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Over the last decades concrete has evolved from a simple mass construction material towards a sophisticated multi-component system. The design parameters for the mixture composition of concrete have significantly increased from strength based towards overall or specific performance based. As a result the variety of concrete has increased yielding a number of special concrete technologies such as Self-Compacting Concrete (SCC), High-Performance Concrete (HPC), Strain Hardening Cement Based Composites (SHCC), and many others. Due to their complex mixture compositions and a multitude of possible interactions between constituents, these concrete types are preferably composed of special components like well-defined powders and sophisticated chemical admixtures. This makes such concrete technology expensive and limits their application to regions with the required material supply chains. The paper puts focus on materials, which are less well studied in conjunction with high performance concrete, but which are available in many developing countries, and in particular sub-Saharan Africa. The paper shows how sec can be designed without polycarboxylate ether superplasticizer and well defined fillers, but with lignosulphonate, cassava starch and rice husk ash. The positive effect of well processed rice husk ashes is demonstrated. Furthermore results are presented of SHCC where typical components like polyvinyl alcohol fibres and fluy ash are replaced by sisal fibres and limestone filler, respectively. The results point out that high performance concrete applications do not have to be limited to a boundary framework with availability of well-defined raw material supply structures and sophisticated admixtures or fibres. Concepts are presented how innovative concrete technologies can be developed based on indigenous materials.
The present work addresses powder bed binder jetting additive manufacturing by selective magnesium phosphate cement activation. Despite the potential of this technology to aid the digitalization of the construction industry, the effect of processing parameters on the mechanical performance of printed materials has not yet been studied to generate a guideline for the further development of the technology. Statistical methodologies were used to screen the effect of four printing process parameters (printing speed, layer thickness, raster angle, and build direction on flexural and compressive strength). As the exploited technology works with constant fluid pressure, the physical interpretation of the effect of each factor can be considered taking into account the interactions between the binder materials in the powder bed. Analysis of variance (ANOVA) indicated that printing speed and layer thickness significantly affect mechanical performances. Furthermore, the layout of samples for the printing process is preferable to be parallel the printhead movement. An anisotropic behavior was observed, and the samples subjected to compressive forces parallel to the layer plane possessed lower strength values. This effect can be interpreted as a result of a weak area of low density in between layers, leading to a pronounced delamination under compression. Even though the strength of the printed material is not suitable for a structural concrete, it can be marginally improved by design of experiment and optimized for non-structural applications, such as for porous artificial stone. Design of experiment coupled with ANOVA methods can be used in the future to support the development of novel material mixtures, thus expanding the fields of application of this novel additive manufacturing technology.
To safely dispose of nuclear waste in underground facilities, engineered barrier systems are needed to seal shafts and galleries. The material used in these barriers must be adapted to the host rock parameters. Shrinking and cracking must be avoided to provide a barrier with almost zero permeability. For repositories in salt rock environments, several types of salt concrete (SC) are possible construction materials.
Within the project SealWasteSafe, we compared the behavior of an innovative alkali-activated material (AAM) with standard SC in their hydration and hardening phase. To monitor the microstructural changes within the two materials SC and AAM, acoustic emission (AE) signals have been recorded for up to ~250 days on 340-liter-cubic specimens.
The phenomenon of AE is defined as the emission of elastic waves in materials due to the release of localized internal energy. Such energy release can be caused by the nucleation of micro-fracture, e.g., in concrete while curing or when exposed to load. The occurrence of AE events gives first rough indications of microstructural changes and potentially occurring cracking and thus, provides insights for structural health monitoring (SHM).
The results show, that for the first 28 days after casting, less AE activity was detected in the AAM compared to SC. After 61 days, in the AAM material, the number of AE events exceeded those observed in the SC. However, the majority of the AE detected and located in AAM was related to surface effects, and not to microstructural changes or occurring cracks within the bulk volume. Additionally, the source location analysis indicated, that despite lower activity in SC, we observed some clustering of the events. In contrast, in AAM, the activity inside the specimen is randomly distributed over the whole volume. The monitoring results help to estimate the material’s sealing properties which are crucial to assess their applicability as sealing material for engineered barriers.
