1. Introduction
In the construction process of actual engineering projects, influenced by various factors such as concrete quality, construction procedures, and climatic conditions, concrete structures are often unable to be poured all at once instead of existing at a certain pouring interval time. This type of staged pouring concrete prepared after a certain interval of pouring time is widely used in engineering construction. For example, wet joints in prefabricated components, continuous casting of rigid frame beams, and repair of damaged structures with high-toughness concrete. Compared with conventional concrete that is finished in a single pour, staged pouring concrete has a special “interlayer weak surface” due to the influence of the pouring interval in the construction process [1,2,3,4,5], which leads to the difference in mechanical properties and durability between staged pouring concrete and conventional concrete. The interfacial bonding strength of this “interlayer weak surface” is provided by the mechanical interlocking force, van der Waals force and chemical bonding resulting from the secondary reaction of new and old concrete at the interface [6,7]. Due to the presence of weak interfaces, the performance of staged pouring concrete differs from that of ordinary concrete. Therefore, clarifying the impact of pouring interval time on concrete structures’ mechanical and durability properties holds significance for construction control in practical engineering projects [8,9,10,11].
Researchers have extensively investigated the mechanical properties of staged pouring concrete. Generally, the mechanical properties of staged pouring concrete are lower compared to ordinary concrete [8]. The interlayer bonding performance determines the overall performance of staged pouring concrete, and factors such as concrete rheological properties, temperature, relative humidity, pouring interval time, and admixtures can affect the interlayer bonding performance of staged pouring concrete [12,13,14,15,16,17,18,19].
The mechanical properties of concrete can be improved by incorporating highly reactive mineral admixtures to enhance the bond strength of staged pouring concrete [14,19]. Li [19] showed that the long-term bond strength of all specimens with added fly ash was higher than that of ordinary Portland cement concrete. Roussel et al. [15] found that the rheological properties of self-compacting concrete (SCC) can influence bonding performance, and it is necessary to ensure that the old concrete meets the flowability requirements when pouring new concrete. Zhang et al. [16] studied the effect of interface roughness on the bonding performance of staged pouring concrete, indicating that increasing roughness is beneficial for enhancing bonding performance. Megid et al. [17] analyzed the effect of pouring interval time on the bonding performance of multi-layer SCC and found that the bonding strength of SCC gradually decreases with increasing time intervals. Yuan et al. [18] analyzed the effect of pouring interval time and curing age on the bonding performance of SCC and ordinary concrete (OC) and found that pouring interval time has a greater influence on the failure mode of SCC and OC than curing age; shorter pouring interval time and longer curing age can achieve higher bonding strength.
In addition to mechanical properties, during long-term service, staged pouring concrete with layered pouring interfaces often experiences more severe durability problems caused by chloride ion ingress compared to ordinary concrete [11,20,21,22]. Shen et al. [21] analyzed the scanning electron microscope images of the transition zone at layered interfaces of staged pouring concrete and the hydration process, revealing the presence of numerous microcracks and voids at the layered interfaces, resulting in lower density in the transition zone, which adversely affects the properties of the concrete matrix. Xia et al. [22] investigated the chloride ion transport behavior of staged pouring concrete and found the presence of microcracks and a large amount of CaCO3 in the pouring interface zone, with higher porosity than the concrete matrix on both sides, leading to deeper chloride ion diffusion into the concrete matrix. Zhang et al. [23], through ultrasonic non-destructive testing and rapid chloride ion migration (RCM) tests, studied the interface bonding performance and chloride ion diffusion resistance of staged pouring concrete, indicating that with increasing pouring interval time, the difference in chloride ion diffusion between the new concrete area and the old concrete area becomes more pronounced. When the strengths of the new and old concrete are the same, chloride ion diffusion at the interface is closer to that of the new concrete. However, most of the existing studies on staged pouring concrete mainly focus on the bonding performance between new and old concrete, conventional mechanical properties, and resistance to chloride ion erosion. There is still relatively little discussion on the influence mechanism of the interface microstructure of staged pouring concrete, and research on the diffusion behavior and chloride ion concentration distribution in staged pouring concrete during long-term service is also relatively lacking.
