How to Choose the Right Adjustable Prop for Slab Support

Understanding Adjustable Props

Adjustable props, also known as acrow props or steel props, are temporary vertical supports used to hold up concrete slabs, beams, and formwork during construction. Choosing the right prop is critical — an undersized prop can fail under load, while an oversized prop wastes money.

Key Selection Criteria

1. Floor Height

Measure the floor-to-ceiling height where the prop will be used. Choose a prop whose adjustment range covers your required height. Always ensure the prop is used within its specified range — never extend beyond the maximum.

• Up to 2.9m: Light duty props (1.6m - 2.9m range)
• Up to 3.5m: Medium duty props (2.0m - 3.5m range)
• Up to 4.5m: Heavy duty props (3.0m - 4.5m range)

2. Load Capacity

Calculate the load the prop needs to support. This includes the weight of wet concrete (approximately 25 kN/m³), formwork, and any construction live loads.

• Light duty: 15 kN — suitable for residential slabs up to 150mm thick
• Medium duty: 20 kN — suitable for commercial slabs up to 200mm thick
• Heavy duty: 30 kN — suitable for thick slabs, beams, and industrial construction

3. Prop Spacing

The spacing between props depends on the slab thickness and the prop's safe working load. A structural engineer should specify the exact spacing, but as a general guideline:

• 100mm slab: props at 1.2m x 1.2m centers
• 150mm slab: props at 1.0m x 1.0m centers
• 200mm slab: props at 0.8m x 0.8m centers

Common Mistakes to Avoid

• Using damaged or bent props
• Extending props beyond their maximum height
• Not using base plates on soft ground
• Removing props too early (concrete needs minimum 28 days to achieve full strength)
• Not bracing props in both directions

Summary

Always consult your structural engineer for specific prop requirements. When in doubt, use a heavier duty prop — the cost difference is minimal compared to the safety margin gained.