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Article

Mechanical Performance of Prefabricated Monolithic Composite Columns with Reinforced ECC Precast Shell

by
Tingting Lu
*,
Bin Wang
and
Haowei Jin
Shaanxi Key Laboratory of Safety and Durability of Concrete Structures, Xijing University, Xi’an 710123, China
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(11), 1777; https://doi.org/10.3390/buildings15111777
Submission received: 22 April 2025 / Revised: 18 May 2025 / Accepted: 20 May 2025 / Published: 22 May 2025
(This article belongs to the Section Building Structures)
Browse Figures
Versions Notes

Abstract

:
To enhance the mechanical properties of a precast monolithic column, the ECC material was made into a prefabricated shell with reinforced bars. Post-cast concrete was poured into the precast parts to form the reinforced ECC precast monolithic composite columns. An axial compression test was conducted to investigate the failure pattern, load-bearing capacity, and deformation performance. The results showed that the R/ECC composite columns had good integrity. The R/ECC prefabricated shell exerted an extra confinement effect on the column and enhanced deformability. At the yield stage, the displacement of the reinforced ECC prefabricated columns was 16.19% greater than that of RC composite columns on average. Additionally, the maximum load point displacement exhibited 15.30% growth. The ECC material delayed the yield time of longitudinal bars and stirrups. Before reaching the maximum load, the strains in the R/ECC composite column rebars were smaller than those in the RC column rebars. At the maximum loading point, the longitudinal reinforcement exhibited a 9.36% greater strain than that of the RC composite column.

1. Introduction

In recent years, with the development of prefabricated buildings, prefabricated concrete structures have relied on “integral” characteristics, promoting rapid development and application. Precast monolithic composite structural elements have also been extensively researched and applied for many years, with high construction efficiency, controllable quality, and significant economic benefits. Therefore, this method is applied in new construction, bridge engineering, and building retrofitting/strengthening projects. However, some mechanical performance problems were presented with the increased application of composite structures. Researchers used various methods to improve the mechanical properties of composite members [1,2,3]. Several studies were also conducted on the mechanical properties of composite structural members under various conditions, and the research results showed that the modified composite members exhibit consistent improvements in mechanical properties [4,5,6].
Engineered cementitious composites (ECCs) have the characteristics of micro-crack development and pseudo-strain hardening. ECCs have great potential to improve and enhance the safety, damage resistance, and seismic resilience of concrete structures. The material is mixed with industrial wastes, including fly ash and silica fume, which are sustainable. Researchers have used ECC materials to reinforce beams and columns or as permanent formwork to form composite members with conventional concrete to improve the bearing capacity, deformation capacity, and damage resistance of concrete structures [7,8,9].
The ECC was prefabricated into a U-shaped permanent formwork with conventional concrete to form a composite beam. The composite beam’s shear capacity was improved, the crack speed slowed down, and the durability was further improved [10,11,12,13]. Other researchers used ECC material to prefabricate permanent formwork and form ECC/RC composite columns with post-cast concrete. The cracks of the composite column showed subtle and dense characteristics, and its horizontal bearing capacity, energy dissipation capacity, and deformation capacity were significantly improved [14,15,16]. Some researchers partially replaced concrete with ECC material to form composite beams in the tensile area at the bottom of the beam, and their crack resistance and bending performance were effectively improved [17,18,19]. The ECC prefabricated formwork provided good restraint for the composite column and enhanced the overall rigidity and peak strength of the composite column [20,21]. The load-carrying capacity of fully encased composite columns showed significant improvement, particularly in short columns. The enhancement was further amplified when the tie reinforcement spacing was reduced [22]. Researchers also used ECC materials to strengthen short concrete columns, investigating the confinement effects of different reinforced thicknesses. The ECC-reinforced short columns exhibited an increased load-bearing capacity, and the failure mechanism resembled that of stirrup-confined short columns, exhibiting similar crack patterns and ductile behavior [23]. The reinforced ECC prefabricated shell and post-cast concrete were used to form composite members in the precast monolithic structures, which can effectively improve their mechanical performance. The flexural bearing capacity was significantly improved, the crack development was effectively controlled, and the ECC material could delay the yield of the steel bars in the shell [24,25].
According to the research mentioned above, most researchers applied ECC materials to the partial areas of components (such as beam–column joints, expected damage position, etc.), only used ECC materials to make components, or used ECC materials to reinforce RC structures to form composite components. The composite members all showed good mechanical properties.
However, studies of the application of ECC materials to prefabricated monolithic structures are relatively rare. In order to enhance the performance of prefabricated structures, this study proposed a novel precast monolithic composite column system. In this paper, ECC material was used for prefabricated parts with welded steel mesh in the shell to form a new R/ECC prefabricated monolithic composite column with post-cast conventional concrete in the core. Using the axial compression experiment, we systematically analyzed the influence of prefabricated shell material and the volumetric stirrup ratio on the compressive characteristics of the composite column. The failure pattern and failure mechanism were studied. The experimental findings provide theoretical foundations for engineering applications of the precast monolithic composite structures.

