Institute of Fundamental Technological Research

Alkali-silica reaction (ASR) is a phenomenon that causes irreversible damage to concrete structures. Since 1940, research has continued to investigate the possibility of eliminating these negative effects. The lack of availability of non-reactive aggregates requires the use of reactive aggregates, characterized by satisfactory physical and mechanical properties, with the introduction of innovative solutions to mitigate the effects of ASR expansion. Recently, fibre-reinforcement has shown to be a promising approach, even if the type and the volume of fibres used to reduce, or eliminate, the deleterious effects of expansion are not well established. For this reason, Miniature Concrete Prism Tests (MCPT) were performed on 4 series of expansive concrete prisms without any fibres and with 0.5% in volume of polypropylene fibres, steel fibres, and recycled carbon fibres, respectively. In addition, 4 series of non-expansive mortar prisms, with and without fibres, were tested in bending. As a result, by using recycled carbon fibres a moderate expansion can be observed after 56 days, in contrast to the high expansion of un-reinforced concrete. The same positive effect cannot be observed in concrete reinforced with steel or polypropylene fibres. This is due to the absence of the deflection hardening capacity of the fibre-reinforcement, as confirmed by both mechanical tests on non-expansive mortars, and by the analysis of microstructure on the post-mortem specimens.

Mortar, Concrete, Bending tests, Miniature Concrete Prism Tests (MCPT), Expansion Microstructure

Three series of tests performed on fibre-reinforced gypsum composites are described herein. Sheep wool fibres and hemp fibres were used as reinforcement. The aim was to evaluate the capability of these biomaterials to enhance the fracture toughness of the gypsum matrix. The mechanical properties were measured by means of flexural tests on small specimens, whereas scanning electron microscopy with energy dispersive spectroscopy and X-ray diffraction were used to analyse the microstructure and composition of the fibres and of the gypsum composites. As a result, wool fibres were shown to improve the mechanical performance of the gypsum matrix, better than hemp fibres. This is due to the high adhesion at the interface of the fibre and gypsum matrix, because the latter tends to roughen the surface of the wool and, consequently to increase the bond strength. This preliminary research carried out shows that this type of biofiber—a waste material—can be considered a promising building material in sustainable and environmentally friendly engineering.

organic waste material, fibre-reinforced gypsum, mechanical properties, microstructure

Natural wool is a good insulating material, both thermal and acoustic. Nevertheless, with the increase in demand for the use of waste materials, other applications, such as the use of wool as a fibre-reinforcement in mortars and concretes, have been found. Unfortunately, wool, like other natural organic materials, dissolve in alkaline environment and, consequently, the performances of the reinforcement cannot be guaranteed for a long time. To solve the above issue, three series of reinforced mortar beams, with various contents of alkalis in cement, are investigated herein. The chemical compatibility, and the effects of alkalinity on the mechanical performances, are investigated by testing the beams in three point bending and, subsequently, by analysing the microstructure of the mortars through a scanning electron microscope equipped with energy dispersive X-ray spectroscopy. The results reveal that the lower the alkalinity of the cement paste, the better the resistance of wool fibres in cementitious matrix, which guarantees larger post-cracking residual stresses in the wool reinforced mortars.

fibre-reinforcement, fracture & fracture mechanics, microstructure, waste valorisation

In this paper, an overview of the latest research activities in the field of cement-based composites incorporating sheep wool reinforcement is presented. First, the characteristics of this type of natural fibre are described. Then, the current use of sheep wool fibres in cement-based composites is discussed. The research problems regarding the properties of cement matrix composites reinforced with sheep wool are divided into four groups: thermal and acoustic properties, mechanical behavior, durability issues, and microstructure aspects. The latter two groups are analysed separately, because both durability and microstructure are of particular importance for future applications of wool reinforcement. Finally, the main directions of future researches are presented.

natural fibres, sheep wool fibres, mechanical properties, durability, microstructure

It is known that natural wool is a good thermal insulating material, but recent results suggest another application: the use of wool as a fiber-reinforcement in mortars and concretes. Indeed, the mechanical properties of wool filaments are comparable to those of some synthetic polymeric fibers (e.g., made with polypropylene). However, wool can dissolve in alkaline environments and, therefore, the performances of reinforced cement-based matrixes cannot be guaranteed for a long time. Accordingly, three series of reinforced mortar beams have been made with low alkali, high alkali, and sulfoaluminate cements. To investigate the chemical compatibility, and the subsequent effects on the mechanical performances, the beams have been tested in three point bending. As a result, the lower the alkalinity of the cement paste, the better the post-cracking capability of wool fibers to arrest the growth of cracks.

