CONCRETE CARBONATION IN ARID CLIMATE

Transcription

1 CONCRETE CARBONATION IN ARID CLIMATE H. AI-Khaiat*, Kuwait University, Kuwait M. N. Haque, Kuwait University, Kuwait N. Fattuhi, England 29th Cnference n OUR WORLD IN CONCRETE & STRUCTURES: August 24, Singapre Article Online Id: The nline versin f this article can be fund at: This article is brught t yu with the supprt f Singapre Cncrete Institute All Rights reserved fr CI Premier PTE LTD Yu are nt Allwed t re distribute r re sale the article in any frmat withut written apprval f CI Premier PTE LTD Visit Our Website fr mre infrmatin

2 29 th Cnference n OUR WORLD IN CONCRETE & STRUCTURES: August 24, Singapre CONCRETE CARBONATION IN ARID CLIMATE H. AI-Khaiat*, Kuwait University, Kuwait M. N. Haque, Kuwait University, Kuwait N. Fattuhi, England Abstract Fifty-fur different cncrete mixes were expsed t ht arid weather in Kuwait t mnitr their extent f carbnatin. Varius parameters affecting carbnatin such as water/cement rati, cement cntent, type f cement, type and dsage f admixture, water-curing perid and type f cating were investigated. Fr many f these variables, the carbnatin depth and rate cnstant (K) are reprted fr an expsure perid f 8.6 years. The results shw that the water/cement rati is ne f the main factrs affecting carbnatin. The values f 'K' ranged frm 2.1 t 7.8 as the water/cement rati increased frm.45 t.8. Cncrete made with white Prtland cement carbnated less than rdinary r sulphate resisting cements. Tw curing cmpunds were effective in reducing carbnatin while tw cating materials prevented carbnatin after eight years f expsure. Keywrds : admixtures, cement, carbnatin, catings, curing cmpunds, field, water-curing, cncrete 1. Intrductin Reinfrcement crrsin is recgnized as the mst imprtant durability prblem f reinfrced cncrete structures. The crrsin f reinfrcing steel is accelerated when the cncrete carbnates. This is due t a reductin in alkalinity f cncrete cver t reinfrcement, which prvides the prtective envirnment. Results f a lng-term investigatin f cncrete expsed t natural weather cnditins in Kuwait are presented in this paper. Cncrete prisms were tested fr up t 8.6 years f expsure. Varius parameters affecting carbnatin are cnsidered in the study such as water/cement rati, cement cntent, type f cement, type and dsage f admixture, water-curing perid and type f cating. It has been generally accepted that the carbnatin depth, d, is related t expsure time, t, by the equatin (1 ) Fkes [1J, hwever, reprts that in ht dry envirnments carbnatin penetrates at abut 1 mm per year depending n the cncrete: it may be a little mre in dry situatins and significantly less in wetter situatins. Als, Rberts [2] has reprted an average Prtland cement cncrete may shw a depth f carbnatin f 5-8 mm after 1 years and 1-15 mm after 5 years. Whilst Sims [3] and Rberts [2] have reprted the K values which may be f mre relevance in temperate envirnment, Haque and AI-Khaiat [4J presented K values which are mre realistic fr the ht dry castal expsure cnditins f Kuwait (r the Gulf regin). Of curse, the carbnatin is a diffusin phenmenn and the rate f penetratin f C2 depends mainly n the cncrete quality and the expsure cnditins. 161

