Materals and Structures/Matéraux et Constructons, Vol. 34, Aprl 2001, pp 150-154 TECHNICAL REPORTS Bt shape geometrc consderatons when samplng by dry drllng for obtanng chlorde profles n concrete M. A. Clment, G. de Vera and E. Vquera Departament d Engnyera de la Construccó, Obres Pùblques I Infrastructura Urbana,Unverstat d Alacant, Ap. 99, 03080 Alacant (Alcante), Span Paper receved: Aprl 3, 2000; Paper accepted: November 3, 2000 A B S T R A C T When obtanng dust concrete samples by dry drllng to determne chlorde content profles, the common concal shape of the bt tp nduces some problems for assgnng the analyzed Cl - content of each sample to a fxed depth from surface, (whch s necessary for applyng Cl - transport models): the amount of concrete removed s not the same throughout the length of a sngle drllng step, and concrete comng from depths lower than the nomnal drllng step s also obtaned. It s proposed that each Cl - concentraton n the profle should be assgned to the gravty centre of the sold sample really removed n each drllng step, nstead of assgnng t to the centre of the nomnal drllng step. A smple method for calculatng the gravty centre of each sample obtaned by dry drllng, has been developed. The calculated values of Cl - dffuson coeffcents from a Cl - content profle may be apprecably lower f the centres of gravty of the samples, nstead of the centres of the nomnal drllng steps, are used for the calculatons. R É S U M É Quand on obtent, au moyen d un forage à sec, des échantllons de béton pulvérsé destnés à établr le profl de concentraton de chlorure, la forme conque de la mèche entraîne des problèmes dans l assgnaton de chaque échantllon à une profondeur fxée, à partr de la surface, (cette assgnaton est nécessare pour applquer des modèles de transport de Cl - ) : la quantté de béton prélevée n est pas la même à travers toute la longueur de chaque échelon du forage. D autre part, on obtent auss de la poudre de béton à des nveaux mons profonds que l échelon nomnal du forage. Pour cela, on propose d assgner chaque concentraton de Cl - du profl au centre de gravté de l échantllon solde réellement obtenu à chaque échelon, plutôt que d assgner cette concentraton au centre de l échelon nomnal. On a développé une méthode smple pour calculer le centre de gravté des échantllons obtenus par forage à sec. Les valeurs ssues du calcul du coeffcent de dffuson de chlorure à partr d un profl de concentraton peuvent être apprécablement nféreures s l on utlse pour ces calculs les centres de gravté des échantllons plutôt que les centres des échelons nomnaux du forage. 1. INTRODUCTION The chlorde concentraton profle n concrete from the exposed surface nwards s a valuable tool for assessng the rsk of corroson of renforcement steel n concrete structures exposed to marne or de-cng salt envronments. Furthermore several test methods for obtanng chlorde (Cl - ) dffuson coeffcents n nonsteady-state condtons are based on fttng the expermental Cl - content profle to a partcular equaton [1]. The frst step for obtanng the profle s samplng, that may be done on-ste or at laboratory on cores prevously taken from the structure or concrete specmen. Several samples must be taken at selected depths from surface n order to get a detaled Cl - content profle. Three knds of methods are most used for obtanng these samples: by dry drllng usng a rotary hammer and a masonry bt [1-12]; by grndng from surface usng especally constructed or adapted devces [1, 10, 13, 14]; and by cuttng wth a saw slces of concrete from a core, whch subsequently wll be crushed to get the powdered samples [2, 10, 15, 16]. The former two procedures may be appled both on-ste or at laboratory. The drllng method s a rather cheap and quck procedure for samplng, although t allows to attan less precson than the grndng methods, whch may yeld samples at depth steps of 0.5 mm, whle the mnmum depth step for the drllng method s about 5 mm. The objectve of ths Edtoral Note Dr. Mguel A. Clment s a RILEM Senor Member. He partcpates n RILEM TC 178-TMC: Testng and modellng chlorde penetraton n concrete. 1359-5997/01 RILEM 150
