INVESTIGATION OF THE
SCHMIDT CONCRETE TEST HAMMER
TA
7
-W34m
6-267
1958
MISCELLANEOUS PAPER NO. 6-267
June I958
o>VV
Ji? o> CP
45
S. Army Engineer Waterways Experiment Station
C ORP S O F EN G IN EE RS
Vicksburg, Mississippi
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Evaluating Concrete Quality and Strength using Schmidt Hammer: A Comparative Study, Lecture notes of Construction
An investigation on the correlation between Schmidt hammer readings and concrete properties such as compressive strength, modulus of elasticity, and velocity. The study involved testing concrete specimens with different cement factors and aggregate types. The results showed that hammer readings could be used as an indicator of concrete quality and strength, with some variations depending on surface moisture and hammer position.
What you will learn
- How does surface moisture affect hammer readings?
- What is the purpose of the investigation presented in the document?
- What were the results of the investigation regarding the correlation between hammer readings and concrete properties?
- How were the concrete specimens prepared for the investigation?
- What are the implications of the study for evaluating concrete quality and strength in practice?
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INVESTIGATION OF THE
SCHMIDT CONCRETE TEST HAMMER
TA
-W34m
MISCELLANEOUS PAPER NO. 6-
June I
o>VV
Ji? o> CP
45
S. Army Engineer Waterways Experiment Station C O R P S O F E N G IN E E R S Vicksburg, Mississippi
LIBRARY
MAY 24 1968
Bureau of Reclamation Denver, Colorado
Miscellaneous Papers art information of considéra but not of a scope suffit Technical Reports or L are reproduced in limit copy to an
PREFACE
The tests described, in this paper were conducted as a part of Item CW 600, General Concrete Research, of the Corps of Engineers Civil Works Investigation program. The tests were conducted at the Waterways Experi ment Station under the general supervision of Mr. T. B. Kennedy, Chief, Concrete Division, and Messrs. J. M. Polatty and B. Mather, Chiefs of the Concrete and Special Investigation Branches, respectively. Messrs. W. 0. Tynes, E. E. McCoy, Jr., and R. A. Bendinelli actively participated in the investigations. This report was prepared by Mr. C. H. Willetts.
CONTENTS P a g e P R E F A C E ............................................. H i SUMMARY....................................................................................................................................................... ........ PART I : BACKGROUND, PURPOSE, AND S C O P E ........................................................................ 1 B a c k g r o u n d .............................................................................................................................. 1
P u r p o s e a n d S c o p e ............................................................................................................... 2
PART I I : MATERIALS, CONCRETEMIXTURES, SPECIMENS, AND TESTS.... 3 M a t e r i a l s ................................................................................................................................... 3 C o n c r e t e M ix t u r e s a n d S p e c im e n s ............................................................................ 3 T e s t s ............................................................................................................................................. A PART I I I : DISCUSSION AND CON CLU SION S............................................................................ 6 D i s c u s s i o n .............................................................................................................................. 6 C o n c lu s io n s .............................................................................................................................. 8 REFERENCES ............................................................................................................................................. 9
TABLES 1-
PLATES 1 - h
INVESTIGATION OF THE SCHMIDT CONCRETE TEST HAMMER
PART I: BACKGROUND, PURPOSE, AND SCOPE
Background
**1. A simple, compact instrument or tool that can measure nonde- structively, and with adequate accuracy and reliability, a property of con crete that may be correlated with results obtained from some accepted stand ard of measurement of that quality, such as compressive or flexural strength, modulus of elasticity, or pulse velocity, has long been needed in concrete construction work. Numerous instruments and techniques have been developed, but none have been entirely satisfactory. One of these test methods that has survived is known as the "hammer test" or "drum test." In this test a concrete surface is struck smartly with a common hammer, and the quality of the concrete is judged by the smartness of rebound and the tone of the sound resulting from the blow. Such classification terms as "drummy" or "sharp" are outgrowths of such a method of testing.
