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SECTION I. METHOD CONSIDERATIONS

SELECTING MIX PROPORTIONS

3-2. Concrete proportions for a particular application are determined by the concrete's end use and by anticipated conditions at the time of placement. Find a moderate balance between reasonable economy and the requirements for placeability, strength, durability, density, and appearance that may be in the job specifications.

BASIC GUIDELINES

3-3. Before proportioning a concrete mixture, certain information about a job is necessary, such as the size and shape of the structural members, the concrete strength required, and the exposure conditions. Other important factors discussed below are the W/C ratio, aggregate characteristics, the amount of entrained air, and the slump.

W/C RATIO

3-4. The W/C ratio is determined by the strength, durability, and watertightness requirements of the hardened concrete. Strength, durability and watertightness are usually specified by the structural-design engineer, but a tentative mix proportion can be determined from knowledge of a prior job. Always remember that a change in the W/C ratio changes the characteristics of the hardened concrete. Use Table 3-1 to select a suitable W/C ratio for normal-weight concrete that will meet the anticipated exposure conditions. The W/C ratios in Table 3-1 are based on concrete's strength under certain exposure conditions. If possible, perform tests using job materials to determine the relationship between the W/C ratio selected and the strength of the finished concrete. If laboratory-test data is not obtainable or experience records for the relationship are unavailable, use the data in Table 3-2 as a guide. In Table 3-2, locate the desired compressive strength of concrete in pounds per square inch (psi) and read across to determine the maximum W/C ratio. If possible, interpolate between the values. When both exposure conditions and strength must be considered, use the lower of the two indicated W/C ratios. If flexural strength rather than compressive strength is the basis for a design, such as a pavement, perform the necessary tests to determine the relationship between the W/C ratio and flexural strength. An approximate relationship between flexural and compressive strength is as follows:

Table 3-1. Maximum W/C ratios for various exposure conditions

Exposure Condition Normal-Weight Concrete
(Absolute W/C Ratio by Weight)
Concrete protected from exposure to freezing and thawing or
the application of deicer chemicals
Select a W/C ratio on the basis of strength, workability, and finishing needs
Watertight concrete*
  • In freshwater
  • In seawater


0.50
0.45
Frost-resistant concrete*
  • Thin sections; any section with less than a 2-inch cover
    over reinforcement and any concrete exposed to deicing salts
  • All other structures


0.45
0.50
Exposure to sulfates*
  • Moderate
  • Severe


0.50
0.45
Concrete placed underwater Do not use less than 650 pounds of cement per cubic yard (386 kg/m3).
Floors on grade Select W/C ratio for strength, plus minimum cement requirements described in Table 3-7.
* For the properties of watertight concrete, frost-resistant concrete and exposure
to sulfates, use designing strength for air-entrained concrete.

 

Table 3-2. Maximum permissible W/C ratios for concrete

Specified Compressive Strength, in psi* Maximum Absolute Permissible W/C Ratios by Water
Nonair-Entrained Concrete Air-Entrained Concrete
2,500 0.67 0.54
3,000 0.58 0.46
3,500 0.51 0.40
4,000 0.44 0.35
4,500 0.38 **
5,000 ** **
NOTE: 1,000 psi = 7 MPa.

*28-day strength. The W/C ratios will provide average strengths that are greater than the specified strengths.

**For strength above 4,500 psi (nonair-entrained concrete) and 4,000 psi (air-entrained concrete), proportions should be established by the trial-batch method.

Where:

f''c = compressive strength, in psi
R = flexural strength (modulus of rupture), in psi
K = a constant, usually between 8 and 10

AGGREGATE

3-5. Use FA to fill the spaces between the CA particles and to increase the workability of the mix. Aggregate that does not have a large grading gap nor an excess of any size but gives a smooth grading curve produces the best mix. Fineness modulus and FA grading are discussed in Lesson 2.

3-6. Use the largest practical size of CA in the mix. The maximum size of CA that produces concrete of maximum strength for a given cement content depends upon the aggregate source as well as the aggregate shape and grading; thus, in most cases, a decrease will take place in the overall cost. The larger the maximum size of the CA, the less paste (water, cement and usually entrained air) that is required for a given concrete quality. The maximum size of aggregate should not exceed one-fifth the minimum dimension of the member or three-fourths the space between reinforcing bars. For pavement or floor slabs, the maximum size of aggregate should not exceed one-third the slab thickness.