Building upon this, the present study conducted compressive strength tests, splitting tensile strength tests, and natural immersion tests to investigate the influence of pouring interval time in layered pouring. Specifically, pouring interval times of 0 h, 3 h, 6 h, 9 h, 12 h, 18 h, and 24 h were considered. The study examined the effects of these intervals on the mechanical strength and chloride ion concentration distribution of staged pouring concrete and analyzed the correlations between pouring interval time, natural immersion age, concrete location and depth, and chloride ion concentration. Additionally, the study revealed how pouring interval time affects the mechanical properties and resistance to chloride ion erosion of staged pouring concrete at the microstructure level. The innovation of this study is to reveal the mechanism of the influence of pouring interval time on the chloride resistance of staged pouring concrete, and it proposed the chloride ion concentration difference ΔC to evaluate the diffusion patterns of chloride ions in staged pouring concrete with different pouring interval times. The results of this research can provide guidance for the study of the mechanical properties and durability of staged pouring concrete in practical engineering construction, as well as for engineering protective measures.
2. Materials and Methods
2.1. Materials
The cement used was PO 52.5 ordinary Portland cement provided by Du’an West River Fish Peak Cement Co., Ltd., Nanning, China. Fly ash with a fineness of 20.3 μm was supplied by Guangxi Guishi Environmental Protection Technology Co., Ltd., Nanning, China. Silica fume with a SiO2 content of 94.09% was provided by Chengdu Qiangbao Construction Engineering Co., Ltd., Chengdu, China. Microspheres, with a CaO content of 7.24%, were produced by Tianjin Zhucheng New Material Technology Co., Ltd., Tianjin, China. The superplasticizer used was a PCA-I type polycarboxylic acid high-performance water reducer manufactured by Jiangsu Subote New Materials Co., Ltd., Nanjing, China, with a total alkali content of 0.44%. Polypropylene fibers were provided by Jiangsu Subote New Materials Co., Ltd., Nanjing, China. Fine sand, medium stone, and small stone were supplied by Guangxi Jintu Quarry Co., Ltd., Nanjing, China, with the fineness modulus of fine sand being 2.90, classified as Zone II medium sand. All technical specifications of medium stone and small stone complied with the requirements of Class I in the standard JTG/T 3650-2020 [24].
2.2. Mixing Proportion and Preparation
This study comprised seven experimental groups to investigate the impact of different pouring interval times, namely 0 h, 3 h, 6 h, 9 h, 12 h, 18 h, and 24 h, on the concrete properties, designated as T0, T3, T6, T9, T12, T18, and T24, respectively. As illustrated in Figure 1, a schematic diagram of staged pouring concrete is presented. The dimensions of the concrete specimens were 100 mm × 100 mm × 100 mm. The control group denoted as T0, represents ordinary concrete poured all at once without any interval. In staged pouring concrete, the lower half layer of concrete of the specimen is poured first. Then, the upper half layer of concrete of the test block is poured according to the pouring interval time so as to obtain the required staged pouring concrete specimens.
The mix proportions of the staged pouring concrete were designed as shown in Table 1. The design strength grade of the concrete is C60, where the water-to-binder of the concrete mixture was set at 0.30, and the volume fraction of polypropylene fibers was fixed at 0.15%. The coarse aggregate was continuously graded gravel with particle sizes ranging from 5 mm to 20 mm, and the ratio of small stones (5 mm to 10 mm) to medium stones (10 mm to 16 mm) was 1:3. The C60 concrete prepared in this case can meet the flowability requirements. After completion of pouring, the staged pouring concrete specimens were demolded after 24 h and cured for 7 days, 28 days, and 56 days under standard curing conditions of (20 ± 2) °C and relative humidity above 95%. The initial setting time of concrete was determined to be 12.5 h through testing, and the flowability of concrete was ensured to meet the requirements throughout the experiment.
2.3. Test Methods
2.3.1. Compression and Split Tensile Tests
As shown in Figure 2, a schematic diagram of the mechanical performance tests for staged pouring concrete is depicted. The compression and split tensile tests were carried out according to the Chinese standard GB/T 50081-2019 [25]. All specimens were cast with dimensions 100 mm × 100 mm × 100 mm. Three replicates were measured in the split tensile test for each mix, and the average value was recorded. The loading rate for both mechanical tests was 2.4 kN/s.