Clment, Vquera paper s to descrbe some consderatons relatve to the assgnment of depth to the samples obtaned by dry drllng, whch may help to mprove the relablty of the calculated transport parameters from a Cl - profle. In ths sense t s worth pontng out the actual nterest n analyzng and standardsng the expermental procedures for testng Cl - penetraton nto concrete [1, 17]. Table 1 Approxmate geometrc parameters of some bts φ (mm) h (mm) α ( ) 12 3 26.6 18 4 24 22 5 24.4 2. PARTICULAR CONSIDERATIONS FOR THE DRY DRILLING SAMPLING METHOD One of the most mportant needs when samplng s to get representatve and non-contamnated samples. Ths means that some mechancal fxaton of the drllng devce to the concrete surface s needed to ensure a perfectly perpendcular drllng, and t s necessary to clean the hole,.e. by blowng compressed ar, after obtanng each sample. These latter condtons must be fulflled n order to avod the contamnaton of a sample wth dust comng from earler depth steps [18]. A mnmum bt dameter, usually 18 mm, s requred to avod excessve contrbutons from large aggregates, when these are present. Another useful means for obtanng a more representatve sample s to perform several drllngs n the surroundngs of each pont to be studed n a structure [19]. Another pont of concern when samplng by dry drllng s the assgnment of depth from surface to each sample. Ths fact s especally relevant f a model s gong to be used to derve Cl - transport parameters. Let us suppose that the drllng s beng performed at 5 mm depth steps. At a frst glance each dust sample would correspond to a cylndrcal pece of concrete wth the same dameter of the bt, and ts Cl - concentraton should be assgned to the centre of the depth step, (.e. 7.5 mm for the second sample obtaned whle the bt tp has moved from 5 to 10 mm depths), when plottng the profle. Nevertheless some geometrc consderatons due to the concal shape of the bt tp must be taken nto account. It may be apprecated n Fg. 1 that the frst sample obtaned from surface corresponds to a cylnder endng wth a cone, whose geometry depends on the geometrc parameters of the bt used. Table 1 collects approxmate data estmated for some bts normally used for dry drllng n concrete. It s nterestng to note that the angle α s always about 25 for ths range of bt cone dameters. Fg. 1 Ideal geometrc projecton of a bore hole performed wth a bt of dameter φ. Fg. 2 Ideal geometrc projecton of the second 5 mm step concrete sample taken by drllng wth a 18 mm dameter bt from 5 to 10 mm depth from surface. Table 2 Volume of concrete powder obtaned at each mm depth nterval when drllng a sngle sample (from 5 to 10 mm depth nterval), wth a 18 mm dameter bt. See Fg. 2 Depth from Volume of % of volume % of volume surface (mm) concrete (ref. to (ref. to (mm 3 ) 5-6 nterval) total) 1 2 V 1 58.3 22.9 4.6 2 3 V 2 153.8 60.4 12.1 3 4 V 3 217.4 85.4 17.1 4 5 V 4 249.2 97.9 19.6 5 6 V 5 254.5 100 20.0 6 7 V 6 196.2 77.1 15.4 7 8 V 7 100.7 39.6 7.9 8 9 V 8 37.1 14.6 2.9 9 10 V 9 5.3 2.1 0.4 TOTAL 1272.5 Fg. 2 shows the deal geometrc projecton of the second 5 mm step concrete sample, taken by drllng wth a 18 mm dameter bt, from 5 to 10 mm depth from surface. An mportant consequence s derved from the concal shape of the bt tp: when drllng the second and followng samples we obtan also dust from depths lower than the nomnal depth step. For the 5-10 mm sample we get some concrete dust comng from the 1 to 5 mm nterval. Table 2 shows the calculated volumes of concrete powder obtaned at each mm depth nterval when drllng the sample correspondng to Fg. 2. These data have been calculated by usng the correspondng expressons for the volumes of a cylnder, a cone and a cone frustum, and by consderng that the volumes of concrete obtaned for each mm nterval between 1 and 5 mm 151
Materals and Structures/Matéraux et Constructons, Vol. 34, Aprl 2001 these facts pont to the concluson that when a concal tp bt s used to obtan the samples for a Cl - content profle, each Cl - concentraton should not be assgned ether to the centre of the depth step or to the depth reached by the tp. A more accurate and realstc approach would be to assgn the Cl - concentratons to the gravty centres of the sold volumes of concrete really taken n each drllng step. The gravty centre of the spatal fgure whose projecton s shown n Fg. 2 may be calculated from the data of Table 2, but an easer way to calculate the gravty centre of each sample has been developed. Fg. 3 Ideal geometrc projectons of the frst sample (A) and the nth sample (B) taken by drllng from a concrete surface n L mm steps. Dashed lnes separate the decomposton elements consdered n each case. Sold ponts ndcate the gravty centres of each decomposton element. See