Construction Joints

Co n s t r u c t io i o n j o i n t s fo fo r m u lt is t o r y s tr u c t u r e s Where to locate locate th em, h ow to form form th em

B Y B RUCE SUPRENANT DEPARTMENT EPARTMENT OF CIVIL ENGINEERING & M ECHANICS NIVERSIT SITY Y OF SOUTH F LORIDA U NIVER TAMPA, FLORIDA

C

o n s t ru ction joints are s t o p p i n g p l a c e s fo fo r c o n c rete work. They’re They’re n eed ed bec aus e it’s it’s no t feasible to place all the con crete for for m ost jobs in o n e p o u r . Br Br e a k in in g t h e j o b i n t o sm aller portions a llows llows the con tra ctor to: Match po ur size with con cre t e d e li ve ry ca pa b ilitie s s

Choose th e be st crew size size

s

Make best use of equipmen t

s

Red uce form w o rk cos ts by in c reas in g form re u s e

Ve rtical an d ho ri zon tal c on stru ction joints m ust be carefully carefully locate d t o p r o d u c e t h e s e b e n e f it it s w it it h o u t weaken ing the stru c t u re. And to fun ction ction prop erly erly the joints joints m ust be b u ilt cor re c t ly.

Where Where to install construction  jo in t s The architect or engineer often specifies specifies constru ction joint location

Figure 1 . The The ACI Code Code requires construct ion joints in elevated slabs t o be located w it hin the middle t hird of spans of slabs, slabs, beams, and girders. Joints Joints must be located at least two beam widths away from beam-girder intersections.

in th e con tract do cum ents. So So m et i m e s, th ough , th e cont racto r is asked asked to subm it a construction construction joint plan a fter th e bid letting. In either c a s e, th e en gine er or con tra c t o r should consider econom y, appeara n c e, stren gth, an d du rability rability wh en choosing joint location. Eco n om y. y. If th e co nt racto r h as inpu t on where to pu t con stru c t i o n  jo in t s, he ca n m a tc h t h e p ou r size t o h is cre w’s w’s ca pa bilities. Also, Also, p lacing a co n str u ction joint in a pra c t ic a l spot, such as between footing an d w a lls, a voids com ple x, cost ly form w o rk . Ap p e a ra ra n ce. Co n st ruc tio n jo in ts can influen ce h ow a stru ct u re looks. Often Often th ey are installed installed in an incon sp icuo us po sition . So m e t i m e s, though, they are used as a feature of  th e stru c t u re. Som e de signers use reglets reglets and rustication gro ov oves es t o hide th e joint joint s. Whatever schem e is used, t he joints shou ld fit fit th e arc h it e c t u ral de sign . St ren gth. A con struc tion joint int rod uce s a weak ve rtical or ho ri zo ntal plane in an otherwise otherwise m on olit olithic hic c o n c rete m em be r. This o bvious slip slip p l a n e m a y re re d u c e t h e s t r e n gt gt h o f   b e a m s, s, colu m n s, an d walls. walls. Test results sh ow th at c onstru ction joints joints redu ce a mem ber’s ber’s shear s t rength bu t no t the flexural flexural o r ben ding strength. Tests Tests con con du cted on simply supp orted beam s with with a ve rtical constru ction joint at the c e n t e r s h o w ed ed t h e s a m e lo lo a d - d e flection lection and ultimate mo m ent chara c t e ristics as an un jointed bea m ( Ref. 1). Me a s u rem en ts o f cra c k  width s also in dicated sim ilar ilar perf o rmance between jointed and un jo in t e d b e a m s. Th e se re su lt s h e ld t rue for a joint tha t was delibe ra t e ly debonded between the two conc rete p ours. The con struction join join t in t hes e te sts was located in a re g io n of negligible negligible sh ear. In a region whe re the sh ear is sigsignificant, nificant, a constru ction joint joint with a s m o o t h s u r fa fa c e r e d u c e s t h e s h e a r s t ren gth of the m em ber by 40%. Bu Bu t a beam having a joint with a ro u ghened surface surface has a failure failure m ode an d ultimate load similar similar to an un jointjoint1 ed beam . A  ⁄ 16-inch texture over over th e

