Introduction
One of the most popular non-destructive methods of concrete testing in the Baltic States is carried out by using the Schmidt rebound hammer. The use of this method is practised on large scale on building sites throughout Latvia, Lithuania and Estonia. This method has gained its popularity by its simple use and the possibility of using it on a single concrete surface without reuiring access to the construction from both sides, as is necessary for ultrasonic testing methods. The main uestion lies in the credibility of results acuired by the Schmidt rebound hammer testing method. !t is often a problem to determine the correlation between the rebound number and the actual compression strength of the construction, as a large number of variables influence the correlation between the rebound number and actual compression strength. These variables must be ta"en into consideration in order to acuire credible testing results. REBOUND HAMMER TEST ON CONCRETE
#ebound hammer test $Schmidt %ammer& is used to provide a convenient and rapid indication of the compressive strength of concrete. !t consists of a spring controlled mass that slides on a plunger within a tubular housing. The operation of rebound hammer is shown in the fig.'. (hen the plunger of rebound hammer is pressed against the surface of concrete, a spring controlled mass with a constant energy is made to hit concrete surface to rebound bac". The e)tent of rebound, which is a measure of surface hardness, is measured on a graduated scale. This measured value is designated as #ebound *umber $rebound inde)&. + concrete with low strength and low stiffness will absorb absorb more energy to yield in a lower rebound value.
Fig: Operation of the reound ha!!er
The
rebound
hammer
test
method
is
used
for
the
following
purposes
$a& To find out the li"ely compressive strength of concrete with the help of suitable corelations between rebound inde) and compressive strength. $b& To assess the uniformity of concrete. $c& To assess the uality of concrete in relation to standard reuirements. $d& To assess the uality of one element of concrete in relation to another. #ebound hammer test method can be used to differentiate the acceptable and uestionable parts of the structure or to compare two different structures based on strength. "rincip#e of Reound Ha!!er Te$t:
#ebound hammer test method is based on the principle that the rebound of an elastic mass depends on the hardness of the concrete surface against which the mass stri"es. The operation of the rebound hammer is shown in figure above. (hen the plunger of rebound hammer is pressed against the concrete surface, the spring controlled mass in the hammer rebounds. The amount of rebound of the mass depends on the hardness of concrete surface. Thus, the hardness of concrete and rebound hammer reading can be correlated with compressive strength of concrete. The rebound value is read off along a graduated scale and is designated as the rebound
number or rebound inde). The compressive strength can be read directly from the graph provided on the body of the hammer. U$ing The Reound Ha!!er
By Lu"e . Snell, E Senior aterials Engineer, (estern Technologies !nc., hoeni) +/ This article was published in the roceedings of The ''th +nnual ongolian 0oncrete 0onference, 1une 23'2. 0ontact Lu"e Snell at l.snell4wt-us.com if you have uestions about this article. The rebound hammer is one of the most popular nondestructive testing methods used to investigate concrete. !ts popularity is due to its relatively low cost and simple operating procedures. The rebound hammer is also one of the easiest pieces of euipment to misuse5 thus, many people do not trust the rebound test results. This article is to discuss the rebound hammer and how to successfully use it to evaluate concrete. rinciples of The #ebound %ammer Ernst Schmidt, a Swiss engineer, developed the modern rebound hammer in '678. The rebound hammer measures the surface hardness of the concrete. This is accomplished by placing the rebound hammer plunger against the concrete surface and releasing a spring loaded weight. The amount the plunger rebounds or bounces bac" is measured. This rebound number is shown on a scale and will be between '3 and '33. The !mpact %ammer is another name for Schmidt %ammer. The surface of concrete gets harder as concrete gains strength5 thus, we have a method of estimating the strength of concrete. + low rebound number will indicate that the surface of the concrete is soft and the concrete is wea". + high rebound number will indicate that the concrete is hard and strong. 9nfortunately, there is no theoretical relationship between surface hardness and the strength of concrete. any things can affect concrete surface hardness, this is discussed later in this article. %owever, +0! :'8-'', Building 0ode #euirements for Structural 0oncrete and 0ommentary $#;.<.;& states =*ondestructive tests of the concrete in place, such as by probe penetration, impact hammer, ultrasonic pulse velocity, or pullout may be useful in determining whether or not a portion of the structure actually contains low-strength concrete. Such tests are of value primarily for comparisons within the same >ob rather than as uan titative measures of strength.? Factor$ That Affect$ Reound Ha!!er Nu!er$
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 a nd thus, the rebound hammer numbers. These factors include '. Smoothness of the surface
@. 0oarse aggregates
2. SiAe and shape of the concrete sample
8. Type of cement
:. The rigidity of the test area
6. orms used
7. +ge of the concrete
'3. 0arbonation
;. Surface moisture
''. Location of the reinforcement
<. !nternal moisture $moisture gradient&
'2. roAen concrete
or these reasons, the user of the rebound hammer must follow e)act procedures and use
engineering >udgment. To illustrate this, the following chart shows how the effects of the
coarse aggregates in concrete of the same strength can have on the rebound hammer. U$ing The Reound Ha!!er To %ocate Re&uiring Additiona# In'e$tigation$
One of the ways to use the rebound hammer is to locate those areas that may need additional investigation. !n this procedure the round hammer is used at several locations to identify those areas that have a lower rebound number. Since the structure would have the same mi)ture, curing history, moisture content, etc., the rebound hammer can identify those areas that appear to have the wea"est concrete $lowest rebound hammer number&.
