Comparison of 2 year carbonation depths of common cement concretes using the modified draft CEN test

Transcription

1 Materals and Structures/Materaux et Constructons, Vol. 34, August-September 2001, pp Comparson of 2 year carbonaton depths of common cement concretes usng the modfed draft CEN test M. R.Jones, M. D. Newlands, A. M. O. Abbas and R. K. Dhr Concrete Technology Unt, Cvl Engneerng Department, Unversty of Dundee, Dundee, Scota,d, UK Paper receved: October 10, 2000; Paper accepted: February 14, 2001 A B S T R A C T Ths paper reports the results of a 2 year study of the development of carbonaton of hardened concrete usng a modfed verson of the draft CEN test method. Ten dfferent common cement mxes, at an equal grade of 37 N/mm 2 have been tested and compared to a benchmark PC/30% PFA mx. The effect of storage condton and the type of cement on carbonaton are determned. RI~SUMI ~ Cet expos~ est le compte-rendu des r&ultats de deux ann&s d'&~de h propos du d&eloppement de la carbonataton du b~ton durc en se servant d'une verson modfde du processus CEN prdlmnare. Dx types de b&on d'un usage courant, chacun d'ml nveau constant de 37 N/mm 2, ont dtd analys& pus cornpar& hun m~lange de r~f&ence Cment Portland / 30% de cendres volantes. Tout cela nous permet de ddtermner l'effet auss ben du genre du b&on que des condtons de stockage, sur la carbonataton. 1. INTRODUCTION 2. EXPERIMENTAL PROGRAMME To enable sustanable and economc renforced concrete buldng constructon, the relable determnaton of the carbonaton rate of hardened concrete s essental to allow desgn decsons to be made. To ths end, CEN TC51/WG12/TG5 have formulated a draft test method, whch s currently beng assessed [1]. In addton to ths, the new European Standards for cement and concrete, namely EN and EN (ncludng the relevant natonal applcaton documents), allow the engneer scope to select cement and concrete characterstcs and requrements approprate to local condtons. In support of ths goal, the CTU has carred out a 2 year study of the carbonaton of concrete contanng 10 dfferent combnatons of common cements, the results of whch are now avalable and reported n ths paper. The objectve of the study was to compare carbonaton rates of concrete subject to dfferent storage condtons, n terms of the degree of wettng appled to the test specmens. The test programme also allowed a 'senstvty' analyss to be carred out n order to determne the effects of dfferent bulk propertes of the concrete. Analyss of long-term publshed data, showed that there s a general agreement that a PC/30% PFA blended cement renforced concrete wth a characterstc 100 mm cube strength 37 N/ram 2 and a cover of 30 mm wll perform satsfactorly gven an ntended desgn lfe of at least 50 years [2]. On ths bass, and reflectng the requrements of EN and famlarty to many engneers [3-7], ths concrete was selected as the 'benchmark' for ths research project to whch other concretes could be compared. A total of 10 dfferent cements were selected reflectng the most commonly used cements, as summarsed n Table 1. The concrete mxes were desgned usng a standard mx consttuent proportonng method [8], to acheve a nomnal 28 day cube strength of 37 N/ram 2. To enable the carbonaton of concrete to be calculated for an exact strength, a normalsaton procedure as prevously establshed and descrbed n detal n Reference [9] was used. In essence, ths procedure used three mxes cast for each cement type wth slghtly varyng water/cement rato and the exact mx proportons nter /

