The effect of temperature rise on cement-based materials:
correlation of experimental data and a simulation approach
A. Susanto1, D.A. Koleva1, K. van Breugel1 1
DelftUniversity of Technology, Faculty of Civil Engineering and Geosciences, Department Materials & Environment, Stevinweg 1, 2628 CN Delft, theNetherlands
Summary
This work reports on the influence of stray current flow on temperature rise in harden-ing cement-based materials and consequently altered cement hydration. To simulate stray current,different levels of electrical current were applied to cement paste and mortar specimens immediately after casting. Isothermal calorimetry tests were car-ried out to quantify the heat release and the degree of cement hydration. The results confirmedan increase in the degree of cement hydration as a resultof temperature increase due to electrical current flow. A simulation approach wasemployed to predict the temperature rise in cement-based materials when higher levels of stray current are involved. By incorporating an “electrical enhancement” factor, the degree of hy-dration and microstructural development of cement-based systems were simulated using HYMOSTRUC3D. These results also confirmed the expected acceleration of cement hydration within temperature rise due to electrical current flow.Densification of the bulk cement matrix would be relevant, following the aforementioned enhanced cement hydration, i.e. a positive effect of low level current densities could be at hand. However, detrimental effects e.g. current and/or temperature associated micro-cracking, internal stress etc., can be expected upon higher current density levels. This paper only focuses on illustrating the approach with regard “electrical enhance-ment” factor implementation in HYMOSTRUC3D and the validation of this approach through collected experimental data.
1 Introduction
One of the major durability-related challenges for civil structures is reinforcing steel corrosion. Well known is that steel corrosion initiation and propagation are not only due to the ingress of aggressive substances (e.g. chlorides or CO2) but canalsobe
caused by stray current flow. The effects of stray current on steel corrosion in rein-forced concrete are well recognised in the engineering practice. The effect of stray current on the bulk matrix alone is, however, rarely considered and therefore scarcely reported.
The effect of stray current depends on the level of the resulting current den-sity. It has been reported that electrical current (including stray current) has negative effect on cement-based materials, for instance: 1) stray current can initiate and en-hance steel corrosion [1]; 2) stray current alters microstructural properties of mortar and concrete bulk matrix [2-4]; 3) stray current has a significant influence on the deg-radation processes in cement-based systems [4] in terms of reduction in mechanical properties and increased permeability. Meanwhile, the effects of electrical current flow can be also positive at initial states e.g. for strength development of cement-based materials as within electrical curing application [5-8].
Considering the aforementioned various effects of stray current, it is important to investigate the threshold for the negative or possibly positive consequences of electrical current flow, when concrete/reinforced concrete structures are concerned. The effect of temperature increase due to electrical current flow will be used as an approach to define this threshold level. With regard to curing and temperature rise,
the maximum curing temperature in concrete should not exceed 60C, which is re-lated to the risk of internal damage e.g. cracks formation [9]. This means that if elec-trical current is applied in view of “electrical curing”, 60C would be a threshold of positive vs negative effects of the current flow.
Based on a series of experimental data, the aim of this work was to determine an “electrical enhancement” factor, which was further usedas thecorrelation between temperature rise and electrical current flow through cement-based materials. A simu-lation approach to predict the temperature rise and subsequently altered cement hy-dration, when various (higher) levels of stray current are involved, was also em-ployed. The simulation approach, using COMSOL Multiphysics and HY-MOSTRUC3D, was validated through the recorded experimental data.
2 Experimental materials and methods 2.1 Materials
Cement paste and mortar prisms of 5,5 cm x 5,5 cm x 29,5 cm (Fig.1) were cast, us-ing OPC CEM I 42.5N with water-to-cement ratio of 0.5 and cement-to-sand ratio of 1:3 for the mortar. The chemical composition (in wt. %) of CEM I 42.5N (ENCI, NL) is as follows: 63.9% CaO; 20.6% SiO2; 5.01% Al2O3; 3.25% Fe2O3; 2.68% SO3; 0.65%
K2O; 0.3% Na2O. The specimens were maintained in sealed conditions after casting
and for the total test duration of 4 days. These specimens were used to monitor tem-perature alterations within cement hydration, with or without stray current involved, through embedded thermocouples.
