1、Standard Method of Test for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading) AASHTO Designation: T 97-18 Technical Section: 3c, Hardened Concrete Release: Group 1 (April) ASTM Designation: C78/C78M-16 American Association of State Highway and Transportation Officials 444 No
2、rth Capitol Street N.W., Suite 249 Washington, D.C. 20001 TS-3c T 97-1 AASHTO Standard Method of Test for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading) AASHTO Designation: T 97-18 Technical Section: 3c, Hardened Concrete Release: Group 1 (April) ASTM Designation: C78/C78
3、M-16 1. SCOPE 1.1. This test method covers determination of the flexural strength of concrete by the use of a simple beam with third-point loading. 1.2. The values stated in SI units or inch-pound units are to be regarded separately as standard. The values stated in each system may not be exact equi
4、valents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in nonconformance with the standard. Note 1For methods of molding concrete specimens, see T 23 and R 39. 1.3. This standard does not purport to address all of the safety concern
5、s, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. 2. REFERENCED DOCUMENTS 2.1. AASHTO Standards: R 18, Establishing and Implementing a
6、Quality Management System for Construction Materials Testing Laboratories R 39, Making and Curing Concrete Test Specimens in the Laboratory T 23, Making and Curing Concrete Test Specimens in the Field T 24M/T 24, Obtaining and Testing Drilled Cores and Sawed Beams of Concrete T 231, Capping Cylindri
7、cal Concrete Specimens 2.2. ASTM Standards: C670, Standard Practice for Preparing Precision and Bias Statements for Test Methods for Construction Materials E4, Standard Practices for Force Verification of Testing Machines 2018 by the American Association of State Highway and Transportation Officials
8、. All rights reserved. Duplication is a violation of applicable law.TS-3c T 97-2 AASHTO 3. SIGNIFICANCE AND USE 3.1. This test method is used to determine the flexural strength of specimens prepared and cured in accordance with T 23, T 24M/T 24, or R 39. Results are calculated and reported as the mo
9、dulus of rupture. The strength determined will vary where there are differences in specimen size, preparation, moisture condition, curing, or where the beam has been molded or sawed to size. Note 2The measured modulus of rupture generally increases as the spe cimen size decreases.1,2,3The result of
10、this test method may be used to determine compliance with specifications or as a basis for proportioning, mixing, and placement operations. It is used in testing concrete for construction of slabs and pavements. 4. APPARATUS 4.1. Testing MachineThe testing machine shall conform to the requirements o
11、f sections on Basis of Verifications, Corrections, and Time Interval between Verification of ASTM E4. Hand-operated testing machines having pumps that do not provide a continuous loading in one stroke are not permitted. Motorized pumps or hand-operated positive displacement pumps having sufficient v
12、olume in one continuous stroke to complete a test without requiring replenishment are permitted and shall be capable of applying loads at a uniform rate without shock or interruption. The testing machine shall be equipped with a means of recording or holding the peak value that will indicate the max
13、imum load, to within 1 percent accuracy, applied to the specimen during a test. 4.2. Loading ApparatusThe third-point loading method shall be used in making flexure tests of concrete employing bearing blocks, which will ensure that forces applied to the beam will be perpendicular to the face of the
14、specimen and applied without eccentricity. A diagram of an apparatus that accomplishes this purpose is shown in Figure 1. Notes: 1 in. = 25.4 mm. This apparatus may be used inverted. If the testing machine applies force through a spherically seated head, the center pivot may be omitted, provided one
15、 load-applying block pivots on a rod and the other on a ball. Figure 1Diagrammatic View of a Suitable Apparatus for Flexure Test of Concrete by Third-Point Loading Method HeadofTestingMachineSteelBall25mmminSteelRodBedofTestingMachine25mmminOptionalPositionsforOneSteelRodandOneSteelBallLoad-Applying
16、andSupportBlocksSteelBallRigidLoadingStructureor,ifitis aloadingaccessory,SteelPlateorChannelSpanLength, LL/3 L/3 L/3Specimend=L/3 2018 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS-3c T 97-3 AASHTO 4.2
17、.1. All apparatus for making flexure tests of concrete should be capable of maintaining the specified span length and distances between load-applying blocks and support blocks constant within 1.3 mm (0.05 in.). 4.2.2. The ratio of the horizontal distance between the point of application of the load
