reducing the specimen size of concrete flexural strength ...docs.trb.org/prp/13-1986.pdf · 1...

Reducing the Specimen Size of Concrete Flexural Strength Test 1

(AASHTO T97) for Safety and Ease of Handling 2

3

By 4

5 Jussara Tanesi, Ph.D. 6

Global Consulting, Inc. 7 Turner Fairbank Highway Research Center/FHWA 8

6300 Georgetown Pike 9 McLean VA 22101 10 Ph: 202 493 3485 11

[email protected] 12 13 14

Ahmad A. Ardani, P.E. (Corresponding Author) 15 Federal Highway Administration 16

6300 Georgetown Pike 17 McLean, Virginia 22102 18

Ph: 202-493-3422 19 [email protected] 20

21

John C. Leavitt 22 Global Consulting, Inc. 23

Turner Fairbank Highway Research Center/FHWA 24 6300 Georgetown Pike 25

McLean VA 22101 26 Ph: 202 493 3111 27

[email protected] 28 29

30

31

32

33

34

Word Count: 35 Text: 2371 36 Figures/Tables: 10 * 250 = 2500 37 -------------------------- 38 Total: 4871 39

TRB 2013 Annual Meeting Paper revised from original submittal.

Tanesi, Ardani & Leavitt 1

Reducing the Specimen Size of Concrete Flexural Strength Test 1

(AASHTO T97) for Safety and Ease of Handling 2

ABSTRACT 3

This study examines the feasibility of using smaller size concrete beam specimens for conducting flexural 4 strength of concrete, using simple beam with third-point loading, according to AASHTO T97 procedure. 5 A total of 22 mixtures, containing four different coarse aggregates (limestone, diabase, gravel and granite) 6 with maximum size varying from ¾ in to 1.5 inch, were prepared. A total of 132 4x4x14 inch specimens 7 and 132 6x6x21 inch standard specimens size were tested. The 4x4 inch specimens yielded higher 8 flexural strengths, as expected from the literature review. Analysis of the flexural strength test data 9 revealed a very good correlation (R

2 =0.93) between the smaller and standard size beams. An equation is 10

proposed to convert the flexural strength of the small size specimen to flexural strength of the standard 11 size specimen. 12

13



INTRODUCTION 1

2 Flexural strength, also known as modulus of rupture, is an important parameter in concrete pavement 3 design. FIGURE 1 shows the influence of the flexural strength on the cracking of a typical jointed plain 4 concrete pavement based on the AASHTO mechanistic-empirical pavement design guide (MEPDG

1). 5

Adequate flexural strength is essential for concrete pavements to resist bending stresses due to combined 6 effects of traffic loading and environmental factors. 7

8

FIGURE 1 Influence of the modulus of rupture (flexural strength) on the percent slabs cracked1. 9

NOTE: 1 psi = 6.89 kPa, 1 ft = 0.305 m and 1 inch = 25.4 mm. 10

11 Flexural strength is measured by applying load to an unreinforced concrete beam with the 12

intention of inducing cracks in accordance with AASHTO T972 and ASTM C78

3 for third-point loading 13

or AASHTO T1774 and ASTM C293

5 for center-point loading. FIGURE 2 shows a schematic view of 14

flexural strength concrete testing specimens for third-point loading and center-point loading. 15



1

FIGURE 2 ASTM C 78 & ASTM C 293- NRMCA6. 2

3 Although, the standard specimen size for conducting flexural strength is normally specified as a 4

6x6x21 inch beam, it is important to note that AASHTO T972, AASHTO T177

4, ASTM C78

3 or ASTM 5

C2935 standards do not specify specimen size; however, they direct the laboratory personnel to follow 6

