Mechanical properties and corrosion of CAC mortars with carbon fibers

Construction and Building Materials 34 (2012) 91–96

Contents lists available at SciVerse ScienceDirect

Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Mechanical properties and corrosion of CAC mortars with carbon fibers Pedro Garcés a, E. Zornoza a,⇑, E.Ga. Alcocel b, Ó. Galao a, L.Ga. Andión a a b

Dpto. de Ingeniería de la Construcción, Obras Públicas e Infraestructura Urbana, Universidad de Alicante, Alicante, Spain Dpto. de Construcciones Arquitectónicas, Universidad de Alicante, Alicante, Spain

a r t i c l e

i n f o

Article history: Received 2 March 2011 Received in revised form 22 February 2012 Accepted 25 February 2012 Available online 29 March 2012 Keywords: CAC Mortar Carbon fiber Silica fume Mechanical strength Corrosion

a b s t r a c t The changes in mechanical properties of Calcium Aluminate Cement (CAC) mortars due to the addition of carbon fibers (CFs) to the mix have been studied. Compressive and flexural strengths have been determined in relation to the amount of CF added to the mix, water/cement ratio (w/c), curing time and porosity. The relationship between the values of ultrasonic pulse velocity (UPV) and compressive strengths for CAC mortars have also been obtained. Additionally the corrosion level of reinforcing steel bars embedded in CAC mortars containing CF and silica fume (SF) has also been investigated. The proportion of CF and SF in the mix, the w/(c + SF) ratio and the type of aggressive media have been chosen as test parameters. The study demonstrates that the addition of small amounts of CF to the mix increases significantly the mortar strength, but at the same time reduces its resistivity and the steel protection capacity, increasing porosity. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction There is a frequent demand for new special properties of concrete in order to comply with new requirements of concrete structures. The most common way of meeting this demand is using admixtures that modify the characteristics of the concrete and improve some properties according to the requirements of a construction industry in constant change. In most cases the presence of such admixtures not only changes the desired properties but also has ‘‘side effects’’ on other properties of the resultant material. Fiber composites are new materials based on carbon and other fiber types with exceptional physical and chemical properties suitable to be used in technologically advanced products. Among inorganic fibers, glass fibers are the most widely used nowadays, based on production data. However the production of carbon fibers has shown a rapid increase in the last 20 years. This fast growing rate is the result of two factors [1]: (a) The continuous improvement of the materials properties. (b) The cut in production cost, due to improvements of manufacturing process. There is a wide range of potential applications of general-purpose carbon fibers (GPCFs). One of the most important, in terms of the amounts involved, is the use in the construction industry as concrete reinforcement. Non-structural functions include ⇑ Corresponding author. Fax: +34 965903876. E-mail address: [email protected] (E. Zornoza). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2012.02.020

sensing, actuation, heating, corrosion protection, thermal insulation, heat retention and electromagnetic interference (EMI) shielding [2,3]. A category of multifunctional cement based materials is electrically conductive cement-based materials [4,5]. One of the main problems that is usually encountered when carbon fibers are added to cement matrix is to obtain an adequate dispersion of the fibers. Furthermore, the effectiveness of short fibers highly depends on their degree of dispersion in the mix. In this respect, it is known that silica fume acts like a dispersant of fibers, contributing to optimize their distribution in the concrete mass [6,7]. Although the cost of Calcium Aluminate Cement (CAC) is higher than that of Portland Cement (PC), its use could be recommended in situations where early strength and/or resistance to sulfate or high temperature are required [8–10]. In spite of the potential advantages of CF, there is no reference in the specialized technical literature about the mechanical properties of CAC mortars fabricated with CF addition. Also, there is very little information regarding the corrosion behavior of reinforcing steel embedded in CF Portland mortars [11–15]. It is therefore the objective of this paper to present data about the mechanical properties and corrosion kinetics of rebars embedded in CAC mortars fabricated with CF addition. For that purpose the behavior of CAC mortars with different proportions of CF and subject to a broad range of experimental conditions was studied in order to characterize their durability. Mortars were also characterized from the point of view of mechanical properties. Compressive and flexural strengths were obtained for different CAC mortars compositions, and curing conditions.

