CHAPTER – V TESTING OF CONCRETE AND QUALITY CONTROL 6.1 Various Strength of Concrete: Tensile, Compressive, Shear and Bond In concrete design and quality control, strength is the property which is generally specified. This is because, compared to other properties, testing of strength is relatively easy. Furthermore, many properties of concrete, such as elastic modulus, water-tightness or impermeability and resistance to weathering agents including aggressive waters, are directly related to strength and can therefore be deduced from the strength data.
Compressive strength of concrete is many times greater than other types of strength and a majority of concrete elements are designed to take advantage of the higher compressive strength of the material. The uniaxial compression tests are the easiest to perform in the laboratory. The other types of strength required for concrete are: -
Tensile Strength
-
Shear Strength
-
Bond and Bearing Strengths
5.2 Compressive Strength Test In order to determine the compressive strength of concrete, three types of specimen can be used: i)
Cubes
ii)
Cylinders
iii)
Prisms
CUBE TESTS Generally, specimens are cast in steel or cast iron moulds of 150 mm dimensions, which should confirm to cubical shape. The dimensions and plainness should be within the limits of tolerance. i)
The cube is filled in three layers and well compacted.
ii)
After compaction, the top surface is made flush with edges of mould and finished by means of trowel.
iii)
The finished surface is left undisturbed for 24 hours at a temperature of 66°F - 70°F and relative humidity not less than 90 percent.
iv)
After 24 hours, the mould is stripped and the specimen is stored in water for further curing. As far as possible the curing temperature should be maintained at 66°F - 70°F.
The specimens are usually cured for 28 days. In some cases, testing of specimens is carried out at the age of 1 day, 3 days, 7 days, 14 days, 21 days, 45 days and 90 days.
CYLINDER TESTS The standard cylinder mould is 150 mm in diameter and 300 mm height made of cast iron or steel. Cylinder specimens are prepared as cube specimens. As the finished surface of cylinder may not be smooth, sulphur caping is done before testing. The strength of cylinder is equal to 4/5th of the strength of a cube, but experiments have shown that there is no simple relation between the strengths of the specimens of the two shapes. L’Hermite suggested the following relation: C linder strength
.76
.2 log
(
f ) 284
Where, fcu = strength of the cube in pounds per square inch. Cylinders are believed to give a greater uniformity of results for nominally similar specimens as their failure is less affected by the properties of the coarse aggregate used in the mix and the stress distribution on horizontal planes in a cylinder is more uniform than on a specimen of square crosssection. EFFECT OF HEIGHT/DIAMETER RATIO ON CYLINDER STRENGTH: Standard c linders are of height ‘h’ equal to twice the diameter‘d’, but sometimes specimens of other proportions are encountered. Height to diameter ratio (h/d)
Strength Correction Factor ASTM C 42 – 77
BS 1881 : 1970
2.00
1.00
1.00
1.75
0.98
0.98
1.50
0.96
0.96
1.25
0.93
0.94
1.00
0.87
0.92
PRISM TESTS Dimensions = 75 mm x 75 mm x 300 mm GAIN OF STRENGTH WITH AGE: The concrete develops strength with continued hydration. The rate of gain of strength is faster to start with and the rate gets reduced with age. It is customary to assume the 28-day strength as the full strength of concrete. Age (in days)
Compressive Strength
1
16 %
3
40 %
7
65 %
14
90 %
28
99 to 100 %
5.3 Tensile Strength Test Concrete being a brittle material is very weak in tension and is not expected to resist tensile forces, in general, from external loading conditions. But, due to restrained conditions of shrinkage and temperature effects, tensile stresses are developed in concrete.
DIRECT TENSION TEST This type of test is carried out under axial loading. The strength obtained under such type of testing is small compared to other types of tensile tests. Generally, this test is not performed. Direct tensile strength, f
.35 √f
Where, fck = characteristic strength of concrete specimen.
