Casting and curing temperature; concreting in Kathmandu

Casting and curing temperature change the speed of hydration and the final structure of the gel. A high temperature gives high early strength but a lower 28-day and long-term strength (non-uniform gel, coarse pores, drying cracks). A low temperature slows early strength but improves later strength, as the gel forms more densely; near freezing, hydration almost stops and fresh concrete can be damaged by ice.

Concreting in Kathmandu

Kathmandu valley has a temperate climate: mild to warm summers (up to about 30 °C) and cool winters (night temperature 0 to 5 °C in December to January) with fog and little rain in winter. The temperature is mostly favourable, but special care is needed in winter and in the pre-monsoon dry period.

1. Season: prefer casting in spring and autumn, between 10 and 30 °C; avoid pouring in the cold early morning of Poush to Magh, or heavy monsoon rain unless protected.
2. Cement: OPC 43 or PPC (Nepal Standard), with fresh stock; PPC is acceptable with extended curing. For durable work follow the minimum cement content and maximum w/c of IS 456 Table 5 for the exposure (mild or moderate: w/c 0.50 to 0.55).
3. Winter: keep the concrete at 5 °C or higher at placing (IS 456, Cl. 14.1); use warm mixing water, cover with plastic and straw/jute at night, delay formwork stripping, avoid casting if frost is expected.
4. Summer (dry spells): cast in early morning or evening, wet the forms, shade, and prevent plastic shrinkage cracking (IS 456, Cl. 14.1).
5. Curing: keep moist for at least 7 days for OPC and 10 days for PPC (IS 456, Cl. 13.5.1) with ponding or wet hessian, and use a curing compound where water is scarce.
6. Mix: use a water-reducing admixture, keep w/c low, compact properly, and cast cubes cured under site conditions for stripping decisions.

Thus normal procedures with attention to cold-night protection and proper curing are enough for Kathmandu.