Testing of concrete and quality control

Quality control (QC) is the set of checks which ensure that concrete meets the specified strength and durability; quality assurance (QA) is the planned system (documents, responsibilities, audits) that gives confidence that QC is done. IS 456 (Cl. 15 and 16) sets the sampling and acceptance rules.

1. Planning and quality assurance

• A QA plan with job specifications, drawings, method statements, responsible persons and test records.
• Trained staff, calibrated equipment, a site laboratory, and an approved mix design (IS 10262) with trial mixes.

2. Materials

• Cement: check manufacturer's test certificate, date of manufacture, storage on a dry raised platform (IS 456 Cl. 10), use in the order received; test for setting time, fineness and strength as needed.
• Aggregates: sieve analysis, silt and organic content, moisture correction, flakiness; stored separately on clean hard floor (IS 383).
• Water: potable, pH not below 6; limits on chloride and sulphate (IS 456 Cl. 5.4).
• Steel and admixtures: approved grade, no loose rust, test certificates.

3. Batching and mixing

• Batching by weight (volume only for small works; IS 456 Cl. 10.2); accuracy of $\pm2\%$ for cement and water, $\pm3\%$ for aggregates.
• Mix in a mechanical mixer for the minimum time (usually 2 minutes); correct for moisture in aggregate.

4. Fresh concrete control

• Slump or compacting factor on every batch, and the temperature (IS 456 Cl. 7).
• Check the w/c ratio; no extra water at site.

5. Transport, placing and compaction

• Prevent segregation, limit free fall to 1.5 m, and place before initial set (30 min to 2 hours).
• Compact with needle vibrators; clean formwork, proper cover blocks, check bar spacing and cover before concreting.

6. Curing and protection

• Cure properly (IS 456 Cl. 13.5) and keep records; follow hot/cold-weather rules (Cl. 14).

7. Sampling, testing and acceptance

• Sample at the point of placing: 1 sample for 1 to 5 m$^3$, 2 for 6 to 15, 3 for 16 to 30, 4 for 31 to 50 and 1 extra for each additional 50 m$^3$ (IS 456 Cl. 15.2).
• Three 150 mm cubes per sample, tested at 28 days; the sample strength is the mean of three (Cl. 15.4).
• Accept using Table 11: mean of 4 consecutive results $\geq f_{ck}+0.825s$ (or $f_{ck}+4$, M20 and above) and every result $\geq f_{ck}-3$.
• If the cubes fail, do NDT or cores (Cl. 17) and load tests.

8. Records and statistical control

Daily records of mix, tests and weather; control charts of cube strength; the standard deviation is monitored and the mix is adjusted. Non-conformities are reported and corrected.

The acceptance (compliance) criteria decide whether the placed concrete meets the specified characteristic strength $f_{ck}$. IS 456 (Cl. 16) deals with them; the strength for each sample is the average of three specimens (Cl. 15.4), a specimen differing by more than 15% from that average being rejected.

Compressive strength (IS 456, Cl. 16.1 and Table 11)

Concrete is accepted if both conditions hold:

Here $s$ is the established standard deviation (Table 8 values may be used) and the value is rounded off to the nearest 0.5 N/mm$^2$. The larger of the two limits is taken.

Flexural strength (IS 456, Cl. 16.2)

Concrete is accepted if

• the mean of any group of four consecutive test results exceeds the specified characteristic flexural strength by at least 0.3 N/mm$^2$, and
• every individual result is not less than the specified characteristic strength minus 0.3 N/mm$^2$.

If the criteria fail (Cl. 16.3 and 17)

The concrete may be accepted at a lower rate by the engineer, or NDT, drilled cores (average of cores at least 85% of $f_{ck}$ and no core below 75%), or load tests on the structure may be done.

Compressive strength test (IS 516)

1. Sampling and casting: take a representative sample from the mix (IS 456, Cl. 15). Fill 150 mm cube moulds in 3 layers, each layer compacted by a table vibrator or tamped with a 16 mm bar (35 strokes per layer for 150 mm cubes). Level and finish the top.
2. Curing: keep the mould covered for 24 ± 0.5 h at 27 ± 2 °C, demould, and cure in water at 27 ± 2 °C until testing (normally 28 days; also 3 and 7 days).
3. Testing: take the cube out of water, wipe the surface, and place it in the compression testing machine so that the load acts on the sides as cast (not the top). Apply the load without shock at 14 N/mm$^2$ per minute (about 140 kg/cm$^2$/min), until the specimen fails. Record the maximum load $P$.
4. Result: $f_{c} = P/A$, where $A = 150\times150 = 22500$ mm$^2$. The test strength is the average of three specimens; if any one differs by more than ±15% from the average, that result is discarded (IS 456, Cl. 15.4).
5. For cylinders (150 mm dia × 300 mm high) the same procedure is followed, with capped ends; $f_{cyl} \approx 0.8\,f_{cube}$.

Tests in tension

Tensile strength of concrete is only about 7 to 12% of its compressive strength, but it controls cracking, shear, bond and the design of pavements, water tanks and prestressed members. Concrete is not designed to carry tension, but the tensile strength decides when the first crack appears.

Direct tension test

A dog-bone or cylindrical specimen with embedded end grips is pulled axially. It is rarely used, because it is hard to avoid eccentricity and secondary stresses from the grips, and the result scatters. Direct tensile strength is about $0.35\sqrt{f_{ck}}$ to $0.45\sqrt{f_{ck}}$ (MPa).

Splitting tension test (Brazilian test, IS 5816)

A standard cylinder (150 mm dia × 300 mm long) is laid horizontally between the platens of the testing machine, with thin plywood or fibre strips (about 3 mm thick and 15 mm wide) along the contact lines. The compressive line load is increased at a rate of 1.2 to 2.4 N/mm$^2$/min (splitting tensile stress) until the cylinder splits along the vertical diameter. The load applied gives a nearly uniform tensile stress across the loaded plane:

$$f_{sp} = \frac{2P}{\pi L d}$$

where $P$ = failure load, $L$ = length and $d$ = diameter of cylinder. Splitting strength is higher than direct tension by 5 to 12%, and about 0.5 to 0.6$\sqrt{f_{ck}}$ in MPa.

Advantages: simple, uses the same cylinders and compression machine as the compression test, gives a more uniform and reproducible result than direct tension, with no grip problem.

