Journal of the American Concrete Institute, Vol. 17, No. 1

J O U R N A L

^■Q51^5lu6 ofthe

AM ERICAN C O N C R ET E INSTITUTE

(ACI PROCEEDINGS Vol. 42)

V o l. 17 September 1945 N o . 1

CONTENTS

Papers and Reports... . 1*-84 Concrete Construction in the National Forests...CLIFFORD A . BETTS 1 Lapped Bar Splices in Concrete Beams...

...RALPH W. KLUGE and EDWARD C. T U M A 13 Tests of Prestressed Concrete Pipes Containing A Steel Cylinder...

... CULBERTSON W. ROSS 37 Field Use of Cement Containing Vinsol Resin...CHARLES E. WUERPEL 49

Job Problems and Practice... 85-92 Non-Skid Concrete Surfacing for Wooden Trestles at Mud Mountain Dam

... H. H. ROBERTS 85 Exposure of Concrete to High Temperature... 88 Derrick Stone and Cobbles in Mass Concrete... 89 Effect of Brine on Concrete... 90

Current Reviews... 93-104

News Letter... 1-12 Report of Nominating Committee • New ACI Charter • New Members • Who’s Who • Honor Roll •

f

to p r o v id e a co m ra d e sh ip in fin d in g the b e s t w ays to d o c o n c re te w ork o f a ll kin d s a n d in sp re a d in g that k n o w le d g e

A D D R E S S ! 7 4 0 0 S E C O N D B O U L E V A R D , D E T R O I T 2, M I C H

DISCUSSION

Discussion closes M a rch 1, 1 946

Concrete Construction in the National Forests— C lifford A . Betts

Lapped Bar Splices in Concrete Beams— Ralph W . Kluge and Edward C. Tuma Tests of Prestressed Concrete Pipes Containing A Steel Cylinder— Culbertson’ W . Ross Field Use of Cement Containing Vinsol Resin— Charles E. W uerpel

1 yQSl(£f%

I V I J O U R N A L

o f the

A M E R IC A N C O N C R E T E IN S T IT U T E

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BOARD O F DIRECTION, A M E R IC A N CONCRETE INSTITUTE

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ROY W . CRUM

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the AC I J o u r n a l

is edited by the Secretary of the Publications Committee under the direction of the Committee

ROBERT F. BLANKS

C h a irm a n

D O UG LAS E. PARSONS

( e x - o ffic io )

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A . T. GOLDBECK

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S T A N T O N WALKER

RODERICK B. Y O U N G

HAR VEY WHIPPLE

S e c re ta ry

I» is th e p o lic y o f th e A m e r ic a n C o n c re te In s titu te to e n c o u ra g e p a r tic ip a tio n b y its m e m b ers a n d o th e rs in th e w o rk o f e x te n d in g th e k n o w le d g e o f c o n c re te a n d re in fo rc e d c o n c re te as a basis fo r im p ro v e d p ro d u c ts a n d structures.

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tu te o b ie c tiv e s w ith in s p a c e re q u ire m e n ts co n s is te n t w ith b u d g e t lim ita tio n s .

A M E R IC A N CONCRETE INSTITUTE

N E W C E N T E R B U IL D IN G D E T R O IT 2 , M I C H I G A N

^ PLEASE NOTE

I

ing a full-blown 1946 Convention, New York City, Feb. 18-21, 1946.

2

aware of the availab ility of separate prints of each paper and report.

M any who are aware of their avail­

a b ility have not been aware that the Institute, organized and financed as it is, is not in a position to make free distribution of its literature. See the new announcement which tops the first page of each paper and report—

any one of them !n the current issue may be had at 25 or 50 cents each.

In quantities the prices are lower— for large quantities much lower.

3

"A m erican Concrete Institute Publi­

cations P o licy" (an 8-page reprint from the September 1941 Journal).

It will be sent without charge, on request.

4

ii

[

Title 42-1 — a part of PROCEEDINGS, A M ER IC AN CONCRETE INSTITUTE Vol. 42

J O U R N A L of the

A M E R I C A N C O N C R E T E I N S T I T U T E

(co p yrig hted )

V ol. 17 No. 1 7 4 0 0 S E C O N D B O U L E V A R D , D E TR O IT 2 , M IC H I G A N September 1945

Concrete Construction in the National Forests '

By CLIFFORD A . BETTSf SYNOPSIS

How U. S. Forest Service applies the fundam entals of good concrete w ithout elaborate control measures to countless small, isolated jobs in th e 175,000,000 acres of N ational forests and along 100,000 miles of roads serving these areas, w ith pictures to show some of the variety of the work done.

C o n crete te ch n iq u e on co m p a rativ ely sm all jobs sc a tte re d over th e 175,000,000 acres of th e N a tio n a l F o rests or along th e 100,000 m iles of ro ad s t h a t serve th ese areas, m u st of necessity be ra d ica lly d ifferent from t h a t used on a large co n c e n tra ted dam p ro jec t or on u rb a n p ro ­ g ram s n e a r tra n s p o rta tio n facilities a n d ce n tra l m ixing p lan ts. T h e t o ta l y a rd a g e p laced on a larg e n u m b e r of m in o r p ro jects m ay , a n d gen­

e rally does, exceed t h a t of sp ectacu lar big jobs. T h e to ta l value of th e sm all buildings, bridges, a n d o th e r stru c tu re s m ay tra n sc e n d t h a t of th e m ore publicized ones, y e t th e o p p o rtu n ity for tech n ical refinem ents an d special eq u ip m e n t is w ith th e big job. A dd to th is th e fa c t t h a t periodic in sp ectio n an d m a in ten an c e are m ore com m on on th e m a jo r in stallatio n s, a n d it becom es e v id e n t t h a t on far-flung w ork pro g ram s such as th o se of th e N a tio n a l F o rests, th e tra in in g of h u n d re d s of sc a tte re d w orkers to b u ild d u rab le, fool-proof stru c tu re s is as difficult as it is desirable.

N ev erth eless, progress is being m ade even on sm all jobs a n d discover­

ies m ad e on th e large p ro jec ts a re .b e in g applied w ith success. H igh- e a rly -s tre n g th cem ent, in te rn a l v ib ra to rs, m o istu re control, ab so rp tiv e form lining, B e n to n ite -c e m e n t g ro u t— all find ap p licatio n s on F o re st Service co n stru c tio n .

