DC current-induced curing and ageing phenomena in cement-based materials

Transcription

1 DC current-induced curing and ageing phenomena in cement-based materials A. Susanto*, D.A. Koleva and K. van Breugel Faculty of Civil Engineering and Geosciences, Section of Materials and Environment, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands Abstract: This paper investigates DC current-induced curing and ageing phenomena in cement-based materials. Two current densities were used in a DC current regime i.e. mortar cubes were subjected to DC current flow of 1 A/m 2 and 100 ma/m 2 ; tap water and calcium hydroxide were external environment. Conditioning ( curing ) was performed for 112 days, during which period, compressive strength, porosity and electrical resistivity of the mortar specimens were monitored. Based on the experimental results, DC curing in the tap water tends to accelerate degradation processes in the mortar specimens i.e. results in increased ageing. Meanwhile, DC curing in calcium hydroxide solution has the potential to accelerate curing without negative side effects. Keywords: DC curing, ageing, water, calcium hydroxide, cement-based materials 1 Introduction Ageing can be referred to as the time-sequential deterioration that occurs in all biological systems, materials and structures. In cement-based materials, ageing is understood to include reduction in mechanical properties (i.e. compressive strength, tensile strength, elastic modulus) and increased permeability. The ageing process in cement-based materials is not only affected by internal factors (i.e. degree of cement hydration and relevant kinetics) but also by external factors that contribute to the degradation process such as chemical attack, temperature/fire, aggressive environment. All these factors have the potential to accelerate ageing/degradation processes of cement-based materials. For instance, when cement-based materials are exposed to high temperature (>200 C), mechanical damage and chemical transformation occur simultaneously, which will change the materials microstructure. Chemical transformation can be attributed to dehydration of calcium silicate hydrate (CSH) and calcium hydroxide (Ca(OH) 2) that cause micro-cracking and irreversible changes in the microstructure [1]. Other factors that contribute to degradation and accelerated ageing of cement-based materials are chemical attacks such as carbonation, external sulphate attack, and chloride ions penetration. Various methods and techniques are used to reduce/prevent degradation of cement-based materials in order to extend service life, e.g. coatings, sealers, more durable and resistant cement-based mixtures, etc. Electrochemical methods (i.e. cathodic protection, realkalisation, electrochemical chloride extraction) are generally applied for steel corrosion protection in reinforced concrete structures, but these (although rarely investigated with this respect) would also affect microstructural properties as a result of the electrical current involved. Among all available methods, accelerated curing is also performed, since if properly executed accelerated curing is not only with beneficial effect on extended service life but also leads to reduced lifecycle cost [2]. * Corresponding author: a.susanto@tudelft.nl 562

2 Several accelerated curing methods of concrete have been reported to achieve rapid gain of compressive strength at an early age due to temperature rise during the curing process [3,4]. Saul developed the maturity concept: Concretes of the same mix at the same maturity have approximately the same strength whatever combination of temperature and age goes to make up that maturity [5]. Various methods of accelerated curing of concrete have been used including steam curing, autoclaving (high pressure steam curing), microwave heating, hot water, hot air heating, infrared heating, and electrical curing [6-9]. This study is part of a larger investigation on the negative and possibly positive effects of electrical (stray) current on cement-based microstructure, where positive and/or negative effects would depend on current density and environment. In other words, possibility exists that DC (including stray current) can be beneficial at certain levels. This paper is focusing on the possible curing effects of electrical current flow i.e. electrical curing (e.g. direct current (DC) curing). In DC curing, electrical current is passed directly through the cement-based systems to produce joule heating effect, thus increasing the initial rate of cement hydration. Bredenkamp [6] stated that DC curing is one of the most energy efficient methods for accelerated curing of concrete and after the initial capital outlay for equipment, the running costs of direct electric curing are substantially lower than that of externally applied heat curing (steam, autoclave, etc). The energy cost of direct (DC) electrical curing with field strength between 300 and 500 V/m and rate of energy input 76kWh/m 3 is about 5% of the total cost for a m 3 of concrete. Other investigators mentioned that energy consumed per m 3 of concrete are (kwh/m 3 ) [10] and (kwh/m 3 ) [11], respectively. As a comparison, this study investigated DC curing with regimes of 100mA/m 2 and 1A/m 2. The electric field strengths used were 4 V/m for the 100mA/m 2 and 40 V/m for the 1A/m 2 regimes respectively. The energy consumed per m 3 of concrete specimen are thus 7.55 kwh/m 3 and 75,5 kwh/m 3, respectively during DC curing for 112 days. Although the benefits of DC curing in cement-based materials have been investigated, there is still limited information about the effect and influence of the external environment during the DC curing. This paper presents results for the effect of DC current flow as a possibility of DC curing for cement-based materials in water and calcium hydroxide solution as external environments, discussing mechanical and microstructural properties and thus differentiating curing phenomena from ageing phenomena when electrical current flow is involved. 2 Experimental materials and methods 2.1 Materials Mortar cubes of 40 mm 40 mm 40 mm (Fig.1) were cast, using OPC CEM I 42.5N with water-tocement ratio of 0.5 and cement-to-sand ratio of 1:3. The chemical composition (in wt. %) of CEM I42.5N (ENCI, NL) is as follows: 63.9% CaO; 20.6% SiO 2; 5.01% Al 2O 3; 3.25% Fe 2O 3; 2.68% SO 3; 0.65% K 2O; 0.3% Na 2O. After casting and prior to conditioning, the specimens were cured in a fog-room of 98% RH, 20 C for 24 hours; after de-moulding they were positioned in the containers. 2.2 Sample designation Three groups of specimens were investigated: 1) group control - no DC current involved; 2) group 100 ma/m 2 and 3) group 1A/m 2, where DC current curing was relevant at the respective current levels (the mortar specimens were subjected to DC current flow, Fig.1). All groups were submerged in water or (Ca(OH) 2) solution as external environment. 2.3 Current regime Figure 1 shows a schematic presentation of the experimental set up for DC curing using current density of 100 ma/m 2 and 1 A/m 2. The experimental set up is as previously used and reported [12]. The current density was adjusted by additional resistors (R 1=2700 ohm and R 2=270 ohm) 563