Co!pari$on of Reound Nu!er$ Re$u#t$
+nother procedure used is to compare rebound numbers of the concrete that you "now is acceptable from a recent placement. This part of the structure has the concrete already evaluated by cores, cylinders or cubes and the concrete strength met the pro>ect reuirements. !n this procedure you would determine the rebound numbers of the concrete "nown to be acceptable. The investigator would then test the concrete with the rebound hammer that needed to be investigated. !f the rebound numbers for concrete being investigated are appro)imately the same or higher than the concrete that had met the pro>ect specifications, the tested concrete can be determined to be acceptable. !f the rebound numbers in the area being tested are lower, then additional investigations would need to be done by the engineer. Ne( De'e#op!ent$ In The Reound Ha!!er
The rebound hammer has had several changes over the years. Some of the latest improvements have been to ma"e the rebound hammer lighter5 use some of the aerospace higher strength metals5 install computer chips to calculate automatically the averages and standard deviations of the readings. +lthough the rebound hammer has gone through several changes and is an e)tremely useful nondestructive testing tool, the user must recogniAe that the rebound hammer measures only surface hardness of concrete. Engineers must determine how to use this information in their investigation of the concrete structure. Engineering )udg!ent and Conc#uding Re!ar*$
The rebound hammer must be recogniAed for what it is able to measure C the surface hardness of concrete. (hen used as part of an investigative process that includes an understanding of concrete being tested, a visual inspection, and documentation from cylinders, cubes or cores, it can be an e)cellent nondestructive testing method. !t is an instrument that reuires engineering >udgment to interpret the reading and to accurately assess the concrete. Engineering >udgment can only be used when an e)act procedure sought as the one outlined in +ST 083; is followed.
Ad'antage$ and di$ad'antage$
Rebound Hammer test. .
$'& Advantages. (a) Simple to use. *o special e)perience is needed to conduct the test. (b) Establishes uniformity of properties. (c) Euipment is ine)pensive and is readily available. !t is relatively simple and ine)pensive to conduct a large number of tests. The euipment for the test is readily available. $2& Disadvantages. (a) Evaluates only the local point and layer $wythe& of masonry to which it is applied. (b) *o direct relationship to strength or deformation properties. (c) 9nreliable for the detection of flaws. (d) Evaluates only the layer $wythe& of masonry to which it is applied, and is unreliable for detection of flaws or for investigation of inaccessible masonry wythes
ht t p: / / bui l di ngc r i t er i a1. t pub. c om/ uf c _3_310_05a/ uf c _3_310_05a0106. ht m
#EBO9*D TEST
The rebound hammer is a surface hardness tester for which an empirical correlation has been established between strength and rebound number. The only "nown instrument to ma"e use of the rebound principle for concrete testing is the Schmidt hammer, which weighs about 7 lb $'.8 "g& and is suitable for both laboratory and field wor". !t consists of a spring-controlled hammer mass that slides on a plunger within a tubular housing. The hammer is forced against the surface of the concrete by the spring and the distance of rebound is measured on a scale. The test surface can be horiAontal, vertical or at any angle but the instrument must be calibrated in this position. 0alibration can be done with cylinders $< by '2 in., '; by :3 cm& of the same cement and aggregate as will be used on the >ob. The cylinders are capped and firmly held in a compression machine. Several readings are ta"en, well distributed and reproducible, the average representing the rebound number for the cylinder. This procedure is repeated with several cylinders, after which compressive strengths are obtained. Limitations and Advantages. The Schmidt hammer provides an ine)pensive, simple and uic" method of obtaining an indication of concrete strength, but accuracy of '; to 23 per cent is possible only for specimens cast cured and tested under conditions for which calibration curves have been established. The results are affected by factors such as smoothness of surface, siAe and shape of specimen, moisture condition of the concrete, type of cement and coarse aggregate, and e)tent of carbonation of surface. http://theconstructor.org/practical-guide/non-destructive-testing-of-concrete/5553/
