Assesment of Relative Surface Hardness of Existing Concrete Member by Schmidt Hammer

Concrete structures as many other engineering structures are subjected to deterioration that affect their integrity, stability, and safety. Faced with the importance of the damages noted on the structures, the current choices are directed towards the repair of the existing structures rather than towards the demolition and construction of new ones. But before any repair work being done, it is a common practice to study the existing structures by analysis after ascertaining the quality of material. Destructive methods of evaluation are inherently limited because specimen removal may be relatively expensive, aesthetically unpleasant and structurally damaging. The number of specimens taken may be limited to a small number for such evaluation method. Thus, potentially, the quantity and quality of the resulting data may be poor and/or inconsistent. Non-destructive Testing is usually undertaken as part of the detailed investigation to complement the other methods. The conclusions of the investigation are based essentially on these tests. The use of Schmidt hammer is commonly accepted for the preliminary assessment of surface concrete strength as the non- destructive test. This article discusses the assessment of existing concrete member by the use of Schmidt hammer and highlights few application importance and limitations for repair and rehabilitation projects.

Concrete structures as many other engineering structures are subjected to deterioration that affect their integrity, stability and safety. Faced with the importance of the damages noted on the structures, the current choices are directed towards the repair of the existing structures rather than towards the demolition and construction of new ones. But before any repair work being done, it is common practice to determine the causes of the deterioration so that successful repair can be done. Many repair work fail because the exact causes of the deterioration was not adequately identified. This identification process comprises many methods including non-destructive testing methods. Non-Destructive Testing (NDT) is usually undertaken as part of the detailed investigation to complement the other methods. The conclusions of the investigation are based essentially on these tests. A correct assessment of a deteriorated structure is a crucial basis for the success of a repair system. Experience has shown that in many cases repairs fail much earlier than expected due to improper diagnosis of the actual situation of the structure, either because the cause of the degradation was not properly understood, or the extent of the damage was under-stated. The strength of the concrete structure will vary by age but its rate may depend on various factors. Since, the use of Schmidt hammer is commonly accepted for the preliminary assessment of concrete surface hardness as the non destructive test, this will provide useful estimation of the uniformity of concrete and measure the strength of existing surface concrete and to detect weak spots. This article discusses the assessment of surface hardness of existing concrete member by the use of Schmidt hammer and highlights few application importance, and limitation for repair, rehabilitation projects.

The Schmidt Hammer was originally developed by Swiss engineer, Ernst Schmidt, in 1948 as a non-destructive method of testing, Figure 1 and measuring the hardness of in situ concrete by the rebound principle (Schmidt 1951). The Schmidt rebound hammer is principally a surface hardness tester with little apparent theoretical relationship between the strength of concrete and the rebound number of the hammer. However, within limits, empirical correlations have been established between strength properties and the rebound number. Since then, Schmidt Hammer has been extended widely in finding the strength of concrete structures as well as rock outcrops (Cargill and Shankar 1990, Amaral et. al. 1999 and Qasrawi 2000). The amount of rebound of the hammer is measured and correlated with the manufacturer’s data to estimate the strength of the concrete (FEMA 306, 1999). The application of Schmidt Hammer to the concrete testing is widely accepted following the procedures given by ASTM C805 (ASTM 1995). IS 13311 Pt-2-1992 as well as BS: 6089-81 and BS: 1881:Part-202 explains the standard procedure for test and correlation between concrete cube crushing strength and rebound number. PROCEQ Company in Switzerland is the renowned manufacturer of non-destructive testing instruments. PROCEQ manufactures the industry’s widest range of hammer types to fit virtually every in situ test application – including the Original SCHMIDT Hammers type N and L, recording type NR and LR, digital types ND and LD DIGI-SCHMIDT. The latest model PROCEQ SilverSchmidt - a Swiss-made instrument - offers unprecedented benefits to users. The new instrument features unrivalled ease of use, higher readability and accuracy, as well as an extended measuring range. A number of user-benefits have been incorporated, such as the automatic correction of readings based upon the impact direction - eliminating the need to refer to impact direction conversion curves. The robust, lightweight unit also makes automatic corrections for carbonation. The Data collection and processing of test results of these equipments comply with major industry standards such as ASTM C805 / BS 1881, Part 202 / DIN 1048, Part 2 / UNE 83.307 / ISO / DIS 8045. The overview of present available PROCEQ Schmidt Hammer models B and U - N and L Version are provided in Figure 3 and Figure 4. It is recommended to use these well accepted instruments for the non-destructive testing of the uniformity of concrete and to measure the surface strength of existing concrete members in situ to control concrete quality and to detect weak spots.

