A real case of steam cured concrete track slab premature deterioration due to ASR and DEF Case Studies in Construction Materials 6 (2017) 63–71 Contents lists available at ScienceDirect Case Studies i[.]
Case Studies in Construction Materials (2017) 63–71 Contents lists available at ScienceDirect Case Studies in Construction Materials journal homepage: www.elsevier.com/locate/cscm Case study A real case of steam-cured concrete track slab premature deterioration due to ASR and DEF Kunlin Maa,b,*, Guangcheng Longa,b , Youjun Xiea,b a b School of Civil Engineering, Central South University, Changsha 410075, China National Engineering Laboratory for Construction Technology of High Speed Railway, Changsha 410075, China A R T I C L E I N F O Article history: Received 29 July 2016 Received in revised form December 2016 Accepted 23 December 2016 Available online 28 December 2016 Keywords: Concrete track slab High speed railway Fatigue load Steam-cured heat damage Alkali-silica reaction Delayed ettringite formation A B S T R A C T Deterioration mechanisms of some premature damaged steam-cured concrete track slabs (CTS) in Chinese railway less than years were investigated Field investigation, raw materials test and suspicious products analysis were carried out Results show that steamcured heat damage (SCHD) of concrete takes place in steam-cured process Expansion products are ettringite in hydrated products and alkali-silica gels between the interface of hydrated products and coarse aggregate SCHD makes CTS surface layer loose, porous and more micro-cracks Long-term fatigue load from high-speed train acting on CTS enlarges concrete microcracks, leading to water penetrating into concrete easily in moist and rainy environment In the process of water ingression, alkali-silica reaction (ASR) and delayed ettringite formation (DEF) take place, hence resulting in CTS cracking and premature deterioration © 2016 Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Introduction Precast concrete elements produced by steam-cured, such as sleepers, track slabs and pre-stressed concrete beams, are mainly used in railway engineering infrastructures The advantages of steam-cured concrete are that it can rapidly improve strength of concrete in early ages and efficiency of template turnover [1,2] However, compared to concrete cured at room temperature, there are some macroscopical and microcosmic disadvantages in steam-cured concrete, such as concrete brittleness developing, concrete surface layer micro cracks increasing and porosity enlarging [3–5] High temperature and moist steam-cured process would give rise to great differences between surface and inner concrete due to temperaturestress difference, heat-mass transfer and non-uniform of hydration of cementitious, being the important reasons for arising SCHD Studies show all these disadvantages resulting from the steam-cured process will put great side effects on concrete [6–8] In this paper, these concrete disadvantages caused by steam-cured are called steam-cured heat damage (SCHD) Delayed ettringite formation (DEF) and alkali-silica reaction (ASR) are two causes of concrete deterioration through internal concrete expansion and cracking Generally, damage due to the DEF allegedly occurs when the material is exposed to temperatures higher than 70 C But, in field, It was found that DEF took place subject to cured temperature no more than 70 C, for being found that DEF was not only influenced by the curing temperature, but also by various other factors, such as cement composition (alkalis, C3S, C3A, SO3, and MgO),fineness, etc If an unfavorable combination of these parameters exists, delayed ettringite formation may occur at temperatures less than 70 C [9] It was also found that DEF became serious under * Corresponding author at: Central South University, School of Civil Engineering, Shaoshan South Road [46_TD$IF]568#, Changsha 410075, China E-mail address: Mark-mkl@163.com (K Ma) http://dx.doi.org/10.1016/j.cscm.2016.12.001 2214-5095/© 2016 Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/) 64 K Ma et al / Case Studies in Construction Materials (2017) 63–71 moist environment [10,11] In some conditions, delayed ettringite formation (DEF) cannot be avoided and will result in unwanted adverse effects on hardened concrete ASR results from the chemical reaction of alkali in cement with potential active siliceous aggregate [12] In order to avoid ASR, the alkali content in concrete must be strictly restrained and low activity of aggregate must be put into use However, some potential active aggregate cannot be avoided totally So alkali-silica reaction in hardened concrete could take place under some conditions [13–15] Studies show that the