UNIVERSITY OF GAZIANTEPFACULTY OF ENGINEERING CIVIL DEPARTMENT CE-550 NONDESTRUCTIVE TESTING AND EVALUATION IN STRUCTURAL ANALYSIS Report About : (Using of Schmidt Hammer as anon destructive test method in structural engineering) Submitted to: Doç.Dr.ESSRA GUNAYISI Prepared by: Chalak Ahmed Mohammed
[email protected] 2014 45056 Date : feb. 2015 1 Introduction: Importance and need of non-destructive testing: It is often necessary to test concrete structures after the concrete has hardened to determine whether the structure is suitable for its designed use. Ideally such testing should be done without damaging the concrete. The tests available for testing concrete range from the completely nondestructive, where there is no damage to the concrete, through those where the concrete surface is slightly damaged, to partially destructive tests, such as core tests and pullout and pull off tests, where the surface has to be repaired after the test. The range of properties that can be assessed using non-destructive tests and partially destructive tests is quite large and includes such fundamental parameters as density, elastic modulus and strength as well as surface hardness and surface absorption, and reinforcement location, size and distance from the surface. In some cases it is also possible to check the quality of workmanship and structural integrity by the ability to detect voids, cracking and delaminating. Non-destructive testing can be applied to both old and new structures. For new structures, the principal applications are likely to be for quality control or the resolution of doubts about the quality of materials or construction. The testing of existing structures is usually related to an assessment of structural integrity or adequacy. In either case, if destructive testing alone is used, for instance, by removing cores for compression testing, the cost of coring and testing may only allow a relatively small number of tests to be carried out on a large structure which may be misleading. Non-destructive testing can be used in those situations as a preliminary to subsequent coring. 2 Situations where NDT is an option to consider for investigation of in situ concrete to investigate the homogeneity of concrete mixing lack of grout in post tensioning ducts to determine the density and strength of concrete in a structure to determine the location of reinforcing bars and the cover over the bars to determine the number and size/diameter of reinforcing bars to determine the extent of defects such as corrosion to determine the location of in-built wiring, piping, ducting, etc. to determine if there is a bond between epoxy bonded steel plates and concrete members to determine whether internal defects such as voids,cracks, delaminations, honeycombing, lack of bonding with reinforcing bars, etc. exist in concrete 3 REBOUND HAMMER (Schmidt hammer) Test EN12504-2: The rebound hammer is one of the most popular nondestructive testing methods used to investigate concrete. Its popularity is due to its relatively low cost and simple operating procedures. The rebound hammer is also one of the easiest pieces of equipment to misuse; thus, many people do not trust the rebound test results. A handy non-destructive testing instrument should be cheap, easy to operate and should have reproducibility for, fairly accurate results. In 1948, a Swiss Engineer, Ernst Schmidt developed a test hammer for measuring the hardness of concrete by the rebound principle. IS: 13311 (part 2): 1992 specified, the rebound hammer method could be used for assessing the likely compressive strength of concrete with the help of suitable corelations between rebound index and compressive strength. Rebound Hammer instrument 4 DESCRIPTION OF THE INSTRUMENT (Rebound Hammer): This is a simple, handy tool, which can be used to provide a convenient and rapid indication of the compressive strength of concrete. The hammer consists of a spring controlled mass that slides on a plunger within a tubular housing. When the plunger is pressed against, the surface of concrete, it retracts against the force of the spring. When completely retracted the spring is automatically released. On the spring controlled mass rebound, it takes the rider with it along the guide scale. By pushing a button, the rider can be held in position to allow readings to be taken. The schematic diagram showing various parts of a rebound hammer is given as Fig. 1. Concrete surface 5. Hammer guide 9. Housing 2. Impact spring 6. Release catch 10. Hammer mass 3. Rider on guide rod 7. Compressive spring 4. Window and scale 8. Locking button 11. Plunger Fig. Components of a Rebound Hammer 5 The use of the rebound hammer test: The rebound hammer method could be used for (IS: 13311 Part 2-1992): a) Assessing the likely compressive strength of concrete with the help of suitable corelations between rebound index and compressive strength. b) Assessing the uniformity of concrete. c) Assessing the quality of the concrete in relation to standard requirements. d) Assessing the quality of one element of concrete in relation to another. Note: The rebound hammer method can be used with greater confidence for differentiating between the questioner and acceptable parts of a structure or for relative comparison between two different structures. The test is classified as a hardness test, and is based on the principle that the rebound of an elastic mass depends on the hardness of the surface against which the mass impinges. The energy absorbed by the concrete is related to its strength. The hardness-strength relationship depend on the following factors: There is no unique relation between hardness and strength of concrete, but experimental data relationships can be obtained from a given concrete. However, this relationship is dependent upon factors affecting the concrete surface such as: 1- Degree of saturation. 2- Carbonation. 3- Temperature. 4- Surface preparation and location. 