Use Joints to Control Floor Cracks_tcm45-346336.pdf

7/25/2019 Use Joints to Control Floor Cracks_tcm45-346336.pdf

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No one likes to see random cracks in concrete floors.T h e y ’re unsightly, they make floor cleaning difficult

and, especially in installations such as food pro c e s s i n g p l a n t s, they can harbor bacteria. Edges of the cra c k smay deteri o rate when the floor is subjected to heavy

u s e. All of this can be avoided by installing joints atproper intervals during floor construction.

Causes of floor crack ing

Two of the most common causes of floor cracking ared rying shrinkage and contraction due to cooling. Both of these cause a shortening tendency in slabs which is re-s t rained by friction between the base and the concre t eresting on it. The result: tensile stress builds up and be-cause concrete is weak in tension it cra c k s.

Cracking can also occur when floor slabs are n’t free tom ove independently of the building elements they’re incontact with. Movement of slabs is likely to be differe n t

f rom that of other stru c t u ral members, and if slabs arerigidly joined to the other members they may be unableto accommodate this differential movement. Againc racking is the re s u l t .

Curling of slabs on grade causes cracking, too. Whenthe top of a slab dries and shrinks more than the bottom,the edges curl up. The weight of these unsupport e dedges plus any other load on the f loor creates tensiles t resses in the top of the slab and cracks occur.

Types of joints and how t hey work

Joints permit concrete to move slightly by cre a t i n g planes of weakness where cracks can form. The idea isthat if concrete cracks or separates at a joint it is less like-ly to crack at other locations. Joints are neater in appear-

ance than random cracks and easier to keep sealed andclean. Also, the edges of a properly maintained joint areless likely to chip or span than the edges of ra n d o mc ra c k s.

T h ree types of joints are used in concrete floor con-s t ruction: isolation joints, control joints and constru c-tion joints.

Isolation joints (sometimes called expansion joints)a l l ow movement in both ve rtical and hori zontal dire c-t i o n s. They separate or isolate concrete slabs fro mc o l u m n s, walls, footings and other points of re s t ra i n tsuch as machine foundations and stairwe l l s. No connec-tion should be made across an isolation joint either by

re i n f o rcement, keyways or bond.C o n t rol joints ( f requently called contraction joints) al-

l ow movement only in the plane of the floor and contro lc racking caused by re s t rained forces resulting from dry-ing or cooling. Load is tra n s f e r red across control jointsby interlock of aggregates in crack faces formed at the joint or by contact of the tongue and gro ove in a keye d joint (Fi g u re 1). Dowel bars are also sometimes used tot ransfer loads in floors which carry heavy wheeled tra f-

Types of joints, how to form themand where to put them

Sawed control joints m ust be deep enough to create a plane ofweakness so that c racks occur at the joint. Aggregate int erlockat the crack helps to transfer loads across the joint.



f i c. When wire mesh is used in the slab it should notc ross a contraction joint. If it does, the joint won’t openp roperly and cracking may occur at some other location.

Construction joints a re stopping places for a day’s w o rk. They can be made to function as isolation joints by lining the bulkhead with a pre f o rmed sheet material or

as control joints by using keyed bulkheads. If constru c-tion joints are located where no movement is wanted, tiebars or welded wire fabric can be used to hold the adja-cent slabs together.

How t o form joints

Isolation Jo i n t s — To permit slabs to move up or dow nve ry slightly re l a t i ve to walls or columns, ve rtical sur-faces of isolation joints are cove red with joint material sothat there is no concre t e - t o - c o n c rete contact. Asphalt-i m p regnated sheets or other expansion joint fillers must

be thick enough to permit the re q u i red move m e n t .Joints with rough concrete surfaces re q u i re thicker jointf i l l e r s.

At columns, isolation joints are boxed out with squareor circular shapes (see Fi g u re 2). When square shapes areused, the wood form is placed so that its corners point atthe control joints along column lines. Unless this isd o n e, cracking can occur as shown in Fi g u re 3. When cir-cular forms are used they are generally made of plastic orf i b e r b o a rd .

