Based upon the different properties of concrete the non-destructive methods have been developed. The methods are: 1. Surface Hardness Test 2. Penetration Resistance Test 3. Pull Out Test 4. Dynamic or Vibration Methods.
Method # 1. Surface Hardness Test:
The hardness of the concrete surface usually is determined by the rebound hammer test. The rebound hammer test was developed in 1948 by a Swiss Engineer named Ernst Schmit. The weight of the hammer Used is about 2 kg and its impact energy 2.2 N.m.
Equipment and its Working:
A typical rebound test hammer is shown in Fig. 19.8. All parts of the equipment are shown on the Fig. It consists of a spring controlled hammer mass that slides on a plunger with in a tubular casing as shown in Fig. 19.8. When the plunger is pressed against the surface of the concrete, the mass rebounds from the plunger. It retracts against the force of the spring. This spring is automatically released when fully tensioned, causing the hammer mass to impact against the concrete through the plunger.
When the spring controlled mass, rebounds it takes with it a rider which slides along a graduated scale and can be seen through a small window in the side of the casing. The rider can be held in any position on the scale by depressing the locking button to record the reading. The distance travelled by the mass is called the rebound number. It is indicated by the rider moving along a graduated scale.
The equipment can be operated vertically or horizontally. The plunger is pressed hard and steadily against the concrete surface to be tested at right angles, till the spring loaded mass is triggered off from its locked position. After impact, the scale index is read while the hammer still is in the test position. The measurement of the distance taken is an arbitrary quantity, known as rebound number.
Each hammer varies considerably in performance. Thus it needs calibration for use on concrete made with the aggregates from the specific source, as the reading of the test is very sensitive to the local variations in the concrete, specially to the aggregate particles near the surface. The hammer may strike an aggregate particle, giving misleading result. Thus it is necessary to take several readings at each test location. B.S. 1881, Part 202 specifies 12 readings over an area of about 300 square mm.
The surface should be smooth, clean and dry. The loose material from the surface must be removed. Fig. 19.9 shows the cross section of rebound hammer and its operating principle. Fig. 19.9 (a) shows the ready state of equipment for test, position (b) shows the body pushed towards the test object, position (c) shows the release of hammer and position (d) shows the hammer rebound.
The test can be conducted horizontally, vertically upwards, or down wards or at any intermediate angle. At each angle the rebound number will be 35 different for the same concrete. Thus separate calibration or correction charts for each condition should be prepared.
Fig. 19.10 shows the relationship between rebound number of test hammer and compressive strength of concrete curve A shows the relation between rebound number in vertically down impact and cylinder strength of concrete. Curve B shows relation of horizontal impact and curve C is shows relation of vertically upward impact, while Fig. 19.11 shows the relationship between rebound number, angle and condition of moisture of test on compressive strength of concrete.
Factors Affecting the Results:
Following factors have been found affecting the results of the test:
(a) Mix Characteristics:
In this group following factors are covered:
(i) Type of cement
(ii) Cement content
(iii) Type and characteristics of coarse aggregate
(b) Member Characteristics:
In this group following factors are covered:
(i) Compaction of concrete
(ii) Age of concrete
(iii) Internal moisture condition of concrete
(iv) Smoothness of surface under test
(v) Shape, size and rigidity of member
(vi) Temperature of concrete etc.
Relation between Rebound Number and Strength of Concrete:
Experimental results have shown that there is a general correlation between the rebound number and compressive strength of concrete. However there is wide range of disagreement among the researchers regarding the accuracy of estimation of strength from rebound numbers. The variation in strength of a properly calibrated hammer has been observed between ± 15% and ± 20%.
Application of the Test:
International survey was conducted during 1965-1967 on the use of Schmitt rebound hammer. During this survey consensus emerged that this test is not fit to be used as acceptance test.
However it is useful for the following applications:
i. For checking uniformity of concrete quality
ii. Approximate estimation of strength
iii. Comparing a given concrete with a specified quality concrete
Method # 2. Penetration Resistance Test:
During 1964-66, a technique or test was developed for concrete testing in the laboratory as well as in-situ. Commercially this test is called as Windsor probe test. The strength of the concrete is estimated from the depth of penetration of a metal rod driven by a standard charge of powder. The under lying principle is that for standard test conditions, the penetration is inversely proportional to the compressive strength of the concrete, but the relation depends upon the hardness of the aggregate.
