NASCLA · Most missed · Episode 8
The Slab Most People Get Wrong
Chapters
- The question
- The table
- The trap
- The answers
- Recap
In short
ACI 318-19 Table 20.5.1.3.1 sets concrete cover by exposure, then by member. Concrete cast against the ground needs 3 inches; concrete exposed to weather needs 2 inches for No. 6 bars and larger and 1-1/2 inches for No. 5 and smaller. Not exposed, slabs, walls and joists with No. 11 bars or smaller need 3/4 inch, while beams and columns need 1-1/2 inches. An elevated slab inside the building with No. 5 bars therefore needs 3/4 inch.
- Inside, slabs, walls and joists: 3/4 in for No. 11 bars and smaller.
- Inside, beams and columns: 1-1/2 in. Weather: 1-1/2 or 2 in.
- Cast against the ground: 3 in.
Transcript
The question
This concrete question has the worst score in our NASCLA bank. Here is the question, and why most people miss it. An elevated office slab, inside the building, with No. 5 bars near the bottom. No weather, no earth contact. 32 of 37 people missed it, and 19 of them gave the same answer: 1 and a half inches. Pause. Which answer would you pick?
The table
Open ACI 318 to the cover table. It sorts by exposure, then by member. Cast against the ground: 3 inches. Exposed to weather: 2 inches, or 1 and a half for No. 5 bars and smaller. Not exposed, and a slab, wall or joist with No. 11 bars or smaller: three-quarters of an inch. This slab is inside, and the bars are No. 5. So the answer is three-quarters.
The trap
So why do so many pick 1 and a half? It is in the table twice. Inside, but in a beam or column. Or outside, with No. 5 bars. Neither is this slab.
The answers
Now the answers. B, 1 and a half: the beam and column number. Out. A, 1 inch: that number is not in this table at all. Out. C, 2 inches: weather exposure, with No. 6 bars and larger. Out. That leaves D: three-quarters of an inch. Read the exposure first, then the member. The slab gets the smallest cover.
Recap
Three-quarters inside a slab, 1 and a half in a beam, 2 in the weather, 3 against the ground. Next: one percent, not ten. Subscribe so you catch it.
Sources
- ACI 318-19 Table 20.5.1.3.1 (Table 20.6.1.3.1 in ACI 318-14)
- EstatePass question bank: nascla-concrete-v22-004
Independent study material. EstatePass is not affiliated with NASCLA, PSI, Pearson VUE, the Florida CILB or the CSLB, and nothing here is legal advice. Any jobs and companies in our episodes are fictional.