Recent durability studies on concrete structure

Cement and Concrete Research 78 (2015) 143–154

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Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp

Recent durability studies on concrete structure S.W. Tang a,b, Y. Yao c, C. Andrade d, Z.J. Li b,⁎ a

State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan, China Department of Civil and Environment Engineering, The Hong Kong University of Science and Technology, Kowloon, Clear Water Bay, Hong Kong c China Building Materials Academy, Beijing, China d Institute of Construction Sciences Eduardo Torroja, Madrid, Spain b

a r t i c l e

i n f o

Article history: Received 4 March 2015 Revised 15 May 2015 Accepted 18 May 2015 Available online 14 June 2015 Keywords: Durability Alkali-aggregate reaction Sulfate attack Steel corrosion

a b s t r a c t The durability of concrete has attracted significant attention over the past several decades and is still a research hotspot until now. This paper reviews and discusses recent research activities on the durability of concrete, including: 1) major durability problems such as alkali aggregate reaction, sulfate attack, steel corrosion and freeze–thaw; 2) durability of concrete in marine environment; and 3) coupling effects of mechanical load and environmental factors on durability of concrete. Moreover, the consideration of durability in concrete structure design (DuraCrete and performance-based specifications) is also briefly reviewed. © 2015 Elsevier Ltd. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Major durability problems . . . . . . . . . . . . . . . . . . . . . . . 2.1. Alkali–aggregate reaction in concrete . . . . . . . . . . . . . . 2.2. Deterioration caused by sulfate attack . . . . . . . . . . . . . . 2.3. Steel corrosion . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Freeze–thaw. . . . . . . . . . . . . . . . . . . . . . . . . . 3. Durability issue in a marine environment . . . . . . . . . . . . . . . . 3.1. Definition and significance . . . . . . . . . . . . . . . . . . . 3.2. Methods to reduce deterioration of concrete in marine environment 4. Consideration of durability in concrete structure design . . . . . . . . . 4.1. Coupling effect of mechanical load and environmental factors . . . 4.2. Loading carrying ability unified service life design philosophy . . . 4.3. Service life prediction based on durability . . . . . . . . . . . . 5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Durability of Portland cement concrete is defined as its ability to resist weathering action, chemical attack, abrasion, or any other process of

⁎ Corresponding author. Tel.: +852 23588751; fax: +852 23581534. E-mail address: [email protected] (Z.J. Li).

http://dx.doi.org/10.1016/j.cemconres.2015.05.021 0008-8846/© 2015 Elsevier Ltd. All rights reserved.

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deterioration to remain its original form, quality and serviceability when exposed to its intended service environment [1]. Durability problems usually appear as the materials deteriorate at the beginning. Although the material deteriorations do not have an immediate safety issue, they will progressively lead to structural damage, which puts a potential danger to the structures. The classification of causes of concrete deterioration can be grouped into three categories, physical, chemical, and mechanical, from which

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S.W. Tang et al. / Cement and Concrete Research 78 (2015) 143–154

major durability issues such as steel corrosion are initiated and developed [2]. To solve the durability problems, many researchers have conducted deep studies on these issues. The studies cover the individual topics of carbonation, alkali aggregate reaction, reinforcing bar corrosion, sulfate attack, CH leaching, and freezing–thawing. As noted by researchers, in most cases, the degradation of a concrete structure is a result of the combined effect of multi environmental factors and loading. Therefore, many researchers have extended their work from a single mechanical loading or environmental factor to the combined effects of multiple factors [3–14]. As a result of durability studies, many countries have proposed durability-based design guidelines [15–17] or durability-loading carrying ability unified service life design method [2]. This paper starts with the discussion on recent researches associated with major durability issues based on selected corresponding activities. After discussion on durability issues for concrete in marine environment, the durability based design philosophy and methodology are introduced. 2. Major durability problems The major durability problems in concrete including alkali–aggregate reaction, sulfate attack, steel corrosion and freeze–thaw are discussed in the following subsections. 2.1. Alkali–aggregate reaction in concrete Alkali–aggregate reaction (AAR) in concrete structure is a reaction between alkalis in pore solution and some active chemicals of aggregates. Two general types of attacks of AAR are classified as: 1) alkali–carbonate reaction (ACR) with active minerals from dolomitic limestone aggregate [18]; and 2) alkali–silica reaction (ASR) with active minerals from amorphous silica. The commonly-used testing methods to assess the AAR are: mortar bar test (ASTM C227), rock cylinder method (ASTM C586), rapid test methods (ASTM C289), accelerated mortar bar method (ASTM C 1260), autoclave testing method [19], chemical shrinkage test [20], concrete prism test (RILEM AAR-3) [21], stiffness damage test [22], dynamic modulus and gel fluorescence test [2]. Apart from these conventional tests mentioned above, Liu and Mukhopadhyay developed a compound activation energy measurement method, as shown in Fig. 1: highly reactive borosilicate glass and/or different types of aggregates were tested with NaOH + Ca(OH)2 solution. It had been proved that volume change in a closed system was in a form of chemical shrinkage as the ASR between aggregates and alkali solution proceed. There, the authors were relating the recorded change volume over time to the risk potential and actual reaction degree of ASR [23].