2.3.2. Natural Immersion Tests
The natural immersion test utilized an improved Volhard titration method [26] to measure the chloride ion concentration at different locations and depths of the staged pouring concrete specimens. After curing the staged pouring concrete specimens with different pouring interval times under standard conditions for 28 days, they were removed and placed in a ventilated area to air dry the surfaces naturally. One face with a crack-bonding surface was used as the diffusion surface, while the remaining five faces were sealed with epoxy resin. In order to accelerate the corrosion rate of chloride ions on concrete, natural immersion was carried out using a 10%/L NaCl solution [27,28]. The prepared specimens were then immersed in a soaking tank containing a 10%/L NaCl solution with immersion times of 28 days, 56 days, and 90 days, followed by testing the chloride ion concentration inside the concrete. In this study, the drilling positions for the staged pouring concrete are illustrated in Figure 3. Among them, the face with a crack-bonding surface of the staged pouring concrete was designated as the middle face, with the left and right sides 25 mm away from the middle face defined as the left and right faces, respectively, for comparison. To accurately compare the chloride ion concentration values between the middle face and the left and right faces, the chloride ion concentration values obtained from the left and right faces were averaged. Additionally, in this study, the drilling and sampling depth of the concrete was 5 mm per layer, obtaining powder samples at depths of 5 mm, 10 mm, 15 mm, and 20 mm, which were dried and titrated to test the chloride ion concentration at different depths of the staged pouring concrete. To ensure accuracy in measurement, at least three points were drilled at each depth.
2.3.3. Microstructure Characterization
As shown in Figure 4, a schematic diagram of sampling for microscopic testing of staged pouring concrete is presented. Fragments located at the samples’ center were chosen randomly and immersed in ethyl alcohol to halt the hydration reaction. Subsequently, these samples were placed in a vacuum oven at 60 °C for an additional 24 h [13]. The pore structure analysis of the staged pouring concrete specimens was conducted using an AutoPore IV 9500 fully automatic mercury intrusion porosimeter, which was provided by Micrometrics instrument, Norcross, GA, USA. They were then examined via an S-3400N Scanning Electron Microscope (SEM), which was provided by Hitachi, Ltd., Tokyo, Japan, to scrutinize the microstructure of the studied mixtures.
3. Results and Discussion
3.1. Mechanical Performance Tests of Staged Pouring Concrete
3.1.1. Split Tensile Strength
The results of split tensile strength for staged pouring concrete are shown in Figure 5. It can be observed that the split tensile strength of the staged pouring concrete specimens decreases continuously with the increase of pouring interval time. Notably, when the pouring interval time exceeds 12 h, the split tensile strength of the staged pouring concrete specimens at the 56 days curing age remains lower than the split tensile strength of ordinary concrete specimens at the 7 days curing age. To further quantitatively analyze the influence of different pouring interval times on the split tensile strength of concrete specimens, taking the whole-poured concrete T0 as the reference, the variation rate of split tensile strength for the staged pouring concrete specimens was calculated, as shown in Figure 5b. It can be observed that with the increase of pouring interval time, the decreasing rate of split tensile strength of concrete specimens at different curing ages continues to increase. When the pouring interval time is 24 h, the split tensile strength of the T24 concrete specimens at 7 days, 28 days, and 56 days curing ages decreases by 40.93%, 32.92%, and 32.93%, respectively. The reason for this phenomenon lies in the fact that with the prolongation of the pouring interval time, the hydration degree of the lower concrete increases, resulting in higher initial yield stress [29], gradual strength enhancement, and denser hydration products [30].
When the upper layer of new concrete is poured, it becomes difficult for the lower layer of old concrete to produce plastic deformation under the gravity of the aggregates in the upper layer of new concrete, leading to poor coordination and compatibility between aggregates and mortar, thereby gradually reducing the bond strength at the interface with the increase of pouring interval time [31]. Additionally, as the pouring interval time increases, the hydration degree of the lower concrete increases continuously, while the moisture at the interface decreases due to consumption or evaporation, resulting in incomplete hydration of the cementitious materials at the interface, a significant decrease in the content of hydration products, and further deterioration of interface properties [32].
