mixdesign review

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Mix Design Review
Review Questions
The weight of a cubic foot of water is
62.4 cu ft
Gallons of water in a cubic foot
62.4 cf
Review Questions
Submit a mix design to NCDOT for
review on form
312U
A process in which an aggregate is
separated into its various sizes by
passing it through a nest of sieves is
gradation
Review Questions
The minimum number of revolutions at
mixing speed required for concrete
batched into a transit mixer is
70
Solid volume of cement is
.48
Review Questions
The water cement ratio needed to
hydrate cement is
.250
The smallest seive opening through
which the entire amount is permitted to
pass is
Nominal Maximum size
Review Questions
The pH of mixing water used in
concrete should be between
4.5-8.5
Definitions
Cement Yield – the volume of concrete
produced from one bag of cement for
one cubic yard.
Water to cement ratio – the number of
pounds of water from all sources per
pound of cement.
Definitions
Unit weight – the number of pounds of
concrete per cubic foot of volume.
Specific gravity – ratio of the weight of
a given volume of material to the
weight of a given volume of water.
Cement factor – number of bags or
pounds of cement in a cubic yard.
Definitions
Absolute or solid volume – volume of a
material in a voidless state.
Durability – resistance of concrete to
deterioration caused by weathering,
chemicals, and abrasives.
Consistency – a condition of concrete which
relates to its cohesion, wetness, or flow.
Slump is a good measure of consistency.
Cementitious Materials - Cement
Portland cement – bonding agent
Portland cement vary by color, fineness
of grind, and chemical composition.
Type I – general all purpose
Type II – low heat of hydration
Type III – high early strength
Cementitious Materials- Cement
1 bag = 94 pounds
A 6 bag mix = 564 lb
1 barrel = 4 bags = 376 pounds
Specific gravity of portland cement =
3.15
Cementitious Materials - Fly Ash
Flyash – a by product from a coal
powered power plant
A good flyash is fine with a low carbon
content
Substitute flyash for cement on a 1.2 to
1 ratio up to a 20% replacement of ash
for cement
Cementitious Materials
Slag and silica fume – by products of
different furnaces
Substitute them for cement on a 1 to 1
ratio up to a 50% replacement of
cement
Aggregates
Abrasion resistance – ability to resist
wearing or deterioration. Resistance is
measured by the L.A. Abrasion test
Gradation – relative amounts of
aggregate particles of consecutively
larger and smaller sizes
Aggregates
Well-graded – a uniform percentage of
material retained on each standard
sieve where the gradation change is
uniform from coarse to fine
Sieve analysis – a process to separate
an aggregate into its various sizes by
passing a sample through screens with
different sized openings
Aggregates
The purpose of sieve analysis is to
determine the distribution of the
separately sized particles
Saturated surface dry – SSD is a
condition where an aggregate will
neither absorb nor contribute water to
the mix
Fine Aggregate
Sand gives workability to the mix and
fills the voids between the coarse
aggregate particles.
Workability – a property which is
measured in terms of ease of placing,
handling, and finishing.
Fine Aggregate
Fineness modulus – an index to the
fineness or coarseness of an aggregate:
the range for medium sand is 2.50 to
2.80.
Coarse Aggregate
Stone is a filler material and aids in
strengthening the concrete mix.
Dry Rodded unit weight – the weight
per unit volume of dry aggregate
compacted in a container by rodding in
3 layers.
Maximum size- the sieve size on which
the largest particles are permitted to
pass.
Coarse Aggregate
Nominal maximum size- the smallest
sieve opening through which the entire
sample will pass.
Rounded aggregate is from a pit.
Angular aggregate is from a quarry.
The size and shape of the coarse
aggregate can affect the strength of the
mix.
Angular shaped aggregate is generally stronger
than rounded shaped aggregate.
Water
Water causes a chemical reaction with
the cement called hydration.
Bleed water – the excess water that
accumulates on the surface of plastic
concrete when aggregates settle.
Water
For a concrete with a given cement
factor, an increase in total mixing water
will cause the strength to decrease.
Specific gravity of water = 1.
The ph of mixing water is 4.5 to 8.5.
Air Content
Entrained air is added to a concrete mix
to improve it’s resistance to freezing
and thawing.