Normally, the degree of AAR is affected by 1) presence of water, ASR only occur in high relative humidity; 2) alkali content; 3) concrete porosity and 4) temperature. Accordingly, there are some recommendations for eliminating the attack of AAR: 1) utilization of nondeleterious aggregates and/or nonreactive aggregates; 2) utilization of low-alkali Portland cement or blended cements with enough pozzolanic materials; 3) keep the concrete dry as much as possible and damage is generally not observed for RH b 80%; 4) utilization of diffusion-control coating; and 5) addition of nitrate salts. Here, the effects of pozzolanic materials and nitrate salts are selected for discussion: 1) The incorporation of sufficient reactive siliceous granular skeleton, fly ash, slag and silica-breccia [24] is beneficial to limit ASR expansion risk [25]. Wright et al. investigated that higher lime content (%CaO N 10%) fly ashes required a greater percentage of cement replacement to reduce ASR expansion to 0.08%[26]. Fly ash with CaO 27.3, 13.5 and 2.42% required 31, 18 and 13% replacement of Portland cement against ASR [26–28]. Besides, mortar and concrete with silica-breccia having a particle size less than 30 μm showed lower expansion than the control mixture in both alkali–silica reactivity and sulfate resistance tests [29,30]. 2) The addition of LiNO3 in cement-based materials can decrease AAR expansion, significantly influence the chemical composition and the morphology of the reaction product. Besides, dense reaction products are generated and served as the effective protective barrier to sulfate attack [31]. Interestingly, some recent researches had observed that initial ASR gel might temporarily densify the cement matrix. 1) With respect to alkali-carbonate reaction, a considerably higher increase in compressive strength was detected for the mortar with activated dolomite aggregates by Stukovnik et al., compared to the one with limestone aggregate. This phenomenon was explained as better interlocking between Portland cement binder and aggregate grains, due to the formation of a new Mg–Si–Al phase both in the ITZ and along pre-existing cracks in the aggregate grains [32]. 2) Krivenko et al. also considered that alkali-susceptible aggregates could be used in the alkali activated cement concretes without any risk in spite that theory suggested the opposite. The authors suggested that the active alumina in the alkali activated cements had a favorable effect allowing to effectively controlling the structure formation process in the interfacial transition zone and to reduce expansion down to admissible levels or completely avoid it [33]. 3) Bektas pointed out that the addition of alkali reactive brick aggregate did not affect engineering properties of concrete although alkali silica gel and secondary ettringite were observed by SEM [34]. 2.2. Deterioration caused by sulfate attack Sulfate attack is one of main factors causing expansion deterioration of concrete structure. Such expansion is attributed to reactions of sulfate ions with some hydration products in concrete structure. Tests on sulfate resistance are generally conducted through storing specimens in a solution of sodium or magnesium sulfate, or a mixture of the two [2]. Currently, the effect of sulfate attack is usually assessed through several indicators: length variations, loss or increase of mass, difference of surface hardness [35], strength and elastic modulus decrease. However, these indicators do not provide sufficient information to assess chemical reactions and understand the damage mechanisms behind [36] and neither the indications can be related to performance in real conditions. Innovative test methods are imperative to study the degradation process of concrete structure caused by sulfate attack.

Fig. 1. The compound activation energy measurement [23].

1) Braganca et al. utilized electrochemical impedance to identify the sulfate attack non-destructively. The impedance tests were


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cured for 6 months stored in water at (5 ± 2) °C. It was noted that such new method could provide a considerably rapid way for thaumasite sulfate attack research [41,42]. 5) Recently, non-linear impact resonance acoustic spectroscopy which can detect changes in materials from the resonance frequency shifts of the vibrational modes of a specimen was utilized for monitoring external sulfate attack process [43]. Fig. 3 illustrates the experimental set-up of non-linear impact resonance acoustic spectroscopy. This technique has been shown to be highly sensitive to micro-cracks of the material and adequate to low and mid expansion damages caused by sulfate attack. The obtained results from non-linear impact resonance acoustic spectroscopy test corresponded to ones from microstructural analysis [43].