It is worth noting that the initial setting time of the concrete tested in this study was 12.5 h. When the pouring interval is within the concrete’s initial setting time, the decrease rate of split tensile strength increases with curing age. However, when the pouring interval time exceeds the initial setting time of concrete, the decreased rate of split tensile strength of staged pouring concrete at the 7 days curing age significantly increases. The reduction rates of split tensile strength for T18 concrete and T24 concrete are 36.8% and 40.9%, respectively, significantly higher than those at 28 days and 56 days curing ages, as shown in Figure 5b. Since the bonding interface strength of staged pouring concrete is controlled by both the initial bonding strength unaffected by time and the hydration degree affected by time [8,33,34,35], when the pouring interval time is less than the initial setting time, the lower concrete can still flow, and the aggregates of the upper concrete can be embedded, resulting in a relatively good overall bonding interface. This is manifested in staged pouring concrete’s relatively good initial bonding strength.
At this time, the factors affecting the split tensile strength with the development of curing age are mainly dominated by the hydration degree of concrete. When the pouring interval time exceeds the initial setting time, the lower concrete cannot flow, and the aggregates of the upper concrete are difficult to embed, resulting in poor overall integrity of the bonding interface or even detachment. Therefore, the initial bonding strength of T18 concrete and T24 concrete is poor, and for the short curing age of 7 days, the development of hydration degree is difficult to compensate for the decrease in split tensile strength due to the poor initial bonding strength. Therefore, the reduction rates of split tensile strength for T18 concrete and T24 concrete are significantly increased. With the increase of curing age, the hydration degree of concrete develops. Therefore, at 28 days and 56 days, the reduction rates of split tensile strength for T18 concrete and T24 concrete decrease, indicating that after the pouring interval time exceeds the initial setting time, the factors affecting the development of split tensile strength are mainly dominated by the early initial bonding strength.
3.1.2. Compressive Strength
Figure 6 illustrates the compressive strength test results of the staged pouring concrete. In contrast to numerous existing research findings [8,12,18,36], the compressive strength of the staged pouring concrete specimens in this study decreased first and then increased with the increasing pouring interval time. Specifically, when the pouring interval time was less than the initial setting time (12.5 h), the compressive strength of the staged pouring concrete decreased with the increasing pouring interval time, consistent with the trend observed in the split tensile strength, mainly due to the decrease in interfacial bonding strength [10,37]. During this period, the compressive strength of T12 concrete was the lowest, with reductions of 25.9%, 34.3%, and 14.7% at 7 days, 28 days, and 56 days curing ages, respectively. However, when the pouring interval time exceeded the initial setting time, the compressive strength of the staged pouring concrete showed an increasing trend with the increasing pouring interval time, although it remained lower than that of the whole-cast concrete specimens.
The reason for this behavior lies in the fact that under compression, when the bonding interface of the staged pouring concrete exhibits good overall integrity, the elastic modulus of the upper and lower concrete layers are similar, resulting in both bearing almost identical stresses and demonstrating a cohesive failure mode upon failure. Conversely, when the staged pouring concrete has poorly performing bonding interfaces under pressure, the interface is the first to undergo expansion and tearing, resulting in a situation where the concrete splits into two separate blocks upon failure [38]. Furthermore, with the continuous increase in pouring interval time, the initial bonding strength of the staged pouring concrete decreases continuously, and the difference in elastic modulus between the upper new concrete and the lower old concrete increases further.
Before the upper concrete pouring is completed, the lower concrete has already undergone a certain degree of hydration, reducing its plasticity and increasing its rigidity, resulting in differences in strength and mechanical performance of the staged pouring concrete [28,34]. Under pressure, the lower old concrete first reaches its maximum stress strength and exits operation, splitting the entire specimen into two blocks. At this point, the stress in the upper new concrete increases sharply, and the aspect ratio of some specimens of the upper concrete increases. At this moment, the size effect of the concrete leads to a similar effect to the compression resistance of two blocks in parallel, increasing the measured compressive strength of the concrete [39]. Therefore, when the pouring interval time exceeds the initial setting time, the difference in elastic modulus of the staged pouring concrete leads to an increase in compressive strength, consistent with the results of Elbakry et al. [38].
