Table 4.1 in ACI 224R-90, “Con-t rol of Cracking in Concre t e

S t ru c t u res,” lists tolerable crackwidths for re i n f o rced concrete undervarious exposure conditions. Section4.4 of ACI 224R-90 indicates that ap o rtion of the cracks in a stru c t u reshould be expected to exceed the val-ues in Table 4.1 by a significantamount, even when other re p o rt re c-ommendations for limiting crackwidth are followed. However, thet a b l e ’s values have been used in courtcases to imply that the presence of anycracks exceeding the widths listed—atany concrete age—indicates inferiordesign or construction practices.

What is the proper use of the val-ues in Table 4.1? We asked EdwardNawy and Randall Poston to sharetheir views. Nawy was the chairm a nof ACI 224 when “Control ofCracking in Concrete Stru c t u re s , ”including Table 4.1, was first pub-lished in 1972. The current table isbased on a crack width table from atechnical paper written by Nawy.Poston chaired ACI 224 from 1991t h rough 1997, when revised word i n gfor the table was proposed to clearlyindicate that the tolerable crackwidths listed are a guide and not to be taken as absolute or mis-i n t e r p reted as“allowable” crackwidths. However,the revision wasn’tadopted.

C C : What doyou consider to bethe proper use ofTable 4.1?

N a w y : Table 4.1 is a generalguide for what could be considere dreasonable crack widths at the tensileface of re i n f o rc e d - c o n c rete stru c t u re sfor typical conditions. These re a s o n-

able crack width values are intendedto serve as a guide—along with soundengineering judgment—for designerswho select re i n f o rcement size andspacing to control expected crackwidths. The table may also be used asa guide for identifying cracks to besealed or re p a i red. It is illogical inc o u rt proceedings to claim that anycrack width exceeding the tabular val-ues indicates inferior design or con-s t ruction practices. The flexuraldesign might be correct, but the long-t e rm detrimental effects on the life ofthe stru c t u re if the cracks were widewould be the issue.

P o s t o n : Simply stated, the pro p e ruse of Table 4.1 is as a guide. Cracksin all types of concrete constru c t i o na re inevitable, and those that do formv a ry in length and width. More o v e r,the crack widths provided in Ta b l e4.1 are surface crack widths. Crackwidths measured optically vary,

depending on the person making them e a s u rement, where the width ism e a s u red along the crack, and thes u rface condition of the concrete atthe measurement location. And asstated in the commentary to the table,crack widths are expected to vary — i nmost cases, increasing with time.

Considering all thesefactors, it’s only logi-cal and practical touse the crack widthsp resented in the tableas a guide.

As an example,consider a 10-year-old stru c t u re withaverage crack width

m e a s u rements that exceed a value inTable 4.1 by 50%. If there are few orno signs of attendant deterioration ors e rviceability problems, the crackwidths might still be considere dacceptable. On the other hand, anaverage of crack width measure m e n t sthat exceeds a value in the table by,s a y, 100% is, in all likelihood, unac-ceptable under any circ u m s t a n c e .

C C : What is the significance of aflexural member containing severalcracks that exceed the limits given inthe table, and should crack repair bere q u i red when crack widths at thetensile face of re i n f o rc e d - c o n c re t emembers exceed the limits?

N a w y : Crack widths incre a s ewith time and could more than dou-ble in width within 2 to 5 years.Several wide cracks could reduce thes t i ffness of structural members, in-c reasing deflection and leading tocontinuous deterioration. Wi d ecracks also can increase corrosion ofre i n f o rcement and, in severe cases,cause spalling of the concrete cover.Repairs are needed to prevent thesel o n g - t e rm eff e c t s .

P o s t o n : The $64,000 question is:By how much do these cracks exceedthe limits? I’ve found in practice thatif a few cracks have average widthsexceeding the limits in the table by

Crack widths: What’s tolerable?

Table 4.1 is a generalguide for what could bec o n s i d e red re a s o n a b l ecrack widths at the t e n-sile face of re i n f o rc e d -c o n c rete stru c t u res fortypical conditions. — E d w a rd Nawy

An average of crack width measurements thatexceeds a value in the table by, say, 100% is, in all likelihood, unacceptable under any circ u m-stance. —Randall Poston

C o n c rete Perspectives

less than 50%, structural implicationsa re unlikely, depending on the ratio ofthe dead to live load, the amount ofactual live load on the stru c t u re, andthe age of the stru c t u re. Thus, thecracks do not have to be re p a i red. Ihave found that the stru c t u re tells thes t o ry. If there is a significant stru c t u r a lissue—such as a lack of re i n f o rc e-ment, a significant structural over-load, or a loss of bond—the crackwidths exceed the limits in the tableby several hundred percent. Whetherthe cracks should be re p a i red can stillbe debated, since cracks may simplyre-crack if they are re p a i red by epoxyinjection. Depending on the cause ofthe crack, strengthening of the stru c-t u re may be re q u i red, with or withoutconcomitant crack re p a i r.

