Effects of high rebar casting temperature on porosity and Ca/Si ratio at steel–concrete interface of reinforced concrete

This study investigates effects of elevated rebar casting temperature on the interfacial porosity and Ca/Si ratio of the steel‐concrete interface by scanning electron microscopy (SEM). Experimental variables including rebar casting temperatures of 25°C, 40°C, 55°C, 70°C, and 85°C, water/cement ratios of 0.30, 0.45, and 0.60, and rebar diameters of 10 and 20 mm are considered. Large capillary pores of steel–concrete interface (SCI) were quantified by backscattered electron (BSE) imaging and Ca/Si ratio of hydration products of SCI were obtained by energy‐dispersive X‐ray (EDX) spectroscopy. The results were analyzed by using statistical methods (ANOVA analysis and Pearson correlation analysis). Results indicate that the average interfacial porosity and the average Ca/Si ratio can increase as a result of heated rebar effects. Heated rebar effects can be enhanced by increasing rebar diameter. Impact ranges of the heated rebar effects are also presented.


| INTRODUCTION
In recent years, increasing interest has been directed towards the steel-concrete interface (SCI). 1 The SCI denotes the interfacial transition zone between reinforcing steel and concrete surrounding the reinforcing steel.A systematic literature review 1 has found that there are 31 types of physical and chemical characteristics for the SCI in terms of reinforcing steel surface, concrete microstructure, temperature changes, etc.An important characteristic of the SCI is the interfacial porosity of concrete. 2The literature indicates that the interfacial porosity of the SCI plays an important role in the chloride-induced corrosion initiation, 3,4 corrosion kinetics, 5,6 cracking or spalling behaviors during corrosion propagation, [7][8][9] and pull-out strength of reinforced concrete (RC) structures. 10,11Therefore, interfacial porosity is at the heart of our understanding on the SCI.
The interfacial porosity of the SCI is characterized by a coarser structure as compared with the bulk concrete.The increased interfacial porosity can be explained by the socalled "wall effect". 12As the steel can act as a flat solid "wall," the packing of the cement grains around the SCI is disrupted.Smaller cement grains tend to accumulate around the SCI, leading to a transitional zone with higher porosity.The thickness of the porous zone due to "wall Discussion on this paper must be submitted within two months of the print publication.The discussion will then be published in print, along with the authors' closure, if any, approximately nine months after the print publication.
effect" is comparable with the size of the cement grains, namely 20-50 μm.An interfacial porous zone 13,14 of the SCI underneath the horizontal rebar with respect to the casting direction ("top bar effect" 1 ) has been found.Water and air bubbles are elevated due to the settlement, segregation and bleeding of the concrete, some of which can be entrapped underneath the horizontal rebar.After concrete hardening, increased porosity or even a macroscopic gap 13 at the SCI underneath the horizontal rebar is formed.The "top bar effect" becomes more significant for horizontal rebars with an high elevation of 800 mm, under which the thickness of macroscopic gap can be up to 650 μm. 13 In a recent study, 15 an interfacial gap underneath a rib of a vertically casting rebar has been found.This is associated with the entrapped water/air bubbles underneath the rib.In conclusion, the pore structure of the SCI can be changed due to geometric and casting direction effects.
A novel factor that may affect the interfacial porosity of the SCI is the elevated rebar casting temperature.In this study, the rebar casting temperature refers to the temperature of rebar prior to concrete casting.The rebar casting temperature is affected by ambient temperature and solar radiation.In summer, rebar casting temperature can be up to 80 C. 10 The corrosion performance of partially carbonated RC and bond strength of RC components have been found to be deteriorated due to the increase in rebar casting temperatures by Chen et al. 16 and Pati, 10 respectively.Mercury intrusion porosimetry (MIP) and Fourier-transform infrared spectroscopy (FTIR) were conducted on bulk samples collected in the rebar-concrete vicinity in. 10 FTIR tests found that heightened casting temperature of the rebar reduced the moisture content in the bulk samples.MIP tests revealed that the porosity of the bulk samples was increased.However, a quantitative and comprehensive understanding on the heated rebar effects has been impeded, since there is a lack of sufficient details on test samples (e.g., the size, distance from the SCI, and collection method) and experimental variables (diameter, water/cement ratios) in. 10 One can expect that the cement hydration around the SCI of fresh concrete may be influenced by an elevated rebar casting temperature, as evidenced by the high temperature effect on cement hydration under steam curing. 17,18As a result, a substantial change in the interfacial porosity of the SCI may occur.Therefore, it is of great significance to get insights on the interfacial porosity of the SCI due to heated rebar effect, e.g., impact range of heated rebar effect under different water/cement ratio and rebar diameters.Influence of elevated rebar casting temperature on the cement hydration in the SCI is also crucial, which is generally characterized by the Ca/Si ratio of hydration products. 19,20However, the above-mentioned knowledge remains unavailable yet necessary.
To fill these research gaps, this study aims to investigate the heated rebar effects on the SCI of RC structures in terms of the interfacial porosity and Ca/Si ratio of hydration products in the SCI of RC by scanning electron microscopy (SEM).Concrete specimens were cast at rebar casting temperatures of 25 C, 40 C, 55 C, 70 C, and 85 C, W/C ratios of 0.30, 0.45, and 0.60, and rebar diameters of 10 and 20 mm.Additionally, 585 backscattered electron (BSE) images and 408 energy dispersive X-ray (EDX) mappings were obtained to statistically study the heated rebar effects on the average porosity and Ca/Si ratio within 100 um of the SCI.The impact ranges of the heated rebar effects were also quantified.

