Chloride-induced corrosion of steel reinforcement is one of the signiﬁcant factors for the durability of the reinforced concrete structures exposed to marine environments. Reinforced concrete is a multiphasecomposite material composed of mortar, coarse aggregate, and rebar. The coarse aggregate and rebarcan both affect the chloride diffusion characteristics of concrete. Nevertheless, the current researches related to the effects of the aforementioned two impact factors on chloride diffusion in concrete either considered the coarse aggregate only, or taken into account the steel bar separately. The coupling effectsof coarse aggregate and steel reinforcement on chloride transport in concrete required to be further explored and discussed. In this paper, an in-door physical experiment for chloride diffusion in plain and reinforced concrete specimens exposed to marine tidal environment was carried out. The specimenswere cast using different volume fractions of coarse aggregate and diameters of steel reinforcement. The chloride concentrations for experimental specimens were measured at various exposure times, and these measurements were adopted to further investigate the common inﬂuences of coarse aggregate volumefraction and blocking effect of rebar on chloride diffusion in concrete. Using the impact factors of coarseaggregate volume fraction f( v), the direct and indirect blocking effect coefﬁcients of rebar a(v),br(U/c)t o improve the analytical solution of Fick’s second law, a prediction model of chloride diffusion in concrete considering the coupling effects of coarse aggregate and steel reinforcement was proposed. The accuracyof this proposed prediction model was validated by comparing the chloride concentrations evaluated by the prediction model with the experimental measurements. The ﬁndings are expected to be useful in realistically predicting the durability of reinforced concrete structures. /C2112019 Elsevier Ltd. All rights reserved. 1. Introduction Chloride ions ingress in concrete is one of the most signiﬁcant factors for reinforced concrete structures when exposed to marineenvironment, and this can cause the steel reinforcement corrosion, ultimately, shorten the durability and serviceability of reinforced concrete structures [1]. Therefore, it is essential to explore the transport behaviors of chloride ions in concrete covers and develop the useful chloride diffusion model in realistically predicting thedurability of concrete structures [2]. Reinforced concrete is a multiphase composite material com- posed of mortar, coarse aggregate, and steel bar [3]. The coarse https://doi.org/10.1016/j.conbuildmat.2019.04.221 0950-0618/ /C2112019 Elsevier Ltd. All rights reserved.⇑Corresponding author. E-mail addresses: yzwang@tju.edu.cn (Y. Wang), gongxlomartin@163.com (X. Gong).Construction and Building Materials 216 (2019) 40–57 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www. elsevier.com/loc ate/conbuildmat aggregate is regarded as one of the vital components of concrete and they can signiﬁcantly affect the chloride diffusion characteris- tics in concrete [4]. Some of the efforts devoted to study the effects of coarse aggregate on chloride migration in concrete. Yang and Cho [5]used the non-steady accelerate chloride migration test (ACMT) to inquire into the effect of lateral surface area of the reg- ular cylindrical coarse aggregate on chloride diffusion coefﬁcient of concrete specimens, and the chloride diffusion coefﬁcients of inter- facial transition zone (ITZ) were indirectly obtained. Yang [6]con- ﬁrmed the volume fraction of coarse aggregate has signiﬁcant inﬂuence on chloride diffusion coefﬁcient through the ACMT. Shah et al. [7]proposed that the essential effects of coarse aggregate on chloride diffusion coefﬁcient for concrete included dilution, tortu- osity, and ITZ effects. Abyaneh et al. [8]developed a three- dimensional meso-scopic numerical model of concrete to investi-gate the effects of w/c; degree of hydration; volume fraction, size, shape and orientation of aggregate; width and percolation of ITZ on chloride diffusion coefﬁcient for concrete using the ﬁnite element method (FEM). Li et al. [9]built three-dimensional concrete numerical model by Monte Carlo method and used the FEM to investigate the effects of shape, grading and volume fraction of aggregates; the thickness and permeability of ITZ on the effective permeability of chloride in concrete models. Wang et al. [10] pro- posed a time-and-depth-dependent chloride diffusion coefﬁcient model (T-D model) for concrete considering the inﬂuences of coarse aggregate volume fraction (CAVF) and maximum size of aggregate (MSA). The T-D model can elaborate the variation