Fig. 2. Schematic diagram of sulfate test [38].

performed by applying an alternating signal with a 25 mV amplitude for a frequency interval from 106 to 5 × 10− 2 Hz in open circuit potential, with 10 points obtained per decade [37]. The electrochemical impedance spectroscopy results obtained from the reinforced mortar exposed in a sulfate chamber indicated that this technique could be used to monitor the decrease in the durability of the system even in the initial stages of deterioration. The formation of monosulfate byproducts in the matrix, which competed with the formation of cement hydrates during the curing process, was identified from the analysis of sulfate profiles obtained and equivalent circuits [37]. 2) The new method proposed by Hachem et al. revealed the relation between the specimen size and the resistance to sulfate attack [38]: the experimental setup, as shown in Fig. 2, was implemented at constant pH and temperature, 7.5 and 20 °C, respectively. The pH was regulated by adding a nitric acid solution at 0.5 mol/L. The mass and length of specimens were measured when the solution was renewed. From the experimental results, they believed that the accelerating effect of sulfate attack was resulted from the increase in leaching kinetics rather than the type of expansive products [38,39]. 3) By utilizing high resolution synchrotron X-ray diffraction (SyXRD), Stroh et al. found that after concretes were immersed in a sulfatebearing soil over a time span 19 years [40], ettringite was identified as the main crystalline phase in the inner zone and gypsum was very close to the surface of concrete [40]. 4) In general, forming a great deal of thaumasite for research usually takes more than one year using an external MgSO4-solution storage method. In order to overcome this time-consuming drawback of external method, Li et al. established a so-called internal adding method to accelerate thaumasite formation in lab [41]. In this method, cement–limestone pastes containing 10% magnesium sulfate were

Fig. 3. Experimental set-up of non-linear impact resonance acoustic spectroscopy [43].

The physical/chemical interaction of sulfate attack is a complicated process and depends on many parameters, including concentration of sulfate ions, ambient temperature, amount of cement/mineral additives, water to cement ratio, diffusivity and/or permeability of concrete, and presence of supplementary pozzolanic admixtures [2,38,44]. Some effective methods that can eliminate negative effects of sulfate attack are presented as: 1) Mineral additions: The incorporation of ground granulated blast furnace slag into the cement is beneficial for the enhancement of diminution of expansion for concrete structure stored in sulfate solutions since slag will consume calcium hydroxide to some degree and lead to diminution of the gypsum and ettringite [45]. It was reported that the incorporation of more than 30% of low reactivity ground granulated blast furnace slag led to a notable enhancement of the mortars efficiency against sodium and magnesium sulfate attacks [46]. Arribas et al. also found out that there was less expansion in slag mortars than in control mortar as a result of the sulfate attack after one year of exposure. Meanwhile, these slag mortars showed a greater increase of strength than control mortar due to the absence of internal damage and the reactivity of the fine fraction of the slag aggregate [24]. For the fly ash case, bottom ash/circulating fluidized bed combustion ash concrete mixtures also showed better resistance to external sulphuric acid attack [47,48]. It was considered when the level of combined high-calcium fly ash and slag reached to 60% of the total cementitious materials, a significant reduction of expansion was observed [49]. With regard to silica fume, the presence of a large amount of nanostructured colloidal silica could enhance the resistance to sulfate attack, delaying the decay evolution [50]. Mortars with fly ash and silica fume had extremely low expansion in sodium sulfate solutions [36]. Additionally, it was reported by Nielsen et al. that the use of mineral admixtures was beneficial to reduce the formation of thaumasite [51]. 2) Burial coating: The increase of depth of burial coating can decrease the amount of concrete deterioration against the thaumasite formation of sulfate attack to some extent [52]. 3) Glass powder: The mortar blended with ground glass powder significantly improved the resistance to sulfate attack, and thus enhanced the durability performance [53]. In the sodium sulfate resistance test of mortar specimens, minimum weight loss occurred when the cement was replaced by appropriate 10% of glass powder [54]. 4) Immersion position: Nehdi et al. investigated the dual sulfate attack performance of concrete partially immersed in a 5% sodium sulfate solution under cycling temperature and relative humidity and concluded that wick action acted as the main transport mechanism of a solution in the partially exposed concrete: the lower portion immersed in sulfate solution could suffer from chemical sulfate attack, while the upper one was sensitive to the intrinsic pore structure and vulnerable to physical sulfate attack [55,56]. 5) Lightweight aggregates and viscosity modifier: Bentz et al. explored two new approaches for increasing a mortar's resistance to sulfate attack: on one hand, fine lightweight aggregates were pre-wetted

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Fig. 4. Schematic diagram of closed microcracks by precipitation of ettringite [60].

to enhance the microstructure of the interfacial transition zone, on the other hand, the isolated pores in the lightweight aggregates might help to accommodate the formation of expansive degradation products, such as ettringite, without creating substantial stresses and subsequent cracking; a viscosity modifier was added to the concrete mixture to increase the viscosity of the pore solution and thus slowed down the ingress of sulfates from the external environment [57]. 6) Magnesium oxide: The addition of magnesium oxide can improve the resistance of concrete to carbonation, chloride attack, and sulfate attack in the long-term [58]. 7) Superplasticizer: Proper amount of superplasticizer in concrete mix can cause high water reduction, thus reducing sulfate attack on the concrete and producing a more robust matrix [45]. 8) Barium carbonate: A new approach to producing sulfate-resistant cements was the addition of barium carbonate to clinker. The main mechanism was utilization of the capacity of barium to immobilize sulfates in the form of highly insoluble barite [59]. Some recent modeling methods have been proposed to describe the process of sulfate attack, these models focus on microcrack bridging or pore filling by the precipitation of expansive products: 1) Bary et al. carried out the chemo-transport-mechanical simulations of mortar specimens subjected to external sulfate attack in numerical integration platform. They considered that microcracks might be bridged or filled by some expansive secondary ettringite under external sulfate attack and restrained force, as shown in Fig. 4 [60]. 2) A diffusion-reaction numerical model was proposed to simulate the response of concrete exposed to external sulfate attack by Ikumi et al. The described approach allowed the consideration of different filling rates and capacities to accommodate expansive products for each pore size considered [61]. 2.3. Steel corrosion Since corrosion of the reinforcing steel is regarded as the most serious durability problem in construction engineering [62,63], how to eliminate the degree of steel corrosion is a headache problem in practical engineering. The combination of concrete quality and concrete cover