Flexure test (modulus of rupture, IS 516)

A plain concrete beam 150 × 150 × 700 mm is supported on a 600 mm span and loaded at the third points (two-point loading) at 0.7 N/mm$^2$/min extreme-fibre stress rate. If the crack occurs in the middle third:

$$f_{cr} = \frac{P\,l}{b\,d^{2}}$$

If the crack is outside the middle third but within 5% of the span from the load point, $f_{cr} = 3Pa/(bd^2)$ ($a$ = distance of crack from the nearer support); otherwise the result is rejected. For single (centre) point loading, $f_{cr} = 3Pl/(2bd^2)$. IS 456 (Cl. 6.2.2) gives the flexural strength $f_{cr} = 0.7\sqrt{f_{ck}}$ N/mm$^2$. The modulus of rupture is greater than direct tension (by about 1.5 to 2 times) since the stress distribution is assumed linear while the actual stress block is plastic.

Modulus of rupture (flexural strength) is the maximum extreme-fibre tensile stress in a plain concrete beam at failure in bending, calculated by the flexure formula $f_{cr} = M/Z$. It is an indirect measure of the tensile strength, used for cracking moment of RC members (IS 456, Cl. 6.2.2: $f_{cr} = 0.7\sqrt{f_{ck}}$ N/mm$^2$) and for the design of pavements.

Laboratory determination (IS 516)

      P/2          P/2
       |            |
   ____v____________v____
  |       beam           |   d = 150
  |______________________|
  ^   l/3  |  l/3  | l/3 ^
  |<------- l = 600 ------>|

1. Cast beams 150 × 150 × 700 mm (or 100 × 100 × 500 for 20 mm aggregate), cure in water and test wet, at 28 days.
2. Place the beam on two rollers at 600 mm span, with the cast side turned to the side. Apply the load at the third points (two-point loading), without shock at 0.7 N/mm$^2$/min extreme-fibre stress (about 400 kg/min).
3. Record the failure load $P$ (total).
4. If the fracture is within the middle third:

$$f_{cr} = \frac{P\,l}{b\,d^2}$$

5. If the fracture lies outside the middle third by not more than 5% of the span, $f_{cr} = \dfrac{3Pa}{bd^2}$ with $a$ the distance from the nearest support; if more than that, the result is discarded.

With single (centre) point loading $f_{cr} = \dfrac{3Pl}{2bd^2}$. The two-point loading is preferred since a uniform moment acts over the middle third and the weakest section governs, as for a direct test; the centre-point test gives about 15% higher values.

Non-destructive testing (NDT) assesses the quality and strength of concrete in a structure without damaging it.

Importance

• Tests the actual in-situ concrete, not only the control cubes, which may not represent the structure's compaction and curing.
• No damage and repeated tests at the same place are possible, so the changes with time can be tracked.
• Quick and cheap; large areas can be surveyed to find weak, honeycombed or cracked zones.
• Assesses old structures for repair, change of use, fire or earthquake damage, and long-term deterioration (corrosion, carbonation).
• Investigates doubts when cube results fail the acceptance criteria (IS 456, Cl. 16), before costly coring or load tests (Cl. 17).
• Checks uniformity, thickness, cover and the position of rebars, and gives a basis for deciding on repair.

NDT methods give indirect estimates of strength, quality and condition without damaging the structure.

Rebound hammer (Schmidt hammer) test (IS 13311 Part 2)

Principle: a spring-loaded mass strikes the concrete surface through a plunger. The rebound of the mass, read as the rebound number $R$ on a scale of 10 to 100, depends on the surface hardness, which is related to strength.

Procedure

1. Select a smooth, clean, dry surface of a structural member at least 100 mm thick; grind off plaster, loose scale and rough texture.
2. Hold the hammer at right angles to the surface (horizontal, vertical up or down need orientation corrections), push the plunger until the mass is released, and read the rebound number.
3. Take at least 12 readings at spots 20 to 50 mm apart; reject readings that differ from the mean by more than 6 units, and average the rest.
4. Read the compressive strength from the manufacturer's calibration curve (for the position of the hammer), which should preferably be made from cubes of the same concrete.

Precautions: avoid edges, honeycombed and wet surfaces, and spots over reinforcement; the instrument should be checked on the test anvil.

Limits: it tests only the surface (about 30 mm), so it is affected by carbonation, moisture, surface finish, type of aggregate and cement, and age. Accuracy of strength is about ±15 to 25%. It is useful for comparing uniformity and for choosing places for coring or UPV.

Ultrasonic pulse velocity (UPV) test (IS 13311 Part 1)

Principle: the velocity of an ultrasonic pulse through concrete depends on its density and elastic properties. Sound, dense concrete transmits the pulse faster than porous or cracked concrete.

Apparatus: pulse generator and receiver with two transducers (usually 54 kHz), a timer reading to 0.1 microsecond, a coupling gel, and a reference calibration bar.

Procedure

1. Calibrate the instrument on the reference bar.
2. Clean the surface and apply grease or gel to ensure good acoustic contact.
3. Put the transducers on the concrete, either directly (opposite faces, most reliable), semi-directly (adjacent faces) or indirectly (same face, least reliable).
4. Measure the path length $L$ and the transit time $T$.
5. Calculate the velocity:

$$V = \frac{L}{T}$$

Take several readings and average them. Avoid paths parallel to reinforcement, as steel carries the pulse faster.

Quality of concrete (IS 13311 Part 1)

A combination of two or more methods gives a better estimate, and the results are best correlated with some cores.

Importance

• Tests the actual in-situ concrete, not only the control cubes, which may not represent the structure's compaction and curing.
• No damage and repeated tests at the same place are possible, so the changes with time can be tracked.
• Quick and cheap; large areas can be surveyed to find weak, honeycombed or cracked zones.
• Assesses old structures for repair, change of use, fire or earthquake damage, and long-term deterioration (corrosion, carbonation).
• Investigates doubts when cube results fail the acceptance criteria (IS 456, Cl. 16), before costly coring or load tests (Cl. 17).
• Checks uniformity, thickness, cover and the position of rebars, and gives a basis for deciding on repair.

Methods

Rebound hammer (Schmidt hammer) test (IS 13311 Part 2)

Principle: a spring-loaded mass strikes the concrete surface through a plunger. The rebound of the mass, read as the rebound number $R$ on a scale of 10 to 100, depends on the surface hardness, which is related to strength.