formation of ettringite would augment the expansion of the mortars caused by the alkali-aggregate reaction [16] In some fields, it was found that DEF and ASR took place in the same time in concrete However, the simultaneous occurrence mechanism of the delayed ettringite formation and the alkali-silica reaction in concrete structures is still not well understood [17,18] In field engineering projects, it was found steam-cured concrete elements often took on premature deterioration In China, a full investigation on steam-cured concrete sleepers showed that the average life of sleepers were only 14 years Some sleepers took on seriously deterioration only used in 5–8 years Concrete cracking, water penetrating, bear capacity decreasing were the main deterioration characters of steam-cured concrete bridges elements in China [19] In the United States, Some steam-cured precast concrete girders used in San Mateo Bridge in San Francisco were found serious deterioration and must be repaired after these beams used 17 years [20] In Texas, many drilled-shaft concrete foundations of high-mast illumination poles (HMIPs) constructed in the late 1980s were found to have premature concrete deterioration due to ASR and DEF [21] From 1998 to 2003, a comprehensive investigation was conducted in Texas on the structural performance of in-service bridges with premature concrete deterioration The premature concrete deterioration was attributed to two expansive distress mechanisms: ASR and DEF [22] However, the premature deterioration mechanism of steam-cured concrete still needs to be further studied In this paper, the underlying deterioration mechanism of steam-cured concrete track slab used no more than years in railway of China was investigated carefully According to field investigation, samples test as well as service environment analysis, a succession of reaction and deterioration mechanisms, including the influence of SCHD, the formation of expansive products and the effects of service environment, are put forward and discussed It is useful to further understand the deterioration mechanism of steam-cured concrete under such complicated conditions Experimental Since the railway was open to traffic in 2010, the railway line was in good condition service for busy traffic in east China However, it was found that some concrete track slabs appeared cracks in 2014, which could induce potential unsafe to transportation For the sake of security, theses cracked concrete track slabs were replaced In the same time, in order to explore the reasons for these cracking concrete track slabs, field investigation and sampling study were carried out 2.1 Field investigation The replaced CTS in the field were brought back to lab and investigated in detail The items of field investigation of CTS included damage characteristics, cracks distribution, production process, raw materials, mix proportion and related examining reports Service environment and climate of CTS were also investigated 2.2 Laboratory investigation of field concrete In field investigation, concrete samples have been cored carefully from CTS surface vertically in locations showing representative deterioration phenomena as well as at positions without any cracks After cutting and drying, fractions of these concrete samples were chosen for microscopical analysis Micromorphology and microstructure of cracks in concrete were observed through scanning electron microscope (SEM) The suspected substance in concrete samples was analyzed by X-ray diffraction (XRD) and energy disperse spectroscopy (EDS) The porosity of concrete was tested through mercury intrusion porosimetry (MIP) Mortar packing coarse aggregate was removed and petrographic analysis was used to analyze mineral compositions of coarse aggregate Results and discussion 3.1 Field investigation The railway was located in the east of China This region belongs to sub-tropical monsoon climate, with an average annual rainfall of about 1200 mm, average annual rainfall of about 140 d and annual average temperature of 15.4 C Therefore, the CTS would be in the moist and rainy environment The railway with design speed 300 km/h was one of the busiest highspeed railways in the world It was reported that train could pass every 3–5 from 7:00 AM to 11:00 PM; result in longterm fatigue load acting on CTS And the fatigue load frequency was about 8–12 HZ According to service environment investigation, it was confirmed that CTS was under the combined actions of high-speed train fatigue load and the moist and rainy environment for a long time Careful inspections were paid to CTS replaced from the railway line The appearance of CTS is shown in Fig (The surface of CTS was brushed waterproofed materials