3D-Printing of Ultra-High-Performance Concrete for Robotic Bridge Construction

Abstract

Automation and robotics are integral parts of many industries but their potential for field implementation has not been significantly recognized by the construction industry. This is mainly attributed to conventional construction and design practices which undermine the benefits offered by these new technologies such as repetitions, precision, time savings, and increased safety. There is a need for advanced materials and 3D-printing systems which are capable of constructing structural elements with performance that emulates conventionally cast elements. This study presents a detailed framework and performance metrics for materials and 3D-printing systems for bridge applications. In addition, a study was carried out on ultra-high-performance concrete (UHPC) which showed sufficient extrudability and workability for 3D-printing applications. A 3D-printing system was developed for 3D-printing of continuous additive layers of UHPC with accelerated heat curing. Accelerated heat curing was used to enhance buildability, expedite the printing of the UHPC layers, and maximize the number of printed layers within the material open time. The effect of heat curing on material properties was also evaluated to obtain the optimal temperature to satisfy compressive strength requirements. This research effort aims to augment automated construction techniques and develop solutions to extend the applications of accelerated bridge construction.

Robotics and automation are widely used in several industries such as automobile, electronics, and aerospace, mainly because of the nature of their design standardization and massive production. Unlike other industries, in the construction industry leaders are reluctant to integrate automation and robotic applications in their construction techniques for reasons including the lack of design standards for automated construction, material limitations, managerial concerns, and the lack of comprehensive proven research findings and guidelines related to robotics and automation in the construction field. The material limitations are related to the inability of most developed 3D-printed materials to sustain the designed structural loads and the challenges of 3D-printing with steel reinforcement, which is essential for structural elements to achieve the desired capacity.

To address challenges related to excessive construction time and poor-to-fair quality of final structural elements, accelerated bridge construction (ABC) techniques have emerged over the last few decades. One ABC technique is based on prefabrication of bridge components in a controlled environment. The prefabricated elements are then shipped to the construction site and erected on-site using dry or wet connections between the bridge elements in a speedy manner. According to Mantawy et al. ( 1 ), the prefabrication of substructure components (footing, columns, and bent caps) can save over 80% of in-situ construction time compared with conventional cast-in-place construction. Thousands of ABC projects had been already constructed including different prefabricated elements such as full or partial depth deck panels with closure joints ( 2 ), prefabricated bent caps, prefabricated columns ( 3 , 4 ), prefabricated bridge foundations, and prefabricated abutments. The advantages of ABC techniques include reduction in the construction time and also improvement of the quality of bridge components as they are prefabricated in a controlled environment.

The implementation of automated construction and robotics techniques to prefabricate bridge components can be highly effective for ABC projects. Construction time can be reduced by replacing some human activities with robots which have no working shift requirements. Robots can perform many construction activities which are especially repetitive or dangerous. The quality of the constructed elements on-site or off-site can be enhanced significantly by eliminating human errors, such as formwork issues and therefore allowing good tolerance. One of the biggest advantage of automated construction and robotics is the introduction of standardization in the construction industry as researchers and engineers can develop a wide range of structural and bridge elements which can be unique, customized, and esthetic. The task of constructing bridge elements through automation and robotics has several challenges, including material strength, bond strength between layers of 3D-printed elements, flexibility and mobility of the automated system, and adaptation of reinforcement. This paper presents a framework for automation and robotics in bridge construction based on a review of the recent advances in automation and robotics. Furthermore, this paper proposes an automated system using ultra-high-performance concrete (UHPC) with an accelerated heat curing approach for 3D-printing of bridge elements along with an evaluation of testing criteria for both material and 3D-printing system for automated construction.

Recent Advancement and Challenges in Rheological Testing Criteria for Automated Construction

For any cementitious material to be suitable for automated construction through robotics or 3D-printing systems, the cementitious mixture should meet rheological criteria as described below.

Extrudability

Extrudability is the ability of the material to be extruded through the printing nozzle and retain the intended shape. Since there is no available standard test for extrudability, researchers have developed methods to evaluate it through the success of extruding continuous filaments. Le et al. ( 5 ) proposed printing of sets of one to five filaments in which each filament is 11.8 in. long using a nozzle of 0.35 in. Ma et al. ( 6 ) used the same proposed method with different length of filaments (7.9 in.) and a total of eight continuous filaments. It should be noted that the length of filaments and the size of the nozzles vary based on the proposed element; therefore, researchers should develop extrudability methods which fit their needs.

