Influence of Reinforcement Bars on Concrete Pore Structure And

materials

Article Inﬂuence of Reinforcement Bars on Concrete Pore Structure and Compressive Strength

Jianmin Hua 1,2, Fengbin Zhou 1, Lepeng Huang 1,*, Zengshun Chen 1, Yemeng Xu 1 and Zhuolin Xie 1

1 School of Civil Engineering, Chongqing University, Chongqing 400045, China; [email protected] (J.H.); [email protected] (F.Z.); [email protected] (Z.C.); [email protected] (Y.X.); [email protected] (Z.X.) 2 Key Laboratory of New Technology for Construction of Cities in Mountain Area (Chongqing University), Ministry of Education, Chongqing 400045, China * Correspondence: [email protected]; Tel.: +86-023-65127725

Received: 3 January 2020; Accepted: 29 January 2020; Published: 1 February 2020

Abstract: In this research, the inﬂuence of reinforcement bars on concrete pore structure and compressive strength was experimentally investigated. Concrete samples with two mixture ratios and nine reinforcement ratios were provided. Tests were conducted on concrete pore structure and compressive strength at three ages (3 d, 7 d, and 28 d). It was found that reinforcement bars changed the concrete pore structure. In terms of size, the pore structure of concrete increased with the increase of reinforcement ratio. At the same age, concrete compressive strength in reinforced concrete specimens saw a gradual reduction when reinforcement ratio increased. A formula was proposed to calculate the compressive strength of concrete in reinforced specimens according to the strength of unreinforced concrete.

Keywords: reinforcement bar; pore structure; concrete strength; strength prediction

1. Introduction Concrete is a cement-based material, whose strength is affected by pores and microcracks, especially under the action of thermal loads [1–5]. Extensive research was carried out to probe into the relationship between concrete pore structure and compressive strength. Li et al. [6] studied the connection between concrete strength and pore structure by introducing an influencing coefficient of pore size. Huang et al. [7] established an expression to calculate concrete compressive strength according to the relative specific surface area of pore structure. Lian et al. [8] investigated the association between concrete pore structure and compressive strength. Gao et al. [9] prepared defective specimens by adding air-entraining agents to concrete, and studied the influence of pore structure on concrete strength. It was generally found that concrete strength was higher for a denser pore structure [6–12]. In engineering practices, concrete is often combined with reinforcement bars to form a reinforced concrete structure. The existing studies showed that there are interfacial transition zones (ITZ) in reinforced concrete structures between reinforcement bar and concrete. The porosity of concrete in ITZ area is higher than that in non ITZ area [13], thereby leading to changes in a series of properties including concrete strength [14–16], especially beneath the horizontal reinforcement and in some places where the reinforcement ratio is high and the concrete cannot be well vibrated. In these areas, the porosity of ITZ is further improved, which further degrades the property of concrete [15,17,18]. Meanwhile, when water is gradually consumed by cement hydration and dries in concrete, the capillary stress generated on concrete pores continuously decreases the spacing between concrete particles and causes the volume of concrete to decrease on a macro scale (this phenomenon is called shrinkage).

Materials 2020, 13, 658; doi:10.3390/ma13030658 www.mdpi.com/journal/materials Materials 2020, 13, 658 2 of 12

Because of the restraint effects of reinforcement bars, restraint stress is generated in the opposite direction of shrinkage inside concrete and decreases concrete shrinkage. Compared with plain concrete, reinforced concrete with a reinforcement ratio of 1 to 7% saw a drop of 30 to 80% in shrinkage after 28 d [19–24]. The pore structure of concrete will be changed by the restraints in its deformation. Previous studies [24,25] revealed that concrete with lateral restraints experienced a significant decline in the values of different pore structure parameters (porosity and average pore size) than that without lateral restraints. However, the effect of restricting the reduction of concrete volume (shrinkage for example) on concrete pore structure remains unknown. Based on the previous findings about the connection between concrete strength and porosity and changes in concrete pore structure by restraints, it could be reasonably inferred that the pore structure of reinforced concrete would be greater than that of unreinforced concrete in the shrinkage process of concrete because of the restraint effect of reinforcement, which caused the strength of reinforced concrete to be lower than that of unreinforced concrete. However, this reference remains to be confirmed by research. In the early phase of construction, it is necessary to carry loads by scaffolding because of low concrete strength. According to Chinese standards [26], the removal of scaffolding during construction depends on concrete compressive strength. Scaffolding is allowed to be removed when the concrete compressive strength of general components and other important components reaches 75 and 100% of design strength respectively. Because of several limitations in engineering practices, the decision of whether concrete compressive strength meets requirements is made by testing concrete blocks that are not restrained by reinforcement bars [26]. Therefore, consideration is not given to the possible influence of reinforcement bars on concrete strength and pore structure. The existing evaluation methods of concrete strength at the construction site may increase the possibility of failure in building structure during construction if research suggested that reinforcement bars would change concrete pore structure and then reduce concrete compressive strength during the shrinkage of concrete. This research experimentally investigated the influence of reinforcement bars on concrete pore structure and compressive strength. Concrete samples with two mixture ratios and nine reinforcement ratios (0–6.56%) were given. At different ages (3 d, 7 d, and 28 d), the mercury intrusion method was adopted to test the pore structure of concrete samples with two mixture ratios and a reinforcement ratio of 0%, 1.14%, 3.24%, and 6.56%. Moreover, the compressive strength of all types of samples was also tested. All specimens were well vibrated before testing. A comparison was made between concrete pore structure and compressive strength with different reinforcement ratios to study the changes in concrete pore structure and compressive strength after reinforcement.