3.2. Natural Immersion Test of Staged Pouring Concrete
3.2.1. Effect of Pouring Interval Time on Chloride Ion Concentration in Staged Pouring Concrete
This study analyzes the variation of chloride ion concentration at depths of 5 mm, 10 mm, 15 mm, and 20 mm in staged pouring concrete. Here, “5 mm” indicates the chloride ion concentration at 0–5 mm from the bonding interface of the staged pouring concrete. In comparison, “5 mm-A” denotes the average chloride ion concentration 25 mm away from the bonding interface on both sides and similarly for other depths.
Figure 7 illustrates the influence of casting interval on the chloride ion concentration in staged pouring concrete. It is observed that regardless of the immersion duration, at the same depth, the chloride ion concentration at the bonding interface of the staged pouring concrete is always higher than that on both sides. With the increase in casting interval, the chloride ion concentration at the bonding interface and on both sides of the concrete continuously increases at the same depth, indicating a gradual decrease in the chloride ion resistance at the bonding interface with the increase in the casting interval. This phenomenon may be attributed to the decrease in bonding strength, increase in porosity, decrease in compactness, and appearance of microcracks at the bonding interface of the staged pouring concrete with the increase in casting interval, which serves as the main channels for chloride ion diffusion. Chloride ions penetrate the concrete through the microcracks at the bonding interface and react with the hydrated calcium aluminate in the concrete to form Friedel’s salt [11,23,32], leading to the continuous consumption and outflow of needle-like hydrated products such as calcium hydroxide at the bonding interface, further increasing the porosity, and accelerating the deterioration of the interface. Chloride ions also diffuse from the bonding interface into the interior of the concrete and then spread to both sides, significantly accelerating the erosion of chloride ions [11]. As the casting interval increases, the deterioration of the bonding interface gradually expands, affecting the surrounding concrete on both sides, resulting in an increase in chloride ion concentration. This is due to the combined effect of chloride ions diffusing along the bonding interface of the staged pouring concrete and spreading to both sides of the concrete [22]. With the increase in immersion time, the chloride ion concentration at the same depth increases to varying degrees, showing an overall increasing trend with the increase in casting interval, and the chloride ion concentration at the bonding interface of the staged pouring concrete is higher than that on both sides. However, with the increase in depth, the difference in chloride ion concentration between the bonding interface and both sides of the staged pouring concrete decreases.
3.2.2. Effect of Casting Interval on Chloride Ion Concentration in Staged Pouring Concrete
Figure 7 presents the chloride ion concentration test results for staged pouring concrete with different casting intervals. To more intuitively reflect the difference in chloride ion content between the bonding interface and both sides of the concrete, a quantitative indicator, the chloride ion concentration difference ΔC, is defined to evaluate the influence of the bonding interface on chloride ion concentration in staged pouring concrete with different casting intervals, as shown in Equation (1).
(1)
where ΔC represents the difference in chloride ion concentration between the bonding interface and the new and old concrete; Ci denotes the chloride ion concentration at the bonding interface of staged pouring concrete, expressed as a percentage; Cnto indicates the arithmetic mean of the chloride ion concentration in the concrete on both sides of the bonding interface, 25 mm away from it.The results of the chloride ion concentration difference between the bonding interface and the concrete on both sides are shown in Figure 8. It can be observed from the graph that, for the same immersion period, ΔC generally decreases with increasing depth, indicating a reduced influence of the bonding interface on chloride ion diffusion with greater depth. When the immersion period is 28 days, the trend of ΔC in the depth range of 0–5 mm shows an initial increase followed by a decrease and then tends to stabilize as the casting interval increases. Concrete samples T3, T6, and T9 exhibit significantly higher ΔC values at the 0–5 mm depth compared to other groups, indicating that the bonding interface performance of staged pouring concrete has a greater impact on chloride ion erosion near the concrete surface when the casting interval is 3 h, 6 h, and 9 h compared to intervals of 0 h, 12 h, 18 h, and 24 h. For immersion periods of 56 days and 90 days, ΔC at different depths increases to varying degrees with increasing casting interval, but the performance of the bonding interface of staged pouring concrete has almost no effect on chloride ion diffusion at the 20 mm depth. This phenomenon is attributed to the presence of minor cracks at the bonding interface of concrete samples T3, T6, and T9, which do not penetrate deeply into the concrete. Consequently, under the conditions of a short immersion period of 28 days, chloride ions are unable to penetrate more than 5 mm into the concrete through unconnected cracks, resulting in a significantly higher chloride ion concentration at the bonding interface of the 0–5 mm depth compared to the concrete on both sides.