S im plification of concrete tech n iq u e s so t h a t a n y capable field m an can p ro d u ce a dense, w ork ab le concrete mix, m akes it possible to g et

* R ec e iv e d b y th e I n s tit u te A u g u s t 1944.

f E n g in e e r , D iv isio n of E n g in e e rin g , U .S. F o r e s t S ervice, W a sh in g to n .

d )

2 JO U R N A L OF THE A M E R IC A N CONCRETE INSTITUTE September 1945

Fig. 1— Smallwood bridge on Spring Creek, Missouri. It is typical of the low water bridges used where the floodwaters cover so much of the wide valleys that high water bridges would be extremely long and their high cost could not be justified by the traffic. Gage posts on each end of the bridge indicate the depth of the water over the bridge in the early stages of the flood and serve as a guide to the motorist as to his chance of crossing the river.

aw a y fro m th e fre q u e n tly in a p p lic a b le 1:2:4 m ix a n d to design econom ­ ical m ixes w ith a m in im u m of effort.

T o th is end, sim ple in stru c tio n s a n d c h a rts h a v e b een c a re fu lly w o rk ed o u t w ith o u t th e scientific refin e m e n ts t h a t a re ju stifie d on th e b ig jo b b u t w ith th e same basic principles. T h e “ W a te r D e v e lo p m e n ts a n d S a n i­

ta tio n H a n d b o o k ” of th e F o re s t S ervice c o n ta in s co m p lete in s tru c tio n s fo r p ro d u c in g im p erv io u s c o n cre te; th e “ T ru c k T ra il H a n d b o o k ” carries specifications fo r co n crete su ita b le fo r c u lv erts, b rid g es, p o sts, a n d p a v e d sectio n s; th e “ Im p ro v e m e n t H a n d b o o k ” in clu d es stu cco a n d p la s te r d a ta as w ell as in fo rm a tio n on rein fo rc ed s tr u c tu r a l co n crete. F o rm s a n d rein fo rc e m en t fa ste n e rs a re describ ed in som e d e tail. I t is in th is d issem in atio n of h an d b o o k s, specifications, a n d in s tru c tio n s t h a t a n o r­

g an iz a tio n su ch as th e F o re s t S ervice h a s c e rta in a d v a n ta g e s o v e r th e in d iv id u a l. T h e re is also th e a id re n d e re d b y c o m p e te n t in s p e c tio n a n d ex change of id eas w ith in th e o rg an izatio n .

B y p ro v id in g field forces w ith la b o ra to ry screens a n d scales to be p assed fro m one jo b to a n o th e r, a n d b y m a k in g p ro v isio n fo r e sse n tia l

m ista k es w ill be discovered a n d n o t recur, local w orkers can tu r n o u t cred itab le , long-life stru c tu re s in large n um bers. M en in te re ste d in p ro ­ ducing good concrete can becom e proficient in a co m p a ra tiv e ly sh o rt tim e a n d th e n replace th e in co m p ete n t. A d eq u ate control t h a t will m a in ­ ta in a u niform s ta n d a rd in sp ite of changes in m a te ria ls is th e m ajo r problem on rem o te jobs w hich are often finished before s ta n d a rd s tre n g th te s ts can be rep o rted .

In a sm u c h as cem en t a n d reinforcing steel are covered b y m a n d a to ry s ta n d a rd g o v ern m en t specifications, considerable a tte n tio n is given to th e selection of aggregates. I t h as been found t h a t it p ay s to p ro sp ect

4 JOURNAL OF THE AMERICAN CONCRETE INSTITUTE S e p t e m b e r 1945

Fig. 5— High Knob Dam, Jefferson Nat'l. Forest, Virginia. Aggregates delivered down hillside by chute to mixer platform; cement stored in log cabin; concrete wheeled to forms,- skip serves various levels.

Fig. 6—GreenPastures Dam, JeffersonNational Fig. 8—QuannahCreekdam, Okla. Forest, Virginia. Fig. 7—HerringtonDamsidechannel spillway,Fig. 9—Donner CreekBridgefromthe West, TahoeCity-Truckee Maryland. Highway, TahoeNational Forest, Calif.

TABLE1—BASICDATAFORDESIGNINGMIX

6 JO U R N A L OF THE A M ER IC AN CONCRETE INSTITUTE September1945

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TABLE 3— TRIAL CONCRETE MIXES Guide for Small Jobs W here T ests are not Justified

E stim ated Comp.

S trength a t 28 days (Lbs. per sq. in.)

M ax Size Aggregate (Inches)

Sacks C em ent P er C.Y.

Concrete

Max. W ater P e r Sack

C em ent (Gallons)

Lbs. of surface- dry, 2.60 sp. gr.

aggregate per sack cement Fine Coarse

2,000 + 1 5 8 275 370

2,000 2 4 ^ 8 295 430

2,000 3 4 8 305 520

2,500 + 1 5% 7 235 320

2,500 2 5 7 245 380

2,500 3 4 y 2+ 7 255 450

3,000 + 1 6 6Lj 215 290

3,000 2 5 ^ 6Li 225 355

3,000 3 5 6 H 235 410

3 ,5 0 0 - 1 6 M 6 185 275

3,500 2 6 6 195 320

3,500 3 5 K 6 215 + 375 +

4 ,0 0 0 - 1 7 + 5 ^ 1 6 0 - 255

4,000 2 646 + 5F2 1 7 0 - 290

4,000 3 6 + 5 H 1 8 0 - 350

4 ,5 0 0 - 1 8 5 135 225

4,500 2 m 5 145 265

4,500 3 7 5 155 300

Mix W eightby

= 2.9 = 3.9

= 3.1 = 4.6

= 3.2 = 5.5

= 2.5 = 3.4

= 2.6 = 4.0

= 2.7 = 4.8

= 2.3 = 3.1

= 2.4 = 3.8

= 2.5 = 4.4 2.0 2.1

2.3 1.7 1.8 1.9

= 2.9

= 3.4

= 4.0

= 2.7

= 3.1

= 3.7 1.4 = 2.4 1.5 1.6

= 2.8

= 3.2 Use m axim um proportion of well-graded coarse aggregate consistent w ith good work­

ability. To increase slump, reduce th e quantities of aggregates, m aintaining w /c ratio specified. W hen specific gravity is n o t 2.60, m ultiply pounds of aggregate by ratio of actu al specific gravity to 2.60.

all availab le sources of san d a n d gravel (p it ru n or crushed), local, or com m ercial. In m o st pits, th e o v erb u rd e n , g ra d atio n , an d s tru c tu ra l p ro p e rtie s of th e m a te ria ls are fa irly easily d eterm in ed even th o u g h some screening of sam ples or ev en la b o ra to ry te sts m ay be involved. N o t only are th e s ta n d a rd A .S .T .M . te sts for “Organic Im purities in Sand”

an d “ Clay and S ilt in San d” used, b u t know ledge acquired th ro u g h use of local san d , gravel, a n d rock on ro ad s is also b ro u g h t in to p lay in recognizing su ita b le aggregates. I t is im p o rta n t to differen tiate betw een d irty , bond -w eak en in g coatings a n d fine rock pow der t h a t fills voids.