thickness is apparently the most important parameter that controls the rate of carbonation and chloride ingress, hence improving the quality of concrete has thus been considered as the primary protection method [64,65]. The protection strategies include increased concrete cover, coating rebar, stainless steel and corrosion inhibitors. 1) Generally speaking, the time-to-corrosion of the embedded reinforcing steel can be significantly influenced by the amount of concrete cover over the rebar [65]. However, as the cover increases, the rebar becomes less effective and the potential for cracking due to tensile stress, shrinkage and thermal effects increases [2]. 2) The coating of the reinforcing steel can enhance the durability performance by serving as a barrier preventing the access of aggressive species to the steel surface and providing electrical insulation to some extent. In a recent study, epoxy-coating distinctly shows a much superior performance against the steel corrosion compared with red-oxide and zinc primer coatings [66], and can be applied in various ways, either as a liquid or as a powder which is fused on the surface. ASTM A775/A775M-04a (Standard Specification for Epoxy-Coated Steel Reinforcing Bars) has addressed the basic requirements for epoxy-coated reinforcing steels by the electrostatic spray method. The amount of damage to the epoxy coating prior to concrete casting has been considered as the major contributing factor to the poor performance [67]. Thus, training is necessary for properly producing, handling and applying the coating, and repairing of field damage to epoxy-coated bars. Besides, the bond properties between the concrete and the epoxy-coated rebar are not as strong as between concrete and conventional rebar, which should be improved [2]. 3) There have been a few investigations on the use of stainless steel bars [68–72]. It has been shown that the chloride threshold value for initiation of corrosion in non-welded AISI 304 (a kind of stainless steel) rebar is three to five times higher than that of conventional rebar. However, welding of the bar reduced the critical chloride level by 50%. It should be emphasized that the use of stainless rebar is an expensive solution [2]. 4) Zinc coating of steel (galvanized steel) is also considered as a good means for providing corrosion resistance. It acts both as a sacrificial and barrier-type coating. But, a disadvantage is that, like other


metal coatings, the zinc coating corrodes over time. The rate of corrosion under the given environmental conditions will determine the loss of coating thickness, and the time period which it will be effective. Generally, there is a fairly linear relationship between the metal thickness and the duration of its effective service life for galvanized steel exposed to an industrial atmosphere [73]. The stability of zinc is dependent on the pH of the surrounding solution where the zinc coating is exposed. The corrosion products of zinc may be deposited at the surface of the zinc coating and seal it, thus arresting the evolution of H2 gas and leading to passivation of the zinc coating. In particular, if the galvanized and ungalvanized bars are used in the same structure, special care should be taken to ensure complete electrical isolation of the two [74]. 5) Corrosion inhibitors are regarded as useful not only as a preventative measure for new structures but also as a preventative and restorative surface-applied admixture for existing structures. Various corrosion inhibitors can be classified into [2]: (a) adsorption inhibitors, which act specifically on the anodic or on the cathodic partial reaction of the corrosion process, or on both reactions, (b) film-forming inhibitors, which block the surface more or less completely, such as the siloxane-based corrosion inhibitor [75–78] and (c) passivators, which favor the passivation reaction of the steel. The mechanistic action of corrosion inhibitors is thus not against uniform corrosion but localized or pitting corrosion of a passive metal due to the presence of chloride ions or a drop in pH value [79]. Corrosion inhibitors admixed to the free concrete can act in two different ways: these inhibitors can extend the corrosion initiation time and/or reduce the corrosion rate after depassivation has occurred [2]. Mixed-in inhibitors are regarded as more reliable since it is easier and more secure to add the inhibitors to the mix. Some laboratory testing has shown that certain corrosion inhibitors do not significantly affect the amount of chloride ions required to initiate corrosion, but can reduce the corrosion rate [2]. Recently, the application of green, sustainable and eco-friendly corrosion inhibitor becomes a promising research hotspot [80]. One green inhibitor is Bambusa Arundinacea leaves extract that exhibits better resistance to steel corrosion compared with calcium nitrite and ethanolamine. This extract can stabilize calcium silicate hydrates (C–S–H) gels that prevent calcium hydroxide conversion to calcite and carboaluminate phases to some degree [80]. The other typical green inhibitor is Anthocleista djalonensis that is a special kind of extracts from leaf, stem bark and root bark. The experimental data and predicted models by Okeniyi et al. showed that only 0.4167% Anthocleista

Fig. 5. Set-up for electrochemical treatment [83].

