Materials and Structures

Donà, Marco; Tecchio, Giovanni; Porto, Francesca

2018 Materials and Structures

doi: 10.1617/s11527-018-1196-x

Previous experimental and numerical works have demonstrated the effectiveness of reinforced masonry (RM) walls in tall single-storey wide-span buildings. Nevertheless, to date, EN1996 does not have a consistent approach to checking the second-order effects due to out-of-plane loads in such structures, and requirements for RM walls are too restrictive. New fibre models have been calibrated on the basis of the above tests and used to carry out parametric pushover analyses to study the influence of wall slenderness, roof vertical load and the percentage of vertical reinforcement on the out-of-plane behaviour of tall RM walls. In this numerical study, new slenderness limits for the safe use of this construction technology are defined, above which second-order moments must be calculated or failures due to instability become predominant, overcoming the shortcomings of current standards. This work also proposes and calibrates some rational approaches for evaluating second-order effects in tall RM walls, based on model column (MC) and nominal curvature (NC) methods. The reliability of the two simplified design methods is checked against the results of the above numerical models. MC method provides the best results, without limitations on the type of failure of the section (i.e. ‘balanced’, as for NC method), with errors usually lower than 25% and always conservative.

Park, S.; Seo, J.; Lee, H.

2018 Materials and Structures

doi: 10.1617/s11527-018-1183-2

The use of circulating fluidized bed combustion in coal power plants is dramatically increasing, and the disposal of the resultant ash is a significant environmental concern. The present study investigated the binder chemistry of sodium carbonate-activated CFBC fly ash. Compressive strength tests showed that strength development was relatively faster in samples with 5% sodium carbonate. The main binder gel in samples with and without sodium carbonate was composed of AFt, AFm and C–S–H, while the formation and Al-substitution in C–S–H was enhanced by sodium carbonate-activation. At an excessive dosage of sodium carbonate, the formation of AFt was significantly hindered due to the precipitation of sodium sulfate, and a notable amount of AFm was formed in its place. The results obtained in this study fill a deficit in the understanding of fly ash activated with near-neutral salts, which are used extensively in several industrial applications, including alkali-activated cements.

Kosalla, Marc; Raupach, Michael

2018 Materials and Structures

doi: 10.1617/s11527-018-1199-7

For determining the chloride threshold level of steel in concrete, the withdrawal of concrete samples is not a trivial task. The elected sampling method, the associated step size of depth intervals and the depth position of the steel with regard to the interval limits can exert a significant influence on the accuracy of the results. Also local variations of chloride ingress may lead to significant deviations from the effective chloride concentration in the depth of interest. In this paper, the chloride profiles from grinding powder samples with small interval sizes of 2 mm are compared to drilling powder samples with interval sizes of 10, 15 and 20 mm. Regression curves according to Fick’s second law were generated and the corresponding chloride diffusion coefficients were determined. The deviations of the curves and the resulting diffusion coefficients were illustrated. The assumption of considering the determined chloride concentration as constant and valid for the entire interval depth was found to lead to huge deviations from the effective profiles which are not acceptable. The implementation of regression approaches from drilling powder samples lead to an extreme reduction of deviations, thus resulting profiles may be suitable for certain inspection tasks, even though most profiles examined do not exhibit a sufficient number of data points from depth ranges completely controlled by diffusion. Additionally, a theoretical modeling of chloride penetration curves was performed. A quantitative approach for the election of depth intervals in practice was developed based on the input parameters diffusion coefficient and exposure time.

