Post on 20-Mar-2018
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CEEN 3144 Construction Materials
Testing strength of hardened concrete and quality control
Francisco AguíñigaAssistant Professor
Civil Engineering ProgramTexas A&M University – Kingsville
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Factors affecting compressive strength
Water/cement ratioTimeMaturity conceptCementAggregateAdmixtures
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Water/cement ratio
Strength of concrete depends on capillary porosity – not easily measurableCapillary porosity is proportional to w/c
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Water/cement ratio
Factors not considered by Abrams law
Degree of hydrationAir contentAggregate effects
Abrams law
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Water/cement ratio
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Time
Compressive strength depends on time and w/cfc at 28 days = f’c = 1.5 fc @ 7 days
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Maturity concept
Maturity = f (t x T)Concretes of same maturity have similar strengths regardless of combination of Tand t leading to the maturityNurse-Saul expression:Maturity (°C x days) = Σ at (T + 10)Where:
at = time of curing – daysT = temperature in °C10 °C = datum T below which hydration stops
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Maturity concept
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Maturity concept
LimitationsDoes not take onto account curing moistureCannot be applied to mass concrete – only ambient temperature is consideredNot useful at low maturitiesNot useful if large T variations during curingStrength affected by cement composition and fineness, and by w/c ratio
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Cement
Chemical composition and finenessCement variability leads to variability in concrete strength
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Aggregate
Important parameters:Shape, texture, and maximum sizeSurface texture affects σ-ε curve but not ultimate strength
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Cement and air contents
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Admixtures
Little effect on strength – per seHowever – admixtures alter w/c ratio and porosityStrength gain modified by accelerating or retarding cement hydration
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Temperature
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Length and diameter of specimen (l/d)
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Cylinder size
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Concrete age
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Strain rate during loading
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Concrete state of stress
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State of stress
Tensile strength
Shear strength
Impact strength
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Shrinkage strain – reduces tensile strength
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Creep strains
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Creep strains
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In-place strength of concrete
Nondestructive methodsSurface hardnessRebound hardnessPenetration resistancePull-out testsUltrasonic pulse velocity
Tests on core samplesStructural load test
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Nondestructive methods
Surface hardnessImpact concrete with a standard massMeasure size of indentationAccuracy between 20 and 30%
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Nondestructive methods
Rebound hardnessSchmidt hammerResults affected by
Surface finishMoisture contentTemperatureMember rigiditySurface carbonationDirection of impact
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Nondestructive methods –rebound hardness
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Nondestructive methods
Penetration resistance - Windsor probe
Harder aggregates yield higher penetration resistance
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Nondestructive methods
Pull-out testsMeasure of concrete shear strengthCorrelated with compressive strength
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Nondestructive methods
Pull-out tests
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Nondestructive methods
Ultrasonic pulse velocityV = (E/ρ)^0.5
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Nondestructive methods
Ultrasonic pulse velocity
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Nondestructive methods
Ultrasonic pulse velocityPulse velocity increases with moistureAffected by surface smoothnessPulse velocity depends on path lengthPulse velocity constant for 5-30°CSteel bars increase pulse velocityFor same pulse velocity – compressive strength is higher for older specimens
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Quality control
Factors that contribute to concrete variability
MaterialsProductionTesting
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Measurement of variability
Concrete distribution can be approximated with a normal or Gaussian distribution
( )⎥⎦
⎤⎢⎣
⎡ −−= 2
2
2exp
21
sx
sy µ
π
nxΣ
=µ
( )1
2
−−Σ
=nxs µ
µsV =
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Measurement of variability
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The normal curve
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The normal curve
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The normal curve
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Area under the normal curve
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Quality control
If concrete compressive strength has a normal distribution
Cannot use µ for designCannot require all strengths be above design strengthMust arbitrarily define acceptable % below design strengthCylinder tests are only estimates of strength
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Quality control
Concrete variability effects are reduced because
Batches are intermixed when placedTests are based on 28-days, strength with timeSteel reinforcement redistributes stresses
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ACI 214 approach to variability
Required average strength
tVf
f ccr −=
1
'
tsff ccr += '
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ACI 214 approach to variability
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ACI 318 approach to variability
Two requirementsThe probable frequency of tests more than 500 psi below f’c should not exceed 1 in 100
Probable frequency of the average of 3 consecutive tests below f’c will not exceed 1 in 100
sff ccr 326.2500' +−=
sfff cccr 343.13
'' +=+=s326.2
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Choice of design strength
ACI - When a strength record exists, increase f’c by
400 psi if s < 300 psi550 psi if 300 psi < s < 400 psi700 psi if 400 psi < s < 500 psi900 psi if 500 psi < s < 600 psi1200 psi if 600 psi < s; or no records availableUse previous equations only after sufficient data is available
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Number of samples
ACI sampling requirementsSamples taken at least once a dayAt least once for every 110 m3 (150 yd3)At least once for every 450 m2 (5000 ft2)Apply procedure for each concrete typeAt least 5 samples for each concrete typeWaive sampling if volume < 40 m3 (50 yd3)
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Specifications compliance