2.2 Sample designation
Two main groups of specimens were investigated: 1) Reference group - no DC cur-rent involved and 2) Stray curcur-rent group with subgroups, reflecting the level of DC current applied i.e. groups“1 A/m2”, “10 A/m2”, “20 A/m2”, “30 A/m2”, “40 A/m2”, “50
A/m2”, and “60 A/m2”.The set-up for DC current application is as depicted in Fig.1.
2.3 Current regime
A simulation of stray current was achieved by applying a DC current at the level of “1 A/m2”, “10 A/m2”, “20 A/m2”, “30 A/m2”, “40 A/m2”, “50 A/m2”and “60 A/m2”. The nega-tive and the posinega-tive terminal of a 80 V source were connected to cast-in metal con-ductors. The current density levels were adjusted by additional resistors. The rele-vant surface area was calculated based on the geometry of the specimens, essen-tially the cross section A (i.e. A=5,5 cm x 5,5 cm). Thermocouples were placed im-mediately after casting of the cement paste (and mortar) to record automatically the temperature development by connecting to the AC/DC converter and the personal computer (PC).
Figure 1 Experimental set-up for measurements of temperature increase in cement paste and mortar specimens due to electrical current flow.
2.4. Temperature development
To simulate stray current, an electrical field was applied to the cement-based speci-mens (paste and mortar prisms) immediately after casting and until approximately 4 days of age. The temperature increase due to both cement hydration and electrical current flow were measured for the specimens in Series A, usingthermocouples, em-bedded in the middle, right and left sections of the specimens (Fig. 1).
3. Simulation the influence of electrical current flow on temperature increase in cement paste
The influence of electrical current flow on temperature increase in cement paste and mortar were modelled, taking into account the following considerations:
3.1. The electrical current flow through amaterial, including a cement-based system, is governed by the following equations [10-12]:
𝐸 = −∇𝑉 (1) 𝐷 = 𝜀0𝜀𝑟 𝐸 (2) 𝐽 = 𝜎 𝐸 +𝜕𝐷 𝜕𝑡 + 𝐽𝑒 (3) 𝜎 = 1 𝜌𝑜(1+𝛼 𝑇−𝑇𝑜 ) (4) ∇. 𝐽 = 𝑄 (𝑄 =𝜕𝜌𝑣 𝜕𝑡 ) (5)
where J is current density (A/m2), vis charge density (C/m
3
) (continuity equation), E is electric field (V/m), is electrical conductivity (S/m); V is a scalar electrical poten-tial (Volt), D is electric flux density (C/m2), r is relative permittivity (F/m) and o is
vacuum permittivity.
3.2. The temperature field in a cement-based material, including effects due to elec-trical current flow, can be determined using the following differential equation [10-15]:
𝜌 𝐶𝑝 𝜕𝑇 𝜕𝑡 + 𝜌 𝐶𝑝 𝑢∇𝑇 = ∇ 𝑘∇𝑇 + 𝑄 (6) where 𝑄 = 𝑄ℎ𝑦𝑑 + 𝑄𝑒𝑙 𝑄ℎ𝑦𝑑 = 𝐶 𝑑𝑞 (𝑡) 𝑑𝑡 where 𝑞(𝑡) = 𝑄𝑚𝑎𝑥𝑒 −𝜏 𝑡 𝛽 𝑄𝑒𝑙 = 𝐽. 𝐸 where 𝐽 = 𝐼 𝐴 and 𝐸 = 𝑑𝑉 𝑑𝑡
3.3. The convective heat flux in cement-based materials can be determined as fol-lows[14, 15]:
−𝑛. −𝑘 ∇𝑇 = ℎ (𝑇𝑒𝑥𝑡 − 𝑇) (7)
Cp: heat capacity (J/kg K) Q: heat source (W/m3)
T: temperature (K) h: heat transfer coefficient (W/m2 K) Text : external temperature (K) J: electrical current density (A/m2)
𝑄ℎ𝑦𝑑: heat of hydration (W/m3) E: electric field (V/m)
𝑄𝑒𝑙: heat power per cubic meter (heat source) from electrical current flow
(W/m3)
: coefficient for adiabatic function (h) : hydration shape parameter
q: function describing the heat produced by the hydration reaction per unit
mass of cement I: electrical current (A)
C: cement content (kg/m3) V: electrical potential (Volt)
where for the case of this work, Qel is called the “electrical enhancement factor” that
contributes to temperature increase in cement-based materials due to electrical cur-rent flow. Simulation of Equation 1-7 were performed using COMSOL Multiphysics to obtain temperature development of cement paste due to electrical current flow.