3 in the electrical circuit. The mortar cubes were completely submerged. Tap water and calcium hydroxide solution were used as external medium. The electrical current was applied from 24h hydration age (immediately after de-moulding of the specimens) and until 112 days of age. mortar Tap water/ca(oh)2 a) _ + I (A) 27 cm 44 cm Electrodes and cables to apply current b) c) Height of water solution=5 cm R Height of mortar cubes=4 cm A 27 cm Current injected in the surface area A 12 V R1=2700 Ω R2=270 Ω 44 cm Figure 1 (a) Experimental set-up for DC curing (b&c) schematic experimental set up [11] 2.4 Methods Standard compressive strength Standard compressive strength tests were performed on mm mortar cubes at the hydration ages of 3, 14, 28, 84 and 112 days. Three replicate mortar specimens were taken out from the conditioning set-up, cloth-dried and tested within a 30 min time interval Mercury intrusion porosimetry (MIP) The sample preparation for MIP tests followed generally accepted procedures [14, 15]. The MIP tests were carried out by using Micrometritics Poresizer 9320 (with a maximum pressure of 207 MPa) to determine the porosity and the pore size distribution of the specimens Mortar electrical resistivity Electrical resistivity of mortar was measured using an AC 2-pin method, where the pins are metal plates with dimensions equal to the sides of the mortar cubes [12]. Resistance of the mortar specimens is measured by applying an alternating current of 1mA at a frequency of 1kHz. R-meter was used to record the resistance of the mortar. For the under current regime (groups 100 ma/m 2 and 1A/m 2 i.e. DC current curing ), the resistance measurements were performed after current interruption of approx. 30 min and surface drying of the cubes. Electrical resistivity 564

4 was calculated using Ohm s Law: ρ=r.a/l, where ρ is the resistivity in Ohm.m, R is the resistance in Ohm, A is the cross-section of the mortar cube in m 2, and l is the length in m. 3 Results and discussions 3.1 Compressive strength Compressive strength is considered as a key property of concrete. It provides a general indication of concrete quality. The strength of a material is defined as the ability to resist stress without failure. Failure is identified with the appearance of cracks. There are several factors that affect strength of cement-based materials such as water/cement ratio, cement type, mixing water, admixtures and curing conditions including humidity, temperature and ageing (time) [16, 17]. The present study is focusing on the influence of DC curing and possible ageing on cementbased materials. DC current flow induces temperature elevation in the cement-based materials due to the Joule heating effect, which leads to the potential for altered mechanical properties. Figure 2 presents the evolution of compressive strength as a function of hydration age for mortar specimens submerged in water and calcium hydroxide solution in control and and DC curing-conditions. Generally, compressive strength increases with cement hydration and age. Figure 2 Compressive strength as a function of hydration age for mortar submerged in (a) water and (b) calcium hydroxide solution Fig.2a) depicts the results for water environment, showing that DC curing leads to compressive strength decrease for the mortar specimens after 112 days of cement hydration, compared to control specimens. It can be deduced that DC curing in water environment accelerates ion migration in the bulk matrix and hydration rate of the mortar specimens, subsequently leading to increased calcium leaching and decreased strength. The influence of current density at the level of 1A/m 2 is more pronounced after 112 days, compared to the effects of current density at 100mA/m 2. In other words, the ageing phenomena in terms of reduced mechanical properties in this case, depend on the level and intensity of applied current density. In contrast, Figure 2b) shows an opposite trend of compressive strength development when Ca(OH) 2 was employed as external environment. In this case, DC curing improves the mechanical properties of the mortar specimens, the most plausible mechanism being related to avoidance of calcium leaching in this environment, accompanied by enhanced ion/water migration and altered cement hydration. As a result, the compressive strength of mortar specimens under both current regimes of 1A/m 2 and 100 ma/m 2 end-up slightly higher than the control cases. 565