Workability

Workability is generally defined as the ability of the mixture to draw a certain level of consistency, avoiding being too stiff or too loose. Different researchers have used different methodologies to measure the workability of 3D-printing material. Le et al. ( 5 ) used a shear vane test to measure workability, which was adopted by Austin et al. ( 7 – 9 ). By correlating between the shear strength and workability, it was found that shear strength between 0.073 pounds per square inch (psi) and 0.08 psi is sufficient to maintain good workability. Many factors such as the percentage of high range water reducer (HRWR, also known as superplasticizer) admixture, retarder dosage, and accelerator dosage affect the mixture workability, for instance, by increasing the percentage of superplasticizer (SP), the workability improved until a limit where the buildability, described below, can no longer be maintained. It is the opposite in case of increasing the dosage of retarder and accelerator. Ma et al. ( 6 ) used slump tests to quantify the workability of their mixtures. Unlike conventional construction materials, where materials need high slump to fill up a prefabricated wood or steel formwork, the materials in automated construction are usually printed in layers so the aim should be to maintain the workability within a slump limit. In the case of UHPC, workability is measured for flowability according to ASTM C1437 test ( 10 ).

Open Time

Open time is the time in minutes needed for the mixture to maintain good extrudability without disruption and loss of workability and buildability. The open time can be evaluated by performing slump tests over a 15 min period, as used by Alhozaimy ( 11 ). Le et al. ( 5 ) used the same method, vane shear test, as mentioned in the Workability section, and concluded that an increase in shear strength from the initial value should not exceed 0.043 psi and the open time was found to be 100 min in case of the use of 0.5% retarder dosage by weight for the proposed cementitious material.

Buildability

Buildability is the ability of the printed layer to retain its shape under the weight of the sequential layers without noticeable layer settlement. Unlike conventional construction materials, materials for automated construction are extruded in layers. Le et al. ( 5 ) quantified the buildability by the number of layers in the filament without noticeable geometrical changes of the initial layers. Ma et al. ( 6 ) used the same approach to define the buildability of their mixtures. After defining the number of layers which are sufficient to maintain the buildability, the extruded layers should be cured before resuming the process.

Mechanical Properties

Mechanical properties in the 3D-printed layers should achieve good compressive strength, tensile strength, bond strength between the 3D-printed layers, durability, and modulus of elasticity, among others, compared with materials constructed using conventional methods. Each mechanical property should be measured in three orthogonal directions by reason of the anisotropic nature of the extruded layers.

Recent Advancement and Challenges in Materials for Automation and Robotics in Construction

Concrete is the most utilized material in construction industries, with isotropic properties and slump allowing it to fill prefabricated formwork to minimize air voids. By adapting concrete to additive manufacturing, many changes are introduced to concrete, such as replacing large aggregates with fine materials; anisotropic properties which mean that the mechanical properties of each direction differ, unlike the isotropic nature of cast concrete, as shown in Figure 1 ( 12 ). To facilitate the use of concrete for additive manufacturing, a low water-to-cement ratio should be used to increase the buildability of the printed layer; however, reducing the water-to-cement ratio decreases the material pumpability and workability. Many researchers have attempted and developed mixtures for cementitious materials and tested them to explore their suitability for 3D-printing.

Figure 1.

Anisotropic properties of the printed concrete from additive manufacturing: (a) loading perpendicular to the layers, (b) and (c) loading along the layers in each direction ( 12 ).