F re q u e n tly th e in sta lla tio n of a screening p la n t or sand w asher, or b o th , is called for a n d p a y s dividends. H ere is w here th e p o rtab le crushing a n d screening p la n ts t h a t tu r n o u t ro ad m aterials can serve a d u a l p u r­

pose b y also p ro d u cin g aggregates.

L a b o ra to ry d e te rm in a tio n s are u su ally lim ited to th e m in im u m ; i.e.:

1) Sieve an aly sis of fine a n d coarse aggregates; 2) Specific g ra v ity an d w eig h t p e r cubic foot of aggregates; 3) A bsorption of aggregates; 4)

JO U R N A L OF THE A M ER IC AN CONCRETE INSTITUTE S e p t e mb e r 1 9 4 5

M o istu re c o n te n t of ag g re g a te s; 5) S tre n g th te s ts — (disclosing s tr u c ­ t u r a l v a lu e of a g g reg a tes).

F in en ess m o d u lu s, t h a t big n am e for a sim ple, co n v e n ie n t w a y of d e­

s ig n a tin g a v e rag e size, is used b y som e field en gineers as a n in d ic a to r of th e re la tiv e coarseness of th e g rading. O th e rs use it fo r ag g re g a te -v o id m ix design, b u t th is is th e ex cep tio n n o t th e rule.

M o re o ften envelope curves are d eveloped to fit local m a te ria ls an d g ra d in g is k e p t w ith in th e se lim its. T a b le s show ing th e allo w ab le p e r­

c en tag es of v a rio u s sizes of ag g reg ates are also su p p lied to field m en . P erm issib le to le ra n c e in size v a ria tio n u su a lly allow s co n sid erab le la t i ­ tu d e in selection. T h e use of w ell-graded ag g reg ates is stre sse d ev en th o u g h it is ra re ly feasible to se p a ra te th e coarse ag g re g a te in to m o re th a n tw o sizes.

(T h e a u th o r p re se n ts T ab le s 1-3 as reference m a te ria l fo r th o se n o t experienced in design.)

T h e choice of m ax im u m size of aggregate follows s ta n d a rd p ractice e x cep t t h a t th e re is a te n d e n c y to utilize “ p lu m s” (n o t closer th a n 12 in.

to each o th e r) w here th e m ixer is u n ab le to h an d le large aggregate an d w here th e cost of h au lin g in cem ent is v e ry high.

M o st of th e concrete m ix design in stru c tio n s h av e been b u ilt aro u n d th e w ate r-c em e n t ra tio a n d tria l b a tc h m eth o d s. In try in g to avoid th e old p itfa ll of a d d in g w a te r w hen th e m ix is u n w o rk ab le, it is c u sto m ary to v a ry th e p ro p o rtio n of san d an d cem ent, re ta in in g th e m axim um a m o u n t of coarse a g g reg ate t h a t is c o n sisten t w ith good w orkability.

T h is a m o u n ts to ad d in g betw een 3 a n d 4 pou n d s of cem en t for ev ery 1 p e rc e n t increase in sand.

W a te r-ce m e n t ra tio s to m eet v ario u s conditions are ta b u la te d for con­

v e n ie n t use. A llow ance is, of course, m ade for m o istu re in aggregates on m o st of th e jobs, som etim es b y w eighing, som etim es b y em pirical rules checked b y a d d in g v ario u s q u a n titie s of w a te r to th e d ry sa n d a n d ob­

serv in g its ap p earan ce.

W h en no facilities are av ailab le for p ro p o rtio n in g b y w eight, volum e­

tric m easu rem en ts are a d ju ste d for bulking. Slum ps of 3 to 4 inches p red o m in ate.

C em en t c o n te n ts are k e p t w ith in prescribed lim its (say 4 to 7 sacks p er cu. y d .) unless w ritte n perm ission to use o th e r a m o u n ts is given.

A d m ix tu res are ra re ly used. D a ta such as t h a t co n tain ed in T ab le 10

“ M ixes for Sm all J o b s ,” A C I Jo u rn a l, N o v em b er 1943, P . I l l , are dis­

tr ib u te d to th e field as guides.

E sp ecial care in curing is req u ire d as th e se are non -tech n ical te c h ­ n iq u es t h a t can be applied in rem o te localities an d h av e sufficient in ­

fluence on th e u ltim a te end p ro d u c ts to ju s tify th e tim e spent.

F in ish in g w ith rough surfaces t h a t harm onize w ith n a tu re is encour­

ag ed ; ru b b in g to a sh in y reflecting surface is frow ned on.

T h e “H andy 40 R ules” are rough b u t helpful:

1. F o rty per cent of the to tal dry weight of sand and gravel equals the weight of sand.

2. F o rty degrees F ahrenheit equals m inim um permissible placing temperature.

3. F o rty seconds equals m inim um m ixing time.

4. F o rty m inutes equals average initial setting time.

5. F o rty days equals desirable curing period.

6. F o rty years equals expected life of concrete w ithout maintenance.

T a k e for exam ple, a ty p ic a l bridge a b u tm e n t jo b in th e m o u n ta in s several m iles a h ead of finished ro ad grading. C em en t m ay be b ro u g h t in on p a c k an im als or b y tra ilb u ild e r an d trailer. S and an d gravel m ay be screened from th e stre a m bed.

10 J O U R N A L O F THE A M E R IC A N CONCRETE INSTITUTE September 1945

A m ix is d esig n ed b y th e fo re st e n g in e er to give th e req u ired strength a n d d e n s ity u tiliz in g a m in im u m a m o u n t of c e m e n t a n d th e b est com­

b in a tio n of a g g re g a te a v a ila b le . Screen sizes are specified.

F o rm s of ro u g h lu m b e r (from a n e a rb y m ill) su p p o rte d b y hewn logs o r poles (c u t on th e jo b ) give a ro u g h finish, b u t th e re is no sloppy con­

c re te , no seg re g a tio n , no p la c in g in freezin g w e a th e r w ith o u t proper p re c a u tio n s.

C o n c re te re s u ltin g fro m th e se sim p le m e th o d s of control has such s tr e n g th a n d d u r a b ility t h a t g re a te r re fin e m e n ts w ould n o t produce com­

m e n s u ra te sav in g s, n o r w o u ld th e y be p ra c tic a l on th e scattered projects in v o lv e d . W ith o u t th e a c h ie v e m e n ts of c o n c re te technicians on major jo b s re c o rd e d in te c h n ic a l lite r a tu r e as a referen ce a n d guide, the suc­

cess of th e se re la tiv e ly ro u g h a n d re a d y m e th o d s w ould not be possible.

