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Effect of Steam Curing Cycles on Strength and Durability of SCC a Case Study in Precast Concrete 2013 Construction and Building Materials

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  Effect of steam curing cycles on strength and durability of SCC: A casestudy in precast concrete A.A. Ramezanianpour, M.H. Khazali ⇑ , P. Vosoughi Department of Civil and Environment Engineering, Concrete Technology and Durability Research Center, Amirkabir University of Technology, Tehran, Iran h i g h l i g h t s  Effects of 36 steam curing regimes on the compressive strength of SCC were studied.   Permeability of steam-cured concrete was investigated.   An optimum steam curing cycle was introduced. a r t i c l e i n f o  Article history: Received 25 July 2013Received in revised form 20 August 2013Accepted 27 August 2013Available online 27 September 2013 Keywords: Steam curingSelf-Compacting Concrete (SCC)Compressive strengthSurface resistivityCapillary absorptionEnergy consumption a b s t r a c t Use of Self-Compacting Concrete (SCC) in pre-cast concrete plants is growing rapidly due to its benefitssuch as reduction in labor and equipment costs, increasing productivity, providing flexibility in fillinghighly reinforced sections and complex formworks, lowering the noise on job site and having superiorsurface quality. Also, considering the critical importance of ‘‘production time’’ in precast plants, acceler-ated curing is considered as an inevitable part of precast concrete elements production.In this study the effects of thirty-six different steam-curing regimes on the compressive strength andpermeability of a self-compacting concrete mixture, used in precast concrete elements of Sadr elevatedhighway was investigated. Compressive strength measurements indicated that in a constant total time,increase in precuring period leads to lower immediate compressive strength. On the other hand, increasein treatment temperature and total cycle time (which means higher energy and time consumption) led tohigher immediate compressive strength. Furthermore, durability tests results demonstrated that applica-tion of cycles with maximum temperature of 70   C imposes negative effect on durability properties of ref-erence SCC, such as surface resistivity and capillary absorption. Finally, on the basis of three criteria(compressive strength, permeability and energy consumption by steam curing cycle), an optimum steamcuring cycle was introduced and utilized in the precast concrete plant.   2013 Elsevier Ltd. All rights reserved. 1. Introduction Self-Consolidating Concrete (SCC) has some advantages overconventional concrete making it suitable for use in pre-cast/pre-stressed concrete plants. These advantages include high workabil-ity (making it possible to omit vibration), reduction of labor costs,making feasible to develop more automated plants, and often pos-sessing higher strength and durability properties. Three essentialcharacteristics of fresh SCC are filling ability, passing ability, andsegregation resistance which make it an ideal choice for use in ele-ments with dense reinforcement or complex geometry [1–4].There are some reasons such as limitation of formworks, facili-ties, storage area, and time which encourage precast concreteplants to obtain high early strength, to speed up the stripping of forms, and to shorten the curing period [5]. Special techniquesare available to this aim including: (I) Using special cements withhigh early strength such as fine or high alumina ones (II) Utilizingsuitable chemical additives (III) Use of accelerated curing (includ-ing increased temperature and humidity).According to economy, availability, and long-term performanceof special materials, the most common method is accelerated cur-ing by means of increased temperature and humidity. Variousmethods have been used including steam curing at atmosphericpressure (temperature less than 100   C), steam curing at high pres-sure (autoclaving), electrical heating of reinforcement, imposingelectrical current to concrete directly, and microwave heating.Among these, steam curing at low pressure is most common, espe-cially for large precast units. A typical steam-curing cycle consistsof a precuring (delay) period after surface finishing, a heating andcooling rate of 11–44   C/h, and a treatment period with constanttemperature for 6–18 h. Maximum treatment temperature in 0950-0618/$ - see front matter    2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.08.040 ⇑ Corresponding author. Tel./fax: +98 21 64543074. E-mail address:  khazali@aut.ac.ir (M.H. Khazali).Construction and Building Materials 49 (2013) 807–813 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat  steam curing is usually limited to 60–90   C [6–10]. The minimumearly compressive strength of concrete, the most important factorfor demolding of concrete elements, is suggested about 25 MPa forthe most cases. Also, the ultimate strength of concrete