Proceedings of the RILEM Spring Convention and Conference 2024

Inhaltsverzeichnis

1. Mechanical Properties and Structural Applications

   1. Frontmatter

   2. Integrating an ANN-Based Tensile Model with a Hybrid Rotating Crack Formulation to Simulate the Behavior of Shear-Critical UHPFRC Structural Elements with Unconventional Cross Sections

      Amjad Diab, Anca C. Ferche

      Abstract

      This paper presents a novel approach to characterize the direct tensile behavior of ultra-high performance fibre-reinforced concrete (UHPC), accounting for the complex relationships between the mix design constituents and the mechanical properties of UHPC. An artificial neural network (ANN)-based predictive model was developed to determine the mechanical properties of UHPFRC such as cracking stress, peak tensile strength, and the corresponding strain. Subsequently, this ANN-based tensile model for UHPFRC was integrated into a hybrid rotating crack model incorporated into an established nonlinear finite element analysis (NLFEA) software. In this formulation, suited for macro-modelling, the fibres are represented as smeared within the material. The proposed procedure was validated against UHPFRC specimens tested in the literature, including 5 membrane elements subjected to pure shear and (34) shear-critical beams. The simulated behaviours were in good agreement with the experimentally measured responses, with an \({R}^{2}\) of 0.97 for all specimens. The model was then utilized to better understand the influence of the cross-sectional shape on the behavior of shear-critical UHPFRC beams. The analysis results facilitated the development of a simplified shape effect parameter that can be integrated into existing shear capacity prediction models for UHPFRC. The proposed shear capacity model showed reasonably accurate results in predicting the capacity of the beams with unconventional cross sections that were tested in the literature.

   3. Cyclic Behaviour of Thin-Walled Pre-cracked HPFRC in Bending

      Sara Bascì, Matteo Colombo

      Abstract

      Applications of High-Performance Fibre-Reinforced Concrete (HPFRC) are gaining popularity both thanks to its enhanced mechanical performance and its capability of guaranteeing pleasant finishing that makes this type of materials a good solution for thin elements like façade panels. These kinds of elements are often subjected to cyclic loads throughout their service life. The impact of cyclic loads on material properties is significant and can potentially lead to fatigue failures, particularly in situations in which the elements have experienced cracks during their service life.

      The research presented here is oriented to studying the cyclic bending behaviour of thin-walled elements made of one specific HPFRC material, also considering different level of pre-cracking that refers both to serviceability limit state and to ultimate limit state condition.

      A set of four-point bending tests were performed on 35 mm thick samples applying a pulsating load with a cycle frequency of 1.3 Hz and considering different load range typical of the serviceability limit state. The pre-crack levels considered correspond to 0.5, 1.5 and 2.5 mm of global crack opening measured astride the constant bending moment region. The crack evolution has been measured during the proceeding of the cycles and, in the cases in which the samples did not experience failure before 100.000 cycles, a monotonic test was performed in order to compare the structural performance after cycles with that in pristine condition.

   4. Quantification of Hemp Concrete Displacement Subjected to Hygrothermal Solicitations Based on 2D DIC

      Haichuan Liu, Remi Legroux, Dmytro Kosiachevskyi, Kamilia Abahri

      Abstract

      Integrating plant aggregate into building envelope insulation materials, such as hemp concrete, provides a promising approach to reducing the energy consumption and carbon footprint of buildings due to its excellent hygrothermal performances and low carbon footprint. Considered a highly hygroscopic material, hemp concrete exhibits susceptibility to hygrothermal solicitation. This characteristic causes dimensional variations (swelling/shrinkage) and consequently leads to damage in the material cracking especially at the interface zone. In addition, this material is highly heterogeneous and therefore requires better consideration of its behavior on a finer scale. The present study aims to improve the precise understanding of how temperature changes impact the morphological variations of hemp concrete. For this purpose, a specialized mini climatic chamber equipped with telecentric lenses, ultra-resolution cameras, and a precise temperature and humidity controller was designed to acquire a high-quality image of hemp concrete under heating-cooling cycles (15 ℃-50 ℃-15 ℃) with the constant relative humidity state. Next, the strain of the hemp concrete was visualized and quantified based on the 2D digital image correlation (DIC) technique. Particular attention was also devoted to the strain uncertainty. The results show that DIC is an accurate method for determining the local deformation characteristics (aggregate and interface) of hemp concrete. The acquired swelling/shrinkage percentage in hemp aggregate and interface zone inside hemp concrete could serve as input for numerical modeling of coupled hygro-thermal properties in the material.

