11th INTERNA T10NAL BRICKlBLOCK MASONRY CONFERENCE

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

STRENGTH AND BEHA VIOR OF LAP SPLICES OF REINFORCEMENT IN HOLLOW UNIT MASONRY

Akira Matsumura 1) Izumi Igarashi 2 ) and Hiromitsu Takao 3)

ABSTRACf

Tests were earried out to clarify the strength and behavior of lap splices within hollow unit masonry structures. Lap splices in 89 wall specimens were tested, concerning vertical reinforcement in the masonry bearing walls . Splices were subjected to tensile stress by applying fleirural loadings in which wall specimens were tilted in plane as if they were deep beams. Test paramctcrs were bar size , lap length, grout cover thickness, and strengths of grout and masonry units. The effeet of eyclic loading was also examined. This paper reports results of tests on eonerete masonry. Pull-pull tests were applied preliminarily. However, they showed that this method was insufficient to estimate the strength of spliees in walls. It is suggested that C. O. Orangun's formula [1] for ordinary reinforeed eonerete members for estimating neeessary lap lengths under various eonditions is also effeetive if it is modified to allow for the influenee of the masonry units surronunding the spliee and grout.

I .INTRODUCTION

1.1 Objeet of Study

Reinforeement must be lap splieed within the eavities of hollow unit masonry struetures. In lap spliees, the tensile stress earried by reinforcement is transmitted by the reinforcing bond of the grout. Despite the importance of this stress transmission in engineering design and eonstruction technique for masonry

KEYWORDS; Reinforced masonry, Lap splice, Lap length, Reinforcing bond

1) Professor, Dept. of Arehit, Kanagawa University,3- 27-1 Rokkakubashi, Kanagawa­ku, Yokohama, 221 Japan.

2) Chief Technician, ditto 3) Structural Engineer, Chubu-Sekisui Industry Co.,Akemi,Toyohashi,Aiehi,440 Japan.

276

structures, the detailed behavior of lap splices has ' not been darified. The authors have investigated this problem mainly by testing the strength and behavior of lap splices of reinforcement in hollow unit concrete masonry.

1.2 Preceding Studies

Many tests of lap splices have been carried out on ordinary reinforced concrete, in the past several decades in the United States and Europe. C.O.Orangun [1] and his colleagues presented a formula for estimating appropriate lap lengths for reinforced concrete in 1977. However, there have been few studies on lap splices in masonry. S. Zoric [2] presented test results of lap splices in concrete masonry using pull-pull tests and suggested an analytical method for determining suitable lap lengths for· various bar sizes in 1988. However, because of the lack of test data and the many and complex parameters in this field, the real circumstances conceming the strength of lap splices are still uncertain .

1.3 Code Provisions in Japan

In the Japanese codes [3], reinforced masonry is divided into two types : partially grouted masonry (PGM) and fully grouted masonry (FGM), using concrete or day hoUow masonry units, respectively. Codes for masonry do not permit lap splices in alI vertical reinforcement in partialy grouted masonry, or in vertical edge flexural reinforcement in fully grouted masonry bearing walls. These provisions are due to the unreliability of the reinforcing bond beca use of the narrowness of the cavities where splices provided. They are also due to the lack of studies conceming the strength of lap splices in masonry.

2. TEST METHOD

2.1 Test Parameters

Test parameters influencing the behavior of lap splices are as follows; a) bar size b) lap length c) grout strength d) masonry units strength e) cover thickness of grout and masonry units to rcinforcement f) loading method (flexuralloading, pull-pullloading, and cyclic loading, etc.)