Procedure

1. Select a smooth, clean, dry surface of a structural member at least 100 mm thick; grind off plaster, loose scale and rough texture.
2. Hold the hammer at right angles to the surface (horizontal, vertical up or down need orientation corrections), push the plunger until the mass is released, and read the rebound number.
3. Take at least 12 readings at spots 20 to 50 mm apart; reject readings that differ from the mean by more than 6 units, and average the rest.
4. Read the compressive strength from the manufacturer's calibration curve (for the position of the hammer), which should preferably be made from cubes of the same concrete.

Precautions: avoid edges, honeycombed and wet surfaces, and spots over reinforcement; the instrument should be checked on the test anvil.

Limits: it tests only the surface (about 30 mm), so it is affected by carbonation, moisture, surface finish, type of aggregate and cement, and age. Accuracy of strength is about ±15 to 25%. It is useful for comparing uniformity and for choosing places for coring or UPV.

Ultrasonic pulse velocity (UPV) test (IS 13311 Part 1)

Principle: the velocity of an ultrasonic pulse through concrete depends on its density and elastic properties. Sound, dense concrete transmits the pulse faster than porous or cracked concrete.

Apparatus: pulse generator and receiver with two transducers (usually 54 kHz), a timer reading to 0.1 microsecond, a coupling gel, and a reference calibration bar.

Procedure

1. Calibrate the instrument on the reference bar.
2. Clean the surface and apply grease or gel to ensure good acoustic contact.
3. Put the transducers on the concrete, either directly (opposite faces, most reliable), semi-directly (adjacent faces) or indirectly (same face, least reliable).
4. Measure the path length $L$ and the transit time $T$.
5. Calculate the velocity:

$$V = \frac{L}{T}$$

Take several readings and average them. Avoid paths parallel to reinforcement, as steel carries the pulse faster.

Quality of concrete (IS 13311 Part 1)

Importance of tensile strength

Tensile strength of concrete is only about 7 to 12% of its compressive strength, but it controls cracking, shear, bond and the design of pavements, water tanks and prestressed members. Concrete is not designed to carry tension, but the tensile strength decides when the first crack appears. It is needed for crack control, shear and bond design, pavement thickness, water-retaining structures (IS 3370) and prestress release.

Direct tension test

A dog-bone or cylindrical specimen with embedded end grips is pulled axially. It is rarely used, because it is hard to avoid eccentricity and secondary stresses from the grips, and the result scatters. Direct tensile strength is about $0.35\sqrt{f_{ck}}$ to $0.45\sqrt{f_{ck}}$ (MPa).

Splitting tension test (Brazilian test, IS 5816)

A standard cylinder (150 mm dia × 300 mm long) is laid horizontally between the platens of the testing machine, with thin plywood or fibre strips (about 3 mm thick and 15 mm wide) along the contact lines. The compressive line load is increased at a rate of 1.2 to 2.4 N/mm$^2$/min (splitting tensile stress) until the cylinder splits along the vertical diameter. The load applied gives a nearly uniform tensile stress across the loaded plane:

$$f_{sp} = \frac{2P}{\pi L d}$$

where $P$ = failure load, $L$ = length and $d$ = diameter of cylinder. Splitting strength is higher than direct tension by 5 to 12%, and about 0.5 to 0.6$\sqrt{f_{ck}}$ in MPa.

Advantages: simple, uses the same cylinders and compression machine as the compression test, gives a more uniform and reproducible result than direct tension, with no grip problem.

Flexure test (modulus of rupture, IS 516)

A plain concrete beam 150 × 150 × 700 mm is supported on a 600 mm span and loaded at the third points (two-point loading) at 0.7 N/mm$^2$/min extreme-fibre stress rate. If the crack occurs in the middle third:

$$f_{cr} = \frac{P\,l}{b\,d^{2}}$$

If the crack is outside the middle third but within 5% of the span from the load point, $f_{cr} = 3Pa/(bd^2)$ ($a$ = distance of crack from the nearer support); otherwise the result is rejected. For single (centre) point loading, $f_{cr} = 3Pl/(2bd^2)$. IS 456 (Cl. 6.2.2) gives the flexural strength $f_{cr} = 0.7\sqrt{f_{ck}}$ N/mm$^2$. The modulus of rupture is greater than direct tension (by about 1.5 to 2 times) since the stress distribution is assumed linear while the actual stress block is plastic.

The three results are related: direct tension < splitting < flexural.

Concrete strength is not a single value; it depends on the type of stress. All are related to the compressive strength, which is the basic one measured.

Compressive strength

Measured on 150 mm cube (IS 516) or 150 × 300 cylinder; $f_{cyl} \approx 0.8 f_{cube}$. Needed for column, beam compression zone and slab design.

Tensile strength

Governs cracking and is found by splitting or flexure tests. Concrete is weak in tension, so steel carries it.

Shear strength

Resistance to sliding failure. Pure shear is difficult to test; diagonal tension controls beam design, with $\tau_v = V/bd$ compared to $\tau_c$ (Table 19) and $\tau_{c,max}$ (Table 20).

Bond strength

Adhesion and friction between steel and concrete that transfers force through the development length $L_d = \phi\sigma_s/(4\tau_{bd})$ (Cl. 26.2.1). It depends on bar surface, concrete strength, cover and bar position.

As the compressive strength rises, the other strengths also rise, but at a lower rate, in proportion to $\sqrt{f_{ck}}$.

Ultrasonic pulse velocity (UPV) test (IS 13311 Part 1)

Principle: the velocity of an ultrasonic pulse through concrete depends on its density and elastic properties. Sound, dense concrete transmits the pulse faster than porous or cracked concrete.

Apparatus: pulse generator and receiver with two transducers (usually 54 kHz), a timer reading to 0.1 microsecond, a coupling gel, and a reference calibration bar.

Procedure

1. Calibrate the instrument on the reference bar.
2. Clean the surface and apply grease or gel to ensure good acoustic contact.
3. Put the transducers on the concrete, either directly (opposite faces, most reliable), semi-directly (adjacent faces) or indirectly (same face, least reliable).
4. Measure the path length $L$ and the transit time $T$.
5. Calculate the velocity:

$$V = \frac{L}{T}$$

Take several readings and average them. Avoid paths parallel to reinforcement, as steel carries the pulse faster.

Quality of concrete (IS 13311 Part 1)

Strength estimation: a correlation curve between $V$ and the compressive strength of cubes of the same mix (made and tested in the laboratory) is used. The accuracy is about ±20% only, because moisture, aggregate type, age and reinforcement also change $V$. Better accuracy is obtained by combining it with the rebound number (SONREB method).

Uses: uniformity of concrete, locating voids and cracks, depth of surface cracks, deterioration by fire, frost or chemical attack, and dynamic modulus.

