2-ACI 211MixDesignGuideHASSIOTIS

7/28/2019 2-ACI 211MixDesignGuideHASSIOTIS

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2-ACI 211.1-91 Standard Practice for Selecting Proportions for

Normal, Heavyweight, and Mass Concrete---Procedure for Mix

Design.

S. Hassiotis (Last updated Fall, 2011)

The ACI 211.1-91 (Reapproved 2009) describes methods to create mix designs for

hydraulic cement concrete. In this summary, I will borrow heavily from the language,

graphs, and tables of the manual to create a guideline for mix design of normal weight

concrete that will be used for educational purposes at Stevens.

2.1 Some Basic Definitions

The ACI method is based on the Absolute Volume Method in selecting the proportions

of the materials needed for normal weight concrete. The Absolute Volume is defined as

the volume a material would occupy if it was solid and without voids

Absolute Volume = Weight /{ S.G. x 62. 4 lb/ft3

}

where S.G. is the specific gravity of a material and 62.4 lb/ft3

is the density of water.

For example, a bag of cement (94 lbs.) in a bulk state occupies approximately 1 cubic

foot of volume. If the cement was consolidated and without voids it would

approximately occupy only 0.48 cubic feet. Therefore, the volume the cement will

actually occupy in a batch of concrete will be its absolute volume of 0.48 cubic feet.

EXAMPLE: What is the absolute volume occupied by cement in a cubic yard of

concrete which contains 588 lbs of cement?

Absolute Volume = 588 lb /{ 3.15 x 62.4 lb/ft3

}=2.99 ft3

where the specific gravity of cement is given as 3.15.

EXAMPLE: What is the weight of cement that occupies an absolute volume of 1 cubicfoot?

Weight = Absolute Volume x S.G. x 62.4 lb/ft3

Weight= 1 x 3.15 x 62.4 = 196.56 lb/ft3


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2.2 Helpful Conversion Factors

One cubic foot of water = 7.5 gallons = 62.4 lbs.

One bag of cement = 94 lbs. (42.6 kg)

Specific gravity of cement is 3.151.308 cubic yards = One cubic meter

1 gallon of water = 8.33 lbs. = 3.78 liters

1 gallon per yard = 5 liters per meter

One cubic yard = 27 cubic feet

One bag of cement = one cubic foot (loose volume)

One bag of cement = 0.48 cubic feet (absolute volume)

2.3 Procedure for Mix Design

The steps for the mix design will be presented in parallel with the following example.

EXAMPLE: Design a batch of 1.25 f3 of concrete for a footing. Design for a strength of4000 psi. The available materials, along with the data that we will need to design the

mix, are given in the table below.

Coarse Aggregate Fine Aggregate Cement

Type Alluvial Rock sand Type I

Absorption 1% 1.1%

Moisture Content 1.95% 6.3%

Specific Gravity 2.62 2.6 3.15

Dry-Rodded Unit Weight 98.5 lb/f3

Max Size 1 in

Finess Modulus 2.65

STEP 1. Choice of Slump

If slump is not specified, useTable 6.3.1 to use an appropriate value. In this example,

for footings the slump should be between 1 and 3 inches. Use a slump of 3 for a mix

that is more workable.

STEP 2. Choice of maximum size aggregate.

The nominal maximum size of aggregate should be the largest that is economically

available and consistent with the dimensions of the structure. (See full report for more

information). In this example, the maximum size is 1 inch.

STEP 3. Estimation of mixing water and air content.

The quantity of water per unit volume of concrete required to produce a given slump is

dependent on: the nominal maximum size, particle shape, and grading of the

aggregates; the concrete temperature; the amount of entrained air; and the use of

chemical admixures. Table 6.3.3 provides estimates of required water. Depending on


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aggregate texture and shape, mixing water requirements may vary above or below

those shown in the table.

Before you can use Tabel 6.3.3 you must decide if you need air-entrained concrete.

Usually, if the concrete is exposed to freeze-thaw, as is the case of concrete placed in

foundations, you will need air-entrainment.

For our example, for a slump of 3 inches and a maximum aggregate size of 1 inch, use

the air-entrained part of the table to find that you will need 295 lbs of water per cubic

yard of concrete. The fine print under the table specifies that the water should be

reduced by 25 lbs if you are using well rounded aggregates. Therefore, in this example

we will use 270 lbs of water.