In contrast, concrete samples T12, T18, and T24 may have interconnected microcracks and pores, allowing chloride ions to enter the concrete and diffuse vertically to both sides of the bonding interface, thus reducing the chloride ion concentration difference between the bonding interface and the concrete on both sides. When the casting interval exceeds the initial setting time of 12.5 h, an obvious increasing trend in ΔC at the 0–5 mm depth is observed for immersion periods of 56 days and 90 days, while ΔC remains almost unchanged for an immersion period of 28 days. This indicates that the bonding interface formed by pouring new concrete after the initial setting time of the old concrete has a greater impact on chloride ion diffusion in long-term immersion scenarios than when new concrete is poured at the initial setting time. This phenomenon is consistent with the dual comprehensive effect described in Section 3.2.1, where chloride ions diffuse along the bonding interface of staged pouring concrete and spread to both sides of the concrete [22].
3.2.3. Variation of Chloride Ion Concentration at Different Depths of Staged Pouring Concrete
Figure 9 illustrates the variation of chloride ion concentration at different depths of the bonding interface in staged pouring concrete. At a curing age of 28 days, the control group T0 concrete shows a chloride ion concentration of 0 at depths of 10 mm to 20 mm, with relatively low concentrations detected at depths of 0 mm–5 mm. As the pouring interval increases to 3 h, the chloride ion concentration in T3 concrete at depths of 15 mm to 20 mm is 0, with a low concentration detected at a depth of 10 mm, only 0.003%. When the pouring interval increases to 6 h, chloride ions are detected in T3 concrete at a depth of 20 mm, indicating that chloride ions have diffused to 20 mm or even deeper into the concrete. Beyond 6 h of pouring time, chloride ions have diffused into T6 concrete to depths exceeding 20 mm. The study indicates that the chloride ion concentration at different depths increases to varying degrees with an increase in the pouring interval. Considering the effect of curing age, at a curing age of 56 days, chloride ions are present at all depths of the staged pouring concrete samples, indicating that chloride ions have diffused to depths exceeding 20 mm within the concrete.
Similar to the results at a curing age of 28 days, different depths show varying trends in chloride ion concentration with changing pouring intervals, and at a curing age of 90 days, the variation pattern of chloride ion concentration in staged pouring concrete is consistent with that at a curing age of 56 days. Notably, the chloride ion concentration in staged pouring concrete decreases with increasing depth, and the rate of change decreases with increasing drilling depth. Fick’s second law can be used to describe its transmission, indicating that the transportation of chloride ions within the concrete is mainly diffusion-based, with chloride ion concentration decreasing with depth. To verify this variation pattern, a quadratic function is used to fit the chloride ion concentration at different depths of the bonding interface in staged pouring concrete. The fitted curve results are shown in Figure 9, with all R-squared values greater than 0.9, indicating a good correlation. Moreover, it can be observed that the rate of change of chloride ion concentration decreases with increasing diffusion depth. Therefore, the variation in chloride ion content at different depths of the bonding interface in staged pouring concrete with different pouring intervals can be represented by a quadratic function.
3.3. Microstructure Analysis of the Bonding Interface in Staged Pouring Concrete
3.3.1. MIP
The pore structure analysis of the staged pouring concrete with a curing age of 28 days was conducted, as shown in Figure 10, illustrating the pore structure analysis results of the staged pouring concrete. According to the effect of the micropore size on the durability of concrete specimens, the pores within the concrete are categorized into non-harmful micropores (<20 nm), harmless micropores (20–50 nm), harmful micropores (50–200 nm), and multi-harmful micropores (>200 nm) according to their sizes, wherein pores with a diameter of less than 50 nm are referred to as harmless micropores, and those with a diameter of more than 50 nm are referred to as harmful micropores [18,40].
