Václav Nežerka

A Micromechanics-Based Model for Stiffness and Strength Estimation of Cocciopesto Mortars

The purpose of this paper is to propose an inexpensive micromechanics-based scheme for stiffness homogenization and strength estimation of mortars containing crushed bricks, known as cocciopesto. The model utilizes the Mori-Tanaka method to determine the effective stiffness, combined with estimates of quadratic invariants of the deviatoric stresses inside phases to predict the compressive strength. Special attention is paid to the representation of C-S-H gel layer around bricks and interfacial transition zone around sand aggregates, which renders the predictions sensitive to particle sizes. Several parametric studies are performed to demonstrate that the method correctly reproduces data and trends reported in available literature. Moreover, the model is based exclusively on parameters with clear physical or geometrical meaning and as such it provides a convenient framework for its further experimental validation.

Fracture-Micromechanics Based Model of Mortars Susceptible to Shrinkage

The addition of stiff aggregates into a relatively compliant matrix results, up to a certain limit, in an increase of the effective mortar stiffness. Conventional micromechanical models are not able to capture this limit and must be coupled with a fracture mechanics model predicting the formation of microcracks between aggregates due to shrinkage of matrix. The proposed model utilizes the Mori-Tanaka (MT) scheme together with an estimation of a crack density, based on requirements for the crack formation determined by a numerical analysis. As a result, the model involves a single phenomenological constant, and provides significant improvement over the basic MT scheme.

Micromechanics-based simulations of compressive and tensile testing on lime-based mortars

Abstract The purpose of this paper is to propose a continuum micromechanics model for the simulation of uniaxial compressive and tensile tests on lime-based mortars, in order to predict their stiffness, compressive and tensile strengths, and tensile fracture energy. In tension, we adopt an incremental strain-controlled form of the Mori–Tanaka scheme with a damageable matrix phase, while a simple J2 yield criterion is employed in compression. To reproduce the behavior of lime-based mortars correctly, the scheme must take into account shrinkage cracking among aggregates. This phenomenon is introduced into the model via penny-shaped cracks, whose density is estimated on the basis of particle size distribution combined with the results of finite element analyses of a single crack formation between two spherical inclusions. Our predictions show a good agreement with experimental data and explain the advantages of compliant crushed brick fragments, often encountered in ancient mortars, over stiff sand particles. The validated model provides a reliable tool for optimizing the composition of modern lime-based mortars with applications in conservation and restoration of architectural heritage.

Use of Open Source DIC Tools for Analysis of Multiple Cracking in Fiber-Reinforced Concrete

Digital image correlation (DIC) became indispensable when monitoring and analyzing adevelopment of displacement or strain fields. The method is capable of capturing strain localization, itis not limited to a relative measurement of discrete points as conventional methods and appears to bemore accurate than measurements by means of extensometers or strain-gauges that often suffer fromimperfect attachment to the measured surface. As open source DIC tools appear, the method becomesfeasible and the development is supported by the growing computational power of modern computers.The presented paper introduces open source 2D DIC tools and simple rules to follow when employingthe method. The presented case study on behavior of fiber reinforced high-performance concretedemonstrates a computational feasibility, accuracy and sensitivity of the method for a relatively lowfinancial cost.

Replacement of Cement with Finely Ground Recycled Concrete: Influence on Mechanical Properties

The waste production from construction sites become very serious problem. Recycling is the best option for disposal of such waste, and the proper sorting and knowledge of the recycled concrete history allows its further use in the construction. The current studies are mostly focused on utilization of recycled concrete in the form of aggregate. The presented work is focused on the utilization of Finely Ground Recycled Concrete (FGRC) used as a filler and partial substitution for binder. Recycled concrete was ground from concrete railway sleepers in the Lavaris Company (Czech Republic). Through the testing of mechanical properties, we demonstrate the influence of FGRC’s amount in cement paste on mechanical properties of the composite. To clearly show the relationship between the amount of FGRC and the composite properties, samples with 33, 67 and 100 wt. % of cement replaced by FGRC were tested. The composite with 33 wt. % of FGRC attained the compressive and flexural strength comparable with reference cement paste without any FGRC additions. The results indicate that the partial substitution of cement by FGRC could lead to a cost reduction of cement composites with minimal impact on their mechanical properties.