ENTRAINED AIR

3-7. To improve workability, use entrained air in all concrete that is exposed to freezing and thawing and, sometimes, to mild conditions. Always use entrained air in paving concrete, regardless of climatic conditions. Table 3-3 gives the recommended total air contents of air-entrained concrete. When mixing water remains constant, air entrainment increases slump. When the cement content and slump remain constant, less water is required. The resulting decrease in the W/C ratio helps to offset possible strength decreases and improves other paste properties, such as permeability. The strength of air-entrained concrete may equal, or nearly equal, that of nonair-entrained concrete when cement contents and slumps are the same. The upper half of Table 3-3 gives the percentage of entrapped air in concrete. The lower half of the table gives the recommended average, total air content, and percent for air-entrained concrete based exposure levels

Table 3-3. Approximate mixing-water and air content requirements for different slumps and maximum sizes of aggregate.

Maximum Aggregate
Size: _____
3/8
Inch
1/2
Inch
3/4
Inch
1
Inch
1 1/2
Inches
2**
Inches
3**
Inches
6**
Inches
Water in Pounds Per Cubic Yard of Concrete*
Slump, in Inches

1 to 2

Nonair-Entrained Concrete
350 335 315 300 275 260 240 210
3 to 4 385 365 340 325 300 285 285 230
6 to 7 410 385 360 340 315 300 285  
  Air-Entrained Concrete
1 to 2 305 295 280 270 250 240 225 200
3 to 4 340 325 305 295 275 265 250 220
6 to 7 365 345 325 310 290 280 270 -
Approximate percentage amount of entrapped air in nonair-entrained concrete
  3 2.5 2 1.5 1 0.5 0.3 0.2
Recommended percentage average and total air content for air-entrained concrete
Anticipated Usage  
Mild exposure 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0
Moderate exposure 6.0 5.5 5.0 4.5 4.5 4.0 3.5 3.0
Severe exposure 7.5 7.0 6.0 6.0 5.5 5.0 4.5 4.0
*These quantities of mixing water are for use in computing cement factors for trial batches.
They are maximums for reasonably well-shaped, angular CA graded within limits of accepted specifications.

**The slump values for concrete containing aggregate larger than 1 1/2 inches are based on slump tests
made after removal of particles larger than 1 1/2 inches by wet screening.

Mild Exposure

3-8. Mild Exposure includes indoor or outdoor service in a climate that does not expose the concrete to freezing or deicing agents. When you want air entrainment for any reason other than durability, such as to improve workability or cohesion or to improve strength in low-cement-factor concrete, you can use air contents that are lower than those required for durability.

Moderate Exposure

3-9. Moderate exposure means service in a climate where freezing is expected, but where the concrete is not continually exposed to moisture or free-standing water for long periods before freezing or to deicing agents or other aggressive chemicals. Structures that do not contact wet soil or receive direct applications of deicing salts are exterior beams, columns, walls, girders, and slabs.

Severe Exposure

3-10. Severe exposure means service where the concrete is exposed to deicing chemicals or other aggressive agents or where the concrete continually contacts moisture or free-standing water before freezing. Examples are pavements, bridge decks, curbs, gutters, sidewalks, canal linings, or exterior water tanks or slumps.

SLUMP

3-11. The slump test (see Appendix B) measures the consistency of concrete in cubic yards. Do not use it to compare mixes having different proportions or mixes containing different sizes of aggregate. When testing different batches of the same mixture, changes in slump indicate changes in materials, mix proportions, or water content. Table 3-4 gives the recommended slump ranges.

Table 3-4. Slumps for various types of construction (with vibration)

Concrete Construction Slump, in Inches
Maximum* Minimum*
Reinforced foundation walls and footings 3 1
Plain footing, caissons, and substructure walls 3 1
Beams and reinforced walls 4 1
Building columns 4 1
Pavements and slabs 3 1
Mass concrete 2 1
NOTE: 1 inch = 25 mm

*May be increased 1 inch for consolidation by methods such as rods and spades.

David L. Heiserman, Editor

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All Rights Reserved

Revised: June 06, 2015