Quality control and quality assurance Archives - OnlineEngineeringNotes https://onlineengineeringnotes.com/tag/quality-control-and-quality-assurance/ A Complete Guide to future Engineers Tue, 08 Nov 2022 13:12:31 +0000 en-US hourly 1 https://wordpress.org/?v=6.5.2 Testing of concrete and quality control: Variability of concrete strength and acceptance criteria and Non – destructive tests https://onlineengineeringnotes.com/2022/11/08/testing-of-concrete-and-quality-control-variability-of-concrete-strength-and-acceptance-criteria-and-non-destructive-tests/ https://onlineengineeringnotes.com/2022/11/08/testing-of-concrete-and-quality-control-variability-of-concrete-strength-and-acceptance-criteria-and-non-destructive-tests/#respond Tue, 08 Nov 2022 13:12:30 +0000 https://onlineengineeringnotes.com/?p=1474 1.1 Various strength of concrete 1. Tensile strength: Tensile strength (Fct) = 0.35 (Fck)1/2 Where, Fck = Characteristics compressive strength 2. Compressive strength: 3. Shear strength: 4. Bond strength: 1.2 Compressive, tensile and bond strength tests Compressive strength tests: 1. Cube test: Compressive strength of concrete = Force at failure / Area 2. Cylinder test: ... Read more

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1.1 Various strength of concrete
1. Tensile strength:
  • Due to brittle nature of concrete it is very weak in tension.
  • Concrete develops cracks when subjected to tensile force. So it important to determine the tensile strength of concrete.

Tensile strength (Fct) = 0.35 (Fck)1/2

Where,

Fck = Characteristics compressive strength

2. Compressive strength:
  • Concrete has high compressive strength.
  • Most of the concrete structure are designed by adopting the value of compressive strength.
3. Shear strength:
  • Occurs to the combination of tensile and compressive stress or due to the application of torsion.
  • Shear strength of concrete is about 12% of compressive strength of concrete.
4. Bond strength:
  • The resistance to slip of the steel reinforcement bars which are embedded in concrete.
  • Higher the grade of concrete higher the bond strength.

1.2 Compressive, tensile and bond strength tests

Compressive strength tests:
1. Cube test:
  • Specimen of 150 mm * 150 mm * 150 mm cube is used.
  • The cube filled in three layers and well compacted.
  • The top is made smooth and the finished surface is left undisturbed for 24 hours at room temperature.
  • Then the mould is removed and the specimen is stored in water for curing usually up to 28 days.
  • The testing of specimen is taken at 1,3,7,14,28,90 and 365 days.
  • The test is performed by standard uniaxial compression test and at each time, minimum three sample are tested.

Compressive strength of concrete = Force at failure / Area

2. Cylinder test:
  • The test is carried out using concrete cylinder of diameter 150 mm and 300mm height.
  • The cylinder is filled in three layer and well compacted.
  • Cube strength is higher than cylinder strength.
Tensile strength test:
  • It is very complex to apply uniaxial tension to a concrete specimen so the tensile strength of concrete is determined by two test.

The test which are performed for tensile strength test are as follows:

1. Flexural test:

  • Evaluates the tensile strength of concrete indirectly.
  • Test the ability of unreinforced concrete beam or slab to withstand failure in bending.

2. Splitting test:

  • Method of determining tensile strength of concrete using a cylinder which splits across vertical diameter.
  • Concrete cylinder is placed horizontally.

Horizontal tensile strength (Fst) = 2P/ πLD

Where,

P = Applied load

D = Diameter of cylinder

L = Length of cylinder

Bond strength test:
  • Bond test can be done by pullout test.

Pullout test:

  • Destructive technique that involves drilling a hole into the hardened concrete and inserting oversized screw into that hole.

1.3 Non – destructive tests

  • Test which are performed without destruction of the sample are known as non – destructive test.

Objective:

  • To determine the strength of concrete.
  • To determine cracks in concrete.
  • To determine permeability of concrete.
  • To determine the thickness of concrete layer.

Merits:

  • Less wastage of material.
  • Accident prevention.

Demerits:

  • Expensive.
  • Dangerous (Radiation hazards)
Method of non – destructive test are:

1. Rebound hammer test:

  • Done to estimate the compressive strength of concrete.
  • Also known as Schmidt hammer test.
  • It is a device used to measure the elastic properties or strength of concrete.

Merits:

  • Easy to carry out.
  • Cheap.
  • Skilled manpower not required.

Demerits:

  • Interpretation of data is difficult.
  • Only measure strength of concrete close to the surface.

2. Ultrasonic pulse velocity method:

  • Used to determine the integrity and quality of concrete by measuring the speed and attenuation of ultrasonic wave passing through the element being tested.
  • There are three ways of measuring pulse velocity through concrete. They are as follows:

a. Direct transmission

b. Indirect transmission

c. Surface transmission

1.4 Variability of concrete strength and acceptance criteria.

Variability of concrete strength:
  • Strength concrete varies from batch to batch over a period of time.

Factors affecting variation of concrete strength:

  • Quality of raw material.
  • W/C ratio.
  • Age of concrete.
  • Compaction of concrete.
  • Curing.
Acceptance criteria:

a. Compressive strength:

  • The mean strength determined from any group of four non – overlapping consecutive test results, complied with the appropriate limits col 2 of table.
  • Any individual test results complies with the appropriate limits in col 3 of table.
Specified grade (1)Mean of group of 4 non – overlapping consecutive test result in N/mm2 (2)Individual test in results in N/mm2 (3)
M15 and aboveFck + 0.825 * (Established standard deviation) Or, Fck + 3 N/mm2 ; which ever is greaterFck – 3 N/mm2
M20 and aboveFck + 0.825 * (Established standard deviation) Or, Fck + 4 N/mm2 ; which ever is greaterFck + 0.825 * (Established standard deviation) Or, Fck + 3 N/mm2 ; which ever is greater

b. Flexural strength:

  • Mean compressive strength of four consecutive test results must be more than equal to characteristic strength of concrete +3 N/mm2.
  • Any test results is not less than specified characteristic strength -0.3 N/mm2.

1.5 Quality control and quality assurance

Quality control:

  • Quality means degree of excellence.
  • Good design is not sufficient but its actual implementation is required for fully quality work and good performance of structure.

Quality can be controlled by:

  • Appropriate mixing.
  • Proper compaction.
  • Correct placing.
  • Adequate curing.

Quality assurance:

  • Systematic process of determining whether the concrete structure meets specified requirement is called quality assurance.
References:
  • Dayaratnam, P. Brick and reinforced brick structure.
  • Neville, A.M. Properties of Concrete. England: Pearson Education Limited.
  • Hendry, A.W., Sinha, B.P. & Davies, S.R. Design of Masonry Structure. London: E & FN Spon.

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