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Cracks in masonry can scar a buildings ar-chitectural appearance. They can be thecause of leaks from wind-driven rain. Orthey can be the first telltale signs of a po-

tential wall collapse. Thus no crack should go unex-amined. For every crack, a trained investigator shouldre c o rd, with photographs and sketches, the followingi n f o r m a t i o n :

Location on the building

Pattern (horizontal, vertical, straight diagonal, orstepped diagonal)

L e n g t h

Width (uniform or tapered; if tapered note how)

Depth (through paint, plaster, or entire wall)

Age (clean crack indicates new; coated with paint ordirt indicates old)

Moving crack or dormant crack

Based on this information, an investigator can assessthe cause of the crack, how harmful it is, and how bestto repair it, if repair is deemed necessary. The location

and pattern of cracks are the most important in deter-mining what caused them. In the drawings shownh e re, the causes of some typical cracks in brick andblock masonry are explained, based on the locationand pattern of the cracks.

Measuring crack width

To measure the width of cracks, most investigatorsuse a crack comparator although a graduated magni-fying device is more accurate. Both of these devices areshown in Figure 1. The crack comparator is easy touse, sufficiently accurate for most jobs, and usually issupplied free by firms specializing in failure investiga-tions. The graduated magnifiers cost $50 to $100.

When is a crack too wide?

Whether a crack is too wide depends on the materi-al, the type of building, the climate, and the type ofcrack. If the owner spared no cost on the buildings ar-chitectural design, cracks that are 0.01 inch wide maybe totally unacceptable. But a homeowner with a 20-y e a r-old brick house would be happyand luckytohave cracks that are only 0.01 inch wide.

A c c o rding to the Portland Cement Association(PCA), a crack that is 0.010 to 0.015 inch wide neitherhurts the surface appearance nor alarms the viewer

Evaluating cracks

To repair cracks in masonry properly, you must first determine what caused them

By Bruce A . Sup rena nt

Figure 1. To measure the width of cracks, mostinvestigators use a crack comparator. Agraduated magnifying device is moreaccurate, but costs $50 to $100. The crackcomparator usually is free from firms that

investigate failures.

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(Ref. 1). Norwegian tests show that wind-driven raindoesnt enter cracks narrower than 0.004 inch (Ref. 2).

Even cracks wider than this may not allow enoughwater penetration to damage masonry or the building.So how does one evaluate the width of cracks? Ta b l e

1 lists typical tolerable crack widths for re i n f o rced con-c rete, modified from Ref. 3. These tolerable crackwidths also should be reasonable for re i n f o rced ma-sonry stru c t u res. Based on these values, a way to clas-sify and evaluate crack widths in masonry is given inTable 2.

Using Table 2, once the investigator classifies the

Cause: Vertical deflection ofconcrete beam

Cause: Shrinkage of mortarand concrete masonry units

TABLE 1 TOLERANCE CRACK WIDTHS, INCHES

Interior exposure

Dry a tmosphere 0.016

Wet or mois t a tmosphere 0.012

Exterior expos ure 0.008

Where w a tertightnes s is req uired 0.004

So urce: Modified from Ref. 3.

TABLE 2 CLASSIFICATION OF CRACK WIDTHS

Cla s s ifica tion Cra ck Width, CW

Very fine (Wa tertight) CW 0.004

Fine (Exterior exposure) 0.004

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crack, he or she also has identified the allowable expo-s u re for that crack. For example, a crack that is 0.014inch wide would be classified as extensive and usu-ally allowed only in a dry interior wall. If a 0.014-inch-wide crack occurs in an exterior wall it pro b a b l yshould be re p a i re d .

Beware of the moving crack

The diff e rence between an active or working crackand a dormant crack is important. An active crack

may open or close, but a dormant crack has stoppedmoving. Dormant cracks tend to be caused by a tem-porary overload. Live cracks are created by re p e a t e doverloading or by temperature or moisture changesthat continually cause the crack to open and close.