92

P. Garcés et al. / Construction and Building Materials 34 (2012) 91–96

The relationship between the values of ultrasonic pulse velocity (UPV) and compressive strengths has also been obtained. Measurements of corrosion rates of reinforcing bars embedded in CAC mortars with different proportions of CF and SF were carried out during the experiments.

Table 2 Chemical composition of silica fume.

2. Experimental programme

Table 3 Physical properties of silica fume.

2.1. Material and specimen preparation

SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

Na2O

K2O

P.F.

90.78

0.50

4.13

0.75

0.23

0.02

0.17

0.41

3.97

2.51 g/cm3 24.5 m2/g 150.8 lm

Density BET surface Mean particle size

Table 1 summarizes the kind of test and the testing conditions carried out for this study. Calcium aluminate cement type VI/55 from Electroland, according to Spanish standards [16], was used. Semi-densified silica fume, whose main characteristics are shown in Tables 2 and 3, was used as cement replacement in corrosion test. Silica sand (with a fineness modulus of 3.63) was always employed as the aggregate and distilled water was always used. The water/(cement + silica fume) ratio (w/c + SF) used was 0.5. The sand/(cement + silica fume) ratio was 3/1 for all specimens. The fiber was previously dispersed in the mixing water. Then, a standard mixing method according to EN-UNE 196-1 was used, plus an additional minute at high speed. The main variables for the curing process were the temperature and the time. The CAC mortar specimens were prepared in the laboratory at 20 °C and 80% R.H. However, materials and molds had been previously stored at target temperatures: 5 °C, 20 °C and 60 °C. For strength testing, prismatic mortars of 40  40  160 mm dimensions were prepared using CAC, all with mechanical compaction in two layers. Immediately after casting and surface finishing, the test specimens in the mold were covered with a polyethylene film, and stored at 5 °C, 20 °C and 60 °C. After demolding 24 h later, test specimens were introduced for curing in water, at the same temperatures as those mentioned above. Carbon fibers were added in the following proportions: 0.5%, 1%, and 5% by cement mass. Commercially available milled carbon fibers (13 lm diameter and 130 lm length) were used in this work (GPCF type). For corrosion testing, prisms of 2  6  8 cm size were cast (Fig. 1), embedding two reinforcing steel bars (diameter = 8 mm) and one graphite bar used as electrodes and counter-electrode, respectively. The resulting thickness of the mortar cover for the electrodes was 6 mm. The exposed area of the bars was 15.08 cm2. Previous to the experiment the bars were cleaned in HCl:H2O 1:1 with hexamethylentetramine solutions, abraded with abrasive paper and degreased in acetone. Adhesive tape was used for limiting the active area. The chemical composition of steel is shown in Table 4. The icorr results are always the average of the two bars embedded in the specimen. Two admixtures were used in the preparation of these specimens: carbon fibers (addition of 0.5% and 2% by cement mass) and silica fume (replacement of 10% and 30% by cement mass). After the curing period, some corrosion test specimens were stored in a chamber holding a 100% CO2 atmosphere at 20 °C and a RH ffi 58% [17]. After 14 days of carbonation, the specimens were totally carbonated as was checked by applying the phenolphtaleine method on small prismatic mortar specimens (2  2  8 cm) that were stored in the same carbonation chamber as the corrosion specimens. The rest of specimens were partially submerged in synthetic seawater (3.5% NaCl solution). The CF porous texture was characterized by gas adsorption (i.e. nitrogen at 77 K and carbon dioxide at 273 K). CF presents negligible nitrogen adsorption, corresponding to non-activated CF, but they contain a small micropore volume detected by CO2 adsorption, which is related with micropores smaller than 0.7 nm [18].

Fig. 1. Scheme of specimen for corrosion test.

Table 4 Reinforcing steel composition. % C = 0.30 % P = 0.02

% Si = 0.33 % Cr = 0.00

% Mn = 0.44 % Ni = 0.00

% S = 0.05 % Mo = 0.00

2.2.2. Compressive strength Six specimens, from the former flexural tests, were tested under compression in laboratory ambient conditions. The crushing load was determined using a 200 kN capacity automatic compressive machine.

