Freezing and thawing resistance of slag alkali activated concrete with different activators

The frost resistance of alkali activated materials is often considered very good. But some opposing results have also been recorded. Despite the good mechanical properties of some slag-alkaline concretes their frost resistance was found bad for field application. One of the reason for the explanation of the discrepancy of the results may be a different composition of the alkaline activator. Very often the content of Na2O (N) or K2O (K) is mentioned as the basic characteristic of an activator. But if water glass is used as an activator the content of SiO2 (S) is also very important. Please click Additional Files below to see the full abstract

Comparison of Properties of Concretes with Different Types and Dosages of Fibers

Concretes with PP fibers 12 mm, construction polymer fibers 25 mm, 3D steel fibers 25 mm, and steel microfibers 12 mm were prepared in dosages 0.5 and 1%. The mechanical properties (compressive strength, bending strength, fracture properties, and modulus elasticity) and the frost resistance of these concretes were tested and they are discussed. The behavior of these concretes is also discussed using graphs load vs. deflection. As bad results of frost resistance are sometimes recorded for concrete with fibers, this property is also evaluated. As was expected, mechanical properties are enhanced with the addition of suitable fibers. Frost resistance is usually comparative with concrete without fibers, but in the case of concrete with 1% of steel fibers, it is reduced

Behavior of shrinkage reducing admixtures based on polyether structure in various alkaline solutions

The several types of alkali activated materials (AAM) are in global research interest for number of previous decades. One of them - alkali activated slag mortars/concretes become attractive due to reduction of the worldwide limestone reserves and rapidly growing carbon taxes1, 2, 3. So the development of these materials in large-scale is nowadays substantial. Granulated blast furnace slag is commonly chosen as a suitable source of latent hydraulicity specie, that can be activated by high alkaline solutions (i.e. NaOH or sodium silicate glass). Although these mortars and concretes possess very durable products with quick strength development and good chemical resistance, the high shrinkage phenomena, drying, autogenous (2 – 4 more times higher than when the ordinary Portland cement is applied), is typically observed4, 5. Various type of polymer admixtures are applied to suppress this phenomenon. One of the possible explanation is attributed to capillary stresses resulting from fine pore size of formed hydration products6. The usage of admixtures, which can lower the surface tension and influenced the pore structure of formed CASH gels, is offered. The addition of shrinkage reducing admixture (SRA) based on polyether type is intensively studied and showed the interesting results in the shrinkage development suppression. The main research is done on the interaction between the SRA and blast furnace slag particles. The behavior, interactions and stability in alkaline solutions are still not clear. This work is related to the study of polyethylene glycol and polypropylene glycol based SRA´ behavior in alkaline environment. The polymers (and corresponding monomers) were mixed with different solutions according to pH and ion composition (H2O, NaOH, sodium silicate glass and synthetic pore solution, respectively) with the mass ratio 1:1. The time dependence study was performed on the samples incubated at 25°C in sealed vials. The part of treated samples was separated after 1, 7, 14 and 28 days after mixing and dried for analyses. The Raman and FTIR spectroscopy was used to assess the stability of the chemical structure. The treated samples were also studied in terms of surface tension characterization. Finally, the time dependence reaction has also affected the rheology of solutions, what can extremely influence the workability and casting of final AAS mortars or concretes. So the rheological behavior was examined. The spectra obtained from Raman and FTIR spectroscopy analyses showed the time influence and pointed to ongoing reactions in the high alkaline system. Especially, the rheology behavior was strongly changed within the time and SRA molecular weight from liquid (in case of low weight) to almost solid state (in case of higher ones). The experimental results showed the essential need to study the polyether compound behavior in alkaline environment. References: 1 van Deventer, J.S.J., Provis, J.L, Duxson, P. (2012) Technical and commercial progress in the adoption of geopolymer cement. Miner Eng 29:89–104 2. Provis, J.L., Geopolymers and other alkali activated materials: why, how, and what? 3 Provis, J.L., Palomo,A., Shi, C. (2015) Advances in understanding alkali-activated materials, Cem Concr Res, Vol. 78, Part A: 110-125, 4 Ye, H., Cartwright, C., Rajabipour, F. Radlińska, A. (2017), Understanding the drying shrinkage performance of alkali-activated slag mortars, Cem. Concr. Compos., 76 pp. 13-24 5 Ye, H., Radlińska, A. (2016) Shrinkage mechanisms of alkali-activated slag, Cem. Concr. Res., 88 pp. 126-135 6 Duran Atiş, C., Bilim, C., Çelik, Ö., Karahan, O. (2009) Influence of activator on the strength and drying shrinkage of alkali-activated slag mortar, Constr. Build. Mater., 23 pp. 548-55

Effect of chemical structure on the efficiency of shrinkage reducing admixtures in alkali activated systems

