Behavior of hollow clay brick masonry walls during fire. Part 2: 3D finite element modeling and spalling assessment

In the first part of this work [1], the fire behavior of masonry walls made with hollow fired-clay bricks was investigated experimentally. Several complex mechanisms occurred during firing are analyzed and identified. In particular, in cases of load-bearing walls, the local spalling of bricks is considered as an important factor governing the fire performance of masonry walls.In this second part, fired-clay masonry is investigated by theoretical approach with the finite element method in order to predict its behavior and its performance under fire exposure. For this purpose, a 3D numerical simulation is conducted with paying a particular attention to the spalling risks of alveolus bricks.     

夹心保温砌体房屋振动台试验研究

黏土砖是我国北方地区广泛使用的主要建筑材料。通过水平灰缝和竖向灰缝连接成整体砖墙,承受竖向荷载和水平荷载。传统的砌体墙热工性能差,在我国最寒冷的地区其厚度可达半米,过大的厚度,一方面使建筑自重过大,抗震能力较差,另一方面也占用较大的居住面积。为解决这一矛盾,文中研究的夹心保温砌体砖墙黏土夹层砖墙是通过在钢筋网连接的两层较薄的砖墙之间插入隔热层,形成夹心保温砌体房屋。为研究其抗震性能,对一个1/2比例的4层黏土夹层砖墙结构进行不同地震烈度下的振动台试验,对其抗震性能进行试验评估,包括动力特性和响应、破坏模式、荷载和变形能力以及损伤演化等。试验结果表明：由于水平拉结钢筋网和构造柱的约束,外墙、保温层和内墙能较好地协同工作;黏土夹芯砖墙结构能满足我国《建筑抗震设计规范》(GB 50011—2010)规定的7度抗震设防的要求。     

蒸压加气混凝土砌块住宅结构体系的应用研究

砌体结构作为我国传统的建筑形式，在住宅建筑中仍占有十分重要的位置。蒸压加气混凝土砌块作为一种替代粘土砖的新型墙体材料，长期以来一直主要用于各类建筑的围护和填充结构中。本文从强度和使用功能方面，结合算例分析，对蒸压加气混凝土承重砌块住宅结构体系的应用进行了探讨。     

表面带凸凹槽蒸压粉煤灰砖砌体受力性能的试验研究

通过对不同强度等级砂浆砌筑的表面带凸凹槽蒸压粉煤灰砖砌体的受剪和受压性能试验,对其抗剪和抗压强度进行了分析。结果表明,表面带凸凹槽蒸压粉煤灰砖砌体的抗剪和抗压强度均不低于粘土砖砌体相应的强度,在抗震设计中可按粘土砖房的相应规定执行,扩大了蒸压粉煤灰砖在地震区的应用范围。     

Experimental and numerical investigation of masonry under three-point bending (in-plane)

The aim of this study is to investigate the failure and post-peak behaviour of masonry panels with low bond strength mortar under three-point bending (TPB) both experimentally and numerically. Full-scale masonry panels with two different mortar strengths were tested under TPB. The material parameters were obtained from compression, TPB and shear tests on bricks and brick–mortar interfaces. The experimental results are detailed. Adopted for the numerical study was a micro-model finite element formulation in which masonry is modelled using expanded brick units with zero thickness brick–mortar interfaces. The simulation of the masonry TPB tests compared well with the experimental results. Investigations were the crack propagation through the masonry and the effects of dilatancy on the post peak response.     