2 Jones, Newlands, Abbas, Dhr polated for 37 N/mm 2, as gven n Table 1. An upper lmt for free water content of 185 kg/m 3 was selected as ths was felt to reflect the typcal upper maxmum used by concrete supplers for contemporary structural concrete. Where requred, a normal range plastcser was used to acheve a fxed workablty of slump Class $2 as defned n EN PREPARATION OF TEST SPECIMENS After castng, the 510 x 100 x 100 mm prsm test specmens were stored n ther moulds for 24 hours under damp hessan. After demouldng, each specmen was ndvdually wrapped n mpermeable plastc flm and stored at 20~ untl a 100 mm cube strength of 18.5 N/mm 2 was reached,.e. half of the standard 28 day cube strength and the curng requrement n ENV : Executon of concrete structures - Part 1: Common rules. Ths ensured that all test specmens were placed n the CEN carbonaton storage envron- Table 1 - Mx proportons for benchmark and test mxes normalsed to a standard cube strength of 37 N/ram 2 Type Benchmark Mx PFA 30% Test Mxes Portland pc(5) GBS GBS 40% GB$ 50% GBS 65% Metakaoln MK 10% MK 15% MK 20% Slca Fume CSF 10% CSF 15% CEM Desgnaton Normalsed Mx Consttuent Proportons (1), kg/m 2 Plastcser Total Water (2) Aggregates I/lOOkg Dosage, Sand (3) 10 mm (4) 20 mm (4) 1 ment at the same strength rather than age. Storage of the specmens was n a sealed room whch had actve control for temperature, relatve humdty and CO 2 concentraton, as descrbed n Reference [9]. The storage condtons used were as follows: Exposure Class 1:350 +_ 50 ppm CO 2, ~ and %RH. Exposure Class2: As Class 1 but test specmens removed from CEN Test Chamber and mmersed n fresh mares water at ~ for 6 hours every 28 days, then surface dred and returned to the CEN Test Room. Exposure Class3: As Class 2 but test specmens mmersed for 6 hours every 7 days. The depth of carbonaton was measured at varous ntervals usng the sprayed phenolphthalen soluton, procedure as defned n the draft CEN method [1] whch also conformed to the RILEM CPC-18 method [11]. Permeaton tests were also carred out usng ntrnsc ar permeablty and water vapour dffusvty tests, both of whch had been prevously developed at the Unversty of Dundee [10, 12] and the Intal w/c rato II/B-V none 0.46 requred I none 0.59 requred III/A III/A none 0.48 III/A requred 0.47 IV/A IV/A IV/A IV/A IV/A () Three mxes were cast for each cement type, a prmary mx assumed to gve a standard cube strength of 37 N/mm 2 at 28 days and two secondary mxes wth cement content + 8% of the prmary mx. The desred proportons to acheve an exact 28 clay strength of 37 N/mm 2 was nterpolated, as shown n the Fgure on the rght. Further detals can be found n Reference [gj. (2) Free water content to acheve slump Class $2 to EN (60-90ram). Plastczer used as shown. (3) Natural sand, zone M gradng to BS 812:1992/pr EN (4) Coarse aggregate: natural gravel (5) Portland s strength class 42.5 N to BS 12 (1996) ;0 LO ;0 LO Example of normalsng PFA 30% concrete mx Normalsed Content for 37 N/ram = mx = 374 kg/m 3 rounded to 375 kg/m Content, kg/m ~ Surface Absorpton Test (ISAT) to BS 1881:Part 208 [13]. The tests were carred out pror to storage n the carbonaton env- ronment and at varous tme ncrements thereafter, as carbonaton proceeded. 4. EXPERIMENTAL RESULTS The CEN test ams to be completed n 1 year, however, the expermental programme was run for 2 years, as t had already been noted that carbonaton depths would be mnmal at 12 months. The depths of carbonaton, normalsed to 37 N/mm 2 for all cement types n the 3 Exposure Classes, are gven n Table 2. For each test prsm, 20 ndvdual measurements of the depths of carbonaton were taken, gvng a total of 40 readngs for each cement type n each Exposure Class. At each test age, the carbonaton depths were statstcally tested to determne whether they were normally dstrbuted by testng the 'goodness of ft' of a normal dstrbuton curve usng the Ch-squared test. The depths of carbonaton had a