after it cracked.) As can be seen from Fig 1, there are map-cracking patterns on K Ma et al / Case Studies in Construction Materials (2017) 63–71 [(Fig._1)TD$IG] 65 Fig Map-cracking of CTS [(Fig._2)TD$IG] Fig White substance in CTS CTS, and some cracks run through all the CTS When concrete is cut out along with cracks, it is found that some white substance on the hardened mortar and coarse aggregate contacting the cracks (Fig 2) In order to learn about concrete actual situation, raw materials, mix proportion and production process of CTS were investigated and analyzed Field investigation information showed that composed raw materials of CTS were P.II 42.5 provided by Jinmao Xinhua cement Co., Ltd satisfied the requirements of Chinese standards GB 175 [23], fine aggregate came from Ganjiang River, coarse aggregate came from Huzhou Zhejiang province, fly ash with grade F satisfied the requirements of Chinese standards GB/T 1596 [24], polycarboxylate superplasticizer with water reducing rate 20.5% produced by Wuhan Geruilin was used and mixing water was tap water Table presents the mix proportion of CTS Field information also showed that in order to ensure good quality all raw materials used in CTS were selected and inspected carefully The results of the rapid expansion of mortar bar test according to GB/T 50733 [25] shown that alkali reactivity of coarse aggregate and sand swelling rate was 0.18% and 0.15% at 14 d ages respectively There were no alkalicarbons reactivity substance exist in aggregate tested through petrographic analysis Alkali content of P.II 42.5 cement (Na2O + 0.658 K2O) was 0.54% Alkali content of fly ash was 0.39% Alkali content of water reducer was 5.86% Total alkali content of concrete was 2.7 kg/m3, satisfied the requirements of Chinese standard of GT/B 50733 The steam-cured method was used to produce CTS The regime of steam-cured treatment was total duration of 13 h, including preheating duration of h, heating duration of h, treating duration of h with a constant temperature of 60 C, and cooling duration of h Fig presents the steam-cured regime Table Mix proportions of CTS (kg/m3) P.II 42.5 cement 432 Coarse aggregate 5–10 mm 10–20 mm 340 795 Fine aggregate Mineral admixture Water Water reducer 695 48 140 6.24 66 [(Fig._3)TD$IG] K Ma et al / Case Studies in Construction Materials (2017) 63–71 o Temperature ( C) 100 80 60 40 20 0 10 Curing time ( h) 12 14 Fig Steam-cured regime of CTS [(Fig._4)TD$IG] Fig Sample of coarse aggregate from crack CTS 3.2 Analysis of raw materials In cracking positions in CTS, mortar embracing coarse aggregate was removed, and coarse aggregate was taken to X-ray diffraction analysis and petrographic analysis Fig was the samples of coarse aggregate taking from the cracking positions in CTS Fig was the tested results of X-ray diffraction of coarse aggregate Fig shows the main mineral compositions in coarse aggregate are quartz (SiO2), albite (NaAlSi3O8) and calcium carbonate (CaCO3) Fig 6(a) & (b) shows the results of petrographic analysis As can be seen from Fig 6, the coarse aggregate was composed of orthoclase, anorthose, quartz, black mica and appetite Orthoclase content is about 35%, displaying plate-like shape and size being no more than mm mm Anorthose content is about 25%, also showing plate-like shape and size being no more than mm mm Quartz content is about 30%, presenting irregular granular shape and average size being about 1.0 mm However, about 1/3 of the quartz particles are small particles with size about 0.1 mm Black mica content is about 10%, exhibiting sheet shape Therefore, it is shown that the aggregate used in CTS belongs to siliceous aggregates In most of the cases in which siliceous aggregates were used, there was extensive damage from the expansion of alkali-silica gel and ettringite [11,21,22] 3.3 Microstructure In the area without macroscopic crack on CTS, samples were cored in the distance from concrete surface mm, 10 mm and 50 mm, and then porosity and microstructure of these samples were tested Fig exhibits the MIP results of concrete samples in different positions As can be seen from Fig 7, the accumulated porosity of concrete samples decrease with the increasing distance from concrete surface to inner Among these tested samples, accumulated porosity of concrete sample K Ma et al / Case Studies in Construction Materials (2017) 63–71 [(Fig._5)TD$IG] 67 Fig X-ray diffraction of coarse aggregate [(Fig._6)TD$IG] Fig Photos of petrographic analysis of coarse aggregate Accumulated porosity ( %) [(Fig._7)TD$IG] 0.09 Distance from concrete surface mm Distance from concrete surface 10 mm Distance from concrete surface 50 mm 0.06 0.03 0.00 10 100 1000 10000 100000 D( nm) Fig Porosity of concrete