Effect of Curing on Durability of Fly Ash Concrete

Transcription

1 TRANSPORTATION RSARCH RCORD ffet of Curing on Durability of Fly Ash Conrete M..A. THOMAS AND J. D. MATTHWS Findings of a 5-year laboratory investigation on the effet of uring on the durability of fly ash onrete are summarized. Three series of onrete mixes were ast suh that onretes within a given series were of similar strength grade (nominally 25, 35, or 45 MPa) but varied in fly ash replaement level ( to 5 perent). Following asting, onrete speimens were subjeted to various uring treatments and subsequently were stored at a range of temperatures and relative humidities before testing. To assess the influene of the various uring and storage regimes on onrete performane, onrete speimens were tested for (a) ompressive strength, (b) oxygen permeability, () arbonation in both internal and external storage, and (d) resistane to hloride ingress and steel orrosion. Results indiated that, whereas all onretes required adequate uring to ahieve their potential properties, onretes ontaining fly ash were more sensitive to poor uring. The differene was espeially marked at higher levels of replaement (5 perent fly ash), where urtailing the moist-uring period resulted in a large inrease in the rate of arbonation and permeability and a marked redution in ompressive strength. Conretes with lower levels of ash (15 to 3 perent) arbonated at slightly higher rates than the ontrol onretes but generally offered lower permeability. Resistane to the penetration of hloride ions inreased as fly ash ontent inreased, irrespetive of the level of uring applied to the onrete. Poorly ured, low strength grade (C25) onretes with 5 perent fly ash provided greater resistane to hloride than higher grade (C45) onretes without ash. Conretes with moderate levels of fly ash (15 to 3 perent) require no additional uring to provide equal, if not improved, durability ompared with ope onrete of the same strength grade. Conretes with higher levels of ash may require extended uring ompared with ope onrete, espeially if the design of the struture and onditions are onduive to arbonation. Adequately ured, high fly ash ontent onretes are likely to offer substantially improved durability beause of their low permeability and high resistane to hloride ion ingress ompared with ope onrete. It is well established that the use of fly ash in onrete an onsiderably improve onrete durability. As with all onrete, adequate uring is essential for fly ash onrete if its potential properties are to be realized. However, beause long-term benefits assoiated with a pozzolani reation are more evident in well-ured onrete, it generally has been thought that fly ash onrete has a greater suseptibility to poor uring than plain portland ement onrete. Consequently, there is some onern about the durability of fly ash onrete under onditions of uring more losely representative of the range of treatments onrete typially reeives on site. Treatment may vary from early stripping of formwork (at low temperatures) with no further uring to keeping onrete overed or spraying with water or uring ompounds for longer periods. This paper reports results from an ongoing laboratory investigation of the effet of uring on the durability of fly ash onrete. Conretes with a range of fly ash levels were subjeted to a wide range M. Thomas, Department of Civil ngineering, University of Toronto, 35 St. George St., Toronto, Ontario, M5S 1A4, Canada. J. Matthews, Building Researh stablishment, Garston, Herts, United Kingdom, WD4 7JR. of uring and subsequent storage onditions before strength and durability testing. Durability testing inluded (a) arbonation, (b) permeability, and ( ) hloride diffusion and orrosion of reinfored onretes in a marine environment (1). The results are summarized and demonstrate the effet of uring on the durability of fly ash onrete exposed to different environments. XPRIMNTAL Three series of onrete mixes were designed using a range of fly ash levels. The mixes' harateristi design strengths were 25 MPa, 35 MPa, and 45 MPa (designated as C25, C35, and C45, respetively) and their slumps ranged from 3 to 6 mm. Mix proportions are presented in Table 1. Cement used for this study was omplied with BS 12 for ordinary portland ement (2). Three soures of fly ash were seleted to provide ashes with a range of 45 mirons sieve retention values (5 to 2 perent retained by mass) and otherwise omplied with BS 3892: Part 1 (3). The hemial ompositions of the ement and fly ashes are given in Table 2. Following asting, speimens were ured for 24 hr in moulds under damp saking and polyethylene, either in the laboratory at 2 C or in an environmental abinet at 5 C. After 24 hr, all the speimens were demoulded, kept at the same temperature, and subjeted to one of the following uring treatments: 1-day ure: air-stored immediately after demoulding, 3-day ure: moist-ured under damp saking and polyethylene for an additional 2 days before air-storag_e, 7-day ure: moist-ured under damp saking and polyethylene for an additional 6 days before air-storage, and water-ured: immersed in water for 28 days. After uring, speimens were stored in air at various temperatures (5 C or 2 C) and relative humidities ( 4 to 9 perent relative humidity for 28 days. The following air-storage onditions were used: Mix Series A,G,andH B D F Temperature ( C) Relative Humidity (perent) At 28 days, the following series of test was arried out: Strength: ompressive strength testing of 1 mm ubes. Carbonation: onrete prisms, 75 X 75 X 2 mm, stored in. the laboratory (2 C and 65 perent RH) or outside (sheltered from