SCHMIDT REBOUND HAMMER TEST
FUNDAMENTA% "RINCI"%E
The Schmidt rebound hammer is principally a surface hardness tester. !t wor"s on the principle that the rebound of an elastic mass depends on the hardness of the surface against which the mass impinges. There is little apparent theoretical relationship between the strength of concrete and the rebound number of the hammer. %owever, within limits, empirical correlations have been established between strength properties and the rebound number. urther, Fole" has attempted to establish a correlation between the hammer rebou nd number and the hardness as measured by the Brinell method. E+UI"MENT FOR SCHMIDT,REBOUND HAMMER TEST
The Schmidt rebound hammer is shown in ig. 7.'. The hammer weighs about '.8 "g and is suitable for use both in a laboratory and in the field. + schematic cutaway view of the rebound hammer is shown in ig. 7.2. The main components include the outer body, the plunger, the hammer mass, and the main spring. Other features include a latching mechanism that loc"s the hammer mass to the plunger rod and a sliding rider to measure the rebound of the hammer mass. The rebound distance is measured on an arbitrary scale mar"ed from '3 to '33. The rebound distance is recorded as a =rebound number? corresponding to the position of the rider on the scale.
!G. 7.'. Schmidt rebound hammer. -ENERA% "ROCEDURE FOR SCHMIDT REBOUND HAMMER TEST
The method of using the hammer is e)plained using ig. 7.2. (ith the hammer pushed hard against the concrete, the body is allowed to move away from the concrete until the latch connects the hammer mass to the plunger, ig. 7.2a. The plunger is then held perpendicular to the concrete surface and the body pushed towards the concrete, ig. 7.2b. This movement e)tends the spring holding the mass to the body. (hen the ma)imum e)tension of the spring is reached, the latch releases and the mass is pulled towards the surface by the spring, ig. 7.2c.The mass hits the shoulder of the plunger rod and rebounds because the rod is pushed hard against the concrete, ig. 7.2d. During rebound the slide indicator travels with the hammer mass and stops at the ma)imum distance the mass reaches after rebounding. + button on the side of the body is pushed
to loc" the plunger into the retracted position and the rebound number is read from a scale on the body.
A""%ICATIONS OF SCHMIDT REBOUND HAMMER TEST
The hammer can be used in the horiAontal, vertically overhead or vertically downward positions as well as at any intermediate angle, provided the hammer is perpendicular to the surface under test. The position of the mass relative to the vertical, however, affects the rebound number due to the action of gravity on the mass in the hammer. Thus the rebound number of a floor would be e)pected to be smaller than that of a soffit and inclined and vertical surfaces would yield intermediate results. +lthough a high rebound number represents concrete with a higher compressive strength than concrete with a low rebound number, the test is only useful if a correlation can be developed between the rebound number and concrete made with the same coarse aggregate as that being tested. Too much reliance should not be placed on the calibration curve supplied with the hammer since the manufacturer develops this curve using standard cube specimens and the mi) used could be very different from the one being tested. RAN-E AND %IMITATIONS OF SCHMIDT REBOUND HAMMER TEST
+lthough the rebound hammer does provide a uic", ine)pensive method of chec"ing the uniformity of concrete, it has some serious limitations. The results are affected by '. Smoothness of the test surface %ammer has to be used against a smooth surface, preferably a formed one. Open te)tured concrete cannot therefore be tested. !f the surface is rough, e.g. a trowelled surface, it should be rubbed smooth with a carborundum stone