| Specimen design
As shown in Table 1, there are three experimental variables in this study, namely W/C ratio, rebar diameter, and rebar casting temperature.Three W/C ratios of 0.30, 0.45, and 0.60 were used by changing the cement content.The studied W/C ratios in this study are selected to represent those in general engineering practices.The mix proportions are shown in Table 2.The cement is 52.5 N ordinary Portland cement as shown in Table 3.The coarse aggregate is crushed angular gravel with a maximum size of 10 mm.The fine aggregate is river sand with a fineness modulus of 3.0.The steel reinforcements are Grade 500B ribbed rebars with diameters of 10 and 20 mm.The existing rust on the rebar was retained.Cuboid concrete specimens with geometrical sizes of 50 mm Â 50 mm Â 100 mm (for 10 mm rebar) and 60 mm Â 60 mm Â 120 mm (for 20 mm rear) were used as shown in Figure 1.The rebars were horizontally casted.The concrete thickness was 15 mm among specimens with different rebar diameters, for consistency.
Ten specimens with same water/cement ratio but different rebar casting temperature (25 C, 40 C, 55 C, 70 C, and 85 C) and different rebar diameters (10 and 20 mm) were casted in a batch.Therefore, there are three batches of specimens in this study.As shown in Figure 1e, a steel formwork was used, in which a rebar was installed prior to heating and casting.For each specimen, the wires of a direct contact thermocouple were attached on the rebar by using a heat insulation tape to make sure the temperature of rebar was accurately monitored prior to concrete casting.Except for reference specimens (25 C), steel formwork and rebar were heated in an oven with a temperature of 100 C for around 20 min.The concrete was mixed at the ambient laboratory environment when the specimens were heated in an oven.The specimens were then taken out from the oven and cooled down at a laboratory environment with a temperature of 25 C and a relative humidity of 70%.Once the desired temperature was achieved, the concrete was poured into the steel formwork immediately.The cooling periods of rebars from 100 C to desired temperatures range from 2 to 15 min.Then, the concrete specimen was fully compacted on a vibration table for 60-180 s.

