Unanticipated cold joints inc o n c rete stru c t u res occa-sionally plague contra c-t o r s, even though super-

p l a s t i c i zed concrete mixes and awide range of vibrating equipmentand techniques are ava i l a b l e. Bu twhen a visible pour line is foundon an exposed concrete surf a c e, itd o e s nt always mean a potentiallytroublesome cold joint is present.

Fo rt u n a t e l y, there are seve ral test-ing methods that can be used toe valuate whether surface lines arejust noncritical surface imperf e c-tions or signs of significant stru c t u r-al defects. By making this determ i-nation, engineers and contra c t o r scan more accurately assess the im-pact on the concre t es stru c t u ral in-t e g ri t y, function, and dura b i l i t y.

Pour Lines vs. Cold JointsBoth pour lines and cold joints

a re common in concrete constru c-tion, but cold joints have more seri-ous consequences. Pour lines ared a rk lines on the formed surface at

the boundary between adjacentbatches of concre t e and typicallyindicate that the vibrator was notl owe red far enough to penetra t ethe layer below the one being vi-b ra t e d (Ref. 1). Cold-joint lines, onthe other hand, indicate the pre s-ence of joints where one layer ofc o n c rete had hardened before sub-sequent concrete was placed anda re typically re vealed by v i s i b l elines on the surfaces of form e dc o n c re t e (Ref. 2).

To even a trained eye, the distinc-tion between the two conditions isusually not apparent at the concre t es u rf a c e. To the engineer, a surf a c eline may indicate an unanticipatedcold joint and a weakened plane forshear and tension. To the contra c-t o r, this surface imperfection may only be a pour line that will b e h a ve as if the concrete we remonolithic, and the flaw can be dismissed as stru c t u rally unim-p o rtant.

The first step in evaluating a sur-face line is realizing that the impor-

tant inform a t i o ndoes not lie at thes u rf a c e. Co re sam-ples must be re-m oved from re p re-s e n t a t i ve locationswithin the stru c-t u ral element un-der considera t i o n .

Evaluating CoreSamples

Co re samplesshould coincidewith the surf a c eline and be located

such that the surface line is at thecenter of each core, as shown inFi g u re 1. Then each core must bevisually examined and tested.

Visual examination of core sam-ples should be perf o rmed both bythe unaided eye and by micro s c o p e.First, visually examine each core tod e t e rmine how far the line extendsb e yond the surf a c e. In many in-s t a n c e s, surface lines are caused byinadequate consolidation betwe e nthe outermost re i n f o rcement laye rand the form w o rk and are limitedto the cover concre t e. In these in-s t a n c e s, the defect is essentially aes-thetic, except in cases where dura-bility is a concern. If the surface linecontinues significantly beyond thes u rf a c e, as shown in Fi g u re 2, the in-t e rface between successive liftsalong each core re q u i res examina-tion. A rough surface with some ag-g regate pro t ruding between liftsand no voids or spaces at the inter-face indicates a pour line and not acold joint, as shown in Fi g u re 3.

Mi c roscopic examination consistsof sampling portions of each coreand constructing thin sections for ex-amination under a stere o m i c ro-s c o p e. Id e a l l y, the thin sections willbe oriented with the interface alongthe center, allowing the gre a t e s tamount of adjacent material on eachside of the line to be studied. A pet-ro g rapher should examine the thinsections to identify evidence of car-bonation, drying, or changes in pastem i c ro s t ru c t u re along the interf a c e.The presence of these conditions in-dicates a possible cold joint; a lack ofthem indicates that the interface ismost likely a pour line. Ca r b o n a t i o n

Are They Pour Lines Or Cold Joints?

BY JEFFERY S. VOLZ, CARLTON A. OLSON, RALPH G. OESTERLE, AND STEVEN H. GEBLER

When contractors detect pour lines on exposed concrete surfaces, theyneed to look beneath the surface to determine the implications

Figure 1. Core sample removal at surface line.

g e n e rally appears as a thin film de-tectable only with a micro s c o p e,e ven in a distinct cold joint. Signs ofd rying include micro c racking per-pendicular to the interface surf a c eand/or a locally lower water- c e m e n tratio in one layer near the interf a c e.