- In 19^8 an instrument for concrete testing was developed by a Swiss engineer, Ernst Schmidt, and the data he obtained with this instru ment were presented to the Swiss Federal Materials Testing and Experiment Institute in 1950 and 1 9 5 1 * ^ ^ This instrument utilizes the principle of a constant force impelling a steel plunger or hammer of uniform cross- sectional area against a concrete surface. The amount of rebound of the hammer after striking the concrete is indicated on a graduated scale. The reading or number indicated on the scale has been designated as "R" and has been correlated by the inventor to the compressive strength of concrete. This correlation in the form of various graphs or curves is contained in an instruction booklet furnished with each instrument. Mr. Gordon W. Green of the Pacific Coast Aggregates, Inc., Fresno, Calif., presented data at the Seventh Regional meeting of the American Concrete Institute in October 195h that had been developed in using the Schmidt hammer to quickly and in expensively check the quality and strength of hardened concrete. The dis cussion of this paper indicated that a great deal of work remained to be
- Raised numbers refer to similarly numbered entries in list of references at the end of this report.**
done in order to prove that a definite range of hammer readings can he cor related with a definite range of the property or quality for which the con crete is being tested. Zoldners'^ work in 1 956 was comprehensive enough to permit the following statement, "The Schmidt hammer provides a simple, in expensive method of nondestructive testing of concrete in place. It cannot substitute for standard methods of compressive tests, but its accuracy is sufficient to determine probable strength limits of the concrete in the structure and detect low-strength batches provided it is calibrated properly O and used competently." Peterson and Stoll state that "the use of the con crete test hammer may save much of the cost and time involved in making of regular compressive strength tests if these tests are being made solely as a check on the constancy of quality of concrete." Two of the conclusions reached by Williams0 were that "concrete specimens from the same aggregate source, tested at the same age, and having nearly the same curing and stor age conditions will produce good correlation between rebound numbers and compressive strength ..." and "when many variables are considered, it would be erroneous to use the curve which was supplied with the instrument (or any modification of the curve) to predict the compressive strength of all con- crete." Mather concluded from tests conducted on precast concrete units in Iceland that "rebound numbers determined on Schokbeton building units are indicative of concrete of high compressive strength." Purpose and Scope
- The purpose of this investigation was to evaluate the rebound readings obtained with a Schmidt hammer on specially prepared concrete specimens by comparing these readings with results obtained by conventional test methods (under the same controlled test conditions) used to determine the qualities of hardened concrete. k. Panels and cylinders made from four concrete mixtures, in which the cement factor and type aggregate were varied, were tested in surface- wet and surface-dry conditions with the Schmidt hammer. The effect of the position (horizontal or vertical) in which the hammer was held to apply the blows was also investigated. The readings obtained were compared with re sults of strength, pulse velocity, and modulus of elasticity tests made on the cylinders and on beams, prisms, and cores cut from the panels.
k
**room where they remained for a period of days.
- After the panels had cured for** lk days, each side of each panel was marked off into twenty 1 -ft squares (k **rows across and 5 rows top to bottom). One side of each panel was then placed on a wet sand mat (6 in. thick) for l4 days. The sand mat was maintained in a saturated condition. This resulted in one surface being kept in a saturated condition while the other was allowed to air-dry. All surfaces were 28 days old before the start of any testing operations. Cylinders
- Eight 6 - by 12-in. cylinders were cast from each of the four con crete mixtures. All cylinders were placed in the curing room for l4 days (wet cured), after which four from each concrete mixture were removed and air-dried in the laboratory for l4 days longer (wet-dry cured). Cores, prisms, and beams**
10. After the panels had been tested, as described subsequently, cores, prisms, and beams were cut from them for additional tests. These test specimens are described in the following subparagraphs. a. Ten 6 - by 12-in. cores were extracted from panels 1, 2, and 3, and 20 from panel 4. Half of the cores from each panel were placed in the curing room for an additional 7 days of moist curing; the other half were air-dried in the labora tory for the same length of time. b. A 60- by 12- by 6 -in. section was sawed from one side of each panel and cut into five 12 - by 12 - by 6 -in. prisms. Three of the five prisms were placed in limewater for seven days, and two were air-dried in the laboratory for seven days. c. The other 60- by 12- by 6 -in. side of each panel was sawed — into four 6 - by 6 - by 60 -in. beams, two of which were soaked in limewater and two were air-dried for a period of seven days.