| Scanning electron microscopy test
After being air cured at a laboratory environment for 90 days, the specimens were sliced by a diamond blade.The samples were then impregnated with epoxy.The samples with hardened epoxy were ground using P320, P1200, and P2000 SiC papers and polished using 3 and 1 μm polycrystalline diamond sticks.
The samples were coated with carbon prior to SEM testing. 21,22A conductive silver paint was applied from the surface of the sample to the metallic sample holder in order to reduce surface charging.The SEM test in this study was conducted using a Hitachi S3400N scanning electron microscope equipped with an EDX detector.The working distance was around 10 mm, with an accelerating voltage of 20 kV.The samples were firstly captured under BSE mode.Before capturing BSE photos, the grayscale histogram was adjusted to spread out as much as possible within the grayscale spectrum, as recommended in Ref. [23], by choosing suitable brightness and contrast values.The beam current was also adjusted to achieve a dead time of around 30%.Upon the capture of BSE photos, EDX mapping was performed to collect the chemical element information of the SCI.The auto-lock function provided by the AzTec software in EDS mapping was used to avoid the drifting problem.
The capture commenced from the bottom direction with respect to the concrete casting direction (from top to bottom).Thereafter, photos were obtained circumferentially with an increase of 45 , except in cases where a position was not well ground and was covered by epoxy, in which case a supplementary photo was taken so that a minimum of eight photos were captured in each sample.
More details on the SEM sample preparation can be found in Ref. [15].

| Image analysis
5][26] The quantification of different phases at various distances from the SCI is one of the most important methods. 1,14In this study, an automatic image analysis program has been developed to calculate the distribution of porosity and Ca/Si ratio around the SCI, based on BSE and EDX photos.The mill scale and existing rusts are regarded as part of steel during image analysis.

| Detection and smoothing of boundary between steel and concrete
Either a BSE photo or an EDX mapping of Fe can be used to obtain the boundary between steel and concrete, as shown in Figure 2a, b.A binary mask of the SCI (Figure 2c) can be obtained according to the difference between the grayscale in the BSE photo 21,22,27 or the Fe intensity in the EDX mapping of Fe.Upon the binary mask of the SCI, the perimeter of the steel area can be extracted and the whole image can be segmented into the steel area and the concrete area, as shown in Figure 2d.As shown in Figure 2e, the perimeter of the steel area was tailored by excluding the segments overlaying the picture frame.As a result, the boundary between the steel area and the concrete area can be obtained.Due to its complexity, the raw boundary between the steel area and the concrete area at the SCI must be smoothed in order to generate a phase-distribution profile.Figure 3 illustrates the smoothing method applied in this study.As shown in Figure 3b, the raw boundary is displayed in yellow and the boundary (after smoothing) is displayed in red.
As shown in Figure 3a, for each boundary, unit vector v iþj connecting point i to point i + j will be defined.A new iteration is established on j and if the following equations are fulfilled, Examples of threshold methods of calculating porosity and anhydrous cement (a) Grayscale threshold of anhydrous cement, calculated by selecting minimum between the peaks of hydration products and anhydrous cement (b) Grayscale threshold of anhydrous cement, calculated using the tangent slope method (c) Cumulative curve of concrete area and grayscale threshold of porosity, calculated using the inflection point method.
Ã cos θ points i + 1 to i + j À 1 will be deleted from the boundary.Once iteration point i from the start point to the end point of the boundary is completed, the smoothing algorithm is reversed from the end point of the boundary to the start point of the boundary.

