Formwork for Beams and Slabs
- Forms are molds to receive concrete in its’ plastic form.
- Formwork is temporary structure, as such, it is not normally shown on the drawings.
Formwork Materials
- Wood
- Either all-wood or some wood components
- Plywood
- Aluminum
- Steel
- Plastics
Lumber
Designated by Cross Sections, Nominal Dimensions (prior to finishing)
After cut length wise, finishing operations reduces actual dimensions
2 x 4 Plank 1 1/2 x 3 1/2 ® 2” by 4” in S4S
Lengths are multiples of 2 ft (8, 10, 12, 14, 16,…)
Specified by type and grade
Type: pine, oak, fir
Grade: Selected (A, B, C, D) and Common (1, 2, 3, 4)
Selected (A best, D poor quality)
Cost ® Kind, grade, size, length, milling, quantity, freight
Aluminum
- Pure aluminum chemically attacked by wet concrete
- Light weight allow larger forming units
- High reuse value
Steel
- For heavy concrete work
- With reasonable care will last indefinitely
- High initial cost and high handling cost
Estimating Issues
- Normally, the forms are used more than once
- More usage of forms reduce the price
- Wood forms have less usage potential than aluminum or steel forms
- Complicated shapes of concrete are more expensive because of labor cost and reuse of forms.
- Majority of cost is _____?? Not ____??
- You have to have a construction plan to determine the reuse cycle
- Number of reused largely controls _____?? Cost.
Estimating Issues (Formwork)
Do not deduct
- Intersection of beams
- Intersection of beams & columns & walls
- Any opening < 100 sq. ft.
Formwork
- Unit of Measurement
Square Foot Contact Area SFCA
- Measure just contact area, not area of formwork
Contact Area= 2h(L+B)
- Wood normally measured:
- Linear feet of one size
- Board Feet
- FBM (Foot Board Measure)
1”x12”x1’(long) or 144 cubic inches
example: 2×8 x 16ft long
= 1.33 BF/LF x 16 = 21.28 FBM
Design and Estimating of Forms
“Design determine Sheathing thickness, stud size, wale size, tie size”
Use of design tables
Watch for
1. Rate of pour
2. Temperature and weather
3. Proportions of mix and consistency
4. Method of placement and vibration
Refer PPT for worked out example