2 1 TRANSPORTATION RSARCH RCORD 1458 TABL 1 Details of Conrete Mixes Mix roortions (km 1 ) 28-day Mix Fly ash Fly Total Thames Valley aggregate Slump Density strength series ontent OPC ash water < 5 mm 5-1.mm 1-2 mm Free w/ (mm) CF (kg/m3 (MPa) A-F (C35) 15%1 Pl % Pl % Pl % P2 As mix A % P3 As mix A G (C25) 15%PI % Pl % Pl % P2 As mix G % P3 As mix G H !C45l 15% Pl % Pl % Pl % P2 As mix H % P3 As mix H TABL2 Chemial and Physial Properties of OPC and Fly Ashes Oxide OPC Si Al Fe Cao MgO 1.53 K2.73 Na2.15 Ti2.26 P2s.19 Mn2 3 <.1 Bao.1 SrO.18 S Cr23 n.d. LOI 1.58 Total 1.39 Free lime.96 Total C (inluded in LOI) 45 µm sieve residue Relative density Bogue omposition C 3 S 57 C2S 16 C 3 A 8 C AF 9 Fly Ash P1 P2 P diret sunlight and rainfall). Depth of arbonation determined by spraying freshly fratured surfaes with phenolphthalein indiator at 9 days, 1, 2, and 4 years ( 4). Permeability: oxygen permeability tests on diss 15 mm in diameter and 5 mm thik ut from 15 X 3 mm ylinders. Speimens onditioned at 2 C and 65 perent RH for 28 days before testing (5). Corrosion: reinfored onrete prisms, 1 X 1 X 3 mm, plaed in tidal zone of marine exposure site (J). Composition of seawater: 2.6 g/l S 4, 18.2 g/l Cl,.4 g/l Ca, 1.2 g/l Mg, 9.74 g/l Na, and.4 g/l K. Speimens removed at 1, 2, and 4 years. Chloride onentration profile determined by inremental drilling and hemial analysis of olleted powder samples. Corrosion of reinforement determined gravimetrially.

3 TABL 3 Summary of Results Ambient Conrete Fly ash 28 day Strength 9 day Strength Indoor Carbonation Outdoor arbonation Oxygen Marine xposure Conditions Grade Content ("' 28 day water- (% 28 day water- 4 years 4 years Pemeability (to 28 days) ured strength) ured strength) (mm) (mm) (x 1-11m2) * C** (x1 8 (wt.% m 2 /s) ement) Temp R.H. days uring day uring days uring days uring days uring days uring ("') C25 none n > C35 none C45 none n n C35 none N.D n C35 none N.D C35 none N.D N.D C35 none N.D N.D N.D. - not determined I * - hloride diffusion oeffiient alulated from hloride onentration profile after 2 years exposure; u - hloride ontent at depth mm after 2 years exposure.