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djalonensis leaf-extract admixture (per weight of cement) exhibited excellent inhibition efficiency [81]. Additionally, Okeniyi and coworkers further employed electrochemical monitoring techniques to assess anticorrosion performance of Phyllanthus Muellerianus leaf-extract on concrete steel-reinforcement in 0.5 M H2SO4 used for simulating industrial/microbial environment. From the experimental electrochemical results and adsorption isotherm predictions, the best inhibition efficiency at inhibiting the concrete steel-reinforcement corrosion was exhibited by the 0.3333% Phyllanthus Muellerianus leaf-extract admixture (per weight of cement) [82]. 6) Since the 1990s, fibrous composites have been introduced into concrete to replace reinforcing steel. Fibrous composites are usually made of continuous fibers (carbon and stable glass) as reinforcement and polymers (epoxy and polyester) as the matrix [2,64]. Application of fibrous composites can completely eliminate the corrosion problem. However, drawbacks of these composites are the high price and high temperature sensitivity [2]. 7) In order to increase the bond strength of the embedded plain steel bars in existing structures without any need for demolition and realkalization, the deep impregnation of cement-based mortars and concrete with a solution of ethyl silicate and with electrochemical treatments by means of a solution of sodium carbonate was investigated [83]. The set-up of electrochemical treatment was presented in Fig. 5. It was reported that the experimental corrosion test showed that corrosion risk classes of some mortar samples after such immersion treatment passed from a high level to a low one [83]. Although a great deal of information associated with steel corrosion has been resolved by means of various prevention techniques, researchers and engineers also intend to monitor the corrosion progress to make maintenance decisions in time. The monitoring methods include visual inspection, half-cell potential measurement, radiography, ultrasonic, magnetic perturbation/flux, and acoustic emission technique [2]. Here, we briefly introduce several cutting-edge methods as follows: 1) A method for determining the reinforcement corrosion occurs (Ccrit) value was proposed by Garcia et al. [84]. Fig. 6 illustrated the test configuration of this method. The method was achieved through capillary suction and a diffusion phase with steel-potential monitoring during the progressive contamination of the concrete by chloride. It was stated that the effect of a sufficient total amount of chlorides on steel potential in compositions with Portland cement was to induce a drop in potential of around 250 mV when steel corrosion was initiated. The method further considered that the presence of sulfide in pore solution had a great impact on the steel potential [84]. 2) In order to evaluate the steel corrosion-based deterioration in an early age, an array low-coherent fiber optic sensor system was developed [85]. This system could effectively monitor the corrosioncaused expansion at the accuracy of sub-micro strains by circled the sensing optical fiber in two configurations. One was to wire the fiber on the surface of steel rock bolt directly; the other was to circle the fiber on a mortar cushion. From the comparison of these two configurations, an interface dominated corrosion process was very sensitive to the uniformity of the interface formed on the steel [85]. 3) Active corrosion with an external anodic current of 5 mA/cm2 was induced by corroding steel embedded in cement paste in an X-ray microcomputed tomography study [86]. The electrochemical setup and X-ray microcomputed tomography examination of this method were presented in Fig. 7. The test samples were placed in sodium chloride in three-electrode system for galvanostatic corrosion and then the corroded samples were taken out from the solution at particular time spans, left at the open circuit potential during X-ray microcomputed tomography imaging. Through this innovative method, the cracked cement paste, the propagating corrosion

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Fig. 6. Test configuration of determination of Ccrit [84].

products, and the corroded steel with a resolution of few micrometers had been investigated in-situ and non-invasively. It was also concluded that after depassivation of steel, the average rate of steel loss recorded by X-ray microcomputed tomography correlated well with the one predicted by Faraday's law of electrolysis [86]. 4) Qian's research group developed an attractive on-line corrosionmonitoring approach that only exploited three heterogeneous tiny energy sources to power commercial-off-the-shelf wireless sensor motes such that the corrosion related data were automatically captured and sent to users via wireless manner [87]. Three tiny energy sources presented in their work were corrosion energy, a cement battery and weak solar energy. Fig. 8(a) showed that the source of corrosion energy was from the half-cell potential and current of carbon steel panel immersed in the simulated concrete pore solution; Fig. 8(b) presented micro-corrosion spots caused by electrochemical corrosion on the surface of the carbon steel. Additionally, packing configuration of the cement battery was demonstrated in Fig. 9. Obviously, this proposed method for monitoring the durability of RC structures located in a corrosive environment through a long-term and human-free manner was very promising. What's more, the

Fig. 7. (a) Electrochemical setup for galvanostatic corrosion; (b) X-ray microcomputed tomography examination [86].

feasibility of use of ambient energy as a power supply for wireless motes was verified by corrosion monitoring of an experimental environment [87]. 2.4. Freeze–thaw Concrete deterioration caused by freezing and thawing is closely linked to the presence of water in concrete but cannot be explained simply by the expansion of water on freezing [88,89]. In general, the loss of mass or decrease of dynamic modulus is used as indexes of degradation. Recently, some advanced test methods have been developed to monitor the degradation process of freeze–thaw attack through other physical quantities.