Rebound hammer (Schmidt hammer) test (IS 13311 Part 2)

Principle: a spring-loaded mass strikes the concrete surface through a plunger. The rebound of the mass, read as the rebound number $R$ on a scale of 10 to 100, depends on the surface hardness, which is related to strength.

Procedure

1. Select a smooth, clean, dry surface of a structural member at least 100 mm thick; grind off plaster, loose scale and rough texture.
2. Hold the hammer at right angles to the surface (horizontal, vertical up or down need orientation corrections), push the plunger until the mass is released, and read the rebound number.
3. Take at least 12 readings at spots 20 to 50 mm apart; reject readings that differ from the mean by more than 6 units, and average the rest.
4. Read the compressive strength from the manufacturer's calibration curve (for the position of the hammer), which should preferably be made from cubes of the same concrete.

Precautions: avoid edges, honeycombed and wet surfaces, and spots over reinforcement; the instrument should be checked on the test anvil.

Limits: it tests only the surface (about 30 mm), so it is affected by carbonation, moisture, surface finish, type of aggregate and cement, and age. Accuracy of strength is about ±15 to 25%. It is useful for comparing uniformity and for choosing places for coring or UPV.

Ultrasonic pulse velocity (UPV) test (IS 13311 Part 1)

Principle: the velocity of an ultrasonic pulse through concrete depends on its density and elastic properties. Sound, dense concrete transmits the pulse faster than porous or cracked concrete.

Apparatus: pulse generator and receiver with two transducers (usually 54 kHz), a timer reading to 0.1 microsecond, a coupling gel, and a reference calibration bar.

Procedure

1. Calibrate the instrument on the reference bar.
2. Clean the surface and apply grease or gel to ensure good acoustic contact.
3. Put the transducers on the concrete, either directly (opposite faces, most reliable), semi-directly (adjacent faces) or indirectly (same face, least reliable).
4. Measure the path length $L$ and the transit time $T$.
5. Calculate the velocity:

$$V = \frac{L}{T}$$

Take several readings and average them. Avoid paths parallel to reinforcement, as steel carries the pulse faster.

Quality of concrete (IS 13311 Part 1)

Strength estimation: read from a calibration curve of $V$ against cube strength for the same mix; accuracy about ±20%.

Formula (IS 5816): $f_{sp} = \dfrac{2P}{\pi L d}$

Given $P = 400$ kN $= 400\,000$ N, length $L = 200$ mm, diameter $d = 150$ mm.

$$f_{sp} = \frac{2\times400000}{\pi\times200\times150} = \frac{800000}{94247.8} = 8.49\ \text{N/mm}^2$$

Answer: splitting tensile strength = 8.49 N/mm$^2$ (MPa).

Formulae: cylinder $f_{sp} = \dfrac{2P}{\pi L d}$; cube (load along the centre line of a face) $f_{sp} = \dfrac{2P}{\pi a^2}$, where $a$ is the side.

$P = 500$ kN $= 500\,000$ N.

Cylinder 300 mm × 150 mm dia

$$f_{sp} = \frac{2\times500000}{\pi\times300\times150} = \frac{1000000}{141371.7} = 7.07\ \text{N/mm}^2$$

Cube 150 mm

$$f_{sp} = \frac{2\times500000}{\pi\times150\times150} = \frac{1000000}{70685.8} = 14.15\ \text{N/mm}^2$$

Answer: cylinder 7.07 N/mm$^2$; cube 14.15 N/mm$^2$. The cube gives the higher value because of the shorter length and the different stress field; the cylinder result is the standard one.

Compressive strength test (IS 516)

1. Sampling and casting: take a representative sample from the mix (IS 456, Cl. 15). Fill 150 mm cube moulds in 3 layers, each layer compacted by a table vibrator or tamped with a 16 mm bar (35 strokes per layer for 150 mm cubes). Level and finish the top.
2. Curing: keep the mould covered for 24 ± 0.5 h at 27 ± 2 °C, demould, and cure in water at 27 ± 2 °C until testing (normally 28 days; also 3 and 7 days).
3. Testing: take the cube out of water, wipe the surface, and place it in the compression testing machine so that the load acts on the sides as cast (not the top). Apply the load without shock at 14 N/mm$^2$ per minute (about 140 kg/cm$^2$/min), until the specimen fails. Record the maximum load $P$.
4. Result: $f_{c} = P/A$, where $A = 150\times150 = 22500$ mm$^2$. The test strength is the average of three specimens; if any one differs by more than ±15% from the average, that result is discarded (IS 456, Cl. 15.4).
5. For cylinders (150 mm dia × 300 mm high) the same procedure is followed, with capped ends; $f_{cyl} \approx 0.8\,f_{cube}$.

Acceptance criteria (IS 456, Cl. 16.1, Table 11)

• Strength of a sample = mean of three specimens; discard if one differs by more than 15% from the mean (Cl. 15.4).
• Mean of every group of 4 non-overlapping consecutive results $\geq f_{ck}+0.825\,s$ (nearest 0.5 N/mm$^2$), but not less than $f_{ck}+3$ (M15) or $f_{ck}+4$ (M20 and above).
• Each individual result $\geq f_{ck}-3$ N/mm$^2$.
• If these are not met the concrete may be checked by cores, NDT and load test (Cl. 16.3, Cl. 17).

Importance in RC design

• Compressive strength is the basis of structural design: the grade (M20, M25 ...) gives the characteristic strength $f_{ck}$ used for design stress $0.446 f_{ck}$ (stress block, IS 456 Cl. 38.1) and $0.45 f_{ck}$ (working stress).
• Other properties are derived from it: $E_c = 5000\sqrt{f_{ck}}$ (Cl. 6.2.3.1), flexural strength $0.7\sqrt{f_{ck}}$ (Cl. 6.2.2), shear $\tau_c$ and bond $\tau_{bd}$ (Cl. 26.2.1.1).
• It is the principal index of the quality of concrete, since it also reflects density, durability and permeability.
• It is the easiest to test and is the criterion of mix design (IS 10262), quality control and acceptance (IS 456 Cl. 16).
• Concrete resists compression in columns, the compression zone of beams and slabs, footings and walls, i.e. the main role in RC members.