AASHTO T237 and ASTM C31

8 for specimens cast in the field and AASHTO R39

9 and ASTM C192

10 7

for specimens cast in the laboratory. Only AASHTO T237 and ASTM C31

8 require this standard size. 8

Standard specimens cast in accordance with AASHTO T237 and ASTM C31

8 weigh 9

approximately 65 pounds, assuming a concrete density of 150 lb/ft3 and when metal molds are used, the 10

total weight becomes anywhere from 120-135 lbs. 11

In many cases, state DOTs prefer using compressive strength for design and quality assurance 12 purposes due to the fact that flexural strength beam specimens are heavy, unsafe to handle and sometimes 13 hard to cast due to the stiff nature of the paving mixtures. In addition, beams require diligent attention as 14 the results of the flexural strength testing are extremely sensitive to the curing and moisture condition of 15 the specimen. To reduce the risk of injury to the testing personnel and to streamline quality assurance 16 testing, many DOTs have established a relationship between compressive and flexural strength test results 17 for their specific mixture design. However, the correlations between the two tests are not all that good 18 and more importantly, concrete primarily fails in bending and not in compression. 19

Since the standard specimens are big and heavy, reducing specimen size could make the handling 20 easier and safer for the testing personnel. Smaller size beams that weigh less than ½ of the standard size 21 beams can encourage DOTs to use them for design and quality assurance. 22

Nevertheless, previous studiesError! Reference source not found.,Error! Reference source not found.,13,14

clearly 23 indicate that the flexural strength of concrete significantly decreases as the beam size increases. For 24 example, Lindner and Sprague

14 showed that flexural strength decreases by a factor of about 1.5, when 25

the beam depth increases from 3 to 40 inch. 26



In order to consider the differences in the size of the specimens tested in the laboratory and the 1 structure in service, Bazant and Novak

13 proposed a change to ASTM C78

3 where different beam sizes 2

would have to be tested or complicated numerical calculations would have to be carried out in order to 3 determine a correction factor. Nevertheless, for pavement design purposes, there is no need to apply the 4 size correction proposed by Bazant and Novak

13 when using standard specimens. The MEPDG models

1 5

were already calibrated using 6x6x21 inch beams and took into account the actual pavement performance 6 of hundreds of sections under the Long-Term Pavement Performance (LTPP) program (LTPP SPS-2, 7 GPS-3, FHWA RPPR and LTPP GPS-5). On the other hand, if a smaller specimen size is used, the size 8 difference should be taken into account and a correction factor applied when using the MEPDG. 9

OBJECTIVES 10

The main purpose of this study is to examine the feasibility of using smaller size specimens (4x4x14 inch) 11 in the laboratory and in the field for flexural strength testing and recommend changes to the current 12 AASHTO standards. 13

EXPERIMENTAL PROGRAM 14

A total of 22 concrete mixtures were prepared. The water-to-binder ratio varied from 0.37 to 0.47 and the 15 cementitious content varied from 521 to 643 lb/yd

3 (309 to 381 kg/m

3). Materials used consisted of 16

portland cement type I/II, natural sand with specific gravity of 2.61, absorption of 1.7% and fineness 17 modulus of 2.76. Type F fly ash was used in one of the mixtures. Four different coarse aggregates were 18 used, which included gravel, limestone, granite and diabase, with nominal maximum size varying from ¾ 19 inch to 1 ½ inch (19 to 38 mm). A variety of air entraining admixtures and water reducers were used. 20

TABLE 1 shows properties of coarse aggregates used in each mixture, TABLE 3 shows the 21 coarse aggregate grading and TABLE 3 shows the mixture proportions. In the mixtures ID, the first 22 number represents the ASTM C33

15 aggregate size (aggregate sizes 67, 57 or 467), the following letters 23

represent the type of aggregate (LS for limestone, DB for diabase, GV for gravel and GT for granite) and 24 the last number represents the water-to-binder ratio. For example, mixture 57DB47 is a mixture with a 25 #57 diabase and a 0.47 water-to-cement ratio. 26

Mixtures were prepared and cast following AASHTO T237 procedure with one exception; 27

concrete containing #467 aggregate was not wet sieved prior to molding 4x4x14 inch specimens 28 (102x102x356 mm), in order to evaluate the effect of bigger maximum size. Three 4x8 inch (102 x 204 29 mm) cylinders were cast for 28-day compressive strength, six 4x4x14 inch (102x102x356 mm) beams 30 and six 6x6x21 inch (152x152x533 mm) beams were cast for 28-day flexural strength (FIGURE 3). Wet 31 burlap was used to protect specimens from moisture loss for the first 24 hours and then demolded and 32 placed in a lime water tank. 33