2.2 Test Method

In the preliminary test [4], the authors tried some tests on concrete masonry and day masonry by the pull-pull method. However, as described later, flexural tension loading was adopted in the main test method. Loading was normally static. One-way or reverse cydic loading was carricd out next. The concrete masonry units used are shown in Fig. 1. An example of a pull-puU test specimen is shown in Fig. 2, and examples of wall test specimens are shown in Fig. 3. Test specimens of ordinary reinforced concrete waUs (WRC) of the same shape and size, and lap splices identical to those of masonry specimens were also made for comparison. The loading apparatus is shown in Fig. 4. Wall specimens were built vertically, and splices were provided at the mid-height of end flexural rebars. After the grout hardened, the wall specimens were tilted at right angles in plane and set on supporting points with the lap splices at the bottom side so that they were

277

• A Section A-A'

~c==6= EE~:E=====E3=p==~-8 AE ~1904 Lap length ( lu )

Fig. 1 Shapes and Sizes of Concrete Masonry Units

Fig. 2 Example of Pull-Pull Test Specimen

1, 1 9 O

195 610 16 O 610 951 I " A,...., 019 ~ 010 I Section A-A'

~:~:' ':;+~'t '~ln~'(' '; :~1~ .. u ,'-1 u 'BarOI9" '----"190 Fully grouted concrete masoruy A'

010 Section B-B' "

,. ri 019 " " / "-I~F= ·-It-·-- I'" rI- _ .

Ir F= I i

A C. ,Y '" " A

Wall of reinforced concrete B' Bar 019 I I

Lap length ( ld ) "'6. : Loading point

~ " " " " " "

-... \l : Supporting point

Fig. 3 Examples of Wall Test Specimens

Bar

610 I

e:::;:::;::::,:0 Oynamic actuater

760 1, O O O

610

H

" " " " " "

o o

Section A-A'

Fig. 4 Outline of Loading Apparatus (For Reverse Cyc1ic Loading)

278

subjected to tensile stress by two symmetrical verticalloads.

2.3 Test Program

The flexuralloading test program is shown in Table 1.

Table 1 Test Program (Concrete masonry) Series Kinds of Number Main parameters and test conditions

structure of walls

FGCM1 1 9 Bar size and grade (D25, -SD345)

I Lap length (5 , 15 , 25 d.b 4)

WRC 21 11 Cover thickness (side cover : 70 mm, bottom cover : 42.5, 107.5, 179.5 mm)

PGCM31 11 Bar size and .grade (D13, SD345)

II Lap length (5, 10,20, 40 d.b) WRC 3 Cover thickness (side cover : 75 mm,

bottom cover : 20, 40, 60 mm) PGCM 8 Bar size and grade (D19, -SD345)

m FGCM 8 Lap length (10, 20 d.b) WRC 6 Strength of grout (A-low, B-medium, C-hig!l)

FGCM 10 Bar size and grade (D19, -SD345)

IV WRC 6 Lap length (10 , 20, 30 d.b) Loading method (monotonic, one side cyclic) Bar size and grade (D19, -SD345)

V FGCM 12 Lap length (15, 25db)

I WRC 5 Confining effect of units (A-weak units, B-or

Loading method (monotonic, reverse cyclic) dinary)

Notes: 1) Fully grouted concrete masonry 2) Wall of reinforced concrete 3) Partially grouted concrete masonry 4) Nominal bar diameter

3. TEST RESULT

In most cases, the lap splices failed by splitting of the grout and masonry units on the sides of the walls along the reinforcement direction. This is assumed to be due to expansion of the grout surrounding the splices . This splitti'lg is assumed to arise from the radial component of diagonal compressive struts of the grout between adjacent rebars to transrnit the tensile stress of the bars . Under this component stress , splitting is assumed to have occurred at the sides of wall where the concrete cover thickness of the wall specimens was a minimum. A similar phenomenon is well known in ordinary reinforced concrete members. Table 2 denotes the strengths of the materials.