Zhang et al. ( 13 ) developed two cementitious materials to facilitate 3D-printing using a team of mobile robots, meeting the rheological printing requirement of yield stresses and velocity. Liu et al. ( 14 ) developed different mix designs by studying the rheological properties of the mixtures using static yield stress (i.e., the critical stresses for the printed material allowing steady-state flow) and dynamic yield stress (i.e., final stress applied before complete stoppage). Tay et al. ( 15 ) studied the bond strength between the printed cementitious material layers and the influence of the time gap between the printed layers. The mixture design met pumpability requirements. Kazemian et al. ( 16 ) proposed four different cementitious mixtures which contain cement, sand, water, and high-range water-reducing (HRWR) admixture. Le et al. ( 5 ) and Paul et al. ( 12 ) also designed several mixtures with different sand/dry mixture proportions (dry mixture includes, sand, cement, silica fume, and fly ash). Weng et al. ( 17 ) designed a cementitious material which contains ordinary portland cement, natural river sand, fly ash, silica fume, water, and HRWR admixture. Arunothayan et al. ( 18 ) developed a mix using viscosity modifying agent (VMA) along with HRWR admixture. Binrong Zhu et al. ( 19 ) developed cementitious composites with ultra-high tensile ductility for digital printing using hydroxypropyl methylcellulose (HPMC). Table 1 shows different mix compositions for several 3D-printed cementitious materials proposed by the above-mentioned researchers.

Table 1.

Different Mix Composition by Weight for 3D-Printed Cementitious Materials

Mix no.	Reference	Cement	Fly ash	Silica fume	Sand	Water	HRWR (SP)	Fiber (lb/yd³)	HPMC	VMA	f’c (ksi)
1	Zhang et al. ( 13 )	0.288	0.135	0.027	0.360	0.189	0	0	0	0	na
2	Zhang et al. ( 13 )	0.196	0.196	0.294	0.201	0.113		0	0	0	na
3	Liu et al. ( 14 )	0.238	0.234	0.019	0.291	0.218	0	0	0	0	na
4	Tay et al. ( 15 )	0.263	0.075	0.038	0.451	0.173	0	0	0	0	na
5	Kazemian et al. ( 16 )	0.268	0	0	0.616	0.116	0.0005	0	0	0	6.48
6		0.243	0	0.027	0.611	0.117	0.0018	0	0	0	7.24
7		0.268	0	0	0.615	0.116	0.0006	2.02	0	0	6.54
8		0.268	0	0	0.615	0.116	0.0017	0	0	0	6.66
9	Le et al. ( 5 )	0.164	0.047	0.023	0.701	0.065	0	0	0	0	na
10		0.194	0.055	0.028	0.646	0.077	0	0	0	0	na
11		0.223	0.064	0.032	0.592	0.089	0	0	0	0	na
12		0.251	0.072	0.036	0.538	0.094	0.01	2.02	0	0	15.95
13		0.280	0.080	0.040	0.488	0.112	0	0	0	0	na
14	Paul et al. ( 12 )	0.131	0.126	0.066	0.548	0.129	0	0	0	0	7.54
15	Paul et al. ( 12 )	0.130	0.125	0.065	0.545	0.128	0	22.75	0	0	6.96
16	Weng et al. ( 17 )	0.345	0.345	0.035	0.172	0.103		0	0	0	7.21
17	Arunothayan et al. ( 18 )	0.321	0	0.138	0.459	0.073	0.0069	***	0	0.0014	18.19
18	Zhu et al. ( 19 )	0.43	0.57	0	0.405	0.28		***	0.001	0	8.09
19		0.43	0.57	0	0.405	0.28		**	0.001	0	7.66
20		0.43	0.57	0	0.405	0.28		*	0.001	0	7.43

Note: ksi = kips per square inch; HRWR = high range water reducer; SP = superplasticizer; HPMC = hydroxypropyl methylcellulose; VMA = viscosity modifying agent; na = not applicable.

1% is added by volume.

1.5% is added by volume.

***

2% is added by volume.

2.36 lb/yd³.

0.58 lb/yd³.

0.012 lb/yd³.

Challenges in Materials for 3D Printing

Many challenges should be addressed to encourage bridge owners and contractors to implement automation and robotics in construction through 3D-printing. The challenges in materials are as follows:

The lack of standard tests to quantify the rheological properties of the extruded layers including extrudability, workability, open time, and buildability. Even though many researchers have investigated methods of quantification for successful 3D-printing of materials, the construction industry would demand standard specifications to gain confidence in the proposed results.

The inter-relation between the rheological properties of the mixtures which could lead to the failure to meet all rheological requirements. For example, increasing the percentage of HRWR admixture to enhance pumpability and workability has a negative effect on extrudability and buildability of the 3D-printed layers.

The unavailability of commercial, proprietary, and premix for the most common materials applicable for 3D-printing such as normal concrete, polymer concrete, high-performance concrete, UHPC, and grout. Once commercial and premix materials are available, a huge step towards the implementation of automation and robotics in construction should take place elsewhere.

