11/12/2008
SE 112A, Fall 2008 Dr. Van Den Einde SE 112A, Fall 2008 Dr. Van Den Einde
Ingredients of Concrete
Portland cement, water, coarse aggregate, fine aggregate, and
What is Concrete? admixtures and air.
Ingredients
Portland Cement
Water
Aggregates
Admixtures
Mix Proportioning
Curing
Properties of Concrete
Concrete Testing
How does this apply to Concrete Canoe? Courtesy of Portland Cement Association
SE 112A, Fall 2008 Dr. Van Den Einde SE 112A, Fall 2008 Dr. Van Den Einde
Portland Cement Water
Courtesy of Portland Cement Association
Portland cement comes to life with water Water required for hydration process. Courtesy of Portland Cement Association
Made from limestone and is essentially fine powder consisting of mainly calcium Quality of hardened concrete greatly influenced by amt of water used relative
silicates and aluminum silicates.
silicates to amt of cement.
cement Higher water contents dilute cement paste (the glue of
Cement and water form paste that coats each particle of stone and sand. concrete).
Through chemical reaction called hydration, cement paste hardens and gains Advantages of reducing water content:
strength. Increased compressive and flexural strength
Character of concrete determined by quality of paste and strength of the paste Lower permeability, thus increased watertightness and lower absorption
depends on ratio of water to cement. Increased resistance to weathering
Water-cement ratio is weight of mixing water divided by weight of the cement. Better bond between concrete and reinforcement
Less volume change from wetting and drying
High-quality concrete produced by lowering water-cement ratio as much as
R d d shrinkage
Reduced hi k andd cracking
ki
possible without sacrificing workability of fresh concrete. Generally, using less water
produces a higher quality concrete provided the concrete is Aggregates
properly placed, consolidated, and cured. Water-to-cement ratio (w/c) usually between 0.40 and 0.70 by weight.
Other than admixtures (used in very small quantitie), cement is most expensive For hydration process, a minimum w/c = 0.25 is required. Usually, w/c > 0.25
ingredient in concrete. required to enhance mobility of water during hydration process and enhance
workability of the concrete mix.
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11/12/2008
SE 112A, Fall 2008 Dr. Van Den Einde SE 112A, Fall 2008 Dr. Van Den Einde
Aggregates Admixtures
Courtesy of Portland Cement Association Courtesy of Portland Cement Association
Aggregates are classified by ASTM C 33 (AASHTO M 6/M 80) as fine or Admixtures are ingredients in concrete other than portland
coarse. cement, water, and aggregates that are added to mixture
Fine aggregate (sand): any material that passes through a No.
No 4 sieve (a sieve
that has four openings per linear inch). Particles are typically smaller than 5mm
immediately
d l bbeforef or during
d mixing to:
or 0.2 in. Reduce cost of concrete construction
Coarse aggregate consists of either (or a combination of) gravel, crushed Modify properties of hardened concrete;
gravel, crushed stone, air-cooled blast furnace slag, or crushed concrete, with Ensure quality of concrete during mixing, transporting, placing, and
particles generally larger than 5 mm (0.2 in.)
curing;
Depending on member dimensions and spacing of reinforcement bars in a
concrete member, maximum size for coarse aggregate is usually 1-1/2 in. Overcome certain emergencies during concrete operations.
Use
U off different
diff aggregate sizes
i can lead
l d to densely
d l packed
k d mixi andd thus
h
reduce quantity of cement required. Aggregates usually constitute 60 to 70%
of the total volume of a hardened concrete.
SE 112A, Fall 2008 Dr. Van Den Einde SE 112A, Fall 2008 Dr. Van Den Einde
Admixtures Admixtures Courtesy of Portland Cement Association
Courtesy of Portland Cement Association Type of Admixture Standard Specifications Desired Effect
Five Functions: To stabilize microscopic bubbles in concrete,
Air‐entraining ASTM C 260 and C 233 (AASHTO
Air-entraining: to purposely place microscopic air bubbles into concrete admixture M 154 and T 157).
which can provide freeze‐thaw resistance and
p g
improve resistance to deicer salt scaling.
Water-reducing:
Water reducing: reduce required water content by ~5-10%
~5 10% so concrete needs
Water reducing Reduce the water content by 5 to 10%, while
less water to reach required slump, and has lower w/c ratio. Can make higher ASTM C 494 (AASHTO M 194)
admixture (WR) maintaining slump characteristics.
strength concrete w/o increasing amt of cement (cheaper).