Figure 2 . Bulkheads form vertical construction joints for elevated slabs (above) and beams (below). Bulkheads are stiffened and braced braced t o resist lateral concrete pressures.

e n tire face qu alified as a ro u gh en ed s u rface in th ese test s (Ref. (Ref. 1). Du ra ra b i l i t y. The loca tion of co n s t ruc tion joints also is a ffecte ffecte d by whether or n ot water and salts can easily easily enter t he joint. Althou Althou gh con c rete h as low perm eab ility ility to water an d sa lts, lts, con struction joints do n’t n’t . Wate r an d salt leaking th ro u g h  jo in t s m a y ca u se r u st in g o f re r e b a r, p re s t ressing ten don s, an d a nch orage h ard wa re in p arkin g s tr u ct u re s. Locate con struction joints joints at h igh igh po ints in th e floor floor away from d rain s. Wh e re d ur ab ility ility is cr itical, u se fewer joints by allowing larger floor p o u r s. Con sider using waters top at som e or all joint locations.

Cod e (Ref. 2, Sect ion 6.4.4) re q u i re s that: “Co “Co n s t ru ctio n joint s in floors shall be located within the m iddle t h ird o f span s o f slabs, bea m s, an d g i rd er s. Joint s in girder s sh all b e offset a minimum distance of two times the width of intersecting b e a m s” ( Figu re 1). 1) . T h e re m ay be high she ar forc e s e ven at midsp an, for instan ce, w h e re a be am int erse cts a gird e r. Th a t’s t’s wh y th e co de re q u ires join ts to be located at least two beam widths away from from a b eam -girder -girder intersection. It’s It’s a good idea to keep c o n s t ru ction joints away from po int loads (such as hea vy equ ipm en t) for for the same reason. reason. Locate construction joints in wall walls and colum ns on t he un derside derside of floor slabs, be am s, or gird gird e r s, an d at th e top s of footings footings or floor slabs.

Figure 3 . Keys are sometimes formed in bulkheads to incr ease joint shear resistance. This joint design also includes a waterstop. Note that the plywood bulkheads bulkheads aren’t not ched. Plywood pieces pieces are cut , positioned under rebars, rebars, and nailed to 2 x4 vert vert ical support support s.

ACI requirements requirements for construction joint design design Becau se constru ction joints are m ost likely likely to red uce she ar st re n gt h , they should be located where shear fo rces a re low. low. Un der un ifor ifor m ly dist ribu ted gravity loads, she ar force s a re low in th e m iddle of a flexura flexura l m em ber spa n. Thu s, the Am eri c a n Co n c re t e In st it u te (ACI) Bu i l d i n g

Plan View

Figure 4 . Detail shows how to form an inconspicuous horizontal horizontal c onstruct ion  joi nt wi t hout a r ust ic at ion st rip ri p ( Ref. 4 ).

To avoid cracking from from settlemen t, pou r concrete in in colum ns an d walls walls at least 2 hours b efore efore placing placing con c rete in slabs or b eam s fram fram ing into th em (Ref. (Ref. 3). 3). Designing Designing concrete m em bers for for la t e ral force s m ay re q u ire sp ec ial de sign treatm en t of con stru c t i o n  jo in t s. Sh ea r k eys , dia d ia go n al d ow e ls, or th e she ar transfer m ethod (AC (AC I Co d e, Section 11.7) 11.7) m ay be use d.

