Shrinkage and creep of masonry mortar

Shrinkage and creep results are presented for different types of masonry mortars having a wide range of strength. The range of values implies that the type of mortar has an appreciable influence on deformation of masonry. The results are analysed, together with other data obtained from other investigations carried out over several years in the same laboratory, and predictive models developed. Factors quantified are strength, volume/surface/ ratio, time of exposure to drying (shrinkage) and time under load (creep). While creep is unaffected, for a given strength, shrinkage of water-cured mortar is greater than shrinkage of mortar that is cured under polythene. When based on 28-day strength, the average error of prediction for shrinkage is 19% but if based on the strength at the start of shrinkage, the error coefficient is reduced slightly to 16%. Creep is estimated with an average error of 24%.     

Influence of building process on stiffness: numerical analysis of a masonry vault including mortar joint shrinkage and crack re-closure effect

Behavior of masonry structures need to be characterized in order to assess masonry walls, towers and bridges. The owners of such constructions would like to understand the mechanical performance of their structures. This article presents the numerical modeling of a vault in the undamaged and damaged domains with a macro-homogenization analysis. The importance of considering the building phases in order to determine the actual stiffness of the structure is highlighted. The numerical tool is a continuous finite element model based on an original anisotropic damage model considering the masonry as a homogenized continuous medium. It computes the localized damage and indicates cracks pattern and their opening in the structure. It is able to predict the cracking zones in large masonry constructions without pre-positioning joint elements. This article focuses on the cracks induced by restrained shrinkage of the joint mortar during the building history. First, the state of the art is described and then the damage model is presented. In order to validate the model and to propose a global methodology, the damage model is applied to analyze a thin masonry vault previously tested in a laboratory. The numerical results are discussed relative to the experimental ones.     

Finite element analysis of notched bar skeletal point stresses and dimension changes due to creep

In this investigation, finite element calculations have been performed to obtain the creep stress distributions generated in circumferentially notched bar test‐pieces. They have also been made to determine the relation between axial extension and notch throat diameter changes. It has been found that an approximate skeletal point can be identified where the stress state is insensitive to the power law stress dependence of creep. Consistent trends in skeletal point stress ratios to those given in an existing Code of Practice for notch bar creep testing have been obtained. Nevertheless updated values, particularly for sharp notches, are proposed and these have now been inserted into a new version of the Code of Practice. In contrast, the link between extension and notch throat diameter changes has been found to depend on the creep stress index as well as the notch geometry. It is anticipated that the analysis can be used to establish the multi‐axial creep stress deformation and rupture behaviour of materials.     

Analysis-oriented model for concrete and masonry confined with fiber reinforced mortar

Analysis-oriented model (AOM) is a theoretical approach intended to analyze and design systems by applying the notions throughout the organization of a series of equations in one or more iterative cycle. The stress–strain curve of an fiber reinforced polymer (FRP) confined column is evaluated using an incremental procedure, which takes into account the interaction between the confining material and the column itself. Many AOMs have been developed for FRP-confined columns, while the prediction of the behavior of the jacketing with fiber reinforced mortar (FRM) is, currently, still a lack of the literature due to the relative recent development of studies about this new confining composite material. The aim of this paper is to present and discuss a new AOM able to deliver the axial stress–strain law of an axially loaded column made of concrete or masonry and with circular or square cross-section, when FRM-confinement is provided. A step-by-step iteration of the axial strain was adopted considering that the column reacts elastically and the FRM confinement remains un-cracked in each single step. The elastic secant modulus of the column was, thereafter, considered in order to catch its non-linear behavior and a further secant modulus was also computed for modelling the damage evolution of the FRM confinement when increasing the axial load. Finally, a parametric study allowed to check the correct interpretation of the phenomenon. Moreover, the theoretical versus the experimental comparison validated the accuracy of the proposed model.     

Compressive strength and elasticity of pure lime mortar masonry

The procedure and findings of an experimental campaign for the mechanical characterization of brick masonry with lime mortar joints are presented. The campaign includes the determination of the properties of the constituent materials and of the resulting masonry composite. The masonry consisted of masonry stack bond prisms made of solid clay bricks and two types of pure lime/sand mortars, material combinations which correspond to the vast majority of historical and existing masonry structures. The paper includes a discussion on the ratio between the elastic modulus and the compressive strength of the masonry constituents and the comparison of these ratios with the ones suggested in design codes. The implications of this comparison are discussed in the context of interventions on historical masonry structures using modern and traditional materials.     

Practical formulation of shrinkage and creep of concrete

Materials and Structures - A set of algebraic formulas is proposed to describe the shrinkage and creep of concrete over the entire range of time durations of interest. The formulas cover: the...     

