ACI Floor Flatness & Levelness (FF/FL Numbers) 2 — Questions and Answers
Question 1: What is the fundamental difference between floor flatness (FF) and floor levelness (FL)?
- FF applies to exterior slabs; FL applies to interior slabs
- FF measures short-interval surface waviness; FL measures long-interval slope relative to a horizontal plane (Correct answer)
- FF is measured while concrete is wet; FL is measured after curing
- FF applies only to formed surfaces; FL applies only to slab-on-grade
Correct answer: FF measures short-interval surface waviness; FL measures long-interval slope relative to a horizontal plane
FF measures local curvature over short distances (bumps and dips), while FL measures the long-range slope or tilt of the floor relative to a horizontal plane.
Question 2: When should F-number measurements typically be taken after concrete placement per ASTM E1155?
- Immediately after finishing while the surface is still plastic
- After 7 days of curing
- Within 72 hours of slab placement (Correct answer)
- After 28-day strength testing is complete
Correct answer: Within 72 hours of slab placement
ASTM E1155 recommends F-number measurements within 72 hours of placement to capture the as-built profile before curling and drying shrinkage alter the surface.
Question 3: A typical minimum FF number for a conventional warehouse floor with random forklift traffic is approximately:
- FF 5–10
- FF 20–35 (Correct answer)
- FF 75–100
- FF 50–75
Correct answer: FF 20–35
ACI 117 and industry standards typically specify FF 20–35 for conventional warehouse floors subject to random forklift traffic.
Question 4: How does finishing concrete before bleed water has evaporated affect floor flatness and surface quality?
- It improves flatness by sealing the surface early and reducing evaporation
- It has no measurable effect on FF numbers or surface quality
- It can trap bleed water beneath the surface, causing dusting, delamination, and compromised FF values (Correct answer)
- It increases FL numbers by raising the overall floor elevation
Correct answer: It can trap bleed water beneath the surface, causing dusting, delamination, and compromised FF values
Finishing over bleed water seals it beneath the surface, weakening the surface layer and potentially causing dusting and delamination, which compromises both surface quality and FF measurements.
Question 5: In the F-number system, what does an F-number of zero indicate?
- A perfectly flat or level floor
- No measurement was taken for that section
- An uncontrolled floor surface with no flatness specification met (Correct answer)
- The floor meets the minimum code requirements
Correct answer: An uncontrolled floor surface with no flatness specification met
An F-number of zero indicates a random, uncontrolled floor surface with no flatness specification being achieved — representing the worst possible condition.
Question 6: The dipstick floor profiler measures elevation differences at what standard interval per ASTM E1155?
- 6 inches (150 mm)
- 12 inches (300 mm) (Correct answer)
- 24 inches (600 mm)
- 36 inches (900 mm)
Correct answer: 12 inches (300 mm)
Per ASTM E1155, the dipstick profiler records elevation readings at 12-inch (300 mm) intervals along defined measurement paths across the floor.
Question 7: Which construction practice most directly improves floor flatness (FF) numbers during placement?
- Using high-water-content mixes for greater workability
- Placing concrete in a checkerboard pattern to allow alternating sections to cure
- Using laser screeds combined with skilled floating technique by experienced finishers (Correct answer)
- Increasing joint spacing to reduce finishing interruptions
Correct answer: Using laser screeds combined with skilled floating technique by experienced finishers
Laser screeds provide precise elevation control during placement, and skilled floating technique smooths out local surface waviness, both directly improving FF numbers.
ACI Floor Flatness & Levelness (FF/FL Numbers) 2
What is the fundamental difference between floor flatness (FF) and floor levelness (FL)?