Evaluating Cracks in Concrete Walls_tcm45-342514

7/28/2019 Evaluating Cracks in Concrete Walls_tcm45-342514

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Cracks in concrete can scar a buildings arc h i t e c t u r a lappearance. They can be the cause of leaks fro mrain or gro u n d w a t e r. Or they can be the first telltalesigns of a potential wall collapse. Thus, no crack shouldgo unexamined.

For every crack, a trained investigator should re c o rd ,with photographs and sketches, the following information:

Location on the building

Pattern (horizontal, vertical, diagonal, eggshell)

L e n g t h

Width (uniform or tapered; if tapered note how)

Depth (through paint, plaster, entire wall)

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

Active (moving) or dormant

Based on this information, an investigator can assessthe cause of the crack, how harmful the crack is, and how

best to repair it, if repair is deemed necessary. The locationand pattern of cracks are the most important factors in de-termining their cause. In the drawings shown here, thecauses of typical cracks in concrete walls are explained,based on the location and pattern of the cracks.

Measuring crack width

To measure crack width, most investigators use a crackc o m p a r a t o r, although a graduated magnifying device ism o re accurate. Both of these devices are shown in Figure 1.

Evaluatingcracks inconcret e walls

B y Bruce A. Suprenanta nd Kim D. Basham

To repair cracks in concreteproperly, first determine what

caused them


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The crack comparator is easy to use, sufficiently accuratefor most jobs, and usually supplied free by companies spe-cializing in failure investigations. Graduated magnifierscost $50 to $150.

When is a crack too wide?

Whether a crack is too wide depends on the type ofbuilding, the climate, wall re i n f o rcement, and the type ofcrack. If the owner spared no cost on the walls arc h i t e c-tural design, cracks that are 0.01 inch wide may be totallyunacceptable. But a homeowner with a 20-year-old con-c rete basement might be satisfied to have cracks that areless than 18 inch wide.

A c c o rding to the Portland Cement Association (PCA), acrack 0.010 to 0.015 inch wide neither hurts the surface ap-pearance nor disturbs the viewer (Ref. 1). Cracks narro w-

er than 0.004 inch are considered watertight (Ref. 2). Evenwider cracks than this may not allow enough water pene-tration to damage the contents of the building.

So how does one evaluate the width of cracks? Table 1lists typical design crack widths for re i n f o rced concre t e ,modified from Reference 2. Based on these values, a wayto classify and evaluate crack widths is given in Table 2.

Using Table 2 to classify the crack, the investigator alsoidentifies the allowable exposure for that crack. For exam-ple, a crack that is 0.014 inch wide would be classified as

extensive and usually allowed only in a dry interiorwall. If a 0.014-inch-wide crack occurs in an exterior wall,it probably should be re p a i red. Naturally, the number andlength of cracks also influences the investigators perc e p-tion of permissible cracking and choice of repair schemes.

Dont focus on one crack. Crack widths vary, so mea-s u re the width of several cracks. For re i n f o rced concre t es t ru c t u res, the coefficient of variation for crack widths in asingle structural element is approximately 40% (Ref. 2).For instance, if the average crack width is 0.050 inch,cracks in a single member could range from 0.040 to 0.060inch in width.

Long-term crack widthsCrack widths in re i n f o rced or unre i n f o rced walls in-

c rease with time. But its difficult to determine the rate ofcrack growth and the maximum crack size. For re i n f o rc e dc o n c rete, a reasonable rule of thumb is to expect the finalcrack width to be about double the initial crack width (Ref.2). Also, about 2 years after the crack occurs, its width atthe rebar is about equal to its width at the surface.

Why do cracks occur?

Cracks result from strains that induce tensile stress inexcess of the materials capacity. Strain can be caused byloads, restraint of volume changes, or material behavior.

Volume changes are created by changes in temperatureand moisture. Influences on material behavior includef reeze-thaw cycles and reactions between cement anda g g re g a t e .

