In this article we will discuss about the tests performed on concrete.
Compression test can be performed on the three types of test specimens, namely cubes, cylinders and prisms. Cubes of standard size of 150 mm are used in India U.K., Germany and many other countries of Europe. Cylinders are the standard specimens in U.S.A. France, Canada, Australia and New Zealand. In Scandinavia tests are made both on cubes and cylinders.
Usually cylinders of 150 x 300 mm size are used. Smaller test specimens may also be used but a ratio of the diameter of the specimen to maximum size of aggregate not less than 3 to 1 is maintained. In case of cubes minimum size of 150 x 150 x 150 mm should be used if the maximum size of aggregate is 20 mm. However a 100 x 100 x 100 mm cube may also be used.
Moulds Used to Prevent Adhesion of Concrete:
Preferably metal moulds made of cast iron or steel should be used. The thickness of the mould should be enough to prevent any distortion. The tolerance between the opposite faces should be ± 0.2 mm. The angle between adjacent internal faces and between internal faces and top and bottom planes of be mould should be 90° ± 0.5°. The internal faces of the mould should be plane surfaces with a permissible variation of 0.03 mm. Each mould has a metal base plate having a plane surface.
While assembling the mould for use, the joints between all sections of the mould are thinly coated with mould oil and a similar coating of mould oil or grease is applied between the contact surface of the bottom of the mould and the base plate in order to ensure that no water goes out during filling it. To prevent adhesion of concrete, interior surfaces of the assembled mould are also thinly coated with mould oil.
The cylindrical mould is made of metal sheet of not less than 3 mm thickness. The internal mean diameter of the mould should be 150 ± 0.2 mm and in no direction the internal diameter should be less than 14.95 cms and not-more than 15.05 cms. The height should be 30 cm ± 0.1 mm.
A 16mm diameter and 60 cm long steel bar with a bullet point at one end is used as tamping rod.
Preparation of Samples:
The mould is assembled, cleaned and oiled. The concrete is filled upto 1/3rd height of the mould. In case of 15 cm cube, each layer should be given at least 35 strokes and in case of 10 cm cube at least 25 strokes of the rod should be given. In case of cylindrical mould each layer should be given each layer of 10 cm at least 30 strokes. Each stroke should penetrate into the under lying layer and the bottom layer is rode throughout its depth. If the voids are left by tamping rod, then sides of the mould are tapped to close the voids.
In case compaction is done by vibrations, then each layer should be vibrated either by means of electric or pneumatic hammer or vibrator or by a suitable vibrating table. Care should be taken that no segregation takes place in the mould which will result in low strength of the concrete.
Capping of Specimen:
Capping is applied to cylindrical specimens only, when the top end of the cylindrical specimen is not plane with in 0.05 mm. The capped surface should be nearly at right angle to the axis of the specimen. The planeness of the cap should be checked with the help of a straight edge and feeler gauge at three different diameters of the specimen. Capping can be done either on completion of casting or a few hours before testing of the specimen.
Materials of Capping:
The test cylinders may be capped with a thin layer of stiff neat port-land cement paste after the concrete has set in the mould, after about 4 hours of casting, so that concrete in the cylinder undergoes plastic shrinkage and subsides fully.
The cap is formed either by a glass plate not less than 6.5 mm in thickness or a machined metal plate not less than 13 mm in thickness and having a minimum surface dimension at least 25 mm larger than the diameter of the mould. In order to prevent the tendency of the cap to shrink, cement for paste is mixed to a stiff paste for about 2 to 4 hours before it is to be used.
The test specimens are stored in place free from vibration in moist air of at least 90% relative humidity and at a temperature of 27 ± 2°C for 24 hours ± 1/2 hours from the time of addition of water to the dry ingredients. After this period, the specimens are marked and removed from the mould and immediately immersed in clean fresh water or saturated lime solution and kept there till taken out just prior to test.
The specimens are not to be allowed to become dry at any time till they have been tested. The specimens prepared as above are known as laboratory prepared specimens. If the specimens are prepared in the field and cured under standard temperature and water, they are called standard specimens. If the specimens are prepared from the concrete to be filled in prototype structure and put alongside it for curing then they are known as companion specimens.
Failure of Compression Specimen:
1. Failure under Uniaxial Compression:
Under uniaxial compression, the cracks developed are approximately parallel to the applied load, but some cracks are also formed at an angle to the applied load. The parallel cracks are caused by a localized tensile stress in a direction normal to the compressive load. The inclined cracks develop due to the collapse caused by the development of shear planes. The cracks are formed in two planes parallel to the load, such that the specimen disintegrates into column type fragments.