The same table also shows that the recommended percent of air content is 6% for

severe exposure (concrete that is exposed to deicing chemicals, or may become highly

saturated by continued contact with moisture or free water prior to freezing-- such as

pavements, bridge decks, foundations, etc.)

STEP 4. Selection of Water-Cement or Water-cementitious materials ratio.

The water/cement (w/c) or water/(cement + pozzolans) [w/(c+p)]determines the

strength and durability of the concrete.

Since different aggregates, cements and cementitious materials (fly ash, slag,silica

fume, etc) generally produce different strengths at the same w/c ratios, it is highly

desirable to have or develop the relationship of strength to w/c ratio for the materials

actually used. In absence of such data, Table 6.3.4(a) and (b) can be used. The

strength shown in the tables is for test specimens cured for 28 days in laboratory

conditions.

In this example, for a 4000psi, air-entrained concrete you should pick w/c = 0.48 (themaximum permissible is 0.5 taken from Table 6.3.4(b))

STEP 5. Calculation of cement content.

Water/cement=0.48

Water weight = 295 lbs (step 3)

Cement=270/0.48=560 lbs

STEP 6. Estimation of coarse aggregate content.

The volume of coarse aggregate for one cubic yard of concrete is given inTable 6.3.6.

For an aggregate size of 1 inch and Fineness Modulus of Sand of 2.65 the volume of

coarse aggregate per cubic yard of concrete is approximately 0.69


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The dry weight of coarse aggregate then is its volume times its dry-rodded weight per

cubic foot, ie,

Weight of coarse aggregate=0.69*98.5 lbs/f3 * 27 f3/yard3=1,840 lbs/yard3

STEP 7. Estimation of fine aggregate content.

At this point all the ingredients of the concrete have been estimated except the fine

aggregates. The ACI committee report allows either the weight method (6.3.7.1) or the

absolute volume method (6.3.7.2) to be used to find the amount of fine aggregates. The

second method is the most accurate and will be summarized here.

Calculate the absolute volumes of all materials using

Absolute Volume=Weight/(S.G.)x 62.4

Dry Weight (lbs) S.G. Absolute Volume (f3)

Water 270 1 4.3

Entrapped Air 6% 0.06 x 27 f3=1.6Coarse Aggregate 1840 2.62 11.3

Cement 560 3.15 2.8

Total 20.0

Calculate the Weight of the Sand

Absolute Volume(f3) S.G. Dry Weight (lbs)

Sand 27-20=7 2.6 7*2.6*62.4=1135

STEP 8. Adjustments for aggregate moisture.

In this example, the total moisture content (absorbed + free water) is 1.95% for thecoarse aggregate and 6.3% for the sand. The absorbed water is 1% for the coarse

aggregate and 1.1% for the sand.

The weights that we calculate above are dry weights. For the batch we will add

Coarse aggregate, Wet=1840*1.0195=1875 lbs

Fine aggregate, Wet=1135*1.063=1206 lbs.

Absorbed water does not become part of the mixing water. However, the free water will

add to the water content so we must account for it.

The surface water contributed by the coarse aggregate is 1.95-1=0.95%

The surface water contributed by the fine aggregate is 6.3-1.1=5.2%

The estimated requirement for added water, therefore, becomes

270 lbs1840(0.0095) - 1135(0.052) = 193 lbs

The batch mix then is


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Water, to be added 193 lbs Note, total wt/yd3=3830. See Table 6.3.7.1

Cement 560 lbs

Coarse Aggregate, Wet 1875 lbs

Fine Aggregate, Wet 1206 lbs


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2.4 Class Problem

Create the mix for one cubic yard of concrete for the following specifications.

Type I cement

Coarse aggregate bulk specific gravity=2.68 and an absorption of 0.5%. Dry unit wt of

coarse aggr.=100 lbs/cubic foot. Maximum aggr. Size =1.5 in

Fine aggregate bulk specific gravity=2.64, an absorption of 0.7% and a Fineness

modulus of 2.8

Concrete is required for a portion of a structure that will be below ground level in a

location where it will NOT be exposed to severe weathering or sulfate attack.

Structural considerations require it to have an average 28-day compressive strength of

3500 psi. A slump of 3 to 4 inches is required.

ANSWER

Water to be added 199 lbs

Cement 484 lbs

Coarse Aggr. 1955 lbs

Fine Aggr. 1451 lbs

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2-ACI 211MixDesignGuideHASSIOTIS