3 l Materals and Structures/Materaux et Constructons, Vol. 34, August-September 2001 Table 2 - Mean carbonaton depths of 2 CEN specmens tested up to 2 years (728 days) wthn the 3 exposure classes normalsed to 37N/mm 2 CEN Mean(l) Depth of Carbonaton from 2 prsms, mm Type Exposure Class lm 3m 6m 9m lyr 1.5yrs 2yrs PFA30% Benchmark PC , GBS 40% GBS 50% , GBS 65% , MK 10% MK 15% MK 20% CSF 10% CSF 15% Statstcal Analyss of 2 year Carbonaton Depths Across Type CEN Exposure Mean, Standard Coeffcent of Class Devaton, o Varaton, V% (2) (I) Mean of 40 readngs, 20 readngs fom each prsm.e. 5 on each face as per CEX Carbonaton Test Method [9] (2) Coeff?cent of varaton where V = ~ Table 3 - Cumulatve ncrease n mass of specmens subjected to wellng and dryng cycle of Exposure Classes 2 and 3 Type PFA 30% PC GBS 40% GBS 50% GBS 65% MK 10% MK 15% MK 20% CSF 10% CSF 15% V% Cumulatve Weght Increase (t) n Specmens Subjected to Wettng and D~ng, % w.r.t specmen weght pror to storage n carbonatng envronment Exposure Class 2 6 months! I year 2 years 2.40 I , ! I months [1) Weght change measured to O. l g and rounded w the nearest 0.5g ton up to a perod of 2 years. Exposure Class 3 1year 4.2 Effect of CEN storage condtons on carbonaton depth 2 years O 3, Fg. 1 compares the carbonaton depths for the dfferent cement types n the 3 Exposure Classes. Storage n Exposure Class 3 (wettest) had the lowest depth of carbonaton and hghest pore saturaton level. Exposure Class 2 resulted n depths of carbonaton between Classes 1 and 3 due to the perod of wettng. Table 3 shows the cumulatve weght ncrease n specmens subjected to cyclc wettng and dryng. In all cases, there s a weght ncrease over the 2 year test perod, wth the mass ncrease beng greater n test specmens subjected to more frequent wettng and dryng cycles The varablty of weght change across cement type showed a coeffcent of varaton at 2 years beng 9.0% n Class 2 and 11.5% n normal dstrbuton both wthn each prsm set and wthn each cement type set.,,-'".. /",~ J Type 4.1 Effect of cement type on carbonaton depth Table 2 shows that wthn Exposure Class 1, where no wettng was carred out, the effect of cement type was most notceable, wth PC and MK 10% mxes havng the lowest depths of carbonaton after 2 years testng. It should be recognsed that under these condtons, corroson s mprobable as the concrete s unlkely to have suffcent pore saturaton. The effect of replacng the PC wth PFA, GBS and the hgher levels of MK and CSF was found to gve ncreased depths of carbona- d~ I -2o ~- ~ -30 ~ -40 ~ -50 "~ -60 ~. ~ -70 eq~ 9 = ~ -8o = Fg. 1 - Comparson of reducton n CEN carbonaton depth at 2 years from Class 1 Exposure to Classes 2 and

4 Jones, Newlands, Abbas, Dhr o Class 1 ~ Class 2 ;~ Class 3 :No change n relatve ~ankng order between Relat!ve rankng changed orller I betwedn 0 and 18 months stoage 18 and 24 months No change n relatve rankng orddr between 0 and 24 mo.'nths storage Stablty Crteron - No change n relatve rankng order over consecutve 6 month storage perods Perod of Carbonaton, Months Fg. 2 - Stablty analyss of relatve rankng order for CEN carbonaton specmens up to 2 years Relatve 'stablty analyss' of test method In order to test after what perod of tme the depth of carbonaton could be relably determned a 'stablty' crteron was defned as 'no change n the rankng of the cement types over a 6 month perod'. Usng Fg. 2, t was establshed that the rankng of cement types changed n Exposure Class 1 up to a perod of 18 months and Exposure Class 2 up to 12 months. Wth Exposure Class 3 the relatve rankng of concrete specmens was constant over the entre test perod. Class 3. The sgnfcance of ths varablty s unclear and further work must be carred out to ascertan ths. The problem, however, for the engneer s whch, f any, of these Exposure Classes represents the natural exposure envronments. At present, there does not appear to be suffcent data to make an nformed judgement and, n any case, there are lkely to be a range of exposure condtons that can support