11 th INTERNA TIONAL BRICKlBLOCK MASONR Y CONFERENCE

TONGJI UNIVERSITY, SHANGHAI, CHINA, 14 - 16 OCTOBER 1997

REINFORCING BAR SPLICES IN HOLLOW BRICK MASONRY

J. Gregg Borchelt, P.E.! and Jeffrey L.Elder, P.E.2

ABSTRACT

This paper presents the results of tests on eleven different splices. The tests were conducted on hollow brick with widths of 100mm (4 in.) and 150mm (6 in.).Reinforcing bar sizes ranged from 13mm (1/2 in.) to 22mm (7/8 in.). Lap splice length varied from 30 to 48 times the bar diameter. Cover ofthe reinforcement varied from 2.0 to 3.2 times the bar diameter. Each test specimen had two lap splices that were tested in tension, with failure occurring in the lower strength splice of the pairo Inc1uded in this paper are material properties, the test procedure, a description . of the failures, and test results. Results are evaluated for the influence of the bar diameter, splice length, and cover depth. Recommended requirements for splice length and cover depth are related to bar diameter.

INTRODUCTION

Background

Reinforced hollow brick masonry combines the compressive and strength of masonry units, mortar, and grout with the tensile strength of steel reinforcement to resist applied load. Reinforcement is placed in vertically aligned cells of hollow brick and in horizontal chases, commonly referred to as bond beams, made by removing part of the end and cross webs. Splices, overlapping length of adjacent reinforcing bars, are necessary to achieve continuity of the reinforcement. Graut serves as a medium for these materiaIs to work together as a single structural element.

KEYWORD: hollow brick masonry, reinforcement, splice length, cover, bar diameter

I Director of Engineering and Research, Brick Institute of America, Reston, Virginia, United States of

America, 20191-1525, WWW.ENG@BIA.ORG

2 Vice-President, Interstate Brick Company, West Jordan, Utah, United States of America, 94084,

A TLASBRICK@AOL.COM

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There are several factors that influence the ability of the reinforcement to transfer stress through the splices. These include: length of overlap of the spliced bars (splice length); amount of masonry between the reinforcement and the face of the masonry (co ver depth); reinforcing bar diameter; levei of stress in the reinforcing bars; and strength of the masonry unit and grout. Failure of the splice may occur in severa! manners. If the splice length is toa short, the bars move along their length and may pull out, perhaps with tensile cracks forming in a radial pattem from the bar cross-section and extending parallel to the bars. If the co ver depth is toa small, crackiTlg and spalling of the masonry parallel to the bars may occur. Finally, the splice strength may be sufficient to achieve the desired stress levei in the reinforcement, to reach the yield strength of the bar, or to fail the bar in tension.

Several aspects of splices influence material and construction costs. Shorter splice lengths obviously reduce the total amount of reinforcement required. They a!so make it easier for the mason to lay the hollow brick since there is less interference by the reinforcement. Reduced co ver permits placing the reinforcement closer to one face of the wall. This improves the efficiency of the cross-section by increasing the distance between the compressive and tensile forces. It also may permit larger reinforcing bars in thinner wa!ls.

The research reported in this paper investigated the performance of reinforcing bar splices in hollow brick masonry. It is part of a project established by the Council for Masonry Research [1]. Hogan, Thomas, and Samblanet [2] reported on the splices in concrete masonry that are part of that research. Parameters for the splices in hollow brick are given in Table 1. Brick thicknesses and reinforcing bar sizes are those typically used in hollow brick construction in the United States. Selection of splice length criteria and cover depth criteria were based on research by Soric and Tulin [3] and the desire to establish lower limits for splice length and cover depth . Lower splice length values were chosen at anticipated minimum performance leveis, below those currently permitted by masonry design standards in the United States. Cover depth was begun at the minimum achievable with unit face shell thickness and minimum grout cover, and increased from that value.