2. Experiments

2.1. Materials Two different concrete mixing ratios were applied in this study (Table1). Cement and fly ash were chosen as cementing materials, whose chemical and physical properties are shown in Table2. Quartz sand was used as fine aggregate, with a specific gravity of 2.8g/cm3, a fineness modulus of 3.0, and a particle size distribution of 0.2–4 mm. Crushed limestone was used as coarse aggregate with a specific gravity of 2.8 g/cm3 and a particle size range of 5–20 mm. The superplasticizer used is polycarboxylate superplasticizer with a water-reducing rate of 30–35%. The information of reinforcement bars used in this study is shown in Table3.

Table 1. Mixing proportion (relative weight ratios to cement).

Mix Cement Water Fly Ash Sand Coarse Aggregate Superplasticizer C30 1.00 0.5 0.26 2.36 2.71 0.03 C60 1.00 0.34 0.26 1.64 2.46 0.03 Materials 2020, 13, 658 3 of 12 Materials 2020, 13, x FOR PEER REVIEW 3 of 13

Table 2. ChemicalComposition and physical %(mass) information Cement of cement Fly Ash and ﬂy ash. CompositionSiO %2 (mass)22.07 Cement 50.22 Fly Ash CaO 64.74 4.52 SiO2 22.07 50.22 CaOAl2O3 5.44 64.74 21.33 4.52 2 3 AlFe2OO3 4.185.44 14.01 21.33 FeMgO2O3 1.394.18 1.14 14.01 MgOSO3 0.54 1.39 1.39 1.14 Specific surfaceSO3 (cm2/g) 34650.54 4598 1.39 Speciﬁc surface (cm2/g) 6465 4598 Density (g/cm3) 3.13 2.21 Density (g/cm3) 3.13 2.21

Table 3. Properties of reinforcement bar. Table 3. Properties of reinforcement bar. Yield ElasticElastic ModulusModulus Yield Strength UltimateUltimate Strength Strength RestraintRestraint 4 Strength ((×1010 4 MPa)MPa) (MPa) (MPa)(MPa) × (MPa) Reinforcement bar 20.6 400 580 Reinforcement bar 20.6 400 580

2.2. Specimens for Concrete Strength Tests Produced by a Teflon mold, specimens whose dimension is 200 mm 200 mm 500 mm were Produced by a Teflon mold, specimens whose dimension is 200 mm ×× 200 mm ×× 500 mm were adopted in the present study (as shownshown in FigureFigure1 1)).. TheThe fourfour sidessides ofof thethe moldmold werewereremoved removedafter after initialinitial concreteconcrete settingsetting (in green in FigureFigure1 1),), andandits its bottom bottom waswas laidlaid withwith twotwo layerslayers ofof 1-mm1-mm thickthick TeflonTeflon filmfilm (in(in yellowyellow inin FigureFigure1 ).1). WithWith thesethese measures, measures, concreteconcrete specimensspecimens werewere onlyonly subjectsubject toto reinforcement bars.bars.

Figure 1. Teﬂon mold for forming specimens (All dimensions are in mm). Figure 1. Teflon mold for forming specimens (All dimensions are in mm). Eighteen kinds of concrete specimens that owned two mixture ratios and nine reinforcement ratiosEighteen (% = 0%, kinds 1.14%, of 1.56%, concrete 2.05%, specimens 2.61%, 3.24%, that ow 3.95%,ned two 5.16%, mixture and 6.56%) ratios wereand nine used reinforcement in the present investigationratios (ρ = 0%, (Table 1.14%,4). 1.56%, Reinforcement 2.05%, 2.61%, bars 3.24%, of di ﬀ3.95%,erent diameters5.16%, andwere 6.56%) evenly were putused in in four the cornerspresent ofinvestigation concrete cross-section (Table 4). Reinforcement (Figure2). For bars each of type,differ threeent diameters specimens were were evenly prepared put in to four test corners concrete of compressiveconcrete cross-section strength at (Figure three ages. 2). For All each specimens type, three were curedspecimens and tested were inprepared a constant to environmenttest concrete (20 2 C, 65 5% RH). compressive± ◦ strength± at three ages. All specimens were cured and tested in a constant environment (20 ±After 2 °C, 365 d, ± 7 5% d, andRH). 28 d, three cylindrical samples whose diameter is 100 mm and height is 200 mm (heightAfter/diameter 3 d, 7 d,= 2)and were 28 d, drilled three fromcylindrical each type samples of specimen whose diameter to measure is 100 the mm concrete and compressiveheight is 200 strengthmm (height/diameter (according to the= JGJ2) /wereT384 standarddrilled from [27]) (Figureeach type2). Theof testspecimen method to of measure concrete strengththe concrete was adoptedcompressive according strength to Chinese(according national to the JGJ/T384 standard standa GB/T50081rd [27]) [28 (Figure]. 2). The test method of concrete strength was adopted according to Chinese national standard GB/T50081 [28].