Recent Advancement and Challenges in Robotics and 3D-Printing for Automation and Robotics in Construction

The building and construction industries are quite backward in implementing robotics and automation advancements compared with other industries such as electronics, aerospace, automobile, and others. Recently there has been an interest in implementing robotics and automation to construct bridge components. To achieve a robust 3D-printing system, the following criteria should be evaluated:

Mobility: The ability of the 3D-printing system to move freely and easily (being portable) and perform the intended operation properly.

Scalability: The ability of the system to scale up the 3D-printing operation to build large-scale elements.

Flexibility: The ability of the system to 3D-print different shapes of elements.

Synchronization: The ability of the system to work simultaneously with another system to 3D-print large specimens to reduce the printing time.

Adaptation of reinforcement: The ability of the 3D-printing system to incorporate fibers and reinforcement in the 3D-printing operation.

Different researchers have developed various automated construction robots and 3D-printing systems. Zhang et al. ( 13 ) proposed the use of a team of mobile robots to construct a truss beam using two robots simultaneously. Each robot was erected over a holonomic platform to allow robot mobility and the robot end had a six-axis robotic manipulator, as shown in Figure 2a. The proposed system enhances flexibility, mobility, and scalability. However, coordination and synchronization between the team of robots in 3D-printing of one element could be challenging in addition to the difficulty in utilizing the 3D-printing of reinforced concrete elements with steel reinforcement.

Figure 2.

Robotics and 3D-printing system suitable for automated bridge construction: (a) team of mobile robots ( 13 ), (b) minibuilders ( 20 ), (c) Apis Cor and SEArch ( 21 ), and (d) Mudbots 3D-Printer ( 22 ).

Minibuilders ( 20 ) developed the concept of a community of small robots to print big structures. A combination of a “foundation robot” to 3D-print a cylindrical structure and a “gripping robot” to 3D-print the rest of the structure is proposed, as shown in Figure 2b. The proposed system allows the construction of many complex structures and enhances mobility, flexibility, and scalability but it has difficulty in utilizing 3D-printing of reinforced concrete elements with steel reinforcement.

Apis Cor ( 23 ) was one of the first companies to develop specialized 3D-printing equipment in construction which allows the construction of entire buildings on-site. They collaborated with SEArch to develop a 3D-printed water tank structure which was also tested for hydrostatic leakage testing as part of a NASA challenge, as shown in Figure 2c ( 21 ). The system has limitations in mobility, flexibility, and adaptation of reinforcement.

Mudbots ( 22 ) also developed commercial 3D-printers, as shown in Figure 2d. The 3D-printers are stationary and can print elements within the printer frame area. As the printer is a stationary system, it has limitations in mobility, scalability, and flexibility. Table 2 evaluates all the four systems described.

Table 2.

Evaluation of Various Construction Robots and 3D-Printing Systems

Robotics and 3D-printing systems	Mobility	Scalability	Flexibility	Synchronization	Adaptation of reinforcement
Zhang et al. ( 12 )	✓	✓	✓	=	✗
Minibuilders ( 20 )	✓	✓	✓	=	✗
Apis Cor ( 23 )	✗	✓	✗	✓	✗
Mudbots ( 21 )	✗	✗	✗	✓	✗

Note: ✓ Possible; = Challenging; ✗ Not possible.

Challenges in Automation and Robotics in Construction

Many challenges are associated with the use of robotics and 3D-printing systems, such as:

Each innovative 3D-printing system and robot is capable of constructing a specific element and performing a specific task. There are no comprehensive 3D-printing or robot systems which can construct various types of structural elements given their complexity.

As shown in Table 2, most of the recent innovations in robotics and 3D-printing system are limited with regard to adaptation of steel reinforcement and printing material with steel fibers. The use of steel reinforcement such as wires, reinforcing bars, stirrups, ties, spirals, and prestressing strands is mandatory in each bridge element. Several attempts were conducted and summarized in Nerella et al. ( 24 ) such as placement of reinforcement between 3D-printed layers, extruding metal cables concurrently with 3D-printed layers, enveloping concrete around reinforcement, and 3D-printing of structural elements with voids and cavities for posttensioning application.

The operation of the 3D-printing system and robots needs skilled labor with good background and knowledge of operation and building information systems (BIM).

