I have a situtation where perimeter masonry walls support an outdoor patio. The owner wants to use a structural reinforced slab instead of slab on grade. I know there are ways around having to shore the slab (light gauge framing supporting slab, composite deck, steel joists, etc.), so this is just a general question. Have you ever been in this type situation where the formwork was required but would be enclosed by the new concrete structure and have to be... abandoned for lack of a better word? Thanks.
Typically use a 6" void form... this can be a cardboard type or my preference, a foamed plastic type. This is placed on moderately compacted granular fill (depending on the soils, sometimes no compaction) and the concrete is cast over it. You can still place a PEVB between the void form and the concrete slab.
Dik
@bigengineer: We have done what we call structural slabs on grade for interior applications where for design purposes the slab is considered as a suspended structural slab. This is possible where the underlying soil/base is capable of supporting the weight of the structural slab and construction loads during the construction phase.
Your post does not mention about your region. If you are in a region where there are no frost protection requirements, then it is possible to place the slab directly on soil/base without using any sacrificial forms or frost protection measures.
Thanks for the replies. Actually, this is an elevated patio, about 5-6' above grade with perimeter masonry walls. The owner doesn't want to use fill inside the masonry walls and wants a free spanning r.c. slab spanning between the walls. There are no access doors, etc to remove form work later. And again, owner is requesting no fill material.
I can use a composite steel deck to achieve the spans he wants, but would be cheaper to do some metal decking (non-composite) as form work that is shored a couple/few times between permanent supports. Like JAE said, this would cause concerns with termites though with abandoned forms. Maybe form work from treated lumber?
You can support metal deck on wood knee walls to cast the concrete and remove the temporary framing after the concrete is sufficiently hard. Metal deck forming can be less costly than plywood... and you may be able to obtain roof deck (not composite) from a local salvage supplier.
Dik
Don't use composite metal decking. The decking would serve as the structural reinforcement but because it is an exterior deck, it will rust and eventually you would have a slab with no reinforcement.
Use form deck and reinforce with rebar or do one of the other ideas above.
I have seen metal decking used as lost formwork on a footbridge project. The slab was reinforced with conventional reinforcement and the metal decking was lest to rust away over time.
Becareful with cardboard void formers. I used them once and they collapsed when the concrete was placed. Very costly.
bigenginnerz:
You should have some openings for ventilation... Can have a louvred opening, or simply a steel grate/wire opening to keep birds and rodents out...
asixth:
Cardboard ones work OK, as long as they're kept dry prior to placing and contractors often wrap them in poly and provide a 1/8" or 1/4" masonite board on top to keep the rebar chairs from poking through.
Dik
Formwork Safety Interview: 50 Questions and Sample Answers
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Formwork Safety Interview: 50 Questions and Sample Answers
1. What is formwork in construction?
Answer: Formwork is a temporary or permanent mold used to support concrete while it sets and gains strength.
2. Why is formwork safety important?
Answer: Ensuring formwork safety prevents collapses, falls, and worker injuries, and helps maintain structural integrity.
3. What are the main hazards associated with formwork?
Answer:
- Collapse of formwork due to weak support
- Falling from heights while assembling
- Struck-by hazards from materials falling
- Exposure to sharp edges and heavy loads
4. What are the different types of formwork used in construction?
Answer:
- Timber formwork
- Steel formwork
- Aluminum formwork
- Plastic formwork
- Fabric formwork
5. What is the role of a Safety Officer in formwork operations?
Answer: A Safety Officer ensures that formwork is properly installed, workers use PPE, and all safety procedures are followed.
6. What PPE is required for working with formwork?
Answer:
- Safety helmet
- Gloves
- Steel-toe boots
- Safety harness (for height work)
- Eye protection (goggles)
7. What is the safe height limit for formwork installation without scaffolding?
Answer: Typically, any work above 2 meters (6.5 feet) requires scaffolding or fall protection measures.
8. What are the basic steps in formwork installation?
Answer:
- Plan the design and layout.
- Prepare the ground and materials.
- Assemble and secure the framework.
- Inspect the formwork before pouring concrete.
- Monitor the structure during and after concrete placement.
9. How can we prevent formwork collapse?
Answer:
- Use proper bracing and supports.
- Ensure formwork components are strong enough.
- Avoid premature removal of supports.
- Conduct regular inspections before and during concrete pouring.
10. What is the minimum curing time before removing formwork?
Answer: It depends on concrete strength, but typically:
- Walls and columns: 24-48 hours
- Slabs (with props left in place): 3-7 days
- Beams and arches: 7-21 days
11. What safety precautions should be taken before pouring concrete into formwork?
Answer:
- Inspect formwork for stability and alignment.
- Check that reinforcement bars are correctly placed.
- Ensure proper access and escape routes.
- Remove unnecessary workers from the area.
12. What is the purpose of formwork bracing?
Answer: Bracing provides stability and prevents deformation or collapse of the formwork during concrete placement.
13. How do you inspect formwork before use?
Answer:
- Check for damaged or weak materials.
- Ensure correct alignment and secure connections.
- Verify that bracing and supports are in place.
14. What should be done if formwork starts to fail during concrete pouring?
Answer:
- Stop concrete pouring immediately.
- Evacuate workers from the danger zone.
- Reinforce weak sections and inspect for further risks.
15. Why should formwork be removed gradually?
Answer: Sudden removal can cause concrete cracking or collapse due to incomplete curing.