From the differential pore size distribution curve in Figure 10a, it can be observed that compared to the control group T0 concrete, although the number of harmless micropores (<50 nm) increases with the increase in pouring interval time up to 12 h, there is a significant increase in harmful macropores (>50 nm). However, for the concrete specimens with a pouring interval time exceeding the initial setting time of 12.5 h, the number of harmful macropores increases significantly, especially for the T24 concrete. The cumulative pore size distribution curves shown in Figure 10b indicate that with the increase in pouring interval time, the cumulative pore distribution curves of staged pouring concrete shift upward, indicating a continuous increase in the cumulative total porosity of staged pouring concrete with the increase in pouring interval time. Further analysis of the changes in porosity and average pore size of the staged pouring concrete are illustrated in Figure 10c, revealing that both the porosity and average pore size of the staged pouring concrete increase with the prolongation of pouring interval time.
Based on the classification of pore size [40], the proportion of pore size distribution of concrete with different pouring interval times was calculated as shown in Figure 10d. It is observed that with the increase of pouring interval time, the proportion of harmful and highly harmful pores in the staged pouring concrete increases to varying degrees. Particularly for T24 concrete, the number of highly harmful pores increases by 60.8% compared to T0 concrete. The reason for this may be that during the pouring process, the lower layer of old concrete is not yet poured with the upper layer of new concrete before the initial setting time. With the increase of pouring interval time, the old concrete remains in the plastic stage, allowing the new cement slurry to fully wrap around the lower aggregate particles, and the aggregates can be densely embedded in each other [31].
Since the hydration of cement particles is a surface-to-interior process when the old concrete loses its plasticity and hardens, the surface slurry has completed partial hydration, and the surface hardness of cement particles gradually increases, making it difficult to wrap around the new concrete aggregates [34]. Thus, a good bond between the aggregate and the slurry cannot be formed. The density decreases with the increase of interval time, leading to an increase in the total porosity and average pore size of concrete, as well as an increase in the number of harmful pores. Therefore, this manifests as a decrease in the bonding interface performance of staged pouring concrete, decreasing its mechanical properties and resistance to chloride ion erosion [11,41].
3.3.2. SEM
As shown in Figure 11, micrographs of the selected representative control groups, T0 concrete, T6 concrete, and T24 concrete, are presented. It can be observed that the microstructure of the T0 concrete exhibits overall integrity, with no apparent voids or defects observed, indicating good overall compactness of the concrete matrix. In T6 concrete, a distinct transition zone at the bond interface can be observed, along with several microcracks. The pore structure is further degraded compared to T0, and the microstructure density is comparatively lower. However, the bond interface is relatively intact, indicating that hydration products provide some chemical bonding strength between the two layers of concrete [34,41]; the microstructure of T24 concrete shows significant structural degradation, with noticeable enlargement of interface defects. The bond interface of the concrete specimen has fractured, losing its integrity, and harmful pores with a diameter of approximately 100 μm are clearly visible, indicating lower compactness of the matrix structure at this stage. Analysis of the pore structure results reveals that with increasing pouring interval time, the bonding strength between the two layers of concrete gradually weakens, leading to continuous deterioration of the concrete pore structure, an increase in pore quantity, and the emergence of weak links at the bond interface of the two layers of concrete. The density of the microstructure gradually decreases [11,18].
From a macroscopic perspective, compared to monolithic concrete pouring, the splitting tensile strength of staged pouring concrete continuously decreases. The resistance to chloride ion erosion also continues to decline, which further confirms the trend observed in Section 3.2.3, where the difference in chloride ion concentration ΔC at depths of 0–5 mm exhibits an initial increase followed by a decrease and then tends to stabilize with increasing pouring interval time. In comparison, the ΔC values at depths of 0 to 5 mm for the 3 h, 6 h, and 9 h groups are significantly higher than those of the other groups, indicating a non-random occurrence.
4. Conclusions
This study conducted compressive strength tests, splitting tensile strength tests, and natural immersion experiments to investigate the effects of time intervals in staged pouring concrete, namely 0 h, 3 h, 6 h, 9 h, 12 h, 18 h, and 24 h, on the mechanical properties and chloride ion concentration distribution of staged pouring concrete. Additionally, the influence of the mechanism of pouring interval time on the mechanical properties and resistance to chloride ion erosion of staged pouring concrete was revealed from a microscopic structural perspective. The following conclusions were drawn:
(1) The splitting tensile strength of staged pouring concrete shows a continuous decrease with increasing pouring interval time. Concrete poured with a 24 h interval exhibited the lowest splitting tensile strength, with reductions of 40.93%, 32.92%, and 32.93% at ages of 7 days, 28 days, and 56 days, respectively. When the pouring interval time was within 12.5 h of the initial setting time of the old concrete, the decrease in splitting tensile strength increased with curing age. However, when the pouring interval exceeded the concrete’s initial setting time, the decrease in splitting tensile strength at an early age (7 days) was significantly greater than at 28 days and 56 days.