Slump tests (AASHTO T11916

), air content tests (AASHTO T15217

), unit weight tests (AASHTO 34 T121

18), compressive strength tests (AASHTO T22

19) using unbounded caps and flexural strength tests 35

(AASHTO T972) were carried out (FIGURE 4). Specimens were demolded at 24 hours and cured in lime 36

water. The 28-day flexural strength tests were carried out as soon as the specimens were removed from 37 curing tank. Water was sprayed on the specimens during testing to prevent any possible drying. 38

39

40

41



TABLE 1 Coarse Aggregate Information 1

Mix ID Coarse

aggregate

ASTM

C33

gradation

Nominal

maximum

size (inch)

specific

gravity

absorption

(%)

57LS37 Limestone 57 1.00 2.71 0.33

57LS42 Limestone 57 1.00 2.86 0.27

57LS47 Limestone 57 1.00 2.71 0.33

57GV37 Gravel 57 1.00 2.58 1.91

57GV42 Gravel 57 1.00 2.58 1.91

57GV47 Gravel 57 1.00 2.58 1.91

57DB37 Diabase 57 1.00 2.97 0.64

57DB42 Diabase 57 1.00 2.97 0.64

57DB47 Diabase 57 1.00 2.97 0.64

467LS37 Limestone 467 1.50 2.71 0.51

467LS42 Limestone 467 1.50 2.71 0.51

467LS47 Limestone 467 1.50 2.71 0.51

67GV37 Gravel 67 0.75 2.57 1.77

67GV42 Gravel 67 0.75 2.57 1.58

67GV45 Gravel 67 0.75 2.57 1.77

67LS37 Limestone 67 0.75 2.83 0.63

67LS42 Limestone 67 0.75 2.83 0.63

67LS45 Limestone 67 0.75 2.83 0.63

67DB37 Diabase 67 0.75 2.97 0.64

67DB42 Diabase 67 0.75 2.97 0.64

67DB45 Diabase 67 0.75 2.97 0.64

57GT45 Granite 57 1.00 2.8 0.5

NOTE: 1 inch = 25.4 mm. 2

3

4

5

6

7

8

9

10

11

12



TABLE 2 Grading of Coarse Aggregate 1

Amounts finer than each laboratory sieve, mass percent

Aggregate 37.5 mm

25.0 mm

19.0 mm

12.5 mm

9.5 mm

4.75 mm

2.36 mm

(1 ½ in.) (1 in.) (3⁄4 in.) (1⁄2 in.) (3⁄8 in.) (No. 4) (No. 8)

57LS 100 95 80 30 7 2 0

57GV 100 100 69 35 17 2 0

57DB 100 95 80 30 7 2 0

467LS 100 88 69 24 11 0 0

67LS 100 100 90 60 20 5 0

67GV 100 100 90 60 20 5 0

67DB 100 100 90 60 20 5 0

57GT 100 100 75 50 25 0 0

NOTE: 1 inch = 25.4 mm. 2

TABLE 3 Mixture Proportions 3

Mix ID

Type

I/II

cement

(lb/yd3)

Fly ash

(lb/yd3)

Coarse

aggregate

(lb/yd3)*

Fine

aggregate

(lb/yd3)*

w/cm WR

(oz/cwt)

AEA

(oz/cwt)

