mix - university of washingtoncourses.washington.edu/cm425/mix.pdf · the most economical mix is...

Concrete TechnologyProportioning Concrete Mixes

Professor Kamran M. NematiWinter Quarter 2015 1

Concrete Technology

MIXPROPORTIONING

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Designing Concrete MixturesCement Water Air FA CA



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Designing Concrete Mixtures

Objective To determine the most economical

and practical combination of readily available materials to produce a concrete that will satisfy the performance requirements under particular conditions of use.

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Designing Concrete MixturesFactors to be considered include: Workability

Cohesiveness, slump Placement conditions Strength Durability Appearance Economy

Minimize the amount of cement, Minimize w/c ratio Minimum amount of water, to reduce cement

content do not sacrifice the quality



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Proportioning Absolute volume method

Most commonly used method

Other methods

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Designing Concrete Mixtures Concrete mixture

proportions are usually expressed on the basis of the mass of ingredients per unit volume.

The unit of volume used is either a cubic yard or a cubic meter of concrete.

Absolute Volume

1 yd

1 yd

1 yd



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Factors to consider- aggregates Economical consideration

Minimize water and cement, Stiffest possible mix Largest practical max size of aggregate, Shape,

Surface Texture Optimize ratio of fine to coarse aggregate Grading (Particle Size distribution, PSD) and its

significance, Consistency, Strength, Finishability Size and shape of members

Max size of aggregate (MSA) Physical properties

Strength Exposure condition

Air entraining or not, sulfate attack

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Factors to consider - aggregates Max size-The size of smallest sieve through which

all Material passes. (Nominal Aggregate Size: One sieve size larger than the first sieve to retain more than 10%).

Nominal max size-The largest size of aggregate present in sufficient quantity to center.

The larger the nominal sizes of aggregate, the lower the water content to produce a given slump.

The most economical mix is the one with the largest possible max size aggregate.

Max size of aggregate: < 1/5 of narrowest dimensions of form or ¾ of spacing between rebars or in unreinforced slabs < 1/3 thickness



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Fineness Modulus of Sand

The fineness modulus is calculated from the particle size distribution of the fine aggregate (sand).

Values for sand suitable for concrete should range between 2.3 and 3.1.

Coarse sand has a higher fineness modulus than fine sand.

The fineness modulus influences the bulk volume of coarse aggregate.

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Bulk Volume of Coarse Aggregate Once the MSA and FM of sand are determined,

these values can be used to determine bulk volume of coarse aggregate (CA) per unit volume.



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Air Content Entrained air must be used in concrete that will

be exposed to freezing and thawing and can be used to improve workability even when not requires.

The amount of air required in concrete depends on: Maximum aggregate size (MSA) Level of exposure

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Air Content Requirements of ACI 318 Building Code and ASTM C 94

Typical (entrapped) air contents in non air-entrained Concrete



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Factors to consider-Water to cement ratio Compressive strength is inversely

proportional to W/C Define fc’ = Specified compressive strength at 28

days. The average compressive strength of concrete tested

at 28 days should be equal or greater than fc’.Typical values fc’ = 4000 - 5000 psi

Water to cement ratio

Strength

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Determining Required w/c Ratio The w/c ratio is determined from:

Durability considerations Required strength

Requirements of ACI 318 Building Code



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Requirements of ACI 318 Building Code for Sulphate Exposure

Determining Required w/c Ratio

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W/C Ratio Required for Strength Use data from field or trial mixtures using the

same materials Where no data are available, estimate using the

table shown below:



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Factors to consider - Durability

Choice of slump Maximum size of aggregate W/C Minimum cement content

Factors to consider: finishability, appearance, water resistance, permeability

Admixtures

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Procedures Slump table 9-1 Max size of aggregates Estimate mixing water and air content

Tables 9-2, 9-3, 9-4 use minimum specified of the two. Required air entrainment from table 9-2

Cement content compute from w/c and water content Coarse aggregate table 9-5 Fine aggregate content from volumetric calculations Adjust for aggregate moisture Trial batches



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Concrete Mix Proportioning Tables

Slump (in.)

Types of Construction Maximum* Minimum

Reinforced foundation walls and footings Plain footings, caissons, and substructure walls Beams and reinforced walls Build ing columns Pavements and slabs Mass concrete

3 3

4 4 3 2

1 1

1 1 1 1

TABLE 9.1- ACI RECOMMENDED SLUMPS FOR VARIOUS TYPES

OF CONSTRUCTION

* May be increased 1-in. for methods of consolidation other than vibration.