Table 2 Strengths of Materiais Yield stress (MPa) Compressive stren,gth (MPa) D25 : 382 - 395 (except series ill) 20 - 36

Reinforcing D19 : 371 - 410 Grout {A : 13 bars D13 : 372 (Concrete) series m B: 20

, C : 38 CompressivOO~)ngth Masonry 34 - 36

mortar Masonry unit (except series V) 52-68 Masonry unit for fully grouted)

series V {~ 27 (for partially 25 68 grouted)

The test results show the following. a) Influence of bar size

A higher maximum avcrage bond strength was achieved in the splice, when a

279

Ê7

Q 6 o C ::::'5

fi 4 § 3

::E 002 c:

~I Id =20db cl5 O

O 5 10 15 20 Curvature (I O"/mm)

Fig. 5(a) Effect of Grout (FGCM)

Ê 7 Z 6 Grout C ~ o

C 4 B ..,

E 3 A o

::E 00 2 c: 'õ I !d =20db c: ..,

O Cl

O 5 10 15 20 Curvature (I0"/mm)

Fig. 5(b) Effect of Grout (WRC)

Ê 7 r

Q 6 Grout C o ::::,5 c 4

~ 3 ::E 2 00 :aI Id =20db c: cl5 O L-__ ~~ __ ~ __ -L __ ~ __ ~ __ ~~

O 5 10 15 20 Curvature (IO"/mm)

Fig. 5(c) Effect of Grout (PGCM)

Ê z 7 ~ := 6

c 5 .., 4 E

o 3 ::E 00 2 c: 'õ c: I ..,

O Cl

O 20 40 60 80 Curvature (I0"/mm)

Fig. 7(a) Influence of One Way Cyclic Loading (FGCM)

100

280

ÊI2

QIO o

8 c

6 .., E o

4 ::E 00

2 c: 'õ c:

O .., Cl

O

70

Id =15db

10 Vertical Displacement (mm)

15

Fig. 6(a) Effect of Cover Thickness (FGCM)

Ê 12

Q 10 o

8 c ..,

6 E o

::E 4 00 c: 2 'õ ld =15db c: .., O Cl

O 10 Vertical Displacement (mm)

15

Fig. 6(b) Effect of Cover Thickness (WRC)

Ê 4 Z 75 ~ := c ..,

2 E o

::E gfl 'õ c: ..,

O Cl

O 2 4 5 Vertical Displacement (mm)

Fig. 6(c) Effect of Cover Thickness (PGCM)

Ê

Q 7 := 6 c 5 E 4 ~ 3 002 c: 'õl c: ld =30db

cl5 O ~----~--~~~~----~----~ O 20 40 60 80

Curvature (IO"/mm)

Fig. 7(b) Influence of One Way Cyclic Loading (WRC)

100

smaller bar size was used. b) Influence of lap length

The longer the lap length , the higher the observed splice. c) Grout sfrength

The higher the grout strength, the higher the lap splice strength. In Figs. 5(a) -5(c), typical moment-curvature graphs show the effect of the grout strength. A,B and C denote low, medium, and high grout strength, respectively. d) Masonry unit strength

The higher the masonry unit strength , the higher the splice strength. In Fig. 9, four types of result are shown : specimens with "No units" (outer units were peeled off from grout) , "WRC" , "Weak" units , and "Ordinary" units. They showed similar initial rigidities but different ultimate states, indicating the effect of the ma­sonry units . e) Influence of cover thickness

The larger the cover thickness of grout plus unit, the larger the plice strength . In Fig. 6(a)-6(c) , typical moment-displacement curves of three different structures are shown, expressing the effectiveness of thicker cover in ali cases . f) Influence of cyclic loading

With one- way cyclic loading, the splice strength seemed to be unchanged. and in some cases, it seemed to have increased (Fig.7) . Meanwhile, with reverse cyclic loading (tension-compression) , it was dear that the lap splice strength was con­siderably decreased (Fig.S) .