If the crack is dormant, then it can be filled with arigid filler, such as epoxy or cement. Working cracks,on the other hand, should be filled with flexiblesealants. If theyre re p a i red with a rigid material,theyll only crack again. If the filler is stronger than them a s o n r y, a new crack parallel to the re p a i red crackp robably will occur.

How do you tell if a crack is active or dormant?T h e re are three ways. The easiest is to measure thecrack width with a crack comparator at regular timeintervals, every day or every week. Record both crackwidth and date of re c o rding. Always measure thecrack width at the same location. Draw a line acro s sthe crack to mark where you measured it. Do this att h ree or four places along the crack. On exterior walls,use a waterproof marker.

Figure 2. Caused by lateral loads, this diagonalshear crack is plastered to determine if the crackis active or dormant. If the crack is active, thepatch will crack at a later date. Dormant crackscan be filled with epoxy or cement, but activecracks that are still moving should be filled with aflexible sealant.

Cause: Foundation settlementor soil heave

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Another inexpensive method is to spot patch thecrack with plaster (Figure 2). Use hot water to speed

the set of the patch. Note the date the patch wasplaced and inspect it at regular intervals to see if it hascracked. A cracked patch shows that the crack is ac-tive. Be sure the patching material doesnt crack fro mdrying shrinkage; use a nonshrink patching material.

A two-piece crack monitor that sells for less than $15also can be used to detect crack movement (Figure 3).One of the two plastic pieces has red cross hairs; theother piece has a grid system with a zero mark at thec e n t e r. The two pieces come taped together so that theintersection of the red cross hairs on the one piece co-incide with the zero mark on the grid of the other

p i e c e .The monitor is laid across a crack and each end is at-

tached to the wall with epoxy or a fast-setting glue. Af-ter the adhesive cures (about 15 minutes for fast-set-ting glue; about 24 hours for epoxy), the tape holdingthe two pieces together is cut. Now, if either side of thecrack moves, one of the plastic pieces moves. The re d

c ross hairs, originally at zero, slide over the grid sys-tem indicating the amount of horizontal and verticalcrack movement. Both the amount and direction ofmovement can be observed. Usually the monitor ischecked at the same time each day.

Evaluate cracks early

If cracks arent examined, evaluated, andif neces-s a r y re p a i red, they can grow from an eyesore into acostly headache. The larger cracks get, the more wind-driven rain they let into the wall. Water in turn canc o r rode metal components, cause eff l o rescence, and ifit freezes it can spall the masonry. Some cracks caneven lead to veneer or wall failures. Cracks are a warn-ing that shouldnt be ignored.

References

1. Building Movements and J oints, 1982, Portland Ceme ntAss oc iation, 5420 Old Orcha rd Rd., S kokie, IL 60077.

2. O. Birkeland and S. D. S evendsen, Norweg ian TestMethods for Rain P enetration through Mas onry Walls, Sym -

posium o n Mas onry Tes ting, S TP 320, 1963, AS TM, 1916Race St., Philadelphia, PA 19103.

3. Control of Cracking in Concrete Structures, ACI 224,ACI Manual of Concrete Practice, Part 3, 1989, AmericanCo ncrete Institute, P.O. B ox 19150, Detroit, MI 48219.

4. Clay ford T. G rimm, Mas onry Cra cks: A Review o f the Lit-erature, Maso nry: Materials, Des ign, Co nstruction, a ndMainten a nc e, S TP 992, 1988, AS TM.

5. R. E. C opeland, Shrinkag e a nd Temperature S tress es inMas onry, J ournal of the America n Conc rete Institute, Feb-ruary 1957.

Bruce A. Sup renant is a consulting engineer, an Adjunc t As-sociate Professor at the University of Colorado at Boulder,

and a contrib uting w riter to this magazine. He also took the

pho tographs for this article.

Figure 3. Laid across a crack and adhered to thewall on both sides of the crack, this two-piececrack monitor detects crack movement. If thecrack moves, the red cross hairs on one of the twopieces becomes offset from the center of the gridon the other piece.

P U B L I C AT I O N # M 9 0 0 0 7 6Copyright 1990, The Aberdeen Gro u p

All rights re s e r v e d