air-entrained aids durability and watertightness, you may also specify air-entrained concrete at this time.
How do I want my plastic concrete to act?
You want your plastic concrete to have workability, uniformity, and homogeneity. Since the slump test is a
measure of workability and uniformity, your next step is to select a slump.
Are my aggregates suitable? How do they affect the workability of the mix?
You especially want your sand and gravel to be clean (this you will check). Additionally, you will determine the
relative fineness or coarseness of your sand, since the finer the sand the greater the workability. Additionally, you
want to make sure that the available gravel is not so large as to induce a plane of weakness in your structure or to
bridge the reinforcing steel, causing honeycombing.
What is the proper mix of ingredients?
You want to find the amounts of cement, water, sand, and gravel required to give you the qualities in the plastic
and hardened concrete you desire. Using the absolute volume mix design procedures, you will determine the
preliminary proportions for the components of your concrete. You will have to adjust this mix design at the job
site due to the moisture condition of the aggregates.
PART A - METHOD OF CONSIDERATIONS
SELECTING MIX PROPORTIONS
Concrete proportions for a particular application are determined by the concrete's end use and by anticipated
conditions at the time of placement. You must strike a balance between reasonable economy and the
requirements for placability, strength, durability, density, and appearance which may be in the job specifications.
Before proportioning a concrete mixture, you must have certain information about a job, such as the size and
shape of structural members, the concrete strength required, and the exposure conditions. Other important factors,
discussed below, are the w/c ratio, aggregate characteristics, amount of entrained air, and slump.
Water and Cement Ratio
The w/c ratio is determined by the strength, durability, and watertightness requirements of the hardened
They are usually specified by the structural design engineer, but you can arrive at
tentative mix proportions 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 2-1 to select a suitable
w/c ratio for normal-weight concrete that will meet the anticipated exposure conditions. Note that the
w/c ratios in Table 2-2 on page 2-4 are based on concrete strength under certain exposure conditions.
If possible, perform tests using job materials to determine the relationship between the w/c ratio you select and
strength of the finished concrete. If you cannot obtain laboratory test data or experience records for the
relationship, use the data in Table 2-2 as a guide. Enter Table 2-2 at the desired f 'c (specified compressive
strength of concrete in pounds per square inch (psi)) and read across to determine the maximum w/c ratio.