6 A rectangular concrete beam has dimensions of 250 mm x 425

6. A rectangular concrete beam has dimensions of 250 mm x 425 mm (width x depth). It is nfored with 4 bars of diameter 16 mm. The clear spacing between them is 30 mm. A mm reinforced concrete slab is integrated into the part of the beam. Given the reject and indicate why. Hint: NMS (6 points) following all-in aggregate samples. State which sample(s) you would accept or Sieve size (mm) 0.3 Sample 1 100 90 70 45 36 30 25 10 Passing (%) Sample 2 100/ 96 | 82 | 56 | 45 | 35 | 30 | 15 Sample 398 80 75 52 47 41 35 12 37.5 19.09.5 4.752.36 1.18 0.6 0.15 0 Cumulative 2 Sample # | Accept or Reject Reasons

Solution

Here, we need to consider the nominal maximum size of aggregate. Nominal Maximum Size of aggregate is defined as the smallest sieve opening through which the entire amount of the aggregate is permitted to pass..

According to ACI 211.1,

Nominal Maximum coarse aggregate size = min.{1/5 times the smallest dimension between sides of forms, 1/3 times the depth of slab, 3/4 times the minimum clear spacing between individual reinforcing bars}

= min.{(250/5 = 50 mm), (175/3 = 58.3333 mm), (3*30/4 = 22.5 mm)}

= 22.5 mm

This means 22.5 mm is the minimum size of sieve opening through which total aggregate should pass. From given data, Sample 3 is having 98 % of cumulative passing through 37.5 mm size sieve. Hence, entire aggregate amount is not going to pass through 22.5 mm sized sieve. Therefore, Sample 3 should be rejected.

Now, in case of Sample 1 and 2, both were passing 100 % from 37.5 mm sieve but having 90 % and 96 % of cumulative passing through 19.0 mm size sieve respectively. Thus, there is possibility of having 100 % of cumulative passing through 22.5 mm size sieve for both the samples. Therefore, Sample 1 and Sample 2 can be accepted (though we need to see the variation of aggregate sizes in between the 37.5 mm and 19.0 mm).