char- acteristics for chloride diffusion coefﬁcient of concrete with expo- sure time and diffusion depth varying. Wang et al. [11] investigated the effects of the heterogeneity of concrete materials (consistent with the inﬂuence of coarse aggregate) on long-term chloride diffusion in concrete using the meso-scopic FEM, and an empirical model for predicting long-term chloride proﬁles in con- crete considering the various volume fractions of coarse aggregate was elaborated. Wang et al. [12] explored the effects of volume fraction and maximum size of natural coarse aggregate on chloride proﬁles and diffusion characteristics for concrete under drying- wetting cycles. A time-dependent model of surface chloride con- centration and a multifactor model of chloride diffusion coefﬁcient for concrete considering the exposure time, volume fraction and maximum size of natural coarse aggregate were developed. Moreover, the blocking effects of steel reinforcement on chlo- ride ions transport in reinforced concrete cannot be neglected as well. Actually, the accumulation phenomenon of chloride ions at the apex of rebar was reported by Hansen and Saouma [13] in 1999. Kranc and Sagues [14] pointed out that the blocking effect of rebar can enhance the chloride concentrations at the apex of steel reinforcement by numerical analysis, and they proposed theattenuation coefﬁcient for chloride concentration due to the steel bar existence to explore the earlier initiation time of steel corro- sion. Oh and Jang [15] used the FEM to estimate the chloride trans- port behaviors in reinforced concrete. They concluded that the larger diameter of rebar can result in greater accumulation of chlo- ride concentrations at the apex of steel bar, as well as the initiation time of steel corrosion can shorten about 30–40% due to the rein- forcement existence. The aforementioned literatures presented a convinced argument for the chloride accumulation at the apex of steel bar and a serious worry about the overestimating the service life of reinforced concrete structures, whereas few physical exper- iments for this problem were carried out. In this context, Wang et al. [16] tested the chloride concentrations at the apex of steel bar embedded in reinforced concrete members under the experi- mental drying-wetting cycles. The authors found the phenomenon of chloride accumulation in front of rebar, as well as the larger diameter of steel bars would result in greater chloride measure- ments. The direct and indirect blocking effects of the steel rein-forcement were deﬁned to reasonably describe the physical obstruction and the chloride diffusion coefﬁcient variation due to the existence of cylindrical bar with various diameters. Despite of all these laudable achievements during the previous investigations, however, the existing researches related to the effects of the coarse aggregate and steel reinforcement on chloride diffusion either considered the coarse aggregate volume fraction (or coarse aggregate content) only [5–12,17] , or taken into account the blocking effect of steel bar separately [13–16] . The coupling effects of coarse aggregate and steel reinforcement on chloride dif- fusion in concrete required to be deeply researched and explored. In this paper, an in-door physical experiment for chloride diffusion in plain and reinforced concrete specimens exposed to marine tidal environment was carried out, and the specimens were cast using different volume fractions of coarse aggregate and diameters ofsteel reinforcement. The chloride concentrations for experimental specimens were measured at various exposure times, and they were used to investigate the common inﬂuence of volume fraction of coarse aggregate and blocking effect of rebar on chloride diffu- sion in concrete. Using the impact factors of coarse aggregate vol- ume fraction, the direct and indirect blocking effect coefﬁcients of steel bar to improve the analytical solution of Fick’s second law, a prediction model of chloride diffusion in concrete considering the coupling effects of coarse aggregate and steel reinforcement was proposed. The accuracy of this proposed prediction model was val- idated through comparing the chloride concentrations evaluated by the prediction model with the experimental measurements. 