carbonaton-nduced corroson actvty n dfferent geographcal locatons for example the north-west coast of the UK s consderably wetter than the south-east coast. Although Class 1 s the worst case condton for carbonaton, t s consdered that ths condton s too dry to support corroson. On the other hand, Class 3 s consdered not only wetter than any natural envronment but the resultng pore saturaton would produce only mnmal corroson actvty. It s recognsed therefore that further work s necessary to defne exposure class envronments whch smulate the natural exposure envronments capable of supportng both carbonaton and carbonaton-nduced corroson. Table 5 - Senstvty analyss of pre-carbonaton water vapour! dffusvty results for test mxes to acheve equvalent depths of CEN carbonaton as PFA 30% mx at 2 years Water Vapour Dffusvty (1), g/mm 2 s mm Hg x 10 "1~ Exposure Class 1 Exposure Class 2 Exposure Class 3 Type At 37N/mm 2 equalto k* benchmark PFA30%.... = : :::::: PC GBS 40% GBS 50% GBS 65% MK 10% MK 15% MK 20% CSF 10% CSF 15% equal to k* equal to k* benchmark benchmark (1) Measured when strength reached 18.5N/mm 2 and pror to commencng the CEN carbonaton test * k factor to acheve smlarpe~rmance to benchmark PFA 3096 mx Table 4 - Senstvty analyss of pre-carbonaton ar permeablty results for test mxes to acheve equvalent depths of CEN carbonaton as PFA 30% mx at 2 years Ar Permeablb] (1), m2x Exposure Class I Exposure Class 2 Exposure Class 3 Type At 37 N/ram 2 equalto k* equalto k* equalto k* benchmark benchmark benchmark ~o% b PC GBS 40% GBS 50% GBS 65% MK 10% MK 15% MK 20% CSF 10% CSF 15% (1) Measured when strength reached 18.5N/mm 2 and pror to commencng the CEN carbonaton test * k factor to acheve smlarpe~formance to benchmark PFA 30% mx Table 6 - Senstvty analyss of pre-carbonaton ntal surface absorpton results for test mxes to acheve equvalent depths of CEN carbonaton as PFA 30% mx at 2 years I Exposure Class 1 Exposure Class 2 Exposure Class 3 Type At 37N/ram 2 equalto k* equalto k* equal to k* benchmark benchmark benchmark ::PFA30% 56 :: Intal Surface Absorpton at 10 mnutes (11, ml/m2/h x 10 "2 PC GBS 40% GBS 50% GBS 65% MK 10% MK 15% MK 20% CSF 10% 44 I CSF 15% 42 I (1) Measured when strength reached 18.5N/mm 2 and pror to commencng the CEN carbonaton test * k factor to acheve smlarpe~rmance to benchmark PFA 30% mx 399

5 Materals and Structures/Mat6raux et Constructons, Vol. 34, August-September SENSITIVITY OF CARBONATION TO DIFFERENT CONCRETE PROPERTIES 5.1 Senstvty to concrete permeaton propertes A number of factors nfluence the resstance of concrete to carbonaton ncludng the varous permeaton propertes commonly used to assess the qualty, of cover concrete [14]. A senstvty analyss of the carbonaton results was carred out to determne ther dependency of partcular concrete propertes n order to acheve a smlar performance to the PFA Reference mx. Tables 4, 5 and 6 show the pre-carbonaton concrete permeaton propertes and what would be requred by dfferent cement types to gve smlar performance to the Reference Mx. For some concretes, namely MK 15%, MK 20%, CSF 10% and CSF 15% the permeaton requrements (k values) are less than was acheved at 37 N/mm 2 grade for other concrete. The 'k' factors are very smlar n magntude for both ntrnsc ar permeablty and water vapour dffusvty, however, although the ISAT 'k' factors show smlar trends to the permeablty tests, the magntudes are somewhat dfferent. The dfferences are hghlghted by the fact that the MK 20% mx requred a reducton n ntrnsc ar permeablty and water vapour dffusvty however, the ISA-10 readng s smlar. Thus, a cautous approach s necessary n attemptng to specfy a sngle permeaton test as an ntal type approval. 