Table 4. Information of specimens. No. Strength Reinforcement Ratio (%) Reinforcement Bars Diameter (mm) C30-R0 0 - C30 C30-R1 1.14 12

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C30-R2 1.56 14 C30-R3 2.05 16 C30-R4 2.61 18 C30-R5 3.24 20 C30-R6 3.95 22 C30-R7 5.16 25 Materials 2020, 13, 658 4 of 12 C30-R8 6.56 28 C60-R0 0 - C60-R1 Table 4.1.14Information of specimens. 12 C60-R2 1.56 14 No. Strength Reinforcement Ratio (%) Reinforcement Bars Diameter (mm) C60-R3 2.05 16 C30-R0 0 - C60-R4 C60 2.61 18 C30-R1 1.14 12 C30-R2C60-R5 3.24 1.56 20 14 C30-R3C60-R6 3.95 2.05 22 16 C30-R4C60-R7 C30 5.16 2.61 25 18 C30-R5C60-R8 6.56 3.24 28 20 C30-R6 3.95 22 C30-R7 5.16 25 2.3. TestC30-R8 Specimens for Concrete Pore Structure 6.56 28 ConcreteC60-R0 pore structure with a reinforcement0 ratio of 0%, 1.14%, 3.24%, -and 6.56% (C30-R0, C30- R1, C30-R5,C60-R1 C30-R8 and C60-R0, C60-R1, C60-R5, 1.14 C60-R8) was tested. The remaining 12 specimens after drillingC60-R2 were used as the samples for concrete 1.56 porosity tests. The concrete sa 14mples were first crushed C60-R3 2.05 16 and thenC60-R4 filtered by aC60 sieve to collect concrete 2.61 particles with a size of 2.5–5 mm. 18 Acetone was applied to preventC60-R5 concrete hydration, and a vacuum 3.24 dryer was used to dry the samples. 20 At last, concrete pore structureC60-R6 was tested by an automatic 3.95 mercury porosimeter that has an 22 aperture measurement range ofC60-R7 0.003–1000 μm, low-pressure analysis 5.16 pressure range of 0–345 kPa, 25 high-pressure analysis pressureC60-R8 range of 101.325–414000 kPa, and 6.56 a resolution ratio of 69 Pa. 28

FigureFigure 2.2. SpecimenSpecimen for for measuring measuring the the pore pore structure structure and and compressive compressive strength strength of concrete of concrete (All (Alldimensions dimensions are arein mm). in mm).

2.3.3. Experimental Test Specimens Results for Concrete and PoreDiscussion Structure Concrete pore structure with a reinforcement ratio of 0%, 1.14%, 3.24%, and 6.56% (C30-R0, C30-R1, C30-R5,3.1. Change C30-R8 of Pore and Structure C60-R0, C60-R1, C60-R5, C60-R8) was tested. The remaining specimens after drillingTable were 5 usedpresents as the the samples pore struct forure concrete parameters porosity of tests.concrete The specimens concrete samples with a reinforcement were first crushed ratio andof 0%, then 1.14%, filtered 3.24%, by a sieveand 6.56% to collect at different concrete ages, particles and withshows a sizethat of the 2.5–5 size mm. of concrete Acetone pore was structure applied togradually prevent concretedeclined hydration, with curing and age a vacuum under the dryer same was mixing used to and dry reinforcement the samples. At ratios. last, concrete For instance, pore structurethe average was and tested median by an pore automatic diameters, mercury critical porosimeter diameter, that and has porosity an aperture of C30-R0 measurement concrete rangeafter 3 of d 0.003–1000were 26.48µ nm,m, low-pressure83.65 nm, 82.73 analysis nm, and pressure 24.31% range respectively, of 0–345 which kPa, high-pressure are 8.97 nm, 57.09 analysis nm, pressure 57.1 nm, rangeand 6.66% of 101.325–414000 higher than kPa, those and after a resolution 28 d respecti ratio ofvely. 69 Pa.Moreover, the proportion of pores whose diameter is below 50 nm at 28 d was noticeably higher than that at 3 d in C30-R0 concrete. Similar 3.patterns Experimental were also Results found and in other Discussion specimens. The size of concrete pore structure gradually decreased 3.1. Change of Pore Structure Table5 presents the pore structure parameters of concrete specimens with a reinforcement ratio of 0%, 1.14%, 3.24%, and 6.56% at different ages, and shows that the size of concrete pore structure gradually declined with curing age under the same mixing and reinforcement ratios. For instance, the average and median pore diameters, critical diameter, and porosity of C30-R0 concrete after 3 d Materials 2020, 13, 658 5 of 12 were 26.48 nm, 83.65 nm, 82.73 nm, and 24.31% respectively, which are 8.97 nm, 57.09 nm, 57.1 nm, Materials 2020, 13, x FOR PEER REVIEW 7 of 13 and 6.66% higher than those after 28 d respectively. Moreover, the proportion of pores whose diameter is below 50 nm at 28 d was noticeably higher than that at 3 d in C30-R0 concrete. Similar patterns were Figure 3 displays the connection between the reinforcement ratio of concrete and different also found in other specimens. The size of concrete pore structure gradually decreased with curing parameters of pore structure (average and median pore diameters, critical diameter as well as age for two main reasons, namely the filling of pores by hydrated products and the decline of the size porosity). Notably, the size of concrete pore structure had a linear relationship with reinforcement between a variety of pore walls and microcracks in concrete because of capillary stress [29–32]. ratio. Therefore, the change of concrete pore structure caused by reinforcement bars could be It was also observed that the placement of reinforcement bars led to a significant change in concrete expressed by the following equation: pore structure whose size saw a great increase when reinforcement ratio increased. For example, the critical diameter of C60-R8 concrete withS aρreinforcement = S0 + nρ ratio of 6.56% was 44.30 nm at 28(1) d, which is higher than those of C60-R5 (% = 3.24%), C60-R1 (% = 1.14%), and C60-R0 (% = 0%) specimens where Sρ is the parameter of concrete pore structure corresponding to the reinforcement ratio ρ; S0 (36.85represents nm, 27.33the parameter nm, and 20.11of concrete nm respectively). pore structur Similare corresponding trends were to alsothe reinforcement observed in C30 ratio concrete of 0%; specimens,n is the increase other in ages, the parameters and pore structure of pore parameters.structure for each unit of increase in reinforcement ratio. FigureRegression3 displays analysis the was connection made on betweenobtained thetesting reinforcement data to get the ratio value of concrete of n (Table and 6). di Figurefferent 3 parametersexpresses the of porerelative structure errors (average between and the median pore pore structure diameters, parameters critical diameter of concrete as well calculated as porosity). by Notably,Equation the (1) size and of the concrete measured pore values. structure It hadwas a observ linear relationshiped that the calculated with reinforcement values well ratio. agreed Therefore, with thethe measured change of ones. concrete pore structure caused by reinforcement bars could be expressed by the following equation: Table 6. ParametersS ρn =forS 0different+ nρ pore parameters. (1) where Sρ is theAverage parameter Pore of concreteDiameter pore Median structure Pore corresponding Diameter Critical to the Diameter reinforcement ratio ρ; Coefficient Porosity S0 represents the parameter(nm) of concrete pore structure(nm) corresponding to the reinforcement(nm) ratio of 0%; n is the increasen in the parameters160 of pore structure for360 each unit of increase in350 reinforcement 105 ratio.