18、and the point of application of the nearest reaction to the depth of the beam shall be 1.0 0.03. 4.2.3. If an apparatus similar to that illustrated in Figure 1 is used: 4.2.3.1. The load-applying and support blocks should not be more than 64 mm (21/2in.) high, measured from the center or the axis of
19、 pivot, and should extend entirely across or beyond the full width of the specimen. Each case-hardened bearing surface in contact with the specimen shall not depart from a plane by more than 0.05 mm (0.002 in.) and should be a portion of a cylinder, the axis of which is coincidental with either the
20、axis of the rod or center of the ball, whichever the block is pivoted upon. The angle subtended by the curved surface of each block should be at least 45 degrees (0.79 rad). 4.2.3.2. The load-applying and support blocks should be maintained in a vertical position and in contact with the rod or ball
21、by means of spring-loaded screws that hold them in contact with the pivot rod or ball. 4.2.3.3. The uppermost bearing plate and centerpoint ball in Figure 1 may be omitted when a spherically seated bearing block is used, provided one rod and one ball are used as pivots for the upper load-applying bl
22、ocks. 5. TEST SPECIMENS 5.1. The test specimen shall conform to all requirements of T 23, T 24M/T 24, and R 39. The specimen shall have a test span within 2 percent of being three times its depth as tested. The sides of the specimen shall be at right angles with the top and bottom. All surfaces shal
23、l be smooth and free of scars, indentations, holes, or inscribed identification marks. 5.2. Provided the smaller cross-sectional dimension of the beam is at least three times the nominal maximum size of the coarse aggregate, the modulus of rupture can be determined using different specimen sizes. Ho
24、wever, measured modulus of rupture generally increases as the specimen size decreases1,2,3(Note 3). Note 3The strength ratio for beams of different sizes depends primar ily on the maximum size of aggregate.3 Experimental data obtained in two different studies have shown that for maximum aggregate si
25、ze between 19.0 and 25.0 mm (3/4and 1 in.), the ratio between the modulus of rupture determined with a 152 by 152 mm (6 by 6 in.) and a 100 by 100 mm (4 by 4 in.) may vary from 0.90 to 1.071for maximum aggregate size between 9.5 and 37.5 mm (3/8and 11/2in.), the ratio between the modulus of rupture
26、determined with a 152 by 152 mm (6 by 6 in.) and a 115 by 115 mm (4.5 by 4.5 in.) may vary from 0.86 to 1.00.2 5.3. The specifier of tests shall specify the specimen size and number of specimens to be tested to obtain an average test result. 5.4. Since the modulus of rupture of a 100- by 100- by 355
27、-mm (4- by 4- by 14-in.) specimen is not equivalent to the modulus of rupture of a 152- by 152- by 533-mm (6- by 6- by 21-in.) specimen, the same specimen size shall be used for qualification and acceptance testing. 2018 by the American Association of State Highway and Transportation Officials. All
28、rights reserved. Duplication is a violation of applicable law.TS-3c T 97-4 AASHTO 6. PROCEDURE 6.1. Moist-cured specimens shall be kept moist during the period between removal from moist storage and testing (Notes 4 and 5). Note 4Surface drying of the specimen results in a reduction in the measured
29、flexural strength. Note 5Methods for keeping the specimen moist include wrapping in moi st fabric or matting, or keeping specimens under lime water in containers near the flexural testing machine until time of testing. 6.2. When using molded specimens, turn the test specimen on its side with respect
30、 to its position as molded and center it on the support blocks. When using sawed specimens, position the specimen so that the tension face corresponds to the top or bottom of the specimen as cut from the parent material. 6.2.1. Center the loading system in relation to the applied force. Bring the lo
31、ad-applying blocks in contact with the surface of the specimen at the third points and apply a load of between 3 and 6 percent of the estimated ultimate load. 6.2.2. Using 0.10-mm (0.004-in.) and 0.38-mm (0.015-in.) leaf-type feeler gauges, determine whether any gap between the specimen and the load
32、-applying or support blocks is greater or lesser than each of the gauges over a length of 25 mm (1 in.) or more. Grind, cap, or use leather shims on the specimen contact surface to eliminate any gap in excess of 0.10 mm (0.004 in.) in width. Leather shims shall be of uniform 6.4 mm (0.25 in.) thickn
33、ess, 25 to 50 mm (1 to 2 in.) width, and shall extend across the full width of the specimen. Gaps in excess of 0.38 mm (0.015 in.) shall be eliminated only by capping or grinding. Grinding of lateral surfaces should be minimized in as much as grinding may change the physical characteristics of the s
