Concrete Properties
What is Concrete

Components
 Aggregates (Sand and Gravel)
 Water
 Cement

Hydration
What are the benefits of
Concrete?
Strong
 Durable
 Waterproof
 Cast into formed shape
 Relatively inexpensive

Typical Concrete Structures
Buildings
 Bridges
 Sidewalks
 Roadways
 Tunnels
 Statues

Famous Concrete Structures
Famous Concrete Structures
Design of Concrete Buildings
Concrete Components

o
What are concrete’s major components?
Aggregate (60%-75% volume)
o
o
o
Cement Paste (23%-33% volume)
o
o
o
o
Coarse Aggregate (gravel)
Fine Aggregate (sand)
Cement (Portland Cement)
Water (potable water)
Admixtures
Air (2%-8% volume)
Decreasing
quantities
by weight
Concrete Components
Aggregates-
Where do they come from?
Natural – Dredged from a pit, river, lake or
seabed
 Crushed – Quarry
 Recycled Concrete

Concrete Components
Aggregates
Selection of Aggregates
Grading
 Durability
 Particle shape and surface texture
 Abrasion and skid resistance
 Unit weights and voids
 Absorption and surface moisture

Concrete Components
Aggregates - coarse
Rounded
and smooth
Equidimensional
Elongated
Flaky
Angular
and rough
Rounded
and smooth
Concrete Components
Aggregates - grading
Concrete Components
Aggregate Distribution
Dense Graded
Open Graded
Concrete Components
Aggregate Distribution
Dense Graded
•
•
•
•
•
Grain-to-grain contact
Low void content
High density
Easy to compact
Used in conventional concrete
Open Graded
•
•
•
•
•
Grain-to-grain contact
High void content
Low, variable density
Difficult to compact
Used in pervious concrete
Concrete Components
Portland Cement Types
Type I
normal
Type II
general-purpose
moderate
sulfate
resistance
Type III High early
strength
Type IV Low heat of
hydration
drainage structures
Type V
Used where soils and groundwater
have high sulfate content
Severe
sulfate
resistance
quick form removal
accelerated curing in cold weather
Minimizes rate and amount of heat
generated through hydration
For massive structures
Concrete Components
Admixtures

Water reducers – workability and strength
 Plasticizers (low range) – 5%-10% water reduction
 Superplasticizers (high range) – 20%-30% reduction
Air entraining – freeze-thaw resistance
 Accelerators – accelerates setting

 Cold weather
 Quick form removal

Retarders – slows setting
 Hot weather
 Difficult placement
Concrete Components
Air

Entrapped air
 macroscopic
 undesirable
 vibration for proper
consolidation
 <1% desirable
Concrete Components
Air

Entrained air
 microscopic
 desirable
 freeze-thaw
resistance (durability)
 2%-8%
Freeze-Thaw Damage
Press-UR-Meter
Concrete Components
Pozzolans and Supplementary Cementitious
Materials

Other cementitious materials that be substituted for
Portland cement?
Flyash
(P)
Silica Fume
(SCM)
Ground Blast Furnace Slag (P)
Concrete Components
Cost

How much does concrete cost?
 approx. $100 /cu.yd. delivered
 Depends on supplier, admixtures, delivery time

What is the most expensive component?
 Cement

How can aggregate grading minimize cost?
 Maximize size of coarse aggregate
 Well graded (dense) mixture of aggregate sizes
Concrete Components
Typical Concrete Batch Mix (1 yd3)
Ingredient*
Conventional
Pervious
400 – 500 lbs/yd3
500 – 600 lbs/yd3
Up to 50%
100-150 lbs/yd3
Coarse Aggregate
1,800-2,100 lbs/yd3
2,400-2,700 lbs/yd3
Fine Aggregate
1,200-1,400 lbs/yd3
0 – 250 lbs/yd3
Water (w/cm)
30-40 gal. (0.4-0.6)
18-20 gal. (0.27-0.34)
Portland Cement
SCM
Admixtures
•
•
•
•
•
water reducer
accelerator
retarder
air entraining
others
•
•
•
•
•
water reducer
viscosity modifier
hydration stabilizer
air entraining
internal curing
Voids
minimal
15% - 35%
Fibers
optional
1.5 – 5 lbs/yd3
* NOTE: Any mix should always be tested by supplier and installer.
Concrete Strength
Setting

What is the hardening process called?
 Setting – the transition from plastic to a solid state

How long does setting take?
 Depends on w/c ratio, temperature, etc.

Can setting time be controlled?
 cement type
 Admixtures

When has concrete set?
 Continuous process
 Bleed water evaporated
 Supports worker with
< ¼” depression.
Concrete Strength
Hydration

What is hydration?
 The chemical bonding of Portland cement and
water forming a cement paste or gel
 Develops in 3 stages
○ Setting
○ Hardening
○ Strength Gain

How long does hydration take to complete?
 Most is within 28 days after pouring
 For life of concrete if sufficient moisture present
( >80% humidity)
 Decreasing rate
Concrete Strength
Hydration

What are the effects of hydration on concrete?
 Improves all engineering properties
○ Increase strength, durability
○ Decrease permeability

Are there any side effects?
 Heat – exothermic chemical reaction

How can concrete temperature be controlled?