INDIRECT TENSION TEST -
Splitting Test
-
Flexural Test – i. Centre Point Loading ii. Third Point Loading
SPLITTING TEST/BRAZILIAN METHOD
A cylinder of the type used for compression test is placed with its axis horizontal between the plates of testing machine and the load is increased until the failure by splitting along the vertical diameter takes place. The load is applied through narrow strips of packing material such as ply-wood. Usually these strips are 3 mm thick and 12.5 mm to 25 mm wide. The horizontal tensile strength is obtained by the formula: T
2 DL
Where, P = compressive load on the cylinder D = diameter of the cylinder L = length of the cylinder T = splitting tensile strength of the specimen This test gives strength very close to the true tensile strength of concrete than the modulus of rupture.
MODULUS OF RUPTURE (FLEXURAL TEST) The tensile strength produced due to bending is known as modulus of rupture. Central point loading and third point loading are two loading schemes in which modulus of rupture is determined. Third point loading is the more appropriate method and gives lesser value compared to central point loading. CENTRAL POINT LOADING:
e have,
Hence,
Where, b= breadth and d=height of section THIRD POINT LOADING:
.
L 6
1 12 bd
d 2
L bd
.
L 4
1 12 bd
d 2
3 L 2bd
5.4 Variabilit of Concrete Strength and Acceptance Criteria Since, strength is a variable quantity, when designing a concrete mix, we must aim at a mean strength higher than the minimum required from the structural standpoint so that we can expect every part of the structure to be made of concrete of adequate strength. Concrete strength follows normal distribution curve.
i. Every sample should have a testing strength not less than the characteristic strength of concrete. Or, ii. The strength of one or more samples though less than the characteristic value, is in each case not less than the greater of the following: a) the characteristic strength minus 1.35 times the standard deviation; that is,
≥ fck – 1.35
s b)
.8 times the characteristic strength; that is,
≥ .8 fck.
Whichever is greater in the above two conditions and the average strength of all the samples should fulfill the following condition: .≥ f here,
[1.65
1.65 √
]
tested strength from supplied concrete, avg.
= average or mean strength of concrete supplied,
fck = characteristic strength, n = number of samples (minimum 30) s = standard deviation
5.5 Non Destructing Testing of Concrete 1. Destructive Tests 2. Non Destructive Tests a. Schmidt’s Rebound Hammer Test b. Ultrasonic Pulse Velocity Test c. Profometer d. Vibroscanner
REBOUND TEST The rebound hammer test measures the elastic rebound of concrete and is primarily used for estimation of concrete strength and for comparative investigations.
It consists of a spring control hammer that slides on a plunger within tubular housing. When the plunger is pressed against the surface of the concrete, the mass rebounds from the plunger, taking the rider with it along the guide scale. By pushing a button, the rider can be held in position to allow the reading to be taken. The distance travelled by the mass, is called the
rebound number. It is indicated by the rider moving along a graduated scale.
The results of this test are affected by: i.
Smoothness of surface under test
ii.
Size, shape and rigidity of the specimen
iii.
Age of specimen
iv.
Surface and internal moisture condition of the concrete
v.
Type of coarse aggregate
vi.
Type of cement
vii.
Type of mould
viii.
Carbonation of concrete surface
ULTRASONIC PULSE WAVE VELOCITY TEST It consists of measuring the time of travel of an ultrasonic pulse, passing through the concrete to be tested. The pulse generator circuit consists of electronic circuit for generating pulses and a transducer for transforming these electronic pulses into mechanical energy having vibration frequencies in the range of 15 to 50 KHz. The time of travel between initial onset and the reception of the pulse is measured electronically. The path length between transducer divided by the time of travel gives the average velocity of wave propagation.
PROFOMETER/COVERMETER TEST Profometer/Covermeter test is conducted to determine the presence of reinforcing bars, laps, transverse steel, metal tie, wires or aggregates with magnetic properties. It determines the size, depth and position of reinforcement buried in concrete. Purpose of application:
Quality control, to ensure correct location and cover to reinforcing bars after concrete placement.
Investigation of concrete members and locating reinforcement as a preliminary to some form of testing, such as, core extraction.
Location of buried ferromagnetic objects other than reinforcement.