3 In this study 'd' is presented in mm, 't' in years and thus 'K' in unit f mm.yea(o.5. The perfrmance f different mixes and curing methds etc. were assessed by cmparing the 'K' values calculated frm the best fit result fr the d-t relatinship. 2. Materials and testing 2.1 Materials and Preparatin f Specimens Ordinary, white and sulphate-resisting Prtland cements were used. Hwever the bulk f the specimens were made with rdinary Prtland cement delivered in ne batch. The fine and carse aggregates cnsisted f natural desert sand and crushed desert gravel. Drinking tap water was used fr mixing and curing the cncrete. Sme prperties f the varius materials used have been described elsewhere (5]. Ten prprietary admixtures, fur curing cmpunds / aids and tw catings specifically designed t reduce carbn dixide diffusin int cncrete were used. Details f the prperties f the cnstituents f the varius cncrete mixes are shwn in Table 1. The varius mixes were prepared in the labratry and 1 x 1 x 5 mm prisms were cast. The prisms were demlded after 24 hurs. Mst prisms were cured in air and / r water fr varius perids. Hwever, sme prisms were brushed with curing cmpunds. Als, sme prisms were brushed with anti-carbnatin catings. Details f the applicatin f the varius curing cmpunds / aids and catings were described by Fattuhi and AI Khaiat [5]. 2.2 Testing T measure the depth f carbnatin, a slice with a minimum thickness f 2 mm was brken ff each 1 x 1 x 5 mm prism. Then a phenlphthalein indicatr slutin was sprayed unifrmly n the freshly brken surface. The depth f the carbnated surface layers f cncrete was measured with a steel ruler f.5 mm accuracy. The average f the fur measurements, frm each edge, was taken t be the depth f carbnatin fr each prism. Each carbnatin depth presented is t he average btained frm the tw slices. Carbnatin testing was perfrmed peridically fr the cncrete. 3. Weather cnditins Kuwait weather is characterized as ht and dry envirnment. Average annual mean temperature is 25.5 C. Fr mnths f May t September, the average mean temperature exceeds 3 C. The maximum mean temperature frm June t August exceeds 4 C. The average rainfall ttals 85 mm. per year. The mean relative humidity ranges frm 2 t 56%. Fr seven mnths a year, the mean maximum relative humidity is less than 5% [6]. The highest mean temperature recrded was 48 C while the lwest mean was 3 C during this perid f 8.6 years. Fr 6% f the perid the maximum mean temperature was higher than 3 C. The relative humidity reached highest values (8-9%) during winter mnths, and its lwest values (5-4%) during the summer mnths. 4. Results and discussin 4.1 Cement and aggregate cntent Tw sets f prisms with a water/cement rati f.5 and water cured fr six days were cnsidered. N admixture was used during the preparatin f the prisms, but the cement cntent f each set f prisms was either 435 r 46 kg/m 3. The results indicate that the carbnatin was marginally lwer fr prisms with a cement cntent f 46 kg/m3 and a fine t ttal aggregate rati f.32. Therefre, by increasing the cement cntent by 5.7% (i.e. frm 435 up t 46 kg/m 3 ), the carbnatin decreased by 5.4% (i.e. 'K' frm 3.11 dwn t 2.95). Hwever, when the fine t ttal aggregate rati in the prisms was increased frm.32 t.39 (i.e. ver sanded prisms), the carbnatin increased by 4.1 % (i.e. 'K' frm 2.95 t 3.7). Of curse, n firm cnclusin can be drawn frm these results and shuld be cnsidered indicative nly. There is nly marginal difference in the cement cntent f the tw mixes tested. 4.2 Cement type There was virtually n difference between the carbnatin depths measured fr cncrete mixes made with rdinary (K =3.23) and sulphate resisting (K =3.11) Prtland cements. Hwever, cncrete made with white Prtland cement (K = 2.72) exhibited the least carbnatin (i.e. abut 19% lwer than cncrete made with rdinary Prtland cement). The maximum carbnatin depths recrded were 15.5, 15. and 13.5 mm fr sulphate resisting, rdinary and white Prtland cement cncrete prisms, respectively. ]62

