Nina Štirmer

Application of ultrasonic methods for early age concrete characterisation

Concrete setting and hardening process are the most critical phases during construction work, influencing to properties of concrete structure, so the application of efficiency non-destructive test methods for early age concrete properties determination is crucial. Penetrometer, pull out and Vicat needle methods are standardised methods for young concrete characterisation. Paper shows that ultrasound methods have potential to be used for these purposes. Measurements of longitudinal compressive wave velocity through concrete and mortar during hardening process are performed. For mixtures preparation different additives were used. Obtained results indicate possibility for hardening process monitoring and time of cementitous materials setting determination.

Shrinkage Behaviour of Fibre Reinforced Concrete with Recycled Tyre Polymer Fibres

Different types of fibres are often used in concrete to prevent microcracking due to shrinkage, and polypropylene fibres are among the most often used ones. If not prevented, microcracks can lead to the development of larger cracks as drying shrinkage occurs, enabling penetration of aggressive substances from the environment and reducing durability of concrete structures. The hypothesis of the present research is that polypropylene fibres, used in concrete for controlling formation of microcracks due to shrinkage, can be replaced with recycled polymer fibres obtained from end-of-life tyres. To test the hypothesis, concrete mixtures containing polypropylene fibres and recycled tyre polymer fibres were prepared and tested. Experimental programme focused on autogenous, free, and restrained shrinkage. It was shown that PP fibres can be substituted with higher amount of recycled tyre polymer fibres obtaining concrete with similar shrinkage behaviour. The results indicate promising possibilities of using recycled tyre polymer fibres in concrete products. At the same time, such applications would contribute to solving the problem of waste tyre disposal.

Residual Mechanical Properties of Concrete Made with Crushed Clay Bricks and Roof Tiles Aggregate after Exposure to High Temperatures

This paper presents the residual mechanical properties of concrete made with crushed bricks and clay roof tile aggregates after exposure to high temperatures. One referent mixture and eight mixtures with different percentages of replacement of natural aggregate by crushed bricks and roof tiles are experimentally tested. The properties of the concrete were measured before and after exposure to 200, 400, 600 and 800 °C. In order to evaluate the basic residual mechanical properties of concrete with crushed bricks and roof tiles after exposure to high temperatures, ultrasonic pulse velocity is used as a non-destructive test method and the results are compared with those of a destructive method for validation. The mixture with the highest percentage of replacement of natural aggregate by crushed brick and roof tile aggregate has the best physical, mechanical, and thermal properties for application of such concrete in precast concrete elements exposed to high temperatures.

The methodology for defect quantification in concrete using IR thermography

AbstractThis paper presents a procedure for detecting and quantifying defects in reinforced concrete structures by using the method of active infrared thermography (IRT). For quantitative analysis, a methodology of thermal stimulation of concrete specimens and post-processing of the gathered data was developed. Presented methodology uses principles of step heating (SH) thermography, pulsed phase (PPT) thermography, principal component thermography (PCT) and correlation operators technique. A short descriptions of the post-processing methods used in the research is also provided in the paper. All three post-processing methods i.e. PPT, PCT and correlation operators technique have shown the possibility to enhance the defect detection in concrete structures in comparison to raw thermograms. According to the data accessible to the authors, in presented research, correlation operators and PCT post-processing techniques are being successfully used for the first time for defect detection within concrete structur...

Utilization of polypropylene fibre reinforced cement composites as a repair material: A review

Environmental and structural loads cause stresses, deformations, and displacements in structures. According to the building codes, they have to be designed and built to safety resist all those actions during entire service life. When deteriorated, structures must be repaired and strengthen so that during usage they do not represent a risk for the users. Due to the low expenses and good mechanical characteristics, cement-based composites are often used for repairs. In structures susceptible to shrinkage and/or exposed to specific load conditions, used cement-based composites are improved with polypropylene fibres. These composites have many beneficial characteristics ; like as reduced occurrence of microcracks, positive effect on autogenous, plastic and restrained shrinkage, increased resistance to spalling during the fire, accessible price etc. Due to those characteristics, they are often used for repair of slabs, pavements and concrete overlays. Detailed state-of-the-art is presented and encompasses both fresh and hardened state properties. Also, a possibility for further improvement of repair materials is discussed.

Management of sewage sludge – new possibilities involving partial cement replacement

An increasing trend of sludge generation at wastewater treatment plants (WWTP) has been observed in developing countries like Croatia. Thermal processing of sewage sludge facilitates its further management, although ash is generated as new waste in the process. The proposed approach, while eliminating the need to dispose ash at non-hazardous waste disposal sites, directly reduces not only the sludge and ash disposal costs, but also the raw cement production costs. All analysed technical and environmental requirements are met when 20 % of cement is replaced with ash.

Effect of lime addition during sewage sludge treatment on characteristics of resulting SSA when it is used in cementitious materials

Final disposal of sewage sludge is important not only in terms of satisfying the regulations, but the aspect of choosing the optimal wastewater treatment technology, including the sludge treatment. In most EU countries, significant amounts of stabilized and dewatered sludge are incinerated, and sewage sludge ash (SSA) is generated as a by product. At the same time, lime is one of the commonly used additives in the sewage sludge treatment primarily to stabilize the sludge. In doing so, the question arose how desirable is such addition of lime if the sludge is subsequently incinerated, and the generated ash is further used in the production of cementitious materials. A series of mortars were prepared where 10-20% of the cement fraction was replaced by SSA. Since all three types of analyzed SSA (without lime, with lime added during sludge stabilization and with extra lime added during sludge incineration) yielded nearly same results, it can be concluded that if sludge incineration is accepted solution, lime addition during sludge treatment is unnecessary even from the standpoint of preserving the pozzolanic properties of the resulting SSA. Results of the research carried out on cement mortars point to the great possibilities of using SSA in concrete industry.

Possible Acceptance of Small Recycled Aggregate Fractions for Concrete Production

In the western region of Croatia, small fractions of recycled aggregate have similar properties like small fractions of crushed natural aggregate regarding grain shape and fines content. The focus of this research was to analyze possibilities of using small fractions of recycled aggregate in the cases where only poor quality natural sand is available for production of concrete. The research is based on available data from concrete production in wider region of western Croatia and also on experimental research carried out on several series of mortar samples prepared with recycled and natural fine aggregate. Influence of two-phase mixing method on mortar properties in fresh and hardened state was analyzed. Based on the results of testing, it is concluded that small recycled concrete aggregate may be suitable for use in the concrete production. In concrete mix design, it is recommended to decrease absolute aggregate absorption determined according to HRN EN 1097 and to use double-mixing method, due to its positive effect on the consistency and strength.

BRIDGE RECONSTRUCTION WITH ONE LAYER CONCRETE OVERLAY

ABSTRACT The paper presents the reconstruction of the damaged reinforced concrete bridge with high performance concrete modified with styrene-butadiene latex and fiber reinforcement. After 30 years in service the severe damages of concrete cover and reinforcement corrosion were observed. The detailed testing has shown poor quality of concrete, low strength and up to 25% reduction of certain number of reinforcing bars. The bridge reconstruction was executed with one layer concrete overlay 20 cm thick. The repair high performance concrete was designed for the following properties and purposes: — to strengthen the span structure by increasing the depth of cross section by 10 cm replacing partly the damaged concrete, insulating layer and asphalt overlay — satisfactory resistance to freezing and deicing chemicals — resistance to wear and abrasion — to serve as the impermeable overlay replacing the insulation.