5 3.2 Microstructural properties Figure 3 reveals porosity development as a function of hydration age for mortar specimens submerged in (a) water and (b) calcium hydroxide solution. As shown in Figure 3a, DC curing contributes to coarsening of the mortar specimens especially after 28 days of age. As expected, the total porosity for specimens conditioned in the regime of 1A/m 2 maintained higher values compared to current density of 100mA/m 2 and control specimens due to accelerated calcium ions leaching. In contrast, for DC curing in calcium hydroxide solution, the total porosity of mortar specimens under both DC current regimes remarkably decreases with conditioning. In this case, the total porosity decrease is larger when higher current density was applied. The pore structure development is in line with the compressive strength results and shows that in water environment the effect of DC current (DC curing respectively) can accelerate ageing phenomena, whereas in Ca(OH) 2 environment, the curing (i.e. positive) effects are prevailing. a) 22 b) 15 porosity (%) Control 100mA/m2 1A/m2 porosity (%) Control 100mA/m2 1A/m2 10 3d 14d 28d 112d Hydration ages 9 14d 28d 84d 112d Hydration ages Figure 3 Porosity as a function of hydration age for mortar submerged in (a) water and (b) Ca(OH) 2 solution 3.3 Electrical resistivity Electrical resistivity measurements are generally used as a non-destructive technique to assess concrete durability [18]. The concrete pore water is essentially an electrolyte, containing mostly K +, Na +, Ca ++, and OH ions [19 21]. Additional ions, such as Cl -, can also be present as a result of exposure to various external sources (such as seawater and de-icing salts), increasing the concentration of ions in the pore water and, if minimal chloride binding is at hand, presumably reducing the electrolytic resistance of the pore water. There are several factors that affect resistivity such as paste volume, concrete composition, water cement ratio, curing temperature, chlorides and moisture content [21, 22]. DC curing promotes temperature increase in the cement-based systems due to the Joule heating effect that contributes to accelerated cement hydration. Accelerating cement hydration influences resistivity development of cement-based materials. Figure 4 reveals the electrical resistivity development of mortar specimens, subjected to DC curing when submerged in water and calcium hydroxide solution. As shown in the Figure 4a, the electrical resistivity of mortar increases gradually with the progress of cement hydration. The electrical resistivity of mortar, subjected to current density of 1A/m 2 is slightly higher compared with 100mA/m 2 and control specimens until 21 days, denoted to enhanced cement hydration in this regime, compared to control and lower current density regimes. However, at later stages, the electrical resistivity of mortar under 100 ma/m 2 and 1 A/m 2 regimes maintained lower values compared to the control cases, attributed to the competing effects of enhanced ion/water transport (accelerated cement hydration) and calcium leaching. The former effects are initially pronounced, whereas the latter are relevant for later stages and determine the lowest electrical resistivity values, recorded in the 1 A/m 2 regime. Calcium leaching contributes to microstructural alterations, including coarsening of the pore network, which 566