Thus the hardness of aggregate has to be determined on MOh’s scale, which can be done without any difficulty. Charts of strength versus depth of penetration are available for aggregates with hardness between 3 to 7 on Moh’s scale. However in practice the penetration resistance should be correlated with the compressive strength of standard test specimen. Fig. 19.12 shows the relation between exposed probe length and 28 days compressive strength of concrete based on the work Ami H.T.
Penetration resistance test basically measures the hardness of the surface like Schmidt hammer test. It also does not yield absolute values of strengths. However it is very use full for determining the relative strength of concrete in the same structure or relative strength in different structures without extensive calibration with specific concrete.
The probes are driven into the concrete in a set of three in close vicinity by firing a precision powder charge cartridge. The exposed depth is measured by calibrated depth gauge. The average of these depths is used in determining the strength. The penetration resistance test can be considered almost nondestructive as the damage to the concrete made by 6.3 mm probe only locally and re testing in the vicinity is possible.
Advantages of Penetration Test:
In this method the hardness of the concrete is measured upto about 6.3 mm depth of concrete and not just at the surface. In this case there is no extra cost of handling, storing, cutting or making good test pieces as in the core test method.
Method # 3. Pull Out Test:
This test measures the force required to pull out a previously cast in steel rod with an embedded enlarged end as shown in Fig. 19.13. The test procedure has been described in ASTMC 900-87. However the reliability of the test has not yet been established. In Denmark this test has been successfully used. The stronger the concrete more is the force required to pull out.
In the operation of test, the core of concrete is pulled out and the force required is related to the compressive strength of the parent concrete.
Due to the shape, the steel rod assembly is pulled out with a lump of concrete, approximately in the shape of a frustum of a cone. The pull out strength is calculated as the ratio of the pull out force to the idealized area of the frustum. Actually the concrete is subjected to tension and shear, but the calculated pull out strength approximately is taken as the shearing strength of the concrete.
The ratio of the pull out strength and compressive strength decreases slightly with the increase in strength of concrete, but for a given strength it is independent of age. However the pull out force or strength correlates well with the compressive strength of core or standard cylinders for a wide range of curing conditions and ages. The typical test results are shown in Fig.19.14.
The ideal way to use pullout test in the field would be to incorporate assemblies in the structure. These assemblies can be pull out at any time, the need felt. It has been suggested that this test need not be carried out to completion. It may be sufficient to apply a predetermined force to the embedded rod, if it does not pull out, the structure is assumed safe.
Method # 4. Dynamic or Vibration Methods:
These methods are important nondestructive methods used for testing concrete strength and other properties as well. The fundamental principle of working of these methods is the propagation of sound velocity through a solid material. A mathematical relationship could be established between the velocity of sound through the specimen and its resonant frequency.
From this relationship, the modulus of elasticity of the material is determined. For deriving this relationship the solid mediums are considered to be homogeneous, isotropic and perfectly elastic. However these relations are also applied to heterogeneous materials like concrete. The velocity of sound V in a solid material is a function of the square root of the ratio of its modulus of elasticity ‘E’ and its density ρ.
where g is the acceleration due to gravity.
The velocity of sound in a solid material can be estimated by knowing the resonant frequency of the specimen or by noting the time of travel of short pulses of vibrations passing through the samples. In nondestructive testing of concrete either resonance method or pulse velocity method may be adopted.
Resonant Frequency Method:
This method is based upon the determination of the fundamental resonant frequency of vibration of a specimen. The resonance is indicated by the point of maximum amplitude for the various driving frequencies generated. The resonant frequency method usually is used in the laboratory. The equipment used for the resonant frequency method usually is known as ‘Sonometer’.
Use of Resonant Frequency Test:
The resonant frequency test may be used for the following purposes:
1. For studying the deterioration effects of concrete subjected to repeated cycles of freezing and thawing.
2. To study the effects due to acidic and alkali reactions.
3. For determining the damage due to fire.
4. To calculate the dynamic Young’s Modulus of elasticity of concrete, but the values obtained are somewhat higher than those obtained with standard static tests.
Pulse Velocity Method:
This method can be sub divided into the following two groups:
1. Mechanical Sonic Pulse Velocity Method:
In this method the measurements of the time of travel of longitudinal or compressional waves generated by a single impact hammer blow or repeated blows are taken. The pulses can be generated either by hammer blows or by the use of an electro acoustic transducer. For generating pulses electro acoustic transducers are preferred as they provide better control on the type and frequency of the pulses generated. Instrument used in this method is called ‘Soniscope’.
In this method when mechanical impulses are applied to a solid mass, three different kinds of waves are generated. Generally these waves are known as longitudinal waves, shear waves and surface waves. All these three waves travel with-different speeds. The speed of the longitudinal or compressional wave is twice that of the other two types, the shear and surface waves. The surface waves are the slowest of the three.