4 12 TRANSPORTATION RSARCH RCORD 1458 RSULTS Results for tests for three types of onrete are summarized in Table 3: (a) no fly ash (ontrol mixes), (b) normal fly ash replaement levels (average results for onretes with 15 to 3 perent fly ash), and () high fly ash ontent (5 perent). Strength The strength of all onretes dereased as their uring period was urtailed (Table 3); however, the strength of fly ash onrete was more adversely affeted by inadequate uring,' espeially at higher levels of fly ash (5 perent and for those onretes exposed to low temperatures or low relative humidities after uring. Other researhers have observed similar trends for fly ash onrete and mortars (6-9). Conretes with traditional levels of fly ash (15 to 3 perent) generally exhibit omparable strength loss to ope onretes, provided the onretes are ured for at least 3 days. Fly ash onretes ured for only 1 day before exposure to low relative humidities (or low temperatures) lose more strength than similarly treated ope onretes. Under favorable exposure onditions (2 C and 8 or 9 perent relative humidity), fly ash onrete requires no additional uring to ahieve strength omparable to ope onrete, even at high levels of replaement. In pratie, only the surfae of large setions of onrete will lose moisture at rates similar to those of small speimens used in this study. The internal bulk of the onrete will retain its moisture for a muh longer period. Consequently, the strength of large setions are less affeted by the quality of uring. In addition, the temperature rise aused by the heat of hydration plays an important part in determining the in situ strength of large onrete setions. xept in slender setions, the effet of a temperature rise is likely to overshadow the effet of low ambient temperatures, whih retard fly ash onrete strength development more severely than ope onrete. Therefore, additional uring required by fly ash onretes in order to ahieve similar strength as ope onretes may be neessary only in slender setions. Permeability The permeability of onrete is an important parameter in determining its durability, beause it provides a measure of the onrete's physial resistane to the ingress of deleterious agents (arbon dioxide, hlorides, and sulphates) and the movement of oxygen and water, whih is required for orrosion. Although there is no sfogle test for assessing the durability of onrete exposed to different onditions, permeability is generally onsidered the best parameter for haraterizing the ability of the onrete to resist deterioration (1). The oxygen permeability results in Table 3 indiate a very marked dependene on the duration of uring. Inreasing the period 1 Strength grade: N ';"'O """ T"" (I) (I) a. : (I) O> Curing period: 1 day 3days 7days I Strength at 9 days (MPa) FIGUR 1 ffet of uring on the strength and permeability of ope onrete.

5 Thomas and Matthews 13 of uring from 1 to 7 days redued permeability by a fator ranging from 2 (for onrete exposed to high temperature and relative humidity after uring) to more than 9 (for onrete exposed to low temperature and relative humidity). Generally, fly ash onretes exhibit greater dependene on uring; onretes with higher levels of fly ash had greater redutions in permeability after extended uring. However, fly ash onretes generally were less permeable than ope onrete of the same strength grade, irrespetive of the level of uring applied. Similar findings have been reported by others testing gas permeability (J J), water sorptivity (J J), and initial surfae absorption (12). In the present study, low strength onretes (C25) with 5 perent fly ash were less permeable than higher strength onretes (C45) without ash. The only ondition that produes a higher permeability in fly ash onrete than in similar grade ope onrete is inadequate uring, (1 day at 5 C) before exposure to low temperature (5 C) and relative humidity (65 perent relative humidity). If the uring period is extended to 3 days the fly ash onrete is less permeable. The behavior of the onretes at 5 C is interesting. Providing adequate uring was given (3 to 7 days), onretes stored at 5 C are less permeable than omparable speimens stored at 2 C. Although the hydration of portland ement is retarded at low temperature, after suffiient uring periods the quality of onrete is inreased beause of the improved dispersion of hydration produts. Figure 1 shows the effet of the uring period on both the strength and the permeability of ope onrete. Inreasing the initial uring period has a muh more pronouned effet on the onrete's permeability than its strength. Ballim (J J) suggested that inreasing the duration of moist uring may be a more effiient way of extending the durability of onrete than inreasing the ement ontent (and design strength). There is no unique relationship between strength and permeability (and hene durability). The relationship is dependent on the level of fly ash replaement, the degree of uring, and the exposure onditions following uring. Consequently, neither design nor in situ strength an be expeted to provide a reliable indiation of onrete durability. Similar onlusions have been reahed by others (J 3, 14). Protetion of Steel Reinf oreinent Durability of reinfored onrete is ontrolled largely by the apability of the onrete over to protet steel reinforement from orrosion. In hardened onrete, the ph of the pore solution typially is in exess of 13, and at suh levels of alkalinity, steel is normally proteted from orrosion by a thin surfae layer of oxide. However, the steel may beome suseptible to orrosion in the presene of hlorides, or if it beomes depassified when the alkalinity of the onrete at the loation of the steel is redued as a result of arbonation. Consequently, it is desirable for the onrete over to adequately resist penetration of hlorides and arbon dioxide. Carbonation Figure 2 indiates the effet of the uring period and storage onditions immediately following uring, on arbonation. Results' ompiled in Figure 2 and Table 3 indiate that uring has a pronouned g (/) 16 (I) >. 14 :6 Q. 12 (I) "U 1 ;g () 'o = Internal storage (2C & 65%AH) --, = %fly ash o 15% P1 t,. 'V Speimens waterured for 28 days 3% P1 5% P1 1 day ure 7 days ure Relative humidity during early storage (%) FIGUR 2 ffet of relative humidity during uring on arbonation of onrete.

