Hardened Concrete - KOCWcontents.kocw.net/KOCW/document/2014/hanyang/ryoujaesuk1/... · 2016. 9....
Transcript of Hardened Concrete - KOCWcontents.kocw.net/KOCW/document/2014/hanyang/ryoujaesuk1/... · 2016. 9....
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Hardened Concrete
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Strength
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Water-to-Cement Ratio
According to Abrams (1918)
fc=k1/(k2w/c)
Where k1 and k2 are
empirically derived
constants
When w/c
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Cement Type
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Cement
Fineness
• Max size ~50µm diam.
• 10-15% particles
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Air Entrainment
Rule of thumb:
Every 1% increase in air content, reduces fc by 5%
However, in very lean mixes, some air entrainment can
improve strength.
Strength, MPa
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Air Entrainment
• Strength is
typically higher at
higher cement
content (richer
mixes)
• Effect of air
entrainment is
greater in richer
mixes
Strength, MPa
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Aggregate
• Aggregate strength not commonly a factor
• Size
• Gradation
• Shape
• Surface texture
• Mineralogy
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MSA
Moist curing period (days)
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MSA
Two competing effects:
• Larger MSA, less water
for a given workability
• Larger ITZ with larger
MSA aggregate, with
more pronounced effect
at lower w/c
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Gradation
• Relative amounts of
coarse and fine
aggregate influence
tendency toward
bleeding and
segregation.
• For a lower fine
aggregate content,
slump may decrease,
but so also might fc
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Texture
• Rougher texture,
better physical
bond with paste
• Small advantage
may be lost by
increased water
necessary to
maintain
workability
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Admixtures
• Accelerators and retarders affect the rate of strength
gain
• Ultimate strength is not really affected
• Some research has shown that retarders can
improve later strength
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SCMs
• Effect on strength varies with type, dosage, and
characteristics of SCM and mix
• Generally, reduced rate of strength gain
• Generally, improved later strength
• Greater relative effect on tensile strength
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Curing
Curing - procedures to promote the hydration of cement
over time (time, temperature, humidity)
• Concrete moist cured
is up to 3x stronger
than concrete air cured
• 7 day moist cure is
recommended
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Curing
• Concrete placed and
cured at varying
temperatures
• The higher the
temperature, the
higher the initial
strength
• Effect diminished
over time
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Curing
• Concrete cast
at varying
temperatures;
cured at 70F
• Concretes
cast at lower
temperatures
achieve higher
strength
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Testing Conditions
• ASTM C39 for compression test
• Specimen size – U.S. standard is 6”x12” cylinder with h/d = 2
• the larger the diameter the lower the measured strength
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Testing Conditions
• With a greater h/d, fc seems lower
• With lower h/d, fc seems higher
• for h/d =1, fc seems 10-15% higher than for the same
mixtures tested with 6x12 cylinders.
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Testing Conditions
• Cylinders should be moist at time of testing
- air dry samples show 20-25% higher fc
• Loading rate should be 20-50psi/sec so that samples
fail within 2-3 minutes
- faster load rate yields higher fc
- load at 1000 psi/sec, fc seems 12% higher
- load at 1 psi/sec, fc seems 12% lower
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Uniaxial Compression
• Linear elastic behavior in
concrete to ~ 0.30fc
• Gradual increase in curvature
up to 0.75 to 0.90 fc, then
flattens, and decreases
• At 0.30-0.50 fc some
extension or growth of pre-
existing cracks in TZ (stable
crack growth)
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Uniaxial CompressionAt 0.50-0.75 fc,
further crack
growth in ITZ,
with some
instability
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Splitting Tension
• Splitting tension test
introduces some
compressive stress
at top and bottom of
(6x12”) cylinder
• Measured strength
is 10-15% higher
than nominal
strength
ft = 2P/πLD
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Third Point Bending
• 6x6x20” beam is loaded at a rate of 125-
175 psi/min.
• MOR = PL/bd2
L= span length
P= max load sustained
b = width
d=depth
• Tends to overestimate tensile capacity
by 50-100% because a linear
relationship between stress and strain is
assumed through the section
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Schmitt Rebound Hardness
• ASTM C805
• Very simple
• Measures rebound of a hardened
steel hammer impacted on the
concrete surface by a spring
• Amount of rebound is related to
strength
• +/- 25% accuracy
• Very sensitive to smoothness,
moisture content, carbonation,
presence of agg at surface
• Good for comparison, not
absolute measurements
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Penetration Resistance
• Windsor Probe ASTM C803
• A powder-actuated gun drives a hardened alloy probe (needle)
into the concrete.
• The exposed length of the probe is measured and related by a
calibration table to the compressive strength of the concrete
• Useful for measuring time to strip forms
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Penetration Resistance
• Less affected by surface
texture and carbonation
• Affected by mix design and
aggregate hardness
• Must be calibrated
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Pullout Test
• ASTM C900
• Steel insert with an enlarged
end is embedded
• Insert is either completely pulled
out or is pulled until a desired
resistance is measured
• Pullout strength (force) ~ 20% fc
• Approximates shear strength
• Can be installed during or after
casting
• Requires patching
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Pullout Test
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Maturity Method
• Not really at test, more of a concept
• Way to estimate the strength of a concrete, knowing
the curing time (t) and curing temperature (T)
• ASTM C1072 (calculating maturity) and C918
(estimating strength)
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Maturity Method
M(t) = Σ(Ta-To)∆t Nurse-Saul equation
M(t) = maturity at some age, t
∆t = time interval, days or hours
Ta = average concrete temperature
during each time interval
To = datum temperature, below which
concrete will show no increase in
strength with time (32 and 14oF are
common)
• Correlate M(t) to fc
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Maturity Method
• Effect of RH is neglected
• Large variations in temperature, which may greatly
affect fc and rate of hardening, are neglected
• Accelerated curing may affect accuracy
• Some uncertainty in knowing datum temperature;
testing often required