ABSTRACT
Digital fabrication methods with concrete have been rapidly developing, with many problems related to component production and material control being solved in recent years. These processes produce inherently layered cementitious components that are anisotropic, and in many cases, produces a weak interface between layers, which are generally referred to as cold joints. While material strength at these interfaces has been well studied in recent years, durability has received less attention, even though cold joints can function as channels for aggressive agents, such as chlorides. This work presents a method using micro-X-ray fluorescence (µXRF) to image chloride ingress into layer interfaces of 3D printed fine-grained concrete specimens produced with varying layer deposition time intervals, and also compares it to neutron imaging of moisture uptake. The results show that cold joints formed after a 1 day time interval are highly susceptible to chloride ingress, and that curing conditions play a major role in how quickly interfacial transport can take place. The µXRF method is also shown to be useful for study of transport of chlorides in cold joints, due to its spatial resolution and direct analysis of an aggressive species of interest.
ABSTRACT
The recent interest in 3D printing with concrete has generated great interest on how inhomogeneities arise and affect performance parameters, in particular strength and durability. With respect to durability, of particular interest is how 3D-printed layer interfaces can impact transport of species of interest, such as moisture, chlorides or carbon dioxide in carbonation processes. This is of particular interest considering that the primary use case of 3D-printed concrete has been as a lost formwork for a cast structural concrete, and thus it is of interest to determine the carbonation resistance. This study consists of a preliminary look at the microstructure after accelerated carbonation of a 3D-printed concrete used as a lost formwork. Preferential carbonation is observed in the layer interfaces compared to the bulk of the printed filaments, possibly related to porosity from air voids or a locally high capillary porosity corresponding to the lubrication layer.
The new technology of 3D printing with concrete has been making a lot of headlines recently due to its great potential to make construction safer, cheaper and faster. It also allows us to make buildings and infrastructure objects that are more materially efficient, meaning that they use much less concrete compared to a more standard construction, so they are less environmentally harmful. However, this is all assuming that the printed concrete will perform similar to normal concrete. A lot of attention has been paid to whether the printed concrete is as strong as normal concrete, however not so much attention has been paid to if the printed concrete is as durable as normal concrete. The aim of this study is to make a first look at this, using the microscope. When we speak of concrete durability, we typically mean the protection of the steel reinforcement in the concrete, which acts to take up tensile stresses that may arise. Concrete acts as a protective barrier to this steel from corrosion, but aggressive species can go through this barrier to attack the steel. One of these aggressive species is carbon dioxide, which acts to reduce the pH around the reinforcement and results in its corrosion. Printed concrete, made in a layer-by-layer process, has many interfaces between these layers where the connection is potentially not as dense as in normal concrete. This study shows that these layer interfaces essentially can serve as highways for carbon dioxide to enter the concrete and attack the reinforcement. This means that any new 3D-printed structures need to take this into account, if the printed concrete is expected to serve as any kind of a protective barrier. We caution the reader that this study is purely observational, however, and a more in-depth study where we can actually make predictions about the printed concrete should be carried out.
ABSTRACT
Digital fabrication with concrete is considered to potentially revolutionize the construction sector and is often presented as a means to reduce its environmental footprint. However, at least in the case of concrete, it encounters significant challenges in terms of material design, since high paste volumes and Portland cement contents are normally used due to process requirements. In this article, the application to layered extrusion of a recently developed low clinker cement containing 50% Portland cement and 50% supplementary cementitious materials, such as limestone, burnt oil shale, and fly ash, is presented. It is found that an accelerator paste composed by Calcium Aluminate Cement (CAC) and anhydrite provides the required hydration and structural build-up for 3D printing, while not compromising the early and long-term compressive strength. Such a low clinker mortar can be successfully retarded, processed, pumped, and extruded just after mixing it in line with the accelerator paste. This accelerated mortar formulation contains only 303 kg/m3 of Portland cement, which is roughly half the amount used in current accelerated formulations used for digital fabrication with concrete.
ABSTRACT
Biocides are used in the building industry to prevent algal, bacterial and fungal growth on polymericrenders and thus to protect buildings. However, these biocides are leached into the environment. To better understand this leaching, the sorption/desorption of biocides in polymeric renders was assessed. In this study the desorption constants of cybutryn, carbendazim, iodocarb, isoproturon, diuron, dichloro-N-octylisothiazolinone and tebuconazole towards acrylate and silicone based renders were assessed at different pH values. At pH 9.5 (porewater) the constants for an acrylate based render varied between 8 (isoproturon) and 9634 (iodocarb) and 3750 (dichloro-N-octylisothiazolinone), respectively. The values changed drastically with pH value. The results for the silicone based renders were in a similar range and usually the compounds with high sorption constants for one polymer also had high values for the other polymer. Comparison of the octanol water partitioning constants (Kow) with the render/water partitioning constants (Kd) revealed similarities, but no strong correlation. Adding higher amounts of polymer to the render material changed the equilibria for dichloro-N-octylisothiazolinone, tebuconazole, cybutryn, carbendazim but not for isoproturon and diuron.