Fig. 8. (a) Source of corrosion energy; (b) micro-corrosion spots caused by electrochemical corrosion [87].


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Fig. 11. 3D digital sample constructed by stacking TXM slice images [94].

Fig. 9. Illustration of the cement battery [87].

1) Guneyisi et al. investigated the deterioration of concrete by variation of ultrasonic pulse velocity during freeze–thaw cycles [90]. They concluded that the differences of ultrasonic pulse velocity might be attributed to modification of pore structure morphology during the freeze–thaw process [90,91]. Meanwhile, in the research group of Aghabaglou, when they evaluated the freeze–thaw resistance of high-volume fly ash concrete, it was found that the dynamic modulus of elasticity corresponded to weight change percentage in freeze–thaw cycles [92]. Besides, two different types of non-linear acoustics techniques (see Fig. 10), the indirect transmission (with incident waves at 90°) and the semi-direct transmission (with incident waves at 45°), were studied by Bui et al. for mortar samples at different levels of damage caused by freeze–thaw cycles [93]. Test results from these two techniques were extremely sensitive to the initiation of cracking caused by freeze–thaw attack. 2) The transmission X-ray microscope (TXM) technique applied by Dai and Ng was employed to perform fast-image acquisition of capillary

pores and micro-damage evolution at 30 nm resolution in situ [94]. The 3D digital sample was constructed by stacking TXM slice images with unit thickness, as shown in Fig. 11; such constructed nanoscale digital sample could be used for fracture simulation and damage mechanism study. It was stated that the crack propagation with predefined crack path under theoretically calculated pore pressure followed the actual crack path captured from the nanoscale TXM images. Also, with the help of this technique, it was demonstrated that the ice crystallization pressures generated in nanoscale pores were sufficient to cause internal-frost damage in cement pastes [94]. In the case of frost damage, few of works on the basic mechanisms have been performed in spite of the still existing questions on it. This situation is perhaps due to the in practice the damage can be avoided by the use of air-entraining agents and the experience on which are the best mix proportions [95,96]. Apart from conventional air-entraining agents, some kinds of additives with appropriate amount are beneficial for the enhancement of the resistance of freeze–thaw in recent researches as well. These additives include synthetic zeolite [97], organic resins [98], encapsulated siloxane [75], fiber [99,100] and nano particles [101].

3. Durability issue in a marine environment 3.1. Definition and significance

Fig. 10. Non-linear acoustics techniques: (a) indirect transmission; (b) semi-direct transmission [93].

Lots of concrete structures are located in the marine environment, such as cross-ocean tunnels, long-span bridge and offshore drilling platform. According to the deterioration mechanisms, the marine environment zones of concrete structure can be classified as submerge, tidal, splash and atmospheric zones [102]. P.K. Mehta in 1980s considered that the tidal zone of concrete structure was the most serious part since it was suffered from both chemical and physical attacks in nature [103,104]. Corrosion of steel and spalling of concrete usually occurred in this zone [103,104]. Safehian et al. also pointed out that the tidal zone was the most aggressive exposure condition followed by the splash zone; the atmospheric zone was considerably less aggressive than the others [105]. As a matter of fact, harmful ions ingress in tidal zones involves adventive transport and is therefore very dependent on the liquid permeability of the material in additional of its diffusion coefficient [106,107]. Moreover, the physical collision, erosion and abrasion of alternating motion of waves and tides is bound to arouse concerns about deterioration of the concrete structure [2].

150


Fig. 12. Conversion of chemical influence to mechanical effects [2].

3.2. Methods to reduce deterioration of concrete in marine environment In this work, some methods to reduce the deterioration of concrete in marine environment are summarized as: 1) Cement matrix: Marine environments are typically aggressive to concrete structures, since sea water contains high concentrations of chlorides and sulfates. The corrosion of steel reinforcement is mainly caused by chloride penetration into concrete, which leads to expansion of steel surface and stress concentration. The additional ettringite crystals are formed since the chemical reaction between sulfate ions in the seawater and AFm in concrete structure. However, these crystals can be quickly dissolved in the seawater and then leach out of concrete structure, causing material loss to some extent [105,108]. Therefore, the utilization of lowpermeability concrete is recommended to eliminate the chloride and sulfate deterioration caused by the marine environment. 2) Mineral additions: For concretes made by the cement blended with certain kind of mineral additions with an appropriate dosage, there may be a decrease in chloride threshold for corrosion, which can be associated with a decrease in its binding ability, lower pH in concrete porous network solution, as well as with its lower buffering effect [102]. With respect to fly ash, ternary Portland cements elaborated with activated paper sludge and fly ash are stable against sodium sulfate attack. This enhancement mechanism is explained as a result of the formation of non-expansive ettringite inside the pores and an alkaline activation of the pozzolanic reaction of additions promoted by the ingress of sulfate and secondary related reactions [109]. Cheewaket et al. indicated that fly ash concretes with water to binder ratio of 0.45 and 15–35 wt.% fly ash replacement exhibited high-quality performance in a marine site [110]. However, for the case of electric arc furnace slag, Arribas et al. found that with regard to the exposure of concretes to sea tides, chloride penetration was greater in electrical arc furnace slag aggregate concrete than that in limestone concrete. Besides, the corrosion of steel rebar in the reinforced electrical were furnace slag aggregate concrete in the tidal seawater environment after a year exposure exhibited greater susceptibility than that in the limestone reference concrete [24]. Therefore, the optimal type and amount of pozzolanic materials should be carefully considered in the marine environment. 3) Temperature and humidity: Zaccardi et al. [111] considered that the influences of ambient temperature and relative humidity were very important for prospective studies of corroding structures in marine environment. The relative humidity by itself is not influencing the corrosion rate because it reflects the water