Tests
Panels
11. Twenty-eight days after casting, each 1-ft square area on both sides of each panel was tested 30 times with the Schmidt hammer, 15 times with the panel in a vertical position and the hammer horizontal and 15 times with the panel in a horizontal position and the hammer vertical,
5
making a total of 1200 individual readings for each panel. The hammer blows were carefully randomized over the 1-ft squares. All tests were made in accordance with the manufacturer's recommendations. The average Schmidt- hammer readings for each square of each panel are listed in tables 2-5. Cylinders
**12. All cylinders were tested for compressive strength and modulus of elasticity at 28 days age. Results of these tests are found in tables 2-5- Cores
- All cores were tested for compressive strength and modulus of elasticity at 35 days age. Results of these tests are found in tables 2-5. Prisms
- After the final 7-hay curing period had been completed, the 6- in. and 12-in. cast faces of the prisms were capped and the prisms placed in the compression machine. Hammer readings were taken at zero load and at every additional 250 -psi loading up to 70 per cent of the ultimate con crete strength as judged by the compressive strength of the cores. Zero load was defined for this test as that compressive load necessary to pre vent movement of the prisms when subjected to impact of the hammer. Five hammer readings on each of the four faces at each load increment and three soniscope readings across both the 12- by 12-in. faces and the 12- by 6-in. faces for each load increment were obtained. Results of these tests are found in tables 2-5. Beams
- After the final 7-hay curing period, each beam was placed on the floor, and hammer readings were obtained on each of the four sides of the beam with the hammer held in a vertical position. Inadvertently, hammer tests were not conducted on the beams from panel 2. Sonic modulus of elasticity E was determined on all beams before they were tested in third-point flexure. Results of these tests are found in tables 2-5.**
7
providing the surface upon which the tests are conducted does not change from wet to dry or vice versa. The influence exerted on the hammer read ings by changes in surface condition is also shown in plates 1 and 2.
- From the data shown in plates 1 and 2, it may be stated that a relationship could be established between hammer readings and the strengths developed by compression tests on concrete cylinders and cores made from the concrete mixtures used for panels 1, 2, 3, and k. In some cases, the curves drawn to best fit the data indicate that a rebound difference of one digit will result in a loss or gain in compressive strength of 500 psi.
- A comparison of the data developed from tests of cores from the structures and the data developed from standard concrete cylinders as a basis for evaluating the hammer readings showed that the core data did not alter any conclusions based only on the data developed from the cylinders.
22. Plate 4 indicates that there is no correlation between hammer
readings and soniscope velocity determinations except that as the hammer readings become progressively larger, the soniscope may or may not indicate a change in the velocity of the transmitted impulse. No relationship is indicated by this limited study between cement content, compressive strength, sound velocity, and hammer readings. Neither are the soniscope determinations nor the hammer-rebound readings influenced by any super imposed loads. 23* Plate 3 shows the relative position of the data developed by this limited series of tests with respect to the data furnished by the manufacturer regarding the correction curve to be used if the hammer is held horizontally or vertically during the test. The data indicate that with the hammer in a horizontal position, a difference of approximately 15C0 psi in compressive strength may result in tests of the same concrete depending on whether the surface of the concrete is wet or dry. The manu facturer's data do not indicate that this might occur. In the cases where the hammer is held vertically and used on wet surfaces, the manufacturer's data do not vary greatly over-all from the data developed by this series of tests except in the very high compressive strength range. However, such is not the case where the hammer is held vertically and used on dry surfaces.