The layer-by-layer mechanism of the 3D-printing system and robots creates cold joints which may cause liquid leakage and salt intervention that causes corrosion for reinforcement and affects the durability of the 3D-printed elements.

Proposed Framework for Automation and Robotics in ABC

A flowchart is proposed for researchers and the bridge industry to investigate the implementation of robotics and automation in ABC, as shown in Figure 3. Researchers should conduct a detailed feasibility study to decide whether a certain bridge element is suitable for automated construction or not, given the recent advancements in automation and robotic construction based on the bridge type and its component. If the selected element is suitable for automated construction, a mix design should be selected from available 3D-printing proven mixtures which can also fulfill testing criteria of 3D-printing such as extrudability, workability, open time, buildability, and mechanical properties. If the mixture is not suitable, another mixture should be evaluated until a suitable mixture is selected. After the selection of a suitable mixture, an automated construction system should be selected through the use of robots or 3D-printing systems. Each commercially available robot or 3D-printing system should be evaluated based on the selected element. The evaluation criteria shall include mobility, scalability, flexibility, synchronization if using multiple systems or team of robots concurrently, and adaptation of reinforcement. If the selected robot or 3D-printing is not suitable, another robot or 3D-printing system should be selected and evaluated until a suitable robot or 3D-printing system is selected. After the selection of bridge element, suitable mixture, and suitable robot or 3D-printing system, automated construction of the selected element should be performed.

Figure 3.

Proposed framework for 3D-printing for robotics bridge construction.

Proposed 3D-Printing System for UHPC with Accelerated Heat Curing

An automated concept for 3D-printing is proposed in this paper, as part of a research initiative at the Accelerated Bridge Construction University Transportation Center (ABC-UTC), which can be used for 3D-printing and prefabrication of different bridge components using UHPC, especially UHPC shells which act as stay-in-place formwork for bridge columns and beams ( 25 ). The process for assembly of such a 3D-printer is shown in Figure 4. The proposed system for 3D-printing of UHPC utilizes accelerated curing by applying heat curing to the material. The proposed system is also evaluated, below, based on the framework proposed in Figure 3 for both material testing criteria and 3D-printing system criteria.

Figure 4.

Proposed 3D-printing system for ultra-high-performance concrete (UHPC) with accelerated curing.

The design of the 3D-printing system for UHPC has been developed in the following stages.

Material for 3D-Printing

Several different UHPC mixes were used and tested to obtain proper workability and flowability for 3D-printing, including both regular and fast setting mixes. Since UHPC is composed of different constituents, the following combination of constituents was used to obtain proper workability and flowability for the material. The proportion of these constituents is also shown in Figure 5.

Ductal^® premix = 88.0% by weight

Super plasticizer = 1.0% by weight

Water = 4.8% by weight

Steel fiber = 6.2% by weight

Figure 5.

Constituents by weight percentage: (a) ultra-high-performance concrete (UHPC) and (b) Ductal^® premix.

The workability of the mixture is measured as flowability according to ASTM C1437 test ( 10 ). The selected mix has a flow of 9 in. as shown in Figure 6b. A flowability of 7 to 10 in. showed good properties of the extruded layer for additive manufacturing, as shown in Figure 6c.

Figure 6.

Workability of ultra-high-performance concrete (UHPC) mix (a) prepared mix (b) flowability test ASTM C1437 ( 10 ) (c) extruded layer.

Optimum Heating Time for Accelerated Curing

Any 3D-printing system should have the ability to print UHPC layers faster to suit the material open time for 3D-printing, therefore, accelerated curing was proposed by heat curing of UHPC using strip heaters, as shown in Figure 7. The resistant thermometer detector probe sensor was attached to the heater to sense the temperature which was further controlled using precision programmable temperature controller proportional integral derivative (PID) to set the required temperature for the 3D-printer head. The optimum time of heating at different temperatures was obtained by hit and trial method based on shape retention. The final temperature and heating time values for shape retention as given in Table 3 was obtained by removing the nozzle at a particular temperature with several different heating time values and by checking the shape of the layer, then one value was selected as the optimum value as presented in Table 3.

Figure 7.

Ultra-high-performance concrete (UHPC) heating setup using strip heater and proportional integral derivatives (PIDs).

Table 3.