prevous fgures and text for detals. Table 3 Comparson between the postons of the centres of drllng depth steps and the postons of the gravty centres of samples obtaned by dry drllng Bt dameter, Drllng depth Centre of drllng depth Gravty centre of sold φ (mm) step, L (mm) step (mm) sample (mm) 1st step 2nd step 3rd step 1st step 2nd step 3rd step 12 5 2.5 7.5 12.5 1.6 5.5 10.5 10 5.0 15.0 25.0 4.0 13.0 23.0 18 5 2.5 7.5 12.5 1.4 4.8 9.8 10 5.0 15.0 25.0 3.7 12.3 22.3 20 10.0 30.0 50.0 8.7 27.3 47.3 22 5 2.5 7.5 12.5 1.3 4.2 9.2 10 5.0 15.0 25.0 3.4 11.7 21.7 20 10.0 30.0 50.0 8.4 26.7 46.7 depths, are complementary to the volumes obtaned between 6 and 10 mm. It s worth notng that an mportant contrbuton of the total volume, (53.4%), s obtaned, n ths case, at depths lower than the nomnal drllng depth step,.e. at depths between 1 and 5 mm. Furthermore the relatve mportance of the concrete volumes obtaned between 8 and 10 mm s rather low. All 152 3. METHOD OF CALCULATION OF THE GRAVITY CENTRES OF CONCRETE SAMPLES OBTAINED BY DRY DRILLING Fg. 3 shows the deal geometrc projectons of the frst sample (A) and the second and subsequent samples (B) taken by drllng from a concrete surface n L mm steps. In order to smplfy the dscusson below we wll suppose that the matter consttutng these fgures s homogeneous. Ths assumpton s always mplct when handlng Cl - concentraton profles, unless separate analyss are performed to determne the bnder and aggregate contents of each sample. It s easly recognzed that the symmetry of Fgs. 3A and 3B mples that ther gravty centres (GC) are located on the drllng axs, whch wll be dentfed as the X axs. Fgs. 3A and 3B may be decomposed nto two or three components of more smple geometry, respectvely. These components are separated n Fg. 3 by dashed lnes. When a spatal fgure can be decomposed nto several fgures of volume V, whose GC le all on the X axs at x, then the X coordnate of the GC of the overall fgure can be calculated as: x GC V x The volumes V may be easly calculated takng nto account that the decomposton elements n Fg. 3 are: cylnders of heght (L-h), cones of heght h, and the spatal element represented on the left n Fg. 3B s complementary wth a cone of heght h for yeldng a cylnder of heght h. Ths means that the volume of the left element n Fg. 3B s the double of that of the cone on the rght. It s not even necessary to calculate the real volumes of the elements f we adopt the unty of volume as that of a cylnder of heght 1 mm and the same radus of the bt, then the volume of cylnders n Fg. 3 wll be (L-h), the volume of the cone wll be (h/3) and the volume of the left element n Fg. 3B wll be (2h/3). The x coordnates of the GC of the elements are located at half the heght for a cylnder, at (h/4) for a cone and at (5h/8) for the = V (1)
Clment, Vquera complementary left element n Fg. 3B. So drect expressons for calculatng x, based only on the knowledge of L and h, can be derved. These expressons are ndcated on the sold ponts of Fg. 3, whch dentfy the postons of the GC of the decomposton elements. Table 3 shows the GC of the sold concrete samples taken wth dfferent bts, from 12 to 22 mm dameter, and wth dfferent L depth steps, between 5 and 20 mm. The postons of the GC have been calculated wth the expressons ndcated n Fg. 3 and data from Table 1. It s worth notng that the GC of the frst sample drlled from a concrete surface, wth these bts, s located at depths lower, between 0.9 and 1.6 mm, than the nomnal centre of the drllng depth step. For the second and subsequent samples the GC s shallower than the nomnal centre of drllng step by a dfference between 2.0 and 3.3 mm. A lmtaton of the valdty of the expressons and calculatons descrbed here s the ntegrty of the bt tp geometry. If a spent bt, whose tp geometrc parameters (see Fg. 1) have been modfed due to the wearng out of the tool, s used for the drllng process the expressons ndcated before would not be applcable. A drect consequence of the consderatons descrbed here s that f Cl - content data obtaned from samples taken by dry drllng, are used to derve transport parameters,.e. dffuson coeffcents, D, dfferent values would be obtaned f each Cl - datum s assgned to the sample GC depth as descrbed before or to the mean depth of the nomnal drllng step. The former case wll yeld lower D values. Some quantfcaton of ths effect would be of nterest. 