Civil Engineering Reference
In-Depth Information
Water
13.12 lb
262
1 x 62.4
Water
=
=
4.20 cu ft
Cement
47.63 lb
Coarse aggregate (2% MC)
126.44 lb
624
3.15 x 62.4
Cement
=
=
3.17 cu ft
Fine aggregate (6% MC)
97.04 lb
Total
284.23 lb
1674
2.68 x 62.4
Coarse aggregate
=
= 10.01 cu ft
The yield of the trial batch is
7.0
100
Air
=
x
27
=
1.89 cu ft
284.23
141.49
= 2.009 cu ft
Total
=
19.27 cu ft
The mixing water content is determined from the added
water plus the free water on the aggregates and is calcu-
lated as follows:
Water added
Fine aggregate volume =
27 - 19.27
=
7.73 cu ft
The weight of dry fine aggregate required is
7.73 x 2.64 x 62.4 = 1273 lb
= 13.12 lb
An air-entraining admixture dosage of 0.8 fluid ounces
per 100 pounds of cement is expected to achieve the 7% air
content in this example. Therefore, the amount of air-
entraining admixture required is:
=
Free water on coarse
126.44
=
x 0.015**
=
1.86 lb
aggregate
1.02*
Free water on fine
97.04
=
=
4.85 lb
x 0.053**
aggregate
1.06*
0.8 x 624
= 5.0 fl oz
100
Adjusted batch weights per cubic yard of concrete are
Water 262 lb
Cement 624 lb
Coarse aggregate (dry) 1674 lb
Fine aggregate (dry)
1273 lb
Total 3833 lb
Air-entraining admixture 5.0 fl oz
Estimated concrete density (unit weight) with the aggre-
gates at SSD:
Total = 19.83 lb
The mixing water required for a cubic yard of the same
slump concrete as the trial batch is
19.83 x 27
= 267 lb
2.009
Batch Adjustments.
The measured 4-in. slump of the
trial batch is unacceptable (more than 0.75 in. above 3-in.
max.), the yield was slightly high, and the 8.0% air content
as measured in this example is also too high (more than
0.5% above 7% max.). Adjust the yield, reestimate the
amount of air-entraining admixture required for a 7% air
content, and adjust the water to obtain a 3-in. slump.
Increase the mixing water content by 5 lb for each 1% by
which the air content is decreased from that of the trial
batch and reduce the water content by 10 lb for each 1-in.
reduction in slump. The adjusted mixture water for the
reduced slump and air content is
(5 x 1) - (10 x 1) + 267 = 262 lb per cu yd
With less mixing water needed in the trial batch, less ce-
ment also is needed to maintain the desired water-cement
ratio of 0.42. The new cement content is
262
0.42
[262 + 624 + (1674 x 1.005) + (1273 x 1.007)]
27
=
= 142.60 lb per cu ft
Upon completion of checking these adjusted propor-
tions in a trial batch, it was found that the proportions were
adequate for the desired slump, air content, and yield. The
28-day test cylinders had an average compressive strength
of 4900 psi, which exceeds the  of 4700 psi. Due to fluc-
tuations in moisture content, absorption rates, and specific
gravity of the aggregate, the density determined by volume
calculations may not always equal the unit weight deter-
mined by ASTM C 138 (AASHTO T 121). Occasionally, the
proportion of fine to coarse aggregate is kept constant in
adjusting the batch weights to maintain workability or
other properties obtained in the first trial batch. After
adjustments to the cement, water, and air content have been
made, the volume remaining for aggregate is appropriately
proportioned between the fine and coarse aggregates.
Additional trial concrete mixtures with water-cement
ratios above and below 0.42 should also be tested to
develop a strength curve. From the curve, a new more eco-
nomical mixture with a compressive strength closer to Â,
can be proportioned and tested. The final mixture would
probably look similar to the above mixture with a slump
range of 1 in. to 3 in. and an air content of 5% to 7%. The
= 624 lb per cu yd
The amount of coarse aggregate remains unchanged
because workability is satisfactory. The new adjusted batch
weights based on the new cement and water contents are
calculated after the following volume computations:
* 1 + (2% MC/100) = 1.02; 1 + (6% MC/100) = 1.06;
** (2% MC - 0.5% absorption)/100 = 0.015; (6% MC - 0.7% absorp-
tion)/100 = 0.053