Compressive strength test (IS 516)

1. Sampling and casting: take a representative sample from the mix (IS 456, Cl. 15). Fill 150 mm cube moulds in 3 layers, each layer compacted by a table vibrator or tamped with a 16 mm bar (35 strokes per layer for 150 mm cubes). Level and finish the top.
2. Curing: keep the mould covered for 24 ± 0.5 h at 27 ± 2 °C, demould, and cure in water at 27 ± 2 °C until testing (normally 28 days; also 3 and 7 days).
3. Testing: take the cube out of water, wipe the surface, and place it in the compression testing machine so that the load acts on the sides as cast (not the top). Apply the load without shock at 14 N/mm$^2$ per minute (about 140 kg/cm$^2$/min), until the specimen fails. Record the maximum load $P$.
4. Result: $f_{c} = P/A$, where $A = 150\times150 = 22500$ mm$^2$. The test strength is the average of three specimens; if any one differs by more than ±15% from the average, that result is discarded (IS 456, Cl. 15.4).
5. For cylinders (150 mm dia × 300 mm high) the same procedure is followed, with capped ends; $f_{cyl} \approx 0.8\,f_{cube}$.

Merits and demerits

Steps for compression testing from sampling

1. Take the sample at the point of placing, as per the frequency of IS 456, Cl. 15.2 (1 sample for 1 to 5 m$^3$, etc.).
2. Fill 150 mm cube moulds in 3 layers, compact by tamping (35 blows each, 16 mm rod) or vibration, level the top, and mark.
3. Keep at 27 ± 2 °C, covered, for 24 hours; demould and cure in water until 28 days.
4. Remove, wipe, place between platens with cast faces on the sides, apply load at 14 N/mm$^2$/min to failure.
5. $f_c = P/A$, average of three; apply the acceptance criteria of IS 456 Table 11.

View: the statement is correct. Many research laboratories are shifting to cylinders because they give a more reliable measure of the real strength of the material.

Reasons in favour of cylinders

1. Less platen restraint. A cylinder of height/diameter = 2 has a central zone not confined by the end friction, so failure is closer to uniaxial compression. The cube (h/d = 1) is completely within the restraint zone and gives a strength 20 to 25% higher.
2. Uniform stress and a well-defined failure mode.
3. Same cylinder is used for splitting tension, modulus of elasticity and Poisson's ratio, and gives consistent results.
4. Better for high-strength concrete and for comparing results among laboratories (ASTM, Eurocode specify cylinders; $f_{ck,cyl}\approx 0.8f_{ck,cube}$).
5. Cores drilled from structures are cylinders, so the same shape allows direct comparison.

Limitations

• Cylinders need capping (sulphur, neoprene pads) or grinding, are heavier, and the test is more sensitive to the end condition and care in moulding.

For quality control on sites in Nepal and India, the 150 mm cube remains the standard (IS 456 and IS 516) for simplicity, but for research and international use the cylinder is rightly preferred.

Effect of height/diameter ratio

In a compression test the platens restrain the lateral expansion of the specimen ends (platen restraint). A short specimen (low h/d) is mostly under this confinement and gives a higher strength; a taller specimen has a free central zone. Hence the measured strength falls as h/d increases from 1 to 2 and is nearly constant beyond 2.

The standard cylinder has h/d = 2. For example, a specimen with h/d = 1 shows about 15 to 25% more strength than the standard. For very large h/d (above 3) buckling and eccentricity reduce the strength.

Splitting tension test (Brazilian test, IS 5816)

A standard cylinder (150 mm dia × 300 mm long) is laid horizontally between the platens of the testing machine, with thin plywood or fibre strips (about 3 mm thick and 15 mm wide) along the contact lines. The compressive line load is increased at a rate of 1.2 to 2.4 N/mm$^2$/min (splitting tensile stress) until the cylinder splits along the vertical diameter. The load applied gives a nearly uniform tensile stress across the loaded plane:

$$f_{sp} = \frac{2P}{\pi L d}$$

where $P$ = failure load, $L$ = length and $d$ = diameter of cylinder. Splitting strength is higher than direct tension by 5 to 12%, and about 0.5 to 0.6$\sqrt{f_{ck}}$ in MPa.

Tensile strength tests

A dog-bone or cylindrical specimen with embedded end grips is pulled axially. It is rarely used, because it is hard to avoid eccentricity and secondary stresses from the grips, and the result scatters. Direct tensile strength is about $0.35\sqrt{f_{ck}}$ to $0.45\sqrt{f_{ck}}$ (MPa).

Splitting tension test (Brazilian test, IS 5816)

A standard cylinder (150 mm dia × 300 mm long) is laid horizontally between the platens of the testing machine, with thin plywood or fibre strips (about 3 mm thick and 15 mm wide) along the contact lines. The compressive line load is increased at a rate of 1.2 to 2.4 N/mm$^2$/min (splitting tensile stress) until the cylinder splits along the vertical diameter. The load applied gives a nearly uniform tensile stress across the loaded plane:

$$f_{sp} = \frac{2P}{\pi L d}$$

where $P$ = failure load, $L$ = length and $d$ = diameter of cylinder. Splitting strength is higher than direct tension by 5 to 12%, and about 0.5 to 0.6$\sqrt{f_{ck}}$ in MPa.

Flexure test (modulus of rupture, IS 516)

A plain concrete beam 150 × 150 × 700 mm is supported on a 600 mm span and loaded at the third points (two-point loading) at 0.7 N/mm$^2$/min extreme-fibre stress rate. If the crack occurs in the middle third:

$$f_{cr} = \frac{P\,l}{b\,d^{2}}$$

If the crack is outside the middle third but within 5% of the span from the load point, $f_{cr} = 3Pa/(bd^2)$ ($a$ = distance of crack from the nearer support); otherwise the result is rejected. For single (centre) point loading, $f_{cr} = 3Pl/(2bd^2)$. IS 456 (Cl. 6.2.2) gives the flexural strength $f_{cr} = 0.7\sqrt{f_{ck}}$ N/mm$^2$. The modulus of rupture is greater than direct tension (by about 1.5 to 2 times) since the stress distribution is assumed linear while the actual stress block is plastic.

Advantage of the split test

It is simple and uses the same cylinder and compression machine as the compression test; it gives a uniform, reproducible tensile stress with no grip eccentricity, and is suitable for cores from structures.

Effect of height/diameter ratio

In a compression test the platens restrain the lateral expansion of the specimen ends (platen restraint). A short specimen (low h/d) is mostly under this confinement and gives a higher strength; a taller specimen has a free central zone. Hence the measured strength falls as h/d increases from 1 to 2 and is nearly constant beyond 2.

The standard cylinder has h/d = 2. For example, a specimen with h/d = 1 shows about 15 to 25% more strength than the standard. For very large h/d (above 3) buckling and eccentricity reduce the strength.