3.4. HYMOSTRUC3D simulation incorporating an “electrical enhancement” factor HYMOSTRUC3D was proposed by van Breugel [13] to simulate the hydration and microstructural development of cement-based systems. It incorporates the fields of cement chemistry, physics and stereology. The 3D microstructure of hydrating ce-ment paste is simulated as a function of the particle size distribution, water/cece-ment ratio, the chemical composition of cement and the initial curing temperature. In HY-MOSTRUC simulation, the 3D microstructure of hydrating cement paste consist of pores, unhydrated cement grains and hydration products as inner and outer hydra-tion products. Figure 2 describes the basic principle of HYMOSTRUC3D. The spheri-cal cement grains are modelled randomly distributed in a three-dimensional body and the hydrating cement grains are simulated as growing spheres. As cement hydration proceeds, the cement grains gradually dissolve and a porous shell of hydration prod-ucts is formed around the cement particle resulting in an outward growth or “expan-sion” of the particles. The hydrates around the cement grains firstly cause the forma-tion of small isolated clusters. Big clusters are formed when small cement particles become embedded in the outer shell of other particles, which promotes the outward growth of these particles. As hydration progresses, the growing particles become more and more connected and the material changes from the state of suspension to the state of a porous solid (see Fig. 2) [13].
Figure 2 Basic principle of HYMOSTRUC3D [13]
The electrical current flow through cement paste (and mortar) increases the temperature development due to resistive heating, or Joule heating effect, resulting in
accelerating the rate of cement hydration. The original version of HYMOSTRUC3D does not contain electrical parameters. In order to simulate the influence of tempera-ture rise in cement-based materials due to electrical current flow and subsequent mi-crostructural development, HYMOSTRUC3D needs to be extended.
In HYMOSTRUC3D, the penetration rate of the reaction front for an individual ce-ment particle at time t is computed with the following basic rate formula [13]:
𝑑𝛿𝑖𝑛 ,𝑖 𝑑𝑡 = 𝐾𝑜 ∗ 1 ∗ 2∗ 3∗ 𝐹1∗ 𝐹2∗ 𝛿(𝛼𝑖) 𝛿𝑡𝑟 (8) Ko (.) is basic rate factor (m/h).
The influence of electrical current flow on the temperature increase in cement paste can be introduced through F1 in Equation 8, that represent the effect of curing
tem-perature on the rate of penetration of reaction front for an individual cement particle. F1 is accounted for with the Arrhenius function as follows [13]:
𝐹1 𝑇, 𝛼, 𝐶3𝑆 = 𝐴 ∗ 𝑒
𝐴𝐸 (𝑇,𝛼 ,𝐶3𝑆)
𝑅 ∗(273 +𝑇) (9)
where AE is the activation energy, R is the gas constant (8.31 103 kJ/mol.K), A is constant.
In this case, variable temperature (T) in F1 was chosen from the maximum
tempera-ture rise of cement paste in condition electrical current flow obtained from simulation and validated with experimental data (Fig. 3 and Fig. 4 in the following section).
