Prestress loss due to creep and shrinkage of high-strength calcium silicate element masonry with thin-layer mortar

Data on creep and shrinkage of high-strength calcium silicate element masonry with thin-layer mortar (CASIEL-TLM masonry) is not currently available in international literature. A novel application of this material in unbonded post-tensioned shear walls requires creep and shrinkage data to predict prestress loss. Therefore, creep and shrinkage were measured on 38 large-scale high-strength CASIEL-TLM specimens, including a TLM bed joint or a wall-floor connection, for a period of 300 days. Initial moisture content of the specimens and temperature and relative humidity of the environment were carefully controlled. Creep and shrinkage data were used to predict prestress loss by means of visco-elastic finite element simulations as well as a simple analytical expression. These predictions were validated by prestress loss experiments, conducted simultaneously with the creep and shrinkage experiments. Predictions for final prestress loss due to creep and shrinkage are below 16 or 24 % for CASIEL-TLM masonry with prestressing strands or prestressing bars respectively.     

碱矿渣陶粒混凝土砌块砌体受压本构关系

为考察碱矿渣陶粒混凝土砌块砌体的受压本构关系及基本力学性能,完成了126个用碱矿渣陶砂砂浆和碱矿渣陶粒混凝土砌块砌筑的砌体试件的轴心抗压试验。研究结果表明:当砌块抗压强度和砌筑砂浆抗压强度相同时,由于碱矿渣陶砂砂浆收缩大,该类砌体的峰值压应变、极限压应变均低于普通混凝土砌块砌体。建立了以砌体抗压强度、砌块抗压强度和砌筑砂浆抗压强度为自变量的这类新型砌体峰值压应变、极限压应变以及弹性模量计算公式。     

A comparative study on the approximate analysis of masonry structures

In this paper, numerical analysis of structural masonry subject to a uniform in-plane tensile stress/strain field is investigated employing various homogenisation techniques. Here, structural masonry is regarded as a composite material with brick, bed joints and head joints as its constituents. Assuming a perfect bonding between constituents. Assuming a perfect bonding between constituent materials, two homogenisation techniques based on the strain energy approach are applied to derive equivalent elastic moduli of masonry. Structural relationships for the constituent materials are next derived to relate strains and stresses in constituents to the average strains and stresses in the masonry. In addition, a slightly different concept of the homogenisation technique based on Eshelby's solution of the ellipsoidal inclusion problem is also applied to compare the results with the energy based methods. The tensile strength of the masonry is found on the basis of the failure of any of the constituent materials. It is shown that tensile strength is a function of the elastic parameters of brick/mortar as well as the tensile strength of mortar. These studies also show that, although initial cracking occurs under horizontal tensile forces, the ultimate strength of the panel is higher in this direction than in the vertical direction.     

藏式石砌体在剪-压复合作用下抗剪性能研究

为研究藏式石砌体抗剪性能,对4组共13个不同压力作用下的泥浆砌筑石墙试件进行双剪试验,分析藏式石砌体沿通缝抗剪强度和破坏机理,研究利用灰缝抗剪强度评估墙体整体抗剪性能的方法。试验结果表明,藏式石砌体灰缝受剪破坏为泥浆层自身变形与界面分离同步发展的延性破坏过程,最终达到完全的界面剪切滑移状态。通过对试验数据的统计回归提出藏式石砌体灰缝的抗剪强度计算公式,计算结果和试验值吻合程度较好。泥浆与水硬性砂浆的主要性能差别在于粘结强度和摩擦系数均较低。通过2种理论模型,分析了砌体灰缝抗剪强度与剪-压复合作用下抗剪强度表达式之间的关系,利用其他研究者的试验数据进行了验证。提出了基于双剪试验结果的藏式石砌体抗剪强度预测表达式,可供藏式石砌体抗剪性能研究参考。     

低温硫酸盐侵蚀下水泥砂浆抗折强度预测模型低温硫酸盐侵蚀下水泥砂浆抗折强度预测模型

为了进一步揭示低温硫酸盐侵蚀条件下水泥砂浆抗折强度的发展规律,本文以不同配合比砂浆试件为研究对象,进行了20℃、10℃、5℃条件下的硫酸盐侵蚀试验,并对不同龄期的抗折强度进行了测试。试验结果表明:侵蚀过程中砂浆试件抗折强度呈现出先上升后下降的变化趋势,且明显受到温度影响,温度越低,侵蚀情况越严重,主要表现为整体强度下降,上升段可达到的最大值减小,劣化开始的时间提前。为考虑温度的影响,在Irassar模型的基础上,引入温度修正系数,提出了低温硫酸盐侵蚀过程中抗折强度的预测模型。模型的计算值与实测值吻合度更高,最大误差为9%,平均误差为2.3%,故该模型可较为准确的预测水泥砂浆在5~20℃硫酸盐腐蚀环境下抗折强度的发展规律。      