2k. The data developed from the tests of the 6- by 6- by 60-in.
beams cut from each panel indicated the same general trend as did the cores
8
and cylinders: increased hammer readings with increased strength, lower readings on wet surfaces than on dry surfaces. Readings were taken only with hammer held in a vertical position. As stated earlier^ no readings were taken on beams cut from panel 2. Readings obtained on the beams from panel ^ (traprock coarse aggregate, natural silica sand, and a cement factor of 5.0 bags per cu yd) agree with the cylinder data in that they are higher than those from panel 1 (cement factor of 3.5 tags per cu yd and limestone aggregate) and are lower than panel 3 (cement factor of 6.5 bags per cu yd and limestone aggregate). The test data for the beams are given
in tables 2-5-
Conclusions
25. From the data derived in this series of tests, it may be stated that: a. The hammer-rebound readings will increase with an increase — in strength of the concrete irrespective of hammer position (horizontal or vertical) or surface condition of the concrete. b. Hammer readings taken on dry concrete surfaces will be gen- — erally higher than readings taken on wet surfaces. c. Hammer readings taken with the hammer held in a horizontal ~ position are generally higher than those obtained with the hammer in a vertical position. d. Hammer readings are not influenced directly by aggregate specific gravity. e. Hammer readings are influenced more by surface moisture than by hammer position. f. Hammer readings could not be correlated with sound velocity — test data except in the most general terms. g. Hammer readings are not influenced by imposed loads other than those necessary to secure the specimen firmly when the hammer is used. h. Impact rebound hammers should be checked and calibrated — against each and every type of concrete on which they are to be used. Curves or data developed by these tests should be used instead of the manufacturer's data which is con tained in an instruction booklet (furnished with each hammer) that contains graphs derived from a statistical analysis of the general data available on hammer performance.
Table 1 Aggregate and Concrete Mixture Data
Physical Property or Mixture Component
______________ Panel 1 ___ 2 3^4
Aggregate Data*
Physical property
Specific gravity
Fine aggregate 2.66 2.66 2.66 2.
Coarse aggregate 2.69 2.69 2.69 2.
Absorption, $
Fine aggregate 1.2 1.2 1.2 0.
Coarse aggregate (^) o .4 0 .4 0 .4 (^) 1.
Concrete Mixture Data
Physical property Average Values from Four Tests
Slump, in. (^) 2 - 1 / b 2-1/4 (^) 2 - 1/4 2-I/ Bleeding, (^) 7.3 1.6 (^) 1.6 (^) 6.
Theo. unit wt, lb/cu ft 153.7 1 5 4. 5 I53.9 160.I
Actual unit wt, lb/cu ft (^) 1^5.6 (^) 147.1 (^) 145-9 151. Air content, $ (^) 5.3 4 .8 5-2 (^) 5- Sand/aggregate ratio, c/o (^) h i (^) 35 34 42 Theo. cement factor, b/cu yd (^) 3-7 5.3 6.85 (^) 5. Actual cement factor, b/cu yd (^) 3.51 5.05 6 .4 9 (^) 5. W/C ratio, gal/bag (^) 8.1 (^) 5 .7 4 .8 (^) 5. 5 w/c ratio by wt (^) 0.72 (^) 0.50 0.43 (^) 0.
Mixture component Weight, lb/cu yd
Cement (^) 329.5 474.7 610.I (^) 472.
Sand 1365.5 II29.2 I O 3 I. 3 1337.
Coarse aggregate
No. ^ to 3 /^-in. (^) 1081.3 (^) II52.9 1101.4 1103. 3 /A-- to l-l/ 2 -in. (^) 918.0 978.7 (^) 935-8 (^) 955. Water (^) 236.5 239.7 259.6 (^) 230. Total (^) 3930.8 (^) 3972.7 3938.2 (^) 4098.