How How t o form construct construct ion  jo in t s Deciding where to p ut con stru ction joints is on ly half the job. The The oth er half is form ing t he m corre c t ly a n d c le le a n i n g t h e j o in in t s u r fa fa c e b e fo re th e n ext concrete placem en t. Bulkheads Bulkheads for constru ction joints a re no rm ally ally form form ed with wood, alt h o u g h e xp xp a n d e d m e t a l m e s h h a s also bee n use d (See (See Co n c rete Co n s t ru ction , Ju Ju ly 1983, pa ge 552; No vem b er 1986, p age 966; Fe b ru a ry 1988, page 214). Fo rm ing co sts a re high because rebar has to pass t h rough the b ulkhead. ulkhead. Wood m em bers are n otched or a gap is lef leftt in th e p lane of th e re b a r. Leaving Leaving a gap a l lows ea ch bu lkhead b oard to be positioned an d re m ov oved ed withou t pu ttin g p re s s u re on the ba r. Exce ss ive p re s s u re m igh t ca u se gre e n c o n cret e to sp a ll o r cr ac k. Be c a u s e they are cut to fit fit an d usu ally ally dam aged when stripped, bulkheads a re n’t re u s a b le.

Co n s t ru ction joints in floor slabs usu ally ally are m ade of 2-by m ateri a ls or p lywood (Fig u re 2). Wall or b eam bu lkhe lkhe ads are usu ally ally mad e of plyplywood . Shea r keys, if re q u ired , can be ma de b y beveling beveling 2x4 2x4 or 2x6 lum ber ( Fig u re 3). 3) . Ho ri zonta l con struc tion joints in walls an d colum n s are gen era l ly s p a c e d o n e s t o r y a p a r t . U se s e c h a lk   line line s or n ails ails d riven on the in side of  wall form s to m ark the joint location. Or use a wood strip n ailed ailed to the form s. For ar c h it e c t u ral co n cre te, m or e c a re is n eeded to get straight a nd le vel h or izon tal joint s (Ref. 4). Nail a 1-inch 1-inch wood strip to the inside face of the form s n ear th e to p (Fi (Figu re 4). Put tie rods abou t 4 inch inch es below the joint to support forms for the next lift. lift. Place c on crete to a leve l slightly above the bottom of the s t ri p. Re m ove th e strip after con c rete has set en ou gh to ho ld its position. When When form s are set for the n ext lift, lift, locate an oth er row of ties ju ju st a b ove th e joint . Ties Ties ab ove an d be low th e joint h old the form tightly tightly against con crete in the lower lift. lift. D o n’t n’t overlap she ath ing m ore t ha n an inch over the lowe r lift. lift. This This also helps pre vent leakage. leakage. Grooves or rustica tions a re a good way t o hid e ho rizon tal joints (Fi (Figu re re 5). Rustication Rustication strip s are b est attached with double-headed nails d ri ven from th e o utsid e. After After n ails

a re pu lled, lled, th e str ips will pull loose loose from form s d uring st ripping an d can b e left left in the concrete longer to p rotect ed ges from sp allin allin g. Ma k e s u re th at reb ar s b eh ind the ru s t ic ation strip have a dequ ate c ove ove r.

Construction joint surface preparation After placing and curin g the con c rete and re m oving oving the bu lkh lkh ead, the harde ned concrete joint su rfa c e m ust be p re p a red . ACI ACI 318-83 req u ires th at : “Su “Su rfac rfac e of c on cret e con stru c t io n  jo in ts sh all al l be b e c lea n ed an d lai ta n ce re m oved. Imm ed iately before n ew c o n cret e is p lace d, a ll co n stru c t io n  jo in t s s h a ll b e we t te d a n d st a n d in g wa te r re m ove d .” .” Use stiff wire br u s h e s, s cab bler s, w a t e r b la s t e r s, or sa n d bla st er s to clean surfaces an d re m ove laitance. Ex p a n d e d m e t a l m e s h b u l k h e a d s left in place also provide a good rough bon ding sur face (Ref. (Ref. 5). 5). Th e 1977 A ACI CI Cod e re q u i red ap plication plication of a n eat cemen t paste to ve rtical con stru ction joint sur fa c e s. This p ro c e d u re was n ot always p ra ctica l an d is n o lon ger re q u ired .