2. SiAe, shape and rigidity of the specimen !f the concrete does not form part of a large mass any movement caused by the impact of the hammer will result in a reduction in the rebound number. !n such cases the member has to be rigidly held or bac"ed up by a heavy mass. :. +ge of the specimen or eual strengths, higher rebound numbers are obtained with a @ day old concrete than with a 28 day old. Therefore, when old concrete is to be tested in a structure a direct correlation is necessary between the rebound numbers and compressive strengths of cores ta"en from the structure. #ebound testing should not be carried out on low strength concrete at early ages or when the concrete strength is less than @ a since the concrete surface could be damaged by the hammer. 7. Surface and internal moisture conditions of concrete The rebound numbers are lower for well-cured air dried specimens than for the same specimens tested after being soa"ed in water and tested in the saturated surface dried conditions. Therefore, whenever the actual moisture condition of the field concrete or specimen is un"nown, the surface should be pre-saturated for several hours before testing. + correlation curve for tests performed on saturated surface dried specimens should then b e used to estimate the compressive strength. ;. Type of coarse aggregate Even though the same aggregate type is used in the concrete mi), the correlation curves can be different if the source of the aggregate is different. +n e)ample is shown in ig. 7.; where correlation curves for four different sources of gravel are plotted. ig. 7.< shows the considerable difference that can occur between correlation curves dev eloped for different aggregate types.
!G. 7.;. Effect of gravel from different sources on correlation curves.
!G. 7.<. 0omparison between correlation curves for crushed limestone and siliceous.
<. Type of cement %igh alumina cement can have a compressive strength '33H higher than the strength estimated using a correlation curve based on ordinary ortland cement. +lso, super sulphated cement concrete can have strength ;3H lower than ordinary ortland cement. @. 0arbonation of the concrete surface !n older concrete the carbonation depth can be several millimeters thic" and, in e)treme cases, up to 23 mm thic". !n such cases the rebound numbers can be up to ;3H higher than those obtained on an uncarbonated concrete surface. http://www-pub.iaea.org/mtcd/publications/pdf/tcs-17_web.pdf INTRODUCTION
I The rebound hammer test is one of the non-destructive tests used to chec" the compressive strength of concrete. I +n empirical relationship has been determined between the absorbed by the concrete when given a high impact and its compressive strength. I The rebound hammer is designed to carry out instant non-destructive test on concrete structure without damage and gives an immediate indication of the compressive strength of the concrete using the calibration curve applied each instrument. I The hammer is simply pressed firmly against the concrete whereupon a powerful internal spring is first compressed and thin tripped to deliver a hammer blow through the hardened concrete trip to the surface being tested.
Reound ha!!er te$t
I #ebound hammer test is done to find out the compressive strength of concrete by using rebound hammer as per !S '::'' $art 2& J '662. I rinciple of the rebound hammer test is The rebound of an elastic mass depends on the hardness of the surface against which its mass stri"es. I (hen the plunger of the rebound hammer is pressed against the surface of the concrete, the Spring-controlled mass rebounds and the e)tent of such a rebound depends upon the surface hardness of the concrete. IThe surface hardness and therefore the rebound is ta"en to be related to the compressive strength of the concrete. IThe rebound value is read from a graduated scale and is designated as the rebound number or rebound inde). IThe compressive strength can be read directly from the graph provided on the body of the hammer.
Reound Ha!!er te$t:
I+ssessing the li"ely compressive strength of concrete . I+ssessing the uality of concrete in relation to standard reuirements. I *DT
Interpretation of Re$u#t$:
The rebound reading on the indicator scale has been calibrated by the manufacturer of the rebound hammer for horiAontal impact.
RE%ATIONSHI" B,. REBOUND HAMMER AND COM"RESSI/E STREN-TH
Cue co!pre$$i'e $trength i$ N,$&0!! p#otted again$t reound nu!er
+& Strength +ssessment ITo assess the relative strength of concrete based on the hardness. I0asting cubes were tested under controlled conditions. IThis is due to hardening of concrete surface due to carbonation. I!t restricted to relatively new structures only. B& Survey of wea" and delaminating concrete I!t helps to identify relative surface wea"ness in cover concrete and to determine the relative compressive strength of concrete. IThis survey is carried by dividing the member into well-defined grid points.
IThe grid matri) should have a spacing of appro)imately :33mm ) :33mm.