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Concrete Mix Proportioning TablesTABLE 9-2 - APPROXIMATE MIXING WATER AND AIR CONTENT REQUIREMENTS FOR DIFFERENT

SLUMPS AND MAXIMUM SIZES OF AGGREGATES

Water, lb./ yd 3 of concrete for ind icated maximum sizes of aggregate Slump, in. 3/8 in.* ½ in.* ¾ in* 1 in.* 1½ in.* 2 in.*† 3 in.*† 6 in.*†

Non-air-entrained concrete

1 to 2 3 to 4 6 to 7 More than 7* Approximate amount of entrapped air in non-air-entrained concrete, percent

350 385 410 -- 3

335 365 385 --

2.5

315 340 360 -- 2

300 325 340 --

1.5

275 300 315 -- 1

260 285 300 --

0.5

220 245 270 --

0.3

190 210 -- --

0.2

Air-entrained concrete

1 to 2 3 to 4 6 to 7 More than 7* Recommended average total air content, percent for level of exposure: Mild exposure Moderate exposure Severe exposure

305 340 365 --

4.5 6.0 7.5

295 325 345 --

4.0 5.5 7.0

280 305 325 --

3.5 5.0 6.0

270 295 310 --

3.0 4.5 6.0

250 275 290 --

2.5 4.5 5.5

240 265 280 --

2.0 4.0 5.0

205 225 260 --

1.5**††

3.5**††

4.5**††

180 200 -- --

1.0**††

3.0**††

4.0**††

* These quantities of mixing water are for use in computing cement factors for trial batches. They are maxima for reasonably well-shaped angular coarse aggregates graded within limits of accepted specifications. † The slump values for concrete containing aggregate larger than 1½" are based on slump tests made after removal of particles > 1½" by wet-screening.



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Concrete Mix Proportioning TablesTABLE 9-3 - RELATIONSHIP BETWEEN WATER/CEMENT RATIO AND COMPRESSIVE

STRENGTH OF CONCRETE

Water/cement ratio, by weight Compressive strength at 28

days, psi Non-air-entrained

concrete Air-entrained

concrete 6000 5000 4000 3000 2000

0.41 0.48 0.57 0.68 0.82

---- 0.40 0.48 0.59 0.74

TABLE 9-4 - MAXIMUM PERMISSIBLE WATER/CEMENT RATIOS FOR CONCRETE IN

SEVERE EXPOSURES

Type of

Structure

Structure wet continuously or frequently exposed to

freezing & thawing*

Structure exposed to seawater

Thin sections (railings, curbs, sills, ledges, ornamental work) & sections with less than 1-inch cover over steel

0.45

0.40

All other structures 0.50 0.45

* Concrete should also be air-entrained.

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Concrete Mix Proportioning TablesTABLE 9-5 - VOLUME OF COARSE AGGREGATE PER UNIT OF VOLUME OF CONCRETE

Maximum size of aggregate

Volume of dry-rodded coarse aggregate* per unit volume of concrete for

different fineness moduli of sand

(in.) 2.40 2.60 2.80 3.00 3/8 1/2 3/4 1

1½ 2 3 6

0.50 0.59 0.66 0.71 0.75 0.78 0.82 0.87

0.48 0.57 0.64 0.69 0.73 0.76 0.80 0.85

0.46 0.55 0.62 0.67 0.71 0.74 0.78 0.83

0.44 0.53 0.60 0.65 0.69 0.72 0.76 0.81

* Volumes are based on aggregates in dry-rodded condition as described in ASTM C29 Unit Weight of Aggregate. These volumes are selected from empirical relationships to produce concrete with a degree of workability suitable for usual reinforced construction. For less workable concrete such as required for concrete pavement construction they may be increased about 10 percent. For more workable concrete, such as may sometimes be required when placement is to be by pumping, they may be reduced up to 10 percent.



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Proportioning of a Concrete Mix Proportioning of a concrete mix consists of

determining the relative amounts of materials which will produce a concrete of desired workability of the fresh concrete, and the desired strength, consistency of volume, durability, and economy in the hardened concrete.

These proportions may be determined either by calculations making use of published data (e.g. American Concrete Institute (ACI) “Recommended Practice for Selecting Proportions of Concrete”, or by direct laboratory test -- called the “trial method”.

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Proportioning of a Concrete Mix In the production of concrete, proportioning of

materials is of primary importance in as much as such procedures provide means of meeting specified requirements of quality.

Proportioning involves also consideration of available materials and costs.

The source and the type both of the aggregate and of the cement have a marked effect upon the quality of concretes produced. It is necessary, therefore, where concrete construction is of considerable magnitude, to make laboratory tests in advance of establishing the desired proportions.