2.2. Testing The types of specimens and tests for the determination of different properties are described below.

2.2.3. Ultrasonic pulse velocity (UPV) A concrete tester CCT-4 (from C.S.I.) was used for measuring the propagation time of a sonic wave through cement mortar specimens. Two standard cylindrical heads were used for such measurements. The measuring surfaces of the testing heads were pressed against the mortar specimens at two exactly opposite points. The surface of the concrete contact area was flat and duly cleaned, using vaseline to guarantee good contact. The speed of the ultrasonic sound wave (V in m/s) is governed by the following formula: V = 106s/t (V: sonic velocity in m/s; s: distance in meters; t: time in microseconds).

2.2.1. Flexural strength All specimens were tested under laboratory ambient conditions, taken directly from the storage container. Three specimens at each specified age, temperature and CF content were broken in 3 point bending. The maximum breaking load was determined using a 63 kN capacity miniflexural machine.

Table 1 Summary of the experimental programme. Test

Specimen size (prisms, mm)

Curing temperature

Testing age (days)

No. of specimens

Flexural strength Compressive strength Porosity UPV

40  40  160 40  40  80 40  40  20 40  40  160

5, 5, 5, 5,

2, 2, 2, 2,

27 54 54 27

20, 20, 20, 20,

60 °C 60 °C 60 °C 60 °C

28, 28, 28, 28,

90 90 90 90

93

P. Garcés et al. / Construction and Building Materials 34 (2012) 91–96

icorr ¼

B Rp

It was stated by Stern and Roth [21] that when using a mean B value of 26 mV, the maximum icorr error factor is 2. For the case of steel embedded in concrete, a value of 26 mV was found adequate [22] for the active state (corrosion) whereas B = 52 mV is more appropriate for passive steel. In this research a B value of 26 mV was assumed. After the specimen preparation, corrosion rate (icorr) and corrosion potential (Ecorr) were monitored periodically until the end of the test. All corrosion potentials are referred to a saturated calomel electrode (SCE). Corrosion rate measurements were made by using a scanning potentiostat model EG&G 362 from Princeton Applied Research. The potential scan was made from 10 mV to +10 mV respect to the corrosion potential of each steel electrode. The scanning rate was 0.5 mV/s.

3. Results 3.1. Effect of CF admixtures on the mechanical properties and porosity of CAC mortars Compressive strength is the most important property of concrete for designing concrete structures. Usually, this strength is also directly related to the development of other engineering properties such as tensile strength and elastic modulus. Nevertheless, not all cements have the same behavior in certain climatic-temperature conditions. It has been demonstrated some time ago that CAC concrete may lose strength over a period of time due to the process of ‘‘conversion’’ [23] among others. Conversion refers to the phenomenon of the transformation of hexagonal phases (phases that remain stable indefinitely at 5 °C) to cubic phases (which are stable at temperatures above 40 °C). A curing process at 20 °C implies a conversion kinetic slower than the one at 60 °C. Fig. 2 shows compressive strength, Rc, and porosity vs. underwater curing time values for CAC mortars cured at different temperatures with 1% of carbon fibers (CFs). It can be observed that mortars cured at 5 °C, which do not experiment ‘‘conversion’’ present the highest compressive strength values. Additionally, as there is not ‘‘conversion’’, compressive strengths do not decrease after 28 days of curing. In mortars cured at 20 °C is very evident the phenomenon of ‘‘conversion’’ in the strength loss between 28 days of curing and 90 days of curing. Mortars cured at 60 °C present the lowest compressive strengths because their porosity is higher, since only cubic phases are formed during the cement hydration. In this case, as it was expected, the ‘‘conversion’’ phenomenon is not observed because no decrease in compressive strength is appreciated, since only cubic hydrates have been formed. The density of hexagonal hydrates is less than 2 g/cm3, whereas the cubic hydrates have a density above 2.5 g/cm3. Consequently the conversion means a considerable reduction of the hydrate

Rc (MPa)