C C : Is it reasonable to apply thecrack widths in Table 4.1 to crackscaused by volume changes due tod rying shrinkage or thermal contrac-t i o n ?

N a w y : No. The tabular valuesa re for flexural cracks in the high-moment zones of structurally support-ed members. Shrinkage and therm a l -contraction cracks are remedied oreliminated by appropriate constru c t i o np ro c e d u res, such as joints, where a ss t ructural members have to developflexural cracks under load, start i n gwith macrocracks at a level as low as20% of the ultimate load. The designeris obliged to prevent these macro c r a c k sf rom increasing beyond re a s o n a b l eand tolerable widths, as re c o m m e n d-ed in ACI 224R-90.

P o s t o n : Yes, but with some re a-sonably applied judgment. The limitsp rescribed in the table were specifical-ly developed based on judgmentsabout flexural-cracking criteria fors e rviceability (e.g., limiting deflection)and durability (e.g., minimizing re i n-f o rcement corrosion). Flexural crack-ing is fundamentally diff e rent fro mvolume-change cracking. The lattergenerally extends through the entirethickness of a member, whereas flex-ural cracking generally extends partway through the thickness from thetensile face to the neutral axis.

In a water-containing stru c t u re ,

the flexural-crack width limit of0.004 inch from Table 4.1 may bea p p ropriate for controlling re i n f o rc e-ment corrosion but too restrictive forp reventing leakage at a thro u g h - c r a c kcaused by volume changes. For leak-age considerations, a crack width of0.010 inch can be tolerated for crack-ing caused by volume changes sincecracks of this width or narrower aregenerally self-healing and thus don’tre p resent a serviceability issue. But ist h e re really much diff e rence betweenthe effects of a 0.004-inch and a0.010-inch crack when consideringthe vagaries of crack formation andm e a s u rements? I contend that therei s n ’t. Consequently, a limit between0.004 inch and 0.010 inch may bec o n s i d e red reasonable in most circ u m-stances, no matter what action caused

the cracking. CC: Rebar corrosion control is an

often-cited reason for limiting crackwidth in re i n f o rced concrete mem-bers. Do wider surface cracks meanthat the structural member is moresusceptible to corrosion damage andthus will be less durable?

N a w y : Flexural cracks at the ten-sile face of the structural memberextend vertically within the membert o w a rd the top compression face.Wider surface cracks enable moistureto seep more easily through the con-c rete cover and initiate corrosion ofthe re i n f o rcement. The rate of mois-t u re propagation is controlled by thei n g ress width of the crack. However,other factors also limit corrosion initi-ation. Thicker cover depths anddenser concrete inhibit the seepagerate within the concrete cover.

P o s t o n : This is a contro v e r s i a lissue, but as a general statement it canbe re g a rded as true.

Some re s e a rchers suggest thatwider flexural cracks perpendicular tore i n f o rcement cause corrosion to startat an earlier age, but in the long-term ,c o n c rete cover and quality govern therate of deterioration due to corro s i o n .F u rt h e rm o re, only cracks directly overre i n f o rcement are of import a n c e .Others believe that crack width corre-lates directly with the time of onsetand the severity of corrosion. In prac-tice, I’ve found both beliefs can bet rue; it depends on the type of corro-sion and the exposure conditions.

For example, corrosion deterio-ration associated with a convention-ally reinforced bridge deck exposedto chemical deicers is governed bymacrocell corrosion, with long-termdurability being controlled not bycrack width but by concrete coverand quality. In contrast, a preten-sioned concrete structure with high-strength prestressing steel exposedto a steady supply of water and oxy-gen at a flexural crack can experi-ence aggressive, localized pitting.This is microcell corrosion and,therefore, more likely to be affectedby crack size. ■

Publication #C01A067Copyright © 2001 Hanley-Wood, LLCAll rights reserve

The $64,000question is: Byhow much dothese crackwidths exceedthe limits?

C o n c rete Perspect ives