A s i t is, v a lu a b le tim e a n d m a n p o w e r are u se d w h ere they will do the m o s t good.

[

Title 42-2 — a part of PROCEEDINGS, AM ERICAN CONCRETE INSTITUTE Vol. 42

J O U R N A L of the

A M E R I C A N C O N C R E T E I N S T I T U T E

(copyrighted)

V ol. 17 No. 1 7 4 0 0 S E C O N D B O U L E V A R D , D E T R O IT 2 , M IC H I G A N September 1945

La p p ed B a r Splices in Concrete Beam s*

By RALPH W . KLUGEt

M em ber A m erican C o n cre te Institute

and EDWARD C. T U M A f SYNOPSIS

An investigation was conducted to determine the general behavior and strength of lapped bar splices which varied in length and m ethod of splicing. T he maximum bond resistance developed in the splice and th e slip of bar was determ ined for two types and sizes of reinforcement.

T he resulting d ata clearly illustrated the m anner in which th e stress was transferred from one lapped b ar to th e other and the relative m erits of the two types of bars as well as the effectiveness of th e two m ethods of splicing was shown.

INTRODUCTION 1. Object and scope of investigation

T h e re is little in fo rm a tio n av ailab le in th e tech n ical lite ra tu re concern­

ing th e b e h a v io r of lap p e d reinforcing b a r splices. As fa r as is know n to th e au th o rs, p u b lish ed d a ta p e rta in to rela tiv e ly long laps involving p lain ro u n d bars. { I t therefo re seem ed desirable to co n d u ct a s tu d y of th is ty p e of splice in clu d in g such v ariab les as len g th , m eth o d of lapping, and ty p e of deform ed b ar.

Specifically, th e p urpose of th e te s ts described in th is p a p e r w as: first, to d e te rm in e th e d istrib u tio n of stress along th e lap p ed bars a n d th e ac co m p an y in g b a r slip ; second, to com pare th e effectiveness of th e la p p e d b a rs spaced l}/2 b a r d iam e ters a p a r t w ith t h a t of th e lap p ed b a rs in c o n ta c t w ith each o th e r; an d , th ird , to com pare th e b eh av io r of tw o ty p e s of reinforcing b a rs, one h a v in g a re lativ ely g reater lug h eig h t th a n

* R eceiv ed b y th e I n s tit u te M a r. 26, 1945.

tN a t io n a l B u r e a u of S ta n d a rd s, W a sh in g to n , D . C.

^ T ech n o lo g ic P a p e rs of th e B u r e a u of S ta n d a rd s N o . 173, “ T e sts of B o n d R esista n ce b etw e en C o n cre te a n d S te e l,” b y W . A. S la te r, F . E . R ic h a rt, a n d G . G . Scofield, or “ S tr u c tu r a l L a b o ra to ry In v e stig a tio n s in R ein fo rc ed C o n c re te M a d e b y C o n c re te S h ip S ectio n , E m erg en cy F le e t C o rp o ra tio n ,” b y W. A. S la te r, P ro c. A m erican C o n c re te I n s tit u te , V. X V , 1919.

(13)

TABLE 1

14 JO U R N A L OF THE AM ER IC AN CONCRETE INSTITUTE September 1945

Sp e c/- ' M E N

£

N

No m/m a l Le n g t h

o f La p, In.

De s/g b De t a/l s o f Be a m s

4 -5 0 A -40 A-3 0 A-20 A-40X A-3QX A-ZOX ' B-40 B -30 B-ZO B-3M B-ZOX'

SO 4 0 3 0 2 0 4 0 3 0 3 0 4 0 3 0 2 0 3 0 2 0

B a r p / a n

3 ' - 3 ■ 5 - 6 " 3 L3 " ,

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A-50 A-40 A-30

A-20

A-40*

A-30X A-ZOX ~

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8-20 B-/0 ~

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/n s p e c /m e n s c /e s /g n p /e e / w ///z / / e /e //e r 'X fo //a tv /p ff s p e c /m e n n a r n /e r /a p p e B B a r s w e r e ir? c o n /p c / w /Y// ep c /z o /A e r /rr a / / oY A erdepm zr b a r s w ere e p p c e c / / J 5 /a . c /e a r .

th e o th e r. B o th b a rs w ere considered to be a m o n g th e b e st a v a ila b le as f a r as b o n d in g p ro p e rtie s w ere concerned.

T h e te s t p ro g ra m w as d iv id ed in to tw o p a r ts as in d ic a te d in ta b le 1.

O ne p a r t covered th e te s ts of large b ea m s co n ta in in g b a rs 1 in c h in d ia m e te r, a n d th e o th e r, te s ts of sm all b e am s reinforced w ith b a rs Y i in.

in d ia m e te r. F o r each size of b a r, tw o se ts of te s ts w ere m ad e, d e sig n a te d series A a n d series B. All te s ts w ere m ad e on each of th e tw o ty p e s of b a rs.

T h e te s ts of series A w ere p rim a rily co ncerned w ith th e m a n n e r in w hich th e te n sile stress or b o n d stress v a rie d along th e lap p ed b a rs a n d th e m a g n itu d e of th e slip a t v ario u s p o in ts. T h e d a ta also serv ed to com ­ p are th e b e h a v io r of th e tw o ty p e s of b a rs a n d th e re la tiv e effectiveness of th e tw o m e th o d s of lap splicing. T h e le n g th of splice in th is series v a rie d fro m 20 to 50-bar d ia m e te rs an d only a single te s t w as m ad e for each le n g th of lap, ty p e a n d size of bar.

Series B w as p lan n ed to p ro v id e values of av erage m ax im u m b o n d stress for several le n g th s of lap. H ow ever, th e re were few w ell defined b o n d failu res an d , n o tw ith sta n d in g th e use of h ig h -y ie ld -stre n g th re in ­ forcem ent, m a n y specim ens failed b y yielding of th e b a rs b e y o n d th e

Fig. 1 — Reinforcing bars (2 No. 1 at top; 2 No. 2 below)

splice. T h ese te sts, how ever, h a d som e value insofar as in d icatin g th e effectiveness of th e s h o rte r laps. In th is series, th e len g th of th e splices v aried from 10 to 40-bar d iam e ters an d in all b u t a few instances d u p licate te s ts w ere m ade.

