@phdthesis{01J0NQES17CTET4Z3G6A4J5PQ6,
  abstract     = {{At the beginning of this century, a new industrial revolution named Industry 4.0 started. Remote connectivity and automation methods were integrated into multiple branches of the existing industry. These core features of the revolution had the function of increasing productivity. This was also applied to the construction industry. One of these newly introduced production methods was additive manufacturing, better known as 3D printing. 
3D concrete printing was adopted into the construction industry and is most known in the form of layered-wise deposition of a mortar mixture through extrusion. The construction method makes it possible to create concrete 
elements in a short time span with a high degree of freedom and without high labor costs. While researchers saw immediate opportunities in this new production methodology, the reaction of the concrete industry was rather 
indifferent. The industry frequently asserts that the substantial variation in material properties introduced by 3D printing significantly diminishes both, production and material quality, compared to the conventional casting method. Although some truth can be found in this statement, this does not mean that 3D printing is not useful for the construction industry. To convince the conservative construction industry of the potential of 3D concrete printing, the application of 3D printed concrete as an advanced formwork technique is introduced in this study. In this application, the benefits of both concrete types can be combined. The high degree of freedom of printed 
concrete is combined with the trustworthiness of conventional cast concrete. Not only can this application reduce the threshold to introduce 3D printing into the conservative construction sector, in addition, an economic and ecological advantage can also be obtained. The most significant advantages are brought forth in the case of complex concrete elements. Even though the idea is straightforward and seems easy to implement, practical 
problems quickly arise. The reliability of the formwork’s performance must be ensured during each design and construction phase for the safety of construction workers and users of the building. However, the interaction between the newly introduced 3D printed concrete and conventional cast concrete is currently unknown which leads to a high uncertainty in the behavior performance of the designed elements. Within this thesis, the interaction between these two types of concrete is investigated to improve the reliability during the different 
construction phases of reinforced concrete with 3D printed formwork. One of the most important aspects is the design of the formwork against failure during filling of the formwork. By performing material and small-scale tests, three different interaction mechanism and their influences were verified. The mechanisms were identified separately and in combination while casting the infill concrete in cylindrical 3D printed formwork. It was observed that 3D printed concrete formwork was always subjected to a load combination resulting from (1) the hydrostatic pressure head, (2) internal swelling of the formwork due to moisture absorption and (3) expansion due to the hydration heat of the infill concrete. This combined load is time-dependent and can increase up to 24 hours 
after casting the infill concrete. The deformation due to moisture absorption and hydration heat was found to be very important during the design phase as cracking of the printed formwork was observed after casting. 
3D printed concrete typically has a higher shrinkage rate than traditionally cast concrete. The shrinkage rate of the infill concrete was observed to be significantly lower compared to the printed material at early ages, while this reversed at middle and later ages. The differential shrinkage profile development was, therefore, observed to be dependent on the time between printing and filling. When printed formwork was filled too early, the core would restrain the shrinkage of the printed material subsequently leading to cracking of the formwork. Contrariwise, having a longer time between printing and filling, the printed formwork will restrain the volume change of the core. This can induce tensile stresses on the interface of the cast and printed material and eventually lead to debonding. The bonding between the printed and infill material is an important aspect to ensure the monolithic behavior of the total element. The bond was evaluated by performing microscopic and destructive testing. Several curing conditions and curing times of the 3D printed formwork were investigated. Microscopic investigation showed that the water absorption of the printed concrete led to a densification of the infill concrete near the formwork. Based on the destructive tests, it was identified that this densification improved the toughness of the 
interface between the cast and printed concrete. The densification improved the shear bond strength, but not the tensile bond strength. To design a structural concrete member, design parameters are needed. Design
parameters of 3D printed concrete obtained by performing standardized tests and by adjusted test methods were compared. The test methods within the current standards were found to be unreliable as they overestimated the 
strength and stiffness of the printed concrete. Unlike the standardized test, the adjusted test method took into account the printed shape and printing methodology during preparation of the specimens. The design parameters
obtained by this adjusted methodology were deemed suitable and more reliable. These design characteristics made it possible to estimate the axial compressive strength of a combined element. The mechanical behavior of concrete structures with 3D printed formwork subjected to pure axial compression and pure bending 
was investigated. The main observation was that the bonding between the cast and printed concrete is important for the stiffness of the designed concrete element. The service life is another important aspect of the design process of concrete structures. As the printed formwork stays in its place after casting, a new function 
could be given to the printed layers. A suitable option would be a concrete cover. The performance of this cover is dependent on the curing conditions of the 3D printed material. Results showed that improper curing of the 3D printed formwork results in an increased porosity, a higher water absorption, and more chloride- and CO2 ingress. This is detrimental to the service life of the concrete structure with a predetermined formwork thickness. Improvement methodologies were investigated to increase the efficiency of the compaction and curing of the produced formwork. Results showed that improving the curing within the first 24 hours after printing, already significantly increased the service life of the element. Improving the compaction by an adjusted nozzle decreased 
the water ingress and lowered carbonation. To show the potential of 3D printed concrete as formwork material, a complex element was designed. During two trials on large-scall elements, all construction phases were analyzed considering the most important influencing factors. Based on the outcome of the preliminary tests, the complex formwork was designed and expectations of the performance were made. The outcome of the final test indicated that the expectations were rather conservative but reliable. In general, it can be concluded that 3D printed concrete is a suitable material for creating complex formwork. The design method is an advantage when unique 
complex reinforced concrete structures are desired. The optimal performance of reinforced concrete with 3D printed formwork is guaranteed by maintaining ideal curing conditions for the formwork and filling it in a manner that aligns with the compatibility of differential shrinkage.}},
  author       = {{Bekaert, Michiel}},
  isbn         = {{9789463558136}},
  language     = {{eng}},
  pages        = {{XX, 309}},
  publisher    = {{Ghent University. Faculty of Engineering and Architecture}},
  school       = {{Ghent University}},
  title        = {{Mechanical behavior and durable aspects of 3D printed concrete  formwork}},
  year         = {{2024}},
}