Testing of Hardened Concrete
Testing of hardened concrete plays an important role in controlling and confirming the quality of cement concrete works. Systematic testing of raw materials, fresh concrete and hardened concrete are inseparable part of any quality control programme for concrete, which helps to achieve higher efficiency of the material used and greater assurance of the performance of the concrete with regard to both strength and durability. The test methods should be simple, direct and convenient to apply.
How to determine the workability of concrete?
One of the purposes of testing hardened concrete is to confirm that the concrete used at site has developed the required strength. As the hardening of the concrete takes time, one will not come to know, the actual strength of concrete for some time. This is an inherent disadvantage in conventional test. But, if strength of concrete is to be known at an early period, accelerated strength test can be carried out to predict 28 days strength. But mostly when correct materials are used and careful steps are taken at every stage of the work, concretes normally give the required strength.
The tests also have a deterring effect on those responsible for construction work when testing of hardened concrete is done. The results of the testing on hardened concrete, even if they are known late, helps to reveal the quality of concrete and enable adjustments to be made in the production of further concretes. Tests are made by casting cubes or cylinder from the representative concrete or cores cut from the actual concrete. Knowledge of the strength of concrete in structure can not be directly obtained from tests on separately made specimens.
Compression test is the most common testing conducted on hardened concrete, partly because it is an easy test to perform, and partly because most of the desirable characteristic properties of concrete are qualitatively related to its compressive strength.
The compression test is carried out on specimens cubical or cylindrical in shape. Prism is also sometimes used, but it is not common in our country. Sometimes, the compression strength of concrete is determined using parts of a beam tested in flexure. The end parts of beam are left intact after failure in flexure and, because the beam is usually of square cross section, this part of the beam could be used to find out the compressive strength.
The cube specimen is of the size 15 x 15 x 15 cm. If the largest nominal size of the aggregate does not exceed 20 mm, 10 cm size cubes may also be used as an alternative. Cylindrical test specimens have a length equal to twice the diameter. They are 15 cm in diameter and 30 cm long. Smaller test specimens may be used but a ratio of the diameter of the specimen to maximum size of aggregate, not less than 3 to 1 is maintained.
Metal moulds, preferably steel or cast iron, thick enough to prevent the distortion are required. They are made in such a manner as to facilitate the removal of the moulded specimen without damage. The angle between the adjacent faces and top and bottom planes of the mould is 90°. Each mould is provided with a metal base plate of 6.5 mm thick. Square mould should have internal distance 15 cm (+- 0.02 mm) between adjacent faces.
During assembling the mould, it is coated with thiny layer of oil. When assembled ready for use; the mean internal diameter of the cylindrical mould should be 15 cm (+- 0.02 mm). The height maintained is 30 cm (+- 0.1 mm). A steel bar 16 mm in diameter and 0.6 m long; bullet pointed at the lower end serves as a tamping bar.
Compacting by Hand
Tamping bar is used for compacting by hands. Concrete should be compacted by giving the uniform number of strokes per layer. For square mould; 35 strokes for 15 cm layer and 25 strokes for 10 cm layer of concrete in cubes. For cylindrical specimen, should have minimum 30 strokes per layer. Where voids are left by tamping bar, the sides of the mould are tapped to close the voids.
Compacting by Vibration
In this each layer is compacted by vibration. This is done by the means of an electric or pneumatic hammer or vibrator or vibrating table. If care is not taken care of then severe segregation takes place in the mould. Segregation results in low strength when cubes are crushed.
The test specimens are stored on the site at a place free from vibrations. It is stored in damp / moist place for 24 hours at 27°C (+- 2°C). After this period, the specimen is removed from mould. Then it is cured in fresh water for next 24 hours at 27°C (+- 2°C).
Making and Curing test specimen in the field
The test specimens are stored under damp matting, sacks or other similar material for 24 hours from the time of addition of water to the other ingredients. The temperature of the place of storage should be within the range of 22° to 32°C. After the period of 24 hours, they should be marked for later identification removed from the mould. Unless required for testing within 24 hours, stored in clean water at a temperature of 24° to 30°C until they are transported to the testing laboratory.
They should be sent to the testing laboratory well packed in damp sand, damp sacks, or other suitable material. So as to arrive there in a damp condition not less than 24 hours before the time of test. On arrival at the testing laboratory, the specimens are stored in water at a temperature of 27° (+- 2°C). Records of the daily maximum and minimum temperature should be kept both during the period the specimens remain on the site and in the laboratory particularly in cold weather regions.
Comparison between Cube and Cylinder Strength
It is difficult to say whether cube test gives more realistic strength properties of concrete or cylinder gives a better picture about the strength of concrete. However, it can be said that the cylinder is less affected by the end restrains caused by platens and hence it seems to give more uniform results than cube. Therefore, the use of cylinder is becoming more popular,
particularly in the research laboratories.
