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An in-depth look into the process of concrete mix design, focusing on the objective, design considerations, and procedures for determining the most economical and practical combination of materials to produce concrete that meets performance requirements. Key factors affecting workability, strength, durability, and appearance are discussed, along with examples using the absolute volume method.
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Design Procedures
(1) Determine Required Average Compressive Strength When the standard deviation of the compressive strength is known, the required average compressive strength, f’cr is equal to the larger of the following two values: f’ (^) cr = f’ (^) c + 1.34 S f’ (^) cr = f’ (^) c + 2.33 S - 500 where f’ (^) c = specified compressive strength, psi S = standard deviation, psi If data are not available to establish the standard deviation of the compressive strength, use Table 9-11.
(3) Determine Mixing Water Requirement & Air Content
Mixing water (Table 9-5) : 300 lb / yd 3 (3-4 in. slump, non-air-entra., nom. max. agg. size: 1.5 in.) Air content (Table 9-5) : 1% (4) Calculate Required Cement Content
Minimum cement content (Table 9-7): 470 lb/yd 3 (This table is applicable since the concrete is to be placed below ground) Cement based on w/c : 300/0.55 = 545 lb/yd 3
Use 545 lb/yd 3
Water Requirement for Various Slumps and Aggregate Sizes
Water Requirement for Various Slumps and Aggregate Sizes
(7) Adjustments for Agg. Moisture Wt. of agg. in natural moisture condition:
Mixing water contributed by agg.: Coarse Agg.: 1917 X (0.02 - 0.005) = 29 lb Fine Agg.: 1266 X (0.06 - 0.007) = 67 lb 96 lb
Bulk Volume of Coarse Aggregate per Unit Volume of Concrete
Over-sanded concrete mix
Check slump of trial mix
Check unit weight of trial mix
Check air content of trial mix
DESIGN PROCEDURES: (1) Determine Required Average Compressive Strength f’ (^) cr = f’ (^) c + 1.34 S = 4000 + 1.34 (450) = 4603 psi f’ (^) cr = f’ (^) c + 2.33 S - 500 = 4000 + 2.33 (450) - 500 = 4549 psi Use a required average compressive strength of 4603 psi.
(2) Determine Water Cement Ratio w/c for strength (Table 9-3) : 0. w/c for exposure (Table 9-1): 0. Design strength (4600 psi) > 4500 psi O.K. (Table 9-2 is not applicable) Use w/c = 0. (3) Determine Mixing Water Requirement & Air Content Mixing water (Table 9-5) : 280 lb / yd 3 (1-2 in. slump, air- entrained, nom. max. agg. size: 3/4 in.) Air content (Table 9-5) : 6% (4) Calculate Required Cement Content Minimum cement content (Table 9-7): 540 lb/yd 3 (For concrete to be placed on ground) Cement based on w/c : 280/0.43 = 651 lb/yd 3 Use 651 lb/yd 3
(5) Determine Coarse Aggregate Content
(F.M. of sand = 2.8, nom. max. size = 3/4 inch) Dry wt. of C.A.per yd^3 of concrete:
(6) Determine Fine Aggregate Content Absolute Volume of ingredients (per yd^3 ) Water 280/62.4 = 4.487 ft 3 Cement 651/(3.15X62.4) = 3.312 ft 3 C.A. 1707/(2.68X62.4) = 10.207 ft 3 Air 0.06 X 27 = 1.62 ft 3 Total = 19.626 ft 3
(7) Adjustments for Agg. Moisture Wt. of agg. in natural moisture condition:
Additional water needed: Coarse Agg.: 1707 X (0.005 - 0.003) = 3.4 lb Fine Agg.: 1215 X (0.007 - 0.005) = 2.4 lb 6 lb