40 120 100 30 80 60 20 40 C30-3days 20 C30-3days C30-7days C30-7days 0 10 C30-28days C30-28days C60-3days C60-3days C60-7days -20 C60-7days Average pore diamete(nm) pore Average C60-28days diameter(nm) pore Median -40 C60-28days 0 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 Reinforcement ratio(%) Reinforcement ratio(%) (a) (b) 120 35 100 30 80 25 60 20 40 15 20 C30-3days C30-3days Relative error(%)

Porosity(%) 3days 7days 28days C30-7days 10 C30-7days 0 C30-R1 0.09 3.81 4.09 C30-28days 5 C30-28days C30-R5 4.97 6.28 1.63 C60-3days C60-3days C30-R8 9.62 6.2 7.29 C60-R1 0.56 4.01 3.83 -20 C60-7days C60-7days 0 C60-R5 8.18 3.2 2.8 -40 C60-28days C60-28days C60-R8 16.7 10.55 4.5

Critical diameter of capillary(nm) Critical diameter of -5 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 Reinforcement ratio(%) Reinforcement ratio(%) (c) (d)

Figure 3.3. Connection between concrete pore structurestructure andand reinforcementreinforcement ratio:ratio: (a) averageaverage porepore diameter; ((b)) medianmedian porepore diameter;diameter; ((cc)) criticalcritical diameterdiameter ofof capillary;capillary; ((dd)) porosity.porosity.

Concrete is a kind of porous material. As water is consumed by hydration and dries in concrete pores, the capillary stress generated on the pore walls of concrete results in concrete shrinkage. In plain concrete specimens (without reinforcement bars), a decline appeared in concrete pore structure because of filling by hydrated products and capillary stress, as shown in Figure 4. Restrictions of reinforcement bars in the free shrinkage of concrete would lead to the occurrence of shear stress on the interface between concrete and reinforcement bars, and generated normal stress (known as

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Table 5. Pore parameters of concrete at diﬀerent ages.

Average Pore Median Pore Critical Porosity Pore Distribution (%) NO. Diameter (nm) Diameter (nm) Diameter (nm) (%) <10 nm 10–50 nm 50–100 nm >100 nm 3 26.48 83.65 82.73 24.31 18.66 21.66 36.23 23.45 C30-R0 7 19.78 51.34 50.49 20.33 18.19 45.29 20.87 15.65 28 17.51 26.56 25.63 17.65 17.58 55.33 14.32 12.77 3 32.96 85.24 88.81 25.53 16.06 14.45 40.96 28.53 C30-R1 7 23.66 58.78 55.03 22.38 12.35 36.77 32.87 18.01 28 19.86 31.45 29.66 19.65 16.52 40.68 26.34 16.46 3 33.86 89.69 90.68 26.4 6.52 9.56 53.37 30.55 C30-R5 7 25.88 72.69 71.54 22.33 18.51 32.89 29.66 18.94 28 21.33 33.15 33.00 21.4 14.52 35.23 30.89 19.36 3 39.68 95.80 96.34 28.46 3.58 15.88 50.28 30.26 C30-R8 7 31.18 78.51 79.11 25.63 17.07 22.54 39.54 20.85 28 25.80 55.65 56.92 22.87 20.64 30.45 30.12 18.79 3 22.36 75.66 79.99 20.56 12.88 60.23 12.29 14.60 C60-R0 7 18.63 35.64 38.54 15.62 15.52 68.44 6.86 9.18 28 15.79 23.56 20.11 12.81 19.29 70.41 3.84 6.46 3 25.69 78.65 85.18 21.88 1.07 63.98 18.44 16.51 C60-R1 7 23.79 45.33 47.68 17.52 13.31 65.66 10.72 10.31 28 20.74 29.68 27.33 13.49 18.86 70.80 5.45 4.89 3 29.66 75.46 79.18 22.15 2.36 62.19 16.28 19.17 C60-R5 7 20.86 50.63 57.65 19.65 1.86 70.47 10.78 16.89 28 19.31 30.23 36.85 15.77 18.01 68.59 3.71 9.69 3 38.92 84.56 90.76 23.52 10.20 47.72 25.13 16.94 C60-R8 7 32.78 55.88 67.25 20.36 6.05 60.35 15.43 18.17 28 28.66 40.79 44.30 18.85 9.30 70.65 10.36 9.69 Materials 2020, 13, 658 7 of 12