4 Results and discussion
4.1. Temperature development of cement paste due to electrical current flow
The heat of cement hydration as well as the temperature development plays a sig-nificant role in determining strength and durability of cement-based materials. Basi-cally, the temperature development in cement-based materials is a consequence from heat evolution due to the exothermic chemical reaction when cement is in con-tact with water i.e. during cement hydration. In-depth understanding of the mecha-nism of heat evolution within cement hydration is beneficial to control the temperature development in cement-based materials. The rate of heat evolution pattern during cement hydration is commonly including five stages under normal conditions, i.e. dis-solution stage, dormant period, acceleration stage, deceleration stage and steady stage [16, 17]. Figure 3 reveals the temperature development in cement paste due to electrical current flow of different levels of current density in comparison to control (no current) conditions. Similarly to the rate of heat evolution, five stages can be dis-tinguished within the temperature development of cement-based materials as well, Fig. 3.
Stage 1 represents the dissolution stage which starts immediately after the contact of cement with water, where dissolved ions and water react with C3A and
gypsum. In this stage, ettringite forms and the initial cement hydration rate maintains lower constant rate towards the end of Stage 1. Within electrical current application at all levels - Fig.3, from 1 A/m2 to 60 A/m2, slightly higher temperature levels were recorded in Stage 1, compared to control conditions. The rapid early reaction rate in Stage 1 was followed by low reactivity in Stage 2, known as the dormant stage. Low reactivity in this stage results in low temperature development.The temperature
evo-lution profile due to electrical current flow presents higher values, following the al-ready observed since Stage 1 trend.Should be noted that in the period of Stage 1, a constant (plateau) region is to be normally observed, rather than the recorded tem-perature drop.The length of the dormant period depends on the fineness of the ce-ment, the temperature and the chemical composition of the cement (including the gypsum content and admixtures [18]).This stage will generally not exceed 5 hours.
Figure 3 Temperature developments in cement paste due to electrical current flow with different level current density
During this period the chemical composition of the aqueous phase will remain more or less constant [13]. According to Bensted [19], there are two concepts to explain the early behaviour and the mechanism which would cause the end of the dormant stage i.e. membrane or protective layer concept" and the concept of "delayed nuclea-tion and growth". For the first concept, the anhydrous cement grains or active sur-faces are assumed to be covered with reaction products which prevent further hydra-tion [20, 21]. Other literature states that formahydra-tion of a membrane or a gelatinous layer over the whole particle surface [22-24].For the second concept, delayed nuclea-tion and growth take place, Ca2+ and OH- ions would dissolve slowly, release little heat, until sufficient supersaturation is achieved to overcome nucleation difficulties [25-28]. Whereas Fierens [29] stated that nucleation at the grain surface is prevented by the existence of a potential barrier. Nucleation could occur as soon as the reaction products have reached a critical size and a Van der Waals attraction commences to dominate the repulsion forces [13].The end of the dormant stage is reachedwhen osmotic pressures at the inside of the membrane rupturethe membrane thus promot-ing acceleration of degree of reaction (known as osmotic pump theory) [30].The for-mer is well known and results from the impeded chemical phenomena within the dormant period, whereas the latter is obviously resulting from the execution of the experiment in this case, probably resulting from fluctuation of the external tempera-ture in the controlled otherwise lab environment. In Stage 3, the alite (C3S) and belite
(C2S) phases start to hydrate and release heat. Cement setting begins and heat
gen-eration is remarkably accelerated. As seen from Fig.2, in this stage that temperature increases rapidly with increase of the level of applied electrical current. The maxi-mum temperature reached is proportional to the square of current density flow through the cement paste. In the stage 4 (i.e. deceleration stage), rate of heat re-lease slow down gradually as the amount of available cement to still react declines. A
shoulder/second peak was observed in this stage which can be attributed to a re-newed formation of ettringite as a result of secondary C3A hydration. The shoulder
feature shifts to the left within increase of the applied electrical current density. In the stage 5 (the steady stage), the rate of hydration is reduced rapidly until reaching a steady state condition. In this stage, the formation of hydration products is very slow.