2. Materials and Methods

2.1. Experimental Design

Based on research requirements and in compliance with the code for design of concrete structures, code for seismic design of buildings, and technical specification for precast concrete structures, seven specimens were designed to investigate the mechanical behavior of the reinforced ECC prefabricated shell monolithic composite columns through axial compression testing. Considering the limitations of experimental conditions, 1/2-scale specimens were adopted for the tests. Factors such as the precast shell material and volume stirrup ratio were investigated. Three columns were R/ECC prefabricated shells, numbered FRC-P-1 to FRC-P-3. The other three specimens were RC prefabricated shells, numbered RC-P-1 to RC-P-3. One cast-in-place RC column was made for comparison. The cross-section of each columns was 250 mm × 250 mm. The height of the column was 750 mm. The prefabricated shell thickness was 30 mm. The detailed reinforcement and parameters of the specimen are shown in Figure 1 and Table 1. The grade of the steel bars was HRB400. The diameter of the longitudinal bars was 12 mm, and the transverse reinforcement consisted of 6 mm diameter hoops with different spacing. The shell was precast first with the reinforced bars. After curing for a certain amount of time, the conventional concrete was poured inside the reinforced prefabricated shell. Each pouring reserved three cubes, three prisms, and three tensile test blocks. The blocks were cured under the same conditions as the specimens cured under natural environmental conditions.

2.2. Materials

The concrete and ECC material mixtures were proportioned to meet the C40 compressive strength grade requirements. Considering castability requirements, commercial fine-aggregate concrete was used for the conventional concrete mixtures. The ECC material was a mixture composed of cement, fly ash, quartz sand, and water. The mix proportion is shown in Table 2. The quartz sand was formulated by blending four gradations: 70–120 mesh, 40–70 mesh, 26–40 mesh, and 16–26 mesh, with a mass ratio of 1:1.48:1.73:2.56. The PVA fibers were added to the material to improve its performance. The PVA fiber property parameters are listed in Table 3. A polycarboxylate-based superplasticizer was used to improve the workability of the composite mixture. The mechanical properties of ECC materials, including tensile performance, were evaluated in accordance with the Standard Test Method for Mechanical Properties of Ductile Fiber-Reinforced Cementitious Composites [26]. From the tensile and compressive stress–strain curves presented in Figure 2, the ECC material showed superior deformability characteristics. The average tensile strength was 4.8 MPa through the standardized dumbbell-shaped specimens, as prescribed in the standard, and the average ultimate tensile strain reached 1.8% (Figure 2a). The average value of the cubic compressive strength fcu,ECC was 51.1 MPa through the 100 mm × 100 mm × 100 mm cubes. The prism compressive strength fc,ECC was 41.5 MPa through the 100 mm × 100 mm × 300 mm blocks, the strain at the peak point was 0.0036, and the ultimate compressive strain reached 0.0055 on average (Figure 2b). The conventional concrete cubic compressive strength fcu,c was 50.4 MPa, and the axial compressive strength fc,c was 39.3 MPa on average; both were evaluated in compliance with the Standard for Test Methods of Concrete Physical and Mechanical Properties [27].
The steel bars were measured according to the Standard Metallic Materials-Tensile Testing—Part 1: Method for tests at room temperature. Each rebar diameter was reserved in three pieces to test the mechanical properties [28]. The grade of the steel bars was of grade HRB400. The diameter of the longitudinal bars was 12 mm, the average yielding strength was 453 MPa, and the ultimate tensile strength was 592 MPa. The transverse reinforcement consisted of 6 mm diameter hoops, with measured yield strength at 442 MPa and ultimate strength at 544 MPa stress levels, as shown in Table 4.