16. What are the risks of using defective formwork materials?
Answer:
- Formwork collapse
- Concrete leakage and instability
- Increased worker injury risks
17. How should timber formwork be stored?
Answer: Keep it dry, stacked properly, and protected from weather to prevent warping or decay.
18. What is shoring in formwork?
Answer: Shoring provides temporary support to keep structures stable before they become self-supporting.
19. How do you ensure proper ventilation while working with formwork in enclosed spaces?
Answer: Use exhaust fans, natural ventilation, or respirators to prevent exposure to harmful fumes.
20. What is the role of tie rods in formwork?
Answer: Tie rods hold the formwork panels together and prevent them from separating under concrete pressure.
21. Why should workers avoid standing on formwork before it is fully secured?
Answer: It may collapse or shift, leading to falls or injuries.
22. How can electrical hazards be avoided during formwork construction?
Answer:
- Use insulated tools.
- Keep formwork away from power lines.
- Inspect electrical cables and connections.
23. How does temperature affect formwork safety?
Answer:
- Cold weather can cause concrete to set slowly, requiring longer curing times.
- Hot weather can cause rapid drying, increasing the risk of cracks.
24. What safety measures should be taken when removing formwork?
Answer:
- Remove in small sections.
- Use proper tools instead of force.
- Ensure workers are not standing under the area being dismantled.
25. How should workers lift and handle heavy formwork panels safely?
Answer:
- Use mechanical lifting equipment.
- Lift with proper body posture.
- Work in teams to distribute the load.
26. What is falsework in formwork construction?
Answer: Falsework is temporary support for formwork, keeping it stable until the concrete is strong enough.
27. Why should rebar and sharp edges in formwork be covered?
Answer: To prevent worker injuries such as cuts and punctures.
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28. How do weather conditions affect formwork stability?
Answer:
- Rain and wind can weaken supports.
- Extreme heat can cause wood formwork to expand.
29. Why is proper drainage necessary around formwork structures?
Answer: To prevent water accumulation, which can weaken the soil and supports.
30. How can noise hazards be controlled during formwork construction?
Answer:
- Use noise-reducing tools.
- Provide hearing protection for workers.
31. What are the risks of using improperly mixed concrete in formwork?
Answer:
- Weak structural integrity
- Formwork failure
- Cracks and defects in the final structure
32. What is the correct way to secure formwork panels?
Answer:
- Use strong fasteners and clamps
- Ensure proper alignment and leveling
- Double-check for loose connections before pouring concrete
33. What should you do if a formwork structure appears unstable?
Answer:
- Stop work immediately
- Reinforce supports and check for defects
- Report to the site supervisor for corrective action
34. How can scaffolding improve formwork safety?
Answer: Scaffolding provides safe access for workers, reducing fall hazards and improving stability.
35. Why is it important to use engineered formwork designs?
Answer:
- Ensures load-bearing capacity is met
- Reduces risk of failure or collapse
- Increases efficiency and worker safety
36. What is the purpose of releasing agents in formwork?
Answer: Releasing agents prevent concrete from sticking to formwork, making removal easier and reducing damage.
37. How can we prevent worker fatigue during formwork operations?
Answer:
- Schedule rotating shifts
- Provide breaks and hydration
- Use ergonomic lifting techniques
38. Why is edge protection required in elevated formwork?
Answer: To prevent falls and ensure worker safety during installation and dismantling.
39. What is the risk of premature removal of formwork?
Answer:
- Structural failure due to weak concrete
- Cracks and deformities in the structure
- Injuries from collapsing materials
40. What should be done in case of an accident involving formwork?
Answer:
- Stop work immediately
- Provide first aid and call emergency services
- Investigate and implement corrective measures
41. How do you ensure formwork stability on sloped or uneven ground?
Answer:
- Level the ground before installation
- Use adjustable props and extra bracing
- Conduct soil stability tests
42. What is the purpose of formwork ties?
Answer: They hold formwork panels together and resist concrete pressure during pouring.
43. How do you prevent trapped air bubbles in concrete poured into formwork?
Answer: Use vibrators to settle the concrete and eliminate air pockets.
44. What should be considered when designing tall formwork structures?
Answer:
- Wind loads and lateral stability
- Strong bracing and tie systems
- Safe access for workers
45. What are the best practices for dismantling formwork?
Answer:
- Remove sequentially from top to bottom
- Use proper tools instead of force
- Ensure no workers are below the removal area
46. How do weather conditions impact formwork safety?
Answer:
- Rain weakens timber and soil stability
- High temperatures cause wood expansion
- Strong winds make tall formwork unstable
47. What are the best safety measures when working at heights on formwork?
Answer:
- Use fall protection equipment (harness, guardrails)
- Secure ladders and platforms
- Train workers on safe height work procedures
48. How can material quality affect formwork safety?
Answer:
- Weak materials can cause collapse
- Poor-quality fasteners may fail under pressure
- Warped wood or rusted metal reduces stability
49. What are the legal requirements for formwork safety?
Answer:
- Follow OSHA and local construction safety standards
- Conduct regular inspections and certifications
- Ensure competent personnel handle formwork operations
50. How do you maintain a safe work environment in formwork operations?
Answer:
- Conduct daily safety briefings
- Implement strict PPE requirements
- Enforce safe work procedures and inspections
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