The effect of ambient air temperature on stripping time of formwork horizontal member of high strength concrete in context of Kathmandu Valley

Abstract

Demand of High Strength Concrete in construction of high rise building is increased, due to the stability and earthquake resistance. Especially in Kathmandu Valley the number of high rise building construction is rapidly increased. The major component of the concrete construction is also the estimation of strength of concrete and the stripping time of the formworks. Maturity method is the appropriate technique for predicting concrete strength based on which formwork removal time is identified. In this thesis present some guide line of stripping time of formwork of concrete, which are identified by the experiment on the lab and the maturity method concept.

The experiment was carried with the Ordinary Portland Cement of 53 grade. Fine Aggregate and Coarse Aggregate collected from Belkhu and Mahadev Besi. powerflow 2239 Plastisizer and micro silica were used for prepared concrete mix grade M40 and M60. The maximum size of the coarse aggregates was 12.5 mm. The concrete mix design was adopted as trial and error method. The concrete mixed proportion for M40 concrete was 0.4: 1: 1.65: 2.92 (W: C: FA: CA) and Admixture (MC-powerflow 2239) 1.2% by weight of cement and micro silica 4% by weight of cement. The concrete mixed proportion for M60 concrete was 0.29: 1: 1.35: 2.19 (W: C: FA: CA) and Admixture (MC-powerflow 2239) 2% by weight of cement and micro silica 4% by weight of cement. The compression test carried out at different age (days) of concrete from 1, 2, 3, 4, 7, 14, 21, and 28 days curing at temperature range of 5 o C -10 o C, 10 o C -15 o C, 15 o C 20 o C, 20 o C -25 o C, 25 o C -30 o C, and 30 o C - 35 o C. All the experiment was conducted in IOE lab.

Stripping time of the concrete grade M40 and M60 was identified on the basis of various literatures and the experiment. On the basis of previous experiment and literatures it has IV been assumed that 70% of design strength of concrete is reasonable to remove the form work especially from the bottom of the slab and beam. This concept has been used to identify maturity of the concrete at deferent temperatures from 5 o C to 35 o C in Kathmandu valley. It has been obtained from the experimental that the stripping time of concrete grade M40 for slab varies from 1.72 days to 31.09 days depending open the ambient temperature From 40 o C to 0 o C and for M40 concrete varies from 1.67 days to 30.17 days. Similarly stripping time of concrete grade M40 for beam varies from 3.44days to 62.18 days depending open the ambient temperature From 40 o C to 0 o C and for M60 concrete varies from 3.34 days to 60.34 days. It was also observed that stripping time of M40 and M60 concrete is 2 to 14 days earlier than the M20 concrete.