ACI Floor Flatness & Levelness (FF/FL Numbers) 2

Question 1

What is the fundamental difference between floor flatness (FF) and floor levelness (FL)?

Accepted Answer

FF measures short-interval surface waviness; FL measures long-interval slope relative to a horizontal plane

Answer

FF applies to exterior slabs; FL applies to interior slabs

Answer

FF is measured while concrete is wet; FL is measured after curing

Answer

FF applies only to formed surfaces; FL applies only to slab-on-grade

Question 2

When should F-number measurements typically be taken after concrete placement per ASTM E1155?

Accepted Answer

Within 72 hours of slab placement

Answer

Immediately after finishing while the surface is still plastic

Answer

After 7 days of curing

Answer

After 28-day strength testing is complete

Question 3

A typical minimum FF number for a conventional warehouse floor with random forklift traffic is approximately:

Accepted Answer

FF 20–35

Answer

FF 5–10

Answer

FF 75–100

Answer

FF 50–75

Question 4

How does finishing concrete before bleed water has evaporated affect floor flatness and surface quality?

Accepted Answer

It can trap bleed water beneath the surface, causing dusting, delamination, and compromised FF values

Answer

It improves flatness by sealing the surface early and reducing evaporation

Answer

It has no measurable effect on FF numbers or surface quality

Answer

It increases FL numbers by raising the overall floor elevation

Question 5

In the F-number system, what does an F-number of zero indicate?

Accepted Answer

An uncontrolled floor surface with no flatness specification met

Answer

A perfectly flat or level floor

Answer

No measurement was taken for that section

Answer

The floor meets the minimum code requirements

Question 6

The dipstick floor profiler measures elevation differences at what standard interval per ASTM E1155?

Accepted Answer

12 inches (300 mm)

Answer

6 inches (150 mm)

Answer

24 inches (600 mm)

Answer

36 inches (900 mm)

Question 7

Which construction practice most directly improves floor flatness (FF) numbers during placement?

Accepted Answer

Using laser screeds combined with skilled floating technique by experienced finishers

Answer

Using high-water-content mixes for greater workability

Answer

Placing concrete in a checkerboard pattern to allow alternating sections to cure

Answer

Increasing joint spacing to reduce finishing interruptions