4. QUANTIFICATION OF THE EFFECT OF THE CONSIDERATION OF BIT GEOMETRY UPON CALCULATED CHLORIDE DIFFUSION COEFFICIENTS Osborne and Sngh [11] obtaned Cl - concentraton profles n specmens of dfferent concretes submtted to exposure to a marne envronment durng four years, as a part of ther expermental program. The samples were taken by dry drllng, but the dameter of the bt used was not ndcated. These authors dd not calculate Cl - transport parameters from ther data. Table 4 contans a set of Cl - concentraton expermental data correspondng to a partcular concrete specmen [11]. For ths set we have calculated the correspondng centres of nomnal drllng depth steps and the correspondng depths of the GC of the samples, followng the expressons of Fg. 3. To ths end we have assumed bt dameters of 12 mm (fourth column n Table 4) and 18 mm (ffth column). Fnally the Cl - dffuson coeffcent, D, and the surface Cl - content, C s, have been calculated by usng the standard error functon soluton to Fck s Second Law of Dffuson for one dmensonal flow nto a 153 sem-nfnte medum [20]. The calculaton nput data have been the expermental Cl - concentratons and three sets of X coordnates correspondng to the centres of the nomnal drllng depth steps, and GC of the samples, see Table 4. It s apprecable from Table 4 that the calculated D value s lowered by about 23% (12 mm bt) or by 32% (18 mm bt) when the centres of the drllng steps are replaced by the GC, whle the C s value s much less affected by ths replacement. In order to get a more detaled assessment of the magntude of ths effect we have extended the calculatons descrbed before to twenty Cl - content profles reported n [11], correspondng to dfferent concrete specmens submtted to full mmerson n sea water or to tdal exposure. The percentage of reducton of D ranges between 6 and 26% (12 mm bt) and between 8 and 35% (18 mm bt). These results confrm the expected non-neglgble nfluence of the consderaton of bt geometry. The dfferent percentages of reducton of D found for the twenty profles treated may be explaned by the dfferent shape of each Cl - profle: a steep profle wth a hgh Cl - content gradent nduces hgher senstvty to the fact of takng nto account or not the geometrc consderatons descrbed here, than a flat profle wth a low Cl - concentraton gradent. The percentages of reducton of D found n these calculatons are not too hgh when compared wth the varablty found n repeated determnatons of D through non-steady-state Cl - mgraton or dffuson tests wth the same type of concrete [1]. Nevertheless the standardsaton of the tests for assessng the resstance of concrete to Cl - ngress, ncludng samplng procedures and calculaton conventons, appears as a clear need f t s desred to compare approprately results obtaned through dfferent methods or by dfferent laboratores. Furthermore the calculatons needed to determne the GC postons of the drlled samples are qute smple and do not mply a sgnfcant complcaton for the applcaton of a transport model. Table 4 Expermental Cl - content profle obtaned from a concrete specmen submtted durng 4 years to mmerson n sea water [11]. Calculated postons of centres of drllng depth steps and gravty centres. Calculated Cl - transport parameters. See text for detals EXPERIMENTAL [11] CALCULATED ( ths work ) % Cl - Depth step Centre of step Bt φ 12 mm Bt φ 18 mm ref. to concrete (mm) (mm) GC (mm) GC (mm) weght 0.811 1 6 3.5 2.6 2.4 0.545 6 11 8.5 6.5 5.8 0.295 11 16 13.5 11.5 10.8 0.089 16 21 18.5 16.5 15.8 0.059 21 26 23.5 21.5 20.8 0.041 26 31 28.5 26.5 25.8 0.031 31 36 33.5 31.5 30.8 D (cm 2 /s) 5.9 x 10 9 4.5 x 10 9 4.0 x 10 9 C s (% Cl - ) 1.07 1.01 0.99
Materals and Structures/Matéraux et Constructons, Vol. 34, Aprl 2001 5. CONCLUSIONS Dust concrete samples from selected depth ntervals from the surface of a concrete structure, exposed to a salty envronment, may be obtaned by dry drllng wth a rotary hammer and a masonry bt, n order to determne the chlorde content profle. Due to the concal shape of the bt tp the amount of concrete dust removed s not the same all along the drllng depth step; and furthermore for the second and subsequent samples, concrete dust from depths lower than the nomnal drllng step s obtaned. For these reasons t s proposed that each Cl - concentraton n the profle should be assgned to the gravty centre of the sold sample really taken n the drllng step, nstead of assgnng t to the centre of the nomnal drllng step. A smple method for calculatng