4 4.3 Water/cement rati Cncrete prisms were made with water/cement ratis ranging frm.45 t.8, and the cement cntent varied between 456 and 256 kg/m 3. Fig. 1 shws that as the water/cement rati increased, there was a significant increase in the depth f carbnatin. The increase in carbnatin depth reached mre than three flds when the water/cement rati was increased frm.45 t.8 (K frm 2.5 t 7.79). This shws the imprtance f selecting and maintaining a lw water/cement rati in cncrete mixes in rder t achieve an adequate durability. The maximum carbnatin depth recrded were 13. and 24. mm fr prisms with water/cement ratis f.45 and.8, respectively. Again, it has t be nted that the difference in the carbnatin depth is nt due t the difference in water/cement rati per se. It has been reprted, that it is, in reality, the lwering f the cmpressive strength and the pen pre structure f the cncrete which enhances the depth f carbnatin with an increase in the water cement rati [7]. Carbnatin depth has been reprted t be nearly a linear functin f the cncrete strength [4]. Accrdingly, the average carbnatin depth f the cncrete mixes with water/cement rati f.45,.5,.6,.7 and.8 has been pltted against the average cmpressive strength f these mixes in Fig. 2. There is a near linear relatinship between the tw which agrees well with the earlier findings [4]. Other factrs remaining unchanged strength is a functin f water/cement rati. 25~ ~ Water/Cement Rati (Summer) -e-.45 (K = 2.5) (K = 3.23) (K = 4.23) - -A- -.7 (K = 5.58) E (K=7.79).r: ~.. Q.. IC "" - CI) - - C 1 /I O~ r-----~------~ r r------'-----~ Time (Days) Figure 1. Carbnatin f Cncrete with varius Water/Cement Ratis 4.4 Type and dsage f admixture Fur main types f admixtures were used in the mixes. These included high-range water-reducing, high-range water-reducing and retarding, water-reducing and retarding, and plasticizing admixtures. Fig. 3 shws the variatin f the carbnatin depth f the varius prisms cntaining admixtures with time. K values were lwest fr prisms cntaining A 7 admixture, a plasticiser based n calcium lignsulfnate. Hwever, K values were highest fr prisms cntaining admixtures AIO and A9. Admixture A9 is a superplasticizer based n cndensate f sdium naphthalene sulfnate with frmaldehyde. Admixture AlO is a superplasticizer and retarder based n cndensate f sdium naphthalene sulfnate with frmaldehyde and these prisms had the highest average carbnatin depth f 9.2 mm at abut 31 days (K =3.19). The difference in the carbnatin depth fr prisms cntaining the varius admixtures was 13%. N definitive cnclusins can be made n the effect differing admixtures used n the depth f carbnatin. In practice, when superplasticisers are intrduced ther cncmitant changes are made in water/cement rati etc. In retrspect it was advisable t keep either the wrkability r the strength f these cncretes cmparable t cmpare the effect n carbnatin. 163

5 s 14 'ti, ~ 12 C- al 'ti 1 I: ;: I'll I: 8.c... 6 I'll U 4 y = -O.3936x cmpressive strength (MPa) Figure 2. Relatinship between Strength and Depth f Carbnatin 4.5 Water-curing perid A further ten sets f prisms with a water/cement rati f.5, were water cured fr, 2, 6, 13 r 27 days and nly half f them cntained admixture A2. Fig. 4 shws the results f the five sets that cntained admixture A2, while Fig. 6 shws the results f the ther five sets f prisms withut the admixture A2. Examinatin f Figs. 4 and 5 indicate that prisms with a cement cntent f 46 kg/m3. and cntaining n admixture exhibited slightly lwer carbnatin than thse with a cement cntent f 41 kg/m 3 with an admixture. Als, the results shw cnsiderable reductin in the carbnatin when the water curing perid was increased frm t 6 days. Haque [8, 9]. has reprted that in ht expsure cnditins 6 days f water curing is almst mandatry fr the prper strength develpment and this study further establishes that at least 6 days f curing is highly desirable t minimize the carbnatin depth and hence t imprve the verall perfrmance and durability f cncrete. 4.6 Curing aids Fur curing cmpunds / aids Cl, C2, C3 and C4 were used t brush prisms. Each set f prisms was demlded, then immersed in water fr a perid f 3-4 hurs. After surface drying they were brushed several times with a curing cmpund accrding t the specificatins f manufacturers. All specimens were then air cured in the labratry fr 27 days. The prisms had a water/cement rati f.5 and cntained 1 % dsage f admixture A2. Curing cmpund Cl was based n sdium silicate, C2 n resin, C3 n wax emulsin, while C4 was based n white resin. The carbnatin results fr the prisms are shwn in Fig. 6. It shuld be nted that prisms brushed with curing cmpunds / aids Cl - C3 were prepared during the summer, hwever prisms brushed with curing cmpund C4 were prepared during the winter. Additinal prisms brushed with curing cmpund C2 and prepared during the winter are dented by C2W. The carbnatin depth measurement results fr the prisms prepared during the summer shwed - - that prisms brushed with curing cmpunds C2 and C3 perfrmed better (K values f 3.6 and 3.7, respectively) than prisms brushed with curing cmpund C1 (K = 4.21). It shuld be nted that similar prisms cntaining admixture A2 and air cured exhibited an average carbnatin depth f 11.6 mm - - after 31 days f expsure (K =4.1). These results indicate that the use f cmpunds C2 and C3 have reduced the carbnatin depth rate by abut 13% in 8-year expsure perid. 164