Retarding: keep concrete workable during placement and delay initial set of Mid‐range water Reduce the water content by 6% to 12%, while
ASTM C 494 (AASHTO M 194)
reducer (MRWR) maintaining slump and avoiding retardation.
concrete. Most retarders also function as water reducers and may entrain some
air in concrete. Good for hot weather. High‐range water
reducer (HRWR) ASTM C 494 (AASHTO M 194),
Accelerating: increase rate of early strength development, reduce time required Reduce the water content by 12% to 30%,
(also called ASTM C 1017
for proper curing and protection, and speed up start of finishing operations. superplasticizer)
while maintaining slump.
G d ffor cold
Good ld weather.
th
Plasticizers (superplasticizers): high-range water reducers (HRWR) reduce water Retarding admixture ASTM C 494 (AASHTO M 194) To decrease the rate of hydration of cement.
content by 12 to 30% and can be added to concrete with low-to-normal slump Accelerating admixture ASTM C 494 (AASHTO M 194) To increase the rate of hydration of cement.
and w/c ratio to make high-slump flowing concrete. Flowing concrete (highly fluid Shrinkage‐reducing Reduce drying shrinkage (and related cracking)
but workable concrete) can be placed with little or no vibration or compaction. admixtures in concrete
Other: for corrosion inhibition, shrinkage reduction, alkali-silica reactivity ASR‐inhibiting Reduce or eliminate deleterious expansion due
reduction, workability enhancement, bonding, damp proofing, and coloring admixtures to alkali‐silica reaction
Corrosion inhibitors ASTM C 1582 Minimize steel reinforcement corrosion
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11/12/2008
SE 112A, Fall 2008 Dr. Van Den Einde SE 112A, Fall 2008 Dr. Van Den Einde
Mix Proportions Mix Proportions
Key to achieving strong, durable concrete is in careful The proportion of different ingredients in a mix is determined
proportioning and mixing of ingredients. with the following considerations.
Mixture with not enough paste to fill voids between aggregates will be Strength: The higher the w/c, the lower the strength will be
difficult to place and will produce rough, honeycombed surfaces and of the concrete.
porous concrete.
Mixture with excess cement paste will be easy to place and will Compressive
produce a smooth surface; but the resulting concrete will likely shrink Strength
more and be uneconomical.
Properly
p y designed
g concrete mixture will possess
p desired
workability for fresh concrete and required durability and
strength for hardened concrete.
Typically, mix is about 10 to 15 percent cement, 60 to 75 percent
aggregate and 15 to 20 percent water. Entrained air in many concrete
mixes may also take up another 5 to 8 percent. W/C
SE 112A, Fall 2008 Dr. Van Den Einde SE 112A, Fall 2008 Dr. Van Den Einde
Mix Proportions Mix Proportions
Workability: The higher the w/c is, the higher the workability
of the mix. Measured by slump tests.
Slump Tests: Conducted by means of cone-shaped metal mold. Top and
bottom of mold are open. Metal mold is set on flat surface and filled
with wet concrete mix. Removed by lifting, and concrete mix allowed
to slump. Slump is measured with respect to top of mold and generally
specified to be between 2 and 6 in.
Too much slump implies excessive water. Too low a slump means poor
workability.. Slump tests should always be conducted prior to the
casting
ti off concretet members.
b
4 in.
Slump
12 in. Metal (2~6 in.)
Mold Concrete Mix
8 in.
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11/12/2008
SE 112A, Fall 2008 Dr. Van Den Einde SE 112A, Fall 2008 Dr. Van Den Einde
Curing Curing
Strength increases with age of concrete. Courtesy of Portland Cement Association Courtesy of Portland Cement Association
Concrete usually reaches 70% of its mature strength by end of first
weekk after
f start off hydration
h d i process. Compressive Strength, f’c
Compressive Strength, f
Final strength of concrete depends on humidity and temperature
conditions during this initial period.
Maintenance of proper conditions during this time is called curing.
Concrete should be protected from loss of moisture for at least 7 days
after casting.
For q
qualityy control, compressive
p strength
g of concrete is usuallyy
measured at age of 28 days using concrete cylinders cured in
temperature and humidity conditions specified in ASTM standards.
Strength increase after 28 days is normally small for ordinary concrete.
However, concrete with large proportion of fly ash (20% by weight) 7 days 28 days Age
can gain strength very slowly over a period much longer than 28 days.
SE 112A, Fall 2008 Dr. Van Den Einde SE 112A, Fall 2008 Dr. Van Den Einde
Reinforcement Properties of Concrete
Concrete is strong in compression, as aggregate efficiently Strength
carries compression load. However, weak in tension as cement Concrete has relatively high compressive strength, but significantly
h ld the
holding h aggregate in place
l can crack,k allowing
ll structure to l
lower tensile
il strengthh (about
( b 10% off compressive
i strength).
h)
fail. Without compensating, concrete would almost always fail from tensile
stresses even when loaded in compression.