vapor content. It is the amount of evaporable (liquid) water which controls the corrosion through the influence in the resistivity [112]. 4) Construction defects: Safehian et al. investigated construction method on the chloride penetration performance in the silica fume concrete in severe marine environmental conditions and found that deep crack formation in reinforced concrete structures due to construction defects might lead to increasing chloride penetration and consequently not reaching to expected service life [105]. 4. Consideration of durability in concrete structure design 4.1. Coupling effect of mechanical load and environmental factors In practice, the prediction of the actual service lifetime of new or existing concrete structures is a global challenge since the concrete structures not only carry the load but also are exposed to various environmental conditions [113,114]. The ocean structure mentioned above under erosion, chloride diffusion, and sulfate attack simultaneously is a good example. Therefore, it is more realistic to study the deterioration mechanism of a concrete structure under the combined effects of mechanical loading and a combination of multi environmental factors. (1) A typical example was illustrated by Lei et al. who explored such combined effects of environmental chloride concentration, time, and load on chloride ingress and the mechanism of concentration distribution. Their results indicated that, with the increase of the ion concentration in the service environment and with the passage of time, chlorine ion content in the protective layer of the concrete went up gradually. They also found that external mechanical load can alter the pore structure properties within concrete and change distribution of tensile stress in favor of chloride ion penetration [115]. (2) Sun et al. had conducted experiments on concrete with a combination of loading, chloride diffusion and freeze–thaw to investigate the combined effect on performance [5]. They noticed that higher strength concrete usually had a better resistance to the different stress ratios and freeze–thaw cycles. Moreover, the incorporation of air entraining might increase the amount of closed pores and relax the pressure during the freeze–thaw cycles [5]. (3) Desmettre and Charron investigated the water permeability of reinforced normal-strength concrete (NSC) and fiber-reinforced concrete (FRC) simultaneously subjected to tensile cyclic loading. The experimental results showed that the water permeability was significantly lower in the FRC than that in the NSC under


Fig. 13. Regularity of concrete structure performance as function time [2].

either constant or cyclic loading [12]. M. K. Rahman et al. studied the impact of compressive stress-induced damage on chloride transport in concrete [14]. They found that there was a significant increase by up to three times in the effective chloride-migration coefficient due to the damage in concrete. Lim et al. studied the chloride permeability of concrete under uniaxial compression and considered that chloride permeability appeared to be associated with critical stress. When the critical stress was exceeded in a concrete specimen, a large chloride permeability coefficient was measured [116].

However, it should be pointed out that such type of research is just in its initial stage, and more effort has to be made in order to deeply understand the deterioration mechanism under a combination of loading and environmental factors [4,5]. With the aim to study the durability of concrete under combined actions, a RILEM Technical Committee RILEM TC 246-TDC has been set up to make service life design more realistically [117]. 4.2. Loading carrying ability unified service life design philosophy The traditional design of concrete usually ignores the environmental factor and only takes the loading-carrying capability of structure into account. Moreover, the design code considers mechanical properties of concrete as time-independent parameters [2]. Since 1990s, durability issue has caused more and more attention and some design code with consideration of concrete durability has been developed. However, in these preliminary attempts, the durability of concrete structure is taken care only by the detailing described in the code such as the cover thickness of a structure under a certain environmental condition. Unfortunately, there is no scientific formulation to assess the effect of each environmental factor. It is imperative that a new design approach of unified load-carrying capability and durability service life design theory should be established. Such theory should include two schemes [2]:1) the quantitative conversion of environmental issues into a design code or transfer these issues to equivalent mechanical loading; and 2) the dynamic assessment of material properties and structure behavior with service time [2]. With respect to the first scheme, one straightforward and possible solution is to build up an explicit method that these environmental factors can be converted to an equivalent distribution of force or stress. Fig. 12 demonstrates such conversion: it may be achieved through thermodynamics, porous media mechanisms or conservative energy

Table 1 Prediction the time of corrosion initiation by various service life models (years) [105]. Exposure conditions