Reasons for the popularity of the compressive strength test

1. The test is simple and quick and needs only a cube mould and a compression machine, which every site laboratory has.
2. Concrete is mainly used in compression, so strength in compression is the main design property.
3. Other properties (tensile, flexural, shear, bond, modulus) can be estimated from $f_{ck}$ (IS 456 Cl. 6.2).
4. It is a good index of overall quality: durability, density and permeability improve as strength rises.
5. Results are reproducible and the standard (IS 516, IS 456 Cl. 15 and 16) is universal, so it is used for mix design, quality control and acceptance.
6. Compressive strength is much higher than tensile strength, so is measured with smaller error.

Methods of obtaining tensile strength

A dog-bone or cylindrical specimen with embedded end grips is pulled axially. It is rarely used, because it is hard to avoid eccentricity and secondary stresses from the grips, and the result scatters. Direct tensile strength is about $0.35\sqrt{f_{ck}}$ to $0.45\sqrt{f_{ck}}$ (MPa).

Splitting tension test (Brazilian test, IS 5816)

A standard cylinder (150 mm dia × 300 mm long) is laid horizontally between the platens of the testing machine, with thin plywood or fibre strips (about 3 mm thick and 15 mm wide) along the contact lines. The compressive line load is increased at a rate of 1.2 to 2.4 N/mm$^2$/min (splitting tensile stress) until the cylinder splits along the vertical diameter. The load applied gives a nearly uniform tensile stress across the loaded plane:

$$f_{sp} = \frac{2P}{\pi L d}$$

where $P$ = failure load, $L$ = length and $d$ = diameter of cylinder. Splitting strength is higher than direct tension by 5 to 12%, and about 0.5 to 0.6$\sqrt{f_{ck}}$ in MPa.

Advantages: simple, uses the same cylinders and compression machine as the compression test, gives a more uniform and reproducible result than direct tension, with no grip problem.

Flexure test (modulus of rupture, IS 516)

A plain concrete beam 150 × 150 × 700 mm is supported on a 600 mm span and loaded at the third points (two-point loading) at 0.7 N/mm$^2$/min extreme-fibre stress rate. If the crack occurs in the middle third:

$$f_{cr} = \frac{P\,l}{b\,d^{2}}$$

If the crack is outside the middle third but within 5% of the span from the load point, $f_{cr} = 3Pa/(bd^2)$ ($a$ = distance of crack from the nearer support); otherwise the result is rejected. For single (centre) point loading, $f_{cr} = 3Pl/(2bd^2)$. IS 456 (Cl. 6.2.2) gives the flexural strength $f_{cr} = 0.7\sqrt{f_{ck}}$ N/mm$^2$. The modulus of rupture is greater than direct tension (by about 1.5 to 2 times) since the stress distribution is assumed linear while the actual stress block is plastic.

Splitting tension test (Brazilian test, IS 5816)

A standard cylinder (150 mm dia × 300 mm long) is laid horizontally between the platens of the testing machine, with thin plywood or fibre strips (about 3 mm thick and 15 mm wide) along the contact lines. The compressive line load is increased at a rate of 1.2 to 2.4 N/mm$^2$/min (splitting tensile stress) until the cylinder splits along the vertical diameter. The load applied gives a nearly uniform tensile stress across the loaded plane:

$$f_{sp} = \frac{2P}{\pi L d}$$

where $P$ = failure load, $L$ = length and $d$ = diameter of cylinder. Splitting strength is higher than direct tension by 5 to 12%, and about 0.5 to 0.6$\sqrt{f_{ck}}$ in MPa.

Advantages: simple, uses the same cylinders and compression machine as the compression test, gives a more uniform and reproducible result than direct tension, with no grip problem.

Platen restraint is the lateral confinement given to the ends of a concrete specimen by the steel platens of the testing machine. Friction between platen and concrete stops the specimen ends expanding sideways (Poisson's effect), putting the ends in triaxial compression, which raises the measured strength.

  platen    ______________          ____________
 (friction)|\            /|        |    |    |
           | \  cone   / |        |  split   |
           |  \ zone  /  |        |   cracks |
           |   \    /   |        |          |
           |   /    \   |        |          |
           | /        \ |        |__________|
           |/__________\|
   cube: hour-glass     cylinder: columnar
   (cone/pyramid) failure   (shear / splitting)

• Cube: both ends restrained, so the sides spall off and the remaining pyramids/cone shapes (hour-glass shape) are left. The restraint extends through the full cube, so a higher strength is read.
• Cylinder with h/d = 2: the central zone is free of restraint, so failure is by vertical splitting or a diagonal shear plane; the strength is closer to uniaxial.
• Remedies: lubricate the platens (grease, rubber or PTFE sheets) to cut friction (specimens then split into columns and the strength is lower), use brush platens, and standardise the loading on the sides of the cast cube.
• Effect: strength increases with decreasing h/d; this is why cube strength is about 1.25 times cylinder strength.

Assumption: the beam of length 750 mm rests on supports 75 mm in from each end, so the effective span is $l = 750 - 2\times75 = 600$ mm (IS 516 standard span for a 150 mm beam). $b = d = 150$ mm.

Single-point (centre) loading

$P = 100$ kN $= 100000$ N. Maximum moment $M = Pl/4$, section modulus $bd^2/6$:

$$f_{cr} = \frac{3Pl}{2bd^2} = \frac{3\times100000\times600}{2\times150\times150^2} = 26.67\ \text{N/mm}^2$$

Two-point (third-point) loading

Total load $P = 50 + 50 = 100$ kN, loads at $l/3$ from each support, constant moment $M = (P/2)(l/3) = Pl/6$ in the middle third (crack assumed within it):

$$f_{cr} = \frac{Pl}{bd^2} = \frac{100000\times600}{150\times150^2} = 17.78\ \text{N/mm}^2$$

Answer: modulus of rupture = 26.67 N/mm$^2$ (single point) and 17.78 N/mm$^2$ (two point). If the full 750 mm is taken as the span the values would be 33.3 and 22.2 N/mm$^2$. The centre-point test gives higher values, as the maximum stress acts only at one section. (These values are above $0.7\sqrt{f_{ck}}$ for ordinary concrete and the data are as given.)

Shear strength is the resistance of concrete to failure by sliding of one part over the adjacent part under forces acting parallel to the plane. In practice pure shear is rare; shear in beams occurs together with flexure, and failure is by diagonal tension.

