Specifications: ACI 211.

4R-7

HIGH-STRENGTH CONCRETE
The high-strength concrete mixtures are applicable to normal weight, non-air entrained
concrete having compressive strengths between 6000 and 12,000 psi. When
proportioning high strength concrete mixtures, the basic considerations are still to
determine the ingredient quantities required to produce a concrete with the desired plastic
properties (workability, finishability, etc.) and hardened properties (strength, durability,
etc.) at the lowest cost. Proper proportioning is required for all materials used. Because
the performance of high-strength concrete is highly dependent on the properties of its
individual components. To achieve high strength, it is necessary to use lowest possible
water-cement ratio, which invariably affects the workability of the mix and necessitates
the use of special vibration techniques for proper compaction.

Steps of High Strength Concrete Mix Design Proportions:

Step 1-Select slump and required concrete strength.
Step 2-Select maximum size of Aggregate-Based on strength requirements.
Step 3-Select optimum coarse aggregate content.
Step 4-Estimate mixing water and air contents.
Step 5-Select w/c in high-strength concrete mixtures.
Step 6-Calculate content of cementitious material.
Step 7-Proportion basic mixture with no other cementitious material
Step 8- Proportion companion mixtures using fly ash.
Step 9-Trial mixtures

Step 1-Select slump and required concrete strength:

Recommended values for concrete slump are given in Table 4.3.1. Although high-
strength concrete with HRWR has been produced successfully without a measurable
initial slump, an initial starting slump of 1 to 2 in prior to adding HRWR is recommended.
For high-strength concretes made without HRWR, a recommended slump range of 2 to
4 in. may be chosen according to the type of work to be done. A minimum value of 2 in.
of slump is recommended for concrete without HRWR.
Step 2-Select maximum size of Aggregate-Based on strength requirements:
The recommended maximum sixes for coarse aggregates are given in Table 4.3.2. ACI
318 states the maximum size of an aggregate should not exceed one-fifth of the narrowest
dimension between sides of forms, one-third of the depth of slabs, nor three-quarters of
the minimum clear spacing between individual reinforcing bars, bundles of bars, or pre
stressing tendons or ducts.

Step 3-Select optimum coarse aggregate content:

The optimum content of the coarse aggregate depends on its strength potential
characteristics and maximum size. The recommended optimum coarse aggregate
contents, expressed as a fraction of the dry-rodded unit weight (DRUW), are given in
Table 4.3.3 as a function of nominal maximum size.
Once the optimum coarse aggregate content has been chosen from Table 4.3.3, the
oven-dry (OD) weight of the coarse aggregate per yd3 of concrete can be calculated using
Eq: (4-1)
weight of coarse aggregate (O.D.) = (% x DRUW) x (% x 27)

Step 4-Estimate mixing water and air contents:

The quantity of water per unit volume of concrete required to produce a given slump is
dependent on the maximum size, particle shape, and grading of the Aggregate, the
quantity of cement, and type of water-reducing admixture used. If an HRWR is used, the
water content in this admixture is calculated generally to be a part of the w/c+p. Table
4.3.4 gives estimates of required mixing water for high-strength concretes made with 3/8
to 1 in. maximum-size aggregates prior to the addition of any chemical admixture. Also
given are the corresponding values for entrapped air content. These quantities of mixing
water are maximums for reasonably well-shaped, clean, angular coarse aggregates, well-
graded within the limits of ASTM C 33.
The values for the required mixing water given in Table 4.3.4 are applicable when a fine
aggregate is used that has a void content of 35 percent. The void content of a fine
aggregate may be calculated using Eq (4-2).
When a fine aggregate with a void content not equal to 35 percent is used, an adjustment
must be made to the recommended mixing water content. This adjustment may be
calculated using Eq. (4-3)
Mixing water adjustment, lbs/yd3 = (V - 35) X 8 Eq. (4-3)

Step 5-Select w/c in high-strength concrete mixtures:

In high-strength concrete mixtures, other cementitious material, such as fly ash, may be
used. The w/c+p is calculated by dividing the weight of the mixing water by the combined
weight of the cement and fly ash. In Tables 4.3.5(a) and (b), recommended maximum
w/c+p are given as a function of maximum-size aggregate.
Step 6-Calculate content of cementitious material:
The weight of cementitious material required per yd3 of concrete can be determined by
dividing the amount of mixing water per yd3 of concrete (Step 4) by the w/c+p ratio (Step
5). However, if the specifications include a minimum limit on the amount of cementitious
material per yd3 of concrete, this must be satisfied.

Step 7-Proportion basic mixture with no other cementitious material:

To determine optimum mixture proportions, the proportioner needs to prepare several
trial mixtures having different fly ash contents. Generally, one trial mixture should be
made with port land cement as the only cementitious material. The following steps should
be followed to complete the basic mixture proportion.
1. Cement content-For this mixture, since no other cementitious material is to be used,
the weight of cement equals the weight of cementitious material calculated in Step 6.
2. Sand content-After determining the weights per yd3 of coarse aggregate, the cement
and water, and the percentage of air content, the sand content can be calculated to
produce 27 ft3, using the absolute volume method.

Step 8- Proportion companion mixtures using fly ash:

The use of fly ash in producing high-strength concrete can result in lowered water
demand, reduced concrete temperature, and reduced cost. However, due to variations in
the chemical properties of fly ash, the strength-gain characteristics of the concrete might
be affected. Therefore, it is recommended that at least two different fly ash contents be
used for the companion trial mixtures. The following steps should be completed for each
companion trial mixture to be proportioned:
1. Fly ash type-Due to differing chemical compositions, the water-reducing and strength-
gaining characteristics of fly ash will vary with the type used, and its source. Therefore,
these characteristics, as well as availability, should be considered when choosing the fly
ash to be used.
2. Fly ash content-The amount of cement to be replaced by fly ash depends on the type
of material to be used. The recommended limits for replacement are given in Table 4.3.6,
for the two classes of fly ash. For each companion trial mixture to be designed, a
replacement percentage should be chosen from this table.
3. Fly ash weight-Once the percentages for replacement have been chosen, the weight
of the fly ash to be used for each companion trial mixture can be calculated by multiplying
the total weight of cementitious materials.
Step 9-Trial mixtures:
For each of the trial mixtures proportioned in Steps 1 through 8, a trial mixture should be
produced to determine the workability and strength characteristics of the mixtures.