the GC of each sample obtaned by dry drllng, has been developed. The method s only based on geometrc consderatons of the bt shape and the real spatal dstrbuton of the concrete removed n each depth step. The nput data requred for ths calculaton are the drllng step length (L) and the heght (h) of the cone that consttutes the bt tp. The calculated values of Cl - dffuson coeffcents may be lowered by as much as 26 to 35%, due to the fact of usng the GC of the samples, nstead of the centres of the nomnal drllng depth steps, for the calculatons. Nevertheless ths reducton may be dependent upon the shape of the Cl - profle. ACKNOWLEDGEMENTS Ths work has been fnancally supported by the Consellería d Educacó Cènca de la Generaltat Valencana through project GV-3224/95 and grant GR00-24. We thank all members of RILEM TC 178- TMC and especally the Charlady Prof. C. Andrade for valuable dscussons. REFERENCES [1] Andrade, C., Castellote, M., Alonso, C. and González, C., Nonsteady-state chlorde dffuson coeffcents obtaned from mgraton and natural dffuson tests. Part I: Comparson between several methods of calculaton, Mater. Struct. 33 (2000) 21-28. [2] West, R. E. and Hme, W. G., Chlorde profles n salty concrete, Materals Performance (July 1985) 29-36. [3] Bamforth, P. B. and Pocock, D. C., Mnmsng the rsk of chlorde nduced corroson by selecton of concretng materals, n Corroson of Renforcement n Concrete, Proceedngs of Thrd Int. Symposum, Warwckshre, U.K., May 1990 (Socety of Chemcal Industry, London, 1990) 119-131. [4] Thomas, M. D. A., Matthews, J. D. and Haynes, C. A., Chlorde dffuson and renforcement corroson n marne exposed concretes contanng pulverzed fuel ash, n Corroson of Renforcement n Concrete, Proceedngs of Thrd Int. Symposum, Warwckshre, U.K., May 1990 (Socety of Chemcal Industry, London, 1990) 198-212. [5] Jaegermann, C., Effect of water-cement rato and curng on chlorde penetraton nto concrete exposed to medterranean sea clmate, ACI Materals Journal 87 (1990) 333-339. [6] Funahash, M., Predctng corroson-free servce lfe of a concrete structure n a chlorde envronment, ACI Materals Journal 87 (1990) 581-587. [7] Lam, K. C., Roy, S. K. and Northwood, D. O., Chlorde ngress measurements and corroson potental mappng study of a 24-year-old renforced concrete jetty structure n a tropcal marne envronment, Magazne of Concrete Research 44 (1992) 205-215. [8] Sand, B. T., The effect of the envronmental load on chlorde penetraton, Nordc Mnsemnar on Chlorde Penetraton nto Concrete (Göteborg, Sweden, January 1993) 113-124. [9] Mustafa, M. A. and Yusof, K. M., Atmospherc chlorde penetraton nto concrete n sem-tropcal marne envronment, Cement and Concrete Research 24 (1994) 661-670. [10] Poulsen, E., Chlorde profles. Analyss and nterpretaton of observatons, AEC Laboratory, Vedbæk, Denmark (1995). [11] Osborne, G. J. and Sngh, B., Chlorde on penetraton and frost resstance of hgh-alumna cement (HAC) and HAC/ground granulated blast furnace slag concretes n marne envronments n Performance of Concrete n a Marne Envronment, Proceedngs of Thrd CANMET/ACI Int. Conference, St. Andrews by-the-sea, Canada, August 1996 (Amercan Concrete Insttute, SP-163, Farmngton Hlls, Mchgan 1996) 295-316. [12] Costa, A. and Appleton, J., Chlorde penetraton nto concrete n marne envronment. Part I: Man parameters affectng chlorde penetraton, Mater. Struct. 32 (1999) 252-259. [13] Germann Instruments, Profle Grnder PF-1100 Manual, Germann Instruments A/S, Copenhagen, Denmark, (1996). [14] Sandberg, P., Tang, L. and Andersen, A., Recurrent studes of chlorde ngress n uncracked marne concrete at varous exposure tmes and elevatons, Cement and Concrete Research 28 (1998) 1489-1503. [15] GjØrv, O. L. and Vennesland,., Dffuson of chlorde ons from seawater nto concrete, Cement and Concrete Research 9 (1979) 229-238. [16] Castellote, M., Andrade, C. and Alonso, C., Chlorde-bndng sotherms n concrete submtted to non-steady-state mgraton experments, Cement and Concrete Research 29 (1999) 1799-1806. [17] RILEM TC-178 TMC, Objectves of the TC, www.rlem.org [18] Vennesland,., Report on Samplng for obtanng chlorde profles, Thrd Meetng of RILEM TC-178 TMC, Graz, Austra, May (1999). [19] Kropp, J., Chlordes n concrete, n Performance Crtera for Concrete Durablty, (RILEM Report 12, E. and F.N. Spon, London, U.K., 1995). [20] Crank, J., The Mathematcs of Dffuson, Second Ed. (Oxford Scence Publcatons, Oxford, U.K., 1983). 154