DESCRIPTION OF TEST SPECIMENS 2. Design of beams

D e ta ils of th e te s t specim ens are show n in th e diagram s of T able 1. As in d ic a te d in th e b a r p lan , each beam co n tain ed a single lap splice and tw o co n tin u o u s bars. T h e beam s w ere designed to p e rm it a clear spacing of lgK -bar d ia m eters betw een all of th e b ars a t a section th ro u g h th e splice an d th e ir effective d e p th was such t h a t th eo retically , for a stress of 20,000 psi in th e co n tin u o u s bars, th e concrete w as stressed to 1,500 psi in com­

pression, assum ing a m o d u lar ra tio of n = 8. T his design resulted in a section, ou tsid e th e region of th e lap splice, c o ntaining 1.4 p ercen t of ten sile reinforcem ent.

T h e sp an of th e large beam s (co n tain in g th e 1-in. b ars) was 12 ft. and t h a t of th e sm all b e am (w ith th e 34 in. b a rs) w as 8 ft. L oad was applied to th e b eam s a t tw o p o in ts, so spaced as to provide a region of c o n stan t m o m e n t sufficient in e x te n t to accom m odate th e longest lap splice of 50-bar diam eters. T h e a c tu a l len g th s of lap were slightly g reate r th a n th e given n o m in al valu es in order to accom m odate a n even m ultiple of a 5-in.

sp acin g of gage holes. T h e ex act values were 34 in- and 34 in- g reater th a n th e n om inal len g th s for th e 34 in- a n d 1 in. bars, respectively.

T h e rein fo rc em en t consisted of tw o ty p es of bars, illu strated in Fig. 1, a n d d esig n ate d b y n u m b er. B a r No. 1 h ad average lug heights of 0.020 in.

a n d 0.027 in., w hereas B a r N o. 2 h a d average lug heights of 0.031 in. an d 0.063 in. for th e 34 in. a n d 1 in. bars, respectively. T h e ap p ro x im ate b ea rin g areas of th e lugs per lineal inch are given in T able 2. F o r th e

16 JO U R N A L OF THE A M E R IC A N CONCRETE INSTITUTE September 1945

TABLE 2— DIMENSIONS OF REINFORCEMENT

Type B ar size

*D iam eter *Area Av. lug h eight

Lugs per yd.

A pproxim ate bearing area of lugs per lineal inch

in. in. sq. in. in. sq. in. per in.

B ar No. 2 Yt. 0 .4 9 0.190 0.031 128 0 .1 6

1 1.00 0.787 0.063 80 0 .4 2

B ar No. 1 Y2 0.51 0.198 0.020 100 0 .0 8

1 1.00 0.785 0.027 50 0.11

*As d e te r m in e d b y w e ig h t-le n g th m e th o d .

TABLE 3— M E C H A N IC A L PROPERTIES OF REINFORCEMENT (Each value is the average of three tests)

T est

Series T ype

Bar size

Yield point

Tensile stren g th

E longation in 8 in.

in. psi. psi. percent

B ar No. 2 Yi 50,000 79,800 2 0 .4

A B ar No. 1 Y2 61,800 90,000 2 2 .0

B ar N o. 2 ¹ 45,000 71,600 26.2

B ar N o. 1 ¹ 51,600 97,200 18.8

B ar No. 2 M 61,800 97,400 16.6

B B ar N o. 1 Y 55,000 92,600 18.0

B ar N o. 2 ¹ 60,400 94,200 18.8

B ar No. 1 ¹ 53,000 92,300 23.1

specim ens of series A, th e rein fo rcin g b a rs w ere of in te rm e d ia te g rad e or b e tte r, w h ereas th e b a rs used in b eam s of series B w ere exclusively of h ig h -stre n g th steel, h a v in g a yield p o in t of from 53,000 to 62,000 psi. T h e ir d im ensions a n d m ech a n ical p ro p e rtie s are liste d in T a b le s 2 a n d 3, respectively.

T h e con crete w as designed for a com pressive s tr e n g th of 4,500 psi a t 28 d a y s a n d consisted of m o d e ra te -h e a t-o f-h a rd e n in g p o rtla n d c e m e n t a n d w ash ed P o to m a c R iv e r san d a n d gravel, th e la tte r h a v in g a m a x im u m size of z/ i in- T h e p ro p o rtio n s of cem e n t, sa n d , a n d g ra v e l in th e m ix w ere, respectively, 1:2.5:3.3 b y d ry w eig h t; a n d th e n e t w a te r c o n te n t, 7 ^ gal- of w a te r p er sack of cem en t, co rresp o n d in g to a w a te r-c e m e n t ra tio of 0.67 b y w eight. The resu ltin g con crete h a d an av e ra g e slu m p of 3 3^ in.

a n d d eveloped an average com pressive s tre n g th of 4,700 psi te s te d d ry a t 28 days.

3. Construction of specimens

F o u r b eam s w ere gen erally c a st a t one tim e , tw o sm all b eam s a n d tw o large beam s, b o th p a irs c o n ta in in g splices of e q u a l le n g th in te rm s of th e

Fig. 2 — Reinforce­

ment for 30-bar d i­

ameter splice. Lap­

ped bars in contact.

Spe c i me n t y p e A-30X.

b a r d ia m e te r. One of each p a ir co n tain e d B a r N o. 1 an d th e o th e r B a r N o. 2. T h e rein fo rc em en t w as rigidly su p p o rte d on w ire chairs as well as on ta p e re d w ooden in serts. R em o v al of th e la tte r from th e c ast beam p ro v id ed openings to th e b a rs for s tra in m easurem ents. Spacing of th e in se rts is show n in Fig. 2 for beam s of series A. T h e beam s of series B co n ta in e d gage holes, sim ilarly spaced, b u t confined to th e region outside th e lim its of th e splice.

T h e free ends of th e lap p e d b a rs w ere saw c u t in o rd er to avoid th e d is to rte d ends generally acco m p an y in g sh ea r cuts. W here no space was p ro v id ed b etw een th e la p p e d bars, th e y w ere firm ly w ired to g e th e r a t th e ce n te r an d n e a r th e ends of th e lap w ith soft iron wire. T h e concrete was v ib ra te d in to place in th e beam s a n d th e control cylinders. T w en ty -fo u r h o u rs a fte r casting, th e side form s of th e beam s an d cylinders were rem oved. T h e te s t specim ens w ere th e n w rap p ed w ith several lay ers of w et b u rla p a n d cured in th is co n d itio n for a perio d of 7 days, a fte r w hich th e y were p e rm itte d to d ry in th e la b o ra to ry for 21 d ay s before testin g .

DESCRIPTION OF TESTS 4. Procedure

T o fa c ilita te th e m e a su re m e n t of stra in , th e beam s w ere te ste d in an in v e rte d p o sitio n on s ta n d a rd b eam su p p o rts re stin g on th e p la te n of a 600,000-lb. ca p a c ity h y d rau lic te stin g m achine eq u ip p ed w ith a 30,000-lb.

c a p a c ity gage. T h e lo ad w as a p p lie d n e a r th e ends of th e specim ens th ro u g h a p a ir of w ide flange I-b e am s restin g on rollers a n d bearing p la te s, w hich can be observed in Fig. 3 a n d 4. T h e w eight of th e loading b eam s a n d a u x iliary eq u ip m e n t was a p p ro x im ately 2,400 lb. an d was con­

sidered as p a r t of th e to ta l ap p lied load.