70

5 ºC 20 ºC 60 ºC

60 50 40 30 20 19

0

20

0

20

40

60

80

100

40

60

80

100

18 17 16 15 14 13

Curing time (days) Fig. 2. Compressive strength, Rc (left), and porosity, P (right), vs. under-water curing time values for CAC mortars cured at different temperatures with 1% of carbon fibers.

volume. This reduction is due to the loss of crystallization water occurred during the hydrate transformation from hexagonal to cubic. As the external dimensions of element paste are practically constant, the conversion process results in a reduction of the paste density and an increase in porosity due to the pores generated when the liberated water evaporates. This means a poorer behavior of the paste as a barrier to the action of external aggressive agents and a higher vulnerability of members to rebar steel corrosion. The relationship between strength of concrete and conversion is complex and is dependent on the temperature and moisture of the concrete and on the water content of the mix [24]. In order to compare the behavior of CAC with CF admixtures the hydration of CAC mortars with various proportions of CF was studied. An evaluation of the compressive and flexural strengths obtained from different curing conditions (5 °C, 20 °C, and 60 °C) is described below. Fig. 3 shows the relationship between compressive strength and porosity vs. time, for CAC mortars with CF. The standard deviation of the results ranged from 1.6 to 2.3 MPa, so the general trend can be described as follows. First, it is observed that the compressive strength generally increases for 0.5% addition and then decreases for higher fiber contents. Also, it is observed that for any curing age, the specimen manufactured with 0.5% CF develops the highest strengths, not presenting, however, the lowest values of porosity. This apparent contradiction could be explained by the fact that different

90

20

0% CF 0.5% CF 1% CF 5% CF

85

19

80 18 75 17

70 65

Porosity (%)

2.2.6. Corrosion measurement techniques The electrochemical technique used to measure the instantaneous corrosion rates was the polarization resistance (Rp) technique, through the well-known Stern–Geary formula [19,20]. The Rp is the slope of the polarization curve around the corrosion potential: Rp = DE/DI when DE ? 0. The Rp value is related to icorr by means of a constant, denominated B by Stern and Roth [21].

Curing Temperature

80

Porosity (%)

2.2.5. Porosity Mortar porosity tests were performed by weighing the specimens in different conditions: dry, saturated in air and saturated while submerged. The percentage taken up by the pores in the specimens volume can be found by the following formula: Porosity (%) = (Wsat  Wdry)/(Wsat  Wsubm) ; with Wsat = weight of the specimens drenched in water (with a vacuum pump) ; Wdry = weight of the specimens dried by heating to 110 °C and later cooled into a chemical desiccator; and Wsubm = weight of the sample in a hydrostatic scale (drenched and submerged).

90

Rc (MPa)

2.2.4. Scanning electron microscope (SEM) The scanning electron microphotographs were taken with a JEOL JSM-840 SEM equipped with an energy dispersive X-ray (EDX) and with 20 kV accelerating voltage. High vacuum evaporation (SCD 004 from Balzers Union) was the method used for producing a thin gold film to make the specimens’ surface electrically conductive.

16

60 15 55 50 0

20

40

60

80

Curing time (days)

100

0

20

40

60

80

14 100

Curing time (days)

Fig. 3. Compressive strength, Rc (left), and porosity, P (right), vs. under-water curing time values for CAC mortars at 20 °C with admixtures of carbon fibers (CFs) in different contents expressed as ratios of cement mass.