Effect of metakaolin on creep and shrinkage of concrete

The effect of metakaolin (MK) on the creep and shrinkage of concrete mixes containing 0%, 5%, 10%, and 15% MK has been investigated. The results showed that the early age autogenous shrinkage measured from the time of initial set of the concrete was reduced with the inclusion of MK, but the long-term autogenous shrinkage measured from the age of 24 h was increased. At 5% replacement level, the effect of MK was to increase the total autogenous shrinkage considered from the time of initial set. While at replacement levels of 10% and 15%, it reduced the total autogenous shrinkage. The total shrinkage (autogenous plus drying shrinkage) measured from 24 h was reduced by the use of MK, while drying shrinkage was significantly less for the MK concretes than for the control concrete. The total creep, basic creep as well as drying creep were significantly reduced particularly at higher MK replacement levels. Compared with estimated values by the CEB 90 model, total creep of all concretes was overestimated, especially in the mixes containing the higher levels of MK. For basic creep, estimates for low levels of MK were acceptable but, for the higher levels, creep was overestimated.     

Effects of mortar layers in the delamination of GFRP bonded to historic masonry

Externally bonded fiber reinforced polymers (FRPs) used as strengthening for shear historic masonry walls increase tensile capacity so as to support combined compression and high shear forces released during earthquakes. The local and global capacity of FRP strengthened shear walls depends on delamination. This paper deals with the anchorage strength of GFRP strips bonded to historic masonry by analysing results obtained in pull–push shear tests carried out on GFRP-to-historic-brick bonded joints. The experimental research also foresaw the analysis of effects deriving from the presence of mortar layers in actual historic brickwork masonry, simulated through the creation of grooves on the surface of mortar filled clay bricks. The experimental results indicated brittle failure of joints due to delamination; results were processed to evaluate failure load values, strain vs. anchorage length diagrams and shear stress vs. slip relationships experimentally. Finally, the anchorage of GFRP-to-historic-brick was theoretically studied to improve the classic solution by incorporating adherent shear deformation. Discussion on the experimental and theoretical data was developed.     

Drying shrinkage of concrete due to capillary action

Summary The effect of capillary tension on drying shrinkage of cement paste and concrete is explained. The theory is based on principles of thermo-dynamics and theory of elasticity.

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Résumé On explique l'action de la pression capillaire sur le retrait de la pate de ciment et du béton. La théorie s'appuie sur les principes de la thermodynamique et la théorie de l'élasticité.

     

Swelling,shrinkage and creep: a mechanical approach to cement hydration

Recent advances in physical understanding of creep and shrinkage of cement paste suggest a novel approach to setting and hardening processes. In high-strength concrete, due to a low water-cement ratio, self-desiccation occurs immediately after setting, and capillary pressure produces compaction of the assembly of hydrating cement grains. For higher watercement ratio, water possibly can withstand cavitation, but then the volumetric balance of hydration requires that water move through the solid skeleton. According to Darcy's law, water only can be displaced by a gradient of hydraulic pressure, as long as the water pressure remains lower than its cavitation threshold. This water pressure only can be equilibrated by a compaction of the solid matrix, with compressive contacts particularly between the C-S-H layers. In both cases, with or without liquid-gas menisci, hydration acts like a distributed pumping of water, the liquid phase is under tension, and the solid skeleton is compressed. The high affinity of C-S-H for water and the viscoplastic behavior of the cement gel can explain the evolution of the contact between two hydrating cement grains and the formation of a continuous and cohesive bridge. Swelling and shrinkage then can be analyzed as two co-existing mechanisms, whose balance is controlled by the permeability of the solid matrix.     

Short-term tensile creep and shrinkage of ultra-high performance concrete

The tensile creep and free shrinkage deformations of ultra-high performance concrete (UHPC) were examined through short-term testing to assess the influences of stress/strength ratio, steel fiber reinforcement, and thermal treatment. The use of fibers and the application of thermal treatment decreased 14-day drying shrinkage by more than 57% and by 82%, respectively. Increasing the stress-to-strength ratio from 40% to 60% increased the tensile creep coefficient by 44% and the specific creep by 11%, at 14 days of loading. Incorporating short steel fibers at 2% by volume decreased the tensile creep coefficient by 10% and the specific creep by 40%, at 14 days. Also, subjecting UHPC to a 48-h thermal treatment at 90 °C, after initial curing, decreased its tensile creep coefficient by 73% and the specific creep by 77% at 7 days, as compared to ordinarily cured companion mixes. Comparison of tensile creep behavior to published reports on compressive creep in UHPC reveal that these phenomena differ fundamentally and that further evaluation is necessary to better understand the underlying mechanisms of tensile creep in UHPC. Results from this study also showed that the effects of both thermal treatment and fiber reinforcement were more pronounced in tensile creep behavior than tensile strength results of different UHPC mixes. This emphasizes the importance of conducting tensile creep testing to predict long-term tensile performance.     

