The truck on site is placing the attached garage floor and the open front porch, and the ticket describes a mix designed for 3,000 pounds per square inch at 28 days with air entrainment. Judging only the specified compressive strength, how should the inspector rule?
- AAccept it, because 3,000 pounds per square inch is the minimum for any concrete placed in a dwelling in a severe weathering area of the map.
Three thousand is the figure for basement walls, foundation walls and interior slabs on the severe weathering map. Flatwork exposed to weather and to de-icing chemicals steps up to 3,500.
- BAccept it, because the entrained air in this mix permits the specified strength to be reduced one step below the value shown in the table.
Air entrainment is an additional requirement for exposed flatwork, not a trade that buys back strength. Both apply at once, so the mix has to carry the air and the 3,500 figure together.
- Reject the mix, because garage floors and porch slabs exposed to weather in a severe weathering area are specified at 3,500 pounds per square inch.
- DReject the mix, because every slab placed on ground has to be specified at 4,000 pounds per square inch in a severe weathering area such as this one.
Four thousand is above anything the table sets for residential work, and interior slabs sit well below it. Rejecting the mix for that number asks the builder for concrete the code does not require.
Why C is correct
The strength table sets a higher minimum for garage floor slabs, porches and carport slabs exposed to the weather in moderate and severe weathering areas, because those surfaces face freeze-thaw cycling and de-icing salts. That minimum is 3,500 pounds per square inch, so a 3,000 mix is short and the pour should be stopped before it goes down.
What this question is testing
Whether the candidate reads the strength table by exposure category rather than by a single remembered number, and knows that air entrainment and compressive strength are independent requirements for exposed flatwork.
On the job
Ticket checking takes thirty seconds and catches the single most common concrete defect, which is the batch plant sending the house mix to a flatwork pour. Salt-damaged garage aprons and scaled porches are among the most frequent early callbacks in cold climates, and they trace directly to strength and air content. Carry the weathering probability map for your jurisdiction; the whole table hangs off which zone you are in.
Memory technique
Anything the weather and the salt truck can reach gets the higher number.
Exam tip
Sort concrete strength questions by where the concrete goes first and by the weathering zone second.
Where to look it up
IRC Chapter 4, the minimum specified compressive strength table under R402.2 and the weathering probability map in Chapter 3.