| Segmentation and phase distribution diagram
The histogram at Figure 4a shows a typical grayscale (0-255) of the SCI after excluding the steel/rust/mill scale area.The three peaks correspond to the anhydrous cement, hydrates and aggregate, and porosity.The grayscale threshold of anhydrous cement is selected as the minimum between the peaks of anhydrous cement and hydrates and aggregate, as shown in Figure 4a.If there is no significant peak of anhydrous cement, as shown in Figure 4b, the tangent slope method 28 is used to determine the grayscale threshold of anhydrous cement.The grayscale threshold of porosity is determined by the inflection point method, 23 as shown in Figure 4c.There is no significant peak for Ca(OH) 2 in the histograms of grayscale in this study.
An example of a phase segmentation of a BSE photo is shown in Figure 5.After excluding porosity (Figure 5b) and anhydrous cement (Figure 5c), the chemical element information collected by EDS mapping is used to segment the hydration products of cement and aggregate.Two thresholds of Ca/Si ratio are used in this study: 0.8 and 3.0.The value of Ca/Si ratio = 0.8 is used to differentiate the hydration products and aggregate (Figure 5d), which is consistent with the literature. 29,30During the hydration of Portland cement, the four main phases-C 3 S, C 2 S, C 3 A, and C 4 AF-react with water 31 to form hydration products: There are four main hydration products: C-S-H gel, which is an amorphous phase with various Ca/Si ratios, Ca(OH) 2 , AFt and AFm phases.A relatively high Ca/Si ratio limit of 3.5 32 was found for the C-S-H gel during the early ages, although the highest Ca/Si ratio of the mature C-S-H gel was around 2.3. 29,30A Ca/Si ratio of 3.0 is used in this study to segment the hydration products of Portland cement.The main hydration product in pixels with a Ca/Si ratio < 3.0 (Figure 5e) is C-S-H gel.For pixels with a Ca/Si ratio >3.0 (Figure 5f), the main hydration products are Ca(OH) 2 , AFt and AFm phases.In this study, the heated rebar effect on the Ca/Si ratio of the hydration products in pixels with a Ca/Si ratio between 0.8 and 3.0 are investigated (Figure 5e).
The boundary between the steel area and the concrete area (Figure 2e) is divided into discrete points.A straight line is extended from each discrete point along vector v ! 2 (Figure 3) until the concrete area is reached, so that the gap between steel and concrete can be identified (if any).Strips are produced from the boundary between the concrete area and the steel area at 1.5 μm intervals to generate the phase-distribution profile, 14,15,25 which represents the distribution of different phases as identified by the segmentation method with the distance from 0 to 100 μm away from the SCI (magnitude: Â500).Where a cutting-induced gap is found, 15 the new boundary between the gap and the concrete area is smoothed and used to generate the phase-distribution profile.

| Heated rebar effects on porosity
An analysis of variance (ANOVA) was conducted to study the effects of the design independent variables (shown in Table 1) on average porosity within 100 um of the SCI.The results of the ANOVA are shown in Table 4.It can be observed that the effects of W/C ratio and rebar diameter are significant on average porosity near the SCI and this is discussed in a previous paper of the authors. 15As shown in Table 4, a p-value of 0.906 for rebar temperature is found, while the p-value for (Rebar diameter) Â (Rebar temperature) is 0.086.This indicates that the rebar temperature is not a main factor, but that there may be an interaction effect for rebar diameter and rebar temperature.A p-value of 0.535 for (W/C ratio) Â (Rebar temperature) is found, indicating that the effect of rebar temperature does not depend on W/C ratio.The results of the ANOVA, as shown in Table 4, indicate that the effect of rebar temperature on average porosity depends on rebar diameter.Therefore, a Pearson correlation analysis was conducted for the effects of W/C ratio and rebar temperature on average porosity within 100 μm of the SCI for different rebar diameters, as shown in Table 5.As indicated by the p-values in Table 5, the effect of rebar temperature on average porosity is only statistically meaningful when the rebar diameter is 20 mm, while W/C ratio has a significant effect on average porosity, regardless of rebar diameter.
Based on the results of the Pearson correlation analysis, Figure 6 presents the effects of rebar temperature on average porosity within 100 μm of the SCI for different rebar diameters.When the rebar diameter is 10 mm, there is no statistical correlation between average porosity within 100 μm of the SCI and rebar temperature, as indicated by the p-value of 0.399 (shown in Figure 6a).On the other hand, the p-value of the Pearson correlation analysis is 0.008 when the rebar diameter is 20 mm, as shown in Figure 6b, indicating only a 0.8% probability that the null hypothesis is accepted.Therefore, one can accept the alternative hypothesis of a statistical relationship between average porosity and rebar temperature when the rebar diameter is 20 mm.The Pearson correlation coefficient is 0.163 for samples with a rebar diameter of 20 mm, as shown in Figure 6b.Compared with the correlation coefficient of 0.330 for W/C ratio with a rebar diameter of 20 mm, as shown in Table 5, one can conclude that the effect of rebar temperature on average porosity is less significant than that of W/C ratio.
Figure 7 shows the effect of rebar temperature on average porosity by analyzing 32 BSE photos under a magnitude of Â100.As revealed by Figure 7, the heated rebar effect on average porosity appears to be limited within the first 250 μm of the SCI.