秸秆石膏渣空心砌块墙体轻质节能

目前,我国的建筑内外墙体仍以粘土砖为主,混凝土空心砌块为辅。混凝土空心砌块属非粘土质新型墙材产品,用于建筑围护结构热工性能差,容重大是较突出的问题。秸杆石膏渣墙体空心砌块容重450-1000kg/m3,导热系数为0.12-0.135W/m·k,作为建筑围护结构不仅容重轻,节约基础用材,且单一材料即可满足建筑节能50%的要求。为小城镇绿色住宅建设就近提供了环保墙材,既可降低生产成本,又能保护耕地,节约能源。其技术、经济、社会效益均十分显著。     

Steady state theoretical model of fired clay hollow bricks for enhanced external wall thermal insulation

This paper proposes a theoretical model to study the steady state thermal behavior of fired clay hollow bricks for enhanced external wall thermal insulation. The study aims at the development of new materials and structural components with good thermal material properties, with respect to energy saving and ecological design. Thermal insulation capacity of two external walls of different thicknesses, constructed of locally produced bricks, is studied. The basic brick units used for the investigation are small-size bricks with eight equal cavities or recesses and big-size bricks with twelve equal recesses. Their recesses configuration has been varied to perform the assessment. The insulation materials injected within brick recesses during the assessment are granulated cork and expanded polystyrene. The improvement in the thermal performance of the walls will be the result of optimization among the various factors such as brick cavity configurations, integration of insulation within brick recesses and the cavity surface emissivities. So emphasis is given to the study of the impact of these factors singly or in combination on the overall thermal resistance of walls in order to find out the best design solutions to maximize their thermal insulation capacity. Computer modeling and calculations performed, for steady state conditions, show that the increase in hollow brick cavity height contributes to the improvement of the overall thermal resistance of the order of 18–20%. The improvement could significantly increase to the range of 88.64% and 93.33%, if the bricks used are injected with the insulating material. If the cavity surface emissivities are lowered to 0.3, the improvement will be 72.73–78.33%. The results have also shown that replacing the cork by expanded polystyrene (EPS), having lower thermal conductivity, would not improve significantly the overall thermal resistance. This improvement is 9.08% for a wall of small-size bricks having configuration BS2CV and 8.34% for a wall of big-size bricks having configuration BB3CV.     

Cracking due to shear in masonry mortar joints and around the interface between masonry walls and reinforced concrete beams

Cracking occurs frequently in building masonry walls, and are usually located in the joints between the units because they represent generally planes of weakness, in some cases due to their low shear resistance. Sometimes cracks can appear in the interface between masonry blocks and reinforced concrete beams. That cracking has negative implications in building performance, namely, due to the possibility of rain penetration through the cracks and to negative aesthetic aspect. So, the objective of this paper is essentially to achieve a better knowledge of the shear behaviour of joints in masonry, for the particular cases of solid lightweight concrete blocks (blocks of aerated autoclaved concrete and blocks of lightweight concrete with expanded clay aggregates) and solid clay bricks, and to find preventive solutions to minimize those defects.This paper presents the principal results and conclusions of an experimental study that concerned the determination of mortar joints shear behaviour. From the results of the experimental tests, it can be concluded that the properties of the referred joints are significant for the wall overall behaviour, and the inclusion of reinforcing elements in these joints may in some cases serve as a preventive measure and can determine positive implications on that behaviour.     

Stochastic spatial modelling of material properties and structural strength of unreinforced masonry in two-way bending

The paper describes the development of a computational method to predict the strength for unreinforced masonry walls subject to two-way out-of-plane bending considering unit-to-unit spatial variability of the material properties of mortar joints and bricks. The study involves conducting a numerical simulation of full-sized walls subject to two-way bending using stochastic analysis in the form of Monte Carlo simulations and comparing the results with experimental work. A 3-D non-linear Finite Element Analysis is used to study how the spatial variability of material properties affect non-load bearing wall failure progression. The numerical results are compared to the experimental results in terms of the wall failure progression and wall capacity. It is shown that the model which considers the spatial variability of brickwork can best capture the failure patterns and predict the cracking and ultimate loads for walls subjected to two-way bending.     

