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CE 39007 Concrete Lab

Mondays/Thursdays/Fridays

Instructors:

Prof. L S RamachandraProf. Sudhirkumar BaraiProf. S ChakrabortyProf. S J HossainProf. Sutapa Das

Tests on Hardened Concrete

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Compressive Strength

z Tests on Cubes and Cylindersz Results of individual cubes should not differ by 15% of the

averagez A correction factor from graph shall be applied to correct cylinder

strengthz Equivalent cube strength = 1.25 times cylinder strength

Correction factor for height/diameter ration

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Failure modes of Cubes:Non-explosive/explosive

Failure modes of Cylinders:Splitting/ Shear (Cone)/ Splitting and Shear (Cone)

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2. Tensile Strength of Concrete

Tensile strength of Concrete

z Tensile strength of concrete ranges from 10 to 18% of thecompressive strength

z Indirect Approaches Standard Beam Test Modulus of Rupture Test Split Cylinder Test

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Determination of Modulus of Rupture

z Both two-point loading and centre point loading can be usedz Normally, tests are conducted in the laboratory after keeping the

concrete in wet condition for 48 hours and at 24 degree Celsiusz Rate of loading is 180 Kgf / minz f cr = pl/bd 2 for the distance between line of fracture and the nearer

support greater than 133 mm (i.e. the fracture occurs within themiddle 1/3 rd of the beam and also when maximum size of aggregate is 20 mm and sample of size 100 x 100 x 500 is used).

z If fracture occurs outside this range, the test should be repeated

Modulus of Rupture Test

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Modulus of Rupture Chart

Determination of Tensile Strengthby Split Tensile Test

z Results are observed to be 5-12% higher than f cr z The main advantage and cause of popularity is that same

specimen and instrument is used for compression testing.z Loading condition produced high compressive stress immediately

below the contact generators but after that almost uniform tensilestresses prevailz Rate of loading is 0.02 to 0.04 MPa per secondz The tensile splitting strength is calculated by

f st =2P/ LDP= The Compressive load on the Cylinder L = Length of the Cylinder D = Diameter of the Cylinder

z The Compressive strength of for an element on the verticaldiameter = 2P/ LD * [D2/r(D-r))-1]

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Split Cylinder Test

Split Cylinder Test

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3. Non-Destructive Tests Rebound Hammer Test

Ultrasonic Pulse Velocity Test

NDT with Schmidts ReboundHammer (IS: 13311 Part 2)

z This is used for measuring surface hardness of existing concrete masswhich in turn is correlated with the grade of concrete, calibration curvesare available to relate the rebound number (distance travelled by themass as a percentage of the initial extension of the spring) with the gradeof concrete for the hammer held either horizontal or vertical (down or up)

for both dry and wet condition of surfacez Unfortunately the result depends upon-

Type and nature of aggregate used Surface and internal moisture condition, presence of void Smoothness of surface

z It can be best used to compare strength of one concrete against another but usually not reliable in determining absolute strength. Moreover, eachhammer varies considerably in performance and require individualcalibration

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Rebound Hammer

Rebound Hammer

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Compressive Strength andRebound Number

NDT with Ultrasonic Pulse VelocityMeter (IS: 13311 Part 1)

z This is used for measuring the time of travel of pulse of vibrationsin ultrasonic ranges, passing through the concrete to judgequalitatively, how good or bad the concrete is. It can be operatedin direct, semi-direct or indirect, i.e. surface mode

z Unfortunately the result depends upon- Heterogeneity of concrete within a short length Presence of reinforcing steel or other impurities in concrete

z It can be best used to judge uniformity of concrete and to establishacceptance criteria, correlation with strength is possible but notpopular

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Ultrasonic Test:Direct Transmission/Semi-direct Transmission/Indirect Surface Transmission

Ultrasonic Test ResultsInterpretation

z > 4500 m/s Excellentz 3500-4500 m/s Goodz 3000-3500 m/s Doubtfulz 2000-3000 m/s Poor z

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Stress-Strain Behaviour of Concrete