| Heated rebar effect on Ca/Si ratio
An ANOVA analysis was also conducted on the effects of the design variables on the Ca/Si ratio within 100 μm of the SCI, as shown in Table 6.It shows that W/C ratio, rebar diameter, and rebar temperature have statistically meaningful effects on the average Ca/Si ratio within 100 μm of the SCI, as indicated by their corresponding pvalues (all lower than 0.001).Moreover, interactive effects exist between these three factors.
A Pearson correlation analysis was conducted on the effects of W/C ratio and rebar temperature on the average Ca/Si ratio within 100 μm of the SCI for different rebar diameters, as shown in Table 7.The average Ca/Si ratio is significantly affected by W/C ratio regardless of   rebar diameter.When the rebar diameter is 10 mm, the p-value of the rebar temperature is 0.929, indicating a probability of 92.9% that the observed differences in samples under different rebar temperatures are caused by random behaviors.When the rebar diameter is 20 mm, the correlation between the average Ca/Si ratio and rebar temperature is more statistically meaningful (p-value of 0.157) as compared with that of rebar diameter of 10 mm.
In conclusion, the heated rebar effect on the average Ca/Si ratio is also affected by rebar diameter.
To study the effects of rebar diameter and rebar temperature on the average Ca/Si ratio within 100 μm of the SCI, a Pearson correlation analysis was conducted for different W/C ratios, as shown in Table 8.The average Ca/Si ratio is negatively associated with rebar diameter.A p-value of 0.024 is found for the effect of rebar temperature on the average Ca/Si ratio within 100 μm of the SCI when the W/C ratio = 0.30, as shown in Table 8, while the p-values for W/C ratios of 0.45 and 0.60 are higher than 0.005.Therefore, one can conclude that the positive correlation between the average Ca/Si ratio and rebar temperature is only statistically meaningful when the W/C ratio = 0.30.
Figure 8 presents the average Ca/Si ratio within 100 μm of the SCI under different W/C ratios, rebar diameters and rebar temperatures.The average Ca/Si ratio appears to increase with rising rebar temperature.This effect is statistically significant, as supported by a pvalue below 0.001 for rebar temperature used as an independent variable, as shown in Table 6.It should also be noted that the heated rebar effect on the Ca/Si ratio is less significant than the W/C ratio and rebar diameter effect on the Ca/Si ratio, as revealed by the F values shown in Table 6.
Figure 9 shows the effect of rebar temperature on the average Ca/Si ratio within 800 μm of the SCI, by analyzing the EDX mapping under a magnitude of Â100.The effect of rebar temperature on the average Ca/Si ratio is still significant at 800 μm from the SCI under a W/C ratio of 0.30 and rebar diameter of 20 mm.It should be noted that the impact range of the heated rebar effect on the average Ca/Si ratio is wider than that of the heated rebar effect on average porosity, as evidenced by comparing Figure 7 with Figure 9.