Regression analysis was made on obtained testing data to get the value of n (Table6). Figure3 expresses the relative errors between the pore structure parameters of concrete calculated by Equation (1) and the measured values. It was observed that the calculated values well agreed with the measured ones.

Table 6. Parameters n for diﬀerent pore parameters.

Average Pore Median Pore Critical Diameter Coeﬃcient Porosity Diameter (nm) Diameter (nm) (nm) n 160 360 350 105

Materials 2020, 13, x FOR PEER REVIEW 8 of 13 Concrete is a kind of porous material. As water is consumed by hydration and dries in concrete pores,restraint the stress) capillary in stressconcrete generated in a direction on the pore opposite walls to of concretethe shrinkage results of in concrete concrete caused shrinkage. by capillary In plain concretestress [23] specimens (Figure 4). (without The equation reinforcement below can bars), be a used decline to calculate appeared restraint in concrete stress pore [23]: structure because of ﬁlling by hydrated products and capillary stress, asε shown·E in Figure4. Restrictions of reinforcement bars in the free shrinkage of concrete would lead to thesh occurrencec of shear stress on the interface σt= between concrete and reinforcement bars, and generatedE normal stress (known as restraint stress)(2) in 1+ c concrete in a direction opposite to the shrinkage of concrete𝜌E caused by capillary stress [23] (Figure4). The equation below can be used to calculate restraint stress [23s ]: where Ec and Es are the elastic moduli of concrete and reinforcement bars respectively; εsh represents εsh Ec the free shrinkage of concrete. σt = · (2) 1 + Ec From Equation (2), it can be seen that restraint stressρEs saw an increase when reinforcement ratio increased, thereby reducing concrete pore structure because of capillary stress. Therefore, the E E whereexperimentalc and resultss are the indicated elastic moduli that the of pore concrete struct andure of reinforcement concrete increased bars respectively; when reinforcementεsh represents ratio theincreased. free shrinkage of concrete.

FigureFigure 4.4. ChangesChanges inin concreteconcrete porepore structure.structure. From Equation (2), it can be seen that restraint stress saw an increase when reinforcement 3.2. Concrete Compressive Strength ratio increased, thereby reducing concrete pore structure because of capillary stress. Therefore, the experimentalTable 7 presents results concrete indicated compressive that the pore strength structure at ofdifferent concrete ages. increased It was when noticeable reinforcement that the ratiocompressive increased. strength of concrete increased with curing age for each specimen. In each group, concrete compressive strength after 3 d reached 56 to 65% of that after 28 d, whereas concrete 3.2.compressive Concrete Compressive strength after Strength 7 d reached 79 to 91% of that after 28 d. Obviously, concrete strength was characterizedTable7 presents by a rapid concrete increase compressive in the early strength age (3–7 at d) di followedﬀerent ages. by a gradual It was noticeabledecrease in that growth the compressiverate. strength of concrete increased with curing age for each specimen. In each group, concrete compressiveHowever, strength compressive after 3 dstrength reached gradually 56 to 65% decreased of that after when 28 d, reinforcement whereas concrete ratio compressive increase in strengthspecimens, after which 7 d reached are the 79 same to 91% in strength of that after but 28diff d.erent Obviously, in reinforcement concrete strength ratio. For was specimens characterized with bythe a same rapid strength increase grade, in the earlythe highest age (3–7 compressive d) followed strength by a gradual was always decrease obtained in growth in specimens rate. whose reinforcement ratio is 0%. For example, the compressive strength of C30-R8 (ρ = 6.56%) at 28 d was 24.6 MPa, which is lower than that of C30-R7 (ρ = 5.16%), C30-R6 (ρ = 3.95%), C30-R5 (ρ = 3.24%), C30- R4 (ρ = 2.61%), C30-R3 (ρ = 2.05%), C30-R2 (ρ = 1.56%), C30-R1 (ρ = 1.14%), and C30-R0 (ρ = 0%) (25.2, 26.6, 27.5, 28.4, 30.3, 31.5, 33.2, and 34.5 MPa respectively).

Table 7. Test results of concrete strength (MPa).

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Table 7. Test results of concrete strength (MPa).