Figure 4a shows the validation simulation of temperature increases due to electrical current flow in cement paste with experimental data. The results indicate a good fit at the early state while at later stage a slight overestimation is at hand. Yet, the simulation as performed shows a good trend in temperature development of ce-ment paste in conditionsof electrical current flow. At early stage, electrical current reduces the length of the dormant stage, resulting in acceleration of temperature de-velopment for current regimes at stage 3 which is maintained higher until the end of stage 4 (Fig. 3 and 4a). The relationship between applied electrical current density with maximum temperature rise (obtained from the peak of curves in Fig. 3, 4a) is given in Fig 4b.
From the curve in Fig. 4b a relationship between the applied current density (J, in A/m2) with maximum temperature increase (T) in the cement paste can be ob-tained as follows:
𝑇 𝐽 = 0.0024𝐽2+ 0.0009𝐽 + 24.729 (10)
Figure 4 Validation of numerical simulation of temperature increase due to electrical current flow with experimental data (a), relationship between applied electrical
poten-tial (b) and electrical current density (c) with maximum temperature increased From equation (10) the level of current density, able to substantially increase the temperature and therefore to possibly create internal damage can be predicted, which is as aforementioned at temperatures higher than 60C. By considering 60C as a threshold of positive vs negative effects of the current flow and by substituting this value to eq. 10 (60 = 0.0024𝐽2+ 0.0009𝐽 + 24.729)the obtained corresponding current density value is 121 A/m2(derived by solving this second order of the polyno-mial equation above). This means that when electrical current flow through the ce-ment paste is higher than 121 A/m2, the maximum temperature increase in cement paste is higher than 60C and the potential to create internal damage is high.
In order to quantify the influence of electrical current flow on the hydration products of cement paste, HYMOSTRUCT3D simulations were also performed. Figure 5a) showsan example of the distribution of hydration productsin cement paste in control conditions and when electrical current density of 60 A/m2 was applied for 100 hours of cement hydration (should be noted that the model is an illustration of enhanced cement hydration, but does not present equal particle configuration for direct com-parison of the chosen cement paste volume). Figure 5b) presentsthe simulation of(accelerated) degree of cement hydration as dependent on the level of temperature rise due to electrical current. The result is increase in the amount of hydration prod-uct of cement paste (Fig. 5a). The influence of temperature rise due to electrical cur-rent flow on the degree of hydration and hydration product is governed by Arrhenius function, F1, in equation (9). With an increase of current density level beyond the
al-ready defined threshold of 121 A/m2, i.e. 150 A/m2for example (Fig.5b) even more pronounced increase of hydration productswould be expected and subsequent even-tually detrimental effects, These however, need to be considered when the same par-ticle configuration is examined within the simulation approach and is subject to on-going investigation.
Figure 5 Microsctructural development of cement paste in control conditions and at 60 A/m2 current density applied(a) and degree of hydration after 100 hours (b).
From Fig. 3, 4, and 5 it can be concluded that the temperature increase in cement paste due to electrical current and the effects on material performance depend on several factors, for instance 1) the initial properties of cement paste i.e electrical properties (e.g. electrical conductivity), thermal properties (e.g. specific heat capacity) 2) the cement content and 3) the level of electrical current flow. Future ssimulation (with experimental validation) will be performed to quantify the material properties of cement-based materials in condition of electrical current flow in order to predict service life, namely: a) simulation of the effect of temperature rise (due to electrical current flow)-induced morphological and microstructural changes of ce-ment-based materials. In HYMOSTRUC3D this can be accounted for through the parameter F2in equation 8; b) simulation of thermal stress due to electrical current
Conclusions
This paper deals with the influence of electrical current flow on temperature increase in cement-based materials. Experimental work was performed to measure tempera-ture increase due to different levels of electrical current density. The experimental results were coupled to simulation of the process. The following conclusions can be drawn:
1) Temperature increase due to electrical current flow accelerates cement hydra-tion, leading to increase in the amount of hydration products of cement paste. The level of temperature increase is proportional to the square of electrical current flow in the cement-based materials.
2) Temperature increase in cement-based materials due to electrical current and its effect on performance depend on several factors as follows:
a) initial properties of cement paste. b) cement content.
c) level of electrical current flow.
Acknowledgements
The financial support from Directorate General of Higher Education Ministry of Edu-cation 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 support with the experimental set up.
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