| DISCUSSION
In this study, elevated rebar casting temperature has been found to increase interfacial porosity and Ca/Si ratio at the SCI of RC.Heated rebar effects on the interfacial properties of the SCI are more significant for specimens with a rebar diameter of 20 mm as compared with those with a rebar diameter of 10 mm.By increasing rebar diameter, the total amount of heat inside a rebar (which is related to the cross-sectional area of a rebar) is quadratically increased.On the other hand, the perimeter of a rebar is linearly increased with increased rebar diameter.Therefore, the normalized amount of heat inside a rebar in the circumferential direction linearly increases with rebar diameter.Based on the results of this study, the heated rebar effect on the interfacial porosity of RC with a rebar diameter of 10 mm can be ignored.For RC with a rebar diameter of 20 mm, the heated rebar effect on interfacial porosity is substantial, albeit less significant than the W/C ratio effect on interfacial porosity.
As shown in Figure 8, the heated rebar effect is found to increase the Ca/Si ratio.It is interesting to find that the Ca/Si ratio can also be increased by decreasing W/C ratio, where the hydration of cement is reduced due to the lack of water. 15Figure 10 presents photos of rebars with different casting temperatures.It is revealed that the hydration product on the rebar with a casting temperature of 85 C (Figure 10b) is drier and more porous than that on the rebar with a casting temperature of 25 C (Figure 10a).Therefore, one may postulate that there is a layer of porous and insufficiently hydrated products of cement attached on the reinforcing steel due to the heated rebar effect, of which the hydration may be ceased at a pre-mature stage.Accordingly, the Ca/Si ratio is increased due to the insufficient cement hydration.Although these less hydrated products may be mixed with well hydrated products which are formed at a later stage upon the dissipation of the heat of a rebar, the heated rebar effects on increasing Ca/Si ratio and interfacial porosity are not insignificant as evidenced in this study, especially for rebars with larger diameter.It should also be noted that the heated rebar effect is regarded negative in terms of cement hydration due to the loss of moisture content in this study.This cannot be confused with a lower local W/C ratio, which is positive in increasing mechanical and durability performances by lowering porosity after water evaporation.
The increased interfacial porosity and Ca/Si ratio due to the heated rebar effects are in accordance with the results found in. 10,19,33Only large capillary pores of a size larger than 5 μum were found to be increased due to the heated rebar effect in. 10 Similarly, the interfacial porosity investigated in this study also ranges from 1 to 20 μm.On the other hand, Famy et al. 19 found that a high curing temperature, especially during the early stages, can reduce microporosity of C-S-H gel with pore size <1 μm and change the chemical composition of the inner C-S-H products of Portland cement mortars.They found that the inner C-S-H formed at 90 C was brighter and had higher S/Ca and Ca/Si ratios compared with the darker C-S-H formed at 20 C. It was stated that the increase in BSE coefficients and analysis total of the lighter C-S-H at 90 C may indicate less microporosity and water content.Therefore, the increased Ca/Si ratio due to the heated rebar effect in this study may indicate less microporosity (pore size <1 μm) for C-S-H gel.

| CONCLUSION
This study investigates the heated rebar effect on the average interfacial porosity and average Ca/Si ratio at the SCI of RC by scanning electron microscopy (SEM).The results are statistically analyzed by ANOVA testing and Pearson correlation testing.Further studies should be conducted in correlating the interfacial performances with macroscopic performances, for example, corrosion and bond strength.
Based on the results of this study, the following conclusions can be drawn: 1.An elevated rebar casting temperature can increase interfacial porosity within 100 μm of the SCI for rebar with a diameter of 20 mm, from 23.5% to 27.0%.This heated rebar effect can be ignored when considering rebar with a diameter of 10 mm. 2. The average Ca/Si ratio within 100 μm of the SCI can be increased by increasing the rebar casting temperature.This increase in average Ca/Si ratio may be induced by a reduction of local moisture content, leading to insufficient cement hydration and less microscopic porosity with pore size <1 μm. 3. The heated rebar effects on the interfacial porosity and average Ca/Si ratio are dependent on rebar diameter and W/C ratio.The heated rebar effects can be enhanced by increasing the rebar diameter.The heated rebar effect on the average Ca/Si ratio is more statistically significant for a W/C ratio of 0.30.4. Interfacial porosity (pore size larger than 1 μm) within 250 μm of the SCI can be affected by the heated rebar effect while the heated rebar effect on the average Ca/Si ratio can be at least 800 μm from the SCI.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.

