Question

Explain the effect of the followings on form work - 1 :design a- Cement type b- Retarders C- Weather condition Provide the specifications on which the form -2 .work should be removed after casting

Answer 1

Framework effecting factors:

Design: Structure members are constructed based on designed load. But before a structure is complete and subjected to all loads assumed during structural design, the structural members are subjected to its self weight and construction loads during construction process.

So, to proceed with construction activities at a quicker rate, it is essential to calculate the behaviour of structure under is self load and construction load. If this can be done and structural member is found to be safe, formwork can be stripped-off.

If these calculations are not possible, then following formula can be used for calculation of safe formwork striking times:

Characteristic strength of cube of equal of maturity to the structure required at time of formwork removal

Cement type: Grade and types of cement

• Grade of cement – Higher cement grade makes the concrete achieve higher strength in shorter time.
• Type of Cement – Type of cement affects the strength development of concrete. For example, rapid hardening cement have higher strength gain in shorter period than the Ordinary Portland Cement. Low heat cement takes more time to gain sufficient strength than OPC.

Retarders:Retarding admixtures are used to slow the rate of setting of concrete. By slowing the initial setting time, the concrete mixture can stay in its fresh mix state longer before it gets to its hardened form. Use of retarders is beneficial for:

• Complex concrete placement or grouting
• Special architectural surface finish
• Compensating the accelerating effect of high temperature towards the initial set
• Preventing cold joint formation in successive lifts.

Thus retarders can significantly increases the time of removal of framework structure.

Wether conditions: The factor that mainly govern is whether temperature.The temperature of whether during the placement affects the lateral pressure greatly because it is one of the factors that govern concrete setting time. When the temperature is low, a greater amount of concrete can be poured before fresh concrete at a lower portion of the form stiffens and support itself.This leads to an increase in liquid head depth which results in higher lateral pressure. That is why cold weather needs to be taken into consideration while forms are designed.

Specifications on which the formwork should be removed after casting:

The removal of concrete formwork also called as strike-off or stripping of formwork should be carried out only after the time when concrete has gained sufficient strength, at least twice the stress to which the concrete may be subjected to when the formworks are removed. It is also necessary to ensure the stability of the remaining formwork during formwork removal.

The rate of hardening of concrete or the concrete strength depends on temperature and affects the formwork removal time. For example, time required for removal of concrete in winter will be more than time required during summer.

Special attention is required for formwork removal of flexural members such as beams and slabs. As these members are subjected to self-load as well as live load even during construction, they may deflect if the strength gained is not sufficient to handle to loads.

To estimate the strength of concrete before formwork removal, the tests on concrete cubes or cylinders should be carried out. The concrete cubes or cylinders should be prepared from the same mix as that of the structural members and cured under same circumstances of temperature and moisture as that of structural member.

When it is ensured that the concrete in the structural members has gained sufficient strength to withstand the design load, only then formworks should be removed. If possible, the formworks should be left for longer time as it helps in curing.