Saturday, May 28, 2011

Tests of basaltic fibre as diffused reinforcement

Tests of basaltic fibre as diffused reinforcement

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Dr. Inz˙. Krzysztof Zielinski (1954) is a graduate ofPoznan´ University of Technology. Since 1978he has been a Lecturer Poznan´ University ofTechnology. He obtained his doctorate in 1983.He is the author of 80 peer-reviewed and publishedpublications in the field of material scienceand technology of concrete. In the years 1997to 2002 he was a scientific consultant at theKreisel AG company. Since 1995 he has been ascientific consultant at Icopal SA

Mgr inz˙. Przemyslaw Olszewski (1980) is agraduate Faculty of Civil Engineering Poznan´University of Technology and a student NicolausCopernicus Uniwesity in Torun, Faculty ofEconomic Sciences and Management. His interestsare fiber reinforced concrete and investmentprocess management

The impact of basaltic fibre on selected physical and mechanical properties of cement mortar

Concrete is a fragile material. On average, the ration of the ultimate compressive strength and tensile strength is 10 to 1. As a rule, materials with fibrous structure are characterized by higher ratio of tensile strength and bending strength than the ultimate compressive strength, when compared with concrete. They are also characterized by lower shrinkage. That is why the idea was developed to add fibres into the concrete structure. At present these are mostly steel, cellulose and plastic (e.g. polypropylene, nylon) fibres as well as asbestos, glass or carbon fibres. However, the use of basaltic fibre as a diffused reinforcement for concrete and mortars is not very popular at the moment. In the article, test results of the laboratory of the Institute of Structural Engineering at Poznan´ University of Technology are presented concerning the impact of the addition of basaltic fibre on bending strength, ultimate compressive strength and shrinkage in the first 28 days of cement mortar curing. The optimum amount of basaltic fibre, allowing them best physical and mechanical properties of the mortar to be achieved, was also defined.

History and characteristics of basaltic fibre
The first building material made of basaltic fibre was mineral basaltic wool. The technology of its production was developed in the 1920s in the laboratories of the American company Johns-Manville. The patent for mineral wool production was purchased and used in practice by the Danish company of Rockwool. In the 1960s in the Soviet Union and the USA, intensive research was started in order to achieve the so called continuous basaltic fibre (BNW). After 20 years of experiments, the technology of its production was patented in Russia in 1991. The research had been financed by the war industry, however, the achieved fibre found application also for civil purposes, for instance in the construction industry as a component of composites characterized by high fire resistance and also as the material used for making a façade grid or surface reinforcement. Basaltic fibre, sometimes called a 21st century material, has very good physical and mechanical properties. Admissible work temperatures range from –260 to +700° C. According to
the producers declaration, the diameter of a single fibre used for tests ranges from 7 to 17 μm (in fact the being 12 to 15 μm – see [Fig. 1]) and the coefficient of elasticity totals 89 GPa. Elongation at break reaches the value of 3.15%. Materials made of the basaltic fibre have significantly better physical and mechanical properties than those made of glass fleece. The use of cut basaltic fibre as diffused reinforcement for concrete and mortar is not currently as popular a practice as the use of steel or polypropylene fibre. Lower interest in the basaltic fibre is mainly due to the lack of clear recommendations as regards the amount of used fibre and the impact that the fibre content has on physical and mechanical properties of mortar and concrete.

The aim and scope of the research
Tests have been carried out in the laboratory of the Institute of Structural Engineering at Poznan´ University of Technology. The aim of the tests was to check the influence of added basaltic fibre on some selected physical and mechanical properties of cement mortar. An attempt was made to calculate the optimum amount of the basaltic fibre, allowing the best mechanical properties of the mortar to be achieved. For mortar preparation cement CEM I 32.5R and standard quartz sand were used (according to PN-EN 196-1). The tests were carried out to check bending strength and ultimate compressive strength after 3, 7 and 28 days. The measurement of shrinkage in the first 28 days of mortar curing was also made. The tests were performed on standard cement mortar prisms with dimensions of 4 x 4 x 16 cm. The results of preliminary tests show that the effective impact of the basaltic fibre on the change of basic physical and Festigmechanical properties of the mortar decreases in case of the fibre amount higher than 2% and lower than 0.2% of the mortar weight. That is why samples for the tests were made by adding to the mortar the basaltic fibre totalling 0.3, 0.8, 1.3 and 1.8% of the mortar weight. The basaltic fibre used for tests was cut into pieces of approx. 6.5 mm (Fig. 2). Base prisms with no fibre content (0.0%) were also made up. During the whole period the samples were stored under laboratory conditions in the temperature of 18°C and a relative air humidity > 90%.