Jepsen, Michael; Damkilde, Lars; Lövgren, Ingemar; Berrocal, Carlos

2018 Materials and Structures

doi: 10.1617/s11527-018-1177-0

During the past decades several inverse approaches have been developed to identify the stress-crack opening ( $${\sigma }-w$$ σ - w ) by means of indirect test methods, such as the notched three point bending-, wedge splitting-, and round panel testing. The aim is to establish reliable constitutive models for the tensile behavior of fiber reinforced concrete materials, suitable for structural design. Within this context, the adaptive inverse analysis (AIA) was recently developed to facilitate a fully general and automatized inverse analysis scheme, which is applicable in conjunction with analytical or finite element simulation of the experimental response. This paper presents a new formulation of the adaptive refinement criterion of the AIA method. The paper demonstrates that the refinement criterion of the nonlinear least square curve fitting process, is significantly improved by coupling the model error to the crack mouth opening and the crack opening displacement relationship ( $$w_{\mathrm{cmod}}-w_{\mathrm{cod}}$$ w cmod - w cod ). This enables an adaptive refinement of the $${\sigma }-w$$ σ - w model in the line segment with maximum model error, which entails significant improvement of the numerical efficiency of the AIA method without any loss of robustness. The improved method is applied on various fiber reinforced concrete composites and the results are benchmarked with the inverse analysis method suggested by the Japanese Concrete Institute (Method of test for fracture energy of concrete by use of notched beam, Japanese Concrete Institute Standard, Tokyo, 2003) and recently adopted in ISO 19044 (Test methods for fibre-reinforced cementitious composites—load-displacement curve using notched specimen, 2015). The benchmarking demonstrates that the AIA method, in contradiction to the JCI/ISO method, facilitates direct determination of the tensile strength and operational multi-linear $${\sigma }-w$$ σ - w models.

Pel, L.; Pishkari, R.; Casti, M.

2018 Materials and Structures

doi: 10.1617/s11527-018-1187-ypmid: 30996649

Salt weathering is one of the major causes of the damage both in cultural heritage as well as in civil engineering constructions. A special case develops when there is a continuous wicking of a salt solution into a material in combination with evaporation of the moisture at its surface. In this study we are interested in the case where the absorption rate is much higher than the evaporation and as a result a salt concentration will build up at the drying surface resulting in crystallization. To this end we propose a simplified model to describe this mechanism. In order to check the model the NaCl concentration profiles were measured non-destructively by Nuclear Magnetic Resonance during a combined wicking and evaporation experiment with limestone. A good correlation was found between the model and the measured NaCl concentration profiles.

Assaad, Joseph; Matar, Pierre

2018 Materials and Structures

doi: 10.1617/s11527-018-1188-x

The use of self-consolidating concrete (SCC) containing recycled concrete aggregate (RCA) considerably increased in sustainable structural applications and civil engineering works. However, current literature and construction practices are not clear regarding the influence of RCA additions and presence of steel reinforcement on formwork pressure exerted by the plastic concrete. This paper reports experimental data obtained from 32 SCC mixtures possessing different stability levels and cast in 1.6-m high formwork containing various combinations of vertical and transverse steel bars. Test results have shown that mixtures incorporating recycled aggregates exhibited reduced initial maximum pressure, given the higher RCA surface roughness that promotes internal friction and material build-up at rest. The decrease in pressure was particularly accentuated in presence of steel bars, suggesting that the reinforcement cage confines the plastic concrete and carries part of its load. The transverse steel was around 1.5-times more influential than vertical steel in reducing the formwork pressure. The rates of pressure drop over time were not altered because of steel, implying that pressure decay is governed by the concrete intrinsic properties such as thixotropy, RCA friction, and cement hydration. Special emphasis was placed to develop regression models and examine suitability of existing ones to predict lateral pressure of RCA-modified SCC cast in formworks containing reinforcing bars.

Fernandez, Ignasi; Lundgren, Karin; Zandi, Kamyab

2018 Materials and Structures

doi: 10.1617/s11527-018-1206-z

Reliable methods are necessary to assess the corrosion level to establish links between structural performance and reinforcement corrosion in concrete structures. Hence, in this study, a set of naturally corroded bars were subjected to metallic brushing, acid immersion, and sandblasting for rust removal. Additionally, 3D optical, CT scanning, and weight loss measurements were used to evaluate the levels of corrosion. The results indicate that sandblasting is an optimal cleaning method. Weight loss measurements are sufficient when detailed information about corrosion is not required, and 3D scanning is preferred if information on corrosion variation along the bar is needed.