2. Ultra Sonic Pulse Velocity Method:
In this method the measurements of the time of travel of electronically generated mechanical pulses through the concrete are taken. Out of the above type of pulse, the ultra-sonic velocity pulse method has become more popular throughout the world.
The ultra-sonic pulse velocity method basically consists of measuring the velocity of electronic pulse passing through the concrete from a transmitting transducer to a receiving transducer. Though a considerable degree of success has been achieved in the determination of the longitudinal wave velocity in concrete, but there is no unique relationship between the velocity and the strength of concrete. However under specific conditions the two quantities are directly related. The common factor is the density of concrete. A change in the density of the concrete results in a change of the velocity of the pulse.
The method is based on the principle that the pulse velocity passing through the concrete primarily is dependent upon the elastic properties of the material and is independent of the shape or its geometry. The pulse velocities range from about 3 to 5 km/sec. The pulse generator circuit consists of electronic circuit for generating pulses and a transducer for transforming these electronic pulses into mechanical energy having vibration frequencies in the range of 15 to 50 Khz. A typical test circuit is shown in Fig.19.15.
The pulses are generated at regular time interval between 10 to 50 seconds. The time of travel of the ultrasonic pulse passing through the concrete is measured accurately with an accuracy of ± 1%. The transit time is displayed either on an oscilloscope or read digitally. The path length between the transducers divided by the time of travel gives the average velocity of wave propagation. For use with concrete, transducers with natural frequencies between 20 and 200 KHz are most suitable.
Recently battery operated fully portable digitized units have been introduced for this purpose. This unit is called portable ultrasonic nondestructive digital indicating tester. In short it is known as ‘PUNDIT’. It weighs about 3 kg only.
Techniques for Measuring Pulse Velocity through Concrete:
For transmitting pulse velocity through concrete following types of transducers have been found to be suitable.
1. Piezo-electric crystal transducer
2. Barium titanate
3. Lead Zirconate titanate transducer, and
Out of these transducers piezo-electrical crystal transducer is most popular.
The transducer is kept in contact with the concrete, so that the vibrations travel through it and picked up by another transducer in contact with the opposite face of the specimen under test.
The transducers generate an- electrical signal, which is fed through an amplifier to a plate of a cathode ray tube. A second plate supplies timing marks at fixed intervals. Thus from the measurement of displacement of the pulse signal and time of travel, the velocity of pulse may be calculate.
Arrangement of Transducers:
Transducers may be arranged on concrete surface in the following three ways as shown in Fig.19.16:
1. On opposite face (direct transmission).
2. On adjacent face (Semi direct transmission).
3. On same face (Indirect transmission).
1. On Opposite Face:
The arrangement of fixing the transducers on opposite sides has been found to be the best for the following reasons:
(a) The maximum pulse energy is transmitted at right angles to the face of the transmitter.
(b) The direct transmission method or opposite side arrangement is most reliable for the measurement of transit time,
(c) The path of pulse travel is clearly defined and can be measured accurately. This approach is recommended for assessing the quality of concrete where ever possible.
2. Semi Direct or Adjacent Face Method:
This method can be used satisfactorily under the following conditions:
(a) If the path length is not large.
(b) The angle between the transducers is not large.
If these conditions are not met the clear signals will not be received due to the reduced or weak transmitted pulse.
3. Same Face or Indirect Transmission Method:
In this case the pulse velocity is measured parallel to the surface of the concrete. In this case energy received is considerably low and the accuracy of readings is correspondingly decreased. In this case the properties of the surface layer only are known. It does not give any indication of strength of concrete at depth. Hence it is least satisfactory.
Transducers making point contact only with the surface of concrete are advantageous as no surface treatment is required, whereas on a flat surface the good contact of transducers must be ensured over a considerable area. The probe transducers are more sensitive to operator pressure.
Accuracy of Measurements:
Generally the ultrasonic concrete tester measures the time of travel through small specimens with an accuracy of 0.1 microseconds. For the specimens of the same length, the accuracy of measurement for ‘SONISCOPE’ and ‘PUNDIT’ is of the order of 0.5 micro-seconds. Thus for laboratory studies under controlled conditions ultrasonic concrete tester is the ideal instrument. For field studies where the path lengths are longer Soniscope and ‘Pundit’ are the best equipments. The Pundit being lighter in weight about 3 kg only, is easy to carry along with.
Application or Uses of Pulse Velocity Methods:
The pulse velocity methods have been used to evaluate the concrete structures and attempts have been made to correlate the pulse velocity with strength and other properties of concrete.