in plants iscommonly considered to be more than 50 MPa [9,10]. Some of common formworks of segments utilized in precast concrete pla-net of Sadr elevated highway are illustrated in Fig. 1.Generally, the maximum total duration of the steam curingcycle in a plant is limited to 18 h, since the production is carriedout 24-h and enough time should be available to prepare the form-work and the arrangement of the reinforcements to continue thedaily production procedure. Besides, it is already shown thatcontinuinga definite steam curing regime for longer hours can leadto detrimental changes in porosity and pore size distribution of concrete [11].Nowadays, durability of concrete structures is widely believedto be major concern and much research work is carried out on thisissue. Mehta and Gerwick [12] investigated the San Mateo bridgeover San Francisco bay after being exposed for 17 years in the envi-ronment; the bridge is comprised of both steam-cured and moist-cured concrete beams with the same mixture proportions andmaterials. The study demonstrated that steam-cured beams hadto be repaired according to corrosion damage, while moist-curedbeams showed no signs of deterioration. Other studies [13,14]illustrated that accelerated curing by excessively increased tem-perature leads to porous concrete with coarse and continuous porestructure, and heterogeneous distribution of hydration products.Consequently, it increases the permeability of concrete againstaggressive ions such as chlorides or sulfates, and decreases thestrength of concrete. Moreover, it can cause initial decomposingof ettringite in fresh concrete, which can recreate Delayed Ettring-ite Formation (DEF) in hardened concrete and produce destructiveexpansion [15–18].To the authors’ knowledge, limited research work is carried outon the effects of steam curing on the durability of SCC. Bingöl andTohumcu [19] compared the compressive strength of SCC mixturescured in the standard situation, by air, and by exposure to steam;the results demonstrated that concrete cured in air had the loweststrength, and the optimum temperature of steam curing is 70   C.Moreover, Reinhardt and Stegmaier [20] studied the pore size dis-tribution of steam cured SCC; it made clear that higher maximumtemperature leads to coarser pores, and the changes are correlatedto (w/c) eq .The object of this study is to investigatethe effects of steamcur-ing on the properties of limestone incorporated SCC, which is usedin a precast plant in Tehran, Iran. For this purpose, compressivestrength and permeability (in terms of surface resistivity and cap-illary absorption) of the reference SCC mixture were investigatedafter exposure to 36 different steam curing cycles. Three mainparameters were selected as variables:(1) Maximum temperature of treatment (50, 60, and 70   C).(2) Total time of steam curing, which is desired to be at the min-imum value, while achieving the required compressivestrength (8, 10, 12, and 14 h).(3) Delay period before commencing the steam curing (1, 2, and3 h).Also, it is to be mentioned that two main constraints were im-posed by the design and project management team, and were con-sidered in this experimental study:(1) A minimum of 24 MPa was required for demolding of pre-cast concrete elements.(2) A maximum steam curing duration of 14 h was allowed.(3) Finally, on the basis of three criteria (compressive strength,permeability and energy consumption by steam curingcycle), an optimum steam curing cycle was introduced andutilized in the precast concrete plant. 2. Experimental program  2.1. Materials The cement used was ASTM C150/C150M-11 [21] Type II Portland cement.Limestone powder was also used as filler. The chemical composition and physicalcharacteristics of Portland cement and limestone filler are listed in Table 1.Natural sand and crushed gravel were used as aggregates. The coarse aggregateshad nominal maximum size of 19 mm, specific gravity of 2.6 gr/cm 3 , and waterabsorption of 1.5%. The fine aggregates had specific gravity of 2.3 gr/cm 3 , absorptionof 2.7%, and fineness modulus of 3.6. The high fineness modulus indicates the lowcontent of fines in the fine aggregates. Therefore, it was decided to add limestonefiller (150 kg/m 3 ) with a maximum particle size of 0.15 mm to compensate forthe lack of fine particles in the local river sand and also to improve the mixture rhe-ological characteristics [22,23]. The specific gravity of the limestone filler was 2.48 gr/cm 3 . The sieve analyses of fine, coarse and final mixture of aggregates usedin the concrete are listed in Table 2.Moreover, a polycarboxylate-ether type High-Range Water Reducer Admixture(HRWRA) with a specific gravity of 1.1 gr/cm 3 and solids content of 47% was con-sumed to achieve the required workability for the mixtures.  