Structural Engineering

Formwork is the term used for the process of creating a temporary mould into which concrete is poured and formed. Traditional formwork is fabricated using timber, but it can also be constructed from steel, glass fibre reinforced plastics and other materials.

There are three types of formwork :-

-Beam formwork :-
Formwork for beams takes the form of a three-sided box which is supported and propped in the correct position and to the desired level. The removal time for the formwork will vary with air temperature, humidity and consequent curing rate. Typical striking times are as follows (using air temperature of 7-16 ºC):

i) Beam sides: 9-12 hours.
ii) Beam soffits: 8-14 days.
iii) Beam props: 15-21 days.

-Column formwork :-
This consists of a vertical mould of the desired shape and size of the column to be poured. As a means of keeping the formwork material thickness to a minimum, horizontal steel or timber clamps (or yokes) are used at equal centres for batch filling and at varying centres for filling that is completed in one pour.

The head of the column formwork can be used to provide support for the beam formwork, but while this gives good top lateral restraint it can make the formwork complex. Alternatively, the column can be cast to the underside of the beams. Later on, a collar of formwork can be clamped around the cast column to complete the casting and support the incoming beam formwork.

-Plastic formwork :-
Re-usable plastic formwork is generally used for quick pours of concrete. The formwork is assembled either from interlocking panels or from a modular system and is used for relatively simple concrete structures. It is not as versatile as timber formwork due to the prefabrication requirements and is best suited for lost-cost, repetitive structures such as mass housing schemes.

Stay-in-place structural formwork is generally assembled on site using prefabricated fibre-reinforced plastic. It is used for concrete columns and piers and stays in place, acting as permanent axial and shear reinforcement for the structural member. It also provides resistance to environmental damage for both the concrete and reinforcing bars.

Proprietary systems are used to support vertical formwork while concrete cures, consisting of a series of tubes and ties.

The small dia reinforcement wrapped around the main or longitudinal reinforcement of columns, restraining the lateral expansion of the concrete under compression and thus strengthening the column. Columns with one helical reinforcement take more load than that of tied column due to additional strength of spiral.

Helical reinforcement can protect columns/piles against seismic loads.