ACI Certification Concrete Mix Design 3

Question 1

In a concrete mix, 'paste volume' refers to the combined volume of:

Accepted Answer

Cement, water, and entrained air

Answer

Coarse aggregate and water

Answer

Fine aggregate only

Answer

Total aggregates and cement

Question 2

When field test records show a standard deviation of 400 psi for concrete with f'c = 4,000 psi, the required average strength (f'cr) using f'cr = f'c + 1.34s is:

Accepted Answer

4,536 psi

Answer

4,200 psi

Answer

5,000 psi

Answer

4,100 psi

Question 3

What is the primary benefit of air entrainment in concrete exposed to freezing and thawing cycles?

Accepted Answer

It creates small voids that relieve hydraulic pressure from freezing pore water

Answer

It increases unit weight and compressive strength

Answer

It slows the rate of cement hydration

Answer

It reduces the amount of cement needed

Question 4

How does increasing the maximum aggregate size generally affect the water requirement for a given slump?

Accepted Answer

Water requirement decreases

Answer

Water requirement increases significantly

Answer

Water requirement remains unchanged

Answer

Water requirement doubles

Question 5

Moisture corrections to batch weights are required whenever aggregate moisture content is:

Accepted Answer

Above or below the SSD condition

Answer

At the saturated surface-dry (SSD) condition

Answer

Only when aggregate is oven-dry

Answer

Only when surface moisture is visibly present

Question 6

What is the approximate specific gravity of normal-weight fine aggregate (natural sand) used in absolute volume calculations?

Accepted Answer

2.65

Answer

2.15

Answer

2.45

Answer

3.15

Question 7

If a trial batch produces a lower slump than specified, the correct adjustment per ACI 211.1 principles is to:

Accepted Answer

Increase both water and cement proportionally to maintain the w/c ratio, or use a water-reducing admixture

Answer

Add water only until the target slump is reached

Answer

Reduce the coarse aggregate content only

Answer

Increase the fine aggregate content only