What Is Non-Destructive Testing of Concrete?
This test provides immediate results, strength, and real properties of the concrete structure. Non-destructive testing of concrete is a method to obtain the compressive strength and other properties of concrete from existing structures.
The standard method of assessing the quality of concrete in buildings or structures is to test cast samples simultaneously for compressive, flexural, and tensile strength.
Although there can be no direct measurement of this strength properties of structural concrete for the simple reason that determination of strength involves destructive stresses, several non-destructive assessment methods have been developed.
This depends on the fact that certain physical properties of concrete can be related to strength and can be measured by non-destructive methods. Such properties include hardness, resistance to projectile penetration, rebound ability, and the ability to transmit ultrasonic pulses and X and Y rays.
Type of Non–Destructive Testing Concrete
The following are different methods of Non-destructive testing in concrete:
• Penetration method
• Rebound hammer method
• Pull-out test method
• Ultrasonic pulse velocity method
• Radioactive methods
• Combined Method UPV & RH Test
• Core Extraction for Compressive Strength Test
1. Penetration Test Method
The Windsor probe is usually considered to be the best way of testing penetration.
The equipment consists of a gun or powder is driven, hardened alloy probes, loaded cartridges, a depth gauge to measure probe penetration, and other related equipment.
A 0.25-inch diameter probe. (6.5 mm) and length 3.125 in. (8.0 cm), is introduced into the concrete by means of a precision powder load.
The penetration depth provides an indication of the compressive strength of the concrete.
2. Rebound Hammer Method
Rebound Hammer strength test, being a non-destructive test, has its advantages of preserving the integrity of the sample.
Besides, the test is relatively simple, versatile, and economical compared to conventional crushing tests.
This method is widely used for testing concretes manufacturers of such rebound hammers usually provide calibrated curves showing this relation of compressive strength and rebound number concrete only.
But these hammers may conveniently be used to test stones and bricks also. This paper presents some models for the correlation between rebound number and compressive strength of bricks.
10 different samples of brick were collected from different manufacturers, and rebound numbers were taken 20 times for each brick, 10 times for horizontal hammer position, and 10 times for vertical.
These samples were then tested in a universal testing machine to obtain their actual compressive strength.
Linear and exponential correlations between average rebound number and compressive strength were established using the least square parabola method.
Regression coefficients for the proposed models were found to vary from 0.87 to 0.96.
The proposed linear model for horizontal hammer position was found to be the better one with a regression coefficient of 0.96, which indicates the acceptability of this model for predicting the compressive strength of bricks.
Concrete Rebound hammer test was first covered by IS 13311 (Part 2) – 1992. Swiss engineer Ernst Schmidt is the name of the scientist who first introduced the Rebound hammer test, and it is also called a Schmidt hammer test or Swiss Hammer Test.
3. Pull-Out Test Method
Pull-out test measures, using a special ram, the force required to pull a specially shaped steel rod in the concrete whose enlarged end was thrown to the concrete to a depth of 3 inches. (7.6 cm).
The concrete is simultaneously in tension and shear, but the force required to pull the concrete may be related to its compressive strength.
The extraction technique can thus quantitatively measure the in situ strength of the concrete when the appropriate correlations have been made.
It had been found, in a wide range of forces, that the tensile forces have a coefficient of variation comparable to that of compressive strength.
4. Ultrasonic Pulse velocity method
A piezoelectric transducer emitting vibration at its fundamental frequency is placed in contact with the concrete surface so the vibrations travel through the concrete and are received by another transducer, which can be in contact with the opposite face of the test object.
This instrument Portable ultrasonic testing equipment is available. The equipment is portable, simple to operate, and includes a rechargeable battery and charging unit.
Typically, pulse times of nearly 6500 OS can be measured with 0.1-OS resolution. The measured travel time is prominently displayed.
The instrument comes with a set of both transducers, one each for transmitting and receiving the ultrasonic pulse.
Transducers with frequencies of 20 to 100 kHz are usually used for testing of concrete.
These transducers primarily generate compressional waves at predominantly one frequency, with most of the wave energy directed along the axis normal to the transducer face.
Factors Affecting Ultrasonic Pulse velocity Test
It is relatively easy to conduct a pulse velocity test. It is important that the test is conducted such that the pulse velocity readings are reproducible and that they are affected only by the properties of concrete under test rather than by other factors.
