Use fine aggregate to fill the spaces between coarse-aggregate particles and increase the workability of a
mix. In general, aggregate that does not have a large grading gap nor an excess of any size that does give
a smooth grading curve, produces the best mix. Ensure that the fineness modules are between 2.3 and 3.1
(see page 1-8). Ensures that the excess fines, material finer than the No 200 sieve, are limited to less than
3 or 5 percent (see Lesson 1).
Use the largest practical size of coarse aggregate in the mix. The maximum size of coarse aggregate that
produces concrete of maximum strength for a given cement content depends on the aggregate source as
well as aggregate shape and grading. The maximum size aggregate should not exceed one-fifth the
minimum dimension of the member of three-fourths the space between reinforcing bars. For pavement or
floor slabs, the maximum size aggregate should not exceed one-third the slab thickness. Equipment limits
the aggregate size also, 16S = 3 inches and M919 = 1 1/2 inches.
Use entrained air in all concrete exposed to freezing and thawing and, sometimes, under mild exposure conditions
to improve workability. Always use entrained air in paving and concrete, regardless of climatic conditions. Table
1-5 on page 1-18 gives recommended total air contents of air-entrained concrete. The upper half of Table 1-5
gives the approximate percent of entrapped air in non-air-entrained concrete, and the lower half gives the
recommended average, total air-content percentages for air-entrained concrete based on level of exposure.
Mild exposure. This includes indoor and outdoor service in a climate that does not expose the concrete to
freezing or deicing agents. When you want air-entrainment for a 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 lower than those required for durability.
Moderate exposure. This means service in a climate where freezing is expected but where the concrete is
not continually exposed to moisture or free water for long periods before freezing, deicing agents, or other
aggressive chemicals. Examples are exterior beams, columns, walls, girders, or slabs that do not contact
wet soil or receive direct applications of deicing salts.
Severe exposure. This means service where the concrete is exposed to deicing chemicals or other
aggregate agents or where it continually contacts moisture or free water before freezing. Examples are
pavements, bridge decks, curbs, gutters, sidewalks, canal linings, or exterior water tanks or sumps.
The slump test (see Part C on page 1-20) measures the workability and consistency of concrete. Do not use it to
compare mixes having completely different proportions or mixes containing different aggregate sizes. When
testing different batches of the same mixture, change in slump indicate changes in materials, mix proportions, or
water content. Table 2-4 on page 2-6 gives recommended slump ranges.