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标题: Luo Xiaoyu et al.: Improving 3D Printed Concrete with Through-Layer Reinforcement and Grout Filling [打印本页]

作者: admin 时间: 2026-10-12 17:30
标题: Luo Xiaoyu et al.: Improving 3D Printed Concrete with Through-Layer Reinforcement and Grout Filling
Source: Journal of Architecture and Civil Engineering.

Mechanical performance of 3D printed concrete enhanced by through-layer reinforcement insertion and grout filling

Luo Xiaoyu¹, Hu Jiawei¹, Yang Min², Yao Xiaofei², Zou Cunjun¹, Zhao Yuqi¹ and Shi Hao¹.

1. School of Highway, Chang'an University, Xi'an, Shaanxi 710064, China.
2. CCCC First Highway Consultants Co., Ltd., Xi'an, Shaanxi 710075, China.

0. Introduction

3D printed concrete eliminates dependence on formwork, reduces pollution and improves construction flexibility, making it an important direction for industrialized construction. However, its bead-by-bead, layer-by-layer process creates inherent mechanically weak interfaces.

Through-layer reinforcement attracts attention because it does not interfere with printing paths. Inserting steel bars inevitably creates voids, however, weakening interaction between reinforcement and concrete. This study proposes void filling with grout. Building on HB through-layer reinforcement, it compares plain cement paste (CP), U-type expansive-agent-modified cement grout (UEC) and nanosilica-modified cement grout (NSC). Macroscopic mechanical tests, mesoscale cross-section analysis and scanning electron microscopy (SEM) reveal filling effects and reinforcement mechanisms, alongside a theoretical model for cracking tensile strength.

1. Reinforcement process and testing

1.1 HB reinforcement and grout filling

HB reinforcement comprises a main bar and short transverse bars. Unlike conventional inserted reinforcement relying on bond, rotation after insertion creates mechanical interlock between transverse bars and concrete. The transverse bars transfer loads to the main bar crossing the layers.

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Figure 1: HB reinforcement.

Pullout tests established the optimum rotation. At 15°, the transverse bars interlocked effectively with concrete and failure occurred by rupture of the main bar. Further rotation did not significantly increase pullout force but increased internal disturbance, so 15° was selected.

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Figure 2: HB reinforcement pullout test results.

To address insertion voids, grout is injected to surround the reinforcement and strengthen interlock between the transverse bars and concrete. Loads can then transfer more effectively through the grout to the main bar. The bar is first wetted with grout, inserted and rotated. Grout is repeatedly injected and allowed to settle until it no longer seeps downward.

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Figure 3: Void filling method.

1.2 Filling grouts

The study balances economy, practicality and innovation. CP is the baseline. UEC compensates for shrinkage through expansion. NSC uses fine particle structure to improve void filling. All three use relatively high water content and water-reducing admixtures to maintain flowability.

1.3 Test methods

A gantry printer produced double-layer strip specimens. After printing, HB bars were inserted every 50 mm and rotated 15°. Voids were filled with CP, UEC or NSC, alongside an unfilled control group. After seven days of standard curing, the strips were cut into 40 mm cubes for three tests:

1. Mesoscale analysis: Macro photography examined filled reinforcement cross-sections, while SEM observed grout microstructure and bonding mechanisms.
2. Direct tension: Adhesively bonded fixtures applied load-controlled loading to measure cracking and ultimate tensile strength.
3. Interlayer shear: A dedicated fixture applied displacement-controlled loading to measure shear strength.

Tests followed relevant national standards and supported analysis of the filling mechanisms.

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Figure 4: Direct tensile testing fixture.

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Figure 5: Shear testing fixture.

2. Mesoscale and microstructural analysis

In the unfilled N control group, clear voids remained between the main bar and concrete, weakening transverse-bar interlock and interlayer reinforcement.

CP filled the upper layer relatively densely, but larger lower-layer voids remained. SEM showed large microscopic pores, limited and short C-A-H structures and insufficiently dense C-S-H. The explanation was initial expansion followed by shrinkage: evaporation increased capillary pressure, producing shrinkage stresses and microscopic voids.

UEC filled well around the main bar, leaving only small voids near the lower transverse bar and bonding effectively with printed concrete. SEM revealed abundant long, interwoven needle-shaped C-A-H structures filling pores and connecting C-S-H gel. Hydration of the expansive agent generated C-A-H to compensate for shrinkage, while curing humidity supported further growth.

NSC gave the densest macroscopic filling because of nanosilica's fine structure. However, a distinct interface remained between grout and printed concrete. Microscopically, many C-S-H particles formed a loose structure, with almost no needle-shaped C-A-H. High water content accelerated the pozzolanic reaction, and nanosilica acted as nucleation sites for abundant small C-S-H particles. Less intimate interface bonding could reduce overall mechanical performance.

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Figure 6: Reinforcement cross-section, N group.

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Figure 7(a): Reinforcement cross-section, CP group.

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Figure 7(b): CP grout microstructure. Observations of CP specimens.

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Figure 8(a): Reinforcement cross-section, UEC group.

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Figure 8(b): UEC grout microstructure. Observations of UEC specimens.

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Figure 9(a): Reinforcement cross-section, NSC group.