Strength (Standard Deviation) Strength (Standard Deviation) NO. NO. 3 d 7 d 28 d 3 d 7 d 28 d C30-R0 22.1(1.05) 29.6(0.12) 34.5(1.10) C60-R0 43.3(2.05) 55.4(1.46) 66.5(2.14) C30-R1 21.3(1.20) 30.2(1.30) 33.2(0.15) C60-R1 42.2(1.20) 54.3(0.87) 64.5(0.91) C30-R2 19.6(0.25) 28.5(0.25) 31.5(1.25) C60-R2 41.4(0.20) 53.1(1.10) 63.8(1.32) C30-R3 18.2(1.15) 27.8(0.31) 30.3(0.15) C60-R3 39.6(2.20) 50.1(1.79) 63.1(0.15) C30-R4 18.9(0.95) 26.2(1.15) 28.4(1.20) C60-R4 36.7(0.35) 52.3(1.52) 63.3(0.17) C30-R5 17.3(1.20) 24.3(0.06) 27.5(1.21) C60-R5 35.3(1.15) 51.5(0.96) 62.5(1.50) C30-R6 17.0(0.21) 23.5(1.25) 26.6(0.15) C60-R6 33.8(0.40) 50.7(1.60) 57.6(0.2) C30-R7 16.5(1.30) 22.1(0.26) 25.2(1.35) C60-R7 32.5(0.32) 48.9(1.05) 55.4(1.07) C30-R8 16.2(0.20) 21.2(0.31) 24.6(0.25) C60-R8 32.4(1.05) 47.3(0.25) 53.6(0.65)

However, compressive strength gradually decreased when reinforcement ratio increase in specimens, which are the same in strength but different in reinforcement ratio. For specimens with the same strength grade, the highest compressive strength was always obtained in specimens whose reinforcement ratio is 0%. For example, the compressive strength of C30-R8 (% = 6.56%) at 28 d was 24.6 MPa, which is lower than that of C30-R7 (% = 5.16%), C30-R6 (% = 3.95%), C30-R5 (% = 3.24%), C30-R4 (% = 2.61%), C30-R3 (% = 2.05%), C30-R2 (% = 1.56%), C30-R1 (% = 1.14%), and C30-R0 (% = 0%) (25.2, 26.6, 27.5, 28.4, 30.3, 31.5, 33.2, and 34.5 MPa respectively). Figure5 presents the connection between the strength and pore parameters of specimens with a reinforcement ratio of 0%, 1.14%, 3.24%, and 6.56%. For concrete specimens of the same strength grade, concrete compressive strength decreased in the whole when pore structure parameters increased, which was consistent with the conclusion that concrete with denser pore structure would have higher strength. In restrained specimens, the existence of constrained tensile stress reduced the effect of capillary pore stress on pore structure, which caused the pore structure of restrained specimens to be larger than that of plain concrete specimens. In restrained specimens, restraint effect would be stronger and concrete pore structure would be larger when reinforcement ratio was higher. Therefore, it was observed that concrete strength decreased when reinforcement ratio increased in tests. The relationship between concrete compressive strength and pore structure (mainly porosity) could be described ideally in a linear, exponential, or polynomial form [10–12]. In Figure5d, the relationship between concrete porosity and compressive strength is fitted by a linear function, which shows that the linear fitting effect of concrete with the same strength grade is better, but the strength of concrete with different strength grades varied greatly when porosity was similar. For instance, the compressive strength of C30 specimen was 27.5 MPa when its porosity was 21.4%, while the compressive strength of C60 specimen was 40.3 MPa when its porosity was 21.88%, which was 12.8 MPa higher than that of C30. Similar phenomena were also found in studies by Li [33] and Gao [34]. Figure6 shows the comparison of pore size distribution between C30 and C60 at 3 d, 7 d, and 28 d, which demonstrated significant differences in their pore size distribution. At each age, the pore diameters of C30 and C60 were mainly distributed between 50–100 nm and 10–50 nm respectively. The porosity of concrete reflected the content of all pores, and different pore size distribution might also lead to the same or similar porosity [33,34]. Meanwhile, previous studies suggested that the compressive strength of concrete would be higher when a small-aperture pore in concrete took up a larger proportion, which thus led to the difference in the strength of C30 and C60 when porosity was similar in tests. Materials 2020, 13, x FOR PEER REVIEW 9 of 13

C30-R4 18.9(0.95) 26.2(1.15) 28.4(1.20) C60-R4 36.7(0.35) 52.3(1.52) 63.3(0.17) C30-R5 17.3(1.20) 24.3(0.06) 27.5(1.21) C60-R5 35.3(1.15) 51.5(0.96) 62.5(1.50) C30-R6 17.0(0.21) 23.5(1.25) 26.6(0.15) C60-R6 33.8(0.40) 50.7(1.60) 57.6(0.2) C30-R7 16.5(1.30) 22.1(0.26) 25.2(1.35) C60-R7 32.5(0.32) 48.9(1.05) 55.4(1.07) C30-R8 16.2(0.20) 21.2(0.31) 24.6(0.25) C60-R8 32.4(1.05) 47.3(0.25) 53.6(0.65)

Figure 5 presents the connection between the strength and pore parameters of specimens with a reinforcement ratio of 0%, 1.14%, 3.24%, and 6.56%. For concrete specimens of the same strength grade, concrete compressive strength decreased in the whole when pore structure parameters increased, which was consistent with the conclusion that concrete with denser pore structure would have higher strength. In restrained specimens, the existence of constrained tensile stress reduced the effect of capillary pore stress on pore structure, which caused the pore structure of restrained specimens to be larger than that of plain concrete specimens. In restrained specimens, restraint effect would be stronger and concrete pore structure would be larger when reinforcement ratio was higher. Therefore, it was observed that concrete strength decreased when reinforcement ratio increased in tests. The relationship between concrete compressive strength and pore structure (mainly porosity) could be described ideally in a linear, exponential, or polynomial form [10–12]. In Figure 5d, the relationship between concrete porosity and compressive strength is fitted by a linear function, which shows that the linear fitting effect of concrete with the same strength grade is better, but the strength of concrete with different strength grades varied greatly when porosity was similar. For instance, the compressive strength of C30 specimen was 27.5 MPa when its porosity was 21.4%, while the Materialscompressive2020, 13 strength, 658 of C60 specimen was 40.3 MPa when its porosity was 21.88%, which was9 of12.8 12 MPa higher than that of C30. Similar phenomena were also found in studies by Li [33] and Gao [34].