T bl 1 R . f a e : em orcmg B S r M .. H 11 B · kM ar spllce atnx m o ow nc asomy Brick Width Bar Diameter, Splice Length Cover Depth

mm (in.) db mm (in.) ratio mm (in.) ratio 100 (4) 13 ('/2) 4 381 (15) 30db 32 (11A) 2.5db

100 (4) 13 ('/2) 4 457 (18) 36db 25 (1)2.0db

100 (4) 13 ('/2) 4 457 (18) 36db 32 (PA) 2.5db

150 (6) 13 ('/2) 4 381 (15) 30~ 38 (IY2) 3.0d 150 (6) 13 ('/2) 4 457 (18) 36db 38 (1 'h) 3.0db

150 (6) 16 eIs) 5 635 (25) 40db 51 (2) 3.2<!t, 150 (6) 16 eIs) 5 762 (30) 48db 38 (1 'h) 2.4~ 150 (6) 19 e14) 6 914 (36) 48db 51 (2)2.7~ 150 (6) 19 e14) 6 914 (36) 48~ 60 (2%) 3.2d 150 (6) 22 eIs) 7 1067 (42) 48d-.b.. 51 (2) 2.3~ 150 (6) 22 (/s) 7 1067 (42) 48d 60(23/~2.7d

307

NOTATION

db = nominal diameter of reinforeing bar, mm (in.)

MATERIALS

Hollow Brick

Hollow briek were 100 mm (4 in.) and ISO mm (6 in.) nominal thiekness. Average measured dimensions and eoring pattems are shown in Figure I . The hollow briek eonformed to the requirements of ASTM C 652-95a, Grade SW, Type HBX, Standard Speeifieation for Hollow Briek (Hollow Masonry Units Made From Clay or Shale)[4]. Physieal properties of the units were evaluated using the procedures of ASTM C 67-94, Standard Methods of Sampling and Testing Briek and Struetural Clay Tile [4] . These results are given in Table 2.

.3 3 mm ( 1.28 In. )

30 mm (1.14 In .)

34 mm (1.28 in .)

~omm ~3.54 In.)

34 mm ( 1.35 In .) ~ I~~

391 mm (15 .42 in.)

150 mm (6 IN .) HOLLOW BRICK

20 mm (0.81 In.)

24 mm (0.94 in .)

17 mm (0 .65 in .)

~ 396 mm (15.57 in.)

~9mm ~3.52 In .)

100 mm (4 IN .) HOLLOW BRICK

Fig. I. Hollow Briek Dimensions and Coring Pattems

T bl 2 Ph . I P a e lyslea ropertles o fHIl B ' k o ow fie

Property I 100 mm (4 in.) Unit 150 mm (6 in.) Unit

%Solid 64.7 63.9 Net Area, mm2 (in2

) 22520 (34.9) 34650 (53.7)

Net Area Compressive 82.6 121.7 Strength, MPa (psi) (11,990) (17,670)

Absorption 24h Cold Water, % 6.1 5.3 5h Boiling Water, % 9.1 7.4 Initial Rate, gmlmin/30in2

3.9 2.5

I A verage of fi ve

308

Mortar

Mortar conformed to ASTM C 270-96, Standard Specification for Mortar for Unit Masonry [4]. Type S by proportions was used, with portland cementlime:sand in the ratio of 1 :Y2:41/2. Mortar was mixed in a paddle-type mixer for three to ten minutes. Retempering was permitted once and the mortar was used within one and one half hours of initial mixing. Water content was seIected by the mason. Mortar air content averaged 3.7%. The average compressive strength of 50 mm (2 in.) mortar cubes was 21.7 MPa (3150 psi) at twenty-eight days.

Grout

Fine graut was delivered to the laboratory in a concrete ready-mix truck by a local supplier. The grout conformed to the proportion specification of ASTM C 476-95, Standard Specification for Graut [4], with a ratio of 1:3 cement:sand. Sand used conformed to ASTM C 33, Standard Specification for Concrete Aggregates . The grout was delivered at a slump of appraximately 150 mm (6 in.). Water was added to the grout in the truck and mixed to praduce a slump in excess of 250 mm (10 in .) for the 100 mm (4 in.) specimens and 225 mm (9 in.) for the 150 mm (6 in.) specimens. Water was added several times to maintain the slump near these leveIs.

Grout compressive strength was measured in conformance with ASTM C 1019-89a(93), Standard Test Method for Sampling and Testing Graut [4] . Specimens were made using units of each thicknesses as the molds. Grout compressive strengths were 43.4 MPa (6300 psi) and 42.6 MPa (6180 psi) for the 100 mm (4 in .) and 150 mm (6 in.) thick units, respectively.