in different concrete samples (a) concrete microstructure from surface mm (b) concrete microstructure from surface 50 mm cored in the distance from concrete surface mm is the largest However, the accumulated porosity of concrete sample cored in the distance from concrete surface 50 mm is the smallest Fig 8(a) exhibits the concrete microstructure in the distance from concrete surface mm As can be seen from Fig 8(a), lots of CH crystals exist in concrete, concrete is loose and porous, and there are lots of pores and micro-cracks in hydration products The picture in Fig 8(b) presents concrete microstructure in distance from concrete 50 mm As can be seen from Fig 8(b), the concrete microstructure is compact, and obvious pores and micro-cracks in hydrated products are hard to find From concrete microstructure investigation and analysis, it is found that there are greatly different between surfaces and 68 [(Fig._8)TD$IG] K Ma et al / Case Studies in Construction Materials (2017) 63–71 Fig Concrete microstructure in different positions inner in steam-cured concrete In the steam-cured process, surface concrete exposed to the steam chamber is loose and porous compared to inner concrete High temperature and moist steam-cured process would give rise to great differences between surface and inner concrete due to temperature-stress difference, heat-mass transfer and non-uniform of hydration of cementitious, being the important reasons for arising SCHD Through testing analysis of the surface and inner concrete of CTS, it is easy to find that compared to inner concrete, there are more pores and micro-cracks in surface concrete Based on the comparison of literature and data [5–8], it can be confirmed that SCHD takes place in the concrete deterioration at present There are great properties and microstructure difference between surface and inner concrete [(Fig._9)TD$IG] Fig SEM photos of cracking concrete surface and related EDS analysis K Ma et al / Case Studies in Construction Materials (2017) 63–71 69 3.4 Expansive products Fig 9(a)–(c) is the microtopography photos in concrete crack taken by SEM Fig 9(a)–(c) show that there’s a great deal of slender acicular and columnar crystals on concrete crack surface The composed elements of the slender acicular and columnar crystals detected by EDS are shown in Fig 9(d) Fig 9(d) illustrates the crystallization products in concrete crack include mainly elements Ca, S and Al According to the crystals shape and its composition, it is definitely confirmed the crystals should be ettringite crystal DEF would take place under high temperature steam-cured condition In high temperature, generally above 70 C, AFt (3CaOAl2O33CaSO432H2O) in hydrated products would turn into AFm (monosulfate), but when the temperature comes to room temperature, AFm would transform into AFt again The newborn AFt would result in an expansion in hardened concrete, so it is one important reason causing concrete cracking A petrographic examination of cracked Swedish concrete railroad ties identified delayed ettringite formation (DEF) as the damaging mechanism This was unexpected because the concrete railroad ties were steam-cured with a maximum concrete temperature below 60 C [9] Fig 10(a)–(c) shows the interface of hydrated products closed to coarse aggregate There are some suspected cracking gel substances on hydrated products surface EDS analysis from Fig 10(d) shows that the gel substances mainly include Si and Ca elements Combining the results of petrographic analysis, it can be confirmed that this gel substances are alkali-silica gels When the alkali-silica gel absorbs water, there will be a large volume expansion Plenty of alkali-silica gels accumulating in interface of coarse aggregate and hydrated products, resulting in uneven expansion happening [(Fig._10)TD$IG] Fig 10 Interface between hydrated products and coarse aggregate 70 K Ma et al / Case Studies in Construction Materials (2017) 63–71 Discussions of deterioration mechanisms 4.1 Service environment Service environment is an important influence factor to concrete materials Severe service environment is bad for concrete For being one of the busiest high-speed lines in the world, there is high frequency and low strength fatigue load acting on CTS acted by passing high-speed train At the same time, this high-speed railway line locates in sub-tropical monsoon climate area, with an average annual rainfall of about 1200 mm, average annual rainfall of about 140 d and annual average temperature of 15.4 C As results, service environment of CTS would be in long-term train fatigue load and moist and rainy environment 4.2 Influence of SCHD Concrete is a kind of non-uniform materials, which includes pores and