57LS37 643 0 1790 1253 0.37 12.0 0.65

57LS42 643 0 1790 1262 0.42 2.0 0.18

57LS47 643 0 1790 1090 0.47 - 0.82

57GV37 643 0 1699 1252 0.37 12.0 0.70

57GV42 643 0 1699 1175 0.42 2.0 0.60

57GV47 643 0 1699 1089 0.47 - 0.30

57DB37 643 0 1699 1481 0.37 6.0 0.15

57DB42 643 0 1699 1397 0.42 5.8 0.58

57DB47 643 0 1699 1315 0.47 - 0.30

467LS37 521 0 1790 1464 0.37 12.0 0.05

467LS42 521 0 1790 1398 0.42 3.2 0.20

467LS47 521 0 1790 1331 0.47 1.7 0.28

67GV37 564 0 1750 1341 0.37 7.0 0.05

67GV42 564 0 1750 1265 0.42 3.2 0.05

67GV45 564 0 1750 1223 0.45 - 0.20

67LS37 564 0 1750 1506 0.37 10.0 0.05

67LS42 564 0 1750 1434 0.42 1.5 0.23

67LS45 564 0 1750 1390 0.45 - 0.20

67DB37 564 0 1750 1581 0.37 11.0 1.00

67DB42 564 0 1750 1509 0.42 1.8 0.20

67DB45 564 0 1750 1465 0.45 0.8 0.22

57GT45 423 141 1823 1264 0.45 1.8 0.77

*Aggregate proportions are expressed in SSD condition. 4



1

FIGURE 3 Comparison between the smaller and standard size specimens. 2

3

4

FIGURE 4 Conducting Flexural strength test using a standard size specimen. 5



RESULTS 1

Fresh properties’ results can be found in TABLE 4. Flexural strength test results for the two specimen 2 sizes, with their respective standard deviation and coefficient of variation, as well as compressive strength 3 results can be found in TABLE 5. Flexural strength is the average of six specimens and compressive 4 strength is the average of three specimens. 5

TABLE 5 shows that in most cases, 4x4x14 inch specimens exhibited slightly higher flexural 6 strength, confirming previous studies

Error! Reference source not found.,Error! Reference source not found.,13,14. A paired t-test 7

comparison was made to test the null hypothesis that the average flexural strength obtained with 4x4x14 8 inch specimens was the same as the average flexural strength of the standard size specimens. The analysis 9 (P= 0.0148) indicates that there is evidence to reject the null hypothesis, with a 5 percent level of 10 significance, in other words, the flexural strength of the two specimen size are statistically different. 11

As it can also be observed in TABLE 5, the coefficient of variation (COV) of flexural strength 12 tests of 4x4x14 inch specimens was, in 18 of 22 of cases, higher than the COV of the tests of 6x6x21 inch 13 specimens with average COVs of 5.3% and 3.4%, respectively. Nonetheless, these COVs are within 14 AASHTO T78 acceptable range. As a consequence, for penalty specifications more specimens per batch 15 might be made and tested for 4x4 compared to 6x6. 16

17

TABLE 4. Fresh Concrete Properties 18

Mix ID slump

(inch)

unit

weight

(lbs/ft3)

air (%)

57LS37 1.00 143.6 7.5

57LS42 3.00 145.3 6.0

57LS47 3.00 144.2 5.5

57GV37 2.50 139.9 7.0

57GV42 2.75 141.4 6.0

57GV47 8.50 137.1 6.0

57DB37 0.50 151.0 6.5

57DB42 2.25 148.4 6.9

57DB47 7.50 145.3 7.0

467LS37 0.25 144.8 6.0

467LS42 0.75 142.9 7.0

467LS47 3.50 142.2 7.9

67GV37 1.25 143.4 6.0

67GV42 4.50 141.4 6.6

67GV45 3.00 142.0 5.5

67LS37 0.25 147.0 4.5

67LS42 1.25 146.9 5.0

67LS45 3.50 144.8 5.4

67DB37 0.25 155.5 4.7

67DB42 2.00 149.7 6.8

67DB45 1.00 152.6 5.0

57GT45 3.00 149.8 5.1

NOTE: 1 inch = 25.4 mm and 1 lb/ft3 = 16.02 kg/m

3. 19



TABLE 5 also shows that, when comparing mixtures, the lower the water-cement ratio, the 1 higher the flexural strength and the compressive strength, with exception of mixtures 67DB42 and 2 67DB45. This was probably due to the higher air content of mixture 67DB42. For the same cement 3 content, water-cement ratio and gradation, mixtures containing limestone aggregate presented the highest 4 flexural strength while mixtures containing gravel presented the lowest flexural strength. 5

FIGURE 5 presents the relation between the flexural strength of 4x4x14 inch specimens and 6 standard size specimens. As it can be observed there is a very good correlation (R

2 = 0.93) between the 7

flexural strength of the two beam sizes, indicating that 4x4x14 inch specimens could be used in lieu of 8 standard specimens. 9