5.2 Senstvty to concrete mx propertes Tables 7 to 9 summarse the requred adjustments to the cement content, w/c rato and compressve strength to acheve a smlar carbonaton resstance to that of the reference mx. Although the trends were smlar across the dfferent cement types, the values were dfferent wth all mxes dsplayng an ncreased depth of carbonaton wth ncreasng water/cement rato. The 'k' values Table 7 - Senstvty analyss of cement content for test mxes to acheve equvalent depths of CEN carbonaton as PFA 30% mx at 2 years Content, kg/m 3 Exposure Class I Exposure Class 2 Exposure Class 3 Type At I I! 37N/mm 2 equalto k* equal to k* equalto! k* benchmark benchmark benchmark PFA 30% 375 PC GBS 40% GBS 50% GBS 65% MK 10% MK 15% MK20% CSF 10% CSF 15% * k factor to acheve smlarpoformance to benchmark PFA 30% mx ~, , ,1 calculated were very smlar n magntude for cement content, w/c rato and compressve strength whch s to be expected gven the degree of nterdependence. It should, however, be noted that gven the range of cement types tested, a smlar level of carbonaton resstance to the Reference Mx could be acheved by adjustng the cement content by no more than + 60 kg/m 3 or - 30 kg/m 3 from the orgnal mx. A smlar performance could be acheved by adjustng the water/cement rato by 0.11 or or by adjustng the compressve strength by + 8 N/mm 2 or - 7 N/mm 2 from the orgnal mx. Tables 10 and 11 show the adjustments that may be made to the PC content and water/pc rato n order to acheve smlar carbonaton resstance to the PFA reference mx. In the majorty of cases, the PC content requres a slght ncrease, however, MK 10% could potentally gve smlar performance wth a slght reducton n PC content. Ths was also seen n GBS 40% (Exposure Class 1) and PC concrete (Exposure Classes 1 and 2). These trends were seen n recprocal where a reducton n water/pc rato was requred n most of the Table 8 - Senstvty analyss of water/cement rato! for test mxes to acheve equvalent depths of CEN carbonaton as PFA 30% mx at 2 years I Type water / cement rato _Exposure Class 1 Exposure Class 2 Exposure Class 31 At 37N/mm 2 equalto benchmark k* PFA 30% PC GBS 40% , GBS 50% ! GBS 65% MK 10% MK 15% 0, MK 20% CSF 10% CSF 15% equal to k* equal to k* benchmark benchmark * kfaaor to acheve smlarpe~ormance to benchmark PFA 30% mx I 1, o o , , , ,9 Table 9 - Senstvty analyss of standard cube strength for test mxes to acheve equvalent depths of CEN carbonaton as PFA 30% mx at 2 years L Type PFA 30% PC 30 GBS 40% 37 GBS 50% 37 GBS 65% 45 MK 10% 30 MK 15% 40 MK 20% 45 Standard Cube Strength, N/mm 2 Exposure Class 1 Exposure Class 2 Exposure Class 3 equal to benchmark k* o equal to equal to benchmark k* benchmark k* , , I I CSF 10% ,1 CSF 15% * k~ctor to acheve smlarpc~ouana! to benchmark PFA 30O/o mx 400

6 Jones, Newlands, Abbas, Dhr Table 10 - Senstvty analyss of Portland content for test mxes to acheve equvalent depths of CEN carbonaton as PFA 30% mx at 2 years Portland Content, kg/m 3 Exposure Class I I Exposure Class 21 Exposure Class 3 Type At 37 N/mm 2 equalto k* equalto k* equalto k* benchmark benchmark benchmark PC GBS 40% GBS 50% GBS 65% MK 10% MK 15% MK20% CSF 10% CSF 15% k factor to acheve smlarpetformance to benchmark PFA 30% mx Table 12 - Comparson of pre-entry and 2 year water vapour dffusvty wth total Ca(OH)2 n the cover zone of he concrete n CEN Class 1, 2 and 3 storage Typell) MK 10% PC PFA 30% GBS 40% CSF 10% GBS 50% CSF 15% GBS 65% MK 15% MK 20% Water Vapour Dffusvty, g/mm 2 s mm Hgx 10 -lo Pre- After 2 year CEN entry Exposure Class V% (1) Note." Rankng n Class 2 Exposure Total Ca(OH)2, % wt of cover zone concrete(2) Pm- A~er2yearCEN Exposum entry Class1 Class2 Class (2) ~ a d~ ~ lo mm 63 Table 11 - Senstvty analyss of water/portland cement rato for test mxes to acheve equvalent depths of CEN carbonaton as PFA 30% mx at 2 years Exposure Class I Exposure Class 2 Exposure Class 3 Type At 37N/ram 2 equalto benchmark PC GBS 