Testing strength of hardened concrete and quality controlusers.tamuk.edu/kfgfa00/CEEN...

CEEN 3144 Construction Materials

Testing strength of hardened concrete and quality control

Francisco AguíñigaAssistant Professor

Civil Engineering ProgramTexas A&M University – Kingsville

Factors affecting compressive strength

Water/cement ratioTimeMaturity conceptCementAggregateAdmixtures

Water/cement ratio

Strength of concrete depends on capillary porosity – not easily measurableCapillary porosity is proportional to w/c

Water/cement ratio

Factors not considered by Abrams law

Degree of hydrationAir contentAggregate effects

Abrams law

Water/cement ratio

Time

Compressive strength depends on time and w/cfc at 28 days = f’c = 1.5 fc @ 7 days

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

Maturity concept

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

Cement

Chemical composition and finenessCement variability leads to variability in concrete strength

Aggregate

Important parameters:Shape, texture, and maximum sizeSurface texture affects σ-ε curve but not ultimate strength

Cement and air contents

Admixtures

Little effect on strength – per seHowever – admixtures alter w/c ratio and porosityStrength gain modified by accelerating or retarding cement hydration

Temperature

Length and diameter of specimen (l/d)

Cylinder size

Concrete age

Strain rate during loading

Concrete state of stress

State of stress

Tensile strength

Shear strength

Impact strength

Shrinkage strain – reduces tensile strength

Creep strains

In-place strength of concrete

Nondestructive methodsSurface hardnessRebound hardnessPenetration resistancePull-out testsUltrasonic pulse velocity

Tests on core samplesStructural load test

Nondestructive methods

Surface hardnessImpact concrete with a standard massMeasure size of indentationAccuracy between 20 and 30%


Rebound hardnessSchmidt hammerResults affected by

Surface finishMoisture contentTemperatureMember rigiditySurface carbonationDirection of impact

Nondestructive methods –rebound hardness


Penetration resistance - Windsor probe

Harder aggregates yield higher penetration resistance


Pull-out testsMeasure of concrete shear strengthCorrelated with compressive strength


Pull-out tests


Ultrasonic pulse velocityV = (E/ρ)^0.5


Ultrasonic pulse velocity


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

Quality control

Factors that contribute to concrete variability

MaterialsProductionTesting

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 =

Measurement of variability

The normal curve

Area under the normal curve

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

Quality control

Concrete variability effects are reduced because

Batches are intermixed when placedTests are based on 28-days, strength with timeSteel reinforcement redistributes stresses

ACI 214 approach to variability

Required average strength

tVf

f ccr −=

1

'

tsff ccr += '


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

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

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)

Specifications compliance

Testing strength of hardened concrete and quality controlusers.tamuk.edu/kfgfa00/CEEN...

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