Service life models

Average

CTDRC and BHRC

DuraPGulf

Life-365

Fib2012-M

Atmospheric Tidal

N200 15

N150 66

55 15

NA 36

135 33

151

principles and provide conveniences for the traditional design formulations based on stress analysis. For the second scheme, the mechanism of materials and/or structure degradation with time should be further clarified, which can be resorted to the experimental study of real structure exposed to various environmental conditions under loading or the virtual computer simulation of concrete structural behavior under different combinations of loading and various environmental factors. Through these studies, the properties and behavior of concrete shall be able to be described as function of time as shown in Fig. 13. With such function, the structural performance of concrete at different service period can be predicted and incorporated into the design accordingly. Fig. 13 sketches the regularity of the performance of concrete system as a function of time. 4.3. Service life prediction based on durability Apart from durability design theories, many mathematical durability models have been proposed in order to achieve a reliable prediction of physical/chemical behaviors of concrete structures during their lifetime [114]. By utilizing these durability models, prompt cost-effective decisions can be made concerning the appropriate time to maintain existing structures. There are some durability models available for concrete service life prediction, such as Life-365™ service life predication model from the consortium consisting of the Slag Cement Association (SCA), the Concrete Corrosion Inhibitors Association (CCIA), National Ready Mixed Concrete Association (NRMCA) and the Silica Fume Association (SFA) [118], DuraPGulf developed by Tehran University [119], LiPred and Life Prob by Andrade [120]. In addition, artificial intelligence neural network approach is used in deterministic equations (CTDRC & BHRC model) proposed by Concrete Technology and Durability Research Center and the Building and Housing Research Center [115,121]. Some mainstream of prediction methods are discussed in detail as: 1) DuraCrete model [122] represents state of the art in the probabilistic design methodology for service life regarding carbonation initiated corrosion. It has considered several models among which one is using concrete resistivity as a decisive factor [122–129], even with consideration of the propagation phase in the assessed situation [130]. This model is empirical using concrete resistivity as a decisive factor. The prediction of the model does not reasonably take the actual environmental climate into account. In particular, initial model inputs, i.e., the concrete resistivity parameter and the corrosion penetration needed for the initiation of a crack, have a significant influence on final model outputs. Therefore, more emphasis must be further focused on the determination of initial model inputs that varies with different service environments and concrete properties [123]. 2) Performance-based specifications have recently been also introduced into European standard EN 206-1 through the equivalent performance concept [131]. This concept can be simply applied by comparing new concrete mixtures with reference mixtures. The main idea of EN 206-1 is illustrated as: it shall be proven that the concrete has an equivalent performance especially with respect to its reaction to environmental actions and to its durability when compared with a reference concrete in accordance with the requirements for the relevant exposure class. EN 206-1 is allowed to use a concrete not formulated as described by the prescriptive approach, and this concrete needs to present equivalent or similar performances with a concrete formulated [132]. It is considered that EN 206-1 is just a concept-based framework and responsibilities of performance-based specifications should be clearly defined in future work [133]. 3) A modified Fib2012-M model for predicting the service life of reinforced concrete structures in tidal zones of marine environment was proposed by Safehian et al.[105]. The degree of deterioration

152


in this model was strongly related to the presence and concentration of chloride ions on the rebar surface [131]. It should be mentioned that the service life predicted by this model was a value determined by material characteristics, cover depth, and severity of service environments. 4) Demis et al. presented a software tool for the estimation of concrete service life by only considering two concrete deterioration mechanisms: carbonation and chloride ingress [134]. The comparison of the estimated time of corrosion initiation for investigated structures from different durability models is shown in Table 1. As can be seen in Table 1, results cannot achieve a good agreement. The actual service life of reinforced concrete in real environment is very difficult to predict/ model theoretically, as it depends on not only environmental conditions but also on the heterogeneous nature of cement-based materials. Therefore, it is important to validate the model by imperative long-term field study in a particular environmental condition [105, 135].

[19]

5. Conclusion

[20]

This review has covered the body of literature that pertains to recent research activities regarding to durability. Based on the works reviewed, the general consensus is adding optimal type and amount of pozzolanic materials is a cost-effective approach to improve the durability performance to some extent. For the case of steel corrosion, the self-power steel corrosion monitoring from tiny corrosion energy leads toward a new horizon of durability assessment. The study of natural green inhibitor for steel corrosion also opens a promising research direction in the near future. With respect to durability design codes, the mainstream codes or methods have more or less intrinsic drawbacks due to the failure to comprehensively consider various coupling effects of mechanical loading and multi environmental impacts. Therefore, it is necessary to develop a new approach of unified load-carrying capability and durability service life design theory for more accurate service life estimate. Acknowledgment The support from the Hong Kong Research Grant Council under grant of 615412 and from the China Ministry of Science and Technology under grant of 2015CB655104 is gratefully acknowledged. Prof. Yao also thanks the finical support from National Natural Science Foundation of China (No. 51320105016).

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