micro-cracks For the sake of SCHD existing in steam-cured concrete, SCHD leads to CTS surface layer becoming loose, porous and more micro cracks Under the effect of long-term fatigue load, micro-cracks in concrete will develop and broaden [26,27], and water absorption of concrete will also augments [28] In the moist and rainy environment, water is easy to penetrate into concrete through these micro cracks 4.3 Formation of ASR and DEF While chemically different, both mechanisms require moisture to drive the expansion process, so water is necessary for the formation of ASD and DEF in concrete [10,11,29] Under the effects of long-term high-speed train fatigue load and moist and rainy environment, SCHD help water penetrate into concrete easily High pH in pore solution favors the presence of monosulfate rather than ettringite However, a decrease of pH in pore solution can lead to the growth of ettringite [11] The ingression of water into concrete dilutes pore solution and decrease pH, which is so called leaching effect A drop in pH due to ASR or carbonation may also promote DEF Loss of alkali from pore solution due to ASR could lead to a situation in which ettringite formation is preferential at selected locations due to locally depression pH and alkali concentration for the sake of formation of ASR gel [30] Another empirical relation shows that DEF index is directly related to the square root of the total alkali content of the cement [31] In many field projects, it was found that both ASR and DEF take place in steam-cured concrete almost at the same time 4.4 Deterioration process of SCT According to the analysis above, the deterioration process of CTS is as follows First, SCHD leads to concrete surface layer being loose, porous and more micro cracks Second, long-term fatigue load from passing high-speed train enlarges these micro cracks Third, water is easy to penetrate into concrete crack under moist and rainy environment Fourth, after the ingression of water into concrete, ASR would take place in siliceous aggregate Fifth, water ingression and ASR induce the decrease of pH in pore solution, increasing the growth of ettringite in concrete Finally, expansion of ASR and DEF result in cracking in hardened concrete Fig 11 exhibits the deterioration process of CTS Conclusions Based on the field and laboratory investigations aimed at cracking CTS, the conclusions of this study are as follows: (1) SCHD exists in the studied CTS SCHD makes concrete surface layer become loose, porous and more micro cracks, resulting in porosity of concrete surface layer is obviously larger than inner concrete Therefore, there are properties and [(Fig._1)TD$IG] Fig 11 Deterioration process of CTS K Ma et al / Case Studies in Construction Materials (2017) 63–71 71 microstructure difference between surface layer and inner concrete Long-term fatigue load on CTS from high-speed train makes micro cracks develop and broaden Moist and rainy climate help more water penetration into concrete (2) The main expansion products in cracking concrete are ettringite and alkali silica gel Acicular and columnar crystals of ettringite appear in hydrated products and plane like alkali- silica gel almost grow up on the interface of coarse aggregate and hydrated products It was also found that DEF would take place under 70 C (3) Effect of SCHD results in initial imperfections including loose, porous and more micro cracks in CTS These initial imperfections develop under the action of a long-term fatigue load of high-speed train, which increase the ingression of water into concrete under moist environment Water penetrating into concrete accelerates the ASR and DEF taking place, hence leading to premature deterioration of CTS Acknowledgments The financial supports under National Basic Research Program of China (“973” Program, Grant No 2013CB036201) and High-Speed Railway Joint Funds of the National Natural Science Foundation (U1534207) are gratefully acknowledged References [1] D.W.S Ho, C.W Chua, C.T Tam, Steam-cured concrete incorporating mineral admixtures, Cem Concr Res 33 (4) (2003) 595–601 [2] Long Guang-cheng, Wang Meng, Xie You-jun, et al., Experimental investigation on dynamic mechanical characteristics and microstructure of steamcured concrete, Sci China Technol Sci 57 (10) (2014) 1902–1908 [3] H.H Patel, C.H Bland, A.B Poole, The microstructure of concrete cured at elevated temperatures, Cem Concr Res 25 (3) (1995) 485–490 [4] B Lothenbach, F Winnefeld, C Alder, et 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removed, and coarse aggregate was taken to X-ray diffraction analysis and petrographic analysis Fig was the samples of coarse aggregate taking from the cracking positions in CTS Fig was the tested