Overloads caused by the dead weight of materials,the live load of equipment and people, soil and waterp re s s u re, wind, snow, and earthquakes can pro d u c ecracks. Deflections and deformations imposed by foun-dations, frames, roof slabs, and other structural ele-ments also cause cracking.

Figure 1. For crack wid th measurement, most investigatorsuse a crack com parator, although more accurate measure-ments are possible with a graduated magnifying device.


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Cracks in reinforced and unreinforced walls

Wall re i n f o rcing doesnt prevent cracking, but ratherc o n t rols cracking. Some cracking is normal in re i n f o rc e dor unre i n f o rced walls. Ty p i c a l l y, re i n f o rced walls haveseveral very narrow cracks at regular crack spacing. Whena re i n f o rced wall cracks, the proper amount and spacingof re i n f o rcing maintains the strength and serviceability ofthe wall.

U n re i n f o rced walls tend to have fewer cracks that are

wider than those for re i n f o rced walls. Aggregate interlockbetween the cracked faces still allows for load transfer andelevation control. If the crack opens too wide, aggre g a t einterlock isnt effective and the cracked faces move sepa-r a t e l y. For unre i n f o rced residential basement walls, theNational Ready Mixed Concrete Association (NRMCA)recommends repairing cracks when they leak or exceed 18inch in width (Ref. 3).

Beware of a moving crack

The diff e rence between an active or working crack anda dormant crack is important. An active crack can open orclose and get longer, but a dormant crack has stopped

moving. Dormant cracks tend to be caused by a tempo-rary overload. Active cracks are created by repeated over-loading or temperature or moisture changes that continu-ally cause the crack to open and close. Also, a crackcaused by a temporary overload can turn into a workingcrack due to changes in temperature .

If the crack is dormant, then it can be fixed with a rigidf i l l e r, such as epoxy or cement. Working cracks, however,should be filled with flexible sealants. If theyre re p a i re dwith a rigid material, theyll only crack again. If the filler is

s t ronger than the concrete, a new crackparallel to the re p a i red crack pro b a b l ywill occur.

T h e re are three inexpensive ways totell if a crack is active or dormant. Theeasiest way is to measure the crackwidth with a crack comparator at re g-ular time intervals, such as every day

or week. Record both crack width andthe date of measurement. Alwaysm e a s u re the crack width at the samelocation. Draw a line across the crackto mark where you measured it. Dothis at three or four places along thecrack. On exterior walls, use a water-p roof marker. Changes in crack widthindicate an active crack.

Another inexpensive method is tospot-patch the crack with plaster (Fig-u re 2). Use hot water to speed the set ofthe patch. Note the date the patch was

placed and inspect it at regular inter-vals to see if it has cracked. A crackedpatch shows the crack is active. To bes u re the patch doesnt crack from dry-

ing shrinkage, use a nonshrink patching material.A two-piece crack monitor that sells for less than $15 al-

so can be used to detect crack movement (Figure 3). Oneof the two plastic pieces has red cross hairs; the other plas-tic piece has a grid system with a zero mark at the center.The two-piece unit is connected by a piece of tape so thatthe intersection of the red cross hairs on the one piece co-incides with the zero mark on the grid of the other piece.

The monitor is laid across a crack and each end is at-

tached to the wall, usually with epoxy or a fast-settingglue. After the adhesive cures (about 15 minutes for the

Table 2. Classification of Crack Widths

C ra ck C la ss ific a tion C ra ck Width C W

Very Fine (Wa tertig ht) C W 0.004

Fine (Exterior Exposure) 0.004 < C W 0.008

Me d ium (Inte rio r Exp osure We t) 0.008 < C W 0.012

Extensive (In terio r ExposureDry) 0.012 < CW 0.016

S evere C W > 0.016

Figure 2. This vertical crack is plastered to determ ine if thecrack is active or dormant. If the crack is active, the patchwill eventually crack. Dormant c racks can be filled withepoxy or cement. Active cracks that are still moving shouldbe filled w ith a flexible sealant.