70 70 C30 C30 60 60 C60 C60 50 50

40 40

30 30

Compressive strength(MPa) Compressive 20 strength(MPa) Compressive 20 10 15 20 25 30 35 40 45 50 20 40 60 80 100 Average pore diameter (nm) Median pore diameter (nm)

(a) (b) 70 70 C30 C30 60 60 C60 C60 50 50

2 40 40 y=-2.87x+104, R =0.92

30 30 Materials 2020, 13, x FOR PEER REVIEW 10 of 13 2 Compressive strength(MPa) Compressive 20 Compressive strength(MPa) 20 y=-1.66x+63.3, R =0.98 The porosity of10 concrete 20 30 40reflected 50 60 the 70 80content 90 100 of 110 all pores, and12 14 different 16 18 pore 20 22 size 24 distribution 26 28 30 might also lead to the sameCritical or diametersimilar of porositycapillary (nm) [33,34]. Meanwhile, previousPorosity(%) studies suggested that the compressive strength of concrete(c) would be higher when a small-aperture(d) pore in concrete took up a largerFigure proportion, 5.5. Relationship which thus between led to compressive the difference strength in the and strength parameters of C30 of porepore and structure:structure:C60 when (a porosity)) averageaverage was similarpore in diameter;diameter;tests. ((bb)) medianmedian porepore diameter;diameter; ((cc)) criticalcritical diameter;diameter; ((dd)) porosity.porosity.

100 100 Figure 6 shows the comparison of pore C30-Psize distribution between C30 and C60 atC30-P 3 d, 7 d, and 28 C30-R1 C30-R1 d, which demonstrated80 significant differences C30-R3 in thei80r pore size distribution. At each C30-R3 age, the pore C30-R6 C30-R8 diameters of C30 and C60 were mainly distributed C60-P between 50–100 nm and 10–50 C60-Pnm respectively. 60 C60-R1 60 C60-R1 C60-R3 C60-R3 C60-R6 C60-R8 40 40 Proportion (%) Proportion (%) Proportion 20 20

0 0 <10 10-50 50-100 >100 <10 10-50 50-100 >100 Pore distribution(nm) Pore distribution(nm) (a) (b) 100 C30-P C30-R1 80 C30-R3 C30-R8 C60-P 60 C60-R1 C60-R3 C60-R8 40 Proportion (%) 20

0 <10 10-50 50-100 >100 Pore distribution(nm) (c)

FigureFigure 6. 6. ComparisonComparison of of pore pore size size distribution distribution between between C30 C30 specimen specimen and and C60 C60 specimen specimen at at three three ages: ages: ((aa)) 3 3 d; d; ( (bb)) 7 7 d; d; ( (cc)) 28 28 d. d.

3.3. Relationship between the Compressive Strength and Reinforcement Ratio of Concrete This study showed that reinforcement bars would reduce the decline of concrete pore structure caused by capillary stress, thereby causing reinforced specimens to be inferior to unreinforced ones in compressive strength. Concrete compressive strength gradually decreased when reinforcement ratio increased. In engineering practices, the risk of accidents may be increased if the removal of scaffolding is determined by the compressive strength of unreinforced concrete. It is difficult to directly test the pore structure parameters of concrete at the construction site. Therefore, the connection between the reinforcement ratio and compressive strength of concrete is established in Equation (3), which can be used to evaluate the strength of reinforced concrete according to the known strength of unreinforced concrete.

f cρ=fc0expbρ (3) where fcρ and fc0 are the compressive strength of concrete with a reinforcement ratio of ρ and 0% respectively; b refers to a calculation parameter and is equal to 5.1 in this study. Figure 7 shows concrete compressive strength predicted by using Equation (3) under the condition of different reinforcement ratios. At 3 d, 7 d, and 28 d, the average prediction errors of C30 were 4.73%, 3.26%, and 3.75% respectively and the maximum error was 9.37%, while the average prediction errors of C60 were 3.35%, 7.96%, and 6.74% respectively and the maximum error was 16.17%. As shown in Figure 6, the predicted results well agreed with the actual test values. It should be noted that the parameter b may also be affected by other conditions, such as the additives used in concrete, the age of concrete, and the external environment. In the future, it will be necessary to carry out further experiments to discuss this issue.