S train -g ag e readings were o b ta in e d w ith a 5-in. W h ittem o re gage for 8 to 10 in cre m en ts of load. In th e te s ts of series A, stra in m easurem ents w ere m ad e a t 2y 2 in. an d 5 in. in te rv a ls on th e y 2 in. a n d 1 in. bars, resp ectiv ely , th e in te rv a ls ex ten d in g from th e free ends of th e lapped b a r to th e lo ad p o in ts a n ^ a t a n u m b e r of selected p o in ts on th e co ntinuous

18 JO U R N A L OF THE AM ER IC AN CONCRETE INSTITUTE September 1945

bars. Slip of th e la p p e d b a rs w as d e te rm in e d w ith th e s tr a in gage b y ob serv in g th e re la tiv e m o v e m e n t b etw een gage holes on th e la p p e d b a rs a n d th o se on th e co n tin u o u s b a rs one gage le n g th rem oved. T h e re a d in g s w ere c o rrec ted for s tra in in th e b a rs as well as for th e angle of th e line of m e a su rem en t. M ea su re m e n ts of s tra in in th e te s ts of series B w ere con­

fined to gage lines o u tsid e th e region of th e splice, th u s a v o id in g th e u n c e rta in effect of openings in th e concrete on th e b o n d stresses alo n g th e la p p e d bars. T h e av erag e b ond stress could be d e te rm in e d fro m th e o b serv ed ten sile stress in th e b a rs a t th e ends of th e lap.

D u rin g th e la tte r stag es of th e te s t, th e beam s were carefu lly sc ru tin iz e d fo r possible lo n g itu d in a l crack s w hich, in som e beam s, fo rm ed in th e c o n ­ crete along th e la p p e d b a rs w hen th e m ax im u m load w as reached.

aa

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Fig. 7 (right)—Distributionofstressalong lappedbars—lengthofsplice 30-bar diameters. Y2 ¡n-bars. Stressvaluesare the product of the modulusof elasticityandobserved strains.

20 JO U R N A L OF THE AM ER IC AN CONCRETE INSTITUTE September 1945

TEST RESULTS OF SERIES A 5. Distribution of stress along lapped bars

T h e stra in s a t c o rresponding gage lines on each of th e tw o b a rs of th e lap w ere av era g ed a n d c o n v e rte d to stress on th e a ssu m p tio n t h a t E a was 30,000,000 psi, a n d lo a d -stress d ia g ra m s p lo tte d for all of th e gage points along th e b a r. T y p ic a l d ia g ram s are show n in Fig. 5. T h e cu rv es are a rra n g e d in th e o rd er t h a t th e gage lines a p p e a r on th e la p p e d b a r, sta rtin g a t th e free end. A fte r in itia l c rack in g of th e co n crete, th e stre ss in th e bar a t v a rio u s p o in ts w as closely p ro p o rtio n a l to th e lo ad u n til e ith e r th e bond stress w ith in th e gage line a p p ro a c h e d a m ax im u m or th e yield p o in t of the stee l w as reach ed . T h e cu rv es for th e p o rtio n of th e b a r n e a r th e free end c le arly show th e m a x im u m ten sile stress, w hich is also a m easu re of the m a x im u m b o n d stre ss t h a t th e b a r w as cap ab le of d eveloping w ith in the first few inches of em b e d m e n t.

T h e v a ria tio n of ten sile stre ss along th e la p p e d b a rs of th e various specim ens is show n in F ig. 6 th ro u g h 13. T h e p lo tte d v alu es w ere tak en fro m th e lo a d -stre ss cu rv es describ ed a b o v e fo r lo ad s n e a r th e m axim um a n d fo r lo ad s w hich p ro d u c e d a stre ss of a b o u t 18,000 to 20,000 psi in the b a rs o u tsid e th e region of th e splice. T h e p a ir of cu rv es show n on each axis, one for each b a r of th e splice, are id e n tic a l ex cep t t h a t th e y are tu rn e d en d for end. B o th th e “ N o. 1” a n d “ N o. 2 “ b a rs are rep resen ted in each figure.

T h e slope a t a n y p o in t on th e curves is ob v io u sly a m e asu re of th e bond stre ss d ev elo p ed b e tw ee n th e b a r a n d th e co n crete a t t h a t p o in t. T h e g re a te st b o n d stress w as d eveloped a t th e free end of th e b a r, a n d th is stre ss decreased fa irly u n ifo rm ly in th e 20- a n d 3 0 -d ia m e te r lap s o v er th e e n tire splice. In th e longer splices, how ever, th e b o n d stress, a f te r a sim ila r decrease re ach ed a low v a lu e w ith in th e m id d le th ird , th e n in creased to w a rd s th e e n d of th e lap. In som e instan ces, w here th e stresses a t th e lo ad e d en d w ere less th a n 30,000 psi, th e b a rs w ith in th e m id d le th ir d of th e splice did n o t develop a n y b o n d stress. T h is w as p a rtic u la rly e v id e n t in th e 4 0 -d iam e ter splices a n d th e 5 0 -d iam eter splices of th e “ N o. 2 ” b a rs. F ro m th e s ta n d p o in t of efficiency, such a splice is lo n g er th a n necessary.

T h e stress in th e co n tin u o u s b a rs a t v ario u s p o in ts in th e region of th e splices is also in d ic a te d on th e d iag ram s b y a sm all cross. E x c e p t for splices 20-bar d iam ete rs long, th e effect of th e la p p e d b a rs w as to d ecrease g re a tly th e stress in th e c o n tin u o u s b ars w ith in th e lim its of th e lap in m a n y in sta n c e s v e ry closely a p p ro a ch in g th e stress in th e p o rtio n of th e la p p e d b a r from th e cen te r of th e lap to its loaded end. I n b eam s con­

ta in in g th e 20-diam eter splices, th e stress in th e c o n tin u o u s b a rs w as, in general, fairly c o n sta n t over th e e n tire lap, th e effective steel a re a of

Spec/mens A-40I _ Spec/mens A-SO^Spec/mirrs ^ A-20

/S 'C ? O O O / is / s w u /S ' d/ys-vajL

Fig. 10(right)—Distributionofstressalong lappedbars—lengthofsplice 20-bar diameters. 1-in. bars. Stressvaluesare the product of the modulusof elasticityandobserved strains.