10

TABLE 5 Flexural Strength and Compressive Strength 11

Flexural Strength Using Third Point Loading Compressive

Strength of

4x8 inch

specimens(psi) 6x6x21 inch specimens 4x4x14 inch specimens

Mix ID Average

(psi)

Stdv

(psi) COV (%)

Average

(psi)

Stdv

(psi) COV (%)

57LS37 935 56 6.0 940 46 4.9 6650

57LS42 828 34 4.1 908 53 5.8 6145

57LS47 775 23 2.9 805 35 4.3 5220

57GV37 689 20 2.8 755 30 4.0 5485

57GV42 675 27 4.0 727 29 3.9 4709

57GV47 586 17 2.8 616 45 7.3 4260

57DB37 880 33 3.8 935 40 4.2 6923

57DB42 743 21 2.8 821 45 5.5 5354

57DB47 674 8 1.2 706 41 5.9 5128

467LS37 1013 44 4.3 1003 60 6.0 7864

467LS42 794 19 2.3 795 69 8.7 5421

467LS47 720 24 3.3 703 26 3.7 4591

67GV37 794 19 2.5 813 70 8.6 6264

67GV42 747 32 4.3 743 46 6.2 5174

67GV45 670 24 3.5 710 33 4.7 4446

67LS37 1112 23 2.1 1042 38 3.7 7713

67LS42 893 33 3.7 908 25 2.8 5612

67LS45 840 39 4.6 850 57 6.7 5018

67DB37 921 36 3.9 904 62 6.9 7536

67DB42 801 20 2.5 801 34 4.3 5571

67DB45 801 25 3.1 811 41 5.0 5732

57GT45 636 32 5.1 667 27 4.1 5038

NOTE: 1 inch = 25.4 mm and 1 psi = 6.89 kPa. 12

13



Nevertheless, since the two specimen sizes do not yield the same flexural strength and the 1

MEPDG models1 were calibrated using flexural strength obtained with standard size specimens, it is 2

important to convert the flexural strength of 4x4x14 inch specimens to standard size flexural strength 3 (EQUATION 1) before using it as a design input, so the pavement is not under designed. 4

5

EQUATION 1

Where: 6 7

8

FIGURE 5 Relationship between flexural strength of 4x4x14 inch specimens and 6x6x21 inch 9 specimens. 10

11 Compressive strength, instead of flexural strength, is also often used as a quality assurance tool, if 12

the relationship between the two properties is determined for the specific mixture design. FIGURE 6 13 shows the relation between flexural strength and compressive strength. As it can be observed, flexural 14 strength obtained with 4x4x14 inch specimens correlates much better (R

2 = 0.84) with compressive 15

strength than the flexural strength obtained with standard size specimens (R2 = 0.63). It is important to 16

note that in levels 2 and 3 of MEPDG1, compressive strength is used as a design input, instead of flexural 17

strength and the compressive strength is used to estimate flexural strength. 18



1

2

FIGURE 6 Relationship between flexural strength of 4x4x14 inch specimens or 6x6x21 inch 3 specimens and compressive strength. NOTE: 1 psi = 6.89 kPa. Error bars indicate ±1 standard 4

deviation of the flexural strength. Top equation corresponds to the 4x4x14 inch specimen 5 correlation while the bottom equation corresponds to the 6x6x21 inch specimen correlation. 6

RECOMMENDED CHANGES TO AASHTO STANDARDS 7

Although AASHTO T972 does not specifically state the specimen size to be used, it refers to AASHTO 8

R399 and AASHTO T23

7. AASHTO R39

9 does not require flexural strength specimens to be 6x6x21 inch 9

but AASHTO T237 does. So, in order to implement the use of 4x4x14 inch specimens, AASHTO T23

7 10

also needs to be revised. 11

Nevertheless, since 6x6x21 inch specimens and 4x4x14 inch specimens do not yield the same 12 flexural strength, it is important to include a note in AASHTO T97

2, recommending the user to either use 13

EQUATION 1 to convert the flexural strength of smaller size specimen into flexural strength of standard 14 size specimen or to establish a relationship between the flexural strength of the two different specimen 15 sizes for the specific mixture design. The specimen size also needs to be included in the report section. A 16 note that the within test COV may be higher with the smaller specimens may be needed. 17