The factors affecting the pulse velocity may be divided into two categories:
(I) Factors resulting directly from concrete properties; and
(II) Other factors. These influencing factors are discussed below:
Applications of UPV Tests
This UPV method has been applied successfully in this laboratory as well as in the field. It is being used for quality control, as well as for the analysis of deterioration.
This applications of the pulse velocity method on a concrete structure are:
• Studies on Durability of Concrete
• Measurement of Surface Crack Depth
• Determination of Dynamic Modulus of Elasticity
• Estimation of Strength of Concrete
• Establishing Homogeneity of Concrete
• Studies on the Hydration of Cement
Effects of Concrete Properties
• Aggregate Size, Grading, Type, and Content
• Water-Cement Ratio
• Age of Concrete
• Transducer Contact
• Temperature of Concrete
• Moisture and Curing Condition of Concrete
• Path Length
• Size and Shape of a Specimen
• Level of Stress
• Presence of Reinforcing Steel
5. Radioactive methods
Radioactive concrete testing methods can be used to detect the location of the reinforcement, measure density, and perhaps establish whether honeycomb has occurred in structural concrete units.
Gamma radiography is increasingly accepted in England and Europe. The equipment is quite simple, and the operating costs are small, even though the original cost can be high.
Concrete up to 45 cm thick can be examined without difficulty.
6. Combined Method UPV & RH Test
Hardness scales are defined steps of the resistance of a material to indentation under dynamic or static load or resistance to scratch, abrasion, wear, drilling, or cutting.
Concrete test hammers evaluate surface hardness as a function of resiliency, i.e the capability of a hammer to rebound or spring back.
The interpretation of this pulse velocity measurements in concrete is complicated by the heterogeneous nature of the material.
The wave velocity isn’t determined directly but is calculated by the time taken by a pulse to travel a measured distance.
A piezoelectric transducer emitting vibration at its fundamental frequency is placed in contact with the concrete surface so that the vibrations travel through the concrete and are received by another transducer, which is in contact with the opposite face of the test object.
7. Core Extraction for Compressive Strength Test
Cores are cut with a drill using a hollow barrel tipped with industrial diamonds. The entire rig has to be firmly fixed in position by weights, anchor bolts, vacuum pads or bracing against other parts of the structure.
The preferred core diameter for strength testing isn’t defined in BS EN 12504-1, BS EN 13791 or BS 6089 however the diameter should be at least 3.5 x the maximum aggregate size.
Sometimes even smaller diameter cores have to be used for strength testing. In this case, the strength results could be variable and a greater number of cores must be extracted.
For strength testing, the length to diameter ratio must be between 1 and 2 and preferably between 1 and 1.2.
When cores are received in the laboratory they might be examined for degree of compaction, cracks, voids, honeycombing and the presence of reinforcement.
Before testing cores for strength, they have to be trimmed to length and the ends prepared so they’re flat and perpendicular to the longitudinal axis.
This is achieved by grinding or, more typically, capping with high alumina cement (calcium aluminate cement) mortar or a sulfur/sand mixture. Cores should be tested in a dry state.
This is an air dry, not oven dry. If tested wet a small positive correction to the strength is made
Purpose of Non-Destructive Tests on Concrete
A variety of non-destructive testing (NDT) methods have been developed or are under development to investigate and evaluate concrete structures.
These methods aim to estimate the strength and other properties, corrosion monitoring and evaluation; measure cracks size and coverage; evaluate the quality of the grout; detect defects and identify relatively more vulnerable areas in concrete structures.
Many NDT methods used for concrete testing originate from testing a more homogeneous metal system. These methods have a solid scientific basis, but the heterogeneity of the concrete makes it difficult to interpret the results.
There may be many parameters, such as materials, mix, labor, and environment, that influence the measurement results.
Objectives of Non-Destructive Testing
• Estimating compressive strength at the site
• Estimating uniformity and homogeneity
• Estimated quality in relation to the standard requirement
• Identification of areas of lesser integrity compared to other parts
• Detection of cracks, voids, and other imperfections
• Monitor changes in the concrete structure that may occur over time
• Identification of the reinforcement profile and measurement of the cover, bar diameter, etc.
• Condition of prestressing / reinforcing steel with respect to corrosion
• Content of chlorides, sulfates, alkalis or degree of carbonation
• Measurement of the elastic modulus
• Grouting condition in prestressing ducts
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