Aggregate temperature
Water temperature
Cement type
SCM’s
Admixtures
Concrete Strength

How is concrete strength
measured?
 Crushing stress
 Cylinder test
 28 day compr. strength ( fc’)
Casting Concrete Cylinders

https://www.youtube.com/watch?v=FS5k
uRHlQNw
Concrete Strength
Cylinder Test
12”
6”
Concrete Strength
Cylinder Test
140,000 lbs
Testing Concrete Cylinders

https://www.youtube.com/watch?v=lWVe
YTOJBzA
Concrete Strength
Stress
Stress = Force/Area
Force
Given:
P = 140,000#
d = 6”
Find:
Area
Reaction
Strength of Test Cylinder
Concrete Strength
Stress
Stress = Force/Area
Force
Given:
P = 140,000#
d = 6”
Find:
Area
Reaction
Strength of Test Cylinder
Solution:
fc= P/πr2
fc = 140,000#/28.3 in2
fc = 4,950 psi
Concrete Strength
Strain
Strain = Change/Length
Length
(L)
Force
Change
(∆L)
Given: At Cylinder Failure
P = 140,000#
L = 12”
∆L = 0.036”
Find:
Average Strain at Failure
Concrete Strength
Strain
Strain = Change/Length
Length
(L)
Force
Change
(∆L)
Given: At Cylinder Failure
P = 140,000#
L = 12”
∆L = 0.036”
Find:
Average Strain at Failure
Solution:
ε= ∆L/L
ε = .036”/12”
ε = .003
Concrete Strength
Strain
Strain = Change/Length
L=42’ concrete column
average ε=.002”/”
Force
Length
(L)
Given:
Find: Total deformation ∆L
Change
(∆L)
Concrete Strength
Strain
Strain = Change/Length
L=42’ concrete column
average ε=.002”/”
Force
Length
(L)
Given:
Find: Total deformation ∆L
Change
(∆L)
Solution:
ε = ∆L/L
∆L = ε x L
∆L = .002 x 42’ x 12”/’ = 1”
Concrete Strength
Testing


What is the sampling frequency for concrete testing
on a construction project? – Slump, Air, Cylinders
Example Specification:






1 test per 50 yd3 or
1 test per day’s pour
4 (or more) cylinders per test
1 slump test
1 air entrainment test
How often are cylinders tested?
 In project specifications
 Some combination of 3,7,14,21,28,56 days from
casting
 One spare is kept from each pour
Concrete Strength
Tension

How strong is concrete in tension?
 Approximately 1/10 compressive strength
Given:
fc’ = 4950 psi
Find:
Concrete Tensile Strength
Concrete Strength
Tension

How strong is concrete in tension?
 Approximately 1/10 compressive strength
Given:
fc’ = 4950 psi
Find:
Concrete Tensile Strength
Solution:
ft= fc’ /10
ft = 495 psi
Concrete Strength
Elastic Modulus

How stiff is concrete?
◦ About 1/10 as stiff as steel
◦ E = 57,000√f’c
Given: fc’ = 4950 psi
Find: Concrete Stiffness E
Concrete Strength
Elastic Modulus

How stiff is concrete?
◦ About 1/10 as stiff as steel
◦ E = 57,000√f’c
Given: fc’ = 4950 psi
Find: Concrete Stiffness E
Solution:
E = 57,000√fc’
E = 57,000√4950
E = 4.01 x 106 psi
Water in Concrete
effects

How is water measured?
 w/cm – ratio of water to
cementitious material by
weight in a concrete volume

How does w/cm affect
workability?
 greater w/cm
○ greater slump
○ More flowable concrete
○ Easier to place
Water in Concrete
effects

How does increasing
w/cm affect engineering
properties?






Decreases strength
Decreases durability
Increases permeability
Increases bleeding
Increases shrinkage
w/cm is the single most important determinant
of the quality of finished concrete
Water in Concrete
effects

Can there be too little water?
 Complete hydration w/c = 0.25
 Practical minimum w/c = 0.50 Given: 5 bag mixture
(94 lbs/bag) per yd3
Find: Minimum water for
complete hydration of 1 yd3
concrete
Solution:
w/c = 0.25
w = 0.25 x c
w = 0.25 x 94 lbs/bag x 5
bags
w = 117.5 lbs / (8.34 lbs/gal)
w = 14.1 gal water
Concrete Consistency
Slump test
Concrete Consistency
Slump test
slump
Concrete Slump Test

https://www.youtube.com/watch?v=zLBs
pIRPKJI
Concrete Consistency
Slump test

What is a “reasonable” slump?
 Around 4”
slump
Questions??