4. DISCUSSION

4.1 CriticaI Lap Length

CriticaI lap length means the minimum lap length which can carry the yielding stress of spliced bars. This denotes an index of the lap splice strength under given conditions, indicating that the shorter the critical lap length, the larger the splice strength. To evaluate this length by test results, test data in which tensile stress in failure did not exeed yielding stress must be used. If the splice yields, the applied load does not register the exact splice strength. In an actual test series, several discrete values of lap length were provided for separe te test specimen(s) when test specimens were constructed. Thus, to determine the critical lap length under some condition, it must be obtained by plotting test data on a graph with the ordinate as the tension ratio (ratio of maximum tensile stress T mar. to yield stress Ty) and the abscissa as the lap length. Assuming a linear relation between failure loads and lap lengths of splices in the range where failures occur before splices yield, the criticaI lap length of each test series can be determined by tracing a straight line from the origin of the graph through point(s) of test datum(data) to the leveI of Tmar. / Ty =1 (paraliel to the horizontal axis of the graph). The abscissa of the intersecting point of the two lines denotes criticallap length.

Figs. 1O(a)-10(c) show examples of the graphs described above, and also show each feature of the structure tested . They also illustrate the effect of grout strength in three different structures. For structures, the strengths of lap splices are shown as FGCM > WRC > PGCM in the same grout condition. The fact that PGCM is the smallest is probably due to the low strength of the units.

4.2 Influence of Reverse Cyclic Loading

281

C Monotonic L> 8 (IOKN'm) E o ~

Cyclic

-20 20 30 40 Vertical Displacement (mm)

Id =25db -8

-20

c E 8 (IOKN'm) ~ ~----------

Monotonic

-6

-8

40 Vertical Displacement (mm)

Id =25db

Fig. 8(a) Influence of Reverse Cyc1ic Loading (FGCM)

Fig. 8(b) Influence of Reverse Cyc1ic Loading (WRC)

o 10 20 30 40 50 Vertical Displacement (mm)

1.6 1.4 1.2

( 1.0

i! 08 I- 0.6

0.4 0.2 0.0

O

24

10 20 30

Lap Length (db)

Fig. 9 Effect of Confinement of Masonry Units Fig. IO(a) Derivation of CriticaI Lap

Length (FGCM)

1.6 1.4 1.2

,: 1.0

" ~ 0.8 I- 0.6

0.4 0.2 0.0

1.6 1.4

26 1.2 ,: 1.0 27

" 0.8 ~ 0.6

I-0.4 0.2 0.0

O 10 20 30 O 10 20 30

Lap Length (db) Lap Length (db)

Fig. IO(b) Derivation of CriticaI Lap Length (WRC)

Fig. IO(c) Derivation of CriticaI Lap Length (PGCM)

~40 :§.. ~30 c '" ~20 '" ...J -;; .:! .'::

li

10

O

J,

Formula (I) :

10 20 30 40 50 100 Converted Concrete Strength (MPa)

Fig. Ii CriticaI Lap Length vs. Converted Concrete Strength

282

As most test results were obtained by monotonic loading, the criticaI lap length has to consider the influence of cydic loadin~, especially that of reverse cyclic loading (tension-compression). If reinforced masonry buildings have suffered from seismic forces in earthquake countries, reinforcement woulá be subjected to reverse cyclic stress due to seismic vibration. The authors' test showed a decrease in splice strength of about 10 - 20% . The design procedure has to consider this decrease.

4.3 Comparison of Pull-Pull Test Result with Flexural Test Result

The authors first tried the lap splice test by the pull-pull method on concrete masonry and day masonry. In the test specimens, except the spliced parts , reinforcement was covered with sheaths of plastic pipe. The strengths of tested splices were as much as about 1.3 - 1.6 times higher than those tested by the flexural method under the same conditions. The reason for this difference is not certain. However, flexural cracks in the grout caused by bending moment might have accelerated splice failure . In the actual buildings under earthquake, walls of them would be subjected to shear and flexure . So that the test results by the flexural method are thorght to be reasonable .