Free,restrained and drying shrinkage of cement mortar composites reinforced with vegetable fibres

Many investigations are realized to establish the basic mechanical properties of vegetable fibre reinforced composites (VFRC) but not their shrinkage and creep behaviour. Some works have been realized to establish the shrinkage of cement mortar matrices reinforced with cellulose fibres, but very few results has been published with regards to shrinkage of VFRC with short sisal and coconut fibres. In this paper a concise summary of several investigations is presented to establish the influence of sisal and coconut fibres on the free and restrained plastic shrinkage, early drying shrinkage cracking, crack self-healing and long-term drying shrinkage of mortar matrices. The free and restrained shrinkage were studied by subjecting the specimens to wind speed of 0.4–0.5 m/s at 40 °C temperature for up to 280 min. The self healing of cracks of the VFRC was studied by using the same specimens as for the study of restrained shrinkage which were kept further in a controlled environment with 100% relative humidity and temperature of 21 °C for up to 40 days. Drying shrinkage tests were carried out at room temperature with about 41% relative humidity for 320 days. The influence of curing method, mix proportions and partial replacement of ordinary Portland cement (OPC) by ground granulated blast-furnace slag and silica fume on the drying shrinkage of VFRC was also investigated. Finally, based on the obtained results on drying shrinkage an equation using the recommendation of ACI model B3 was adjusted and compared well with the obtained experimental data.     

Early chemical shrinkage due to dissolution and hydration of cement

New investigations into very early volume change of aqueous cement dispersions allow us to understand better what is meant by the general term ‘chemical shrinkage’. In fact, early chemical shrinkage should be subdivided into dissolution shrinkage and hydration shrinkage, the latter taking place before as well as after the induction period. Shrinkage is evaluated by picnometry and by linear measurement carried out in three directions by means of an experimental set-up specially built to measure endogenous shrinkage right from the time after mixing. As expected, the volume of dissolved cement is a function of w/c-ratio, whereas the volume of early hydrated cement is independent of w/c-ratio. This relation, is linear and the slope as well as the intersection of the straight line with the ordinate vary depending on the cement composition. Therefore, these values may be considered to be indicating characteristic cement properties. In this way it becomes possible to classify cements and to predict the extent of early chemical shrinkage of mortar and concrete by just knowing their w/c-ratio.     

Application of fresh mortar tests to poultices used for the desalination of historical masonry

An European enquiry revealed that workability and adhesion of poultices is the main criteria for conservators to select poultice recipe. The poultice needs first to adhere to the substrate and be easy to use and prepare before presenting a real effect on salt extraction. However, a procedure to test the consistency and workability of fresh poultices does not exist neither in practice nor is it documented. Users usually rely on their empirical know—how to determine the most adapted consistency of poultices for a given type of substrate. The purpose of the following study was to scientifically determine the consistency and workability of fresh poultices. A test procedure was adapted from the fields of mortar, cement and concrete. In the same way as for cement and mortar standards, poultices can be classified in different categories according to their composition. It has been demonstrated that the determination of consistency helps to optimise poultice water content, while the workability has a direct influence on the adhesion of the poultice to the substrate. The knowledge of these properties is essential to determine the best conditions for a desalination treatment. Finally, optimal ranges of consistency and workability are established for the main categories of poultices (cellulose and mineral) and may help end-users to improve their own poultice recipes. Shrinkage limits of poultices are determined to insure their good adherence of the substrate during drying.     

Practical prediction of creep and shrinkage of high strength concrete

Model for practical prediction of creep and shrinkage of normal strength concrete, developed previously, is extended to high strength concrete. It is found that only a minor adjustment for the concrete strength effect is needed in the formulas for drying creep. The formulas for basic creep and shrinkage need no adjustment. The prediction model is compared with test data for creep and shrinkage obtained recently by Ngab, Nilson and Slate, and by Collepardi, Corradi and Valente, and a satisfactory agreement is demonstrated. The coefficient of variation of the deviations from test data is not larger than that for the normal cient of variation of the deviations from test data is not larger than that for the normal strength range. However, the existing data are rather limited and further testing is desirable.     

Simulation of shrinkage distress and creep relief in concrete repair

This paper addresses the problem of stress buildup in the repair layer of a concrete patch repair system resulting from moisture diffusion. As moisture evaporates from the repair layer into the surrounding ambience of known relative humidity, the hardened concrete substrate restrains free shrinkage movement of the repair layer. As a consequence, primary tensile stresses are set up in the repair layer together with shear and peeling stresses at the interface of the repair layer-concrete substrate. The repair layer under non-uniformly increasing tensile shrinkage stresses undergoes restrained creep in tension, which results in the development of secondary stresses in the system. The secondary stresses due to restrained creep being of opposite sign to that of restrained shrinkage serve to relieve the primary shrinkage stress field and the net or combined stress buildup as a result is reduced.A finite element based computer program used for computing the time dependent moisture loss profile in the repair system is interfaced with a finite element based 2-D stress analysis program for computing the time dependent restrained shrinkage and creep stresses.Variation of normal and shear stresses across depth and width at critical locations in the patch repair and temporal variation of these stresses are presented. Influence of ultimate free shrinkage strain ε_sh^∞ and the buildup of tensile stresses versus the evolution of tensile strength capacity f′_t of the repair is highlighted. Also, possible zones of failure are identified in the repair layer and at the interface of the patch repair system.