2.3. Test Setup and Instrument

All the prefabricated composite column specimens were loaded on a 5000 kN electro-hydraulic servo-compression testing machine in the Shaanxi Provincial Key Laboratory of Safety and Durability of Concrete Structures. The test was conducted under displacement control at a loading rate of 0.5 mm/min. An initial displacement of 0.3 mm was applied at each level, after which the load was maintained for 120 s. When the longitudinal reinforcement yielded, the loading displacement of each stage was changed to 0.5 mm, and then the load was held for 120 s. The experiment was concluded once the load declined to 60% of the maximum load.
The strain of steel reinforcement was monitored by electrical strain gauges (120 Ω, 3 mm gauge length). The displacement meter D0 was placed to monitor the actual displacement values applied by the loading device. The vertical deformations of the middle region were monitored by displacement meters (D1~D2, measurement range: ±50 mm, resolution: 0.01 mm, accuracy: ±0.1%), as shown in Figure 3. The lateral deformations of the middle region were monitored by displacement meters (D3~D4, measurement range: ±50 mm, resolution: 0.01 mm, accuracy: ±0.1%). The strain gauges S1 and S2 (120 Ω, 60 mm gauge length) were also arranged in the middle of the column to monitor the transverse strain of the concrete and ECC material. The strain gauges S3 and S4 (120 Ω, 60 mm gauge length) were also arranged to monitor the longitudinal strain of the concrete and ECC material.