Entrained air also improves workability.
The air content measured at the site for
structural or incidental concrete will be
from 4.5 to 7.5 %.
Air Content
To much air entraining agent will result
in to many air voids which will reduce
the strength of the concrete mix.
The volume occupied by 6% air is 1.62
cubic feet.
You can calculate the air content by the
unit weight method.
Air Content
Increasing the cement will cause the air
content to decrease.
Increasing the temperature of the mix
causes the air content to decrease.
Retarding Admixtures
Retarders are chemicals that are added
to a concrete mix to slow down the set
and allow more time to place and finish
the concrete.
To much retarder will cause a longer
delay in the initial set of the concrete. It
will have little affect on the long term
(28 day) strength of the concrete.
Retarding Admixtures
According to NCDOT table 1000-2,
using a retarder will allow about 45
extra minutes of placement time.
Mix Design Review
Problem Solving
How many cubic yards did this mix produce?
•3055 Lb Cement
•5660 Lb. Wet Sand ( 5390 Lb. SSD), at 5.0% Free Moisture
•10,205 lb. Wet Stone ( 10,095 Lb SSD) at 1.1% Free Moisture
•150 Gal. Metered Water
•6% Design Air
•20 Gal. Water added at jobsite
•5.8% air by pressure meter
•147.65 pcf field unit weight
Solution: Find the Total Weight of All Materials
in the Mix and Divide by the Field Unit Wt.
3055 + 5660 + 10205 + ( 150 gal x
8.33) +(20 gal x 8.33) = 3055 + 5660
+ 10205 + 1250 + 167 = 20,337 lbs.
20,337 lbs / 147.65 pcf = 137.74 cu.Ft.
137.74 cu.Ft / 27 = 5.1 cu yds.
Design air not a factor because the air
is taken into account in the field unit
weight.
Find Yield, Cement Factor, and the Water
Cement Ratio for This Same Mix
Cement Yield: 3055 lb cem / 94 = 32.50
bags
137.74 cu.ft. / 32.50 bags = 4.24
CF: 27 / 4.24 = 6.37 bags
6.37 x 94 = 599 lb cement
W/C: (5660- 5390) + (10,205 – 10,095)
+ (150 gal x 8.33) + (20 gal x 8.33) =
1797 lb
1797 lbs water / 3055 lbs cem = .588
Given: 1050 lb SSD sand
Specific Gravity = 2.61
Total Moisture = 6.4%
Absorbed Moisture = 0.6 %
Find wet weight of sand.
6.4% - 0.6% = 5.8 %
5.8 / 100 = .058.
.058 x 1050 lb = 61 lb free water.
1050 lb + 61 lb = 1111 lb batch sand.
Find dry weight of sand.
.006 x 1050 lb SSD = 6 lb absorbed water.
1050 lb SSD – 6 lb = 1044 lb dry sand.
Find absolute volume of
SSD sand(same problem)
1050 lb SSD
2.61 x 62.4
=
6.45 cu.ft.
Fly Ash Mix Problem
Given: Class B non vib mix 15% fly ash
replacement. Angular aggregate is used.
Minimum cement from table is 545 lbs.
What is the new cement and cementitious
wt?
.15 x 545 lb = 82 lb cement replaced.
82 lb x 1.2 = 98 lb ash for cement.
New cement is 545 – 82 = 463 lbs.
463 lb cem + 98 lb ash= 561 lbs cemetitious
Water(same problem)
What is the new water?
W/C = .545 for angular aggregate.
.545 x 561 = 306 lb of water
306 lb / 8.33 = 36.7 gallons.
Find the % solids and voids in a
coarse aggregate.
Given: Dry rodded unit weight = 95 pcf
Given: Specific gravity = 2.79
2.79 x 62.4 = 174.10 pcf.
(95 pcf / 174.10 pcf) x 100 = 55%
Solid.
100 –55 = 45% voids
Trial Mix
Class A, vib,AE, rounded aggregate, 3.5
inch slump, 37.5 gallons water. How
much cement is needed?
Maximum W/C = .488
37.5 gal x 8.33 = 312 lb water
312 lb water / .488 = 639 lb. cement

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