Various uses of the pulse velocity methods are discussed below:
1. Establishing the Uniformity of Concrete:
For this purpose, the ultrasonic concrete tester is an ideal tool for laboratory specimens and soniscope and Pundit are excellent tools for laboratory and field studies both.
2. Establishing the Acceptance Criteria:
The high velocity value in concrete is an indication of good quality of concrete. Table 19.5 gives the pulse velocity rating as suggested by central water and power research station Khadakwasla and Table 19.6 gives the rating as suggested by Leslie and Cheesman.
According to Jones, the lower limit for good quality concrete is between 4.1 and 4.7 km/seconds. If the pulse velocity is below 2.0 km/sec, the concrete quality is very poor.
Estimation of Strength of Concrete:
There is no unique relationship between the pulse velocity and strength of concrete. It has been observed that there is a wide variation in the pulse velocity of concrete of a given quality. This variation of pulse velocity has been observed due to the influence of the coarse aggregate. Both the quantity and type of aggregate have been found to affect the pulse velocity. However for a constant water/cement ratio, the influence of the coarse aggregate on the strength of concrete comparatively has been observed small.
Thus for different mix proportions, different relations between the pulse velocity and strength would be obtained as shown in Fig.19.17. Jones has suggested that at a constant water/cement ratio, the higher the volume of aggregate, the higher the pulse velocity. Thus the estimated strength would be more. It has been found that for each 1% increase in volume of aggregate results 0.9 MPa increase in the strength of concrete.
Determination of Pulse Modulus of Elasticity:
By measuring the pulse velocity and knowing the poisson’s ratio the dynamic modulus of elasticity can be determined by the following relation:
v = pulse velocity
ρ = density of concrete
µ = poisson ratio
the value of Poisson ratio can be calculated from the following relation:
n = resonant frequency (Hz)
L = length of beam in mm
v = pulse velocity in mm/sec.
The value of poisson ratio varies from 0.2 to 0.24. This value is a little higher than found by static method.
The value of dynamic modulus of elasticity determined from ultrasonic pulse velocity method is somewhat higher than determined from static method as dynamic modulus is not affected by creep. Creep also is not found to affect significantly the initial tangent modulus in static method. Hence the values of dynamic modulus and initial tangent modulus are more or less equal. The relation between static modulus Ec and dynamic modulus Ed in GN/m2 is-
Ec = 1.25Ed – 19 …(iii)
An empirical relationship between the pulse velocity and static young’s modulus is shown in Fig. 19.18. From this relation the value of young’s modulus may be determined for those points in the structure where pulse velocity measurements have been taken.
Estimation of Setting Characteristics of Concrete:
Soniscope has been used widely to determine the rate of setting of concrete.
Estimation of Durability of Concrete:
The effects on durability of concrete of freezing and thawing of water, aggressive environmental effects such as sulphate and acidic effect etc. have been studied by many researchers using the pulse velocity method. Lack of compaction and change in water cement ratio can be detected by these studies.
Detection and Measurement of Cracks:
The detection and measurement of cracks, and voids, deterioration due to frost or chemical action in massive concrete structures by the use of ultrasonic pulse technique is very important. The basic principle of crack detection is that if the depth and width of the crack is appreciable no signal will be received at the receiving transducer. If the depth of crack is small compared to the distance between the transducers, then the pulse will be deflected around the crack and the signals will be received by the transducer.
However this increase in length will result in reduced pulse velocity in comparison with straight travel path. The difference in pulse velocity is used to estimate the path length. The difference between this path length and the straight path length will give double the crack depth.
Estimation of Age of Concrete:
As shown in Fig. 19.20 the strength of concrete increases with the age of concrete, resulting the increase in pulse velocity. Fig. 19.20 shows the relation between age and pulse velocity for different w/c ratios.
Estimation of Fire Hazard:
To estimate the hazard of fire, experiments were conducted on 88 x 102 x 406 mm prisms. The specimens were exposed to fire for 1 hour at temperatures ranging from 100°C to 1000°C. The specimens then were removed from furnace and allowed to cool to room temperature. Then velocity was measured by ultra-sonic method and the specimens were tested for flexure strength. The percentage loss in pulse velocity was found more or less same as the loss in flexural strength of test prisms after exposure to fire.
The pulse velocity technique may also be used for the estimation of pavement concrete thickness. At the end it would be fair to add that the use of ultrasonic pulse measurement as a means of quality control on construction job is not practicable, as no satisfactory correlation exists between the variability of the compression test specimen and the variability of pulse measurements.