! 1
is defined, the direction of which leads from the start point to the end point of the boundary.Another unit vector v ! 2 , which is perpendicular to v ! 1 and points to the concrete area, can thus be obtained.The smoothing begins from the start point of the boundary.For point i of the boundary, the vector v !i,iþ1 connecting this point to the following point i + 1 is checked.If the following equations are fulfilled,

F
I G U R E 1 Geometrical size of concrete specimens (unit: mm): (a) cross section of specimens with 20 mm rebar; (b) side view and cutting positions of specimens with 20 mm rebar; (c) cross section of specimens with 10 mm rebar, (d) side view and cutting positions of specimens with 10 mm rebar and (e) photo for steel formwork and rebar.F I G U R E 2 Illustration of steps to detect boundary between steel and concrete: (a) photo under BSE mode; (b) EDS mapping of Fe; (c) binary mask of steel-concrete interface; (d) perimeter of steel area (the remaining area of this image is the concrete area); and (e) boundary between steel and concrete.F I G U R E 3 Schematic diagram of smoothing of boundary between steel and concrete at SCI.

F I G U R E 6
Heated rebar effect on average porosity within 100 μm of SCI.Magnitude of BSE images: Â500.Number of BSE images: (a) 273 and (b) 312.Rebar diameter = (a) 10 mm and (b) 20 mm.W/C ratio = 0.30, 0.45, and 0.60.F I G U R E 7 Impact range of heated rebar effect on porosity.Magnitude of BSE images: Â100.Number of BSE images: 32.W/C ratio = 0.30, 0.45, and 0.60, rebar diameter = 20 mm.

F I G U R E 8
Heated rebar effect on average Ca/Si ratio of hydration product (0.8 < Ca/Si ratio <3.0) within 100 μm of SCI.Magnitude of EDS mapping: Â500.Number of EDX mappings: 408.

F I G U R E 9
Heated rebar effect on average Ca/Si ratio of hydration product (0.8 < Ca/Si ratio <3.0) in Â100 EDX mappings (distance from SCI <800 μm).Number of EDX mappings: 11.W/C ratio = 0.30, rebar diameter = 20 mm.F I G U R E 1 0 Comparison of rebars with different casting temperatures.Rebar casting temperature: (a) 25 C and (b) 85 C.
Chemical compositions and physical properties of 52.5 N ordinary Portland cement.
Value is a statistical value to indicate the probability under which the null hypothesis can be accepted; generally, a p-value lower than 0.05 indicates that the sample means are statistically different, which is emphasized in the table.Pearson correlation analysis of porosity based on different rebar diameters (<100 μm from SCI).
T A B L E 4 ANOVA analysis results of porosity (<100 μm from SCI).Note: p-Value is a statistical value to indicate the probability under which the null hypothesis can be accepted; generally, a p-value lower than 0.05 indicates that the sample means are statistically different, which is emphasized in the table.
T A B L E 6 ANOVA results of Ca/Si ratio (<100 μm from SCI).Note: p-Value is a statistical value to indicate the probability under which the null hypothesis can be accepted; generally, a p-value lower than 0.05 indicates that the sample means are statistically different, which is emphasized in the table.Pearson correlation analysis of Ca/Si ratio based on different rebar diameters (<100 μm from SCI).
Note: p-Value is a statistical value to indicate the probability under which the null hypothesis can be accepted; generally, a p-value lower than 0.05 indicates that the sample means are statistically different, which is emphasized in the table.T A B L E 8 Pearson correlation analysis of Ca/Si ratio based on different W/C ratios (<100 μm from SCI).Note: p-Value is a statistical value to indicate the probability under which the null hypothesis can be accepted; generally, a p-value lower than 0.05 indicates that the sample means are statistically different, which is emphasized in the table.