3. Results and Discussion

3.1. Failure Pattern

The RC composite columns exhibited apparent damage, such as concrete spalling under axial compression, while the R/ECC composite columns showed good integrity and deformability. The excellent bonding between ECC materials and conventional concrete and the composite stirrups connecting the longitudinal reinforcement ensured effective composite action between the R/ECC precast shell and cast-in-place concrete without noticeable bond-slip behavior.
  • For specimen FRC-P-1, at an axial load of 1014.32 kN, initial vertical cracking appeared from the top of the column. The cracks developed with the increasing load. When the load reached 2448.84 kN, the vertical cracks on the right side of the specimen extended to the middle, and the transverse crack appeared at the edge of the column. The longitudinal reinforcement yielded at a load of 2625.80 kN. At the peak load, the transverse crack extended to the other side. The transverse cracks were connected with the vertical cracks on the left and right sides to form an “H” shape. Until the test stopped, the maximum crack width was about 3 mm. The final state is shown in Figure 4.
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Figure 4. Failure diagram of specimen FRC-P-1. (a) Integral failure pattern; (b) Localized damage.
Figure 4. Failure diagram of specimen FRC-P-1. (a) Integral failure pattern; (b) Localized damage.
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  • For specimen FRC-P-2, when the load was 679.77 kN, small vertical cracks occurred at the upper part. With the increasing load, continuous crack formation and extension were observed. When loading to 2652.38 kN, transverse cracking was initiated on one side of the column. The longitudinal reinforced bars yielded at 2908.29 kN. At peak load capacity, the transverse crack propagated through the full section width, and the vertical crack on the upper right side of one surface developed into the transverse crack. At test termination, the specimen exhibited a transverse crack present in the shell, and there was a vertical crack extending in the middle, as shown in Figure 5.
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Figure 5. Failure diagram of specimen FRC-P-2. (a) Integral failure pattern; (b) Localized damage.
Figure 5. Failure diagram of specimen FRC-P-2. (a) Integral failure pattern; (b) Localized damage.
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  • When the load of specimen FRC-P-3 reached 1030.28 kN, transverse cracking was observed at three separate positions in the central region of the specimen. The longitudinal reinforced bars yielded at a load of 2039.07 kN. Upon reaching the ultimate load, a slender vertical crack appeared from the top surface to the central section. The edges and corners at the upper part were broken. The central transverse crack propagated through the entire cross-section, resulting in full penetration of both specimen surfaces. When the specimen failed, there were obvious vertical and transverse cracks in the mid-height region of the column, as illustrated in Figure 6. However, the ECC material of the shell did not fall off, and the specimen still maintained good integrity.
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Figure 6. Failure diagram of specimen FRC-P-3. (a) Integral failure pattern; (b) Localized damage.
Figure 6. Failure diagram of specimen FRC-P-3. (a) Integral failure pattern; (b) Localized damage.
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  • For specimen RC-P-1, vertical cracks appeared at a load of 438.40 kN. The vertical cracks continued to occur and extend with the increase in load. The reinforced bar yielding was initiated at a measured load of 2406.67 kN. At 2549.28 kN, longitudinal cracks also developed in the specimen’s lower region. Simultaneously, existing vertical cracks propagated fully, creating a continuous through-fracture. When it reached the peak load, dense vertical cracks appeared in the concrete shell, and the concrete began to spall off. When the specimen failed, the longitudinal cracks propagated vertically between the column’s end regions, and the shell at the edges and corners broke and fell. Crushing occurred at the edges and corners of the lower side of the column (Figure 7a). Extensive concrete spalled off, and the reinforcement was exposed, as shown in Figure 7b.
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Figure 7. Failure diagram of specimen RC-P-1. (a) Integral failure pattern; (b) Localized damage.
Figure 7. Failure diagram of specimen RC-P-1. (a) Integral failure pattern; (b) Localized damage.
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  • Vertical cracks began to appear at the bottom of specimen RC-P-2 at precisely 561.08 kN. Primary vertical cracks formed in the upper region at the load of 1325.52 kN. The longitudinal bars reached yielding when they reached 2788.29 kN. Upon reaching 3287.28 kN, transverse cracking and concrete extrusion occurred at the mid-height region, indicating compressive failure. The column exhibited diagonal crack failure. The column exhibited concrete cover spalling, resulting in the peeling of the shell (Figure 8a). In the end, the concrete at the edges and corners of the column cracked and spalled off. The steel bars were exposed. The final failure pattern and damage distribution are illustrated in Figure 8b.
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Figure 8. Failure diagram of specimen RC-P-2. (a) Integral failure pattern; (b) Localized damage.
Figure 8. Failure diagram of specimen RC-P-2. (a) Integral failure pattern; (b) Localized damage.
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  • For column RC-P-3, the first visible vertical cracks formed at an applied load of 430.20 kN, and the vertical cracks began to propagate bidirectionally from the top and bottom toward the midspan when the load reached 1640.96 kN. The reinforced bars reached yielding at 1980.68 kN. At the ultimate load of 3014.24 kN, the main diagonal crack formed (Figure 9a), generating dense cracks around the vertical cracks, and the concrete began to fall off. When the test stopped, extensive concrete spalled off, resulting in full exposure of the reinforcement, as shown in Figure 9b.
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Figure 9. Failure diagram of specimen RC-P-3. (a) Integral failure pattern; (b) Localized damage.
Figure 9. Failure diagram of specimen RC-P-3. (a) Integral failure pattern; (b) Localized damage.
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  • For specimen RC-1, vertical cracks occurred at the top of the column when the load reached 667.35 kN. At approximately 2621.15 kN, vertical cracks developed through the full specimen height, and the number of cracks increased. At 2654.32 kN, the reinforcement yielded. At the ultimate load of 3254.28 kN, the vertical cracks gradually developed to form oblique cracks. The concrete in the middle of the specimen was crushed, and the concrete began to spall off (Figure 10a). When the loading was stopped, vertical cracks at the upper and lower column regions progressively connected through diagonal fracture paths. A large amount of concrete fell off, exposing the longitudinal bars, as shown in Figure 10b.
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Figure 10. Failure diagram of specimen RC-1. (a) Integral failure pattern; (b) Localized damage.
Figure 10. Failure diagram of specimen RC-1. (a) Integral failure pattern; (b) Localized damage.
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From the analysis of the specimens mentioned above, it can be observed that the RC precast shell composite columns exhibited typical diagonal crack-induced compression failure under axial compressive loading with severe damage, accompanied by significant concrete spalling and exposed reinforced bars. The columns exhibited brittle failure behavior. Even the conventional cast-in-place reinforced concrete specimen RC-1 showed diagonal crack failure, along with substantial concrete spalling and exposed reinforced bars. In contrast, the columns with R/ECC precast shells, due to the strain-hardening characteristics and superior deformation capacity of ECC material, provided additional confinement to the post-cast concrete inside the column. At the final failure stage, specimens FRC-P-1 and FRC-P-2 did not exhibit spalling of the ECC material, and the overall integrity of the specimens remained good. For specimen FRC-P-3, which had the lower stirrup ratio, the damage was more severe, only limited to the middle part of the specimen, with minor cracks appearing at the upper and lower regions of the column. Failure analysis revealed ductile characteristics in the R/ECC precast monolithic composite columns. Since the ECC material exhibited greater strain capacity at peak compressive stress compared with conventional concrete, the R/ECC precast shell ultimately developed transverse cracks due to the compressive failure of the post-cast concrete inside the column.