Characteristics of cement-soil mortars

Cement-soil mortars are commonly used for the construction of soil-cement block masonry. The paper focuses on an experimental study in understanding the various characteristics of cement soil mortars in fresh and hardened state. Workability, strength, water retentivity, shrinkage and stress-strain characteristics of cement soil mortars and bond strength of soil-cement block couplets using such mortars are examined. Characteristics of 1:6 cement mortar and 1:1:6 cement lime mortar are also examined for the purposes of comparison. Workability of mortars has been quantified by conducting flow table tests. Results of flow values obtained for mortars from various construction sites are reported. There is a linear relationship between flow and water cement ratio of the mortars. Flow increases with increase in water-cement ratio. Very high flow value of 130% can be achieved for cement soil mortars and cement lime mortars. Reduction in flow value from 100% to 80% leads to increase in strength and modulus of mortars. Clay fraction of the mortar mix controls the flow, strength, density, shrinkage value and modulus of cement soil mortars. Cement-soil mortars lead to better tensile bond strength for soilcement block couplets when compared to the cement mortar and cement lime mortar.     

Stress rate sensitivity of stone masonry units bound with fibre reinforced hydraulic lime mortar

It is well established that most construction materials behave differently under static and dynamic loading. However, the literature on the time-dependent response of masonry joints is scarce, particularly with regard to the bond behaviour in historical stone masonry. This paper describes the dynamic response of sandstone masonry units bound with hydraulic lime mortars (HLMs). A drop weight impact machine was used to generate stress rates up to 10⁷ kPa/s. The dynamic impact factor and stress rate sensitivity were evaluated for the flexural strength of the sandstone, mortar and for the bond strength of the unit and further, the pattern of failure was noted in the units for each mortar mix and loading rate. Based on a related study on the fracture toughness of HLM, polypropylene micro-fibres were incorporated at 0, 0.25 and 0.5% volume fraction into the mortar. Results show that the flexural bond strength was more sensitive to stress rate than the flexural strength of the mortar, at similar rates of loading. Further, the stress rate sensitivity of the bond strength decreased with an increase in the fibre content. Also, whereas the mode of failure in the masonry units under quasi-static loading was through fracture at the mortar-block interface, the failure plane transferred to within the mortar under dynamic loading, particularly in the presence of fibre reinforcement.     

Numerical derivation of homogenized dynamic masonry material properties with strain rate effects

Masonry is a composite material composed of bricks and mortar disposed in a regular arrangement. It is commonly used as load bearing or partition walls in building structures. Owing to limitations of computer power, detailed distinctive modelling of brick and mortar of a realistic masonry structure or a structure with masonry infilled walls is usually not possible. Moreover, no dynamic masonry material model can be found in the open literature. Dynamic masonry material properties are important for an accurate prediction of masonry failure and fragmentation under dynamic loads. In this paper, a continuum damage model with strain rate effect is developed for masonry materials based on the homogenization method. The equivalent elastic properties, strength envelope and dynamic increase factors (DIFs) of strength and moduli for the homogenized masonry material are numerically derived from the simulated responses of a representative volume element (RVE). A numerical model of an RVE is analyzed with detailed distinctive modelling of brick and mortar with their respective dynamic material properties obtained from laboratory tests. The homogenized material model can be used to analyse large-scale masonry structures subjected to dynamic loading.     

Flexural bond strength of masonry using various blocks and mortars

This paper deals with an experimental study on flexural bond strength of masonry using various blocks in combination with different mortars. Flexural bond strength of masonry has been determined by testing stack-bonded prisms using a modified bond wrench test set-up. The effect of mortar composition and strength on the masonry's flexural bond strength using three types of masonry units (stabilized mud blocks, stabilized soil-sand blocks and burnt brick) has been examined. The effect of the masonry unit's moisture content on flexural bond strength has also been studied. Increases in mortar strength lead to increased flexural bond strength for cement mortar, irrespective of the type of masonry unit. It has been found that combination mortars, such as soil-cement mortar and cement-lime mortar, lead to better bond strength compared to cement mortars. The moisture content of the masonry unit at the time of casting has displayed significant influence on the flexural bond strength of the masonry. It has been found that for each type of masonry unit, an optimum moisture content exists, beyond which the flexural bond strength falls off quickly.     