Optimum Heating Time of Ultra-High-Performance Concrete for Shape Retention

Sr. no.	Temperature (°F)	Heating time (minutes)
01	100	45
02	125	30
03	150	20
04	175	15
05	200	10
06	225	08
07	250	05

Optimum Heating Temperature for Accelerated Curing

One of the main advantages of heat curing of UHPC is the acceleration of layer printing; however, imposing heat curing could affect mechanical properties and needed to be studied. The optimum heating temperature was one of the main parameters to obtain proper mechanical properties of the material to fulfill the testing criteria. The optimum heating temperature was decided based on the compressive strength of a cubical specimen tested at different temperatures for post-printing curing condition in dry (ambient condition) and wet environments. The 2 in. cubical specimens were cut as shown in Figure 8a from the specimen obtained after cure-heating and then compression testing was conducted to obtain the compressive strength of all samples at different temperatures. Some of the tested samples are shown in Figure 8b.

Figure 8.

(a) Preparation of heated samples and (b) tested sample at 175°F.

The average compression strength results of the cubes prepared at different temperatures (for three repetitions per certain temperature), which were tested after seven days, are shown in Figure 9 and those tested after 14 days are shown in Figure 10. The results indicate that there is a prompt decrease in UHPC strength by heat curing for samples that were left in ambient conditions (dry condition) after printing but if the cubes are cured in wet conditions after printing, then the UHPC compressive strength was almost the same for temperatures ranging from 125°F to 225°F. Compressive strength of 15 kips per square inch (ksi) (at 14 days) can be achieved even by heating the material to 225°F which can fulfill the strength criteria needed for 3D-printing of bridge components.

Figure 9.

Compressive strength of ultra-high-performance concrete (UHPC) at different temperatures from heat curing (7 days).

Figure 10.

Compressive strength of ultra-high-performance concrete (UHPC) at different temperatures from heat curing (14 days).

Design of 3D-Printer Nozzle Head

The nozzle head was designed in solid works and then 3D-printed. The nozzle head was designed to print UHPCs layer of 12 in. in length, 1.0 in. in thickness and 1.5 in. in height at a single time. The final design of the nozzle includes the following components, as shown in Figure 11.

Rollers for separation membrane

Mounting head with the actuator

Strip heater for heating

Side plates to control material flow

Figure 11.

Nozzle head of 3D printer with separation membranes.

Conceptual Design of 3D-Printer

A 3D-printing system, including horizontal and vertical actuators, mounted on a raptor track drive is proposed in this research, as shown in Figure 12. Mounting the system on raptor drive provides mobility, scalability, and flexibility properties for the 3D-printing. The buildability is achieved through accelerated curing. UHPC has steel fibers and it also has high strength properties so that it fulfills the mechanical testing criteria for 3D-printing. This 3D-printing system is flexible enough to print different bridge elements such as cap beam shells ( 25 – 27 ) or column shells ( 28 , 29 ).

Figure 12.

Conceptual design of 3D-printer on mobile platform. 1. Raptor track drive; 2. Horizontal actuator; 3. Vertical actuator; 4. Printing nozzle; 5. Heating plates; 6. Printed layers.

Components of Proposed 3D-Printer

The following items are the main components of the proposed 3D-printer which are also mentioned in Figure 11.

Raptor track drive

Horizontal actuator

Vertical actuator

Printing nozzle

Heating plates

Printed layers

3D-Printed Wall

A small 3D-printed sample wall specimen is shown in Figure 13. The wall is printed in the same manner as additive manufacturing using layer-on-layer and in several segments using a small-scale stationary frame for proof of concept. The final heating temperature was selected by considering two parameters: (i) reduction in 3D-printing time, and (ii) highest compressive strength possible.

Figure 13.

Wall sample manufactured using the proposed 3D-printing system.

At 150°F, the material showed good compressive strength (>15 ksi) along with a significant reduction in heating time, so this temperature was selected to 3D-print the wall, as shown in Figure 13. The length of each segment was kept at 12 in. with a height of 1.5 in. The wall consisted of three segments and four layers resulting in a total length of 36 in., and a total height of 6 in. The thickness of the wall was 1 in. This wall was printed for demonstration purposes and to observe the efficiency and printing time of the 3D-printing system. Each UHPC segment was printed with heat curing at a temperature of 150°F with a heating time of 20 min, resulting in 4 h of printing time which makes it quite a quick process to 3D-print a UHPC wall compared with other available 3D concrete printers discussed above. The evaluation of material testing and the 3D-printing system for the given wall is discussed below.