Reinforced concrete solves these problems by adding metal
Concrete elements subjected to tensile stresses must be reinforced
reinforcing bars, glass fiber, or plastic fiber to carry tensile with materials that are strong in tension
loads.
Ultimate strength is influenced by the w/c ratio, design
constituents, and mixing, placement and curing methods.
All things being equal, concrete with lower w/c ratio makes a stronger
concrete than that with a higher ratio.
The density varies, but is around 150 pounds per cubic foot
(2400 kg/m³)
http://en.wikipedia.org/wiki/Image:Trebar.jpg
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11/12/2008
SE 112A, Fall 2008 Dr. Van Den Einde SE 112A, Fall 2008 Dr. Van Den Einde
Properties of Concrete Properties of Concrete
Elasticity
Modulus of elasticity of concrete is function of modulus of elasticity of
aggregates and d cement matrixi and
d their
h i relative
l i proportions.
i
Relatively constant at low stress levels but starts decreasing at higher
stress levels as matrix cracking develops.
The elastic modulus of the hardened paste may be in the order of 10-
30 GPa and aggregates about 45 to 85 GPa.
The concrete composite is then in the range of 30 to 50 GPa.
E = 57,000 f c'
SE 112A, Fall 2008 Dr. Van Den Einde SE 112A, Fall 2008 Dr. Van Den Einde
Properties of Concrete Properties of Concrete
Expansion and shrinkage Cracking
Concrete has a very low coefficient of thermal expansion. If no All concrete structures will crack
provision
i i iis maded ffor expansion,
i very large
l forces
f can be
b created,d Concrete cracks due to tensile stress induced by shrinkage or stresses
causing cracks. occurring during setting or use.
As concrete matures it continues to shrink, due to ongoing reaction To overcome this:
(rate of shrinkage falls relatively quickly and keeps reducing over time Fiber reinforced concrete: uses fine fibers distributed throughout the mix
(for all practical purposes concrete is usually considered to not shrink or larger metal/reinforcement elements to limit the size and extent of
due to hydration any further after 30 years cracks.
Because concrete is continuously shrinking for years after it is initially In many large structures joints or concealed saw-cuts are placed in the
placed,
l d it isi generally
ll accepted
t d th
thatt under
d ththermall lloading
di it will
ill never concrete as it sets to make the inevitable cracks occur where they can be
expand to its originally placed volume. managed and out of sight.
Water tanks and highways are examples of structures requiring crack
control.
AND CONCRETE CANOE’S!!!
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11/12/2008
SE 112A, Fall 2008 Dr. Van Den Einde SE 112A, Fall 2008 Dr. Van Den Einde
Properties of Concrete Concrete Testing
Creep Engineers usually specify the required compressive strength of
Permanent movement or deformation of a material in order to relieve concrete, which is normally given as the 28-day compressive
stresses within
i hi the
h material.
i l strength
h in MPa
MP or psi.
Concrete which is subjected to long-duration forces is prone to creep 3-day and 7-day strengths can be useful to predict the ultimate
Short-duration forces (such as wind or earthquakes) do not cause 28-day compressive strength of the concrete.
creep.
A 25% strength gain between 7 and 28 days is often observed with
Creep can sometimes reduce the amount of cracking that occurs in a
ordinary Portland cement mixtures
concrete structure or element, but it also must be controlled.
The amount of p primaryy and secondaryy reinforcingg in concrete
structures contributes to a reduction in the amount of shrinkage,
creep and cracking.
SE 112A, Fall 2008 Dr. Van Den Einde SE 112A, Fall 2008 Dr. Van Den Einde
Concrete Canoe Concrete Canoe
Ingredients we use:
Cementious Materials
Key rules for Mix Design:
Portland Cement Cementious materials - maximum of 50% by mass of hydraulic cement
i any concrete mix
in i
Metakaolin
Silica Fume Cementious materials - no more that 400 lb/yd3 total (hydraulic
Fly Ash
cement)
Aggregates Aggregates - minimum of 15% of the total weight of any concrete
mixture.
Poraver (0.1 mm - 0.3 mm)
Poraver (0.25 mm - 0.5 mm) Aggregates - no more than 5% by weight may pass No. 100 sieve (0.15
mm)
Poraver (0.5 mm - 1 mm)
Poraver (1 mm - 2 mm) Aggregates - recycled material content of at least 25%
K1 Glass Bubbles, B-Lites Water - Maximum allowable water to cement ratio for any concrete
Water mixture is 0.40.
Admixtures Air - minimum required air content for any concrete mixture is 6.0%
Plasticizer
Latex
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11/12/2008
SE 112A, Fall 2008 Dr. Van Den Einde
Questions?