Global energy consumption will increase in the future necessitating both fossil fuels and renewable energy choices - especially wind energy. Such high energy demand requires installation of offshore energy structures, rigs, platforms and towers, which are susceptible to adverse environmental conditions along with maintenances. Due to their large size and remote locations, cylindrical grouted joints are often adopted between substructure and foundation in these offshore platforms and wind structures such as monopiles. However, these connections are composite structures with exterior sleeve, interior pile and infill mortar. Degradation and settlements were reported inside similar connections, which were installed in last three decades. Besides, grouting in the offshore sites were proven difficult to obtain ideal load bearing capacity. In-situ loading conditions were also found to be affecting the failure mechanism inside such connections. This study aims at characterizing the nature of cracks generated in these grouted connections under both static and cyclic loading. Scaled grouted joints were manufactured using a novel reusable mold, and connections were loaded to failure to visualize the main failure patterns. An assessment between failure under these two types of load is drawn along with comparison to previously available literature.
The authors are currently investigating the possibility to apply compaction grouting for offshore pile foundations (Jacket piles as well as monopiles) as a possible retrofitting technique for an optimised foundation concept. In this research project, we are developing a design approach aiming to predict the ideal amount and properties of a grout for a specific soil situation and desired improvement of pile bearing capacity after Installation and during service time. Both numerical and experimental tests have been carried out to investigate the injection process during which a highly viscous grout is injected into the soil under high pressure to displace and compact the surrounding soil without fracturing it. The implicit Material Point Method (MPM) based on a mixed formulation is the numerical technique chosen to deal with the expected large deformations and the arbitrary shape of the developing grout bulb. The usage of MPM prevents both the need of remeshing and the numerical instability induced by extensive mesh distortion. For validation with experimental results, we have constructed a testing chamber with one transparent sidewall. This chamber enables us to observe the injection process directly at the transparent vertical window and to measure the in-plane soil displacements and strains by means of the Digital Image Correlation (DIC) technique.
The results already reveal the interrelation of soil and grout properties for a successful usage of this common ground improvement technique.
Superplasticizer and Shrinkage Reducing Admixture Dosages for Microfine Cement in Grout Systems
(2019)
Grouts have numerous applications including crack repair as maintenance in construction industries. Microfine cements are intensively used for high strength mortar and grout products. They are ideal for injection grouting in structural repair. Such grouts should have suitable rheological properties to be injectable, especially those used in repair and rehabilitation. The use of superplasticizers (SP) in these products is thus becoming increasingly crucial to achieve favorable workability and viscosity properties. A difficulty in such grouts is the plastic shrinkage due to finer particles used. It is thus necessary to determine optimum SP and shrinkage reducing admixture (SRA) dosages for a microfine cement based grout. In this study, a saturation dosage was decided from two Polycarboxylate ether (PCE) based SPs in relation to neat cement using slump flow and rheological parameters. A range of grout mixtures was formulated containing micro silica (MS) and fly ash (FA), and tested for suitable rheological and mechanical parameters. Based on the results, a grout mixture with MS and FA was selected to determine optimum SRA content. According to the results, a SP dosage of 3% by weight of neat cement is sufficient to achieve saturation. The grout material including MS and FA can produce comparable properties to neat cement grout. MS is found to improve compressive strength within the range considered, whereas a higher FA content provides favourable rheological properties. Finally, a SRA dosage of 4%, which could reduce the shrinkage by about 43% after 28d days, is determined for the grout system.
Grouting is a universal repair and strengthening technique, which is constantly used for structural remediation of concrete components, trenches, mine subsidence, dam joints, restoration of masonry structures, and geological stabilizations. Having an extremely small particle size of only few microns, ultrafine cements are ideal for grouting applications due to their superior permeability and compressive strength properties of the hardened cement paste compared to that of the less-expensive, but coarser ordinary Portland cements. Supplementary cementitious materials (SCMs) are often used to replace ultrafine cement in order to modify certain properties and to reduce costs. The aim of this experimental study is to investigate the effect of three supplementary materials: microsilica (MS), fly ash (FA), and metakaolin (MK) on the workability, and mechanical properties of an ultrafine cement based grout with a constant water-binder ratio and constant superplasticizer content. Maximum percentages of replacement with ultrafine cement were 6% by volume of cement for MS and 16% for FA, and MK. In general, results suggest that the workability is improved by addition of FA, whereas is reduced, when modified with MS and MK. The compressive strength of grout after cement replacement remains comparable to that of pure cement grout. However, there is a tendency of the MS to positively affect the compressive strength opposite to FA, whereas flexural strength is positively affected by FA. Based on the results, it is evident that grouts with Hägerman cone flow more than 500 mm and compressive strength of more than 90 MPa after 28 days can be produced.