Cylinders are cast and tested in the same position, whereas cubes are cast in one direction and tested from the other direction. In actual structures in the field, the casting and loading is similar to that of the cylinder and not like the cube. As such, cylinder simulates the condition of the actual structural member in the field in respect of direction of load.
The points in favour of the cube specimen are that the shape of the cube resembles the shape of the structural members often met with on the ground. The cube does not require capping, whereas cylinder requires capping. The capping material used in case cylinder may influence to some extent the strength of the cylinder.
Flexural Strength of Concrete
Concrete as we know is relatively strong in compression and weak in tension. In reinforced concrete members, little dependence is placed on the tensile strength of concrete since steel reinforcing bars are provided to resist all tensile forces. However, tensile stresses are likely to develop in concrete due to drying shrinkage, rusting of steel reinforcement, temperature gradients and many other reasons. Therefore, the knowledge of tensile strength of concrete is of importance.
A concrete road slab is called upon to resist tensile stresses from two principal sources- wheel loads and volume change in the concrete. Wheel loads may cause high tensile stresses due to bending, when there is an inadequate subgrade support. Volume changes, resulting from changes in temperature and moisture, may produce tensile stresses, due to warping and due to the movement of the slab along the subgrade.
Stresses due to volume changes alone may be high. The longitudinal tensile stress in the bottom of the pavement, caused by restraint and temperature warping, frequently amounts to as much as 2.5 MPa at certain periods of the year and the corresponding stress in the transverse direction is approximately 0.9 MPa. These stresses are additive to those produced by wheel loads on unsupported portions of the slab.
Determination of Tensile Strength
Direct measurement of tensile strength of concrete is difficult. Neither specimens nor testing apparatus have been designed which assure uniform distribution of the “pull” applied to concrete. After number of investigations beam tests are found to be dependable to measure flexural strength of concrete.
The mould should be metallic i.e. steel or cast iron. The standard size of the specimen is 15 x 15 x 70 cm. Alternatively, if the largest nominal size of the aggregate does not exceed 20 mm; the size of the specimen 10 x 10 x 50 cm may be used. The tamping bar should be of steel, weighing 2 kg. It has length of 40 cm, ramming face 25 mm square.
Test specimens are stored in water at a temperature of 24° to 30°C for 48 hours before testing. They are tested immediately on removal from the water whilst they are still in a wet condition. The dimensions of each specimen should be noted before testing. No preparation of the surfaces is required.
Specimen is placed in the machine in such a manner that the load is applied to the uppermost surface. Axis of specimen is carefully aligned with the axis of loading device. Load is applied without any shock and increasing continuously. It is applied at the rate of 400 kg/min for 15 cm specimen; and 180 kg/min for 10 cm specimen. Load is kept on increasing until the specimen fails. Maximum applied load is recorded.
Indirect Tension Test Methods
1. Cylinder Splitting Tension Test
This is also known as “Brazilian Test”. This test was developed in Brazil in 1943. This test is carried out by placing a cylindrical specimen horizontally between the machine. Load is applied until the failure of the cylinder, along the vertical diameter.
The main advantage of this method is that the same type of specimen and the same testing machine as are used for the compression test can be employed for this test. That is why this test is gaining popularity. Splitting strength gives about 5 to 12% higher value than the direct tensile strength.
2. Ring Tension Test
Another test for finding out the tensile strength of concrete is known as “Ring Tension test”. Briefly, a hydro-static pressure is applied radially against the inside periphery of 15 cm diameter, 4 mm thick and 4 mm high concrete ring specimen.
3. Double Punch Test
Yet another test to find out the indirect tensile strength of concrete is known as “double punch test”. In this test, a concrete cylinder is placed vertically between the loading plates of the compression test machine. And is compressed by the steel punches located concentrically on the top and bottom surfaces of the cylinder.
The test specimen, cube or cylinder is made from the representative sample of concrete used for a particular member, the strength of which we are interested. As the member can not be in fact tested, we test the parallel concrete by making cubes or cylinders. It is to be understood that the strength of the cube specimen cannot be same as that of the member because of the differences with respect to the degree of compaction, curing standard, uniformity of concrete, evaporation, loss of mixing water etc. At best the result of cube or cylinder can give only a rough estimate of the real strength of the member.
To arrive at a better picture of the strength of the actual member; attempts are made to cut cores from the parent concrete and test the cores for strength. Perhaps this will give a better picture about the strength of actual concrete in the member.
Core can be drilled at the suspected part of the structure; to detect segregation or honey combing; to check the bond at construction joint or to verify the thickness of pavement.