6 14 TRANSPORTATION RSARCH RCORD 1458 effet on the arbonation of onrete. Fly ash onretes arbonate at a faster rate than ope onretes _of the same strength grade; the effet is more marked at higher replaement levels and under poor uring onditions. For onretes with a moderate level of fly ash (15 to 3 perent) the differenes are small, generally less than 2 mm after 4 years in most ases, ompared with the differenes resulting from hanges in uring or ambient onditions. The results support the view of other researhers (15-22) that onretes ontaining fly ash arbonate to a similar or slightly higher degree ompared with ope onrete of the same strength grade, even when the onretes are poorly ured. At higher levels of fly ash, differenes in arbonation rate are more signifiant, espeially for poorly ured lower grade onretes. Inreases of more than 12 mm were observed for 25N onretes with 5 perent fly ash ompared with equal grade ope onrete stored internally for 4 years. Figure 3 illustrates the relationship between the arbonation depth of a onrete speimen at 2 years and the atual ompressive strength at 28 days of identially ured ompanion speimens from the same mix. There is a single relationship between these two properties for eah exposure environment for all the onrete mixes tested, and the relationships are independent of the level of fly ash and the onditions of uring and early storage. Carbonation results appear to be in onflit with the permeability results; fly ash onrete arbonates at an inreased rate despite being less permeable ompared with ope onrete of similar strength (Figure 4). This may be explained by the proess of arbonation, whih ours mainly by gaseous diffusion of C 2, aompanied by hemial interation between the C 2 and the ement hydrates, partiularly Ca(OH) 2 The redued Ca(OH) 2 ontent of fly ash onrete means that there is less C 2 onsumed; onsequently, C 2 may diffuse more quikly ompared with an ope onrete of similar permeability. However, as there is little hemial interation between oxygen and ement hydrates, permeability to oxygen is a funtion of the pore struture. Chloride ion penetration Table 3 gives hloride diffusion oeffiients for onretes ured for 1 day. The oeffiients were alulated from hloride onentration profiles after 2-years' exposure, using Crank's solution to Fik's seond law (1). The duration of initial uring was observed to have little effet on hloride penetration (1), and diffusion oeffiients alulated from onentration profiles indiated little variation with duration of uring. Those results are in ontrast to the permeability and arbonation test results, whih show a marked dependene on the initial uring period. The marine-exposed onretes remained in a saturated ondition for the duration of the test, and the ontinued ement hydration and pozzolani reation tended to mask the initial differenes between speimens beause of initial uring onditions. The redued impat of uring on diffusion results ompared with other permeation properties has been observed by others (23,24) g (/) >. 2 :6.. "O : 15 Internal storage (2C & 65o/oRH) 16 : <> 1 (..) o day strength (M Pa) «> FIGUR 3 Relationship between 28-day strength and arbonation for all onrete mixes.