2. Experimental studies 2.1. Materials and mixture proportions During the experiment, the Ordinary Portland cement (P.O. 42.5) with the density of 3100 kg/m 3, which was produced by Tian- jin Cement Plant, was used as the cementitious material for differ- ent experimental specimens. River sand with continuous grading, which can be considered as no initial chloride ions, was used as the ﬁne aggregate, as well as the apparent density and ﬁneness modulus for sand were 2610 kg/m3and 2.7, respectively. Natural crushed limestone with continuous grading was used as the coarse aggregate, in which its nominal size range and apparent density were 5–20 mm and 2690 kg/m3. Distilled water was utilized dur- ing the process of experimental specimens casting, curing, and testing in order to avoid the interference of additional chloride ions within the raw materials on the ﬁnal experimental measurements. To explore the coupling effects of coarse aggregate volume frac- tions and diameters of steel bar on chloride diffusion characteris-tics of concrete, the mixture proportions for concrete specimens (100 /C2100/C2400 mm 3) were designed by the standard [18]. The volume fractions of coarse aggregate v= 0, 0.2, 0.3, 0.4, and 0.5, and the details of mix proportions were listed in Table 1 . The con- crete can be generally simpliﬁed as a heterogeneous composite material consisting of three-phases, namely, cement paste, aggre- gate, and the interfacial transition zone (ITZ) at the meso-scale per- spective [8]. In fact, the mortar can be also treated as a composite material consisting of cement paste, ﬁne aggregate, and ITZ. How- ever, the mortar is generally considered as a homogeneous chloride transport medium in concrete because of the smaller sizes of gel pores, capillary pores, and ﬁne aggregates than those of coarse aggregate [19]. Therefore, the effect of aggregate on chloride diffu- sion in concrete is signiﬁcantly attributed to the coarse aggregate. For this paper’s study, the inﬂuence of ﬁne aggregate on chloride diffusivity can be reasonably neglected. The hot-rolled plain round steel bars (HPB300) with diameters of U= 0, 8, 12, 16, and 20 mm were used and embedded in experimental specimens. The detailY. Wang et al. / Construction and Building Materials 216 (2019) 40–57 41 information of the steel bar properties were provided in Table 2 . The experimental specimens with U= 0 mm means there is no steel bar installed in concrete, and these types were named as the plain concrete specimens. Also, the others with U–0m m were called as the reinforced concrete specimens. To sum up, dur- ing this paper’s experiment, the numbers of coarse aggregate vol- ume fractions and steel bar diameters are both 5, namely, the total experimental groups are equal to 25, as exhibited in Table 3 . 2.2. Casting, curing and preparation for specimens Plain and reinforced concrete specimens, all measuring 100/C2100/C2400 mm3sizes and considering the concrete cover of 20 mm for reinforced concrete, were cast in plastic molds, subse- quently, compacted on a vibrating table. After casting for 24 h kept temperature of 20 ± 3 /C176C and over 90% relative humidity, experi- mental specimens were demoded and moved to water saturated with Ca(OH) 2at 20 ± 3 /C176C for 28 days curing. Then, all the speci- mens were sealed on the ﬁve sides using epoxy polyurethane based coating to expose only one side (100 /C2400 mm2) to make sure chloride ions would diffuse only in one-dimension. For this paper’s study, the slump values ( Sv) of three mixture proportions of concrete with v= 0.3, 0.4, and 0.5 were measured to evaluate the effect of coarse aggregate on the workability and rheological properties of fresh concrete. They were determined as Sv(v= 0.3) = 131 mm, Sv(v= 0.4) = 123 mm, and Sv(v= 0.5) = 115 mm per the procedure of GB 50164-2011 [20]. An increase in design volume fraction of coarse aggregate from v= 0.3 to 0.5 resulted in a lower slump, and the slump values decreased from 131 to 115 mm, representing a 13.9 % reduction, when the coarse aggregate volume fraction was increased from 0.3 to 0.5. This is due to lower cement paste content in concrete mixes, providing less lubrication for the coarse aggregates and reduced workability. In addition, the compressive strength results of 28 days ( fc) for the aforementioned three uniform mixture proportions of concrete are tested based on three specimens and their average results are fc(v= 0.3) = 38.3 MPa, fc(v= 0.4) = 46.4 MPa and fc(v= 0.5) = 53.2 MPa. The 28 days compressive strength increased with the volume fraction of coarse aggregate increasing. This indicated thatthe increased coarse aggregate volume fractions led to compressive strength of concrete improvement, which is consistent with the reality. 