Strength of Cores
The reduction in strength of cores appear to be greater in stronger concrete. The reduction in the strength can be as high as 15 per cent for 40 MPa concrete. Generally, a reduction of 5 to 7 per cent is considered reasonable. It has been reported by man investigators that in situ concrete gains very little strength after 28 days. Tests on high strength concrete show that, although the strength of cores increase with age, the core strength, even up to the age of 1 year, remains lower than the strength of standard 28 day cylinders.
Non-Destructive Testing Methods
Non-destructive methods have been in use for about four decades. In this period, the development has taken place to such an extent that it is now considered as a powerful method for evaluating existing concrete structures, also with regard to their strength and durability apart form assessment and control of quality of hardened concrete when testing. In certain cases, the investigation of crack depth, micro-cracks, and progressive deterioration are also studied by this method.
Though non-destructive testing methods are relatively simple to perform, the analysis and interpretation of test results are not so easy. Therefore, special knowledge is required to analyses the testing of hardened properties of concrete. In the non-destructive methods of testing, the specimen are not loaded to failure. And as such the strength inferred or estimated cannot be expected to yield absolute values of strength. These methods, therefore, attempt to measure some other properties of concrete from which an estimate of its strength, durability and elastic parameters are obtained.
Based upon the above, various non-destructive methods of testing concrete have been developed:
1. Surface hardness tests:
These are of indentation type, include the Williams testing pistol and impact hammers, and are used only for estimation of concrete strength.
2. Rebound test:
The rebound hammer test measures the elastic rebound of concrete and therefore, is primarily used for estimation of concrete strength and for comparative investigations.
3. Penetration and Pull out techniques:
These include the use of the Simbi hammer, Spit pins, the Windsor probe, and the pullout test. These measure the penetration and pullout resistance of concrete and are used for strength estimations, but they can also be used for comparative studies.
4. Dynamic or vibration tests:
These include resonant frequency and mechanical sonic and ultrasonic pulse velocity methods. These are thus used to evaluate durability and uniformity of concrete and to estimate its strength and elastic properties.
5. Combined methods:
The combined methods involving ultrasonic pulse velocity and rebound hammer have been used to estimate strength of concrete.
6. Radioactive and nuclear methods:
These include the X-ray and Gamma-ray penetration tests for measurement of density and thickness of concrete. Also, the neutron scattering and neutron activation methods are used for moisture and cement content determination.
7. Magnetic and electrical methods:
The magnetic methods are primarily concerned with determining cover of reinforcement in concrete, whereas the electrical methods, including microwave absorption techniques, have been used to measure moisture content and thickness of concrete.
8. Acoustic emission techniques:
These have been used to study the initiation and growth of cracks in concrete.
9. Surfaces Hardness Methods:
The fact that concrete hardens with increase in age, the measure of hardness of surface may indicate the strength of concrete. Various methods and equipments are devised to measure hardness of concrete surface. William testing pistol, Frank spring hammer, and Einbeck pendulum hammer are thus some of the devices for measuring surface hardness.
Schmidt’s Rebound Hammer
Schmidt’s rebound hammer developed in 1948 is one of the commonly adopted equipments for measuring the surface hardness.
It consist of a spring control hammer that slides on a plunger within a tubular housing. When the plunger is pressed against the surface of the concrete, the mass rebound from the plunger. It retracts against the force of the spring. The hammer impacts against the concrete and the spring control mass rebounds, therefore 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 therefore called the rebound number. It is indicated by the rider moving along a graduated scale.
Each hammer varies considerably in performance and needs calibration for use on concrete made with the aggregates from specific source. The test can be conducted horizontally, as well as vertically-upwards or onwards or at any intermediate angle. At each angle the rebound number will be different for the same concrete and thus will require separate calibration or correction chart.
Although, rebound hammer provides a quick inexpensive means of checking uniformity of concrete, it has thus serious limitations and these must be recognized. The results are thus affected by:
- Smoothness of surface under test.
- Size, shape and rigidity of the specimen.
- Age of specimen.
- Type of cement.
- Surface and internal moisture condition of the concrete.
- Type of coarse aggregate.
- Type of Mould.
- Carbonation of concrete surface.
Following are more tests related to above, we’re not going to discuss all these in detail. So find the names of the relevant tests as follows –
- Pullout Test
- Dynamic or Vibration Methods
- Resonant Frequency Method
- Pulse Velocity Method (Mechanical sonic PVM and Ultrasonic PVM)
- Combined Methods
- Radioactive Methods
- Nuclear Methods
- Magnetic Methods
- Electrical Methods
Testing on Composition of Hardened Concrete
- Determination of cement content
- Determination of the original water / cement Ratio
- Physical Methods
- Accelerated Curing Methods