13. (30 pts) You have been tasked to design a concrete mix for a warehouse floor

Question

13. (30 pts) You have been tasked to design a concrete mix for a warehouse floor slab. The slab is 8" thick (to accommodate the heavy floor loads), so you have chosen to use a coarse aggregate with a 2" NMAS. You need a slump of 3-4" without admixtures and a design compressive strength (f'e) of 4800 psi. Since this will be indoors, you do not need to add air entrainment. The coarse aggregate is a subangular stone and the fine aggregate is a river sand that meets the ASTM C33 specification. The aggregate has the following properties: Coarse Fine Aggregate Aggregate Unit Weight (Ib/ft3) Bulk Specific Gravity Apparent Specific Gravity Fineness Modulus Absorption 102 2.66 2.68 106 2.62 2.65 2.90 1.3% 0.9% Determine a trial mix design using the estimated weight method. DO NOT ADJUST FOR ABSORPTION OR MOISTURE CONTENT. You must show all of your work. Spreadsheer solutions are not acceptable. Continue your calculations on the next page if necessary.

Explanation / Answer

Trial mix using estimated weight approach as per ACI

Nominal maximum aggregate size (NMAS) = 2"

Mixing water as per table 10.9 = 285 lb/ yd3

Let us assume 1 yd3 total volume of mix

Thus mixing water required = 285 lb

Now w/c by weight as per table 10.3 = 0.57-((0.57-0.48)*(4800-4000)/(5000-4000)) = 0.498

Thus cement content= 285/ 0.498 = 572.289 lb

Thus volume of cement assuming specific gravity of 3.15 is = 572.289/(3.15*62.4) = 2.91 ft3

Volume of water = 285/ 62.4 = 4.567 ft3

Volume of air = zero ( non air entrapped)

Thus volume of coarse and fine aggregates = 27- 2.91-4.567-0= 19.523

As per table 10.5 the volume of coarse aggregates based on fineness modulus of fine aggregates =0.74- ((0.74-0.72)*(2.9-2.8)/(3-2.8))=0.73 *19.523 = 14.25 ft3

Thus volume of fine aggregates = 27- 2.91-4.567-0-14.25 = 5.273 ft3

Thus weight of water = 285 lb

Weight of cement = 572.289 lb

Weight of fine aggregates = 106*5.273 = 558.938 lb

Weight of coarse aggregates = 102*14.25 = 1453.5 lb

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