2.2. Mixture proportions The mix proportions were selected exactly the same as the reference SCC mix-ture which was used in the precast concrete plant. These proportions were alreadyselected based on a comprehensive study of local materials and evaluation of a var-ious number of mixtures regarding fresh and hardened properties. The mix propor-tions and properties of the reference SCC mixture are listed in Tables 3 and 4,respectively.In order to study the effects of different steam curing cycles, 36 batches wereprepared according to the reference SCC mix proportions. In order to control thevariations and provide the maximum similarity between mixes, ‘‘slump flow diam-eter’’ and ‘‘28-day compressive strength of water cured specimens’’ were measuredeach time. In order to consider a mixture as ‘‘acceptable’’, 28-day compressivestrength of control specimens had to be in the range of ±5% of reference SCC mix-ture (59 MPa). Furthermore, slump flow diameter had to be in the range of 680–700 mm while HRWRA dosage was maintained between 0.75 to 0.85 percentagesof cement mass. If either condition was not satisfied for a mixture,it was consideredas ‘‘rejected’’ and another batch was prepared and tested. Fig. 1.  Precast concrete element made with self-compacting concrete for Sadr elevated highway, Tehran, Iran.808  A.A. Ramezanianpour et al./Construction and Building Materials 49 (2013) 807–813  The SCC mixtures were produced in a horizontal pan mixer with 100 l capacity.A specific mixing sequence was applied for all mixtures; which consisted of drymixing the coarse and fine aggregates, limestone filler and Portland cement for1 min. Then the whole water was added to the dry mixture and mixed for 2 min.Finally, the HRWRA was introduced to the mixture and mixed for another 2 min.After testing the fresh mixture for being self-compactible, cylinder and cube spec-imens were cast without using any vibration.  2.3. Steam curing regimes In order to study the effects of different steam curing cycles on the hardenedproperties of reference SCC mixture, three parameters were selected as variables:(1) Precuring (delay) period.(2) Peak temperature.(3) Total time of steam curing.Considering the precast factory limitations, values were assigned to the param-eters and 36 different steam curing regimes were designed (see Table 5). Precuringtemperature was kept constant at 20   C, while any temperature variation (heatingand cooling) occurred during constant period of 2 h.It is common knowledge that decreasing the steam curing duration is highly de-siredinthelargescaleproductionofprecastelements,sinceitleadstolowerproduc-tion cost and also higher productivity. In this study, a maximum allowable steamcuring duration of 14 h was imposed by precast plant project management team.In addition, ‘‘Energy Index’’ was defined as the multiply of time by extra tem-perature (above ambient temperature which was considered 20   C) during eachsteam curing regime. This was used as an indicator of the energy consumption byeach cycle.High-accuracy automatic climate simulators were used to impose varioussteam curing cycles on SCC specimens (see Fig. 2). The devices are capable of con-trolling both ‘‘humidity’’ and ‘‘temperature’’ in the range of   20   C to +120   C withhigh accuracy. Also, the results ofactual humidity and temperature values duringthe steam curing cycle are reported in MS-Excel format.  2.4. Tests procedures 100  100  100 mm cubic specimens were tested for compressive strengthimmediately after steam curing cycle and at the ages of 7 and 28 days. Furthermore,maturity index (ASTM C1074 [24]) was utilized to study the effect of temperature–time history on compressive strength of steam cured specimens. This value was cal-culated using:  Table 1 Materials properties. Chemical composition (%) Cement Limestone fillerCaO 62.08 50.17SiO 2  21.10 3.12Al 2 O 3  4.18 1.19Fe 2 O 3  3.34 0.53MgO 3.79 3.46SO 3  2.84 0.20K 2 O 0.69 0.25Na 2 O 0.14 –(Na 2 O) eq  0.59Loss on ignition (%) 3.12 40.31 Physical properties Specific gravity (gr/cm 3 ) 3.17 2.48Blaine (cm 2 /g) 3519 –  Table 2 Sieve analysis of aggregates (cumulative percentage passing). Sieve size (mm) River sand Gravel Aggregate mixture19 100.0 100 10012.5 100.0 62.6 89.39.5 100.0 38.3 82.44.75 92.8 1.1 67.12.38 63.6 0.4 47.91.19 40.6 0 32.80.6 26.3 0 22.90.3 13.3 0 13.60.15 4.0 0 6.5  Table 4 Properties of reference SCC mixture. Fresh properties Hardened properties (water cured specimens)Wet density (kg/m 3 ) 2370 Slump flow at 45 min (mm) 550 1-Day compressive strength (MPa) 17Slump flow-Avg. of two diameters (mm) 690 U-box (mm) 10 3-Day compressive strength(MPa) 35 T  50 (s) 2.5 J-ring (h2–h1) (mm) 5 7-Day compressive strength(MPa) 48V-funnel (s) 9 L-box (%) 0.85 28-Day compressive strength(MPa) 59V-funnel at 15 min  (s) 14  Table 5 Steam curing regimes. No. Curingregime IDPrecuringperiod (h)Peaktemp. (  C)Total timeof steamcuring (h)Energy index(min   C)CTL – – – –1 T50-1-8 1 50 8 1802 T50-1-10 50 10 2403 T50-1-12 50 12 3004 T50-1-14 50 14 3605 T50-2-8 2 50 8 1806 T50-2-10 50 10 2407 T50-2-12 50 12 3008 T50-2-14 50 14 3609 T50-3-8 3 50 8 18010 T50-3-10 50 10 24011 T50-3-12 50 12 30012 T50-3-14 50 14 36013 