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This paper presents experimental investigations and analyses on the early-age-behavior of zero-slump concrete, such as compaction behavior and green-strength. First, the influence of the granulometric properties of the fines is discussed in detail. For this purpose, the early-age behavior of two different fines (quartz flour and fly ash) is investigated by means of the intensive compaction test (IC-test). The tests on the influence of the fines focus on effects caused by differences in the particle shape and the use of a plasticizing admixture. The conducted tests on the compaction behavior of the fines and their corresponding green-strength are extended to continuously graded granular mixes. Here, the influence of optimized particle packing on the early-age behavior is presented and a comparison on the basis of the aforementioned quartz flour and fly ash is made. In this consideration, experimental investigations on the early-age behavior of a zero-slump concrete mix and possible effects on the hardened concrete properties are included.
This paper addresses the characterization of six different amorphous silica samples with respect to their application in cement paste. Different mixes are compared and analyzed using the mini spread-flow test. Also the granular properties, different void fraction states of packing and distribution moduli q are analyzed and compared using a mix design tool. A deformation coefficient is derived from the spread-flow test, which correlates with the value of specific surface area computed from the particle size distribution, and intrinsic density of the samples. Finally, the thickness of a constant water layer of 25 nm around the particles is computed at the onset of flowing. The granular analysis demonstrated that it is possible to decrease the water demand of the cement paste when nanoparticles are added and the resulting grading follows the modified Andreasen and Andersen curve (q = 0.5), and the concentration of nano silica is less than 5% bwoc.
Waste sludge, a solid recovered from wastewater of photovoltaic-industries, composes of agglomerates of nano-particles like SiO2 and CaCO3. This sludge deflocculates in aqueous solutions into nano-particles smaller than 1 µm. Thus, this sludge constitutes a potentially hazardous waste when it is improperly disposed. Due to its high content of amorphous SiO2, this sludge has a potential use as supplementary cementitious material (SCM) in concrete. In this study the main properties of three different samples of photovoltaic's silica-rich waste sludge (nSS) were physically and chemically characterized. The characterization techniques included: scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), nitrogen physical adsorption isotherm (BET method), density by Helium pycnometry, particle size distribution determined by laser light scattering (LLS) and zeta-potential measurements by dynamic light scattering (DLS). In addition, a dispersability study was performed to design stable slurries to be used as liquid additives for the concrete production on site. The effects on the hydration kinetics of cement pastes by the incorporation of nSS in the designed slurries were determined using an isothermal calorimeter. A compressive strength test of standard mortars with 7% of cement replacement was performed to determine the pozzolanic activity of the waste nano-silica sludge. Finally, the hardened system was fully characterized to determine the phase composition. The results demonstrate that the nSS can be utilized as SCM to replace portion of cement in mortars, thereby decreasing the CO2 footprint and the environmental impact of concrete.
In this study two different types of nano-silica (nS) were applied in self-compacting concrete (SCC), both having similar particle size distributions (PSD), but produced through two different processes: fumed powder silica and precipitated silica in colloidal suspension. The influence of nano-silica on SCC was investigated with respect to the properties of concrete in fresh (workability) and hardened state (mechanical properties and durability). Additionally, the densification of the microstructure of the hardened concrete was verified by SEM and EDS analyses. The obtained results demonstrate that nano-silica efficiently used in SCC can improve its mechanical properties and durability. Considering the reactivity of the two applied nano-silicas, the colloidal type showed a higher reactivity at early age, which influenced the final SCC properties.
Alkali-activated materials are ideal for the repair of concrete structures in harsh environmental conditions due to their high durability in chemically aggressive environments. However, slag-based mortars, in particular, are prone to shrinkage and associated cracks. In this respect, the application of steel fibres is one solution to reduce the formation of shrinkage induced cracks and to improve post cracking behaviour of these mortars. This study investigated the influence of two different types of steel fibres on the tensile properties of two alkali-activated mortars. Direct tensile tests and single fibre pull-outs were performed to analyse the determining failure modes both on macro and micro scale. Mechanical testing was accompanied by non-destructive testing methods such as digital image correlation and acoustic emission for a detailed analysis of the fracture process.
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.