Properties of alkali-activated mortars with salt aggregate for sealing structures in evaporite rock
(2021)
Concrete structures for sealing of tunnels in the host rock are an essential part of systems for nuclear waste storage. However, concretes based on blended cements or magnesium oxychloride cements, which are commonly considered for this application, can deteriorate severely due to a significant heat of hydration and associated deformation and cracking. Alkali-activated materials (AAMs) offer a potential solution to this problem because of their low heat release during hardening. To explore their suitability for the construction of sealing structures in evaporite rock, various AAMs with salt aggregate were studied regarding fresh properties, heat release, mechanical properties and microstructure. The heat of reaction of the AAMs was up to 55% lower than that of a blended cement designed for sealing structures, indicating significant benefits for the intended application. Other relevant properties such as mechanical strength and permeability depended strongly on the mix-design of the AAMs and curing conditions.
One of the major tasks in nuclear waste management is to design safe and reliable sealing structures for radioactive waste repositories. Particularly for salt as a host rock, engineered barrier systems for underground waste disposal must be constructed of well-understood (cementitious) materials that will sustain environmental conditions and ensure high durability.
Within the SealWasteSafe project, we studied two materials with a potential for sealing structures for nuclear waste repositories: an innovative alkali-activated material (AAM) and standard salt concrete (SC). To analyze the development of microstructural changes within the materials, we monitored microseismicity (aka acoustic emission monitoring) occurring during the hardening and setting period in two 340-liter-cubic specimens for up to ~250 days.
The monitoring results showed, that in the first 61 days, fewer events occurred in AAM compared to SC. After this time the number of events in AAM sharply increased and significantly exceeded those observed in SC. However, the source localization analysis revealed that the increase of microseismicity in AAM material was not related to the formation of macro-cracks within the material but was mainly caused by the surface effects. Accompanying analysis of the test specimens with additional methods (such as e.g., X-ray CT) proved that no macro-cracks were observed inside the AAM specimen. Further analysis in the time and frequency domains helped to characterize the tested materials and to estimate their potential to be used for engineered barriers in nuclear waste repositories.
Overall, our study shows the potential of microseismicity monitoring for feasibility studies and quality assurance in a broad range of applications, also in structural health monitoring.
The aim of an ongoing research project is to develop a design approach for typical offshore driven piles (e.g. Jacket piles) based on the application of injections by compaction grouting directly at the pile shaft. The paper aims to present the results of laboratory and in-situ tests, which reveal the efficiency and the promising potential of the optimised foundation concept for a more economic dimensioning of pile foundations and to increase their bearing capacity in non-cohesive soil at any moment after installation.
Current trend suggests that global energy consumption will increase in the future. This growing energy demand and advancement of technology lead to explore all potential offshore fossil and non-fossil energy sources, necessitating erection of exploration and production structures, rigs, platforms and towers, which are susceptible to adverse environmental conditions along with their maintenances. Cylindrical grouted joints provide suitable connections between steel substructure and foundation in these offshore platforms and wind structures especially monopiles for ease of installation. However, these are composite connections with exterior sleeve, interior pile and infill grout. The capacity of these connections is affected by number of factors. The literature over last four decades by numerous researchers has shown the development of these connections with increasingly higher capacities and influences on these capacities due to various factors. This paper provides a comprehensive review on the factors affecting the connection capacity along with technical challenges for the future. Critical aspects and shortcomings of the current connection systems and potential solutions may be sought after for these issues are also discussed.