P. Garcés et al. / Construction and Building Materials 34 (2012) 91–96

proportions of CF imply developing different intrinsic strengths, because the hydrated components of the paste would have different strength bonds with different proportions of CF, being higher for those of 0.5% CF and lower for those of 5% CF. Another possibility would be that a different degree of fiber dispersion is obtained, i.e., when the amount of fiber is increased, its dispersion is worsened. These results are somewhat different to the ones obtained with Portland cement [12]. Additionally, it should be considered that there exists a positive contribution of fibers as reinforcement and as filler that presents an optimum value for 0.5% of substitution. In addition, if the fiber proportion is high there is a lack of cohesion in the microstructure of the cementitious paste. On the other hand, at the same temperature, after 28 days, the compressive strength values are maximum for all specimens. These maximum values should also be considered as a transitory stage, during which a provisional excess in strength is produced. This effect is due to the process of ‘‘conversion’’ of hexagonal phases to the cubic ones along the curing period of time. To be in the safe side strength values at the end of the process (fully converted) along with other factors should be considered in engineering calculations. The sound propagation velocity is a function of the mineral composition, the intercrystalline connections, the humidity content and, fundamentally, the pore volume of mortar/concrete specimen. Fig. 4 demonstrates the coherence between these general bases and the results. The great similarity of UPV curves and compressive strength curves vs. time suggests the hypothesis that it is possible to use a non-destructive test as an empirical determination of compressive strength for mortar with the same proportions of CF/cement/water, as those used in the test. Fig. 5 shows the influence of CF content and curing time on the flexural strength (Rf) of mortars. For any CF addition, a decrease in the Rf value is observed for any curing time. Due to the short length of the CF (130 lm), the length of anchorage is short. Since the bond between the CF and the CAC matrix is supposed to be not as high as to produce an effective reinforcement of the flexural strength, a higher length would be necessary. Regarding the evolution with curing time, a decrease similar to that observed in Rc values is also registered for Rf, due to the conversion process of the aluminates. 3.2. Corrosion tests 3.2.1. Carbonated samples The evolution of corrosion current density (icorr) of steel bars embedded in CAC mortars fabricated with CF and SF admixtures in different proportions (including control specimens without

10.0 9.5 9.0 8.5

Rf (MPa)

94

8.0 7.5 7.0 6.5 0% CF 0.5% CF 1% CF 5% CF

6.0 5.5 5.0 0

20

40

60

80

Fig. 5. Flexural strength, Rf values, vs. under-water curing time at 20 °C of CAC mortars fabricated with different contents of CF expressed as percentage of cement mass.

additions) was also investigated. In corrosion tests, SF was used to improve the fiber dispersion, since mechanical tests had shown that poor dispersion of high amounts of fiber could be a source of the observed strength loss for CF additions higher than 0.5%. Samples were first subjected to an accelerated carbonation process and then stored in an ambient environment with 100% relative humidity (RH). icorr values were first recorded immediately after sample demolding and then periodically until stabilized values. Due to the rapid carbonation process, which took place in a few days time because of the accelerated conditions, the analysis of the results is based on the corrosion propagation period. Fig. 6 shows the icorr evolution of CAC mortars with CF and SF admixtures subjected to an accelerated process of carbonation and then stored at 100% HR conditions and fabricated with a w/ (c + SF) ratio of 0.5. A log scale has been used for icorr values and the band 0.1–0.2 lA/cm2 above which the steel bars are considered to be in active corrosion state has been marked [20]. At the beginning, icorr values were similar for all samples, under 0.1 lA/cm2, meaning passive state of bars. Then after a few days, all of them presented a sudden increase in icorr values, meaning significant corrosion levels. This sudden increase marks the end of the

0% SF - 0% CF 10% SF - 0.5% CF 10% SF - 2% CF 30% SF - 0.5% CF 30% SF - 2% CF

10 90

100% RH

80

4.42

75

4.40

70

4.38

65

Icorr (µA/cm2)

4.44

UPV (km/s)

85

Rc (MPa)

CARBONATION

0.5% CF 5% CF

100

Curing time (days)

1

0.1

4.36 60 4.34

55

0.01 50 0

20

40

60

80

Curing time (days)

100

0

20

40

60

80

4.32 100

0

20

40

60

80

100

120

140

160

Time (days)

Curing time (days)

Fig. 4. Compressive strength, Rc values (left) and ultrasonic propagation velocity (UPV) values (right) vs. under-water curing time at 20 °C of CAC mortars fabricated with different contents of CF expressed as percentage of cement mass.

Fig. 6. Evolution of corrosion current density (icorr) vs. time in CAC reinforced mortars with carbon fibers (CFs) and SF additions in contents expressed as percentages to the cement mass, subject to an CO2 saturated atmosphere at 20 °C for 90 days. w/(c + SF) ratio 0.5.