Team effort Understanding the why, where, an d how of construction joints for for m u l t i s t o ry st ru c t u re s in vo lve s a team a pproach . The The architect wants  jo in t s th a t a r e c o n s ist is t e n t wi th th e

SHEAR WA LLS NEED CONSTRUCTION JOINTS JOINTS TOO

Figure 5 . Rustication strips at construction joints hide joint lines and add to the architect ural ural effect ( Ref. Ref. 4).

a rc h it e c t u ral d esign . Th e en gin ee r wants joints joints that do not reduce the s t rength or du rability rability of t he stru ct u re. The con tract or is looking for  join  jo in ts t h a t e n h a n ce h is p r o d u c t i vit y. A good con stru ction joint p lan i n vo vo lves all th ree p ar t ie s. References

1. Monks, W. L., and B. M. Sadgrove, “ The Effect Effect of Construction Joints on the Performance of Reinforced Concrete Beams,” Technical Report, Cement and Concrete Association (London), 1973. 2. Building Code Requirements for Reinforced Conc rete, ACI 318-83, (Re(Revised 1986), American Concrete Insti-

Ea rth quakes have shown en gigineers and contractors that cons t ruction joints in shea r walls can fo rm the weakest link in t he loadresisting resisting m echan ism of a stru ct u re. Du ring th e 1964 Alaska an d 1971 San Fe rn an d o ea rt h q u a k e s, h o ri zon tal con stru ction joints in both m edium and low low - rise buildings we re dam aged, som e b eyon d re p a i r. Esp e c ia lly t ro u b le s o m e we re light light weight con crete floor floor slab s con n ecte d to n orm a lwe i g h t c o n c rete she ar walls th rou gh two c o n s t ru ction joints (one at th e bottom of the slab slab and o ne at the top of the slab). An ad equate con struction joint joint fo r b e a m s a n d c o lu lu m n s m a y n o t be sat isfactor isfactor y for for walls that n eed to resist large shea rs gene ra t e d d u rin g ea rt h q u a ke s. Ea rt h q u a ke s g e n e rat e m an y cycles of h igh igh -inten sity she ars t ha t c an de teri o ra t e tute, Box 19150, Detroit, Michigan, 48219, 1987. 3. Specifications for Structural Concrete for Buildings, ACI 301-84, (Revised 1985), American Concrete Institute, 1986. 4. Formwork for Concrete, Publication SP-4, Fourth Edition, (Revised (Revised second printing), American Concrete Institute, 1981.

 jo i n t s. Th is re p ea te d lo a d in g re du ces t he joint inte rface ro u g hn e s s, th u s red u cin g t he fric t io n a l re s i s t a n c e. D o n’t n’t re ly on joint r ou ghn ess to resist resist seismic sh ears. Use Use sh ear keys o r ext ra ve rt ica l re in fo rc i n g ba rs place d a t t he joint inte rfa c e. Min im ize th e n um ber of ho ri zo ntal construction joints by slipfo rm ing th e she ar wa ll tower or by making each concrete pour two or t h ree stories h igh. igh. Consider designing signing th e constru ction joint joint u sing the shear friction concepts fro m th e ACI Build in g Co d e. Th e shear wall is an im po rtant load-resisting mechanism and the designer signer spends m any hou rs analyzanalyzing the forces and stresses in the wall. The weakest link, the cons t ruction joint, need s som e d esign esign time too.

5. Building Movements and Joints, Portland Cement Association, 5420 Old Orchard Road, Skokie, Illinois, 60077, 1982.

PUBL BLICATION# N#C880577 Copy right © 1988 1988,, The Aberd een Gr ou p All right rig htss re ser ved