Materials 2020, 13, 658 10 of 12

3.3. Relationship between the Compressive Strength and Reinforcement Ratio of Concrete This study showed that reinforcement bars would reduce the decline of concrete pore structure caused by capillary stress, thereby causing reinforced specimens to be inferior to unreinforced ones in compressive strength. Concrete compressive strength gradually decreased when reinforcement ratio increased. In engineering practices, the risk of accidents may be increased if the removal of scaﬀolding is determined by the compressive strength of unreinforced concrete. It is diﬃcult to directly test the pore structure parameters of concrete at the construction site. Therefore, the connection between the reinforcement ratio and compressive strength of concrete is established in Equation (3), which can be used to evaluate the strength of reinforced concrete according to the known strength of unreinforced concrete.

bρ fcρ = f c0exp (3) where fcρ and f c0 are the compressive strength of concrete with a reinforcement ratio of ρ and 0% respectively; b refers to a calculation parameter and is equal to 5.1 in this study. Figure7 shows concrete compressive strength predicted by using Equation (3) under the condition of diﬀerent reinforcement ratios. At 3 d, 7 d, and 28 d, the average prediction errors of C30 were 4.73%, 3.26%, and 3.75% respectively and the maximum error was 9.37%, while the average prediction errors of C60 were 3.35%, 7.96%, and 6.74% respectively and the maximum error was 16.17%. As shown in Figure6, the predicted results well agreed with the actual test values. It should be noted that the parameter b may also be aﬀected by other conditions, such as the additives used in concrete, the age of concrete, and the external environment. In the future, it will be necessary to carry out further Materialsexperiments 2020, 13 to, x discussFOR PEER this REVIEW issue. 11 of 13

40 70 35 60 30 25 50 20 40

15 30 10 3days 3days 7days 5 20 7days Compressive strength(MPa) Compressive 28days Compressivestrength(MPa) 28days 0 10 012345678 012345678 Reinforcement ratio (%) Reinforcement ratio (%) (a) (b)

FigureFigure 7. 7. ComparisonComparison of of predicted predicted and and test test results: results: ( (aa)) C30 C30 concrete concrete ( (bb)) C60 C60 concrete. concrete.

4.4. Conclusion Conclusions TheThe present present study study experimentally experimentally investigated investigated the the effects eﬀects of of reinforcement reinforcement bars bars on on concrete concrete pore pore structurestructure and and compressive compressive strength strength that were were tested at three three ages ages in in the the case case of of various various reinforcement reinforcement ratios.ratios. The The main main observations observations are are presented presented below: below: (1)(1) TheThe size size of of concrete concrete pore pore structure structure gradually gradually decreased decreased during during cement cement hydration hydration because because of the of fillingthe ﬁlling of hydrated of hydrated products products and capillary and capillary stress. stress. The proportion The proportion of pores of whose pores diameter whose diameter ranges fromranges 0 to from 50 nm 0 to increased, 50 nm increased, whereas whereasthat of pores that ofwhose pores diameter whose diameteris above 50 is abovenm declined 50 nm noticeably.declined noticeably. (2)(2) TheThe size size of of concrete concrete pore pore structure structure graduall graduallyy increased increased when when reinforcement reinforcement ratio ratio increased. increased. A linearA linear relationship relationship was was found found between between pore pore structure structure parameters parameters (average (average and and media media pore pore diameters,diameters, critical critical di diameterameter and and porosity) porosity) and and reinforcement reinforcement ratio. ratio. (3) At the same age, the compressive strength of reinforced concrete specimens was inferior to that of unreinforced ones and gradually declined with the increasing reinforcement ratio. (4) In engineering practices, the risk of accidents may be increased if the removal of scaffolding is determined by the compressive strength of unreinforced concrete. A formula was proposed in the present study to forecast the strength of reinforced concrete using unreinforced concrete strength and reinforcement ratio. The predicted results well agreed with the measured ones.

Author Contributions: Formal analysis, L.H.; investigation, L.H., Z.C., Z.X., and Y.Z.; methodology, J.H.; project administration, J.H.; writing–original draft preparation, L.H.

Funding: This research was funded by National Natural Science Foundation of China, grant number 51778087 and 51808071.

Acknowledgments: The authors would like to thank Hui Cao of Chongqing university for his assistance with the experimental method.

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Beaudoin, J.J.; Feldman, R.F.; Tumidajski, P.J. Pore structure of hardened portland cement pastes and its influence on properties. Adv. Cem. Based Mater. 1994, 1, 224–236. 2. Röβler, M.; Odler, I. Investigations on the relationship between porosity, structure and strength of hydrated portland cement pastes I. Effect of porosity. Cement Concrete Res. 1985, 15, 320–330. 3. Zhang, G.; Li, Z.; Zhang, L.; Shang, Y.; Wang, H. Experimental research on drying control condition with minimal effect on concrete strength. Constr. Build. Mater. 2017, 135, 194–202. 4. Tufail, M.;Shahzade, K.;Gencturk, B. Effect of elevated temperature on mechanical properties of limestone, quartzite and granite concrete. Int. J. Concr. Struct. Mater. 2017, 11, 17–28. 5. Szeląg, M. Properties of cracking patterns of multi-walled carbon nanotube-reinforced cement matrix. Materials 2019, 12, 2942.

Materials 2020, 13, 658 11 of 12

(3) At the same age, the compressive strength of reinforced concrete specimens was inferior to that of unreinforced ones and gradually declined with the increasing reinforcement ratio. (4) In engineering practices, the risk of accidents may be increased if the removal of scaﬀolding is determined by the compressive strength of unreinforced concrete. A formula was proposed in the present study to forecast the strength of reinforced concrete using unreinforced concrete strength and reinforcement ratio. The predicted results well agreed with the measured ones.

Author Contributions: Formal analysis, L.H.; investigation, L.H., Z.C., Z.X., F.Z. and Y.X.; methodology, J.H.; project administration, J.H.; writing–original draft preparation, L.H. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Natural Science Foundation of China, grant number 51778087 and 51808071. Acknowledgments: The authors would like to thank Hui Cao of Chongqing university for his assistance with the experimental method. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

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