95

P. Garcés et al. / Construction and Building Materials 34 (2012) 91–96

3.2.2. Samples immersed in seawater The evolution of corrosion current density for CAC samples fabricated with SF and CF admixtures and submerged in seawater was also studied. Fig. 7 shows the results for samples with 0.5 w/(c + SF) ratio. The icorr value trend shows that they were first growing and then reached different stationary levels which allow meaningful comparisons. In this case higher CF content means higher corrosion rates which are reduced using higher SF contents, although icorr values are always higher than the ones of control samples.

Table 5 Porosity of mortars used in corrosion tests after 28 days of curing time in humid chamber. Mortar

Porosity (%)

0% CF–0% SF 0.5 % CF–10% SF 2% CF–10% SF 0.5% CF–30% SF 2% CF–30% SF

15.2 16.3 16.7 15.6 15.9

0% SF - 0% CF 10% SF - 0.5% CF 10% SF - 2% CF 30% SF - 0.5% CF 30% SF - 2% CF

10

Icorr (µA/cm2)

initiation stage and, subsequently, the beginning of the corrosion propagation stage. After 57 days (carbonation period), the corrosion levels of all samples reached a stationary state. The highest icorr value (1.22 lA/cm2) is for a sample with 30% SF and 2% CF admixtures. The lowest icorr value (0.48 lA/cm2) is for samples with no admixtures (control samples). When samples were stored in 100% RH ambient, icorr values showed again variations but reached stationary values after 100–120 days. In this second phase it can be observed that the increase of CF admixture implies an increase in registered icorr values. For example, samples with 10% SF and CF ratios of 0.5% and 2.0% show icorr values of 0.43 lA/cm2 and 1.46 lA/cm2, respectively. This increase of icorr values is attributed to the higher porosity of mortars for higher fiber contents (Table 5), meaning a higher permeability to oxygen and the external aggressive agents such as carbon dioxide, which provokes steel depassivation. In that case, a higher extension of steel surface would be depassivated (and more rapidly) and a higher oxygen concentration would be expected at the steel surface. Assuming these two considerations, the cathodic reaction would have been enhanced (because of the higher oxygen concentration at the steel surface); and the electrolytic resistance between cathodic and anodic areas would have been lowered (because of the higher porosity of the mortar cover). Therefore, the increase in the CF content will result in an increase in the corrosion level in the propagaton period. In general, it can be drawn from results that the increase of CF ratio means an increase in corrosion current density, as can be observed by comparison of 0.5% CF samples with 2% CF samples both with the same SF ratio. This increase is due to the drop of mortar resistivity provided by the presence of carbon fibers and the increase in sample porosity. When samples with different SF ratios and the same CF ratio are compared it can be observed that the increase of SF ratio means lower corrosion current density values, due to the lower mortar porosity and a better protection of embedded steel. The silica fume, due to the small size of its particles, can fill in the voids between cement grains and between CF particles and cement, improving this way the interfacial bond of fibers and cement paste and making a more compact and denser mix. Nevertheless the addition of SF does not completely counteract the effect of CF admixture because in all cases icorr values are higher than the ones of control samples.

1

0.1

0.01 0

20

40

60

80

100

120

140

160

180

Time (days) Fig. 7. Evolution of corrosion current density (icorr) vs. time in CAC reinforced mortars with carbon fibers (CFs) and SF additions in contents expressed as percentages to the cement mass, submerged in seawater. w/(c + SF) ratio 0.5.