Water vapour permeability of clay bricks

The water vapour permeability of clay bricks has been experimentally measured in order to draw a representative outline of industrial products without pore-forming additives. The correlations between water vapour permeability and the main compositional and microstructural parameters of both bricks and clay bodies have been investigated. A statistical model was set up in order to predict with reasonable precision and reliability, the water vapour permeability on the basis of open porosity, bulk density, mean pore size and pore specific surface values of bricks, and the finer particle size of clay bodies.     

In-plane retrofitting of masonry panels with fibre reinforced composite materials

Research activities carried out during the past years concerning the use of fibre reinforced polymers (FRP) as external reinforcement of masonry walls have shown that this system considerably improves structural stability and ductility with minimum increase in the load transmitted to foundations. However, different aspects of this retrofitting system should still be analyzed.The mechanical behaviour under in-plane compression and diagonal compression of clay masonry panels reinforced or repaired with carbon fibre reinforced polymer laminates is experimentally assessed in this paper. The results show that if correct retrofitting schemes are chosen, reinforcement and repairing with fibre reinforced polymers improves masonry behaviour, increasing ductility and, in some cases, ultimate strength and even stiffness. In this way, brittle behaviour and sudden failure of unreinforced masonry can be avoided.     

Modeling post-fire behavior of aluminum structural components using a maximum temperature approach

Aluminum alloys exhibit permanent degradation of material strength subsequent to reaching temperatures above 200 °C. While modeling of aluminum structural components during fire is becoming more heavily researched, modeling response of aluminum structures following fire events has still not been explored. Previous research suggests that the post-fire strength of AA6061 is heavily correlated to the maximum temperature reached during the fire. This research uses the maximum exposure temperature as an input to finite element simulations of a small scale aluminum beam to predict the post-fire behavior of the beam. Results of the simulations were compared to a set of post-fire thermo-mechanical experiments. Samples were heated using a radiative source, water quenched, and then loaded to failure in a four point bending configuration. The simulated thermal response of the aluminum was within 10% of measured values across the entire side of the beam. Prediction of the unexposed mechanical response was within 5% of experimentally measured response. The post-fire peak bending load from simulations generally over-predicted the experimentally measured value while predictions of the flexural stiffness were within 15% of experimental results.     

Equilibrium moisture content of clay bricks: The influence of the porous structure

The comprehension of the influence exerted by the material microstructure on the hygrometric properties of clay bricks plays a fundamental role in order to control the condensation phenomena and to avoid the deterioration of the masonry structure. The equilibrium moisture content (MEq) of ordinary and lightweight clay bricks was measured and the correlation with microstructure and pore morphology was investigated. The influence of the pore size and specific surface on the amount of MEq was found to be prevalent when compared to the other physical variables. A statistical model was also set up in order to predict the MEq values.     

An experimental fixture for continuous monitoring of electrical effects in moist masonry walls

A purposely designed experimental fixture for simultaneous and reliable monitoring of moisture, salts and spontaneous electrical polarization in masonry walls is proposed, based on the combined use of permanent sampling points and electrodes. Two fixtures were set up in laboratory with two different commercially available masonry bricks, in order to investigate the physical–chemical effects of capillary water rise as a function of the main bricks features (microstructure and salts nature and content).The research is aimed to contribute to a better understanding of the electrical effects in moist masonry walls, in order to possibly set up a reliable and scarcely invasive on-site monitoring procedure to be used in ancient building walls affected by rising damp, to lead restoration works to higher awareness.     

Mechanical properties characterization of different types of masonry infill walls

It is remarkable, the recent advances concerning the development of numerical modeling frameworks to simulate the infill panels’ seismic behavior. However, there is a lack of experimental data of their mechanical properties, which are of full importance to calibrate the numerical models. The primary objective of this paper is to present an extensive experimental campaign of mechanical characterization tests of infill masonry walls made with three different types of masonry units: lightweight vertical hollow concrete blocks and hollow clay bricks. Four different types of experimental tests were carried out, namely: compression strength tests, diagonal tensile strength tests, and flexural strength tests parallel and perpendicular to the horizontal bed joints. A total amount of 80 tests were carried out and are reported in the present paper. The second objective of this study was to compare the mechanical properties of as-built and existing infill walls. The results presented and discussed herein, will be in terms of strain-stress curves and damages observed within the tests. It was observed a fragile behavior in the panels made with hollow clay horizontal bricks, without propagation of cracks. The plaster increased the flexural strength by 57%.     

