same temperature and humidity conditions', and tested before stripping. Our stage-wise quality checklist lists what else to record at each pour.

Does a Cold Slab Need Less Curing?

It needs different curing, not less. In near-freezing conditions, clause 8.3 says water curing 'is not necessary' because evaporation is slight, and it warns that concrete just released from insulated formwork must 'on no account' be saturated with cold water; the surface temperature is to be brought to the air temperature gradually, under plastic sheet or tarpaulin lapped and made wind-tight. Delhi's winter days are not near freezing, and a slab in January sun at 20°C does dry, so the sheet is the right answer at night and light wetting under it is the right answer by day; ponding a slab with cold water at dusk is the one thing not to do. The minimum periods in IS 456 clause 13.5.1 still apply, and they are longer than many sites allow in any season: at least 7 days of moist curing for ordinary Portland cement, at least 10 days where mineral admixtures or blended cements are used, and an extension to 14 days recommended for blended cements. In cold weather slow hydration makes each of those days count for less, which is why the code links the removal of protection to measured strength rather than to the calendar.

Which Cement, and Can You Use an Accelerator?

The code's preference is for cement that gives heat and early strength. Clause 4.4 says rapid-hardening Portland cement is preferable for winter concreting, and clause 5.1 notes that extra ordinary Portland cement or an accelerating admixture, 'when used with proper precautions', shortens the protection period and the formwork cycle. The other side of that coin is in IS 456: blended cements, which contain fly ash or slag, gain strength more slowly and need the longer curing described above. For a structural slab poured in late December or January we therefore specify ordinary Portland cement of 43 or 53 grade and plan the formwork around the cold-weather table, and keep Portland pozzolana cement for the masonry, plaster and non-structural work that follows; our guide to cement types explains what each standard guarantees. On accelerators, clause 5.2 permits calcium chloride within the chloride limits of IS 456, but lists the conditions under which the corrosion risk to reinforcement rises: cracked concrete, alternate wetting and drying, small cover and permeable concrete. Clause 5.2.7 is blunt that admixtures 'will not lower the freezing point of concrete to any significant degree' and that harmful materials used to protect concrete from freezing 'shall not be permitted'. On a house site the safer route is the one the code puts first: a slightly richer mix, warm water on a cold morning, and patience with the props.

How Does GRAP Fit Around a Winter Pour?

GRAP decides whether you can pour at all, and the cold decides how long the pour takes to harden. The Commission for Air Quality Management's winter restrictions apply from the day a stage is invoked, and last winter Stage III, which stops most outdoor construction, came in three spells between 11 November 2025 and 22 January 2026, about six weeks in all, as our GRAP construction guide documents. On 28 September 2026 the Commission revised the schedule for the coming season: it merged the repeated dust-control actions in Stage I, moved the actions on diesel generator sets up from Stage II to Stage I, and kept unchanged the rule that construction sites on plots of 500 square metres and more must register on the dust-control web portal. For a house under construction the sequence that works is to pour the slab before a Stage III order is likely, keep it covered and protected through the ban (the Stage III list targets dust-generating work, so check the current order for what it allows on your site), and use the ban weeks for the indoor trades the stage still permits. A slab cast on a Tuesday needs its props for weeks either way; the difference in winter is that the next slab's shuttering, bar bending and casting may not be permitted when the props come off, so the floor-to-floor cycle stretches further than the cold alone would stretch it. Our month-by-month guide to construction timing explains why we recommend reaching the roof slab before November and why a February start suits most owners.

What Should the Site Diary Record?

The code is specific. Clause 9.1 asks inspection staff to record the date, time, outside air temperature, the temperature of the concrete at placing and the general weather, with readings at several points on the concrete surface, corners and edges. Clause 9.3 asks for the concrete temperature at placing, at the time the protection goes on, and three times a day until the concrete is safe from frost. Clause 10.3 recommends that before winter work starts the chosen mix is tested in the laboratory cured at 5°C, and clause 10.4 that site-cured cubes are tested before stripping. For a homeowner the checks are simpler: ask what time the pour will start and finish (late morning to mid-afternoon in January, so the concrete goes in warm and is covered before dusk), see the hessian and sheet go on the same evening, note the date the slab was cast against the date the props come out, and ask for the cube results before the next floor is loaded. The construction agreement should already name the tests and the engineer responsible for them.

On the houses we build, Nirman Ved plans the frame so that the slabs most exposed to the cold are cast early in the day, covered the same evening and left on their props for the cold-weather periods rather than the warm-weather ones. Winter adds time to a project, not a surcharge: our house construction packages start from ₹1,500 per sqft of built-up area, and the schedule written into the agreement allows for the slower cycle and for the GRAP weeks; our construction timeline guide shows how the stages add up. We are trusted by DAV Pitampura and DPS Dwarka for campus civil repairs and maintenance. To plan a build that starts this winter or in February, call +91-7838355055 or book a free site visit.