3.2. Load Deformation Analysis

The load–deformation curves of the specimens could reflect the fundamental axial compression behavior of the columns. Figure 11 presents the axial stress–strain curves for each specimen. All load–deformation curves were nondimensionalized, with stress and strain normalized to their respective reference values.
  • For specimen FRC-P-1, which used ECC material for the precast shell, the yield load increased by 6.36% compared with the specimen RC-P-1 with an RC precast shell, and the corresponding compressive strain increased by 20.42% in Table 5. Although peak load differences were negligible, FRC-P-1 showed a 23.62% higher compressive strain at the ultimate load. Compared with RC-P-2, FRC-P-2 exhibited a 7.96% higher yield load capacity and a 20.61% enhancement in the corresponding compressive strain. There was also an 8.97% increase in the compressive strain at peak load. For specimen FRC-P-3, the yield load slightly increased compared with RC-P-3, with a 7.54% increase in the corresponding compressive strain. At maximum load, specimen FRC-P-3 achieved a 13.31% greater compressive strain compared with the reference.
  • Compared with RC precast shell composite columns, the deformation capacity of R/ECC precast shell composite column specimens was significantly improved without a sudden drop in load-bearing capacity. The R/ECC prefabricated shell provided additional confinement to the core concrete originating from the tensile strain-hardening characteristics of the ECC material beyond the confinement provided by the stirrups. The yielding of longitudinal reinforced bars was postponed, effectively enhancing the deformation capacity of the columns. The yield and ultimate load capacity of specimen FRC-P-1 were close to those of the cast-in-place specimen RC-1, with no significant difference in compressive strain. It indicated that the monolithic composite columns with prefabricated R/ECC shells exhibit essentially equivalent bearing capacity and deformation capability compared to the specimen RC-1.
  • With the increase in the volumetric stirrup ratio, the ultimate load of the specimens increased slightly, and the deformation capacity was significantly enhanced. Compared with FRC-P-3, the volumetric stirrup ratio of FRC-P-1 increased by 0.31%, resulting in a 28.72% enhancement in yield load and a 46.14% improvement in compressive strain. The compressive strain at peak load increased by 24.75%. Compared with FRC-P-1, FRC-P-2 showed a 14.69% rise in yield load and a 24.20% improvement in corresponding compressive strain. At the ultimate load, the corresponding compressive strain improved by 13.25%. For the composite columns with RC precast shells, compared to RC-P-3, RC-P-1 exhibited a 22.32% increase in yield load and a 30.51% increase in corresponding compressive strain. The specimen RC-P-2, with a 0.43% higher volumetric stirrup ratio than RC-P-1, showed a 12.99% improvement in yield load and a 24.01% enhancement in corresponding compressive strain. The compressive strain at peak load increased by 31.41%. It showed that the influence of ECC materials on both yield capacity and compressive ductility of composite columns varied depending on the volumetric stirrup ratio.