Under biaxial compression, failure takes place in one plane parallel to the applied load and results in the formation of slab type fragments.
Here it should be noted that the fracture pattern is for direct stresses only. Thus there should be no restraint from the platens of the testing machine, while in practice these platens of the machine introduce some lateral compression due to the friction generated between the steel platens and the concrete. In an ordinary machine, it is difficult to eliminate this friction. However its effect may be minimized by using a specimen whose length/diameter ratio is greater than 2.
2. Actual Testing:
The testing under uniaxial compression is the ideal mode of testing, but in actual compression testing more complex system of stresses is imposed. These stresses are imposed mainly due to the lateral forces developed between the end surfaces of the concrete specimen and the adjacent steel platens of the testing machine. These forces are induced due to the restraint of concrete, which tries to expand laterally (Poisson’s effect). This expansion is restrained by the steel platens which are several times stiffer than concrete and have much smaller lateral expansion.
The degree of platen restraint on the concrete section depends on the following two factors:
1. The friction developed at the interfaces of concrete and steel platens.
2. The distance from the end surfaces of the concrete.
Thus in addition to the imposed uniaxial compression, there is a lateral shearing stress also. The effect of this shearing stress is to increase the apparent compressive strength of concrete.
The influence of platen restraint can be seen by the typical failure modes of test cubes. The effect of shear is always present, though the shear effect decreases towards the centre of the cube. Thus the sides of the cube have nearly vertical cracks or completely disintegrate leaving a relatively undamaged central core (Fig.19.1 (a)). This mode of failure takes place when the test is performed in a rigid testing machine. A less rigid machine can store more energy, resulting in a more explosive failure as shown in Fig.19.1 (b).
In this case, one face touching the platen cracks and disintegrates forming a cone or pyramid. Failures of other type than those shown in Fig.19.1 (a) and (b) given above are regarded as unsatisfactory and indicate a probable fault in the machine. Failure shown in Fig. 19.1 (a) is called non explosive while that of 19.1 (b) as explosive.
If the ratio of height to width or height to diameter of the specimen increases, the influence of shear becomes smaller, so that the central part of the specimen may fail by lateral splitting. This situation arises in a standard cylinder test where the height/diameter ratio is 2. The possible modes of failures of cylinder test are shown in Fig. 19.2. Out of these failures the most usual failure is by spitting and shear as shown in Fig. 19.2 (c).
The compressive strength of concrete is determined by placing the specimen centrally in between the platens of the machine. The rate of loading as per IS 516, should be 0.233 MPa/second or 2.33 kg/cm2 per second.
As the influence of platen restraint on the mode of failure is greater in a cube than in a standard cylinder, the cube strength approximately is taken as 1.25 times the cylinder strength, though it varies from 1.33 to 0.96 depending upon the strength of concrete and moisture condition of concrete at the time of testing. If the end friction is eliminated, the effect of height/diameter would disappear, which is very difficult to achieve in a routine test and is not feasible for the range of strength normally encountered.
Advantages of Cylinder as a Test-Specimen:
Cylinders are believed to give a greater uniformity of results for nominally similar specimens due to the following reasons:
(a) The failure of cylinders is less affected by the end restraint of the specimen.
(b) The strength of cylinder is less influenced by the properties of aggregate used in the mix.
(c) The stress distribution over the cross section (horizontal planes) in cylinders is more uniform than on a specimen of square cross section.
For these reasons, the cylinder strength probably is closer to the true uniaxial compressive strength of concrete than cube strength. Further in structural compression members the situation is similar to that existing in a test cylinder. Thus the tests on cylinders are more realistic.
Further it may be noted that cube is loaded or tested on the surface at right angle to the axis of the cube as cast, whereas cylinders are cast and tested in the same position. However the relation between the directions as cast and as tested has been found having no effect on the strength appreciably of cubes made with homogeneous and un-segregated concrete.
The strongest advantage in favour of cube is that it does not need capping. The capping procedure is expensive and time consuming. The capping material may also influence the strength of the specimen.
Reinforcement of Concrete Strength:
Due to the presence of reinforcement, the measured strength of concrete is found reduced upto 10%. Thus, as far as possible reinforcement from the core must be avoided.