5- Type of surface finish. Concrete must be approximately the same age, moisture conditions and same degree of carbonation (note that carbonated surfaces yield higher rebound values). It is clear then that the rebound number reflects only the surface of concrete. The results obtained are only representative of the outer concrete layer with a thickness of 30– 50 mm. 6 Principle: The method is based on the principle that the rebound of an elastic mass depends on the hardness of the surface against which mass strikes. When the plunger of rebound hammer is pressed against the surface of the concrete, the spring controlled mass rebounds and the extent of such rebound depends upon the surface hardness of concrete. The surface hardness and therefore the rebound are taken to be related to the compressive strength of the concrete. The rebound value is read off along a graduated scale and is designated as the rebound number or rebound index. The compressive strength can be read directly from the graph provided on the body of the hammer. The impact energy required for a rebound hammer for different applications is given below: Impact Energy of Rebound Hammers Depending upon the impact energy, the hammers are classified into four types, i.e. N, L, M & P. Type N hammer having an impact energy of 2.2 N-m and is suitable for grades of concrete from M-15 to M-45. The type L hammer is suitable for lightweight concrete or small and impact sensitive part of the structure. Type M hammer is generally recommended for heavy structures and mass concrete. Type P is suitable for concrete below M15 grade. 7 Type N Measuring range 10 to 70 N/mm ² compressive strength (below 25 N/mm ² type P is better suited). Impact energy= 2,207 Nm. Rebound values are read from a dial. Testing the compressive strength of a prefabricated concrete girder. Rebound values are recorded by an assistant who will calculate mean values and read compressive strength values from a conversion diagram. Type NR Measuring range 10 to 70 N/mm ² compressive strength. Impact energy = 2,207 Nm. Rebound values are recorded as a bar chart on a paper strip. One roll of paper strip o_ers space for 4000 test impacts. A bridge concreted in several stages is tested for uniform concrete quality. The engineer will perform a series of tests at intervals of 10 m each. Type L/LR Measuring range 10 to 70 N/mm ² compressive strength (0,735 Nm). Handling and dimensions as for types N and NR, but with a three times smaller impact energy. These types are used for testing thin walled (< 100 mm) or small components, but 8 also cast stone components sensitive to impact Type DIGI-SCHMIDT Measuring range 10 (ND) / 18 (LD) to 70 N/mm ² compressive strength. Rebound values are measured by an electronic method and may be read directly as compressive strength values. For further information, see lea_et Nr. 810 340 01E. TEST METHODOLOGY: For taking a measurement, the hammer should be held at right angles to the surface of the structure. The test thus can be conducted horizontally on vertical surfaces and vertically upwards or downwards on horizontal surfaces, as shown below: (Various positions of Rebound Hammer) 9 The following should be observed during testing (a) The surface should be smooth, clean and dry. (b) The loosely adhering scale should be rubbed off with a grinding wheel or stone, before testing. (c) The test should not be conducted on rough surfaces resulting from incomplete compaction, loss of grout, spoiled or tooled surfaces. (d) The point of impact should be at least 20mm away from the edge or sharp discontinuity Factors That Affects Rebound Hammer Numbers Since the rebound hammer measures the surface hardness of the concrete, it is important to understand all the items that might affect surface conditions of the concrete and thus, the rebound hammer numbers. These factors include: 1- Type of cement: Concrete made of high alumina cement can given strengths up to 100% higher, whereas supersulphated cement concrete can give 50% lower strength compared to a calibration obtained on Portland cement cubes. It is necessary to recalibrate the hammer for different types of cement. 2- Age of concrete: In very old and dry concrete the surface will be harder than the interior, giving rebound values somewhat higher than normal. New concrete with moist surface generally has a relatively softer surface, resulting in lower than normal rebound. 3- Carbonation of concrete surface: 10 Surface carbonation of concrete significantly affects the rebound hammer test results. In old concrete where the carbonation layer can be up to 20 mm thick, the strength may be overestimated by 50% Limits. 4- Smoothness of the surface. 5- Size and shape of the concrete sample. 6-The rigidity of the test area. 7- Surface moisture. 8- Internal moisture (moisture gradient). 9- Coarse aggregates. 10- Forms used. 11- Location of the reinforcement. 12- Frozen concrete. 13- hammer type. 14- hammer inclination. Relationship between 28 day compressive strength and rebound number for limestoneaggregate concrete obtained with Type N-2 hammer 11 A typical correlation procedure is, as follows: (1) Prepare a number of 150 mm × 300 mm cylinders (or 150 mm3 cube specimens) covering the strength range to be encountered on the job site. Use the same cement and aggregates as are to be used on the job. Cure the cylinders under standard moistcuring room conditions, keeping the curing period the same as the specified control age in the field. (2) After capping, place the cylinders in a compression-testing machine under an initial load of approximately 15% of the ultimate load to restrain the specimen. Ensure that cylinders are in a saturated surface-dry condition (3) Make 15 hammer rebound readings, 5 on each of 3 vertical lines 120° apart, against the side surface in the middle two thirds of each cylinder. Avoid testing the same spot twice. For cubes, take 5 readings on each of the 4 molded faces without testing the same spot twice. (4) Average the readings and call this the rebound number of the cylinder under test. Repeat this procedure for all the cylinders. (5) Test the cylinders to failure in compression and plot the rebound numbers against the compressive strengths on a graph. Fit a curve or a line by the method of least squares 12 Correlation curves produced by different researchers. (Greene curve used Type N hammer; others used Type N-2) If concrete compression test fails, should Schmidt hammer test be adopted as an alternative test to prove the concrete strength? The Scmidt hammer test is based on the elastic rebound of hammer which presses on concrete surface and it measures the surface hardness of concrete. Since the test is very sensitive to the presence of aggregates and voids at the concrete surface, it is necessary to take more than 10 readings over the area of test. However, it should be noted that Schmidt hammer test measures surface hardness only but not the strength of concrete. Therefore, it may not be considered a good substitute . Regards... 13 14