Another step in core eva l u a t i o nconsists of perf o rming split-cylin-der tests in accordance with ASTMC 496-96 (Ref. 3). Co res must betested with the pour line or cold-joint line oriented ve rtically at thecenter of the test machine, ass h own in Fi g u re 4. If the line doesnot coincide with the anticipatedve rtical failure surface due to ana p p reciable va riation from the cen-t e r, the test result for that core cantbe used to evaluate the tension ca-pacity of the interf a c e.

In addition to obtaining the split-cylinder test va l u e s, inve s t i g a t o r sshould examine the fra c t u re pat-t e rns visually. Nu m e rous fra c t u re da g g regate particles and a ro u g h-ened, fra c t u red paste surface indi-cating adhesion along the failureplane are signs of monolithic con-c re t e, not a cold joint.

The specimens splitting tensiles t rength can also be compared tothe expected splitting tensiles t rength for concrete with the c o n t ract-specified compre s s i ves t rength. ACI 318-95 (Ref. 4) andNeville (Ref. 5) present the necessaryrelationships between concre t ec o m p re s s i ve strength and splittingtensile strength. A statistical eva l u a-tion of the tensile test values willp redict the lowe r-bound va l u e,which is then compared with the ex-

pected strength of monolithic con-c re t e. If the value exceeds the ex-pected strength, the strength of thein-place material is considered ac-c e p t a b l e. If this value falls below theexpected strength, stru c t u ral impli-cations should be inve s t i g a t e d .

Fo l l owing are two case histori e sthat illustrate how to apply thesec o re test methods to re a l - w o r l dc o n s t ruction pro b l e m s.

Case HistoriesUn n e c e s s a ry grade-beam re-

m ova l . At a Texas prison, 600 lin-eal feet of concrete grade beamwe re re m oved based onlyon a visual inspection ofthe surf a c e, which showe da defect resembling eithera cold joint or pour line( Fi g u re 5). But the deci-sion to re m ove theseg rade beams based solelyon the limited visual sur-vey was a mistake, as welater discove re d .

The project contra c t o renlisted our services to de-t e rmine whether it had indeed been necessary tore m ove the beams. In ve s t i-gators inspected the con-c rete surfaces and re m ove d18 randomly selected core sat the level of the suspect-ed cold joint for testing andvisual examination. Sp l i t -cylinder test results ra n g e d

f rom 340 to 535 psi with a mean of445 psi and standard deviation of68 psi.

A statistical evaluation of theseresults indicated a 95% pro b a b i l i t ythat the true population ave ra g ewas equal to or greater than 410 psiand that no single value would beless than 300 psi. Expected splittingtensile strengths for the contra c t -specified concrete compre s s i ves t rength (fc) of 3000 psi are approx-imately 200 to 275 psi, or less thanthe values indicated.

Fu rt h e rm o re, the fra c t u re pat-t e rns had numerous fra c t u red ag-

Figure 2. Visual examination of the core sample revealshow far the line extends beyond the surface.

Figure 3. Signs of a pour line: Interface exhibits a roughsurface with aggregate protruding between lifts and novoids or spaces.

F i g u re 4. P roper orientation ofspecimen for split-cylinder test.

g regate particles along the failurep l a n e. A few specimens exhibitedsmall areas lacking adhesion alongthe failure surf a c e. Though this con-dition probably reduced the split-cylinder stre n g t h s, the values stille xceeded the expected value forc o n c rete with a 3000-psi compre s-s i ve strength. There f o re, the in-placec o n c rete at the pour line would havep e rf o rmed at least as well as mono-lithic concrete with the specifiedc o m p re s s i ve strength of 3000 psi.

We also investigated the gra d eb e a m s, assuming that their per-f o rmance at the pour line wouldbe somewhat less than monolithicc o n c re t e. The grade beams we ree valuated as composite sectionsfor hori zontal shear tra n s f e ra c ross the pour line. In accor-dance with ACI 318, the hori zo n t a lshear was tra n s f e r red through ashear friction mechanism. The re-q u i red coefficient of friction for ashear friction transfer mechanismwas calculated and compared towhat could conserva t i vely be ex-pected at the pour-line region. Inall cases the capacity exceeded there q u i red stre n g t h .

Su rface defect on a re i n f o rc e d -c o n c rete wall. At a Ma s s a c h u s e t t ss t o rm w a t e r- runoff management fa-c i l i t y, concern was raised after re-

m oval of form w o rk along 100 linealfeet of re i n f o rc e d - c o n c rete wall re-vealed a surface defect resembling ei-ther a cold joint or pour line (Fi g u re6). The contractor enlisted our firmto examine the defect. In ve s t i g a t o r sinspected the concrete surfaces andre m oved randomly selected cores atthe level of the suspected cold jointfor testing and visual examination.