34、pecimens. Capping shall be in accordance with T 231. 6.3. Load the specimen continuously and without shock. The load shall be applied at a constant rate to the breaking point. Apply a load at a rate that constantly increases the maximum stress on the tension face between 0.9 and 1.2 MPa/min (125 and
35、 175 psi), until rupture occurs. The loading rate is calculated using the following equation: r = Sbd2/L (1) where: r = loading rate, N/min (lb/min); S = rate of increase in extreme fiber stress, MPa/min (psi/min); b = average width of specimen mm (in.); d = average depth of specimen mm (in.); and L
36、 = span length, mm (in.). 7. MEASUREMENT OF SPECIMENS AFTER TEST 7.1. To determine the dimensions of the specimen cross section for use in calculating modulus of rupture, take measurements across one of the fractured faces after testing. The width and depth are measured with the specimen as oriented
37、 for testing. For each dimension, take one measurement at each edge and one at the center of the cross section. Use the three measurements for each direction to determine the average width and the average depth. Take all measurements to the nearest 1.3 mm (0.05 in.). If the fracture occurs at a capp
38、ed section, include the cap thickness in the measurement. 2018 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS-3c T 97-5 AASHTO 8. CALCULATIONS 8.1. If the fracture initiates in the tension surface within
39、 the middle third of the span length, calculate the modulus of rupture as follows: 2RPlbd= (2) where: R = modulus of rupture, kPa (psi); P = maximum applied load indicated by the testing machine, N (lbf); l = span length, mm (in.); b = average width of specimen mm (in.); and d = average depth of spe
40、cimen mm (in.). Note 6The weight of the beam is not included in the above calculatio n. 8.2. If the fracture occurs in the tension surface outside of the middle third of the span length by not more than 5 percent of the span length, calculate the modulus of rupture as follows: 23RPabd= (3) where: a
41、= average distance between line of fracture and the nearest support measured on the tension surface of the beam, mm (in.). See Note 6. 8.3. If the fracture occurs in the tension surface outside of the middle third of the span length by more than 5 percent of the span length, discard the results of t
42、he test. 9. REPORT 9.1. The report shall include the following: 9.1.1. Identification number; 9.1.2. Average width to the nearest 1 mm (0.05 in.); 9.1.3. Average depth to the nearest 1 mm (0.05 in.); 9.1.4. Span length in millimeters (inches); 9.1.5. Maximum applied load in newtons (pounds-force); 9
43、.1.6. Modulus of rupture calculated to the nearest 0.05 MPa (5 psi); 9.1.7. Curing history and apparent moisture condition of the specimens at the time of test; 9.1.8. If specimens were capped, ground, or if leather shims were used; 9.1.9. If specimens were sawed or molded and any defects in the spe
44、cimens; and 9.1.10. Age of specimens. 2018 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS-3c T 97-6 AASHTO 10. PRECISION AND BIAS 10.1. Precision: 10.1.1. Single-Operator PrecisionThe single-operator sta
45、ndard deviation has been found to be 0.26 MPa (38 psi) and to be independent of beam size used in the ILS. Therefore, the modulus of rupture from two properly conducted individual determinations by the same operator on the same material (batch of concrete), using the same size specimen, is not expec
46、ted to differ by more than 0.73 MPa (106 psi). Note 7This number represents the difference limit (d2s) as described in ASTM C670. 10.1.2. Multi-Laboratory PrecisionThe multilaboratory coefficient of variation has been found to be as shown in the third column of Table 1. The coefficient of variation
47、was found to be similar for both specimen sizes used in the ILS for modulus of rupture between 4.1 and 5.5 MPa (600 and 800 psi). A higher multilaboratory coefficient of variation was observed for beams of width of 100 mm (4 in.) with modulus of rupture greater than 6.9 MPa (1000 psi). Therefore, th
48、e modulus of rupture from two properly conducted tests (Note 8) by two different laboratories on specimens of the same material (batch of concrete) and beam size used in the ILS are not expected to differ from each other by more than the value in the fourth column of Table 1. The acceptable differen
49、ce between two test results is expressed as a percentage of their average. Table 1Multilaboratory Precision Beam Depth, in. (mm) Modulus of Rupture, psi (MPa) Coefficient of Variation Acceptable Difference between Two Test Results (percentage of their average)a100 mm (4 in.) 4.1 to 5.5 MPa (600 to 800 psi) 6.1% 17.1% 100 mm (4 in.) 6.9 MPa (1000 psi) 11.4% 31.8% 152 mm (6 in.) 4.1 to 6.9 MPa (600 to 1000 psi) 6.7% 19.3% aThese numbers represents the difference limit (d2s%) as described in ASTM C670. Note 8The precision of this test method was determin