7 Thomas and Matthews 15 3 g :6 16 a. 'O 14 : : 1 (.) %P1 / /i / / o o ""o _.,, """" - -- / /> / / / / I I Oxygen permeability (m2) FIGUR 4 Relationship between arbonation and permeability. In addition to hloride diffusion oeffiients, the hloride onentration at depth (26 to 31 mm) is also given in Table 3. The results indiate that the fly ash onrete has a muh greater resistane to the penetration of hloride ions than does ope onrete, espeially at high replaement levels. Indeed, low grade onretes (C25) with moderate or high levels of fly ash perform better in this environment than (C45) ope onretes. Figure 5 shows the effet of strength grade and fly ash ontent on hloride penetration. Inreasing the strength or fly ash ontent redues hloride penetration. However, inreasing the level of fly ash is far more effetive; low strength (C25) onretes offer greater resistane than higher strength (C45) ope onrete. Figure 6 shows the hange in hloride onentration with time. Differenes between fly ash and ope onrete beome more marked with time; hloride levels at all depths inrease with exposure for ope onretes. The inrease in hloride ontent at a given depth between 1 and 4 years is of the order of 2 times for ope onrete [i.e., the hloride ontent inreases proportionally with the square root of time onsistent with Fikean law (25)]. In ontrast, there is little ingress of hlorides beyond the first year for onretes with high levels of fly ash. A hloride ontent below 26 mm indiates no signifiant inrease between 1and4 years. This finding does not onform with Fikean diffusion, and it has been suggested that the initial profile may be the result of the absorption of hlorides beause of the unsaturated ondition of the onretes before exposure (1). Consequently, the diffusion oeffiients for fly ash onretes atually may be lower than those reported in Table 3. Inreased resistane to the penetration of hlorides in fly ash onrete led to substantially redued orrosion of the reinforing steel in marine exposed speimens (1). Redued hloride diffusion of fly ash onretes observed in the present study is onsistent with previous studies that have indiated lower diffusion rates for pastes (26-29), mortars (3), and onretes (3-34). PRACTICAL SIGNIFICANC OF RSULTS Curing has a signifiant influene on the strength and durability of laboratory-size onrete speimens. In pratie, only the outer surfaes of larger elements will be affeted by early uring, the internal bulk of the onrete will retain its moisture for longer periods (35). Consequently, the overall strength of a onrete element may not be adversely affeted, and the greater suseptibility of fly ash onrete strength is of pratial signifiane in thin setions exposed to adverse uring environments. Reent studies by the authors (36) of fly ash and ope onretes in the field have shown fly ash onrete to invariably have a greater in situ strength than omparable ope onrete (equal design strength or ementitious material ontent) taken from the same struture. Furthermore, studies of high-volume fly ash onrete strutures (up to 56 perent ash), examples of whih have existed in the United Kingdom for more than a deade, show these onretes to have strengths onsiderably in exess of the d.esign strength or measured 28-day strength (37).

8 5 5 5 % Fly Ash I 3% P1 I I 5% P1 I :;::- : () eie "O g :: (.) +C35 C45 o......, o......, o _ Average sample depth (mm) FIGUR 5 Chloride onentration profiles for ope and fly ash onretes after 4 years marine exposure C45 - % Fly Ash C45-3% P1 C45-5% P1 :;::- : () eie i () "O ' 1 :: (.) o---..i.--'-- o.... o Average sample depth (mm) FIGUR 6 ffet of age and fly ash ontent on hloride onentration profiles in onrete.

9 Thomas and Matthews 17 Properties of the outer surfaes or "overrete" of onrete elements have a major influene on the durability of the struture, as suh surf aes provide protetion to the steel reinforement and bulk onrete. The results of this study indiate that fly ash onrete may be less permeable and onsiderably more resistant to the penetration of hloride ions than ope onrete of the same strength grade, irrespetive of the level of uring provided. Differenes in arbonation depths were not marked for onretes with fly ash ontents between and 3 perent. The results orrelate well with field studies (36), whih show fly ash onrete to be_ less permeable and more resistant to the penetration of hlorides from deiing salts or seawater than is omparable ope onrete with the same exposure. Fly ash onretes in the field exhibit slightly higher arbonation also. Overall, the results indiate that onretes with normal levels of fly ash (15 to 3 perent require no additional uring, as ompared with ope onretes of the same strength grade. High fly ash ontent onretes (5 perent) show exellent durability properties, for inreased arbonation rates. The differenes an be signifiant under ertain uring and exposure onditions, so partiular attention should be paid to the uring fly ash onretes if the design of the struture and onditions of exposure are onduive to arbonation. All the onretes studied had improved durability when the uring period was extended. The importane of providing adequate uring protetion annot be overemphasized. Although, inadequate uring may be ompensated for by dereasing the water to ementitious material ratio of the onrete, this will usually result in higher ement ontents and onomitant inreases in shrinkage, hydration temperature, alkali ontents, and material osts. 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