*** Panels** 1 , 2 , and 3 contained limestone fine and coarse aggregates, panel 4 contained natural fine and traprock coarse aggregate.
Table 2 Summary of Hammer Rebound Readings and Strength, Modulus of Elasticity, and Velocity Data Obtained from Hardened Concrete Panel 1 Concrete Mixture: Limestone Fine and Coarse Aggregate, 3»5 Bags Cement/cu yd Cylinder Data Core Data Panel Rebound Readings* 20 -day Compr 35 -day Compr Cone Hammer Surf. Hammer Wet ConeHammer Surf. Hammer Dry Strength,Wet- W - D * ^ psi^ Static^ Strength,^ psi^ Static Vert Horiz^ Vert^ Horiz^ cured^ Cured^ x^ E^10^^ -6^ çuredWet-^ W-DCured^ x^ ®^10 - 15^16 20^23^1920 3 .^ 3^^2220 3.8 7 17^20 22 26 l6l0^ 3 .3 0^^1800 3. 18 20 22 25^1820 2 .6 7^ I75O^ 3. to 20 22 22 26 1680 3 .1 1 I98O^ 3.7 9 (^22) 25 27 29^2220 3.20^ 28to^ 5. l b (^16) 19 23^1710 2.95^^2020 3. I k^18 20 22 1820 3.19^ 23^0^ 3. I k (^18) 19 25^1780 3.02^2660 5. (^16) 19^20 2 b^^2220 3. 20 2 b^ 23 25 223O^ b. 2 9 l b l6 (^) 19^22 li. 18 22 2 b 15^18 21 2 b 17^18 22^25 22 22 22 27 I k l6 21 23 15^18^23 2 b l6 (^18) 23 2 b 17^20^23 21 2 k^ 2 b^^30
A v g l6.8 (^) 19 .3 21 .7 2 b. 9^^1760 1830 2120 229O SDf 2. k (^) 2 .7 1-9 2.1^ 120.5^ 198.8^ 396.8^ 210. CVttl ^ -3 ltoO^ 8.8^ Q. k^ 6.8^ 10.6^ I8.7^ 9.
Test Load lb
Hammer Rdngs Cone Cone Surf. Wet Surf.Dry
Prism Data (Pulse) Velocity (fps x 10 ~ 3 ) (^) StrengthCompr. psi
Flexural^ Beam Strength, psi
Data Sonic x IO"
Hammer 12- x 12-^ Read ings in. face 6- in. x face12-^ Wet- cured W-D Cured 0 20 16.5 15.3 1730 to^5 5.5O^^2 b 25O 21 16.2^ 15 .^ to^5 5OO 20 16.3 15 .^^ too^^18 750 19 1 6. k^ 15.3 350 18 1000 21 16.2^ 15. 0 29 16.2^ 15.3^2570 25O (^) 31 16.7 15.3 52O^ 5.26^26 5OO 32 16.^ 15.3 to5^26 750 31 *** 16.5^ 15.3^^3 to^^25 1000 32 16.5 15. 0 2 b 16.^^ 15.7 2670 250^2 b^ 16.3^ 15.8^ 5to^ 5 .1 1^^22 500 26 1 6. k^ 15.7^370 (^750) 25 16.5^ 15.8^ too^19 (^1000) 25 16.^ k^ 15. 0 32^1 6. b^ 16.O^^3310 (^250) 3 ^ 16.7 16.O^ too^ 5.01^28 500 36 16.8^ 16.I^ too^25 750 35 16.6^ I6.I^^355 1000 3 ^ 17.O^ 16.O^^365 (^0) 27 17.3 16.6^ 237O 250 28 17.3 16. (^500 28) 17.3 1 6. 750 28 17.7 16. 1000 28 17.6^ 16.
*** Each hammer reading reported is the average of the** 10 best suited of 15 readings. ** t Wet-dryStandard cured, deviation. ■ft Coefficient of variation.