Specimens A-30 0nSpecimens A-40 Specimens A-50

JO U R N A L OF THE AM ER IC AN CONCRETE INSTITUTE

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Fig. 14 (left)— Distribution of stress along lapped bars for various lengths of splice.

Fig. 15 (right)— Relation between bond stress at free end of bar and tensile stress at limit of lap.

Stress va lu e s a re th e p ro d u c t o f th e m o d u lu s o f e la s tic ity a n d o bse rve d strains.

th e la p p e d b a rs a p p a re n tly being e q u iv alen t to a single b a r a t a n y section.

A d e tailed com parison betw een th e stress d istrib u tio n curves for each len g th of lap is show n in Fig. 14. T h e curves rep resen t th e stress dis­

trib u tio n for a load of 10,000 lb. on th e beam s co n tain in g th e Yi in- b a rs a n d 45,000 lb. for th e beam s co n tain in g th e 1-in. bars. T hese loads stressed th e rein fo rcem en t bey o n d th e splice to 45,000 psi a n d 37,000 psi, respectively. T h e slopes of th e curves in th e v ic in ity of th e origin, w hich rep resen ts th e free end of th e b a r, are a lm o st id e n tic a l for each set of curves, co n seq u en tly th e b o n d stress developed w ith in th e first few inches of b a r e m b ed m e n t w as p ra c tic a lly alike, regardless of th e le n g th of splice for a given load a n d ty p e of b ar.

I t is to be expected t h a t som e rela tio n sh ip exists betw een th e b o n d stress a t th e free end of th e lap p ed b a r a n d th e tensile stress in th e b a r a t th e o th e r lim it of th e splice. Fig. 15 in d icates, w ith in c ertain lim its, a lin ea r re la tio n sh ip w hich, for each ty p e of b a r, a p p a re n tly is in d e p e n d e n t of th e le n g th of splice for laps of 30-bar d iam eters or m ore. T h u s, th e d a ta for th e 30-, 40- a n d 5 0 -d iam eter laps a p p e a r to group them selves a b o u t in d iv id u a l s tra ig h t lines, one for B a r N o. 1 a n d one for B a r N o. 2, b etw een tensile stresses of 5,000 psi a n d 40,000 psi. T h e 20 d ia m e te r lap splices also show a sim ilar relatio n sh ip , b u t th e slopes of th e lines are, in som e instan ce s, g re a te r th a n th o se of th e longer splices. W here th e y differ, th e d a ta for th e 20 d ia m e te r lap are in d ic a te d b y a bro k en line.

24 JO U R N A L OF THE AM ER IC AN CONCRETE INSTITUTE September 1945

Fig. 16 (left)— Distribution of stress along lapped bars compared for “ No. 1” and “ No. 2"

Y¿ in. bars.

Fig. 17 (right)— Distribution of stress along lapped bars compared for “ No. 1 ” and "N o.

2” 1 in. bars.

Stress va lu e s a re th e p ro d u c t o f th e m o d u lu s o f e la s tic ity a n d o b se rve d strains.

T h e d is trib u tio n of ten sile stresses along th e la p p e d b a rs fo r a given lo a d a n d for v a rio u s le n g th s of lap is co m p ared in g re a te r d e ta il fo r b o th ty p e s of b a r in Fig. 16 a n d 17. In ev e ry in sta n ce, th e N o. 2 b a rs p ick ed u p stress m o re ra p id ly a t th e ir free ends th a n th e N o. 1 b a r, a lth o u g h th e r a te of increase of stress b e y o n d an em b e d m e n t of 5 -b ar d ia m e te rs w as n o t sig n ifican tly different. In a well designed splice, th e normal stress, t h a t is, th e stress th e b a r w ould n o rm a lly c a rry if i t w ere co n tin u o u s, sh o u ld be reach e d a t th e end of th e splice. I n Fig. 18 a n d 19, th e ob serv ed stresses in th e la p p e d b a rs a t th e end of th e splice are p lo tte d w ith re sp e c t to th e lo ad ap p lied to th e beam s for each le n g th of lap. Also show n b y solid lines is th e lo ad -stress re la tio n sh ip t h a t w ould o b ta in if th e b a rs w ere continuous. T h is normal stress w as assu m ed to be th e sam e as th e a v e r­

age stress observed in b o th th e co n tin u o u s a n d la p p e d b a rs w ell o u tsid e th e lim its of th e splice. T h e d ev ia tio n of th e o bserved stre ss fro m th e normal stress b ey o n d a v alu e of a b o u t 35,000 psi is e v id e n t for m o st of th e beam s reinforced w ith N o. 1 bars. T h e exceptions a re th e 5 0 -d ia m e te r lap an d possibly th e 4 0 -d iam eter lap. On th e o th e r h a n d , w ith th e excep­

tio n of th e 2 0 -d iam eter lap, th e o bserved stresses in th e N o. 2 b a rs are p ra c tic a lly id en tical w ith th e normal stresses u p to th e y ield p o in t of th e steel, w hich h a d a value of a b o u t 50,000 psi a n d 45,000 psi for th e in.

a n d 1-in. bars, respectively.

I t is e v id en t th a t, if th e stress in th e lap p e d b a r a t th e en d of th e splice is n o t equal to th e normal stress, th e a d ja c e n t co n tin u o u s b a rs assum e a g re a te r p ro p o rtio n of th e to ta l tensile stress. A ctu ally , th is w as n o t a serious m a tte r in th e beam s w hich w ere te ste d w ith lap s g re a te r th a n 20 d ia m e te rs; for even a t th e hig h er stresses th e difference b e tw ee n th e ac tu a l an d normal stress w as n o t m ore th a n 8 to 10 p ercen t, p ro b a b ly h a lf 0f

Fig. 18 (left)— Load-stress relation for lapped bars at limits of the splice— Beams of series A . in. bars.

Fig. 19 (right)— Load-stress relation for lapped bars at limits of the splice— beams of series A . 1 in. bars.

Stress va lu e s a re th e p ro d u c t o f th e m o d u lu s o f e la s tic ity a n d o bse rve d strains.

w hich w as assum ed b y th e tw o continuous b a rs a t t h a t p o in t. C on­

seq u en tly , th e load w hich w ould n o rm ally stress th e reinforcem ent, b ey o n d th e splice, to its yield p o in t w as n o t m a te ria lly affected b y th e slig h t sh iftin g of stress. T his w as n o t tru e , how ever, of th e beam s con­

ta in in g th e 20-diam eter lap splices, w here th e load a t w hich yielding of th e rein fo rcem en t occurred as a rule w as som ew h at low er th a n for th e o th er beam s.