Bending beams and slabs are typical examples for structural elements used for reinforced concrete structures such as bridge girders, T-beams and bridge decks. Their strength related failure modes at maximum loading can be divided into bending and shear failure. The failure of beams loaded in bending can occur with or without indication. Therefore, conventional design concepts aim on failure modes with sufficient indication (e.g. large deflections or cracks), as it occurs in the case of secondary flexural compression failure. These indicating factors can also be used for Structural Health Monitoring (SHM) of civil infrastructure systems (e.g. bridges) to identify structural changes. In this context, non-destructive testing (NDT) methods offer different techniques for measuring deflections or crack formation and opening. However, profound knowledge on the determining failure modes of bending beams and their detection by NDT methods is required for the reliable application of SHM. Different NDT methods have been used in this study for analysing the load-bearing behaviour of a reinforced concrete beam in bending. The different measuring techniques are briefly described and their applicability is discussed by means of experimental results. For this purpose, the load-bearing behaviour of a reinforced concrete beam having a span of 2.75 m was investigated in a four-point bending flexural test at laboratory scale. The focus is on the characterization of determining failure modes by optical NDT and the comparison with classical measuring techniques (e.g. deformation measurements by displacement transducers). The bending beam was equipped with two single-mode (SM) sensor fibres. One fibre served as Distributed Optical Fibre Sensor (DOFS), whereas the other fibre contained Fibre Bragg Grating (FBG) sensors. In addition, optical deformation measurements using Digital Image Correlation (DIC) and Stereophotogrammetry (SP) were conducted.
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.
The tensile behavior of strain-hardening cement-based composites (SHCC) is usually investigated on macroscopic scale by means of direct tension tests or bending tests. Additionally, the micromechanical properties of the composites are often described based on single fiber tension and pull-out tests. Such investigations, performed both on macroscopic and microscopic scales, are based on ‘classical’ force and displacement measuring techniques. Advanced test methods such as digital image correlation (DIC) and acoustic emission analysis (AE) may facilitate the identification and the analysis of the failure mechanisms in SHCC, which is important for both monitoring loaded SHCC elements and further material development and optimization. In this study, these two techniques are combined to characterize the failure mechanisms of three different types of SHCC in direct tension tests. The results are related to data of stress and strain measurements. It is shown that DIC provides detailed spatially resolved and stress related strain measurements. Furthermore, it is demonstrated that AE allows for the localization of active cracks, quantification of the damage accumulation under increasing stresses, and characterization of the dominant crack bridging mechanisms and failure modes observed in the different types of SHCC.
Damage mechanisms analysis of reinforced concrete beams in bending using non-destructive testing
(2019)
Beams that are loaded in transverse direction are one of the main structural elements used in reinforced concrete structures. Bending beams are used, for example, in bridge girders or joists. The strength related failure modes at maximum loading can be divided into bending and shear failure that could occur with or without indication. Conventional design concepts used in practice for designing structural elements subjected to bending aim at failure modes with sufficient indication (e.g. large deflections or cracks), as it occurs in the case of secondary flexure-compression-failure. These indicators can also be used to identify structural changes of civil infrastructure systems (e.g. bridges) using non-destructive testing methods for Structural Health Monitoring (SHM). However, profound knowledge on the determining failure modes of bending beams is required for the reliable application of structural health monitoring. Therefore, resulting effects of different thermal and mechanical loads on the response of a structure are investigated by means of a bridge demonstrator. The results of this investigation are used for developing a monitoring system. Different non-destructive testing (NDT) methods have been used for analyzing the deformation behavior of a reinforced concrete beam in bending. The suitability of the different measuring techniques will be discussed by means of the obtained experimental results. For this purpose, the deformation behavior of a reinforced concrete beam having a span of 2.75 m was investigated in a four-point bending flexural test at laboratory scale. The focus is on the characterization of determining failure modes by the NDT methods and the comparison with classical measuring techniques (e.g. deformation measurements by displacement transducers). The bending tests have been accompanied by Acoustic Emission analysis (AE), optical deformation measurements using Stereophotogrammetry (SP) and Digital Image Correlation (DIC). The conducted tests have shown that the three-dimensional detection of deformations and resulting strains with optical techniques is beneficial for analyzing the damage mechanism of loaded structures and related crack formation and propagation. Optical techniques can be supplemented by acoustic emission testing which gives detailed information on crack formation and progressing damage. The combination of both techniques offers the application as efficient monitoring system for larger structures.