2.3. Exposure environment To simulate the marine environment in diurnal tidal zone, the artiﬁcial marine tidal environment automatic simulation device, which developed by the research team of Harbour Engineering Structure Laboratory in Tianjin University, is used to realistically simulate the real-time tidal cycles. This device contained a series of components including the test tank, storage tank (water level indicator), ﬂow rate control system (ﬂow meter, water pump, and solenoid value, etc.), air-blast system, temperature control sys- tem, humidity control system, spray system (sprayer, sprinkler), operating system, etc., which connected with each others based on the programmable logic controller (PLC), shown in Fig. 1 . Com- paring to the totally wet and totally dry conditions in traditional wetting-drying cycles [10,12,16,17] , this device can automatically, effectively and realistically simulate the real-time rising and ebb tide cycling simulation during the long-term test periods, which is more approximated to the true circumstance. Some signiﬁcant controlling parameters, for instance the number of tidal cycles, the high- and low-water level values, the temperature and relative humidity in test tank, etc., can be set through the parameters input interface of operating system, and the automatic simulation device would complete the requested action by receiving the command from the programs compiled into the PLC controller. This device can not only control the water level to go up and down gradually in marine tidal zone but also adjust the temperature, relative humidity, and other ocean environmental impact factors, which realized the test automation and greatly improved the experimen- tal efﬁciency. 3.5% sodium chloride solution was conﬁgured and used as the seawater, and all experimental specimens were verti- cally placed on a corrosion-resistant table (200 mm high) located in test box of this device for chloride ions diffusion. In terms of the geometric sizes of the specimens, the high and low tidal levelswere set as 600 ± 20 mm and 200 ± 20 mm respectively. The tidal cycle period was 24 hours (one day), including 12 hours for risingTable 1 Mixtures details for concrete specimens with various volume fractions of coarse aggregate. w/c Cement (kg/m3) Water (kg/m3) Fine aggregate (kg/m3) Coarse aggregate (kg/m3) volume fraction of coarse aggregate, v 0.54 345 639 1173 0 0 277 512 932 538 0.2242 448 816 807 0.3208 384 699 1076 0.4173 320 583 1345 0.5 Table 2 Properties of steel bar. Type Eigenvalue dr(mm) dr(m)(mm) Weight (kg/m) Rel(MPa) Rm(MPa) A(%) Agt(%) HPB300 Max 8 8.24 0.416 390 515 39.5 22.5 12 12.18 0.91916 16.32 1.613 20 20.24 2.574 Min 8 7.92 0.385 335 460 31.5 16 12 11.86 0.84116 15.76 1.52620 19.82 2.430 Average 8 8.08 0.401 362.5 487.5 35.5 19.3 12 12.02 0.88016 16.04 1.570 20 20.03 2.502 d rand dr(m)denote the theoretical diameter and the measured diameter of rebar; Reland Rmmean the yield strength and the tensile strength of rebar, respectively; Aand Agt are the fracture elongation and the elongation at maximum load, respectively.42 Y. Wang et al. / Construction and Building Materials 216 (2019) 40–57 tide and other 12 hours for ebb tide. The exposure times for exper- imental specimens of U= 0 mm (no rebar existence) were 30, 70, 100, 140, and 180 consecutive days, as well as for the specimens ofU= 8, 12, 16, and 20 mm were 100, 140, and 180 consecutive days, respectively. 2.4. Samples and testing The real-time tidal cycles can signiﬁcantly affect the chloride ions distributions along the elevation direction of concrete struc- tures. According to an in-site test for a certain wharf concrete structure, the tested chloride concentration results decreased with the structural elevation increasing, and the most unfavorable chlo- ride concentrations were distributed at the areas about 1/4–1/2 elevation from the bottom of the structure (the midway down of the wharf structure) [21]. On the basis of the aforementioned sum- mary, after each required chloride exposure time completed, the related experimental specimens were taken out of the device’s test box and were averagely divided into four parts numbered ①–④ from the bottom to the top, and the part ②was used as the repre- sentative to drill the concrete powders, as shown in Fig. 2 . To make sure the test accuracy, the concrete cubes of part ②were again divided into two parts (see in Fig. 2 ), and the powders for the two parts in any of the same holes were respectively drilled and obtained. The chloride concentrations were measured using the two powders which obtained in same sample holes, and the aver- age values of the two were treated as the ﬁnal chloride ion results in this hole position. In addition, all the powders obtained were dried for 2 h in the drying box (105 ± 5 /C176C) after being sieved through the sieve with a 0.63 mm aperture, subsequently, they were separately tested to determine the water-soluble chloride concentration (free chloride) at each depth by the standard [22]. 