Evaluating Material Testing Criteria for Proposed 3D-Printing System

Accelerated curing of UHPC is used in this research for 3D-printing of bridge components; it provides many advantages in fulfilling material testing criteria for 3D-printing system. Table 4 evaluates all the material testing criteria of UHPC with accelerated curing for automated construction.

Workability: UHPC is a flowable material, as shown in Figure 6a, which fulfills workability and pumpability requirements for 3D-printing.

Open time: Heat curing of UHPC maximizes the number of printed layers and segments within the open time of the UHPC mix which reduces material wastage. The optimum heating time at different temperatures is listed in Table 3.

Extrudability: The proposed system is flexible so that extrudability is obtained by altering the nozzle for the required size.

Buildability: Buildability is achieved through heat curing in the proposed system. Heat curing increases the buildability of the layers in the proposed system.

Mechanical properties: Sufficient compressive strength of UHPC can be achieved as per requirements of the bridge component by varying the heating temperature but there is still a need for a research study to ensure the highest bond between layers and segments of the printed element which is included in the next phase of this research project.

Table 4.

Evaluation of Material for the Proposed 3D-Printing System

Ultra-high-performance concrete for 3D-printing system	Workability	Open time	Extrudability	Buildability	Mechanical properties
	✓	✓	✓	✓	=

Note: ✓ Possible; = Challenging.

Evaluating Testing Criteria for Proposed 3D-Printing System

The proposed 3D-printing system has several advantages to fulfill the proposed framework for the testing criteria of the 3D-printing system. Table 5 evaluates all the testing criteria of the proposed system.

Mobility: The proposed system has horizontal and vertical actuators and is mounted on a raptor track drive which provides mobility benefits in any direction. The height of the vertical actuator can be designed to satisfy specimen height.

Scalability: The proposed system is fully scalable and can print full scale and scaled bridge elements.

Flexibility: The proposed system is fully flexible for printing different structural elements.

Synchronization: There is no requirement of synchronization as the proposed system can operate solely to print different structural elements.

Adaptation of reinforcement: The proposed system is successfully used to print UHPC wall which includes 2% steel fibers by volume which provides strength benefits. Furthermore, to incorporate typical reinforcement, the system nozzle can be modified as per specific job requirement; however, further research is being conducted as part of this research project.

Table 5.

Evaluation of Testing Criteria of the Proposed 3D-Printing System

Proposed 3D-printing system	Mobility	Scalability	Flexibility	Synchronization	Adaptation of reinforcement
	✓	✓	✓	✓	=

Note: ✓ Possible; = Challenging.

Conclusion

The proposed automated 3D-printing system using UHPC to print different bridge components has many advantages compared with the currently available systems. The proposed system and material fulfill material testing criteria for 3D-printing such as workability, open time, extrudability, and buildability using an accelerated heat curing technique. The material also satisfies compressive strength requirements through wet curing of the 3D-printed elements; however, there is need for further research related to mechanical properties to satisfy bond strength requirements in 3D-printing. The proposed system is also advantageous in enhancing mobility, scalability, and flexibility along with the incorporation of steel fibers which provide strength and ductility to the printed layers. The proposed automated system and framework can be considered a promising step toward the implementation of 3D-Printing and can encourage stakeholders and contractors to implement automated construction in ABC projects.

Footnotes

Author Contributions

The authors confirm contribution to the paper as follows: study conception and design: Ali Javed, Islam M. Mantawy, Atorod Azizinamini; data collection: Ali Javed; analysis and interpretation of results: Ali Javed, Islam M. Mantawy, Atorod Azizinamini; draft manuscript preparation: Ali Javed and Islam Mantawy. All authors reviewed the results and approved the final version of the manuscript.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This project is supported by the U.S. Department of Transportation through the Accelerated Bridge Construction University Transportation Center (ABC-UTC) at Florida International University with grant number 69A3551747121.

The opinions, findings and conclusions expressed in this paper are those of the author(s) and not necessarily of the sponsor. This paper does not constitute a standard, specification, or regulation.

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