wool reinforcement, low alkali cement, high alkali cement, sulfoaluminate cement

The necessity of tailoring more environmentally friendly materials has prompted researchers and practitioners to explore new and more sustainable components for cement-based mixtures. Some of these materials are in their natural state and they can also be used to improve the mechanical performances of cementitious composites. Sheep wool, which is nowadays considered a special waste, can substitute some polymeric fibres to increase the fracture toughness. However, in alkaline environment, wool fibres tend to degrade within a few days after casting, so fibres can lose the reinforcement function of concrete and mortars due to long term durability issues. A series of tests have been performed with the aim of examining the microstructure and measuring the mechanical properties of wool-reinforced pastes made with various types of cement. By linking the results of microstructural analyses with those of the mechanical tests, it is possible to argue that the lower the pH of the paste the larger the efficiency of wool fibres.

wool-reinforced paste, microstructure analyses, residual strength, degradation, sustainability, eco-friendly material

cement, durability, fracture toughness, microstructure analysis, natural fibers

Depending on the intended use, some cement-based construction materials, such as paste, mortar and concrete, need to be fibre reinforced. In these materials, fibres play the same mechanical role as ossein, the elastic collagen fibres in animal bones that guarantees the resistance to fracture. Although commonly used fibres are made of various materials, such as steel, glass, polymers etc., animal and plant fibres can also be used in building materials. Among them, wool of sheep, a waste material in several countries, can effectively reinforce pastes, mortars and concretes. In addition to the research already performed in the field of cement-based composites, the use of sheep wool as reinforcement of gypsum-based composite is experimentally investigated herein for the first time. As a result, sheep wool reinforcement provides high fracture toughness, due to an excellent adhesion, and could be a valid alternative to the current industrial fibres in reinforced gypsum manufacts.

sheep wool fibres, gypsum-based composite, mechanical properties, microstructure

It is known that natural wool is a good thermal insulating material, but recent results suggest another application: the use of wool as a fiber-reinforcement in mortars and concretes. Indeed, the mechanical properties of wool filaments are comparable to those of some synthetic polymeric fibers (e.g., made with polypropylene). However, wool can dissolve in alkaline environments and, therefore, the performances of reinforced cement-based matrixes cannot be guaranteed for a long time. Accordingly, three series of reinforced mortar beams have been made with low alkali, high alkali, and sulfoaluminate cements. To investigate the chemical compatibility, and the subsequent effects on the mechanical performances, the beams have been tested in three point bending. As a result, the lower the alkalinity of the cement paste, the better the post-cracking capability of wool fibers to arrest the growth of cracks.

Wool reinforcement, low alkali cement, high alkali cement, sulfoaluminate cement

The addition of natural fibers residue in cement based materials can be a sustainable technological alternative for traditional dispersed reinforcement, and can improve the performance of brittle matrix materials. The presence of a wool reinforcement can increase the fracture toughness and, at the same time, can reduce the environmental impact of cementitious mortars. The beneficial effects are similarly to those observed in presence of vegetal fibers (e.g., hemp), which have been largely investigated in the literature. However, there are some limits in the use of wool fibers due to their chemical compatibility with the cement matrix, as they can dissolve in alkaline environments. In the present paper, to investigate the compatibility between wool fibers and cementitious mortars, laboratory prototypes have been taken into consideration. Three series of wool-reinforced mortar beams have been cast and cured in water (20°C) or in dry conditions (temp. 20 °C, 50% R.H.) for some days. Portland-limestone cement CEM II has been used, whereas the content of fibers has been limited to about 1% in volume to maintain the workability of the mortars. To investigate the chemical compatibility, and the subsequent effects on the mechanical performances, prototypes have been tested in three point bending. After the mechanical test, the mortars microstructure was evaluated through SEM images and by thin section in transmitted light, in order to individuate a possible relationship between the dissolution of wool and curing conditions. The microstructure observation revealed the capability of wool fibers to bridge the cracks, and to reduce the brittleness of plain mortars. The differences in the mortars microstructure due to alternative curing conditions were also observed and described in the paper. Accordingly, wool could be effectively used to reduce the plastic shrinkage of cementbased composites, like the industrially manufactured polypropylene fibers.

Wool fibers, Plain cement-based mortar, Fiber-reinforced mortar, Polypropylene fibers, Three point bending tests, SEM analyses

AgriculturalWaste, Animal Origin Fibres, Cement-BasedMaterials, Microstructure, Mechanical Properties