T60-1-8 1 60 8 24014 T60-1 -10 60 10 32015 T60-1-12 60 12 40016 T60-1-14 60 14 48017 T60-2-8 2 60 8 24018 T60-2-10 60 10 32019 T60-2-12 60 12 40020 T60-2-14 60 14 48021 T60-3-8 3 60 8 24022 T60-3-10 60 10 32023 T60-3-12 60 12 40024 T60-3-14 60 14 48025 T70-1-8 1 70 8 30026 T70-1-10 70 10 40027 T70-1-12 70 12 50028 T70-1-14 70 14 60029 T70-2-8 2 70 8 30030 T70-2-10 70 10 40031 T70-2-12 70 12 50032 T70-2-14 70 14 60033 T70-3-8 3 70 8 30034 T70-3-10 70 10 40035 T70-3-12 70 12 50036 T70-3-14 70 14 600  Table 3 Mix proportions of reference SCC mixture. Constituent QuantityCement (kg/m 3 ) 400Filler (kg/m 3 ) 150Sand (kg/m 3 ) 921Gravel (kg/m 3 ) 714Water (kg/m 3 ) 156w/c 0.39HRWRA a (%) 0.8 a Superplasticizer percentage is presented by Portland cement mass.  A.A. Ramezanianpour et al./Construction and Building Materials 49 (2013) 807–813  809  M  ð t  Þ¼ X t  0 ð T  a  T  0 Þ D t   ð 1 Þ where  M  ( t  ) is the temperature–time factor at age ( t  ), degree-days or degree-hour; D t  is a time interval, days or hours;  T  a  is average concrete temperature during timeinterval,   C;  T  0  is datum temperature,   C. It is commonly assumed equal to   10   C[24]. Capillary absorption was carried out at 28 days age. The capillary absorptionrate of specimens were calculated using their weight after 48 h being in 110   C asinitial weight, and the measured weight after 72 h of being partly in contact withthe water (on side exposed to water at 5 mm depth) [25].Electrical resistivity is one of the intrinsic specifications of concrete which canbe related to its permeability. In 1915, Wenner [26] presented a practical method tomeasure the earth electrical surface resistivity by means of a four probes apparatuswhich for the first time, has been standardized to use on concrete in 2004 [27].Since it is a non-destructive, rapid, low-cost, and reliable method, it is a wide-ac-cepted technique to investigate durability properties of concrete. Electrical resistiv-ity refers to the resistance that any electrical charge experiences while passingthrough the concrete. The increased electrical resistivity of concrete impedes themovement of electrons from the anodic to the cathode regions, and consequentlydelays the propagation of the corrosion process[28]. As presented in Table 6, FM 5-578 test method [27] defines chloride ion permeability ratings according to thesurface resistivity test results.The electrical resistivity meter (Fig. 3) was used to measure the surface resistiv-ity of specimens at the ages of 1, 7, and 28 days age. Three saturated 100  200 mmcylinders were tested at each age. The test was carried out by the four-point Wen-ner array probe technique [29]. 3. Results and discussion  3.1. Compressive strength and maturity index The compressive strength test was conducted on 100 mm cubespecimens immediately after steam curing and at the ages of 7 and28 days. The results are presented in Figs. 4–6.As observed in Fig. 4, increase in precuring period has led tolower immediate compressive strength values. For instance, con-sidering the total cycle time of 8 h, 2 h increase in delay time hasbrought about 5 MPa decrease in immediate strength of referenceSCC mixture. Furthermore, as expected, increase in temperatureand total cycle time (which means higher energy and time con-sumption) led to higher immediate compressive strength. This isdue to the accelerated hydration reactions and rapid formation of Calcium–Silica–Hydrate, C–S–H gel, the most important bindphase in hardened concrete [30] in the presence of moisture andhigh temperature.Also, the average ratio of initial compressive strength to 28-daycompressive strength is calculated as 39%, 46% and 53% at the max-imum temperatures of 50   C, 60   C and 70   C, respectively. By tak-ing 28-day results into account, it proves that increasing themaximum cycle temperature has negative effect on compressivestrength of concrete at later ages, while it improves the immediatestrength after curing.Figs. 5 and 6 illustrate the strength measurements for the spec-imens exposed to cycles with maximum temperatures of 60   C and70   C, respectively. The same immediate strength pattern as for50   C cycles is observed for these cycles, too. Considering thestrength development until the 7 days age, it is observed that in-crease in total cycle time reduces the strength development during Fig. 2.  Automatic climate simulator devices used for steam curing of specimens.  Table 6 Permeability classes based on surface resistivity by FM 5-578[27]. Chloride ion permeability Surface resistivity (k X cm)High <12Moderate 12–21Low 21–37Very low 37–254Negligible >254 Fig. 3.  Electrical resistivity meter. Fig. 4.  Relative compressive strength values ofthe cycles with 50   Cpeak temper-ature (Mean values are presented in MPa).810  A.A. Ramezanianpour et al./Construction and Building Materials 49 (2013) 807–813

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Jul 23, 2017
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