Sustainable earth walls to meet the building regulations

The thermal conductivity and diffusivity of un-fired clay bricks, a straw clay mixture and straw bales have been measured using a thermal probe technique, with an iterative method for data analysis. The steady-state air-to-air thermal transmittance, or U-value, and the time-dependent thermal properties of some proposed sustainable earth wall constructions are presented. Sustainable cavity walls of un-fired clay bricks with paper, straw or wool cavity insulation have thermal transmittances less than 0.35 W/m² K, and therefore meet the current United Kingdom Building Regulations. A review of possible methods for thermally up-grading existing earth walls, by adding an internal insulated timber frame construction, again demonstrates possible compliance with the current UK thermal regulations.     

Local post-strengthening of masonry structures with fiber-reinforced polymers (FRPs)

In-plane loaded masonry structures can be post-strengthened effectively with fiber-reinforced polymers (FRP). This applies to shear walls under vertical and horizontal loading, as well as to walls with additional cut-outs or single loads. A mechanical model is required in all cases for the detailing and design of post-strengthening measures and to characterize load transfer within the wall and, from this, to calculate stresses of masonry and FRP materials. To establish a three-step model for local post-strengthened masonry walls, extensive testing on different scales has been carried out at the University of Kassel within the last few years. Firstly, the load transfer between single masonry units and FRP was addressed. Overall, 91 bonding tests were carried out with seven types of bricks and blocks to examine failure modes and the bonding strength for a broad variety of bricks and blocks. Two different types of adhesive were used in combination with four types of glass- and carbon–fibers. Based on the results of the bonding tests, 24 anchoring tests overall on two different types of masonry – clay brick and calcium–silicate – were carried out under different geometrical and loading conditions. The test results of all test series will be explained by a combination of fracture mechanics and strut-and-tie modeling. A mechanical model based on fracture energy provides the background for the theoretical explanation of the debonding phenomena. The model can be used to predict failure of bonding on single bricks as well as bonding geometries with more than one brick where the bonding area is separated by bed or head joints. Comparison of data from calculations and testing exhibited good correlation.     

Masonry exposed to high temperatures: Mechanical behaviour and properties—An overview

The paper summarises the available information about the behaviour of masonry assemblies and components exposed to high temperatures and fire. A general frame is provided by fire resistance issues in current code provisions and observations from real events, theoretical as well as experimental researches on materials' performance during and after exposure to high temperatures. In particular, the newly acquired engineering approach to fire design has raised the need for improvements in the knowledge of fire and post-fire behaviour of structures at a material scale; thus, the paper particularly focuses on the mechanical properties of masonry materials, that are still scarcely investigated, especially in the residual (i.e. post-fire) situation. Detailed comments are given about the temperature-dependence of the relevant mechanical properties for masonry and its components, that are put into comparison. Then, the main research needs are outlined in the conclusions.     

Numerical investigation of historic masonry walls under normal and shear load

The behaviour of masonry under seismic loads has recently become of great interest. New methods for design and analysis, as well as for resistance, have been developed. The main focus has been on experimental and analytical research for the development of new brick material and mortar. With the introduction of Eurocode 8, the behaviour of old masonry structures under seismic load became an interesting topic for engineers. In particular, the available resistance capacity of existing masonry structures is highly relevant to the restructuring of existing buildings. Laboratory tests on masonry walls were performed in order to determine the mechanical behavioural characteristics of old brick material.