Basu and Aydin (2004) have described operation principle of the Schmidt hammer. The Rebound hammer test as per ASTM C805 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. It consists of a spring-loaded piston of a steel mass. When the hammer is pressed orthogonally against a surface, the piston is automatically released onto the plunger and the rebound height of the piston is considered to be an index of surface hardness. A small sliding pointer indicates the rebound of the hammer on the graduated scale. Method of Testing consists of preparing the instrument for a testing, by releasing the plunger from its locked position by pushing the plunger against the concrete and slowly moving the body away from the concrete. This causes the plunger to extend from the body and the latch engages the hammer mass to the plunger rod Figure 2 A. The plunger is held perpendicular to the concrete surface and slowly push the body toward the test object. As the body is pushed, the main spring connecting the hammer mass to the body is stretched Figure 2B. When the body is pushed to the limit, the latch is automatically released, and the energy stored in the spring propels the hammer mass toward the plunger tip Figure 2C. The mass impacts the shoulder of the plunger rod and rebounds. During rebound, the slide indicator travels with the hammer mass and records the rebound distance Figure 2D. A button on the side of the body is pushed to lock the plunger in the retracted position, and the rebound number is read from the scale.

The test can be conducted horizontally, vertically upward or downward, or at any intermediate angle. Due to different effects of gravity on the rebound as the test angle is changed, the rebound number will be different for the same concrete and will require separate calibration or correction charts.

Using the Schmidt hammers has following advantages:

i. A small amount of structure damage occurs in testing, usually negligible.

ii. It makes possibility of testing concrete strength in structures where cores cannot be drilled. (For example thin walls, densely reinforced walls etc).

iii. It has an application of less expensive testing equipment, Low power consumption.

iv. Simple operation doesn’t need high consumption of labor, or intensive training.

v. Ideally suited for on-site testing, Handy for difficult to access or confined test areas (i.e. working overhead).

In the field site, the Schmidt hammer are used for all identified concrete member for obtaining the relative strength of concrete based on the hardness at or near its exposed surface and to reduce the error, the following procedures are generally applied:

i. This testing is to be carried out on each identified member in a systematic way by dividing the member into well-defined grid points and well defined test points. The grid matrix should have a spacing of approximately 300mm x 300mm.

ii. The concrete test surfaces are made smooth by an abrasive stone at predefined locations of testing within the grids.

iii. Rebound values are obtained from each of the predefined locations of testing within the grids.

iv. About 3-5 readings of rebound number are measured and the average of them is found at each test point. At least 10 readings must be taken from each tested grid area. Extremely higher and lower values were ignored. The consistency in the rebound value is expected from several tests.

v. The strike of the hammer was introduced from various directions normal to the surface of structural element to obtain surface hardness.

vi. In some cases, additional readings are taken if initial rebound values are not consistent.

vii. The rebound value is converted into compressive strength value using the provided conversion chart. The obtained values are then reduced using the suitable time factor.

Carrying out periodic calibration of rebound hammer using standard anvil is desirable. However for new and retrofit concrete construction, rebound hammer is calibrated on concrete test cubes for a given source of constituent materials (viz. cement, sand and stone aggregate), this calibration data can be used with reasonable accuracy in arriving at equivalent in-situ cube strength of relatively new concrete (i.e. not more than three months old concrete). This calibration exercise may be carried out in a concrete lab by casting cubes of designed mix and testing these under controlled condition with rebound hammer as well as test to destruction in compression. Calibration graphs then can be drawn. Large number of readings is desirable to reduce the effects of variability in readings due to various localized as well as instrument factors. For the entire test trails the atmospheric condition, duration of exposure, etc., must be recorded, as this method may give highly erroneous results for concrete whose surface is exposed to atmosphere for longer periods say more than three months. This is due to hardening of concrete surface due to carbonation, which may cause overestimation as much as 50% for old structure.

Although, rebound hammer provides a quick inexpensive means of checking the uniformity of concrete, it has serious limitations and these must be recognized. The results are affected by:

i. Weak & delaminating Concrete: As the test requires a flat surface and large number of readings to reduce variability, this test is not generally suitable for use on spalled concrete surfaces of distressed structures. However, comparison of Rebound numbers, which indicate the near surface hardness of the concrete, will help to identify relative surface weaknesses in cover concrete and also can be used to determine the relative compressive strength of concrete. Locations possessing very low rebound numbers will be identified as weak surface concrete and such locations will be identified for further investigations like corrosion distress, fire damage and/or any other reason including original construction defects of concrete.