• Direct (pure) shear strength of plain concrete is about 20 to 30% of the compressive strength (higher than tensile strength). It is hard to test because shear and bending are mixed (the double shear test of the plate/prism is used).
• Beam shear: the nominal shear stress $\tau_v = V_u/(bd)$ (IS 456, Cl. 40.1). The design shear strength of concrete $\tau_c$ depends on grade and the tension steel percentage $100A_s/bd$ (Table 19; for M20 it ranges from 0.28 N/mm$^2$ at 0.15% steel to 0.82 N/mm$^2$ at 3%). If $\tau_v > \tau_c$, shear reinforcement is provided (Cl. 40.4); $\tau_v$ must not exceed $\tau_{c,max}$ (Table 20, 2.8 N/mm$^2$ for M20), otherwise the section is changed.
• Factors: compressive strength, steel percentage (dowel action), aggregate interlock, depth of beam (size effect) and axial load (compression raises, tension reduces it).
• Punching shear in footings and flat slabs: $\tau_c = k_s \cdot 0.25\sqrt{f_{ck}}$ (Cl. 31.6.3).
• Shear failure is brittle and sudden; so design is conservative with minimum stirrups (Cl. 26.5.1.6).

Shear strength

Resistance of concrete to sliding of one part over another. Pure shear is about 20 to 30% of $f_c$; in beams the failure is by diagonal tension. IS 456 (Cl. 40) uses the nominal shear stress $\tau_v = V_u/bd$, compared with the permitted $\tau_c$ (Table 19, depends on grade and tension steel %) and $\tau_{c,max}$ (Table 20); stirrups carry the excess.

Bond strength

Bond is the adhesion and friction between the reinforcement and concrete, and mechanical interlock with the ribs of deformed bars, which transfers force between the two materials.

• Design bond stress $\tau_{bd}$ (IS 456, Cl. 26.2.1.1): 1.2 N/mm$^2$ for M20 plain bars in tension; increased by 60% for deformed bars (1.92 N/mm$^2$), and by 25% in compression.
• Development length: $L_d = \dfrac{\phi\,\sigma_s}{4\,\tau_{bd}}$ (Cl. 26.2.1).
• Bond is increased by higher concrete strength, ribs on bars, adequate cover, lower position of the bar (better compaction), hooks and anchorage; it is reduced by rust scale, oil, and top-cast bars with bleeding.

Bond between steel and concrete is the grip which allows stress transfer from concrete to the reinforcing bar, so that the two act together in a member. It has three parts: chemical adhesion of cement paste to steel, friction caused by shrinkage-induced gripping, and mechanical interlock between ribs of deformed bars and concrete (the biggest contribution).

The bond stress is $\tau_b = \dfrac{T}{\pi \phi L}$, and the design bond stress for M20 is 1.2 N/mm$^2$ for plain bars (IS 456, Cl. 26.2.1.1), 60% more for deformed bars. The development length $L_d = \phi\sigma_s/(4\tau_{bd})$ provides the required anchorage. Bond is better with higher $f_{ck}$, deformed bars, adequate cover, and well-compacted concrete; it is reduced by oil, loose rust and by bleeding under horizontal top bars. It is tested by the pull-out test.

Characteristic strength is the value of strength below which not more than 5% of the test results are expected to fall (IS 456, Cl. 2).

Formula: characteristic strength (95% confidence, IS 456 Cl. 2.2.2 and Annex) $f_{ck} = f_m - 1.65\,s$, with the sample standard deviation $s = \sqrt{\sum(x-f_m)^2/(n-1)}$. Assumption: results follow the normal distribution, and the sample standard deviation (n-1) is used.

$n = 10$, $\sum x = 216$, mean $f_m = 216/10 = 21.60$ N/mm$^2$.

$$\begin{aligned} s &= \sqrt{\frac{204.40}{10-1}} = 4.766\ \text{N/mm}^2 \\ f_{ck} &= 21.60 - 1.65\times4.766 = 13.74\ \text{N/mm}^2 \end{aligned}$$

Answer: characteristic strength $f_{ck} \approx$ 13.7 N/mm$^2$.

Characteristic strength is the strength below which not more than 5% of test results are expected to fall.

Formula: characteristic strength (95% confidence, IS 456 Cl. 2.2.2 and Annex) $f_{ck} = f_m - 1.65\,s$, with the sample standard deviation $s = \sqrt{\sum(x-f_m)^2/(n-1)}$. Assumption: results follow the normal distribution, and the sample standard deviation (n-1) is used.

$n = 11$, $\sum x = 263$, mean $f_m = 263/11 = 23.91$ N/mm$^2$.

$$\begin{aligned} s &= \sqrt{\frac{82.91}{11-1}} = 2.879\ \text{N/mm}^2 \\ f_{ck} &= 23.91 - 1.65\times2.879 = 19.16\ \text{N/mm}^2 \end{aligned}$$

Answer: characteristic strength $f_{ck} \approx$ 19.2 N/mm$^2$.

Definition: the characteristic strength of concrete is the value of the compressive strength of 150 mm cubes at 28 days, below which not more than 5% of the test results are expected to fall (IS 456, Cl. 2). It is $f_{ck} = f_m - 1.65\,s$ (normal distribution).

Calculation

Formula: characteristic strength (95% confidence, IS 456 Cl. 2.2.2 and Annex) $f_{ck} = f_m - 1.65\,s$, with the sample standard deviation $s = \sqrt{\sum(x-f_m)^2/(n-1)}$. Assumption: results follow the normal distribution, and the sample standard deviation (n-1) is used.

$n = 10$, $\sum x = 285$, mean $f_m = 285/10 = 28.50$ N/mm$^2$.

$$\begin{aligned} s &= \sqrt{\frac{70.50}{10-1}} = 2.799\ \text{N/mm}^2 \\ f_{ck} &= 28.50 - 1.65\times2.799 = 23.88\ \text{N/mm}^2 \end{aligned}$$

Answer: characteristic strength $f_{ck} \approx$ 23.9 N/mm$^2$.

Circumstances for NDT

• When the cube/cylinder results fail the acceptance criteria (IS 456, Cl. 16) or are doubtful.
• Suspected poor compaction, honeycombing, voids, or cracks in the member.
• Lack of records of quality control, or the specimens were not properly cured.
• Assessment of old structures for change of use, extension, and rehabilitation.
• Damage by fire, earthquake, impact, corrosion, chemical or freeze-thaw attack.
• Checking uniformity of concrete, early stripping time, and cover/rebar location.
• Monitoring strength gain or deterioration over time.