Bond behaviour between textile reinforcement and mortar under tensile loading / Verbundverhalten zwischen textiler Bewehrung und Mörtel unter Zugbeanspruchung

Extensive experimental investigations are currently being carried out on various selected materials covering a wide range of properties to achieve a deeper knowledge about the bond performance of textile reinforced mortar (TRM) for masonry strengthening. The objective of the tests includes investigations of the bonding behaviour between alkali‐resistant glass textile reinforcement and mortar under tensile loading to determine the required anchorage and overlapping lengths of the reinforcement in the mortar‐based material. This article describes the test methods used as well as the results obtained so far. This research will also examine debonding of the mortar‐based reinforcement system and the masonry surface under shear load. The definition of these bond parameters is necessary for the design of textile‐reinforced masonry components, which will be developed in the near future. The research is also intended to contribute to the finding or even designing of matching alkali‐resistant glass textiles specifically for use in masonry.     

我国农村典型砖砌体墙片拟静力试验研究

农村房屋抗震是我国防震减灾工作的重要内容。砖砌体结构作为农村房屋的主要结构形式,如何提高其抗震能力具有重要研究意义。为研究砂浆强度、构造措施和窗洞口对砖砌体抗震性能的影响,该文首先对农村砖砌体房屋常用的几种砂浆和砖砌体进行了抗压强度试验,得到了相应材料及构件的强度参数;然后对不同砂浆强度、不同构造措施和开洞的12个墙片进行了拟静力试验,对比分析了不同因素对墙体力学性能影响。试验结果表明,砂浆强度影响最大,构造措施和开洞影响次之。最后依据试验分析结果为我国农居防震减灾工作提出建议。该文相关研究可为农居的抗震设防、标准制定以及施工提供参考。     

砌体结构双轴拉压应力强度试验研究

为弥补砌体结构双轴拉压应力强度理论研究的不足,以多孔砖砌体为研究对象,基于材料主应力与平面应力的对应关系,考虑不同灰缝角度及拉压主应力比的影响,重点开展了砌体双轴拉压应力宏观单元试件的强度试验。从中揭示了砌体双轴拉压应力状态下的破坏特征与规律,分析得出双轴拉压应力强度破坏曲线及对应平面应力状态的强度破坏准则函数表达式。试验结果验证了砌体结构材料各向异性的力学特性,试验方法为砌体结构抗拉强度的测试提供借鉴,试验数据为丰富砌体结构强度理论提供参考,研究成果为建立完善的砌体结构破坏准则提供依据。     

核爆冲击波作用下空心砌块墙对主体结构的作用

该文采用显式动力有限元软件LS-DYNA对核爆冲击波作用下高层建筑混凝土小型空心砌块外墙(以下简称砌块墙)的破坏模式进行了数值模拟,计算模型考虑了材料非线性、接触非线性、大应变、大变形等主要特征。在此基础上讨论了砌块墙在破坏飞散过程中传给结构的荷载,得到了不同冲击波超压作用下砌块墙传给结构的荷载时程。计算结果表明:核爆冲击波作用下,砌块墙的破坏模式与冲击波超压的大小有关;其传给结构的荷载时程基本呈三角形分布,荷载峰值随超压的增加而增大,墙体顶部与结构构件之间接触面的允许抗剪强度对荷载峰值时间和持续作用时间起关键作用。     

Influence of shear bond strength on compressive strength and stress–strain characteristics of masonry

The paper is focused on shear bond strength–masonry compressive strength relationships and the influence of bond strength on stress–strain characteristics of masonry using soil–cement blocks and cement–lime mortar. Methods of enhancing shear bond strength of masonry couplets without altering the strength and modulus of masonry unit and the mortar are discussed in detail. Application of surface coatings and manipulation of surface texture of the masonry unit resulted in 3–4 times increase in shear bond strength. After adopting various bond enhancing techniques masonry prism strength and stress–strain relations were obtained for the three cases of masonry unit modulus to mortar modulus ratio of one, less than one and greater than one. Major conclusions of this extensive experimental study are: (1) when the masonry unit modulus is less than that of the mortar, masonry compressive strength increases as the bond strength increases and the relationship between masonry compressive strength and the bond strength is linear and (2) shear bond strength influences modulus of masonry depending upon relative stiffness of the masonry unit and mortar.     