At present, Wind Turbine Generators (WTGs) operating in onshore and offshore wind farms are primary sources of renewable energy around the world. Cylindrical grouted sleeve connections are usually adopted in these WTG structures to connect the upper structure and foundation for ease of installation. These structures including grouted connections experience considerable adverse loading during their lifetimes. Settlements were reported inside similar connections used in energy structures especially oil and gas platforms, which were installed in last three decades. Thus, repair and rehabilitation of such connections in existing wind structures should also be planned ahead to keep them operating in the future. The nature of failure and crack generation in grouted connections are crucial prior to adopt a strengthening strategy. This pilot study is carried out to actualize the failure mechanism in the grouted connection, when subjected to axial loading. A novel reusable scaled cylindrical grouted connection with shear keys was designed and tested for its load bearing behaviour. The mechanical test was accompanied by classical measuring techniques (e.g. displacement transducer) as well as non-destructive measuring techniques (e.g. digital image correlation (DIC), acoustic emission analysis (AE)). The failure mechanism incorporating slippage of the shear keys and cracking of the grout was investigated. The capacity and applicability of such test mould were also discussed. The knowledge is expected to pave way towards repair of deteriorated grouted connections with similar geometry and failure pattern.
Remediation of Cracks Formed in Grouted Connections of Offshore Energy Structures under Static Loads
(2018)
The future energy demand necessitates the exploration of all potential energy sources both onshore and offshore. Global trend has shifted towards offshore energy, which can be obtained from either carbon intensive or renewable options, hence requiring structures such as rigs, platforms, and monopiles. Most of these structures adopt easily installable construction techniques, where lower foundation need to be connected with the super structure by mean of grouted composite joints. Generally, these composite connections have exterior sleeve, interior pile and infill grout. Being located in remote offshore conditions, connections can experience considerable adverse loading during their lifetimes. Degradations were reported inside similar connections, which were installed in last three decades. Besides, grouting in the offshore sites may often be proven difficult, which eventually leads to reduced capacity of connections in the long run. Thus, repair and rehabilitation of such connections should be planned ahead to minimize operational delays and costs in the future. This study aims at characterizing the nature of crack generation in grouted connections and thereby identifying the potential of repair using suitable repair material. Scaled grouted joints were manufactured using a novel mold, and connections were loaded under static load to visualize the main failure pattern. The failure mechanism and loading capacity are found compatible to previous results from earlier literature. Grouted connection was then repaired using cementitious injectable grout. The effectiveness of the repair system is also discussed.
Grouts have numerous applications in construction industry such as joint sealing, structural repair, and connections in precast elements. They are particularly favoured in rehabilitation of structures due to penetrability and convenience of application. Grouts for repair applications typically require high-performance properties such as rapid strength development and superior shrinkage characteristics. Sometimes industrial by-products referred as supplementary cementitious materials (SCM) are used with neat cement due to their capabilities to provide binding properties at delayed stage. Micro silica, fly ash and metakaolin are such SCMs, those can modify and improve properties of cement products. This study aims at investigating long-term mass loss and linear shrinkage along with long-term compressive and flexural strength for grouts produced from ultrafine cement and SCMs. A series of mixtures were formulated to observe the effect of SCMs on these grout properties. Properties were determined after 365 days of curing at 23oC and 55% relative humidity. The effect of SCMs on the properties are characterised by statistical models. Response surfaces were constructed to quantify these properties in relation to SCMs replacement. The results suggested that shrinkage was reduced by metakaolin, while micro silica and fly ash had positive effects on compressive and flexural strength, respectively.
Grouted connections are intensively used in offshore rigs, platforms as well as jacket and monopile offshore wind turbine structures. Being located in remote offshore conditions, these connections can experience considerable adverse loading during their lifetimes. Degradation was reported inside similar connections, which were installed in the last three decades. Grouting in the offshore sites may often be proven difficult, which eventually leads to reduced load-bearing capacity of connections in the long run. Thus, repair and rehabilitation of such connections should be planned ahead to minimize operational delays and costs. In this study, scaled grouted connections were manufactured using a novel mould, whose integrity were monitored using digital image correlation (DIC). The connections were loaded under static load to visualize the main failure pattern using distributed fibre optic sensors and acoustic emission (AE) analysis. Grouted connections were then repaired using a cementitious injectable grout. The effectiveness of the grout injection was monitored using dye penetration technique. Finally, specimens are reloaded to identify the potential of such repair for grouted connections.