Reinforcing Steel

SteeI reinforcement was Grade 60 deformed bars meeting the requirements of ASTM A 615-95, Standard Specification for Deformed and Plain Billet-Steel Bars for Concrete Reinforcement. Bar diameters were 13 mm (\12 in.), 16 mm eis in.), 19 mm el4 in.), and 22 mm C/s in.). Ali bars were straight and cut to specified lengths. One end was threaded with the diameter of the threaded end increased to prevent tensile failure. The threaded end was used to apply tensiIe load to the splice during testing. Steel yield and breaking strengths are given in Table 3.

T bl 3 T a e enslOn T R est esu ts on S I R . ~ tee em orcmg B ars

Bar Size Yield Strengthl.2

mm (in.) # MPa (psi)

13 (Y2) 4 461.6 (67,000)

16 eIs) 5 480.8 (69,780)

19 (lA) 6 445.2 (64,620)

22 (/s) 7 499.1 (72,440)

I A verage of three speclmens 2 Based on nominal bar area

Breaking Strength l•2

MPa (psi)

647.7 (94,000)

724.6 (105,160)

703.6 (102,120)

692.4 (100,500)

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Total Elongation, %1

17.5

16.6

15.6

18.0

TEST SPEClMENS AND PROCEDURES

Specimen Construction

Ali test specimens were constructed by an experienced mason using good construction practices. Ali units were laid in running bond using face shell bedding. Mortar joints were 10 mm el8 in.) ± 3 mm e/8 in.). Joints were cut flush and tooled with a concave jointer on the front and back of the specimens. Mortar that protruded more than 6 mm (1,4 in.) into the cells was removed with a wooden stick after completion of the specimen.

Specimen height was determined by desired splice length. The number of courses was selected so that the specimens were just high enough to completely embed the splices. The distance from the end of the splice to the top and bottom of the specimen varied from 25 mm to 64 mm (1 in. to 2.5 in.). Each specimen was two and one half units in length, approximately 1.0 m (40 in.) . Three specimens were built for each combination of unit thickness, bar diameter, splice length, and cover depth.

-Splices of reinforcement were formed by extending one end of adjacent bars out of the bottolI! and top of the specimen. The embedded end of each bar was loosely tied to the adjacent bar to form a contact splice. The adjacent bars were parallel to the face of the specimen. Cover depth was measured from the face of the wall and was maintained by templates 'at the top and bottom of the specimens. Two splices were placed in each specimen, approximately 400 mm (16 in.) apart. Specimen arrangernent is shown in Fig. 2.

Prism Compressive Strength

The specimens were fully grouted in a single lift using scoops to fill the cells. The grout was mechanically vibrated at placement. Grout was reconsolidated from 15 to 45 minutes later, depending on the rate of stiffening. Grouting was accomplished three days after the 100 mm (4 in.) brick were laid and eight days after the 150 mm (6 in.) brick were laid. ~he specimens were tested sixty-nine days after grouting. Curing was in laboratory air, without special curing procedures.

Compression prisms were fabricated at the time of specimen construction using the same materiais. The prisms were one unit long, three units high, and laid in stack bond with full mortar bedding. Construction and testing conformed to ASTM C 1314-95, Standard Test Method for Constructing and Testing Masonry Prisms Used to Determine Compliance with Specified Compressive Strength of Masonry [4]. Compressive strengths of the ungrouted prisms adjusted to an h/t of 2.0 are 29.7 MPa (4310 psi) for the 100 mm (4 in.) and 43.8 MPa (6360 psi) for the 150 mm (6 in.) units.

Test Procedures

A test frame was built of steel members bolted together to form a rectangle around the specimen. The frame was laid on the laboratory floor. The specimen was placed horizontally, with the face having the specified co ver depth in full view. The specimen

310

rested on rollers. Threaded couplers were attached to each of the extended reinforcing bars. At the top of the specimen these were attached to rams controlled by a hydraulic pump. At the bottom of the specimen the couplers were attached to the test frame. The loading set-up exerted the same force to each of the individual splice, the forces differed by no more than 5%. Displacement between the extended bars of each splice and the tensile force on each splice was recorded electronically. Thus, the displacement includes both the slip between the bars and grout, and the e1ongation of the bars themselves. Testing apparatus and monitoring equipment are shown in Fig. 2.