In case the reinforcement is present in the core, the measured core strength should be corrected as follows:
ɸr = diameter of reinforcement bar
ɸc = diameter of the core
h = distance of bar axis from the nearest end of core
l = uncapped length of core
In case core has multiple bars, then ―
If the spacing of two bars is less than the diameter of the larger bar, then the bar with higher value of (ɸr, h) should be considered.
Estimation of Cube Strength from Core Strength:
The equivalent cube strength can be estimated in two steps as follows:
1. To convert the core strength to equivalent standard cylinder strength, a correction for the effect of length/diameter ratio is applied.
2. To convert the equivalent standard cylinder strength obtained in step (i) a proper relationship between the strength of cylinder and cube is used. Generally the cube strength is taken as 1.25 times the cylinder strength for l/d = 2.0 = λ.
Taking into account the strength difference of 6% between a core with cut surface relative to cast cylinder and strength reduction of 15% for weaker top surface zone of a corresponding cast cylinder adopted by B.S. 1881, Part-120-1983.
(i) For Vertical Drilled Core:
(ii) For Horizontal Drilled Core:
where fc is the measured strength.
However for the potential or target strength of a standard specimen made from a particular mix is about 30% higher than the actual fully compacted in situ strength. The potential strength is the strength equivalent to the 28 day strength of the standard test specimen.
Then the expression for the cube strength is given as:
(iii) For Vertical Drilled Core:
(iv) For Horizontal Drilled Core:
IS 516-1959 describes the method of preparation of core after drilling and the procedure of test. According to IS 456-1978, the concrete in the member represented by the core test shall be considered acceptable if the average equivalent cube strength of the core is equal to at least 85% of the cube strength of the grade of concrete specified for the corresponding age, and no individual core has the strength less than 75%.
Disadvantages of Core Method:
Following are the disadvantages of core testing method:
1. The structural integrity of the concrete across the full cross section may be affected.
2. The ratio of length/diameter other than standard ratio 2.0 will have its effect on strength.
3. Capping of both ends of the core is also liable to affect the strength of concrete.
4. Existence of reinforcement will also have its effects on the strength of concrete.
Accelerated Curing Test:
Concrete mixes usually are designed for 7 and 28 days compressive strength. The concrete test results are available only on the expiry of 7 or 28 days period. During this period a considerable quantity of additional concrete may have been placed in the structure. At this late stage any remedial action, if the concrete is found too weak is impossible. If it is too strong then the mix was un-economical. This is the main disadvantage of the standard compression test of concrete.
Thus it would be advantageous to be able to predict the 28 days strength with in few hours of casting, 8 hours period is found most suitable. Unfortunately, the 1 to 3 day strength of a given mix cured under normal conditions is not reliable in this respect as it is very sensitive to small variations in temperature during the first few hours of casting and to the fineness of cement. Thus the 28 day concrete strength after casting can be predicted by the accelerated curing methods.
The following are the three methods of accelerated curing test for cylinders:
1. Warm Water Method:
In this method, the cylinders after casting and finishing are immersed immediately in water at 35°C temperature in covered condition. After 24 hours of immersion, the cylinders are taken out, capped and tasted.
2. Boiling Water Method:
In this method, the cast cylinders are cured for 23 hours in moist environment at 21°C and then immersed in boiling water for 3½ hours. After curing in boiling water for 3½ hours, specimens are taken out and allowed to cool for 1 hour. After cooling for 1 hour, specimens are capped and tested at the age of 28½ hours.
3. Autogeneous Method:
In this method the specimens are cured by insulation for 48 hours, then capped and tested at the age of approximately 49 hours.
B.S. 1881 Part-112-1983 also gives following three methods:
In this case in all the three methods the test cubes are cured in covered condition at temperatures of 35°C and 55°C and 82°C respectively.
1. 35°C Temperature Cured Method:
In this method the cubes cast and immersed in covered condition in water at 35°C temperature after 15 minutes of casting. The specimens are cured at 35°C temperature for 24 hours. After this period the specimens are tested as usual.
2. 55°C Temperature Cured Method:
In this method the specimens are allowed to remain undisturbed at 20°C for 1 hour after casting. Then the specimens are immersed in covered condition in water at 55°C and allowed to remain there for 20 hours. Then the specimens are allowed to cool in water at 20°C for 1 to 2 hours before testing.