Sangiorgio, Filippo

2018 Materials and Structures

doi: 10.1617/s11527-018-1200-5

This paper aims to provide the reader a simple graphical method for the estimation of the concrete strength class (CSC) in existing reinforced concrete structures, which may be adopted even using limited concrete core drillings (less than three per test region) when compared to actual practice (European standards). The method is based on a combined probabilistic/empirical model for material properties of in situ concrete compressive strength and age-hardening effects. Assumptions are according to Eurocodes. The procedure has been subjected to a rigorous validation process. The principle of this approach is to use two graphs to mimic the role that in situ compressive strength test results play in determining the CSC of existing structures at the ages of the core drilling and of the construction time (28 days). Basis for the method are few considerations on: (a) age of concrete; (b) exposure environment; and (c) type of cement. Information from the two graphs is combined allowing valid and reliable estimates to be drawn. The proposed method is intended to be used as a clear alternative to classic methodologies and will be useful for professionals working in the field of structural rehabilitation. It reduces invasiveness, time expenditure and labour cost while maintaining high accuracy in estimations. Furthermore, it provides extra information about the concrete strength development that may be used to explore alternative scenarios in safety analyses.

Nzabonimpa, J.; Hong, Won-Kee

2018 Materials and Structures

doi: 10.1617/s11527-018-1207-y

This study investigated the structural behavior of a mechanical joint with laminated concrete and metal plates for moment connections which can be used for the rapid erection of reinforced concrete precast columns. A concrete filler plate was placed between the metal column plates to transfer loads and protect nuts threaded with rebars. Nonlinear numerical finite element analyses considering concrete damaged plasticity was also performed to evaluate the load–displacement relationship, plate deformation of the joints with concrete filler plates, and rates of strain increase of the structural components. The influence of the column and concrete filler plates on the rate of strain increase of the structural components attached to the column plates was explored to determine how concrete, rebars, and steel sections, were activated relative to the stiffness of the metal plates. The strain values of structural elements that were attached to plates with sufficient stiffness values were found to be higher than those of the structural elements that were attached to plates with smaller stiffness values. These strains were evident in the nonlinear finite element analyses and experimental investigations. It can be inferred that laminated mechanical plates consisting of metal and concrete plates can be implemented and used to replace conventional precast connections.

Wallau, Wilma; Pirskawetz, Stephan; Voland, Katja; Meng, Birgit

2018 Materials and Structures

doi: 10.1617/s11527-018-1205-0

The susceptibility of concrete structures due to alkali–silica reaction (ASR) can be assessed by means of ASR concrete prism testing at 60 °C, according to RILEM AAR 4.1. There, expansion of concrete prisms indicates alkali-reactivity of the examined concrete mix. This work applies in situ expansion measurement to accelerated concrete prism testing. Automated measuring facilitates both storage without the usually necessary interruptions for manual measurement and acquisition of quasi-continuous expansion data. A comparative experimental programme showed that conventional testing resulted in stronger expansion and leaching of alkalis than automated testing. Experimental simulation of interruptions, typically associated with manual measurements in conventional testing, could prove the influence of these cooling–heating cycles. Two phenomenological approaches, frequently used for describing reaction kinetics of ASR by linking it to expansion results from ASR-testing, were validated with continuous expansion data of three types of aggregate. Experimental expansion depicted s-shaped curves similar to them of the modelling approaches. However, strong swelling recorded in the beginning of the test was not covered by the model curves. Auxiliary measurement of acoustic emissions and ultrasonic velocity helped characterising mechanisms such as hydration and cracking, which also influence prism expansion. The proposed modification of the measurement procedure provides an extended basis to analyse expansion mechanisms. Regarding data for validation of ASR-expansion models, continuous expansion results are preferable to conventional test results.