Ultrasonic pulse velocity (UPV) test (IS 13311 Part 1)

Principle: the velocity of an ultrasonic pulse through concrete depends on its density and elastic properties. Sound, dense concrete transmits the pulse faster than porous or cracked concrete.

Apparatus: pulse generator and receiver with two transducers (usually 54 kHz), a timer reading to 0.1 microsecond, a coupling gel, and a reference calibration bar.

Procedure

1. Calibrate the instrument on the reference bar.
2. Clean the surface and apply grease or gel to ensure good acoustic contact.
3. Put the transducers on the concrete, either directly (opposite faces, most reliable), semi-directly (adjacent faces) or indirectly (same face, least reliable).
4. Measure the path length $L$ and the transit time $T$.
5. Calculate the velocity:

$$V = \frac{L}{T}$$

Take several readings and average them. Avoid paths parallel to reinforcement, as steel carries the pulse faster.

Quality of concrete (IS 13311 Part 1)

Strength estimation: a correlation curve between $V$ and the compressive strength of cubes of the same mix (made and tested in the laboratory) is used. The accuracy is about ±20% only, because moisture, aggregate type, age and reinforcement also change $V$. Better accuracy is obtained by combining it with the rebound number (SONREB method).

Uses: uniformity of concrete, locating voids and cracks, depth of surface cracks, deterioration by fire, frost or chemical attack, and dynamic modulus.

Rebound hammer (Schmidt hammer) test (IS 13311 Part 2)

Principle: a spring-loaded mass strikes the concrete surface through a plunger. The rebound of the mass, read as the rebound number $R$ on a scale of 10 to 100, depends on the surface hardness, which is related to strength.

Procedure

1. Select a smooth, clean, dry surface of a structural member at least 100 mm thick; grind off plaster, loose scale and rough texture.
2. Hold the hammer at right angles to the surface (horizontal, vertical up or down need orientation corrections), push the plunger until the mass is released, and read the rebound number.
3. Take at least 12 readings at spots 20 to 50 mm apart; reject readings that differ from the mean by more than 6 units, and average the rest.
4. Read the compressive strength from the manufacturer's calibration curve (for the position of the hammer), which should preferably be made from cubes of the same concrete.

Precautions: avoid edges, honeycombed and wet surfaces, and spots over reinforcement; the instrument should be checked on the test anvil.

Limits: it tests only the surface (about 30 mm), so it is affected by carbonation, moisture, surface finish, type of aggregate and cement, and age. Accuracy of strength is about ±15 to 25%. It is useful for comparing uniformity and for choosing places for coring or UPV.

Importance of NDT

• Tests the actual in-situ concrete, not only the control cubes, which may not represent the structure's compaction and curing.
• No damage and repeated tests at the same place are possible, so the changes with time can be tracked.
• Quick and cheap; large areas can be surveyed to find weak, honeycombed or cracked zones.
• Assesses old structures for repair, change of use, fire or earthquake damage, and long-term deterioration (corrosion, carbonation).
• Investigates doubts when cube results fail the acceptance criteria (IS 456, Cl. 16), before costly coring or load tests (Cl. 17).
• Checks uniformity, thickness, cover and the position of rebars, and gives a basis for deciding on repair.

Rebound hammer (Schmidt hammer) test (IS 13311 Part 2)

Principle: a spring-loaded mass strikes the concrete surface through a plunger. The rebound of the mass, read as the rebound number $R$ on a scale of 10 to 100, depends on the surface hardness, which is related to strength.

Procedure

1. Select a smooth, clean, dry surface of a structural member at least 100 mm thick; grind off plaster, loose scale and rough texture.
2. Hold the hammer at right angles to the surface (horizontal, vertical up or down need orientation corrections), push the plunger until the mass is released, and read the rebound number.
3. Take at least 12 readings at spots 20 to 50 mm apart; reject readings that differ from the mean by more than 6 units, and average the rest.
4. Read the compressive strength from the manufacturer's calibration curve (for the position of the hammer), which should preferably be made from cubes of the same concrete.

Precautions: avoid edges, honeycombed and wet surfaces, and spots over reinforcement; the instrument should be checked on the test anvil.

Limits: it tests only the surface (about 30 mm), so it is affected by carbonation, moisture, surface finish, type of aggregate and cement, and age. Accuracy of strength is about ±15 to 25%. It is useful for comparing uniformity and for choosing places for coring or UPV.

Precautions for the rebound (Schmidt) hammer test (IS 13311 Part 2):

1. Check the hammer on the standard test anvil (rebound number about 80 ± 2) before and after the test.
2. Select a smooth, clean, dry surface; grind rough or plastered surfaces and remove loose scale and finishing.
3. Test a member at least 100 mm thick that is rigidly held (a thin or loose member will vibrate and give low readings).
4. Keep the hammer perpendicular to the surface; if it is not horizontal, apply the orientation correction.
5. Avoid edges (at least 20 mm away), honeycombed areas, voids, and spots directly over reinforcement or aggregate pockets.
6. Take at least 12 readings per area, 20 to 50 mm apart, never at the same point twice; reject readings that differ by more than 6 units and average the rest.
7. Do not use on wet, frozen or carbonated surfaces without correction (carbonation raises the number), and note the age and moisture.
8. Use the calibration curve with the same type of hammer and concrete, and handle the instrument carefully; clean the plunger regularly.

Destructive tests (DT) are tests in which the concrete specimen is loaded or cut to failure, so it is damaged or destroyed. They give the direct measure of strength and are the basis of acceptance (IS 456, Cl. 15 and 16).

1. Compressive strength test on 150 mm cubes or 150 × 300 mm cylinders (IS 516): the main test.
2. Split tensile test on cylinders (IS 5816): indirect tensile strength.
3. Flexural strength test on beams 150 × 150 × 700 mm (IS 516): modulus of rupture.
4. Core test (IS 516, ASTM C42): 100 mm cores drilled from the structure and tested in compression; the average of cores should be at least 85% of $f_{ck}$, no core below 75%.
5. Pull-out and bond tests: a steel bar pulled from concrete to find bond strength.
6. Load test on the structure or member (IS 456, Cl. 17.6): the member is loaded with the full dead load plus 1.25 times the imposed load for 24 hours, and the deflection and recovery are checked.
7. Modulus of elasticity test and shear tests.

Comparison with NDT: DT is accurate but expensive, slow and damages the member, while NDT is quick, cheap, and non-damaging but gives only indirect results.