3.3. Strain Analysis

3.3.1. Stirrup Strain

When short columns were subjected to axial compression, the load–stirrup strain curves could be analyzed to understand the confinement effect and deformability, as shown in Figure 12.
  • When the load was less than 500 kN, the development trends of stirrup strain for specimens FRC-P-1 and RC-P-1 were basically the same. For loads from 500 kN to the peak load, the stirrup strains of specimen FRC-P-1 were consistently lower than those of specimen RC-P-1 at equivalent loading stages. The stirrup strain of specimen FRC-P-1 showed a sudden increase in growth rate at the load level of 2294.28 kN, while the stirrup strain of column RC-P-1 exhibited a similar sudden increase at a load of 2065.87 kN. The stirrups in specimen RC-P-1 yielded at the peak load point, whereas those in specimen FRC-P-1 did not. After the peak load, specimen RC-P-1 exhibited brittle post-peak behavior, while the stirrup strain of specimen FRC-P-1 increased rapidly, and the curve declined more slowly. Compared with specimen RC-P-1, the stirrup strain of specimen FRC-P-1 at the peak load was reduced by 22.28%.
  • When the load was less than 1000 kN, there was no significant difference in stirrup strain between specimen RC-P-2 and specimen FRC-P-2. From the load of 1000 kN to the peak load, the stirrup strain of specimen RC-P-2 began to exceed that of specimen FRC-P-2, and the difference gradually increased. Due to the high stirrup ratio of the two specimens, the stirrups of the two specimens did not yield at the ultimate load point. After that, the stirrup strain of specimen FRC-P-2 increased rapidly, and the curve declined more slowly than that of specimen RC-P-2.
  • The stirrup strains of specimens RC-P-3 and FRC-P-3 yielded before the peak load. At the peak load point, the stirrup strain of specimen FRC-P-3 was 15.76% smaller than that of RC-P-3. Then, the load of specimen FRC-P-3 decreased more slowly than that of specimen RC-P-3. Because the volumetric stirrup ratios of specimens RC-P-3 and FRC-P-3 were smaller than those of other specimens, the stirrup strains of specimens RC-P-3 and FRC-P-3 were larger than those of other specimens. The confined effectiveness was relatively weaker.
The tensile strain-hardening characteristic of the ECC improved the cooperative performance between the prefabricated shell and stirrups, leading to measurable enhancements in confinement effectiveness. The yield of the stirrup was delayed. Thus, the restraint effect was improved. Then, the deformability of the composite columns was enhanced.

3.3.2. Longitudinal Reinforced Bar Strain

The change in the strain curves of the longitudinal reinforced bars could correspond with the variation in the vertical strain development of the columns. The diagram of the axial load versus reinforced bar strain relationship is illustrated in Figure 13.
  • The longitudinal reinforcement strain in RC composite columns exhibited a slower growth trend with increasing stirrup ratios. At pre-peak loading stages, RC-P-3 developed the highest longitudinal reinforcement strains, whereas RC-P-2 maintained the lowest recorded values. As the loading approached its maximum load, specimen RC-P-2 displayed an accelerated strain development. At peak load, RC-P-1 exhibited an 8.85% greater longitudinal strain than RC-P-3, while RC-P-2 showed a 33.22% increase compared with RC-P-3. During the post-peak load stage, RC-P-2 maintained the most stable load-bearing behavior, with its load–strain curve descending much more slowly than that of other specimens.
  • For R/ECC specimens, the development pattern of longitudinal reinforcement strain was similar to that of RC composite columns. However, distinct differences were observed in strain values at the maximum load point. Compared with FRC-P-3, FRC-P-1 exhibited a 9.36% improvement in longitudinal bar strain, while FRC-P-2 showed a more substantial 44.59% enhancement.
  • Before the peak load point, the longitudinal reinforced bars’ strains in the composite column with the R/ECC prefabricated shell were lower than those in the RC counterpart at equivalent loading stages. When the stirrup ratio was relatively large, the longitudinal reinforcement strain value was quite different. The longitudinal strain difference between RC-P-3 and FRC-P-3, which had the smallest stirrup ratio, was relatively small, as shown in Figure 13. The ECC material delayed the yield time of the longitudinal reinforced bars. At equivalent longitudinal reinforcement strain levels, the R/ECC composite columns showed a 9.23% higher load-carrying capacity on average compared with RC counterparts. However, at the peak load, the corresponding strain of the longitudinal reinforced bars in the specimen FRC-P-1 was 6.68% larger than that of RC-P-1, the strain corresponding to the specimen FRC-P-2 was 15.21% larger than that of RC-P-2, and the strain of specimen FRC-P-3 was 6.18% larger than that corresponding to RC-P-3. The R/ECC composite columns exhibited better deformability than the RC composite columns. After the peak load, the curve decline of the R/ECC composite columns was slower compared with the RC prefabricated shell composite columns. This observation also showed that the volumetric stirrup ratio notably influenced the longitudinal reinforcement strain in R/ECC composite columns. It indicated effective cooperative behavior between the ECC precast shell and transverse reinforcement in providing complementary confinement.

