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.
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.
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
Table 3-2. Maximum permissible W/C ratios for concrete
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.
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.
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.
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.
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.
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)
|David L. Heiserman, Editor||
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Revised: June 06, 2015