6 E 8 -:5 Admixture Type --&-A6 (K =3.8) -.- A7 (K =2.68) ~ A8 (K = 3.16) -.If-. A9 (K =3.19) -.. A1 (K = 3.19) Q. CI) C 6 4 t Time (Days) Figure 3. Carbnatin f Cncrete Cntaining Different Admixtures - E E 8 - J: c. 4) 6 C Water Curing Perid (days) (withut Admixture) --&-27 (K = 2.78) (K = 3.4) ~ 6 (K =3.7) - 1f- 2 (K =3.3) -- (K = 4.5) G< 4 2, Time (Days) Figure 4. Carbnatin f Cncrete Cured fr Varius Perids: Withut Admixtures The results als shwed that prisms C2W which were prepared during the winter and brushed with curing cmpund C2, exhibited a higher carbnatin depth rate (K = 4.3). On the ther hand, prisms brushed with curing cmpund C4 exhibited a lwer carbnatin depth rate (K = 3.34). In summary, the curing cmpunds used have been fund t effective in mitigating the carbnatin depth f cncrete expsed t ht ambient cnditins. 165

7 Cnclusins 1. The water/cement rati was the main parameter affecting the rate f cncrete carbnatin. The K value varied frm abut 2.1 t 4.2 fr w/c =.45 t Six days f water curing was fund necessary and adequate t lwer the carbnatin depth t a level cmparable t 13 r even 27 days f water curing. 3. All the fur curing cmpunds brushed nt cncrete prisms were fund effective in reducing the carbnatin f cncrete. 4. White Prtland cement cncrete appeared t exhibit the lwest rate f carbnatin. Hwever, rdinary and sulphate resisting Prtland cement cncretes appeared t have had a similar rate f carbnatin. References: [1] Fkes, P.G., "Cncrete in Ht Dry Salty Envirnments", Cncrete, 29 (1995) [2] Rberts, M.H., "Carbnatin f Cncrete Made with Dense Natural Aggregates", BRE, Infrmatin Sheet, (1981 ). [3] Sims, I., "The Assessment f Cncrete fr Carbnatin", Cncrete, 28 (1994) [4] Haque, M.N. and AI-Khaiat, H.. "Carbnatin f Cncrete Structures in Ht Dry Castal Regimes". Cement and Cncrete Cmpsites, 19 (1997) [5] Fattuhi, N.I. and AI-Khaiat, H., "Shrinkage f Cncrete Expsed t Ht & Arid Climate", Jurnal f Materials in Civil Engineering (ASCE) 11 (1) (1999) [6] Meterlgical Department, "Climatlgical Annual Summary", Climatlgical Divisin Reprt, Kuwait Internatinal Airprt (1996). [7] AI-Khaiat, H. and Haque, M.N., "Carbnatin f Sme Castal Cncrete Structures in Kuwaif'. ACI Material Jurnal, 94 (1997) [8] Haque, M.N., "Sme cncretes need seven days initial curing". American Cncrete Institute, Cncrete Internatinal: Design and Cnstructin, 12(2) (199) [9] Haque, M.N., "Give it a Week: Seven days initial curing". ACI, Cncrete Internatinal, 2(9) (1998) [1] Davies, "Prtectin f cncrete in the Arabian Gulf', Paint and Ink Int. 8(3) (1995)