6. Bond strengths

T h e load-stress curves for th e gage line n e a re st th e free ends of th e la p p e d b a rs in d ic a te d t h a t th e tensile stress th e re reach ed a m ax im u m valu e a t ap p ro x im a te ly th e sam e load in all of th e specim ens tested . Also, th e m ax im u m stresses a t th is p o in t w ere reaso n ab ly alike for a given ty p e an d size of b a r regardless of th e len g th of lap splice. H ow ever, th ere was a m a rk e d difference in th e values for th e tw o ty p e s of bars. U sing th e m a x im u m ten sile stresses scaled from th e curves, th e b ond stre n g th developed b y th e lap p ed b a rs for an em bedded len g th of ab o u t 3 in. w as co m p u ted fo r each specim en a n d th e b o n d stresses th u s o b ta in e d are listed in T ab le 4. T h e values are som ew hat higher th a n w ould be expected from s ta n d a rd p u ll-o u t te sts, b u t com pare fa v o ra b ly w ith resu lts from un p u b lish e d d a ta of sim ilar b ars in b eam tests.

26 JO U R N A L OF THE A M ER IC AN CONCRETE INSTITUTE September 1945

TABLE 4— M A X IM U M BOND STRESS DEVELOPED A T FREE END OF LAPPED BARS

(Av. stress for an em bedm ent of approx. 3 in.)

Specimen

14-in. bars 1-in. 1rars

No. 2 N o. 1 No. 2 No. 1

A-50 850 745 880 755

A-40 970 840 960 655

A-30 880 810 1,010 705

A-20 890 745 1,010 755

Average 895 785 965 718

,A -40x 880 675 755 630

*A-30x 920 675 1,010 755

*A-20x 950 810 1.000 760

Average 917 720 922 715

♦ L a p p e d b a r s in c o n t a c t w ith e a c h o th e r.

O S /O 1 5 2 0 2 5 " D is ta n c e / r o o t fr e e e n o ' o f ¿ to r , i n c h e s

D is ta n c e f r o m f r e e e n d o f ¿> arf i n c h e s

Fig. 20 (left)— Distribution of stress along the lapped bars of two types of lap-splices. T iin . bars.

Fig. 21 (right)— Distribution of stress along lapped bars of two types of lap-splices. 1 in.

bars.

Stress va lu e s a re th e p ro d u c t o l th e m o d u lu s o f e la s tic ity a n d o b s e rv e d stra ins.

7. Comparison between methods of lapping

T h e re su lts of th e tw o m e th o d s of lap p in g , one in w h ich th e la p p e d b a rs are spaced 1 Yi d ia m e te rs clear a n d th e o th e r w here th e y are in c o n ta c t, are co m p ared in Fig. 20 a n d 21. H ere th e stre ss d is trib u tio n curves are show n for a given lo ad a n d for each le n g th of splice. T h ere is p ra c tic a lly no difference in th e stre ss b etw een th e cu rv es re p re se n tin g each ty p e of la p a t a n y p o in t for th e 1-in. b a rs an d little difference for th e 3^-in. b ars.

T h ese differences are p ro b a b ly no g re a te r th a n w ould be o b ta in e d in d u p lic a te te s ts . A fu rth e r com parison is show n in Fig. 18 an d 19, in w hich th e lo ad -stress curves for th e lap p ed b a rs a t th e lim its of th e lap are show n fo r b o th ty p e s of splice. H ere, again, th e differences are sm all a n d in m o st in sta n c e s negligible. I t is also ev id en t fro m T ab le 4 t h a t it m a d e little difference in th e b ond s tre n g th in th ese te s ts w h e th e r th e la p p e d b a rs w ere sp aced l j ^ d iam eters a p a r t or in c o n ta c t.

8. Distribution of slip along lapped bars

T h e re w ere larg e differences in slip betw een ce rta in specim ens an d th e se differences w ere n o t alw ays consistent. I t is highly p ro b ab le t h a t w ide v a ria tio n s in slip d a ta , o b ta in e d in th e m an n er described herein, w ould also be observed in d u p lica te te sts. I t should be n o te d t h a t th e slip a t each p o in t along th e la p p e d b a r w as reckoned from a po in t d irec tly opp o site i t on th e a d ja c e n t contin u o u s b a r, an d co nsequently does n o t necessarily in d ic a te th e re lativ e m ovem ent of th e b a r w ith resp ect to th e concrete.

Slip s ta rts , q u ite n a tu ra lly , a t th e free end of th e b a r w here th e bond stress is alw ays th e g re a te st an d progresses along th e b a r as th e tensile stresses increase a t th e loaded end. In th is respect, th e b e h av io r of th e b a r differs from th a t in th e u su al p u ll-o u t or beam te sts for bond. T h e d istrib u tio n of th e slip along th e lap p ed b a rs for various len g th s of splice is show n in Fig. 22 an d 23. T h e d a ta are p lo tte d for tw o loads, one of w hich p ro d u ced a tensile stress of a p p ro x im ate ly 18,000 psi a n d th e o th e r ab o u t 40,000 psi a t th e loaded ends of th e lap p e d bars. M o st of th e evidence seem s to in d ica te th a t th e d is trib u tio n an d m a g n itu d e of th e slip is in d e p e n d e n t of th e len g th of th e splice. T h ere ap p ears to be a serious d e p a rtu re from th is principle w ith th e N o. 1 b ars a t higher loads.

T h e d iscrep an cy is n o t read ily acco u n ted for except in so fa r as a p a r t of th e v a ria tio n m ight be ac cid en tal. W ith in th e ran g e of w orking stresses, how ever, th e le n g th of lap seem s to h av e little or no effect u p o n th e m a g n itu d e of th e slip for t h a t b ar.

W ith one exception, th e re w as little difference in slip betw een th e N o. 1 an d N o. 2 j/y-in. b a rs fo r a given tensile stress a t th e loaded ends of th e la p p e d bars. T h e exception w as th e 20-diam eter lap p ed N o. 1 b a r w hich show ed a co nsiderably g re a te r slip th a n a n y of th e o th e r lap p ed b a rs of th is size. T h e 1-in. N o. 1 ba rs, how ever, exh ib ited a decidedly g re ater slip for a given lo ad or e n d stress th a n th e 1-in. N o. 2 b a rs or e ith e r of th e in. bars. T h e only e x p lan a tio n t h a t can be offered for th e dissim ilar b e h a v io r of th e tw o sizes of N o. 1 b a rs is th e possible effect of th e lug heig h t. T ab le 3 show s t h a t th e 1-in. N o. 1 b a r h a d a m u ch low er ra tio of lug h eig h t to d ia m e te r of b a r th a n th e 3drhi. b a r of th e sam e m ake, w hereas th is ra tio for th e tw o sizes of N o. 2 b a rs w as alm ost alike.