-

• • • •

• e

• •

'---

CENTER BORE-HOLE HYDRAULlC

RAMS~

i--- ~ -~

'~FT - U~_L~1i

~~H~ ! 1 J I

I 1 :I: I I >-

'" I I z :I: ... >-...J

I 1 '" >-z ... z

i I ...J ...

:::E ... ... I Li

u u ::::; :'i <l. <l. I I Vl Vl õ I 1

I i ~?nOM / i 1 BonOM

EFT --='= COUPlER -: RIGHT

/ REINFORCIIo BARS -

-

,--

'"

• • • • f\\ \

• • • •

'----

STRUCTURAL STEEL FRA ME

V

~ HYDRAULlC

CP

Fig. 2. Splice Length Specimen and Testing Apparatus

311

TEST OBSERVA TIONS RESULTS

T bl 4 S I' T t R lt I a e ~lce es esu s

Splice Max. Max. Max. Max. Max. Splice Descrip. Tensile Stress in Splice Stress + Displ. + dM mm (in.) Force ReinC. Displ. Specified Displ. at • length (xdb) in Splice MPa (psi) mm (in.) Yield Specified • cover (xdb) kN (Ib) Strength Yield

100 rnrn-.L4 in~HolJow Brick 13 (V2) - 30 - 2.5 69.61(15,184) 523 (75,922) 5.13 (0.202) 1.27 19.9

13 (~) - 36 - 2.0 68.81 (15,469) 533 (77,345) 5.99 (0.236) 1.29 7.2

13 (~) - 36 - 2.5 74.24 (16,646) 575 (83,228) 8.71 (0.343) 1.39 23.9

150 rnrn-.L6 in,lHolJow Brick 13 (V2) - 30 - 3.0 73.84 (16,369) 564 (81,843) 7.62 (0.300) 1.36 22.3

13 (V2) - 36 - 3.0 81.53 (18,225) 628 (91,123) 13.7 (0.539) 1.52 32.0

16 C/8) ,40 - 3.2 115.6 (25,777) 573 (83,151) 11.8 (0.465) 1.39 17.9

16 ('/8) - 48 - 2.4 \08.5 (24,278) 540 (78,316) 7.87 (0.3 \O) 1.31 13.9

19 (%) -48 -2.7 170.4 (38,044) 596 (86,463) 16.5 (0.648) 1.44 11.3

19(%) - 48 - 3.2 166.3 (37,006) 579 (84, \05) 18.2 (0.716) 1.40 12.0

22 eis) - 48 - 2.3 203.2 (45,690) 525 (76,150) 6.30 (0.248) 1.27 4.0

22 eis) - 48 - 2.7 204.9 (45,819) 526 (76,364) 8.03 (0.316) 1.27 5.2

I A verage of both sphces m 3 speclmens

100 mm (4 in.) Hollow Brick Specimens

As the load was applied, first cracking was observed in the face shells at the top and bottom courses. These cracks were parallerto the splices at the extended ends of the bars. These occurred at loads ranging from44.5 kN (10,000 Ibs) to 62.3 kN (14000 Ibs .) which correspond to reinforcing bar tensile stresses of 345 MPa (50,000 psi) to 482 MPa (70,000 psi). Bed joint and head joint cracking was observed as loading increased. Yielding of the reinforcement began at a load of approximately 62.3 kN (14,000Ibs.). Failure occurred suddenly by splitting of the face shells or by separation of a triangular wedge of grout and face shells along the length of one splice. One specimen with the 36db splice length and 2.5db cover depth failed with a diagonal crack in bed joints, head joints, and face shells in a stair step fashion between the bottom of the right splice to the top of the left splice.

Values of maximum tensile force in the splice, stress in the reinforcement, and splice displacement are given in Table 4. These values are the average of the measurements on the two splices in a single specimen. Plots of the average tensile load and average splice displacement are shown in Fig. 3. These plots are for the specimen with the median average maximum displacement of the three specimen set.