40% GBS 50% GBS 65% MK 10% MK 15% MK20% CSF 10% CSF 15% w/pc rato k* equal to k* equal to k* benchmark benchmark ~N! k factor to acheve smlarperformance to ber~hmark PFA 30% mx mxes n order to acheve smlar performance as the Reference Mx, Table ASSESSING THE POTENTIAL CARBONATION RESISTANCE OF CONCRETE To assess the potental durablty of concrete n a carbonatng envronment a combnaton of both the physcal and chemcal propertes of the concrete must be taken nto account. The senstvty analyss showed that assessng carbonaton performance on a sngle permeaton test alone may not gve the true carbonaton performance of a concrete as the alkalne nature of the pore fluds wll also play an mportant part n resstng the progress of carbonaton. Although there s no permeaton test whch wholly determnes the mcrostructure, mosture accessblty and nterconnectvty of the cover concrete pore system, water vapour dffusvty was found to be the most senstve test method. Due to the dffcultes assocated wth the extracton of pore fluds and determnaton of pore flud alkalnty, the total Ca(OH)2 content of concrete to a depth of 10 mm was measured by means of thermogravmetrc analyss. The 10 mm depth was taken as t was felt that the carbonaton front would not progress beyond 10 mm after 2 years' testng. Table 12 compares the varaton n water vapour dffusvty measured pror to storage n the carbonaton envronment and the total Ca(OH)2 n the cover 10 mm. The cement types are ranked n order from the lowest depth of carbonaton to the hghest n Exposure Class 2; however, t was clear that the GBS mxes were actng dfferently from the pozzolanc cement combnatons. Although the PC mx had a very hgh ntal water vapour dffusvty, the hgh Ca(OH)2 concentraton of the cover prevented rapd progress of the carbonaton front. The Ca(OH)2 content of the cover also depleted slghtly over the test perod; however the concentraton remaned relatvely hgh at 2 years. The MK 10% mx showed the combned effect of a low water vapour dffusvty and hgh ntal Ca(OH)2 concentraton. One would expect ths to outperform the PC mx however wth tme, the Ca(OH)2 concentraton depleted, due to a combnaton of carbonaton and the pozzolanc reacton, leadng to a smlar performance to that of the PC mx. The remanng pozzolanc mxes, PFA, MK 15%, MK 20%, CSF 10%, CSF 15% showed a relatvely smlar performance n carbonaton. The MK and CSF mxes had a low water vapour dffusvty however the Ca(OH)2 concentraton was also very low and depleted wth tme. Subsequently, the carbonaton was relatvely smlar wthn these concretes. The PFA Reference Mx had a moderate water vapour dffusvty and a moderate Ca(OH)2 concentraton however performance was smlar to that of the MK and CSF mxes. The spread of carbonaton depths across these mxes s 2.0 mm at 2 years. Takng account of the senstvty of the measurement of carbonaton depth, water vapour dffusvty and Ca(OH)2 concentraton, these concretes were assumed to have a 401

7 Materals and Structures/Mate.raux et Constructons, Vol. 34, August-September E d 8 ~ s ~ 4 I PFA 30% 9 GBS 50% 4 Q PC Bulk 9 GBS 65% * 9 GBS 40% -I- MK 10% 9 ~ C I Class 1 ass 2... ~<...~.l]ss Water/PCrato MK 15% 9 CSF 15% MK 20% CSF 10% ~ Class I Class 2 Class 3 [ I! I L., I Fg. 3 - Relatonshp between 2 year CEN carbonaton and water/pc rato for Exposure Classes 1, 2 and 3. Arbtrary lmtng values are smlar across all exposure classes. smlar performance n carbonaton. The GBS mxes appear to be actng dfferently to the pozzolanc mxes, possbly due to the dfferent hydraton characterstcs and hydraton products assocated wth GBS concrete. There are obvous dffcultes wth the assessment of water vapour dffusvty and the determnaton of the hydroxyl on concentraton n the concrete pore fluds. It s hypothessed that t may be possble to estmate lkely carbonaton resstance for a partcular concrete