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Figure 9(b): NSC grout microstructure. Observations of NSC specimens.

3. Mechanical performance

3.1 HB reinforcement and grout filling

Figure 10 shows the N group's insertion voids reduced reinforcement effectiveness, producing the lowest cracking and ultimate tensile strengths. All three grouts improved tensile strength. UEC produced the greatest improvement, with equal cracking and ultimate tensile strength values.

The load-deformation curves in Figure 11 have three stages.

First is elastic loading, during which concrete carries most of the load and grout contributes relatively little.

Second is the cracked working stage. Bonding and enclosure by grout transfer loads, with microstructure determining cracking tensile strength. CP's lower-layer voids and shrinkage reduce transfer. UEC's expanding needle-shaped C-A-H extends into concrete, providing strong bonding and effective transfer. NSC fills densely but its loose small-particle C-S-H structure and limited C-A-H lead to weaker bonding and the least reinforcement benefit among the grouts.

Third is final failure. After cracking, transverse bars transfer load to the main bar, so grout has limited influence on capacity. N and CP show sudden load drops, supporting the value of effective filling. UEC reaches ultimate load at the end of stage two, mobilizing reinforcement tensile capacity before concrete cracking and showing ductile failure characteristics.

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Figure 10: Specimen tensile strengths.

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Figure 11: Direct tensile load-deformation curves.

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Figure 12: Load transfer in direct tension.

3.2 Shear tests

All three grouts increased shear strength, with UEC giving the largest improvement and NSC the smallest. Overall improvements were limited.

The curves comprise interlayer concrete shear failure followed by reinforcement bending. Strength mainly comes from the interface and HB bars. Because grout occupies little shear-plane area, its direct shear contribution is negligible. Its bonding and interlock reduce bar slip and convert part of the shear into reinforcement tension.

Stress concentration makes grout at the shear plane a weak point. After it crushes, bars deform and the interface fails in shear. CP shrinkage creates pores that promote crack propagation. UEC's needle-shaped C-A-H improves bond and density, giving the best shear performance. NSC's low C-A-H content and loose microstructure produce the weakest result among the filled groups.

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Figure 13: Specimen shear strengths.

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Figure 14: Shear load-deformation curves.

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Figure 15: Load transfer in shear.

4. Cracking tensile strength formula

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The original calculation formula is reproduced in the figure above.

5. Conclusions

1. Filling and microstructure differ substantially. CP shrinkage leaves poorer filling and larger pores. UEC expansion compensates for shrinkage, with abundant C-A-H filling microscopic voids. NSC has the densest macroscopic filling but small, loosely arranged C-S-H and weaker interface bonding.

2. All three grouts improve cracking tensile, ultimate tensile and shear strengths. UEC gives the largest gains and NSC the smallest. Improved bond and interlock enhance cooperation between HB reinforcement and concrete. UEC is the most suitable grout in this comparison because of its expansion characteristics and dense microstructure.

3. A semi-empirical, semi-theoretical cracking tensile load formula was established from the data. Calculation errors were within ±9%, providing a reference for further research.

4. Grout filling significantly improves the mechanical performance of through-layer reinforced components and may also improve corrosion and freeze-thaw durability, warranting further study.

Citation
Luo Xiaoyu, Hu Jiawei, Yang Min, et al. Mechanical performance of 3D printed concrete enhanced by through-layer reinforcement insertion and grout filling. Journal of Architecture and Civil Engineering, 2026, 43(4): 188–200.

Author biography

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Luo Xiaoyu was born in October 1979 in Qingyuan, Guangdong. He is an associate professor and master's supervisor in Chang'an University's School of Highway, secretary of the second faculty Party branch in bridge engineering, and a Ministry of Education recognized teacher for curriculum-based ideological and political education. He earned a doctorate in bridge and tunnel engineering from Tongji University in 2013. His research focuses on bridge aesthetics and form design, and intelligent bridge design and construction.

In teaching research, he led a 2018 centrally administered university Double First-Class undergraduate reform project on VR-based interactive bridge engineering teaching; a 2019 teaching development project creating a MOOC on bridge aesthetics and form design; and a 2020–2022 key China Transportation Education Research Association project on curriculum-based education mechanisms in transportation universities.

In scientific research, he led landscape and form design studies for major bridges including section A of the Shenzhen-Zhongshan Link, Shuidong Bay Bridge, Wuxue Yangtze River Bridge, the China-Maldives Friendship Bridge and the third Panama Canal bridge. As a principal drafter, he contributed to the Ministry of Transport's first highway bridge landscape design specification, JTG/T 3306-03-2018.

He also led and completed centrally funded work on topology and size optimization of high-performance steel-concrete composite T-beams with diagonal bracing; a National Mountain Highway Engineering Technology Research Center project on vehicle-collision simulation for composite bridge-pier protection; and a Shaanxi social sciences project on bridge aesthetics and industry innovation under the transport-strength strategy. He participated as a principal researcher in a national 973 Program project on fundamental theories for safety design and evaluation of very long-span bridges, and leads the Shaanxi Natural Science Foundation project on interfacial load transfer in printed concrete, 2023-JC-YB-410.

AI-translated from an article by Kuaixiong on Nanjixiong, originally published on 2026-10-10. Text and images retain the original source attribution.
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