4.4 Appllication to Formula for Estimating Lap Length for Reinforced Concrete

As described before, Oraogun [1] presented a empirical formula for ordinary reinforced concrete to estima te the criticaI lap splice length. This is shown below as formula (1) for the case without transverse reinforcement, although the formula is thought to be derived from monotonic loading test results.

fYy 50 n = 0.33ffc (1)

1.2+3 C~n where, n : Lap length (as multiple of nominal bar diameter cib)

fYy: Yield stress of reinforcement (MPa) Fc : Compressive strength of concrete (MPa) Cmin : Minimum cover thickness of concrete to reinfOIcement (cm) cib : Nominal bar diameter (cm)

If concrete cover thickness Cmin is replaced with minimum cover thickness of grout plus face shell of masonry unit , the formula is thought to be applicable to masonry. Critical lap lengths of test as expressed in 4.1 agreed roughly to values calculated with this formula . However, the effect of masonry unit strength is not evaluated. As can be observed from Fig. 10, not only grout strength but also the structure differences should be taken into account, although the gaps are oot large. As the masonry unit strength is generally larger than the grout strength, it is possible that masonry units provide confioemeot to the splice. To evaluate this effect , concrete strength Fc in formula (1) shall be replaced by something influenced by the strength of the units. One way to evaluate this effect is to introduce a weighted mean value taking into account the effect of the strength of the grout and the masonry units . In a vertical section of the specimen, assume one cirle with its center at the centroid of sections of two spliced rebars, and its radius the minimum distance from the center to the wall surface. Then consider that the areas of the grout and the masonry units within this cirde contribute to the splice confinement according to their area and strength. This weighted mean can be expressed as formula (2).

283

r. '- AgFg+AuFu (2) cc - Ag+Au

where, Fc': weighted mean of the strength of grout and masonry units (MPa) Ag : area of grout in the assumed circle (cm2 )

Au : area of uintsin the assumed circle (cm2 )

Fg : grout strength (MPa) Fu : masonry unit strength (MPa)

Fig. 11 shows the relations between the critical lap length and converted concrete strength Fc' as explained above. The correlation seemes in good agreement, although the authors are not yet sure that this method is applicable. Further verification tests are required.

5. CONCLUSIONS

Test results lead to the following conclusions.

1) If masonry unit strength is higher than grout strength, lap splices in concrete masonry show higher strength than those in WRC specimens, even with the same concrete (grout) strength. ,

2) One-way cyclic loading does not affect lap splice strength in concrete masonry. 3) Reverse cyclic loading (tension-compression) greatly influences the lap splices.

Data from monotonic loadings should be decreased as much as 10 - 20 percent in designo

4) An empirical formula for ordinary reinforced concrete to estimate the critical lap length is roughly applicable to masonry by replacing the concrete strength with the grout strength. If the effect of the confinement of splices by the masonry units could be introduced into this formula, i.e. with a converted concrete strength such as weighted mean of strength of grout and units shown in formula (2), better estimates would be obtained.

5) Pull-pull tests are not appropriate for evaluating splice strength in masonry. Flexural tests are much better. Test data obtained from pull-pull tests showed strength as much as about 1.3 - 1.6 times higher than those obtained by the flexural method.

RFFERENCES

[1] Orangun, C.O., Jirsa, J .O., and Breen, J.E., "A Reevaluation of test data on Development Length and Splices," ACI Joumal, March 1977, pp. 114 - 122.

[2] Soric, Z., and Tulin, L.G" "Length of Lap for Spliced Reinforcement in Masonry Structures," Proc. 8th Int1. BricklBlock Masonry Conference, Ireland, Sept.1988.

[3] Architectural Institute of Japan, "AlJ Standards for Structural Design of Masonry StIllctures,-Englich Version 1989 Edition-," Architectural Institute of Japan, 1994.

[4] Matsumura, A., Igarashi, 1., et al., "Test of Lap Spliced Bars in Oay Masonry," Summaries of Technical Papers of Annual Meeting, Archit. Inst. of Japan, 1989.

(in Japanese) Matsumura, A., Igarashi, 1. , et al. , "Test of Lap Spliced Bars in Concrete Ma­sonry, Part 1-3," Summaries of Technical Papers of Annual Meeting, Archit. Inst. of Japan, 1990-1992. (in Japanese)

284