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Owners dont like cracks inconcrete, and homeownersare particularly paranoid aboutcracks in basement walls. Everyhomeowner has heard a horrorstory about a sinking foundationor a flooded basement, so a crackin new concrete is a serious redflag. But which cracks are accept-able and which are signs of trou-ble? For some answers, we askedan engineer, Willard Norton of Norton& Schmidt Consulting Engineers,Kansas City, Mo., and a repair con-tractor, Jim Bushby, president ofStructa-Bond of Maryland Ltd., Jessup,Md., for their views on the causes, sig-nificance, and repair options for foun-dation cracking.

Concrete Construction: Why do

basement walls crack? Can you deter-mine the cause by simply looking atthe crack?

Norton: Basement walls crack foressentially four reasons: drying shrink-age, thermal stress, lateral pressure,and differential settlement. You oftencan determine the cause of cracks bytheir location, orientation, and thetime they occurred.

Shrinkage cracks are usually verti-cal and often appear shortly after theforms are stripped. The footing, which

has already undergone initial shrink-age, resists wall shrinkage and a crackpropagates from the bottom up. Inmany cases, the crack does not extendall the way to the top of the wall.

Contractors can prevent mostshrinkage cracks by putting enoughhorizontal rebar in the wall. Unfor-tunately, most contractors put #4rebar on 2-foot centers in an 8-inch-

thick wall,which isntenough topreventshrinkagecracks. Placing the rebar on 12-inchcenters is sufficient.

Thermal cracks also are verticaland almost always occur at corners.They can occur at any time but usual-

ly appear during construction, whenthe foundation is exposed to ambienttemperature swings.

Cracks caused by lateral pressuretypically occur during or shortly afterbackfilling. If the basement wall isproperly restrained at the top (thejoists are nailed to the sill plate,which is bolted to the foundation),backfill pressure should not causecracking if the wall is adequately rein-forced. As I mentioned earlier, manycontractors place #4 bars on 2-foot

centers. This usually is enough rein-forcement to prevent flexural crackingin an 8-foot-high wall. If theres notenough reinforcement, a horizontalflexural crack can form. The crackwill turn upward at each end of the

wall, so it looks like a smile.If the top of the wall is not

anchored to the framing, lateral pres-sure can push it in under the sill plate.In this case, diagonal cracks canextend up from the bottom corners ofeach end of the wall. In extremecases, you also get a vertical crack inthe middle of the wall. In addition,

the wall may bulge and inducea bending moment at each cor-ner, causing outside verticalcracks in the wall at each endof the bulging wall.

Finally, cracks caused bydifferential settlement can bevertical or on an angle, andthey usually dont appear untilafter the framing has beenloaded onto the walls. Unless ahouse is on firm material, suchas rock or shale, the foundationwill settle a little, and its notnecessarily going to settle uni-

formly because different parts of thehouse are different in weight.

Bushby: Most foundation wallcracks are caused by mechanical over-load from backfilling green walls

walls that have not gained enoughstrength to resist the lateral pressure.Many walls are backfilled within 3 to5 days, which is too early. The wallbows inward at the center and oftentears on the outside corners. On theinside, you get diagonal cracking fromthe corners and sometimes a verticalcrack in the middle of the wall.

Another cause of cracking isminor settlement of the foundation.The cracks usually appear within thefirst year or so after construction.

Settlementcracks typicallyappear at theweakest areasof the wall,such as the

Concrete Perspectives

Basement wall cracks:No problem?Or signs of trouble?

You often can

determine the

cause of cracks

by their location,

orientation, and

the time they

occurred.

Willard Nor ton

Many walls are backfilled within

3 to 5 days, which is too early.

Jim Bushby

Solut ions

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corners of steel beam pockets, win-dows, and door frames.

Many people blame concreteshrinkage for basement wall cracking,but rarely will you find shrinkagecracks in a poured wall. Those thatdo occur almost never penetrate allthe way through the wall, so leakageis not a problem.

CC: When do cracks requirerepair, and how do you repair them?

Norton: Regardless of their cause,foundation cracks require repair onlyif they continue to get wider or theyleak. Cracks increasing in width are asign of a structural problem, which

must be addressed. Leaking cracks area result of poor drainage, whichshould be corrected before the individ-ual cracks are treated.

Shrinkage and thermal cracks arecommon and dont require repairunless theyre leaking. The stresses arerelieved when the concrete cracks andno further problems occur. This oftenis true for cracks caused by backfillpressure and settlement. However,these cracks must be monitored andremedial action taken if there is an

ongoing condition.To see if a crack is getting wider,

I like to place a plaster patch acrossthe crack. If the crack changes, theplaster will crack and the time andamount of movement can be recordedfor evaluation of repair needs, if any.

There are two ways to stabilizewalls pushed in by lateral pressure.Outside the foundation, you can holdthe wall back by installing earthanchors, or deadmen. Inside the base-ment you can install vertical steel

columns by burying their bottoms inthe concrete slab and then bracingthem off into the floor frame. If thewall is moving in only at the top, youcan prevent further movement bybolting angle irons to the bottom ofthe floor joists. Ongoing differentialsettlement requires underpinning thefoundation with concrete or steelpiers that extend down to stable soil.

If a crack is leaking, you firstneed to solve the underlying cause,

which is typically poor drainage. Theway you solve that is to provide apositive grade slope away from thehouse and to make sure the drain tileis open by injecting water into thedrain tile with a deep root feeder. Ifthe drain tile is clogged, you canrelieve pressure by drilling throughthe bottom of the wall and divertingwater into a sump pump system.

If a crack is still leaking, I recom-mend sealing it with silicone from theinside. Using a caulking gun, squirtthe silicone into the crack and scrub

the excess in with a gloved hand.Some people recommend epoxy injec-tion as a way to structurally repairthe crack and stop leaks. But unlessthe underlying cause of a structuralcrack is eliminated, the wall will sim-ply crack again if the crack is epoxy-injected. As far as stopping leaks, Ithink silicone does just as good a joband costs much less.

Bushby: In the case of lateraloverload, the backfill must be exca-vated and the wall straightened. I

always recommend building the housebefore backfilling the foundationbecause the weight of the house willhelp keep the wall from bowing.

If settlement cracks are wide anddisplaced or out of plane, check thefooting to see if its cracked as well.That could be a sign of ongoing set-tlement. In that case, the footing mayneed to be underpinned before thecracks are repaired.

Once the wall is stabilized, youcan repair the individual cracks.

Hairline cracks that dont leak maynot require repair. Otherwise, we typi-cally epoxy-inject basement wallcracks. The walls are designed to bemonolithic, and epoxy injectionreturns them to that state. However,the cracks should be injected onlyafter the house is built to allow theweight of the house to stabilizestresses within the wall.

Publication #C00A072Copyright 2000, The Aberdeen Groupa division of Hanley-Wood, Inc.All rights reserved