mx based on the water/pc rato. Fg. 3 plots the water/pc rato aganst 2 year carbonaton depth and t can be seen that mxes wth a hgher water/pc rato generally have hgher carbonaton depths. Usng the Reference Mx as a benchmark t s possble to provde a lmtng value for water/pc rato whch would result n smlar or lower depths of carbonaton occurrng for any other combnaton of cements for concrete of the same compressve strength. It s recognsed that further work to prove ths hypothess s necessarv but the authors would welcome any data to assst n provng the hypothess or otherwse. 7. CONCLUSIONS Concrete wth dfferent cement types showed the broadest spread of results n Exposure Class 1. Across 10 cement combnatons a spread of 5 nun was found after 2 years' testng. As the frequency of cyclc wettng and dryng ncreased, so the spread of carbonaton depth reduced across cement type. In Exposure Class 3, the 2 year depths of carbonaton were low and very smlar, the spread of results beng 1.5 mm across 10 cement combnatons. It was also found that concrete specmens subjected to contnuous wettng and dryng ncreased n mass over a perod of tme. Specmens subjected to wettng every 7 days showed slghtly larger ncreases n mass compared to those n Exposure Class 2. A 'relatve stablty' analyss showed that the relatve rankng of cement types contnually vared wth tme 1.50 dependng on the rate of wettng and dryng cycles. The rankng of cement combnatons n Exposure Class I vared up to 18 months whereas n Exposure Class 3, the relatve rankng was constant over the 2 year test perod. Although Class 1 may be deemed a worst case scenaro for carbonaton t s unlkely that corroson would occur. However, n Class 3 the concrete may be deemed to be too wet to allow carbonaton thus t s also unlkely to lead to a corroson problem. Further work s needed to make an nformed judgement as to whch exposure class s representatve of the real external envronment; however, results suggest that the level of exposure to mosture may be somewhere between Class 1 (no wettng) and Class 2 (wettng every 28 days). of the 'senstvty' of mx proportons Analyss showed that no sngle factor amongst those most commonly specfed.e. cement content, compressve strength, cement type and water/cement rato had overall control of the carbonaton rate. A further parameter, water/pc rato showed that, wth the excepton of GBS mxes, all cement types could be grouped wthn a specfc concrete grade to gve a tentatve lmtng water/pc rato. For all cement types (wth the excepton of GBS), a maxmum water/pc of 0.65 would gve a smlar carbonaton performance to that of the Reference mx and ths was smlar n all Exposure Classes. To provde a 'satsfactory' carbonaton performance smlar to that of the Reference Mx, GBS mxes allowed a hgher water/pc lmt of Although ths lmt showed a slght varaton n Classes 2 and 3 the lmt stll gave carbonaton depths smlar to the other cement types. The potental carbonaton resstance of concrete was determned to be dependant on both permeaton and alkalnty of the cover concrete. A comparson of water vapour dffusvty and total Ca(OH)2 n the cover concrete showed that concretes wth lower water vapour dffusvty propertes stll exhbted hgher carbonaton depths due to the fact that the cover concrete alkalnty was relatvely low. ACKNOWLEDGEMENTS Ths research was carred out as part of a UK Government's, Department of Envronment, Transport and the Regons, Partners n Technology (PIT) project. The authors would lke to acknowledge the support and advce provded by Ash Resources Ltd, Castle Ltd, tous Slag Makers Assocaton, ECC Internatonal Ltd, PowerGen (PFA Sales) plc and Rugby Ltd. The authors would also lke to acknowledge the techncal advce gven by Professor T. A. Harrson of Quarry Products Assocaton. 402

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