3.3.3. Strains of Concrete and ECC Material

The axial load versus concrete strain relationships of the composite columns are presented in Figure 14. The transverse concrete strain measurements were recorded as positive values and plotted conventionally on the right side of the ordinate axis, and vertical contraction strains were plotted as negative values on the left ordinate axis.
Compared with specimens FRC-P-1 and RC-P-1, the strain of specimens FRC-P-1 and RC-P-1 exhibited linearly elastic strain development during the initial loading in the tests, with nearly identical curve progression trends. Under the same load, the strains of the ECC precast shell composite columns were smaller than those of the RC precast shell composite columns. At the peak load point, the vertical strains corresponding to the R/ECC composite columns were larger than those of the RC composite columns. For the transverse strain, the strains corresponding to the ECC precast shell composite columns were smaller than those of the RC composite columns. After the peak load, there were no data on the transverse strain of the RC composite columns due to the phenomenon of cracks and concrete spalling. However, the decline in curve data could still be measured because the ECC composite columns still had good integrity.

4. Conclusions

This experimental program systematically evaluated the axial compressive performance of prefabricated monolithic composite columns. The axial compressive performance and influencing factors of R/ECC composite columns were comparatively analyzed through axial compression tests on three R/ECC prefabricated composite columns, three RC prefabricated composite columns, and one cast-in-place column. The key findings can be summarized as follows.
  • The R/ECC prefabricated shell monolithic composite columns exhibited good mechanical properties under axial compression. The R/ECC composite columns exhibited excellent structural integrity throughout testing, and the final failure was manifested as transverse cracks in the shell but no spalling with characteristic ductile failure behavior. However, the RC composite columns showed a typical diagonal compression failure. The concrete spalled off, and the reinforced bars were exposed. The damage was serious.
  • Compared to the specimens with an RC precast shell, the specimens that used ECC material for the precast shell showed improved deformability. The yield load increased slightly, and the corresponding compressive strain increased by 16.19% on average. Additionally, the compressive strain at the ultimate load improved by 15.30% on average. Using ECC material in the precast shell enhanced the restraint effect, thereby improving the deformability and integrity of the composite column.
  • With the enhancement in the volumetric stirrup ratio, the deformability of the R/ECC prefabricated shell composite columns showed a measurable improvement. Compared with specimen FRC-P-3, the volumetric stirrup ratio of specimen FRC-P-1 increased by 0.31%, its yield load was enhanced by 28.72%, and the corresponding compressive strain increased by 46.14%. The compressive strain corresponding to the peak load showed a 24.75% enhancement. Compared with column FRC-P-1, the yield load of specimen FRC-P-2 increased by 14.69%, and the corresponding compressive strain increased by 24.20%. The corresponding compressive strain at the maximum load exhibited a 13.25% improvement. The experimental measurements showed that ECC materials’ enhancement effects on column yield strength and compressive strain varied with changes in the volumetric stirrup ratio.
  • ECC materials could delay the yield time of the reinforced bars. Before the maximum load, the longitudinal reinforced bar strains in the composite column with the R/ECC precast shell were lower than those of the RC prefabricated shell composite columns at the equivalent load level. At equivalent longitudinal reinforced bar strain levels, the R/ECC precast shell composite column showed consistently higher load-bearing capacity than the RC precast shell counterpart. During the post-ultimate load stage, the R/ECC precast shell composite column exhibited a more gradual load degradation compared with the RC precast shell composite column system.
The experimental investigation in this study was limited to the axial compression performance of R/ECC prefabricated shell composite columns, specifically investigating the influences of shell material and volumetric stirrup ratio. Future studies should address the effects of different ECC material strengths, varying shear span ratios, and other parameters. Additionally, it is necessary to study the seismic performance of R/ECC composite columns and the influencing factors.

Author Contributions

Conceptualization, T.L. and H.J.; Investigation, B.W. and H.J.; Data curation, B.W. and H.J.; writing—original draft preparation, T.L. and B.W.; writing—review and editing, T.L. and B.W.; supervision, T.L.; funding acquisition, T.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Science Foundation of Shaanxi Province, grant number “2021JQ-873”, and the Youth Innovation Team of Shaanxi Provincial Department of Education, grant number 21JP140.