Visual inspection of the core sam-ples indicated that the concrete waswell-consolidated, with no evidenceof cold joints along the pour-line ar-e a s. The line had a rough surf a c e, withsome aggregate pro t ruding betwe e nlifts and no voids or spaces at the in-t e rf a c e. There we resome areas of iso-lated honeyc o m b-ing, but they we relocated within thec over concrete andjudged as minorand easily re-p a i ra b l e.

Sp l i t - c y l i n d e rtest re s u l t sranged from 380to 460 psi with amean of 410 psi.A statistical eva l-uation of these re-sults indicated a95% pro b a b i l i t y

that the true population ave ra g ewas equal to or greater than 390psi and that no single value would be less than330 psi. Expected split-cylinders t rengths ranged from 250 to 315 psi for the contra c t - s p e c i f i e dc o n c rete compre s s i ve strength (fc)of 4000 psi. Thus, based on the vi-sual observations and results ofthe test data, the in-place concre t eat the pour line would perf o rm atleast as well as monolithic con-c rete with the specified compre s-s i ve strength of 4000 psi.

Remedial AlternativesAs the case histories illustra t e,

c o re testing and visual examinationa re re l a t i vely simple to implementand can save both time and moneyon a construction project. Its im-p o rtant to note, howe ve r, that coldjoints may not seriously affect thep e rf o rmance of a concrete element,depending on the elements stru c-t u ral demands and service condi-t i o n s. Even when testing and in-spection indicate that a surface lineis indeed a cold joint, each specifico c c u r rence must be investigated tod e t e rmine if the cold joint re d u c e sthe stru c t u ral capacity below thes t ru c t u ral demand of that part i c u l a relement. If the stru c t u ral capacitydoes not meet the demand, re m e d i-al repairs should be investigated asfinancially viable altern a t i ves tocomplete concrete re m oval and re-placement.

Figure 5. Saw operator removes grade beams for a Texas prison after visualinspection revealed a defect resembling either a cold joint or pour line. Coretesting later determined that beam removal was unnecessary.

Figure 6. Core tests revealed that this surface defect in areinforced-concrete wall was only a pour line and that theconcrete would meet performance demands.

The authors are all with Constru c-tion Technology Laboratories Inc.(CTL), Skokie, Ill.:J e ff e ry S. Vo l z , S.E., P.E., is a seniors t ructural engineer who specializesin analytical investigations, nonde-s t ructive testing, and inspection ofexisting stru c t u res for gravity, seis-mic, thermal, and wind loading.Carlton A. Olson, senior evaluatione n g i n e e r, has perf o rmed field eval-uations of various re i n f o rc e d - c o n-c rete stru c t u res, including dams,bridges, roadways, parking garages,industrial facilities, and buildings.Ralph G. Oesterle, S.E., P.E., is a se-nior principal structural engineerwho directs teams of CTL engineersinvolved in lab and field testing,s t ructural analysis, and re s t o r a t i o nof re i n f o rced- and pre s t re s s e d - c o n-

crete structures.Steven H. Gebler, P.E., is a seniorprincipal evaluation engineer and aLevel III concrete inspector who spe-cializes in concrete pro p e r ties andconstruction-related problems.

References1. ACI Committee 309, Guide for Con-solidation of Concrete (ACI 309R-87),American Concrete Institute, Farming-ton Hills, Mich., 1987, p. 309R-21.2. ACI Committee 116, Cement andConcrete Terminology (ACI 116R-90),American Concrete Institute, 1990, p.116R-14.3. ASTM C 496-96, Standard TestMethod for Splitting Tensile Strengthof Cylindrical Concrete Specimens,ASTM, West Conshohocken, Pa.,1996.4. ACI Committee 318, Building CodeRequirements for Structural Concrete

(ACI 318-95) and Commentary (ACI318R-95), American Concrete Insti-tute, 1995.5. A.M. Neville, Properties of Con-crete, 3rd ed., Longman Scientific &Technical, London, 1981.

PUBLICATION #C970369Co py right 1997, The Ab e rdeen Gro u pAll rights re s e rve d