As in the case of carbonated sample, once the steel is depassivated the corrosion rate is governed by the resistivity of the cementitious paste and the porosity of the mortar cover. Therefore the higher the CF content, the higher is the paste conductivity and the higher is the porosity of the mortar cover, so the icorr stationary level observed is higher. This observation is consistent with mortar porosity values shown in Table 5. Once again, higher porosity would provoke higher chloride concentration at the steel surface (although this fact was not measured). Therefore, a higher chloride concentration at the surface will develop a higher extent of corrosion by means of increasing the anodic areas. Besides, the higher permeability which comes from the higher porosity of mortars with high levels of CF, would also produce higher oxygen diffusion rates, thus favouring oxygen reduction process. These phenomenons are added to the fact that ions produced in the oxidation and reduction reactions migrate more easily between cathodic and anodic areas when the porosity is high. As a result, when the CF fiber content is increased, the porosity of the mortar is increased and the corrosion process occurs at higher rates. On the other hand, for a certain amount of CF, the addition of silica fume reduces the porosity of the mortar and this is translated into lower corrosion rates in the propagation period. In principle three aspects need to be considered for discussing the corrosion behavior of embedded steel bars. First, as it is well known, the initiation of the corrosion of steel embedded in concrete exposed to seawater is linked with the arrival of sufficient chlorides at the rebar surface. Chlorides lead to a local breakdown of the protective oxide film on the reinforcement in alkaline concrete, so a subsequent localized corrosion attack takes place. Second, increasing the content of conductive materials (CFs), in concrete should imply the reduction of concrete electrical resistivity. This fact should contribute to activate the corrosion kinetics and the increase of measured icorr values. Third, on the other hand, the galvanic couple between steel and added conductors such as CF has to be considered. The contact between two conductors of different nobility implies that the nobler material will develop lower corrosion levels as compared to the case in which this material is the only conductor in the same media. In our case the nobler material is carbon and the less noble steel. Therefore an increase in the CF content ratio should contribute to increase the measured icorr values. Finally the filler effect, that is the void filling in the cement paste by the addition of particles, should provide a denser, less permeable material. The porosity tests deny that assumption in the

96

P. Garcés et al. / Construction and Building Materials 34 (2012) 91–96

case of CF but it is confirmed for SF: an increase of specimen porosity is registered when the carbon material content ratio increases, and a decrease in the case of the addition of SF. Concerning to the contribution of chlorides in the level of corrosion registered, it should be considered that the higher is the porosity of the mortar, the higher is the amount of chlorides reaching the steel surface [25–27]. Since an increasing addition of CF implies higher porosity, its effect would be a higher corrosion rate. This fact along with the contribution of the other three effects above commented, should imply an increase of the corrosion level when the content ratio of CF addition is increased. In other words, an increase of the specimen conductivity, an increase of the carbon-steel galvanic couple and an increase of the specimen porosity and consequently an increase of chlorides reaching the steel surface, should lead to an increase of the mean corrosion level. 4. Conclusions The following conclusions can be stated based on the valuation of the experimental data: 1. Carbon fiber (CF) admixtures in calcium aluminate cement mortars provide an optimum value of compressive strength for 0.5% of cement substitution by CF. 2. The increase of CF content ratio from 0.5% to 5%, with respect to cement mass does not provide a significant increase in mechanical strengths. This small CF ratio of 0.5% will also produce an increase in mortar porosity. 3. The presence of CF increases the corrosion rate of steel embedded in CAC mortars with SF. If the quantity of CF is increased, the corrosion rate increases, but higher amounts of SF reduce the corrosion rate for a given proportion of CF. 4. The great similarity of UPV vs. time with compressive strength vs. time, allows establishing the hypothesis that it is possible to use a non-destructive test as an empirical determination of compressive strength for CAC mortar with addition of CF. 5. The higher the CF content, the higher is the paste conductivity and the higher is the mortar cover porosity, so the higher is the icorr long term level. 6. The addition of SF does not fully counteract the negative effect (from the steel corrosion point of view) of the presence of carbon fibers in the mix, but it helps to reduce icorr values.

Acknowledgments Authors would like to acknowledge financial support received from the Generalitat Valenciana (Spain) (CTIDIB/2002/164) from Ministerio de Ciencia y Tecnologia (MAT 2003-06863) and from Ministerio de Fomento (Spain). E. Zornoza would also like to thank to ‘‘Juan de la Cierva’’ Programme (Ministerio de Ciencia e Innovación of Spain) for the given support.