3. 82°C Temperature Cured Method:
In this method the specimens are allowed to remain un-disturbed for at least 1 hour after casting. After this period the cubes are placed in an empty curing tank. Then the tank is filled with water at ambient temperature. Then the temperature of this water is raised to 82°C in a period of 2 hours. This temperature is maintained for further 14 hours. After this period, the water is discharged quickly and the cubes are tested with in 1 hour of discharge of water, while they are still hot.
The accelerated strength obtained by any of the above methods is all different and are lower than the 28 day strength of standard specimens. However for a given mix, the accelerated strength obtained by any method can be correlated with the standard specimen strength either at 7 days or 28 days.
Indian Methods of Concrete Testing:
The Indian Bureau of standards IS 9013-1978 has suggested the following two methods.
Any of the two methods may be adopted for accelerated curing of concrete:
1. Warm Water Method:
In this method the specimens are immersed in water for 1½ to 3½ hours after moulding.
Curing water temperature = 55 ± 1°C Curing period = 20 hours ± 10 minutes
After the curing period, the specimen (cube) are de-moulded and cooled at 27 ± 2°C for 1 hour before testing.
2. Boiling Water Curing Method:
In this method following procedure in adopted Standard moist curing for 23 hour ± 15 minutes Curing water temperature = 100° C
Curing period = 3½ hours ± 5 minutes
Cooling period = 2 hours
The actual correction of accelerated test results at 28 day normally cured specimens depends upon the curing method, chemical composition of cement and mix proportions of concrete. It is recommended that relation between the two strengths should be established prior to placing concrete in the structure, so that accelerated test can be used as a rapid quality control test for detecting variations in the mix proportions. The relation suggested can be used to predict the 28 day compressive strength of the normally cured concrete with in ± 15% limits.
Fixed Set Method:
In Canada, following relation between the accelerated strength Ra and 28 day cylinder strength R28 has been established, which is independent of type of cement, mix proportion, and type of admixtures etc.
In this case, the accelerated curing of concrete is delayed till a fixed set has taken place. It is measured by the proctor needle penetration under load of 24 MPa. After a delay of 20 minutes, the moulded standard cylinder is placed in boiling water for 16 hours. After this period the specimen is de-moulded and allowed to cool for 30 minutes. The cylinder is capped and tested after removal from boiling water.
Disadvantages of the Method:
Following are the disadvantages of this so called fixed set method:
(a) Time is wasted to ascertain the set of concrete.
(b) The erratic nature of the needle penetration test.
Thus it can be said that the standard compression test is really only a relative measure of the strength of concrete used in the structure. It can also be argued that there is no inherent superiority of the standard 28 days test over other tests. In fact there is a school of thought which argues that the accelerated strength test should be considered as a basis for acceptance of concrete, rather than nearly a means of predicting the 7 or 28 days strength.
Bond to Reinforcement:
Determination of Bond Strength:
Actually it is Difficult to Define Bond Strength. In this test a 19 mm (3/4″) diameter deformed bar is embedded in a 150 mm cube. The bar is pulled out relative to the concrete till the bond fails or concrete splits or a minimum slip of 2.5 mm takes place at the loaded end of the bar. The bond strength then is taken as the load on the bar at failure divided by the nominal embedded surface area of the bar.
Surface protective treatment of reinforcement bars may reduce the bond strength due to the absence of rusted surface of the reinforcement, which enhances the bond strength. However the bond of galvanised reinforcement was found as good as that of ordinary steel bars and wires. The thickness of a galvanised coating usually varies from 0.03 mm to 0.1 mm.
Due to this galvanised coating the steel is protected against corrosion even when cover to reinforcement is reduced upto 25% of the nominal value. With the galvanised coating the light weight aggregate concrete can be used without increased cover. Fig.19.7 shows relationship between compressive and bond strength of plain and deformed bar.
Non-destructive testing methods have been in use for the last about 40 to 50 years. During this period progress in this field has been made very fast. Now it is considered that nondestructive testing methods are a powerful means for evaluating the strength, durability and quality control of the existing concrete structures. In addition to above, the depth of cracks, micro cracks and progressive deterioration can also be studied by this method.
Though nondestructive testing methods are relatively simple to perform, but the analysis and interpretation of these tests is tedious. Thus special knowledge is required to analyse the properties of hardened concrete. In the case of nondestructive testing, the specimens are not loaded upto failure and the strength estimated cannot be expected to yield absolute values of strength.
