8  PCC - ACI Method

The American Concrete Institute (ACI) mix design method is but one of many basic concrete mix design methods available today.  This section summarizes the ACI absolute volume method because it is widely accepted in the U.S. and continually updated by the ACI.  Keep in mind that this summary and most methods designated as "mix design" methods are really just mixture proportioning methods.  Mix design includes trial mixture proportioning (covered here) plus performance tests.

This section is a general outline of the ACI proportioning method with specific emphasis on PCC for pavements.  It emphasizes general concepts and rationale over specific procedures.  Typical procedures are available in the following documents:

• The American Concrete Institute's (ACI) Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete (ACI 211.1-91) as found in their ACI Manual of Concrete Practice 2000, Part 1: Materials and General Properties of Concrete.
• The Portland Cement Association's (PCA) Design and Control of Concrete Mixtures, 14th edition (2002) or any earlier edition.

The standard ACI mix design procedure can be divided up into 8 basic steps:

1. Choice of slump
2. Maximum aggregate size selection
3. Mixing water and air content selection
4. Water-cement ratio
5. Cement content
6. Coarse aggregate content
7. Fine aggregate content
8. Adjustments for aggregate moisture

8.1  Slump

The choice of slump is actually a choice of mix workability.  Workability can be described as a combination of several different, but related, PCC properties related to its rheology:

• Ease of mixing
• Ease of placing
• Ease of compaction
• Ease of finishing

Generally, mixes of the stiffest consistency that can still be placed adequately should be used (ACI, 2000).  Typically slump is specified, but Table 5.14 shows general slump ranges for specific applications.  Slump specifications are different for fixed form paving and slip form paving.  Table 5.15 shows typical and extreme state DOT slump ranges.

Table 5.14: Slump Ranges for Specific Applications (after ACI, 2000)

Table 5.15: Typical State DOT Slump Specifications (data taken from ACPA, 2001)

8.2  Maximum Aggregate Size

Maximum aggregate size will affect such PCC parameters as amount of cement paste, workability and strength.  In general, ACI recommends that maximum aggregate size be limited to 1/3 of the slab depth and 3/4 of the minimum clear space between reinforcing bars.  Aggregate larger than these dimensions may be difficult to consolidate and compact resulting in a honeycombed structure or large air pockets.  Pavement PCC maximum aggregate sizes are on the order of 25 mm (1 inch) to 37.5 mm (1.5 inches) (ACPA, 2001).

8.3  Mixing Water and Air Content Estimation

Slump is dependent upon nominal maximum aggregate size, particle shape, aggregate gradation, PCC temperature, the amount of entrained air and certain chemical admixtures.  It is not generally affected by the amount of cementitious material.  Therefore, ACI provides a table relating nominal maximum aggregate size, air entrainment and desired slump to the desired mixing water quantity.  Table 5.16 is a partial reproduction of ACI Table 6.3.3 (keep in mind that pavement PCC is almost always air-entrained so air-entrained values are most appropriate).  Typically, state agencies specify between about 4 and 8 percent air by total volume (based on data from ACPA, 2001). 

Note that the use of water-reducing and/or set-controlling admixtures can substantially reduce the amount of mixing water required to achieve a given slump.

Table 5.16: Approximate Mixing Water and Air Content Requirements
for Different Slumps and Maximum Aggregate Sizes (adapted from ACI, 2000)

8.4  Water-Cement Ratio

The water-cement ratio is a convenient measurement whose value is well correlated with PCC strength and durability.  In general, lower water-cement ratios produce stronger, more durable PCC.  If natural pozzolans are used in the mix (such as fly ash) then the ratio becomes a water-cementitious material ratio (cementitious material = portland cement + pozzolonic material).  The ACI method bases the water-cement ratio selection on desired compressive strength and then calculates the required cement content based on the selected water-cement ratio.  Table 5.17 is a general estimate of 28-day compressive strength vs. water-cement ratio (or water-cementitious ratio).  Values in this table tend to be conservative (ACI, 2000).  Most state DOTs tend to set a maximum water-cement ratio between 0.40 - 0.50 (based on data from ACPA, 2001).

Table 5.17: Water-Cement Ratio and Compressive Strength Relationship
(after ACI, 2000)  

8.5  Cement Content

Cement content is determined by comparing the following two items:

• The calculated amount based on the selected mixing water content and water-cement ratio.
• The specified minimum cement content, if applicable.  Most state DOTs specify minimum cement contents in the range of 300 - 360 kg/m³ (500 - 600 lbs/yd³). 

An older practice used to be to specify the cement content in terms of the number of 94 lb. sacks of portland cement per cubic yard of PCC.  This resulted in specifications such as a "6 sack mix" or a "5 sack mix".  While these specifications are quite logical to a small contractor or individual who buys portland cement in 94 lb. sacks, they do not have much meaning to the typical pavement contractor or batching plant who buys portland cement in bulk.  As such, specifying cement content by the number of sacks should be avoided.

8.6  Coarse Aggregate Content

Selection of coarse aggregate content is empirically based on mixture workability.  ACI  recommends the percentage (by unit volume) of coarse aggregate based on nominal maximum aggregate size and fine aggregate fineness modulus.  This recommendation is based on empirical relationships to produce PCC with a degree of workability suitable for usual reinforced construction (ACI, 2000).  Since pavement PCC should, in general, be more stiff and less workable, ACI allows increasing their recommended values by up to about 10 percent.  Table 5.18 shows ACI recommended values.

Table 5.18: Volume of Coarse Aggregate per Unit Volume of PCC
for Different Fine aggregate Fineness Moduli for Pavement PCC (after ACI, 2000)

8.7  Fine Aggregate Content

At this point, all other constituent volumes have been specified (water, portland cement, air and coarse aggregate).  Thus, the fine aggregate volume is just the remaining volume:

8.8  Adjustments for Aggregate Moisture

Unlike HMA, PCC batching does not require dried aggregate.  Therefore, aggregate moisture content must be accounted for.  Aggregate moisture affects the following parameters:

1. Aggregate weights.  Aggregate volumes are calculated based on oven dry unit weights, but aggregate is typically batched based on actual weight.  Therefore, any moisture in the aggregate will increase its weight and stockpiled aggregates almost always contain some moisture.  Without correcting for this, the batched aggregate volumes will be incorrect.
2. Amount of mixing water.  If the batched aggregate is anything but saturated surface dry it will absorb water (if oven dry or air dry) or give up water (if wet) to the cement paste.  This causes a net change in the amount of water available in the mix and must be compensated for by adjusting the amount of mixing water added.

8.9  Summary

The ACI mix design method is one of many available methods.  It has been presented here to give a general idea of the types of calculations and decisions that are typical in PCC mix design.