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Proportioning of a Concrete Mix

The constituent materials themselves must satisfy quality requirements Aggregates should be sound, clean, well-

shaped and properly graded. The portland cement should be of the

appropriate type, have normal setting characteristics and should fulfill specification requirements.

Economy is, of course, always a consideration in mix proportioning.

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Proportioning of a Concrete Mix The purpose of selecting proportions for a

concrete mix is not necessarily to produce a concrete of the highest possible quality, but only of adequate quality required for the intended use, consistent with the economics of the project. A reinforced concrete bridge or a building, for

example, require concrete of comparatively high strength, whereas in the construction of a dam, a low rate of heat generation rather than strength is of primary importance.



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Data Needed for Proportioning Concrete Mixes There are four principal properties of

aggregates which are needed in the proportioning concrete mixes by the ACI method. These properties are given as follows: Free moisture and Absorption Specific Gravity Unit Weight of Aggregate in Bulk Gradation of aggregate

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ACI Method of Proportioning Concrete Mixes

The ACI (American Concrete Institute) Standard 211.1 is a “Recommended Practice for Selecting Proportions for Concrete”. The procedure is as follows: Step 1. Choice of slump Step 2. Choice of maximum size of aggregate Step 3. Estimation of mixing water and air content Step 4. Selection of water/cement ratio Step 5. Calculation of cement content Step 6. Estimation of coarse aggregate content Step 7. Estimation of Fine Aggregate Content Step 8. Adjustments for Aggregate Moisture Step 9. Trial Batch Adjustments



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For details on ACI Method of Proportioning Concrete Mixes, refer to the handout posted on the class website.

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Determine the job parameters Step 1: Slump

Table 9.1 Step 2: Max aggregate size

Size 1/5 < Min dimensions ¾ clear spacing bars and strands 1/3 Slab Depth

Step 3: Estimate mixing water and air content Table 9.2 Slump, Aggregate Size, Water content lbs/yd3 and

air entrainment



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Step 4: Water to Cement ratio, w/c Strength table 9.3, 28 day strength Durability table 9.4 Exposure conditions

Minimum w/c

Step 5: Cement content calculate from w/c and water content Minimum cement content

Step 6: Estimate coarse aggregate For the same workability, Vcoarse aggregate , F.M. of

fine aggregate and Nom. Max. Size, Table 9-5


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Step 7: volume of fine aggregates Vfine agg. = Vtotal - Vwater-VCement-V Coarse Agg. - Vair

V = Weight / (BSG *62.4)

Step 8: Consider the air content volume

Step 9: Adjustment for moisture in aggregates

Step 10: Volumetric Calculations




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Example For a residential street Paving in Boston, MA,

6” thick pavement, unreinforced, subjected to freezing and thawing and deicing agent anticipated. The required fc’ = 4000 psi. Vibration will be used.

Sand FM = 2.7 Aggregate available = 1 ½” BSG (FA)= 2.58 BSG (CA)= 2.6, = 105 lbs/ft3

BSG (cement)= 3.15

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Example1) Nominal Max size- 1/3 * 6 = 2”

since 1 ½” MSA is available, use 1 ½” 2) Air entrainment, definitely used, max

Table 9-2 5.5% (%vol. of conc.) Table 9-1 (since vibrating) Slump 1” – 2”

3) W/C Table 9-3, 9-4 0.48 4) Estimate water content Table 9-2 250 lbs/yd3

5) Calculate the cement content 250 /0.48 = 521 lbs/yd3

6) Estimate the coarse aggregate Table 9-5 0.72 ft3/ft3 of concrete

0.72 (27) = 19.44 cu.ft. of CA (OD condition)Weight of C.A. = (19.44 cu.ft.) (105 lbs/ft3) = 2040 lbs/yd3



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Absolute volume measurement Cement = 521/(3.15*62.4)= 2.65 ft3 of cement / yd3 of

concrete. Water =250/ 62.4 = 4.01 ft3 / yd3 of concrete. CA = 2040/(2.6*62.4) = 12.57 ft3 / yd3 of concrete. Air content = (0.055)(27 ft3/yd3) = 1.49 ft3/yd3 of concrete. Abs. volume – fine = 2.65 + 4.01 + 12.57 + 1.49 = 20.72 ft3 Fine Aggregate Abs. volume = 27 – 20.72 = 6.28 ft3/yd3 of

concrete Weight of F.A. = (6.28)(2.58)(62.4) = 1011 lbs/yd3

For 1 cubic yard Cement 521 lbs Water 250 lbs FA (dry) 1011 lbs CA (dry) 2040 lbs

mix - university of washingtoncourses.washington.edu/cm425/mix.pdf · the most economical mix is...

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