AD/ANTA-E
ISimple to use. *o special e)perience is needed to conduct the test. IEstablishes uniformity of properties. IEuipment is ine)pensive and is readily available. I+ wide variety of concrete test hammers is available with an operational range of '3 to @3. Ior rehabilitation of old onuments DISAD/ANTA-E
IEvaluates only the local point and layer of masonry to which it is applied. I*o direct relationship to strength or deformation properties. I9nreliable for the detection of flaws. I0leaning maintenance of probe and spring mechanism
Conc#u$ion
IThe rebound value can be measured discretionary, whereas the number of crushed specimens is limited. I The combination of both methods is the best and most reliable procedure to determine the compressive strength of concrete structures. IThe method does not damage the structure li"e the classical method, where cores must be ta"en for the evaluation of the compressive strength. I!t is a fast, ine)pensive and easy to perform method using a light and portable test euipment.
https://www.academia.edu/83!5/"ebound_#ammer_$estpriciple_procedure_cons_and_pros.....
A$tract
One of the most widely spread techniues to estimate the c ompressive strength of concrete is the rebound hammer test, also "nown as Schmidt %ammer test. !n spite of a large number of scientific wor"s trying to calibrate the test, to identify the parameters affecting its results and to estimate its reliability, the original Schmidt curve is still provided by the producers along with the hammer and is used in Structural Engineering +pplications. This paper discussed an e)tensive research, and application, of this techniue to a large number of cubes provided by the Laboratory for Building aterials of the 9niversity of Genoa, !taly, showing that several phenomena strongly affect the test moisture content, maturity, stress state among the others. Strength estimates may differ as much as @3H if these parameters are not ta"en into account. Besides, several in situ investigations on e)isting buildings were affected by a large dispersion of data, so that we should conclude that the #ebound %ammer is unable of giving a reliable estimate of the concrete strength. This is probably due to the very limited area of the material on which the test is performed that allows also small local inhomogeneity to affect uite strongly the test. Therefore, the rebound hammer seems to be useless in the estimation of concrete
compressive strength, being only a rough tool for estimating material homogeneity inside a specific concrete type. Introduction
!n 0ivil Engineering practice, the estimation of concrete u ality is needed both for uality controls of new buildings and for rapid surveys of e)isting structures. +mong the *DT and DT procedures, the Schmidt, or rebound, %ammer test is largely the most commonly used worldwide. The reason for such a success is not the reliability of the tests, that may be easily showed to be less than :3H, but the simplicity of the procedure, the low price of the euipment and its easy of use. The strength estimation of concrete on the basis of its surface hardness dates bac" more than '33 years. *evertheless, a simple and low cost procedure was proposed only at the beginning of the ;3Ks gaining immediate attention from either the scientific and professional world. The worldwide use of the procedure soon raised some doubt on the reliability of the test so that a vast number of research pro>ects have been developed trying to better calibrate the Schmidt %ammer test, either dating bac" from the <3sK till recent years. + comprehensive bibliography can be found in !n the first years, calibration has been performed on a large number of specimens cured in standard conditions but without separating the contribution of the different factors affecting the test, such as concrete maturity and hardening conditions, moisture, surface finishing, concrete composition, aggregate type and hardness, etc. The fundamental assumption was that these parameters only slightly affect the strength estimate. Only recent wor"s, in the last two decades, separated the effects of different parameters. igure ' summariAes the up-to-date "nowledge, displaying the calibration curves that can be found in literature. The most recent results of scientific research show that the #ebound %ammer might provide some information on concrete uality provided that it is calibrated on the specific concrete type it is used on. 9nluc"ily, these conclusions did not yet enter common 0ivil Engineering practice. !n this paper, the calibration of the #ebound %ammer is studied by means of a series of laboratory and field tests gathering the e)perience of the Laboratory of Building aterials of the 9niversity of Genoa, !taly. Several parameters are ta"en into account surface finishing, moisture content, concrete maturity, distance from the free edges, dimension and mass of the structural element, stress state. 0alibration is performed either on concrete specimens specifically built for the research $ideal conditions& and on concrete cubes delivered to the laboratory for uality controls $actual commercial production&. +lso field data, from e)isting structures of different types and age, are considered in order to allow a rational estimation of the test reliability by comparison of the available data.
http://www.hrpub.org/download/%&131&/cea.%&13.&1&3&3.pdf