Thus these methods are used to measure some other properties of concrete from which its strength, durability and elastic parameters are estimated. Some of the properties of concrete are hardness, resistance to penetration of projectiles, resonant frequency and its ability to allow ultrasonic pulse velocity to propagate through it. With the help of the electrical properties of concrete and its ability to absorb, scatter and transmit x-rays and Gamma-rays etc. moisture content, density, thickness and cement content of concrete can be estimated.
Tests on the Composition of Hardened Concrete:
Sometimes disputes arise about the quality of hardened concrete. The more pertinent question arises whether the cement content is there as specified. To resolve this dispute chemical and physical tests are carried out on a sample of hardened concrete.
Determination of Cement Content:
ASTM Standard C 85-66 has prescribed a method for the determination of cement content in the hardened concrete. This method is based on the fact that silicates present in the port-land cement are much more readily decomposed and are made soluble in dilute hydrochloric acid than the silica compounds normally contained in aggregates. Similarly the lime contained in cement is much more soluble than lime contained in aggregates, with the exception of lime stone aggregates.
A representative sample of concrete is crushed and dehydrated at a temperature of 550°C for 3 hours. A small portion of this sample is taken and treated with 1:3 hydrochloric acid. By the chemical reaction with the hydro-chloric acid, the silica contained in the cement is liberated. The quantity of silica so liberated is determined by standard chemical methods.
The filtrate from the silica determination contains soluble calcium oxide from the aggregate and cement. Further calculations depend on whether the aggregate is largely siliceous or not. If the original aggregate is available its solubility should be tested.
From the contents of soluble silica and calcium oxide, the cement contents in the original volume of the sample can be calculated. The results thus obtained are reliable and can be used to check the cement contents of different parts of a structure, when it is desired to check whether or not segregation of cement has taken place.
However the accuracy of the test is lowest for lean mixes with low cement contents and often in this category of mixes the exact value of cement content is required. Further the test depends on the knowledge of the chemical composition of aggregate, which may not be available for testing. When large amounts of soluble silica and calcium oxide both are liberated from the aggregate, the method is not reliable. It should be noted that chemical tests are more expensive. Thus such tests should be used only to resolve the disputes and not as a means for quality control of concrete.
Determination of Original Water/Cement Ratio for Concrete:
For the hardened concrete under test at present, the method for estimating the water/cement ratio that existed at the time of placing the concrete has been developed by Brown. In essence, the method involves the determination of the volume of the capillary pores and the weight of cement and combined water in the concrete.
A sample of the concrete is oven dried at 105°C and the air is removed from the pores under vacuum. Then the pores are refilled with carbon tetrachloride and the weight of sample is calculated. Thus the weight of water which originally occupied the pores can be determined.
As the voids formed by air entrainment are discontinuous, they remain filled with air at the time of applying vacuum and no water is absorbed in them. Thus the result is not affected by the air entrainment.
The sample now is broken up and the carbon tetrachloride is allowed to evaporate and the aggregate is separated and weighed. The loss on ignition and the carbon dioxide content of the remaining fine material are determined. From these two quantities the weight of combined water can be calculated.
The sum of the combined water and pore water gives the original amount of mixing water. The quantity of hydrous cement can be determined either together with this method or by the method described in the last test. From these quantities of water and cement, the water/cement ratio can be determined with an accuracy of about 0.02 of the true value. The technique has been described into standard method B.S. 1881, Part-6-1971.
Physical Method for Determining Cement Content in Concrete:
For determining the cement content, total aggregate content and fine/coarse aggregate ratio, Polivka carried out experiments on a sawn and varnished surface of a dried concrete specimen and developed a method known as ‘point count method’. This method is based on the fact that the relative volumes of the constituents of a heterogeneous solid are directly proportional to (i) their relative areas in a plane section, and (ii) to the intercepts of these areas along a random line.
Further the frequency with which a constituent occurs at a given number of equally spaced points along a random line is a direct measure of the relative volume of that constituent in the solid. Thus a point count by means of a stereo microscope can rapidly give the volumetric proportions of a hardened concrete specimen.
The aggregate and voids containing evaporable water or air can be identified. The remainder material is assumed as the hydrated cement. To convert the quantity of this hydrated cement to the volume of un-hydrated cement, the knowledge of the specific gravity of dry cement and the non-evaporable water content of the hydrated cement is required.
The test determines the cement content of concrete within 10% accuracy but the original water content or voids ratio cannot be estimated by this method, as no distinction between the air and water voids has been made.