MIX PROPORTIONING

Prepared by Marcia C. Belcher

Construction Engineering Technology

Design Consideration: Obtaining Good Workability

Design Consideration: Obtaining Good Workability

Obtaining Good Workability

Obtaining Good Workability

Design Consideration: Prevent Segregation & Bleeding

Well Graded Aggregates Will Reduce Segregation & Bleeding

Well Graded Poorly Graded

Measuring Workability: Slump

The Slump Cone

Approximate Required Slump Values (PCA)

Concrete Construction

Slump (Max.) Slump (Min.)

Reinforced foundation walls/footings

3 in 1 in

Plain footings and substructure walls

3 in 1 in

Beams & reinforced walls 4 in 1 in

Building columns 4 in 1 in

Pavement & slabs 3 in 1 in

Mass concrete 2 in 1 in

Economics of Mix DesignGoals:

Maximize strength = minimize water= control bleeding & segregation

Reduce Cost= use largest gravel possible for the job = minimize paste requirement

Provide good durability= use well graded aggregates= maximize void packing= reduced segregation

Cost Comparison of Constituents

Guidelines We Use For Mix Design

PCA Manual• Tables for w/c ratio based on compressive strength

requirement & slump (workability)

• Volume of stone required based on max. agg. size and sand fineness.

• Water required based on max. agg. size, slump & w/c ratio (compressive strength)

Design Method We Will Use:Absolute Volume Method

Assumes no air voids in concreteAmount of concrete is sum of solid volumes:

1 CUBIC YARD• Cement• Sand • Coarse aggregate• Water• Air

WATER CORRECTION

Any water content in aggregates above SSD water content must be subtracted from the water requirements

Any water requirement of aggregates (below the SSD water content) must be added to the water requirements

Material Values & Constants Needed For Design:

SSD (Absorption) of SandUnit Weight & SG of SandSSD (Absorption) of StoneUnit Weight & SG of StoneDensity of Cement = 195 pcfSG Cement = 3.15Density of Water = 62.4 pcf1 Cubic Foot Water = 7.48 gal1 Gal. Water = 8.34 lbs

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

ACI Standard Mix Design Method

Step #1: Select Slump

Table 9.6

Step #2: Determine Mixing Water and Air Content

Table 9.5

DEFINITION: Nominal maximum aggregate size is the largest sieve that retains some of the aggregate particles.

ACI Limits:

1/3 of the slab depth

3/4 of the minimum clear space between bars/form

1/5 minimum dimension of non-reinforced member

Aggregate larger than these dimensions may be difficult to consolidate and

compact resulting in a honeycombed structure or large air pockets.

Step #3: Max. Agg. Size Check

Step #4: Select W/C Ratio

Table 9.3

Step #5: Cement Content

The calculated cement amount is based on the selected mixing water content and water-cement ratio.

W/C= Wt. of Water

Wt. of Cement

Step #6: Coarse Agg. ContentTable 9.4

Step #7: Fine Agg. Content

Step #8: Batch Weight & Water Adjustment

Aggregate weights.  Aggregate volumes are calculated based on oven dry unit weights, but aggregate is batched in the field by actual weight. 

Any moisture in the stockpiled aggregate will increase its weight.

Without correcting for this, the batched aggregate volumes will be incorrect.

Amount of mixing water.  If the batched aggregate is anything but saturated surface dry it will absorb water (if 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.

Mix Design Example: 10” Thick Unreinforced Pavement Slab

Properties of Concrete Specified By Engineer:

Slump =1.0 inch

28-day strength of 5000 psi

Air content: 4.5 - 6.5 percent

Information About Materials:Coarse aggregate we are using (ODOT #467):

• nominal maximum size = 1.5 inch (see Agg. Size Table)

• dry-rodded weight = 100 lb/ft3

• specific gravity = 2.68

• moisture content = 1.0 percent

• absorption = 0.5 percent

Fine aggregate: • fineness modulus = 2.80

• specific gravity = 2.64

• moisture content = 5 percent

• absorption = 0.7 percent

Step #1: Select Slump

Engineer Specified 1” (correlates w/table)

Table 9.6

Step #2: Determine Mixing Water and Air Content

1.5” Stone

1” Slump

Table 9.5

Weight of Water = 250 lbs/yd3

Volume of Water = 250 lbs/yd3 = 4 ft3

62.4 lbs/ft3

Volume of Water = 4 ft3 per cubic yard of concrete

Step #2: Determine Mixing Water and Air Content

ACI Limits:1/3 of the slab depth

10”/3 = 3.33 inches > 1.5” OK

Step #3: Max. Agg. Size Check

Step #4: Select W/C Ratio

Table 9.3

Step #5: Cement Content

W/C= Wt. of Water

Wt. of Cement

Wt. of Cement = 250 lbs/yd3

.40

=625 lbs/yd3

Volume of Cement = 625 lbs/yd3 (Concrete)

3.15 x 62.4 lbs/ft3

Volume of Cement = 3.18 ft3 per cubic yard of concrete

SG Cement

Step #6: Coarse Agg. ContentTable 9.4

Step #6: Coarse Agg. Content

Weight (Dry) =.71 x 27 ft3/yd3 x 100 lb/ft3 = 1,917 lbs

Volume = 1,917 lbs = 11.46 ft3

2.68 x 62.4 lbs/ft3

Dry Rodded Unit Wt of Stone

SG Stone

Step #7: Fine Agg. Content

27 ft3 Cubic Yard of Concrete

4 ft3 Water

1.49 ft3 Air (.055 x 27 ft3)

3.18 ft3 Cement

11.46 ft3 Stone

6.87 ft3 Sand

Wt of Sand(Dry) = 6.87 ft3 x 2.64 x 62.4 lbs/ft3 = 1,131.7 lbs.

SG Sand

Step #8: Aggregate Batch Weights & Water Adjustment

Wt of Stone(Wet) = 1,917 lbs x 1.01= 1,936.2 lbs

Wt of Sand(Wet) =1,131.7 lbs x 1.05= 1,188.3 lbs

1% Moisture

5% Moisture

Mixing water needs to be adjusted. Both the coarse and fine aggregate are wet of SSD and will contribute water to the cement paste.

Water from Stone = 1,917 lbs. x (.01-.005) = 9.59 lbs

Water from Sand= 1,131.7 lbs x (.05-.007) = 48.66 lbs

Water = 250 lbs – 9.59 lbs – 48.66 lbs = 191.75 lbs

Step #8: Aggregate Batch Weights & Water Adjustment

MoistureDry Wt. Absorption

Dry Wt. Moisture Absorption

Final Batch Wts. (1 Cubic Yard)

Water 191.75 lbs = 23 gallons

Cement 625 lbs

Stone 1,936.2 lbs

Sand 1,188.3 lbs