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CONCRETE MIX DESIGN

Objective of concrete mix design:

To determine the most economical and

practical combination of available materials

(i.e. aggregates, cements, water, admixtures,

etc.) to produce a concrete that will satisfy

the performance requirements.

Design Considerations:

Acceptable workability of fresh concrete.

Adequate durability and strength of

hardened concrete.

Uniform appearance of hardened concrete.

Economy.

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Most Important Factor Affecting Workability: Water

Content

Most Important Factor Affecting Strength: Water-

Cement Ratio

Most Important Factors Affecting Durability: Water-

Cement Ratio and Cement Content

Most Important Factor Affecting Appearance:

Proportioning of fine and coarse aggregates

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ABSOLUTE VOLUME METHOD

EXAMPLE 1

Design Criteria:Structure: Foundation

Exposure: Below ground, mild climate, no sulfates

Specified compressive strength, fc:

3,000 psi @ 28 days

Data not available to establish the standard deviation of

compressive strength.

Slump of fresh concrete: 3 - 4 inches

Properties of aggregates to be used:Coarse Agg. Fine Agg.

Spec. gravity 2.68 2.64

Absorption 0.5% 0.7%

Natural moisture 2% 6%

Dry-rodded unit wt. 100 pcf -

Nom. max. size of agg. 1.5 inches -

Fineness modulus - 2.8

Cement used: Type I, non-air-entraining

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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, fcr is

equal to the larger of the following two values:

fcr = fc + 1.34 S

fcr = fc + 2.33 S - 500

where

fc = specified compressive strength, psi

S = standard deviation, psi

If data are not available to establish the standard deviationof the compressive strength, use Table 9-11.

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(3) Determine Mixing Water Requirement & Air

Content

Mixing water (Table 9-5) : 300 lb / yd3 (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/yd3

(This table is applicable since the concrete is to be

placed below ground)

Cement based on w/c : 300/0.55 = 545 lb/yd3

Use 545 lb/yd3

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Water Requirement for Various Slumps and Aggregate Sizes

Water Requirement for Various Slumps and Aggregate Sizes

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(7) Adjustments for Agg. Moisture

Wt. of agg. in natural moisture condition:

Coarse Agg.: 1917 X 1.02 = 1955 lb

Fine Agg.: 1266 X 1.06 = 1342 lb

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

Adjusted mixing water: 300 - 96 = 204 lb

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Trial Mix

Under-sanded mix

Concrete mix with proper amount of sand

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Check unit weight of trial mix

Check air content

of trial mix

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EXAMPLE 2

Design Criteria:Structure: Loading-dock slab

Exposure: Severe freeze-thaw, no deicers or

sulfates

Specified compressive strength (fc) : 4,000

psi @ 28 days. Standard deviation of

compressive strength is 450 psi .

Slump of fresh concrete: 1 - 2 inches

Properties of aggregates to be used:

Coarse Agg. Fine Agg.

Spec. gravity 2.68 2.64

Absorption 0.5% 0.7%

Natural moisture 0.3% 0.5%

Dry-rodded unit wt. 102 pcf -

Nom. max. size of agg. 3/4 inch -

Fineness modulus - 2.8

Cement Used: Type I with air-entraining admixture

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DESIGN PROCEDURES:

(1) Determine Required Average Compressive Strengthfcr = fc + 1.34 S

= 4000 + 1.34 (450)

= 4603 psi

fcr = fc + 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) : 0.43w/c for exposure (Table 9-1): 0.45

Design strength (4600 psi) > 4500 psi O.K.(Table 9-2 is not applicable)

Use w/c = 0.43(3) Determine Mixing Water Requirement & Air Content

Mixing water (Table 9-5) : 280 lb / yd3 (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/yd3(For concrete to be placed on ground)

Cement based on w/c : 280/0.43 = 651 lb/yd3

Use 651 lb/yd3

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(5) Determine Coarse Aggregate Content

Dry-rodded volume (Table 9-4) 0.62 yd3/yd3

(F.M. of sand = 2.8, nom. max. size = 3/4 inch)Dry wt. of C.A.per yd3 of concrete:

0.62 X 27 X 102 = 1707 lb.

(6) Determine Fine Aggregate Content

Absolute Volume of ingredients (per yd3)

Water 280/62.4 = 4.487 ft3

Cement 651/(3.15X62.4) = 3.312 ft3

C.A. 1707/(2.68X62.4) = 10.207 ft3

Air 0.06 X 27 = 1.62 ft3

Total = 19.626 ft

3

Absolute Volume of Sand = 27 - 19.626 = 7.374 ft3

Dry Wt. of sand = 7.374 X 2.64 X 62.4 = 1215 lb.

(7) Adjustments for Agg. MoistureWt. of agg. in natural moisture condition:

Coarse Agg.: 1707 X 1.003 = 1712 lb

Fine Agg.: 1215 X 1.005 = 1221 lb

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

Adjusted mixing water: 280 + 6 = 286 lb

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Summary of Mix Ingredients for 1 yd3 of concrete

Water 286 lb

Cement 651 lb

Coarse Agg. 1712 lb

Fine Agg. 1221 lb

3870 lb

Calculated Unit Wt.: 3870/27 = 143.3 pcf