312

80

70

! 60 .., ~ 50

.!! li 40 I:

{! 3. 30 f li

~ 20

10

O

O

~ , I • ---Ir-

5 10 15 20

Average Spllce Dlsplacement (mm)

Fig. 3. Load-Displacement of Splice with

13(%)-36-2.0

13(Y2)-30-2.5

13(%)-36-2.5 bar diameter, mm (in.) - splice length Xdb - cover depth Xdb

Median Maximum Displacement in 100 mm (4 in.) Hollow Brick

150 rnm (6 in.) Hollow Brick Specimens

For ali of the 150 mm (6 in.) specimens, first cracking occurred in bedjoints near midheight. These appeared at loads ranging from 53.4 kN (12,000 lbs) to 88.9 kN (20,000 lbs.). In severa! instances other bed joint cracks appeared. These bed joint cracks did not open significantly beyond the initial size. First cracking in the face shells or head joints near the projecting bars occurred after yieIding of the 13 mm (Y2 in.) and 16 mm eis in.) bars and before yielding ofthe 22 mm eis in.) bars. For the 19 mm el4 in.) bar, the steeI stress at first cracking near the projecting bars depended on cov~r depth. Cracks radiating from the reinforcing bar were visible in the grout core at the top and bottom of most specimens. Cracking parallel to the reinforcing extended toward the center of the specimen as the bar yielded. As with the 100 mm (4 in.) walls, failure occurred suddenly. Face shell splitting or separation of a triangular wedge of grout and face shell along the length of the splice occurred with the 13 mm through 19 mm (Y2 in. through % in.) bars. For specimens with the 22 mm (7fe in.) bars, stair step cracking developed at the bottom of the specimens.

Values of maximum tensile force in the splice, stress in the reinforcement, and splice displacement are given in Table 4. These values are the average of the measurements on the two splices in a single specimen. Plots of the average tensile load and average splice displacement are shown in Fig. 4. These plots are for the specimen with the median average maximum displacement of the three specimen set.

313

250r---------,---------,---------,-------__ ~

0 __ -------4--------~--------~------~ o 5 10 15 20

Average Spllce Dlsplacement (mm)

Fig. 4. Load-Displacement of Splice with

~ 13(%)-30-3.0

---- 13(Y2)-36-3.0 -Ir- 16(%)-48-2.4

""""*- 16(%)-40-3.2 ___ 19(3/4)-48-2.7

~ 1ge/4)-48-3.2

-+- 22(/8)-48-2.3

_ 22(/8)-48-2.7 bar diameter, mm (in.) - splice length Xdb - cover depth Xdb

an Maximum Displacement in 150 mm (6 in.) Hollow Brick

DISCUSSION

Table 4 also contains ratios useful in discussing the results. These are: maximum stress in reinforcement divided by specified yield strength and maximum splice displacement divided by splice displacement at specified yield strength. A similar displacement ductility ratio was employed by Blake, et ai [5] as an evaluation of splice performance. Displacement at specified yield strength was determined from the load-displacement data. The specified yield strength was multiplied by the bar area. This load was used to determine the actual displacement at yield.

100 mm (4 in.) Hollow Brick Specimens

For alI of.the splices ofthe 13 mm (Y2) bars, specified yield strength ofthe reinforcing bars was exceeded. The ratio of maximum tensile stress to specified yield strength ranged from 1.27 to 1.39.

Maximum displacement divided bydisplacement at specified yield ranged from 7.2 to 23.9. At a given splice length criterion, displacement ductility increased as cover depth increased. Similarly, at a constant cover depth criterion, the displacement ducti:ity increased with splice length. The change in co ver depth had a more pronounced effect on displacement ductility than did the change in splice length.

As expected, the splice with the highest splice length criterion (36db) and the highest cover depth criterion (2.5db) gave the best performance. Overall , these results indicate that ali of the splices of 13 mm (Y2 in.) bars perform adequately in 100 mm (4 in.) walls.

314

150 mm (6 in.) Hollow Brick Specimens

As with the 100 mm (4 in.) specimens, specified yield strength of the reinforcing bars ,vas exceeded for ali 150 mm (6 inch) specimens. Ratio of maximum tensile stress to specified strength ranged from 1.27 to 1.52. With the exception of the 19 mm (3/4 in.) bar, this ratio decreased with increasing bar diameter.