6 results in the observed variation of electrical properties of the bulk matrix (electrical resistivity of the mortar was measured in unsaturated condition). Figure 4 Electrical resistivity evolution of mortar submerged in (a) water and (b)ca(oh) 2 Figure 4b shows that the electrical resistivity of mortar specimens under DC curing in Ca(OH) 2 tends to increase gradually and maintained higher after 14 days, compared to the control specimens. This environment prevents or minimises calcium leaching, therefore a pronounced effect in this case is enhanced ion/water migration and accelerated cement hydration within DC current flow, resulting in increase in electrical resistivity of the bulk matrix. In contrast to accelerated aging phenomena in water, the DC curing effects, as positive such, are only observed for the specimens in Ca(OH) 2 environment. 4 Conclussions This paper discussed DC current-induced curing and ageing phenomena in cement-based materials. Based on the experimental results, several conclusions can be drawn: a. DC current induced-curing in tap water tends to accelerate degradation process in the mortar specimens and is therefore responsible for enhanced ageing of the cement-based matrix in terms of reduced compressive strength, coarser pore structure and reduced electrical resistivity. b. DC current induced-curing (at the hereby investigated levels) in calcium hydroxide solution has the potential to accelerate curing and would result in a good quality concrete. c. Further investigation is needed to determine the threshold of positive and/or negative influence of DC current on the properties of cement-based materials. Curing (positive) or ageing (degradation) phenomena when DC current is involved need to be studied also with respect to the composition of the external environment and/or for sealed conditions. 5 Acknowledgements The financial support from Directorate General of Higher Education Ministry of Education Republic of Indonesia is gratefully acknowledged. The authors would like to thank technicians of Microlab, Section of Material and Environment, Delft University of Technology for supporting an experimental set up. 6 References [1] Zhang, Q., Microstructure and deterioration mechanisms of portland cement paste at elevated temperature. Thesis (Ph.D). Delft: Delft University of Technology. 567

7 [2] Cusson D, Lounis Z, Daigle L, Benefits of internal curing on service life and life-cycle cost of highperformance concrete bridge decks -A case study, Cement & Concrete Composites 32 (2010) [3] Federation Internationale de la Precontrainte, Acceleration of concrete hardening by thermal curing: guide to good practice, 1982, pp [4] J.G. Wilson, N.K. Gupta, An overview of the methods of accelerated curing of concrete by elevated temperatures with emphasis on electroheat techniques, in: Universities Power Engineering Conference Proceedings, University of Bath, UK, September 1992, pp [5] A.G.A. Saul, Principles underlying the steam curing of concrete at atmospheric pressure, Mag. Concr. Res. 2 (6) (1951) [6] Bredenkamp S., Kruger D. and Bredenkamp G.L., 'Direct electric curing of concrete'. Magazine of Concrete Research, Vol. 45, No. 162, March 1993, pp [7] lan Heritage, Direct electric curing of mortar and concrete, Napier University, Edinburgh, UK, [8] John GW, Narendra KG (2004) Equipment for the investigation of the accelerated curing of concrete using direct electrical conduction, Measurement 35 (2004) [9] J.G. Wilson, N.K. Gupta, Analysis of power distribution in reinforced concrete during accelerated curing using electroheat, IEE Proceedings Electric Power Applications 143 (2) (1996) [10] Kafry, I. D., 'Direct electrical curing of precast products', Precast Concrete, 1980, Vol. II, no. 7. [11] Wadhwa S.S., Srivastava L.K., Gautam D.K.,Chandra D., Direct electric curing of in situ concrete, Batiment International, Building Research and Practice, 1987, 15:1-6, [12] Susanto A, Koleva DA, Copuroglu O, van Beek K, van Breugel K, Mechanical, electrical, and microstructural properties of cement-based materials in condition of stray current flow, Journal of Advanced Concrete Technology Vol. 11, , April [13] Susanto A, Koleva DA, van Beek K, van Breugel K, The effect of electrical stray current on material properties of mortar specimens, Proceeding the 6th Civil Engineering Conference in Asia Region: Embracing the Future through Sustainability, Jakarta August [14] Sumanasooriya, M.S., Neithalath,N., Stereology- and morphology-based pore structure descriptors of enhanced porosity (previous) concrete, ACI Materials Journal. 106 (5) (2009) [15] Hu J., Porosity of concrete, morphological study of model concrete, PhD thesis, Delft University of Technology, Delft [16] P. Kumar Mehta and Paulo J. M. Monteiro, Concrete Microstructure, Properties and Materials, October 20, [17] Nehdi and Soliman, Early-age properties of concrete: overview of fundamental concepts and state-ofthe-art research, Construction Materials 2011, 164: [18] Millard SG, Gowers KR. Resistivity assessment of in-situ concrete: the influence of conductive and resistive surface layers. Proc Inst Civil Eng Struct Build 1992;94(4): [19] Woefl GA, Lauer K. The electrical resistivity concrete with emphasis on the use of electrical resistance for measuring moisture content. J Cem, Concr Aggr 1979;1(2):64 7. [20] Andersson K, Allard B, Bengtsson M, Magnusson B. Chemical composition of cement pore solutions. Cem Concr Res 1989;19(3): [21] Elkey W, Sellevold EJ., Electrical resistivity of concrete. Norwegian Public Roads Administration, Publication No. 80; [22] Weydert Rand Gehlen C. Electrolytic resistivity of cover concrete: relevance, measurement and interpretation. In: Lacasse MA, Vanier DJ, editor. Durability of building materials and components 8, vol.1. Ottawa, Ontario, Canada; p