utilities

Formwork Technical Data

Formwork Design

The design and final proportions of GRC permanent formwork takes into account factors such as the detail of the permanent works, the imposing loads from those permanent works and from construction operations, plus self-weight and other environmental factors.

Once this criteria has been decided, the formwork panels can be designed to accommodate any span width up to 1200mm – or wider by incorporating deeper corrugations – and in thickness capable of carrying varying depths of concrete to meet DOT requirements.

The table below provides typical examples:

Overall Span (mm) [1]
Concrete Depth (mm)
GRC Formwork
Style
Thickness (mm)
Corrugations (mm) [2]
Panel Weight (Kg) [3]
700
200
Flat
16
30
700
200
Corrugated
8
30
15
800
200
Flat
18
38
800
200
Corrugated
10
30
21
1000
200
Corrugated
8
40
21
1300
200
Corrugated
14
40
48

 

  1. Clear span is normally 80-100mm less than overall span
  2. Corrugation can be adjusted to vary the height/corrugation above or below horizontal
  3. All panels assumed to be 1200mm long

Material Properties

GRC formwork combines the well-established compressive properties of cement-based mortars with the valuable flexural and tensile properties contributed by the glass fibre to give a material with the following characteristics:

  • dense and crack resistant
  • non-combustible with excellent resistance to fire and surface spread of flame
  • low water, water vapour, and air permeability – contributing to low carbonation rates and resistance to both sulphate and chloride attack
  • good abrasion resistance
  • freedom from rot, corrosion, and UV degradation
  • easy to handle on – site, yet heavy enough to be stable
  • minimum maintenance

Strength and Deflection Criteria

GRC formwork is designed to support twice the sum of the following nominal loads:

(i) self weight of formwork

(ii) weight of wet concrete of deck slab(assumed concrete density of 2400kg/cu.m)

(iii) minimum superimposed load of 1.5KN/sq m

(iv) the effects of wind load where appropriate

The minimum superimposed load of 1.5KN/sq m covers situations described in Clause 27.3.1 of BS5975: 1982 and this means that the formwork is able to resist the effects of normal concentrated loading e.g. due to reinforcing spacer blocks or operatives walking on the reinforcement.

Deflections of the GRC permanent formwork four hours after completion of concreting will not exceed 1/300 of the span of the formwork unit.

If necessary, compliance with the strength and deflection criteria can be demonstrated by load testing.

Sealing

GRC permanent formwork is sufficiently rigid to prevent the loss of grout during construction. This is subject to the use of approved methods of seating and sealing the panels, e.g. by bedding on mortar, taping the joints, or the use of compressive foam strip.

 

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