Data Availability Statement

The data that support this study are available from the corresponding author upon reasonable request.

Acknowledgments

The authors express their gratitude to those who provided the equipment and technical support for the testing in the paper.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Specimen dimension diagram (Unit: mm). (a) General dimension; (b) Cross-section.
Figure 1. Specimen dimension diagram (Unit: mm). (a) General dimension; (b) Cross-section.
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Figure 2. ECC material stress–strain curves. (a) Tensile curves; (b) Compressive curves.
Figure 2. ECC material stress–strain curves. (a) Tensile curves; (b) Compressive curves.
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Figure 3. Test layout. (a) Loading equipment schematic diagram; (b) Actual loading setup.
Figure 3. Test layout. (a) Loading equipment schematic diagram; (b) Actual loading setup.
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Figure 11. Load–deformation curves of each specimen.
Figure 11. Load–deformation curves of each specimen.
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Figure 12. Load–strain curves of stirrups in specimens.
Figure 12. Load–strain curves of stirrups in specimens.
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Figure 13. Load–strain curves of longitudinal rebars in specimens.
Figure 13. Load–strain curves of longitudinal rebars in specimens.
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Figure 14. Load–strain curves of concrete and ECC material.
Figure 14. Load–strain curves of concrete and ECC material.
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Table 1. Specimen design parameters.
Table 1. Specimen design parameters.
NumberShell MaterialLongitudinal Reinforcement (mm2)Stirrup Spacing
(mm)		Volumetric Stirrup Ratio
(%)
FRC-P-1R/ECC904701.05
FRC-P-2R/ECC904501.48
FRC-P-3R/ECC9041000.74
RC-P-1RC904701.05
RC-P-2RC904501.48
RC-P-3RC9041000.74
RC-1Cast-in-place904701.05
Table 2. The amount of each component in the ECC material.
Table 2. The amount of each component in the ECC material.
Cement (g/L)Fly Ash (g/L)Quartz Sand (g/L)PVA Fiber (g/L)Water Reducer (g/L)Water (g/L)
6306304062015436
Table 3. Parameters of PVA fiber.
Table 3. Parameters of PVA fiber.
Length (mm)Diameter (µm)Tensile Strength (MPa)Elastic Modulus (GPa)Elongation (%)
12401560416.5
Table 4. Average values of reinforced bars.
Table 4. Average values of reinforced bars.
Diameter (mm)Yield Strength (MPa)Ultimate Strength (MPa)
6442544
12453592
Table 5. Loads and strains at critical points.
Table 5. Loads and strains at critical points.
NumberYield PointPeak Load Point
Load (kN)Compressive Strain (10−6)Load (kN)Compressive Strain (10−6)
FRC-P-12624.807771.183199.829511.28
FRC-P-23010.479652.343293.8410,772.33
FRC-P-32039.135317.333074.387624.52
RC-P-12467.886453.333181.437693.67
RC-P-22788.628002.673287.2810,110.76
RC-P-32017.454944.473014.246728.87
RC-12654.327738.673254.289558.67
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Lu, T.; Wang, B.; Jin, H. Mechanical Performance of Prefabricated Monolithic Composite Columns with Reinforced ECC Precast Shell. Buildings 2025, 15, 1777. https://doi.org/10.3390/buildings15111777

AMA Style

Lu T, Wang B, Jin H. Mechanical Performance of Prefabricated Monolithic Composite Columns with Reinforced ECC Precast Shell. Buildings. 2025; 15(11):1777. https://doi.org/10.3390/buildings15111777

Chicago/Turabian Style

Lu, Tingting, Bin Wang, and Haowei Jin. 2025. "Mechanical Performance of Prefabricated Monolithic Composite Columns with Reinforced ECC Precast Shell" Buildings 15, no. 11: 1777. https://doi.org/10.3390/buildings15111777

APA Style

Lu, T., Wang, B., & Jin, H. (2025). Mechanical Performance of Prefabricated Monolithic Composite Columns with Reinforced ECC Precast Shell. Buildings, 15(11), 1777. https://doi.org/10.3390/buildings15111777

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