1

CE 39007 – Concrete Lab

Mondays/Thursdays/Fridays

Instructors:

Nirjhar Dhang (CE, Monday)

Sudhirkumar Barai (CE, Monday)

S K Bhattacharyya (CE, Thursday)S K Bhattacharyya (CE, Thursday)

Biswanath Banerjee (CE, Thursday)

Sushanta Chakrabarti (CE, Friday)

A George (ARP, Friday)

Tests on Hardened Concrete

2

Tests on Hardened Concrete

1. Compressive Strength of the Concrete

2. Tensile Strength of Concrete

3. Non-Destructive Tests –Rebound Hammer Test/Ultrasonic Pulse Velocity Test

Sh t t St ti M d l f El ti it4. Short-term Static Modulus of Elasticity

1. Compressive Strength Test

3

Compressive Strength

Tests on Cubes and Cylinders

Results of individual cubes should not differ by 15% of the average

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

Equivalent cube strength = 1.25 times cylinder strength

Correction factor for height/diameter ration

4

Failure modes of Cubes: Non-explosive/explosive

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

5

2. Tensile Strength of Concrete

Tensile strength of Concrete

Tensile strength of concrete ranges from 10 to 18% of the compressive strength

Indirect Approaches

– Standard Beam Test – Modulus of Rupture Test

– Split Cylinder Test

6

Determination of Modulus of Rupture

Both two-point loading and centre point loading can be used

Normally, tests are conducted in the laboratory after keeping the concrete in wet condition for 48 hours and at 24 degree Celsius

Rate of loading is 180 Kgf / min

fcr = pl/bd2 for the distance between line of fracture and the nearer support greater than 133 mm (i.e. the fracture occurs within the middle 1/3 rd of the beam and also when maximum size ofmiddle 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).

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

Modulus of Rupture Test

7

Modulus of Rupture Chart

Determination of Tensile Strength by Split Tensile Test

Results are observed to be 5-12% higher than fcr

The main advantage and cause of popularity is that same specimen and instrument is used for compression testing.

Loading condition produced high compressive stress immediately below the contact generators but after that almost uniform tensile stresses prevail

Rate of loading is 0.02 to 0.04 MPa per second The tensile splitting strength is calculated by

fst =2P/LDP= The Compressive load on the CylinderL = Length of the CylinderD = Diameter of the Cylinder

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

8

Split Cylinder Test

Split Cylinder Test

9

3. Non-Destructive Tests –Rebound Hammer Test

Ultrasonic Pulse Velocity Test

NDT with Schmidt’s Rebound Hammer (IS: 13311 Part 2)

This is used for measuring surface hardness of existing concrete mass which in turn is correlated with the grade of concrete, calibration curves are available to relate the rebound number (distance travelled by the mass as a percentage of the initial extension of the spring) with the grade of concrete for the hammer held either horizontal or vertical (down or up) for both dry and wet condition of surface

Unfortunately the result depends upon-– Type and nature of aggregate used– Surface and internal moisture condition, presence of void– Smoothness of surface

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

10

Rebound Hammer

Rebound Hammer

11

Compressive Strength and Rebound Number

NDT with Ultrasonic Pulse Velocity Meter (IS: 13311 Part 1)

This is used for measuring the time of travel of pulse of vibrations in ultrasonic ranges, passing through the concrete to judge qualitatively, how good or bad the concrete is. It can be operated in direct, semi-direct or indirect, i.e. surface mode

Unfortunately the result depends upon-– Heterogeneity of concrete within a short length

– Presence of reinforcing steel or other impurities in concretePresence of reinforcing steel or other impurities in concrete

It can be best used to judge uniformity of concrete and to establish acceptance criteria, correlation with strength is possible but not popular

12

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

Ultrasonic Test Results Interpretation

> 4500 m/s Excellent

3500-4500 m/s Good

3000-3500 m/s Doubtful

2000-3000 m/s Poor

<2000 m/s Very poor

13

4. Short-term Static Modulus of Elasticity

Modulus of Elasticity Test

14

Stress-Strain Behaviour of Concrete