CONCLUSIONS 18

In the current study, the flexural strength of smaller size specimens was slightly higher and statistically 19 different than the flexural strength of standard size specimen. Since the MEPDG models were calibrated 20 using flexural strength of standard specimen size, if 4x4x14 inch specimens are used, it is necessary to 21 apply a correction so the pavements are not under designed. 22

Smaller specimens 4x4x14 inch appear to be a viable alternative specimen size and are easier and 23 safer to handle by testing personnel. 24



ACKNOWLEDGMENTS 1

The authors would like to express their sincere appreciation to Gary Crawford and Richard Meininger for 2 their valuable inputs and for providing excellent comments. 3

REFERENCES 4

1. ARA, Inc., ERES Division, “Guide for Mechanistic-Empirical Design of New and Rehabilitated 5 Pavement Structures,” Final Report NCHRP 1-37A, March 2004. 6

2. AASHTO T97-10. Standard Method of Test Flexural Strength of Concrete (Using Simple beam 7 with Third-Point Loading). Washington, D.C., 2010. 8

3. ASTM C78-08. Standard Test Method for Flexural Strength of Concrete (Using Simple Beam 9 with Third-Point Loading), West Conshohocken, PA, 2008. 10

4. AASHTO T177-10. Standard Method of Test for Flexural Strength of Concrete (Using Simple 11 Beam with Center-Point Loading). Washington, D.C., 2010. 12

5. ASTM C293-08. Standard Test Method for Flexural Strength of Concrete (Using Simple Beam 13 with Center-Point Loading). West Conshohocken, PA, 2008. 14

6. National Ready Mixed Concrete Association (NRMCA), “Concrete in Practice - What, Why & 15 How? – CIP 16 Flexural Strength Concrete”, 2000. 16

7. AASHTO T23-08. Standard Method of Test for Making and Curing Concrete Test Specimens in 17 the Field. Washington, D.C., 2008. 18

8. ASTM C31/C31M-08a. Standard Practice for Making and Curing Concrete Test Specimens in the 19 Field. West Conshohocken, PA, 2008. 20

9. AASHTO R39-07. Standard Practice for Making and Curing Concrete Test Specimens in the 21 Laboratory. Washington, D.C., 2007. 22

10. ASTM C 192/C192M-07. Standard Practice for Making and Curing Concrete Test Specimens in 23 the Laboratory. West Conshohocken, PA, 2007. 24

11. Zhou, F.; Balendran, R. and Jeary, A. Size Effect on Flexural, Splitting Tensile, and Torsional 25 Strengths of High-Strength Concrete. Cement and Concrete Research, Vol. 28, No. 12, pp. 1725–26 1736, 1998. 27

12. Denneman, E.; Kearsley, E. and Visser, A. Size-effect in high performance concrete road 28 pavement materials Advances in Cement-Based Materials - Proceedings of the International 29 Conference on Advanced Concrete Materials, Stellenbosch, South Africa, 2009. 30

13. Bazant, Z. and Novak, D. Proposal for Standard Test of Modulus of Rupture of Concrete with its 31 Size Dependence, ACI Materials Journal/January-February 2001. 32

14. Lindner, C. and Sprague, I. Effect of Depth of Beams upon the Modulus of Rupture of Plain 33 Concrete, ASTM Proceedings, V.55, pp. 1062-1083, 1956. 34

15. ASTM C33-07. Standard Specification for Concrete Aggregates. West Conshohocken, PA, 2007. 35

16. AASHTO T119M/T119-10. Standard Method of Test for Slump of Hydraulic Cement Concrete. 36 Washington, D.C., 2010. 37

17. AASHTO T152-10. Standard Method of Test for Air Content of Freshly Mixed Concrete by the 38 Pressure Method. Washington, D.C., 2010. 39



18. AASHTO T121M/T121-09. Standard Method of Test for Density (unit Weight), Yield, and Air 1

Content (Gravimetric) of Concrete. Washington, D.C., 2009. 2

19. AASHTO T22-10. Standard Method of Test for Compressive Strength of Cylindrical Concrete 3 Specimens. Washington, D.C., 2010. 4


reducing the specimen size of concrete flexural strength ...docs.trb.org/prp/13-1986.pdf · 1...

Documents