ii. Smoothness of surface under test: Surface texture has an important effect on the accuracy of the test results. When a test is performed on a rough textured surface, the plunger tip causes excessive crushing and a reduced rebound number is measured. More accurate results can be obtained by grinding a rough surface to uniform smoothness with a carborundum stone.

iii. Size, shape and rigidity of the specimen: If the concrete section or test specimen is small, thin and slender is prone to movement under the impact and this movement will lower the rebound readings. In such cases the member needs to be rigidly held or backed up by a heavy mass.

iv. Age of specimen: It is emphasized that when old concrete is to be tested, direct correlations are necessary between the rebound numbers taken on the structure and the compressive strength of cores taken from the structure. Experimental studies has indicated that the rate of gain of surface hardness of concrete is rapid up to the age of 7 days, following which there is little or no gain in the surface hardness; however, for a properly cured concrete, there is significant strength gain beyond 7 days. It has been confirmed that for equal strength, higher rebound values are obtained on 7-day-old concrete than on 28-day-old concrete. The use of the Schmidt hammer for testing low-strength concrete at early ages, or where concrete strength is less than 7 MPa, is not recommended because rebound numbers are too low for accurate reading and the test hammer badly damages the concrete surface.

v. Surface and internal moisture condition of the concrete: The degree of saturation of the concrete and the presence of surface moisture has a decisive effect on the evaluation of test hammer results. Studies has demonstrated that well-cured, air-dried specimens, when soaked in water and tested in the saturated surface-dried condition, show rebound readings lower than when tested dry. When the same specimens were left in a room at 70°F (21.1°C) and air dried, they recovered ~3 points in 3 days and ~5 points in 7 days.

vi. Type of coarse aggregate: It is generally agreed that the rebound number is affected by the type of aggregate used. For equal compressive strengths, concretes made with crushed limestone coarse aggregate show rebound numbers lower than those for concretes made with gravel coarse aggregate, representing lower compressive strength. Lightweight concrete made with expanded shale aggregate yielded at equal compressive strengths, different rebound numbers from concrete made with pumice aggregate. But for any given type of lightweight aggregate concrete, the rebound numbers proved to be proportional to the compressive strength.

vii. Type of cement: The type of concrete significantly affects the rebound number readings. High alumina cement concrete can have actual strengths 100% higher than those obtained using a correlation curve based on concrete made with ordinary portland cement. Also, supersulfated cement concrete can have 50% lower strength than obtained from the ordinary portland cement concrete correlation curves.

viii. Type of mould: When cylinders cast in steel, tin can, and paper carton molds were tested, there was no significant difference in the rebound readings between those cased in steel molds and tin can molds, but the paper carton-molded specimens gave higher rebound numbers. This is probably due to the fact that paper molds withdraw moisture from the fresh concrete, thus lowering the water-cement ratio at the surface and resulting in a higher strength. As the hammer is a surface hardness tester, it is possible in such cases for the hammer to indicate an unrealistically high strength. It is therefore suggested that if paper carton molds are being used in the field, the hammer should be correlated against the strength results obtained from test cylinders cast in similar molds.

ix. Carbonation of concrete surface: Surface carbonation of concrete significantly affects the Schmidt rebound hammer test results. Rebound measurement depends on the hardness of surface of the concrete. An increase in surface hardness increases the rebound values. The carbonation effects are more severe in older concretes when the carbonated layer can be several millimeters thick and in extreme cases up to 20 mm thick. In such cases, the rebound numbers can be up to 50% higher than those obtained on an uncarbonated concrete surface. The increase of this surface hardness, however, has no influence on the compressive strength of the sample or structural concrete. Therefore, the rebound values measured on a carbonated surface must be reduced by a certain time factor or Suitable correction factors otherwise overestimation of concrete strength will result. Concrete strength becomes harder close to the surface.

The rebound hammer developed by Schmidt provides an inexpensive and quick method for non-destructive testing of concrete in the laboratory and in the field. This test is conducted to assess the relative strength of concrete based on the hardness at or near its exposed surface.The limitations of the Schmidt hammer should be recognized and taken into account when using the hammer. It cannot be overstressed that the hammer must not be regarded as a substitute for standard compression tests but rather as a method useful in checking uniformity of concrete and comparing one concrete against another but it can only be used as rough indication of concrete strength in absolute terms. Estimation of surface strength of concrete by the rebound hammer within an accuracy of ±15 to ±20% may be possible only for specimens cast, cured, and tested under conditions similar to those from which the correlation curves are established. Rebound hammers test the surface hardness of concrete, which cannot be converted directly to compressive strength, flexural strength, elastic modulus, etc and also because of the inherent uncertainty of estimating strength with a rebound number, the test is not intended as the basis for acceptance or rejection of concrete.