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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.

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mix proportioning

MIX PROPORTIONING

Prepared by Marcia C. Belcher

Construction Engineering Technology

economics of mix design
Goals:

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

Economics of Mix Design
guidelines we use for mix design
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
Design Method We Will Use:Absolute Volume Method
  • Assumes no air voids in concrete
  • Amount of concrete is sum of solid volumes:

1 CUBIC YARD

      • Cement
      • Sand
      • Coarse aggregate
      • Water
      • Air
water correction
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
Material Values & Constants Needed For Design:
  • SSD (Absorption) of Sand
  • Unit Weight & SG of Sand
  • SSD (Absorption) of Stone
  • Unit Weight & SG of Stone
  • Density of Cement = 195 pcf
  • SG Cement = 3.15
  • Density of Water = 62.4 pcf
  • 1 Cubic Foot Water = 7.48 gal
  • 1 Gal. Water = 8.34 lbs
aci standard mix design method
ACI Standard Mix Design Method
  • The standard ACI mix design procedure can be divided up into 8 basic steps:
    • Choice of slump
    • Maximum aggregate size selection
    • Mixing water and air content selection
    • Water-cement ratio
    • Cement content
    • Coarse aggregate content
    • Fine aggregate content
    • Adjustments for aggregate moisture
step 3 max agg size check
Step #3: Max. Agg. Size Check
  • 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.
slide22

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 8 batch weight water adjustment
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.
properties of concrete specified by engineer
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
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
slide29

WE ARE DESIGNING BATCH WEIGHTS FOR ONE CUBIC YARD

  • Step #1: Select Slump
  • Engineer Specified 1” (correlates w/table)

Table 9.6

step 2 determine mixing water and air content31
Step #2: Determine Mixing Water and Air Content
  • 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 3 max agg size check32
Step #3: Max. Agg. Size Check
  • ACI Limits:
    • 1/3 of the slab depth
  • 10”/3 = 3.33 inches > 1.5” OK
slide34

SG Cement

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

slide36

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 content37
Step #7: Fine Agg. Content

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
Step #8: Aggregate Batch Weights & Water Adjustment

Since there is moisture in both coarse & fine aggregate,

their batch weights must be adjusted

1% Moisture

5% Moisture

  • 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
step 8 aggregate batch weights water adjustment39

Step #8: Aggregate Batch Weights & Water Adjustment

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

Dry Wt.

Moisture

Absorption

Dry Wt.

Moisture

Absorption