Tragfähigkeitserhöhung bei Mauerwerk durch textilglasgitterverstärkte Mörtelfugen

Increase of the vertical load carrying capacity of masonry due to mortar bed joints with textile glass mesh reinforcement From a structural point of view, one of the most important material parameters in the construction sector is the vertical compressive strength of masonry, which consists of the compressive strength of the bricks as well as of the mortar bed. The interaction between the bricks and the mortar beds is the main reason for compression failures of masonry walls. A close analysis of the deformation behavior of the two components shows that different transverse strains in the contact surface between the bricks and the mortar are the main cause for compression failures. However, the load‐bearing capacity of masonry walls can be increased by using some reinforcement in the mortar beds which counteracts lateral expansion. The impact of textile glass mesh reinforcement on the load‐bearing capacity of masonry was analyzed in a test program on masonry columns with different numbers of textile glass mesh reinforced mortar beds. The results of the analyses show that the load‐bearing capacity of the columns rises with an increased ratio of reinforcement, regardless of the type of bricks used. From the ratio of the height of the reinforcement layers to the thickness of the wall it can be deduced that a higher degree of reinforcement has a positive effect on the load‐bearing capacity of the masonry. On this basis, an increase of the strength and load‐bearing capacity of masonry walls is formulated to be on the safe side.     

Composite modelling of masonry deformation

Based on the measured properties of individual bricks, blocks and mortar specimens, composite model expressions are presented for elasticity, creep and moisture movement of masonry in two directions. The model demonstrates the influence of creep in horizontal moisture movement, and allows for anisotropy of brick or block. Simulation of the role of moisture diffusion in masonry is a requirement for statisfactory predictions by the model, this being achieved by testing specimens having the same volume/exposed surface area ratio as the masonry component phases. Experimental verification is shown for clay brickwork walls and piers, and for calcium silicate walls.     

Prediction of solid block masonry prism compressive strength using FE model

Masonry strength is dependent upon characteristics of the masonry unit, the mortar and the bond between them. Empirical formulae as well as analytical and finite element (FE) models have been developed to predict structural behaviour of masonry. This paper is focused on developing a three dimensional non-linear FE model based on micro-modelling approach to predict masonry prism compressive strength and crack pattern. The proposed FE model uses multi-linear stress–strain relationships to model the non-linear behaviour of solid masonry unit and the mortar. Willam–Warnke’s five parameter failure theory developed for modelling the tri-axial behaviour of concrete has been adopted to model the failure of masonry materials. The post failure regime has been modelled by applying orthotropic constitutive equations based on the smeared crack approach. Compressive strength of the masonry prism predicted by the proposed FE model has been compared with experimental values as well as the values predicted by other failure theories and Eurocode formula. The crack pattern predicted by the FE model shows vertical splitting cracks in the prism. The FE model predicts the ultimate failure compressive stress close to 85% of the mean experimental compressive strength value.     

贯入法检测砌筑砂浆抗压强度曲线误差验证

本文根据《贯入法检测砌筑砂浆抗压强度技术规程》JGJ／T136—2001第5．0．3条要求,验证规程中附录D的砂浆抗压强度换算表误差是否在规程第E．0．10条规定的范围内。     

Mechanical behavior of masonry assemblages manufactured with recycled-aggregate mortars

Mortars containing recycled aggregate, instead of quartz sand, were characterized to find an alternative application for the fine recycled-aggregate fraction coming from building debris processing. Tests on bond strength of mortar to masonry units were carried out, as well as tests on compressive and shear strengths of masonry assemblages. The results obtained were related to the mechanical properties of mortars and brick. On the basis of the characterization results and performance evaluations, recycled-aggregate mortar appears to be superior to ordinary mortars in terms of mortar–brick bond strength and shear strength of masonry assemblages. This improved performance is of particular interest for the masonry structures in zones of seismic activity. In addition, the use of fine recycled aggregate is in accordance with the sustainable development concept, where recycling of building rubble plays a key role in ending the building life cycle.