Effect of Pva Modification on Properties of Cement Composites

Polymers are used for modification of the cement-based composites and others building materials since the thirties of 20th century. Based on the conclusions of recent studies, it is assumed that even water soluble polymers could be used as an admixture for such modification. Currently, there exist and are exploited several possibilities for polymer modification of mortars, wood-based products or bituminous asphalts. Various options differ in the way of modification, which can be basically applied to the entire volume or just a surface, but also in the form of the polymer used – either in the form of solution or fibers. The aim of our study was to investigate the influence of volume modification by the water soluble polymers, such as polyvinyl alcohol (PVA), on the properties of cement paste and find an optimum additive. It turned out that the addition of PVA solution into fresh cement paste results in an increase of porosity and therefore a stiffness and compressive strength reduction. On the other hand, the bending strength of PVA-rich specimens was significantly higher and their water absorption decreased, which may consequently result in enhanced frost resistance.

Utilization of Recycled Fine-Ground Concrete from Railway Sleepers for Production of Cement-Based Binder

The environmental awareness and potential cost reduction have promoted the recycling of materials in civil engineering. This paper is dealing with the recycling and secondary use of old concrete railway sleepers for reconstruction of old railway lines. In particular, it is focused on the investigation of material properties of a binder prepared from finely crushed old concrete sleepers. This material could be used for strengthening of subsoil and embankments supporting the railway structures. The study shows that the compressive and bending strength of the investigated material is sufficient for this purpose and that the strength can be further increased by suitable curing conditions. These findings could contribute to the utilization of old concrete sleepers during railway reconstructions and safe money spent for the purchase of new materials and disposal of old sleepers.

Mechanical Properties of Single and Double-Layered PVA Nanofibers

Multi-layered nanofiber textiles can be utilized in many applications. In such case the individual layers are laid in more stages and the question arises whether the connection is perfect. Two kinds of samples of PVA nanotextiles having the weight of 1.3 g/m2 (single-layered, AI) and 2.8 g/m2 (double-layered, AII), respectively. It was shown that mechanical properties, in particular the average tensile strength (24 N/mm for AI and 51 N/mm for AII) and stiffness (950 N/mm for AI and 1600 N/mm for AII), are independent of the number of layers, only their weight per unit area matters. This indicates that the bond between the individual layers is perfect.

Influence of Aggregate Stiffness on Fracture-Mechanical Properties of Lime-Based Mortars

Addition of relatively stiff aggregates into lime-based mortars is responsible for an increase of the effective mortar stiffness and stress concentrations around aggregates during mechanical loading. To observe the damage development during the three-point bending and splitting tests a 2D plane-stress nonlinear finite element analysis utilizing isotropic damage model was carried out and the results were validated against experimentally obtained data. The study revealed that the finite element model is able to capture the trends observed during the experimental investigation. The results of the numerical modeling and experimental investigation show the advantages of the use of relatively compliant crushed brick aggregates in ancient structures.

Contact Angle Measurement Tool Based on Image Analysis

We present a cheap solution for assessment of contact angles on sessile drops or menisci formed around partially submerged fibers. The proposed system consists of a simple optical set combined with an open-source software CAMTIA. The fully automatic assessment of contact angles is based on image binarization, identification of regions of interest, boundary smoothing, and contour differentiation. After initial setting of calculation parameters, there is no need for further interaction with the user. This eliminates the need for expensive commercial solutions or tedious manual placement of tangents, guarantees consistency in the assessment procedure, and allows fast bulk processing of images.