Link to Report: Coming Soon
Background :
Concrete 3D printing has gained rapid momentum as an emerging construction method offering reduced labor, faster construction, enhanced design flexibility, and minimal formwork requirements. However, its broader adoption in structural and bridge applications remains limited due to inherent weaknesses, including insufficient interlayer bonding, lack of conventional reinforcement, and early-age shrinkage cracking. These limitations reduce durability and restrict the load-carrying capacity of 3D-printed elements. Currently, IBT/ABC-UTC at FIU has pioneered the development and successful field deployment of pneumatic spray technology for ultra-high-performance concrete (UHPC). This technique provides high-quality, rapid, and cost-effective strengthening solutions for deteriorated or deficient bridge components. Recent full-scale demonstrations have shown that sprayed UHPC can deliver superior bonding, enhanced toughness, and substantial improvements in structural performance. Despite the individual advancements of concrete 3D printing and UHPC spray technology, their integration remains largely unexplored. This project aims to evaluate the feasibility of using pneumatically sprayed UHPC as an external reinforcement layer for 3D-printed concrete components. The research will: (1) quantify the interfacial bond strength between sprayed UHPC and 3D-printed substrates; (2) assess structural enhancements through flexural strength, ductility, and post-cracking performance; and (3) investigate improvements in durability, including resistance to chloride penetration and freeze-thaw deterioration. The findings will provide foundational knowledge for advancing a hybrid construction approach that merges concrete 3D printing with sprayable UHPC. The expected outcomes will support the development of practical guidelines, promote adoption in bridge infrastructure, and enable future field-scale validation studies.
Objectives :
This research aims to establish the scientific basis and practical feasibility of integrating pneumatically sprayed UHPC with 3D-printed concrete to overcome inherent material and structural limitations. The approach combines targeted experimental testing, systematic characterization of interfacial bonding behavior, and evaluation of mechanical and durability improvements resulting from the UHPC overlay. The overall objective is to determine whether sprayed UHPC can significantly enhance interlayer bonding, structural performance, and durability of 3D-printed elements.
Quantitative metrics including pull-off strength, direct tension response, rapid chloride permeability, freeze–thaw performance, flexural strength, flexural toughness, and post-cracking behavior will be measured. Failure modes and crack development will also be documented to gain mechanistic insight.
Scope :
Task 1: Fabrication of 3D-printed concrete specimens and application of sprayed UHPC overlay
This task involves the fabrication of 3D-printed concrete specimens followed by the application of a pneumatically sprayed UHPC overlay. The objective is to establish consistent baseline specimens for subsequent mechanical and durability evaluations and to ensure uniform overlay–substrate conditions for bond characterization. A series of 3D-printed concrete elements will be fabricated using proprietary SIKA printable mortar. The printing process will be conducted using a robotic arm. Printing parameters including layer height, nozzle speed, extrusion rate, and interlayer time interval will be optimized to ensure repeatable surface quality and representative interlayer interfaces. Specimens will be printed in standardized geometries suitable for bond testing, flexural testing, and durability assessment. Following fabrication, the printed elements will be sprayed using an optimized UHPC mix designed for pneumatic spraying.
Task 2: Evaluation of interfacial bonding between sprayed UHPC and 3D-printed concrete
This task will quantify the interfacial bonding behavior between pneumatically sprayed UHPC and 3D-printed concrete substrates. The objective is to determine whether UHPC overlay can enhance both the overlay-to-substrate adhesion and the intrinsic interlayer bonding of the 3D-printed elements. A series of 3D-printed specimens will be fabricated using proprietary SIKA printable mortar. Multiple sets of specimens will be prepared with UHPC overlays. The UHPC will be applied using the pneumatic spray system developed at IBT/ABC-UTC.
Bond strength will be measured using pull-off tests as shown in Figure 1 (ASTM C1583). A steel disc will be epoxied to the UHPC surface, and tensile load will be applied perpendicular to the surface until failure. Results will include maximum tensile stress and observed cracking patterns. Direct tension tests will also be conducted to assess improvements in load transfer across weak printed interfaces. Comparison with control (unsprayed) specimens will quantify the contribution of the UHPC overlay to interlayer strengthening.
Task 3: Evaluation of flexural performance of sprayed UHPC
The goal of this task is to determine the extent to which the UHPC overlay improves the flexural performance of 3D-printed concrete where interlayer weakness and limited reinforcement typically govern failure.
Flexural tests will be carried out on 3D-printed prism specimens both with and without sprayed UHPC overlays. Measured parameters will include peak flexural strength, flexural toughness, stiffness, post-cracking behavior, and energy absorption.
Task 4: Durability assessment of 3D-printed concrete strengthened with sprayed UHPC
This task examines whether the UHPC overlay enhances the durability of 3D-printed concrete. Durability issues such as chloride ingress and freeze–thaw damage are particularly critical for transportation structures exposed to deicing salts and temperature extremes.
Resistance to chloride ion penetration (see Figure 2(a)) will be evaluated using the rapid chloride permeability test (ASTM C1202) to assess permeability reduction provided by the UHPC overlay. Freeze–thaw resistance (see Figure 2(b)) will be assessed following ASTM C666 with specimens subjected to repeated cycles of freezing and thawing. Performance indicators will include mass loss and interfacial bond characteristics. Comparative analysis between strengthened and unstrengthened samples will quantify improvements in durability performance and surface protection.
| (a) rapid chloride permeability | (b) freeze-thaw resistance |
Task 5 – Final report preparation
The final task will focus on the preparation of a comprehensive final report that documents methods, experimental data, analysis, and recommendations.
Research Team :
Principal Investigator: Mahyar Ramezani, Ph.D.
Co-Principal Investigator: Kingsley Lau, Ph.D., and Arslan Khan, Ph.D.