The ratio of maximum splice displaeement to displacement at specified yield strength ranged from 4.0 to 32.0. This displacement ductility decreased with increasing bar diameter and the decrease was more pronounced than that of the stress ratio.

Several of the specimens have the same splice Jength criterion (48db). In general both the stress ratio and displacement ductility decreased with increasing bar diameter. As cover depth inereased for a given bar diameter the stress ratio did not exhibit any relationship. The displacement ductility increased, and the increase was Jess pronounced with larger reinforcing bar. The largest bar had the lowest stress ratio and the lowest displacement ductility. These changes are not a linear function of bar diameter.

Ali Specimens

Examining ali specimens with common parameters reveals the following trends:

Spliees with nearly equal cover depth cri teria: as bar diameter increased with the same spliee criterion, the stress ratio and displaeement ductility decreased. The greater teduetion oecurred in displacement ductility. as bar diameter inereased and splice length increased, both ratios decreased. The greater reduetion oecurred in the displacement ductility. as spliee length inereased with the same bar size, the stress ratio and the displaeemt:nt ductility increased. Again, the greater ehange is in ductility ratio.

Splices with the same splice length cri teria: bar diameter had little influenee on stress ratio. Displacement ductility decreased as bar diameter increased. as bar diameter increased at the same cover depth eriterion, both stress ratio and displacement ductility decreased. as cover depth eriteria increased, the stress ratio increased. Displacement ductility increased for the smallest bar at both splice length criteria, but decreased for the two largest bars at a single spliee length.

There is not sufficient data to determine the influence of masonry compressive strength.

CONCLUSIONS

Aeeeptable performance as measured by the ratio of maximum bar stress to specified yield strength ean be obtained with cri teria of 30db for spliee length and 2.5db for eover depth or 36db splice lengthand 2.0db cover depth for 13 mm (Y:z in.) in a 100 mm (4 in.)

315

wall and of 48db for a splice length and 2.3db for cover depth for 22 mm (1fs in.) bars in a 150 mm (6 in.) wall .

Performance based on displacement ductility in excess of 6.0 can also be obtained with a 13 mm (Y2 in.) bar with a cri teria of 30db splice length and 2.5db cover depth or 36db

splice length and 2.5db cover depth in a 100 mm (4 in.) wall . The 22 mm (1fs in.) bars cannot achieve this displacement ductility in a 150 mm (6 in.) wall. Ali other splices exceeded this value.

Splice length is not a linear function of bar diameter. A suggested criterion for splice length based only on bar diameter is 2.4db

2 where db is in mm, and 60db2 where db is in

inches.

Cover depth is not a linear function of bar diameter. A suggested criterion for cover depth based only on bar diameter is (2.1 + 0.31db) db where db is in mm, and (2.1 + 0.8db)db where db is in inches.

The recommendation of Soric and Tulin [3] that the maximum reinforcing bar size, as expressed in eighths of an inch (#4 for V2 in. bars, #7 for 1fs in. bars), should equal the nominal thickness of the wall is validated.

Splice length and co ver depth criteria currently permitted by masonry design standards in the United States are found to be conservative.

REFERENCES

1. Thomas, R.D.,"Evaluation of Minimum Splice Criteria for Hollow Brick and Concrete Masonry", Report to Council for Masonry Research, August 1997.

2. Hogan, M.B., Thomas, R.D., and Samblanet, P.J., "Research Evaluation of Reinforcing Bar Splices in Concrete Masonry", 11 th Intemational BrickIBlock Masonry Conference, Tonji University, Shanghai, China, October 1997.

3. Sorie, Z., and Tulin, L.G., "Bond and Splices in Reinforced Masonry", Report No. 6.2-2, U.S. - Iapan Coordinated Program for Masonry Research, University of Colorado, Boulder, CO., August 1987.

4. Annual Book of Standards, Volume 04.05, American Society for Testing and MateriaIs, Philadelphia, PA., Iune 1996.

5. Blake, I.D., Marsh, M.L., and McLean, D.I., "Lap Splices in Flexurally Loaded Masonry Walls", The Masonry Society Ioumal, February 1995, p. 22-36

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