Figure 1. Control joint s may provide load transfer throughaggregate interlock, keyways or dowel bars.

Figure 2. Isolation jo in t s separ at econcrete slabs fromcolumns allowing

movement in both ahorizontal and vert icaldirection. Note thatwhen s quare-shapedboxouts are used theyare rotated so thatthe corners meet t hecontrol joints in t heslab.

Figure 3. Crack ing at a re-entrant corner created by failureto rotate the boxout at an isolation joint. This type ofcrack ing can be avoided by proper joint layout.



C o n t rol Jo i n t s — Co n t rol joints can be produced in sev-e ral ways: by saw-cutting, tooling, or inserting plastics t ri p s. (See box for description of a recently deve l o p e dmethod using a plastic ri b b o n . )

Cutting a gro ove with a power saw is the most com-mon method of making a control joint. A va riety of dia-

mond-tipped blades is available to cut concrete con-taining hard or soft aggregates; blade suppliers can re c-ommend the best choice. Lowe r-cost re i n f o rced abra s i ve(silicon carbide) blades are suitable for use when thec o n c rete is made with soft, free-cutting aggre g a t e s. On e way of getting longer life from saw blades is to “p re - c u t”the joint with a pointing trowel after the fresh concre t ehas been bull floated. This moves the aggregate part i-cles aside so that when the joint is sawed later, only the

m o rtar is cut. A straight board can be used as a guide forthe trowel cutting. Fu rther floating and troweling follow,making it important to mark exactly where the pre - c u t was made so the saw operator will know where to saw the joint.

Saw cutting must be done as soon as possible after thec o n c rete hardens; otherw i s e, random cracks may formin the slab. The concrete is hard enough when it isn’tt o rn or damaged by the blade. Saw operators shouldmake trial cuts starting a few hours after the concre t eh a rd e n s, then observe the re s u l t s. If aggregate part i c l e scome loose, it is too soon to begin sawing. Joints shouldbe sawed to a depth of about 1 ⁄ 4 the slab thickness. Sh a l-

l ower cuts are unlikely to weaken the concrete suffi-ciently at the joint and this can cause cracking betwe e n

j o i n t s. Another method of forming control joints is by tool-

ing. This is u sually less efficient and less economicalthan sawing but may be done on smaller jobs. To o l e d joints are formed with a jointer having a bit deep enoughto cut 1 ⁄ 4 the thickness of the slab. When making the

JOINTS MADE BY PLASTIC RIBBONPLACED IN FRESH CONCRETE

Like other methods of forming a control joint, aplastic ribbon placed on edge just beneath the sur-face of a freshly placed concrete slab creates avertical plane of weakness in the slab. As the con-crete hardens and the slab shrinks, the slab willcrack along this plastic ribbon.

The ribbon is placed in the concrete with thetool shown here. The float pan of the tool ispushed across the slab with a wood handle orpulled across by a string attached to the front of

the float pan. As the float pan is moved acrossthe concrete slab, the vertical steel blade under-neath the float pan cuts a groove in the slab andthe ribbon is fed into this groove through a slenderopening in the steel blade. Concrete is then fin-

ished over the top of the ribbon, and the ribbon isleft in place, even after the slab cracks. If specifi-cations require that joints be sealed, the manufac-turer recommends a butyl or silicone sealant. Be-cause the crack is so narrow less sealant isneeded than would be required in other types of

joints. To be certain the ribbon is placed in a straight

line, a screed or board can be used as a guide forthe float pan, or a stringline can be struck on theconcrete and used as a guideline. The ribbon isusually installed immediately after the concretehas been screeded, but it can also be installedwhen the concrete is just starting to set and is

able to support the weight of a worker.Any of three different ribbon widths, up to 11 ⁄ 2inches, can be placed by the same tool with onlyminor alterations. Regular surveyor’s tape in a 13 ⁄ 16-inch width can be purchased from most build-ing materials suppliers for use in the tool.

By this method, a joint 25 feet long can report-edly be installed in about 30 seconds, and thecost to install joints is said to be less than 3cents per foot. The tool used to place the ribboncosts about $100.