3. Results analysis and model establishment 3.1. Tested results (1) Chloride proﬁles in plain concrete specimens ( U= 0 mm)For the plain concrete specimens (denotes the diameter of steel barU= 0 mm), the tested chloride proﬁles against diffusion depths in concrete specimens with various volume fractions of coarse aggregate at different exposure times are exhibited in Fig. 3 . The free chloride concentration results shown in Fig. 3 and the follow- ing ﬁgures for this paper’s study are all presented as a percentage of the concrete mass (%). The chloride concentrations in concrete specimens with various coarse aggregate volume fractions, as expected, increase with the exposed time increasing, whereas decrease with the diffusion depth increasing. These variation trends for chloride proﬁles are consistence with the results reported in literatures [5,11,12,16,17] . (2) Chloride concentration distributions in reinforced concrete specimens ( U–0 mm) The chloride concentration distributions within the reinforced concrete specimens with different diameters of U= 8, 12, 16, and 20 mm are all measured during the experiment procedure. A rep- resentative, in which the volume fraction of coarse aggregate, diameter of rebar and exposure time are v= 0.2, U= 12 mm and t= 180 d respectively, is used to exhibit the tested chloride concen- tration isolines, as shown in Fig. 4 . The data involved in Fig. 3 pre- sent the chloride concentration values located on various isolines. Fig. 4 illustrates that the phenomenon of chloride accumulation is occurred at the apex of rebar, and the impact region is about 3– 5 mm from the apex of rebar to the exposure surface of reinforced concrete specimen. Moreover, the chloride concentrations are tended to be consistence in a same diffusion depth (sample holes in an uniform xcoordinate with various ycoordinates) with the distance from the apex of rebar increasing. Thus, it means that the steel reinforcement embedded in concrete can signiﬁcantly affect the chloride ion concentration distributions around the rebar. 3.2. Prediction model of chloride proﬁles considering the effect of coarse aggregate When the reinforced concrete structures are exposed to the marine aggressive environments, the chloride ions can ingress inTable 3 Codes of plain and reinforced concrete specimens. Code Diameters of rebar, U(mm) 0 8 12 16 20 Volume fractions of coarse aggregate, v 0 U0-0 U8-0 U12-0 U16-0 U20-0 0.2 U0-0.2 U8-0.2 U12-0.2 U16-0.2 U20-0.2 0.3 U0-0.3 U8-0.3 U12-0.3 U16-0.3 U20-0.3 0.4 U0-0.4 U8-0.4 U12-0.4 U16-0.4 U20-0.4 0.5 U0-0.5 U8-0.5 U12-0.5 U16-0.5 U20-0.5 Fig. 1. Artiﬁcial marine environment automatic simulation device.Y. Wang et al. / Construction and Building Materials 216 (2019) 40–57 43 concrete through multiple mechanisms, such as diffusion, convec- tion, adsorption, permeation and surface deposit of airborne salts.However, among these reasons, the main transport mechanism for chloride ions in concrete is diffusion [23,24] , which can be mod- eled using Fick’s second law of diffusion: @C @t¼@C @xD/C1@C @x/C18/C19 ð1Þ where Cdenotes chloride concentration (%); Dmeans chloride dif- fusion coefﬁcient of concrete (m2/s); tis exposure time (s); x denotes the diffusion depth (m). According to a large number of investigations, the boundary condition of C=Cs(for x=0 , t/C210) and the chloride diffusion coefﬁcient Dshould be both considered as time-dependent parameters resulting in Cs=Cs(t) and D=D(t) [25], where Csmeans the surface chloride concentration (%). Formu- las of Cs(t) can be expressed as linear, square-root, power, exponent, and logarithmic functions [11]. Also, the time-dependency of chlo- ride diffusion coefﬁcient D(t) is more appropriately modeled based on a power function proposed by Thomas and Bamforth [26] as follows: DðtÞ¼D28t28 t/C18/C19m ð2Þ where D(t) means the chloride diffusion coefﬁcient at the exposure time t (m2/s);D28denotes the chloride diffusion coefﬁcient at the exposure time t=2 8d( m2/s), and D28can be also deﬁned as the ref- erence chloride diffusion coefﬁcient; mis the aging factor. Substi- tuting the Eq. (2)and Cs=Cs(t) into Eq. (1)and considering the actual initial condition C=C0(for x>0 , t= 0) (where C0denotes the initial chloride concentration of concrete before exposure to the marine environment (%)), the solution of Eq. (1)consideringthe time-dependency of the chloride diffusion coefﬁcient and the surface chloride concentration is determined as: Cðx;tÞ¼CsðtÞ1/C0erfx 2ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ DaðtÞ/C1tp/C18/C19 /C20/C21 DaðtÞ¼D28 1/C0mt28 t/C0/C1 m8 >< >:ð3Þ where Dadenotes the apparent chloride diffusion coefﬁcient (m2/s). Dais also a time-dependent parameter, Da=Da(t)[27–30] , and the expression form of Da(t) can be based on the