Alkali activated binders, especially those based on alkali activated blast furnace slag (AAS), have the potential to become an alternative construction material to ordinary Portland cement binders. Nevertheless, AAS has some disadvantages which prevent its broader practical applications. An extensive shrinkage is one of the main limiting factors. Therefore, the study of chemical admixtures mitigating especially the drying shrinkage is necessary to be performed. The efficiency of suitable shrinkage reducing admixtures depends on the chemical structure of used surfactants. The study is consequently focused on the molecular architecture of amino alcohol surfactants which are closely associated with their ability to effectively reduce shrinkage. The molecular structure of used chemical compounds is shown in Figure 1. The influence of different substituents bounded to the secondary amine group was studied in terms of their effect on alkali activation, mechanical properties, microstructure arrangement and in particular on the enhancement of drying shrinkage reduction. It was determined that the addition of any tested admixture delayed the CASH gel formation which negatively influenced the flexural as well as compressive strengths in the early stages of hydration process (1 – 7 days). However, only slight decrease in strengths compared to reference sample was measured after 28 days of curing. The deeper insight into the microstructure (Figure 2) confirms previous results. It is obvious that in the case of reference sample the consistent matrix of binding phase is created after 24 hours. On the other hand, only thin layer of hydration products is formed in samples containing the admixture, which increases the porosity of material and tends to the deterioration of mechanical properties. Finally, the study confirms that the reduction of surface tension in pore solution occurs primarily with admixtures containing branched substituents, which further decreases the capillary tension responsible for the shrinkage according to Young-Laplace equation. The presented study highlights the essential role of molecular structure of shrinkage reducing admixtures contributing to the development of a new range of additives designed especially for alkali activated materials. Please click Additional Files below to see the full abstract

Comparative Evaluation of Mechanical Properties of Fibre-Reinforced Concrete and Approach to Modelling of Bearing Capacity Ground Slab

The use of steel fibre-reinforced concrete is becoming gradually widespread in the engineering design of buildings. Typical cases include concrete foundations or floors. The actual design approach is often different. The proposal significantly encompasses the knowledge and range of the material properties of steel fibre-reinforced concrete. This article presents a comprehensive research programme, which has been focused on extensive laboratory testing, in situ testing and advanced numerical modelling using computing models and nonlinear models of concrete. It aims at a comprehensive description of the material properties of concrete, according to the degree of reinforcement, using 8 types of laboratory tests for description. In total, over 74 specified laboratory tests and four slab tests in situ are carried out. Selected evaluated material properties are also provided for the regression curve according to the degree of reinforcement, 0 to 3%. Subsequently, the detailed description of steel fibre is used in advanced modelling of tests of concrete slabs in situ. Numerical models simulate the behaviour of the steel fibre-reinforced concrete base structure in the interaction with the subsoil, where the objective was to verify the total carrying capacity of the slab structure

Measurement and Utilization of Acoustic Emission for the Analysis and Monitoring of Concrete Slabs on the Subsoil

The article deals with the field of use of acoustic emission (AE) measurement in engineering structures. The research particularly focuses on the assessment of acoustic emission during an experimental test of the load-carrying capacity of concrete slabs on the ground. A wider field of research includes structural and material optimization of advanced engineering structures. The tests of concrete slabs are then carried out in an alternate solution which differs in the used concrete or steel fibre reinforced concrete (FRC). The experimental program then includes typical measurement methods using displacement sensors and strain gauges. Non-destructive methods of measurement including acoustic emission have been used with an eye to the configuration of the experiment and deeper understanding of the actual behaviour and damage to the structure allowing for subsequent optimization and non-linear simulation of slab computation. The aim of the submitted article is to present and assess the acoustic emission as a non-destructive method which can be used to detect damage and determine the load-bearing capacity of the selected type of a FRC structure

Identification of Mechanical Fracture Parameters of Alkali-Activated Slag Based Composites During Specimens Ageing

The aim of the paper is to present the results of the experiment focused on the development of the mechanical fracture characteristics of alkali-activated slag (AAS) based composites within the time interval from 3 days to 2 years of ageing. Two AAS composites, which differed only in the presence of shrinkage reducing admixture (SRA), were prepared for the purpose of experiments. The composites were prepared using ground granulated blast furnace slag activated by water-glass with silicate modulus of 2.0, standardized quartzite sand with the particle grain size distribution of 0−2 mm, and water. Commercially produced SRA was added into the second mixture in an amount of 2 % by weight of slag. The test specimens were not protected from drying during the whole time interval and were stored in the laboratory at an ambient temperature of 21 ± 2 °C and relative humidity of 60 ± 10 %. The prism specimens made of the abovementioned composites with nominal dimensions of 40 × 40 × 160 mm with the initial central edge notch were subjected to the fracture tests in a three-point bending configuration. The load F and displacement d (deflection in the middle of the span length) were continuously recorded during the fracture tests. The obtained F−d diagrams and specimen dimensions were used as input data for identification of parameters via the inverse analysis based on the artificial neural network, which aim is to transfer the fracture test response data to the desired material parameters. In this paper, the modulus of elasticity, tensile strength, and fracture energy values were identified and subsequently compared with values obtained based on the fracture test evaluation using the effective crack model and work-of-fracture method

Non-Linear Analysis of an RC Beam Without Shear Reinforcement with a Sensitivity Study of the Material Properties of Concrete

A detailed analysis of concrete structures requires knowledge of the mechanical properties of the materials used. In the case of a non-linear analysis, the scope of the information needed is even greater. In particular, the tensile strength and fracture-mechanical parameters are required for the concrete. Prospective approaches that could increase the informative value of detailed analyses include the use of stochastic modelling. It particularly enables the definition of the effects of individual input parameters on the load capacity, failure mode, and general behaviour of the structure. The presented paper aims at a detailed analysis of a reinforced-concrete beam without shear reinforcement, which is based on a complex set of laboratory tests and non-linear analyses with a sensitivity study. The laboratory program includes different types of laboratory tests. Selected and missing material parameters of the concrete are calculated according to recommendations in scientific papers and the valid standards. The results are compared and discussed