Table 1. Typical Design Crack Widths, inches

Interior Exposure

Dry a tmosphere 0.016

Wet or mois t a tmos phere 0.012

Exterior Exposure 0.008

Whe re w ate rtig htnes s is req uired 0.004

Modified from Reference 2.


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glue and 24 hours for the epoxy), cut the tape holding thetwo pieces together and re c o rd an initial value. Now, if ei-ther side of the crack moves, one of the plastic piecesmoves. The red cross hairs, originally at the initial value,slide over the grid system indicating the amount of hori-zontal and vertical movement. Both the amount and di-rection of movement can be observed.

Evaluate cracks early

If cracks arent examined, evaluated, andif neces-sary re p a i red, they can grow from an eyesore into acostly headache. The larger cracks get, the more wind-dri-ven rain they let into the wall. Wa t e r, in turn, can corro d e

metal components, cause eff l o rescence, and, if it fre e z e s ,spall the concrete. Some cracks can even lead to wall col-lapse. Cracks are a warning that shouldnt be ignore d .

References

1. Building Movements and J oints, 1982, Portland Cem entAss oc iation (P C A), 1982.

2. ACI 224R-92, Control of Cracking in Concrete Struc-tures, American Concrete Institute (ACI), 1992.

3. What, Why &How? Crac ks in Conc rete Bas ementWalls, Concrete in Practice, CIP 7, Na tiona l Ready MixedCo ncrete Ass oc iation (NRMCA).

4. Non-structural Crac ks in Conc rete, C oncrete S oc ietyTec hnica l Rep ort No. 22, Report of a Conc rete SocietyWorking Party, The Co ncrete S oc iety, Terminal House ,Grosveno r Ga rdens , London S W1W 0AJ .

Kim Basham is an assistant professor of civil engineering atthe University of Wyoming, Laramie.

Examp les of Nonstruct ural Cracks in Concrete Walls(Modified from Ref. 4) These cracks usually dont affectthe structural integrity of the wall or lead to significantconc rete deterioration.

A. Plast ic Set t lement - Reinforc ingAfter initial placement and vibration, conc rete continuesto consolidate. Plastic concrete may be locally restrainedby rebar, previous pours, or formw ork. Concrete consoli-dates around these local restraints, causing cracks adja-cent to the restraining element.

B. Plast ic Set t lement - Arching

See previous explanation for drawing A.

C. Plast ic Shrinkag e

Occurs at the exposed top of the wall when the freshconcrete loses moisture rapid ly. High wind, low hum idity,and high temperatures evaporate surface mo isture fasterthan its replaced by b leedwater. Typical plastic shrinkagecracks are shallow and short and oc cur in all directions.

D. Ear ly Thermal Con tract ionCement hydration produces heat, causing the tempera-t u re of the concrete to rise. A fter a few days, the concre t ecools, causing a con traction. If this contraction is re-strained b y previous pours or inte rnally by d iff e rences incooling, c racks may develop. Early thermal cracking usu-ally occurs w ithin a few weeks after concrete placement.

E. Long-term Drying Shr inkage

Drying shrinkage, caused by loss of moisture, induc esvolume changes that are restrained b y rebar and inter-connecting walls and slabs. The comb ination of shrinkageand restraint causes tensile stresses that can crack theconcrete. Drying shrinkage cracks typically occur 2 to 4weeks or more after concrete placement.

F. Crazin gLocalized drying shrinkage. Low -permeability form s(steel or p lastic) or early form removal increase the o c-c u rrenc e o f crazing.

G. Alkali -aggregate React ions

Alkali in t he c emen t can react with silica in the aggre g a t eto form a swelling gel. The swelling gel creates an ex-pansion, produc ing tensile stresses that can cause thec o n c rete to crack.

Figure 3. Laid across a crack and adhered t o the wall onboth sides of the crack, this two- piece crack monitor detectscrack movement. If the crack moves, the red cro ss hairs be-come o ffset from the center of the grid on the o ther piece.

P U B L I C ATION #C930016Copyright 1993, The Aberdeen Group, All rights re s e r v e d

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Evaluating Cracks in Concrete Walls_tcm45-342514