   5. Development of Shrinkage/Crack-Controlled Alkali-Activated-Oil-Fibre Repair Mortars for Composite Applications

      Ognjen Rudić, Seyrek Yunus, Bernhard Freytag, Joachim Juhart, Cyrill Grengg, Florian Mittermayr

      Abstract

      One of the main weaknesses of alkali-activated materials is their higher drying shrinkage behavior compared to Portland cement-based construction materials. This limits their applications as repair materials, especially in composite action with OPC. In this study, an innovative approach to develop sprayed AAM exhibiting low drying shrinkage and no crack formation is presented. In order to develop proper spray formulation, fresh metakaolin-slag AAM is mixed with vegetable oil and reinforced with hemp and basalt fibers. The impact of oil and different fibers on various properties of the AAM composite is investigated experimentally: (i) workability, (ii) mechanical (compressive strength), (iii) porosity (low temperature nitrogen adsorption), (iv) drying shrinkage and (v) the crack formation due to constrained shrinking (analyzed on cylindrical disc specimens). Addition of oil does not degrade the properties of fresh mortars, but shows a beneficial effect on the shrinkage behavior by significantly reducing the porosity; especially of gel pores and small capillary pores below 20 nm of pore sizes. This led to reduced drying shrinkage rates by 25–30%. The addition of hemp and basalt fibers lead to changes in workability of the fresh mixtures, and a reduction in crack formation after 7 days of curing. By the combination of oil, fibers and air-entrainment agents the drying shrinkage related crack formation allowed the crack-free application of thin AAM layers on concrete substrates.

   6. Investigation of Shear Capacity to Facilitate More Efficient Short-Span R/UHPC Beams

      Timothy Frank, Peter Amaddio, Alexis Tri, Elizabeth Decko, Darcy Farrell, Cole Landes, Joshua Kates

      Abstract

      Ultra high performance concrete (UHPC) is a cementitious material containing a large percentage of cement, and 1–3% by volume of randomly distributed steel fibers. When UHPC is reinforced with longitudinal steel, R/UHPC members can develop significantly higher strength for a given cross-sectional area than traditional concrete members. Due to the compressive properties of UHPC, larger longitudinal reinforcement ratios than commonly used in reinforced concrete lead to more efficient use of both the UHPC and steel materials. The objective of this research is to explore various methods of providing shear capacity in R/UHPC beams subjected to high shear demand. Specifically, this study explored short-span R/UHPC beams where steel fiber content and longitudinal steel reinforcement ratio were held constant at 0.5% and 3.9%, respectively. Transverse steel spacing, transverse steel grade, and placement method were varied in seven small scale R/UHPC beams, which were experimentally tested to failure. Results indicated that traverse steel spacing had the greatest impact on specimen response. More transverse steel did not significantly increase load carrying capacity, however, it increased ultimate drift by 24%. Increasing the grade of the transverse steel did not significantly impact response, indicating that the shear force capacity within each transverse reinforcing bar was not a limiting factor to specimen performance. While some differences in fiber orientation within the R/UHPC beams were observed, the impact of placement method on beam response was negligible.

   7. Test and Analysis of Prestressed Ultra High Performance Concrete Beams

      Jens Peder Ulfkjaer, Daniel Peter Brosbøl, Rasmus Larsen, Johan Clausen

      Abstract

      Ultra-high-performance steel fibre reinforced concrete (UHPSFRC) combines the benefits of ultra-high-performance concrete with the added reinforcement of steel fibres. The durability of UHPSFRC can contribute to sustainable construction practices. Its strength can lead to longer-lasting structures, reducing the need for frequent repairs or replacements. Using UHPSFRC poses the possibility of a decrease in Global Warming Potential (GWP) compared to normal-strength concrete per available m² in residential and office buildings. Implementation of prestressing could reduce GWP even further. Deformations are often a problem when designing structures. From this came the idea of combining the two. In the present study 9 beams with the same reinforcement arrangement have been tested. Three were not prestressed, three were pre-tensioned, and the last three were post-tensioned. Cylinders and small beams were tested for reference parameters, strength, and fracture energy. The results of the tests are presented as load-displacement curves at several points along the beam axis and at the mid-point.