Figure 4. Butt-type construction joints may be formed withor wit hout provision for load transfer. Bonded construc tion jo in t s are oc cas ionall y neede d and can be buil t us ing t iebars instead of dowels.



g ro ove, a straight board can be used as a guide to ensurethat the joint is stra i g h t .

Pre f o rmed plastic strips can also be used to form con-t rol joints. Be f o re the strip is inserted into freshly placedand bull floated concre t e, a stringline is snapped at each

joint location. A slot should then be cut in the fresh con-c rete along this line. Ca re must be taken to keep the stri pve rtical when placing it in this slot. If placed at a seve reangle the plastic strip forms a wedge-shaped lip of con-c rete that may later break off. As with all other contro l j o i n t s, the depth of the strips should be at least 1 ⁄ 4 of theslab thickness.

Construction Jo i n t s — Butt-type construction joints(see Fi g u re 4) are formed by a bulkhead and are satisfac-t o ry for thin floors that are n’t heavily loaded. Dowels canbe added at butt-type joints to provide load transfer infloors carrying heavier loads.

The side forms or bulkheads for slab-on-grade con-

s t ruction can also be keyed so that the slab will have atongue and gro ove construction joint after concrete hasbeen cast on both sides. These keyed joints act as contro l

j o i n t s, permitting slight hori zontal movement but no

ve rtical movement. On the surf a c e, a keyed joint oftenlooks like a ragged line unless both sides of the joint areedged or a sawcut is made as shown in Fi g u re 5.

Spacing and layout of joint s

Fi g u re 6 shows a typical layout of joints in a floor ong ra d e. It is common practice to construct floors in long

lanes and to locate control joints, whether sawed ork e yed, along column lines. The contractor can start atone end and place each successive lane after the pre v i-ous one has hardened; another approach is to constru c t

a l t e rnate lanes, but this may re q u i rem o re side form s.

Joint spacing should also be cho-sen so that concrete sections are ap-p roximately square. If sections havelength-to-width ratios greater than11 ⁄ 2 to 1, they are likely to crack be-t ween joints.

Often, locating joints only along column lines doesn’t provide closeenough spacing to pre vent cra c k i n g b e t ween joints. Curling is also mores e ve re when joints are too far apart . A rule of thumb for plain slabs is that joint spacing in feet shouldn’t exc e e dtwo and one-half times the slab thick-ness in inches. Howe ve r, other factorsbesides thickness can influence the

Figure 5. Tongue and groove or keyed joints can be formedby attaching bevelled 1 by 2-inch strips to the side forms.A longitudinal sawcut in the strip permits the key form toswell sl ightly wit hout cracking the concrete.

Slab Less than Larger than Slump lessthickness, 3 ⁄ 4 -inch aggregate: 3 ⁄ 4 -i nch aggregate: than 4 i nches:

inches spacing, feet spacing, feet spacing, feet

4 8 10 12

5 10 13 156 12 15 18

7 14 18 21

8 16 20 24

9 18 23 27

Note: Given spacings also apply to the distance from control joints to parallel isolation joints or to parallel construction joints. Spacings greater than 15 ft. show a marked loss ineffectiveness of aggregate interlock to provide load transfer across the joint.

SUGGESTED SPACING OF CONTROL JOINTS

Figure 6. Joints should be locat ed along column lines and atintermediat e points so t hat panels are as nearly square asis practical. Construction joints in this layout function ascontrol joints. Note also the isolation joints around walls,columns and mac hinery bases.



re q u i red spacing. These include theslump of the concrete and the max-imum size of coarse aggre g a t e. Su g-gested joint spacings based uponthese factors are given in the table.

Another consideration in plan-ning joint layouts is the need toeliminate re - e n t rant corn e r s. Ass h own in Fi g u re 3, cracking is ve ry

likely to occur at any location wherea sharp inside corner exists. Jo i n t sshould be located where cracks aremost likely to appear.

PUBLICATION #C840553Co py right © 1988, The Ab e rdeen Gro u p All rights re s e rve d

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Use Joints to Control Floor Cracks_tcm45-346336.pdf