(2) The compressive strength of staged pouring concrete initially decreases and then increases with increasing pouring interval time. The lowest compressive strength was observed at a pouring interval time of 12 h, with reductions of 25.90%, 34.34%, and 14.69% at ages of 7 days, 28 days, and 56 days, respectively. The compressive strength decreased with increasing pouring interval time when the interval was less than the initial setting time of 12.5 h but increased with interval time exceeding the initial setting time, although it remained lower than that of monolithic concrete specimens.
(3) The chloride ion concentration at the bond interface and on both sides of staged pouring concrete increases continuously with increasing pouring interval time. Moreover, with increasing immersion time, the effect of pouring interval time on the chloride ion concentration at the bond interface is greater than that on the sides of the concrete. The chloride ion concentration at the bond interface of staged pouring concrete is always higher than that on both sides, and the difference in chloride ion concentration between the bond interface and the sides of the concrete decreases with increasing diffusion depth.
(4) The chloride ion transport at the bond interface of staged pouring concrete conforms to Fick’s second law, with diffusion being the primary mode of transport, leading to a decrease in concentration with increasing depth. Additionally, a quadratic function curve can be used to characterize the variation trend of chloride ion concentration at different depths of the bond interface of staged pouring concrete.
(5) The chloride ion concentration difference ΔC is proposed to evaluate the influence of bond interface performance on chloride ion concentration in staged pouring concrete. The influence of bond interface performance on chloride ion concentration decreases to different extents with increasing depth but varies with different immersion ages. For a short immersion age of 28 days, the influence of bond interface performance on chloride ion concentration near the concrete surface is greater at pouring interval times of 3 h, 6 h, and 9 h than at 0 h, 12 h, 18 h, and 24 h. For immersion ages of 56 days and 90 days, the influence of bond interface performance on chloride ion concentration increases with increasing pouring interval time. Still, it has almost no effect on chloride ion concentration at a depth of 20 mm.
Conceptualization, Z.C. and J.W.; methodology, Z.H.; validation, Z.C. and J.W.; formal analysis, Z.H.; investigation, Z.H.; writing—original draft preparation, Z.H. and G.Z.; writing—review and editing, J.W. and Y.T.; visualization, Y.T.; supervision, Z.C.; project administration, Z.C. All authors have read and agreed to the published version of the manuscript.
The data that support the findings of this study are available on request from the corresponding author.
Author Zhaoqi Huang is employed by the Guangxi Yidong Expressway Company Limited, Guangxi Communications Investment Group Corporation Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Footnotes
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Enlarge this image.Figure 2. Schematic diagram of mechanical performance tests for staged pouring concrete.
Enlarge this image.Figure 4. Schematic diagram of sampling for microscopic testing of staged pouring concrete.
Enlarge this image.Figure 8. Chloride ion concentration difference between the bonding interface and both sides of staged concrete.
Enlarge this image.Figure 9. Chloride ion content at different depths of the bonding interface in staged pouring concrete.
Mix proportion design of staged pouring concrete kg/m3.
| Cement | Fly Ash | Silica Fume | Microspheres | Polypropylene Fiber | Medium Stone 10–16 | Small Stone 5–10 | Manufactured Sand | Superplasticizer | Water |
|---|---|---|---|---|---|---|---|---|---|
| 325 | 87 | 25 | 52 | 1.25 | 797 | 265 | 852 | 8.3 | 146 |
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; Tang, Yunchao 3 1 School of Civil Engineering and Architecture, State Key Laboratory of Featured Metal Materials and Life-Cycle Safety for Composite Structures, Guangxi University, Nanning 530004, China;
2 School of Civil Engineering and Architecture, State Key Laboratory of Featured Metal Materials and Life-Cycle Safety for Composite Structures, Guangxi University, Nanning 530004, China;
3 School of Civil Engineering and Architecture, State Key Laboratory of Featured Metal Materials and Life-Cycle Safety for Composite Structures, Guangxi University, Nanning 530004, China;