References [1] Chung DDL. Cement reinforced with short carbon fibers: a multifunctional material. Compos B Eng 2000;31:511–26. [2] Chung DDL. Functional properties of cement–matrix composites. J Mater Sci 2000;36:1315–24. [3] Chung DDL. Cement–matrix composites for thermal engineering. Appl Therm Eng 2001;21(ER16):1607–19. [4] Chen W, Chung DDL. Improving the electrical conductivity of composites comprised of short conducting fibers in a non-conducting matrix: the addition of a non-conducting particulate filler. J Electron Mater 1995;24(1):47–51. [5] Chung DDL. Electrical conduction behavior of cement–matrix composites. J Mater Eng Perf 2002;11(2):194–204. [6] Chen P-W, Fu X, Chung DDL. Microstructural and mechanical effects of latex, methylcellulose, and silica fume on carbon fiber reinforced cement. ACI Mater J 1997;94(2):147–55. [7] Chen P-W, Chung DDL. Concrete reinforced with up to 0.2 vol% of short carbon fibres. Composites 1993;24(1):33–52. [8] Robson TD. High alumina cements and concretes. London: Contractors Record Ltd.; 1962. [9] Sanjuan MA. Effect of curing temperature on corrosion of steel bars embedded in calcium aluminate mortars exposed to chloride solutions. Corros Sci 1999;41(2):335–50. [10] Sanjuan MA. Formation of chloroaluminates in calcium aluminate cements cured at high temperatures and exposed to chloride solutions. J Mater Sci 1997;32(23):6207–13. [11] Hou J, Chung DDL. Effect of admixture in concrete on the corrosion resistance of steel reinforced concrete. Corros Sci 2000;42:1489–507. [12] Garcés P, Fraile J, Vilaplana-Ortego E, Cazorla D, Andión LGa. Effect of carbon fibers on the mechanical properties and corrosion levels of reinforced Portland cement mortars. Cem Concr Res 2005;35:324–31. [13] Garcés P, Andión LGa, Varga I, Catalá G, Zornoza E. Corrosion of steel reinforcement in structural concrete with carbon material addition. Corros Sci 2007;49:2557–66. [14] Alcaide JS, Alcocel EGa, Vilaplana E, Garces P. Mechanical characterization of Portland cement mortars containing petroleum or coal tar. Mater de Constr 2007;57(287):53–62. [15] Alcaide JS, Alcocel EGa, Puertas F, Lapuente R, Garces P. Carbon–fibre reinforced, alkali-activated slag mortars. Mater de Constr 2007;57(288):33–48. [16] UNE-EN 197-1. Cemento. Parte 1: Composición, especificaciones y criterios de conformidad de los cementos comunes; 2000. [17] ASTM E-104-71. Maintaining constant relative humidity by means of aqueous solutions; 1998. [18] Cazorla D, Alcañiz J, De la Casa MA, Linares A. CO2 as an adsorptive to characterize carbon molecular sieves and activated carbons. Langmuir 1998;14(16):4589–96. [19] Stern M, Geary AL. A theoretical analysis of the shape of polarization curves. J Elect Soc 1957;104(1):56. [20] Andrade C, González JA. Techniques electrochimiques qualitatives et quantitatives pour mésurer les effects des additions sur la corrosion des armatures. Silic Ind 1982;47:289–95. [21] Stern M, Roth RM. Anodic behaviour of iron in acid solution. J Electrochem Soc 1957;104(6):390–2. [22] Andrade C, Alonso C. Corrosion rate monitoring in the laboratory and on-site. Constr Build Mater 1996;10(5):315–28. [23] George CM. Industrial aluminous cements. In: Barnes P, editor. Structure and Performance of Cements. London: Applied Science Publishers; 1983. [24] Garcés P, Chinchón S, Alcocel EGa, Andreu CGa, Alcaide J. Effect of curing temperature in some hydration characteristics of calcium aluminate cement in comparison with Portland cement. Cem Concr Res 1997;27(9):1343–55. [25] Zornoza E, Paya J, Garces P. Chloride-induced corrosion of steel embedded in mortars containing fly ash and spent cracking catalyst. Corros Sci 2008;50(6):1567–75. [26] Mejia R, Delvast S, Gutierrez C, Talero R. Chloride diffusion measured by a modified permeability test in normal and blended cements. Adv Cem Res 2003;15(3):113–8. [27] Sanchez I, Novoa XR, de Vera G, Climent MA. Microstructural modifications in Portland cement concrete due to forced ionic migration tests. Study by impedance spectroscopy. Cem Concr Res 2008;37(7):1015–25.