      A semi-analytical model governed by beam theory and the principle of virtual work is proposed. A software package developed in MATLAB allows for a wide range of possible beam cross-sections and non-linear material models. It is a proposal for a faster evaluation of the deflection behaviour of a concrete beam compared to i.e. FEM. The general idea of the methods is that a numerical iterative method can be utilised to investigate the member’s method, calibrate the parameters, and predict the response. The model assumes that individual beam cross-sections remain planar, equilibrium between internal and external forces and non-linear constitutive models for the materials, including a Fictitious Crack Model for UHPSFRC. The agreement between tests and the model is very good.

      The GWP is compared between a beam of normal-strength concrete (C40) and a pre-stressed UHPSFRC with the same performance parameters and shows a reduction in GWP of 20.7%.

   8. A Study on the Effects of Fatigue Loading on the Mechanical Properties of UHPFRC

      Yusuke Nagai, Toshimichi Ichinomiya, Takashi Kosaka, Liberato Ferrara

      Abstract

      A waffle-shaped UHPFRC deck slab has been designed and tested by running wheel fatigue tests to assess its fatigue resistance under traffic loading within the intended operational period. However, these tests did not result into destructive outcomes, and the specific failure mode remains undisclosed. Consequently, constitutive laws of the UHPFRC have been experimentally identified to elucidate the failure mechanism and the progression of damage in the UHPFRC deck slab by Non-Liner Finite Element Method (NLFEM) analysis. In this paper, various fatigue loading tests employing UHPFRC specimens were conducted to comprehend the evolution of the mechanical characteristics of the material under fatigue. The results indicated a reduction in the elastic modulus of the UHPFRC and an increase in displacement corresponding to an increase in the number of fatigue loading, thus signifying the potential for establishing these shifts of mechanical properties as governing parameters for fatigue dependent constitutive laws.

   9. Advances in Wind Turbine Tower Design and Optimization

      Yara Alzoubi, Giovanni Muciaccia, Liberato Ferrara

      Abstract

      This work provides a succinct overview of recent advancements in wind turbine tower design and optimization. Recognizing the critical role of tower structures in enhancing the efficiency of wind energy harvesting, the review traces the historical evolution from traditional designs to modern tubular steel, concrete, and hybrid towers. A focus on taller towers highlights the significance of accessing higher wind speeds for optimal energy output. Materials science innovations, including the use of high-strength alloys and composites and of high-performance cement-based materials, are discussed for their role in achieving lighter yet robust and better constructable tower structures. In addition, new construction techniques are examined. Optimization methodologies, including computational modelling and machine learning, are examined, considering genetic algorithms, computational modelling, and simulation tools, coupled with machine learning algorithms. The paper concludes with a forward-looking perspective on future developments, anticipating progress in materials, construction techniques, and ongoing research initiatives. This concise review serves as a valuable resource for researchers, engineers, and industry stakeholders engaged in advancing wind turbine tower technologies.

   10. Exploring Shear Strength of Concrete: A Novel Z-Test for Direct Shear Testing of Hpfrc

       Ziad N. Sahlab, Nicholas S. Burello, Alfredo A. Flores G., Jorge C. Diaz, Davide Zampini

       Abstract

       With the advent of new concrete materials and advances in structural design, new thin structural elements are becoming increasingly popular. However, the lack of a standardized shear characterization process impedes full utilization of the inherent shear strength of these novel concrete mixes. This paper aims to address this by introducing a novel direct shear test for fibre reinforced concrete (FRC) that uses standardized samples and loading method. The shear behaviour of the cementitious matrix and FRC samples having different fibre dosages was thoroughly captured using the test. Furthermore, the new testing method can detect the effect of fibres on the shear strength, with samples with higher dosages exhibiting greater shear strength. Additionally, residual strength was found to depend on both the fibre dosage and type, with longer fibres exhibiting slightly better performance and higher ductility. This study contributes to the advancement of shear strength characterization in concrete and provides valuable information on the behaviour of fibre-reinforced concrete under shear loading, potentially facilitating the direct use of concrete shear strength in the design, especially for thin structural elements without conventional longitudinal and shear steel reinforcements.