Test results for the bending strength
The results achieved during the tests are presented in (Table 1) and (Fig. 3). The presented results are the arithmetic average of measurements made on six samples. Standard deviation of the obtained results of bending strength is between 1.6 to 8.3%. The analysis of the data included in (Table 1) demonstrates that the addition of basaltic fibre increases the bending strength by 13% on average. The achieved increase is practically independent from the amount of added basaltic fibre. After 7 days of mortar curing the bending strength is highest. The highest increase of the bending strength (of approx. 6.5%) in comparison with the base mortar is achieved by adding basaltic fibre equalling 0.8% of the mortar weight. After 28 days of curing the achieved bending strength was lower than after 7 days. The reason for this was probably the use of “R” type cement for tests (cement of high initial strength). For prisms with an additive of fibre equalling 0.3%, 1.3%, 1.8%, the bending strength decreased by another 7.5% in comparison with the base prisms (0.0%). The lowest decrease (approx. 4.5 %) was noticed in samples with the fibre content totalling 0.8% of the volume. It may be assumed that the main reason for this is high fragility of the used basaltic fibre, its relatively small elongation at break and high adherence to the mortar. The mortar which was used to make samples is characterized by fairly high shrinkage. After more than twelve days of curing, the shrinkage strength causes cracking of the basaltic fibres. This would explain a high increase of bending strength after 3 days of curing and a lower increase after 7 days as well as a significant decrease after 28 days of mortar curing. Observation of the prisms’ cross-sections after the tests revealed that the basaltic fibre was diffused in the mortar at random and spatially. During breaking of the samples, approx. 90% of the fibre got broken and the remaining 10% were torn out of the mortar. This demonstrated considerable adherence of the mortar to the basaltic fibre.

Ultimate compressive strength
The achieved test results are presented in (Table 2) and (Fig. 4). The presented results are the arithmetic average of
measurements made on 12 samples. Standard deviation for achieved results of ultimate compressive strength is between 3.0 and 8.1%. After 3 days of curing a significant increase of ultimate compressive strength (of 10%) was observed only for the amount of 0.8% of the fibre in the mortar. For the remaining tested prisms, with the fibre content of 0.3%, 1.3% and 1.8%, the ultimate compressive strength remains on the same level in comparison to the base samples or is slightly higher, which is within the measurement error. After 7 days of curing we can observe stabilization of the strength for 0.3%, 0.8% and 1.8% of the fibre content. In case of the fibre amount equalling 1.3%, we can see a slight decrease of the ultimate compressive strength. However, the reason of such results might be a measurement error. After 28 days of curing, test results are much more differentiated. For small contents of fibre we can observe a significant increase (over 20% for 0.3% of the fibre content and almost 8% for 0.8% of the fibre content). For the fibre amount equalling 1.3%, the strength compared to that of the base samples did not change and for the content reaching 1.8% there was a visible decrease of the strength (of over 15%). The mortar with 1.8% of the fibre content was characterized by much worse workability and worse possibility of its thickening than others. This might have had a decisive impact on its lower ultimate compressive strength.

Test results concerning shrinkage
The shrinkage values given in (Table 3) are the arithmetic average of measurements made on 3 samples. The measurements were made with the accuracy of ± 0.005 mm. From the achieved results we may see that the highest shrinkage value is achieved in the specimens with no fibre content. Shrinkage after 28 days decreases proportionately to the increasing fibre content in the mortar. Measurements made after 3, 7 and 14 days of curing show that the proportionality of the shrinkage to the fibre content is maintained during the whole 28-day period of measurements. It proves that the phenomenon of breaking of the basaltic fibre, observed during the tests of bending strength after the first week of mortar curing, practically has no impact on weakening of its anti-shrinkage properties. The fibre, broken into small pieces, considerably decreases the shrinkage of the mortar. (Fig. 5), presenting the data from (Table 3), shows the dependence between the fibre content in the cement mortar and its 28-day shrinkage. The dependence has the form of the linear function y = a + bx, where a = 0.074, b = – 0.024 and the correlation coefficient equals 0.95. Thus it may be assumed that in the tested range of the basaltic fibre content in the mortar, increasing the fibre amount by 0.1% of the cement mortar weight, results in lower shrinkage of the mortar by approx. 0.0024 mm/m, i.e. by approx. 3.1% in comparison to the mortar with no fibre content.

The analysis of the test results as well as the observation made during these tests allow us to make the following conclusions: n The addition of basaltic fibre causes noticeable increase of 3 and 7 day bending strength; after 28 days of curing, there is a decrease in bending strength n The highest bending strength in comparison to the base samples was achieved in case of 0.8% of the fibre content after 7 days of curing and the highest ultimate compressive strength was achieved in case of 0.3 to 0.8% of the fibre content in the cement mortar after 28 days of curing n exceeding the amount of the fibre addition by 1% results in significant worsening of mortar workability and possibility of its thickening n adding the basaltic fibre to the mortar causes a smaller shrinkage, proportionate to the fibre content (in the tested range of the fibre content in the mortar) Summing up the above presented conclusion, we may say that the optimum amount of the basaltic fibre in the mortar, allowing the best mechanical properties to be achieved, ranges from 0.5% to 0.8% of the cement weight. The addition of basaltic fibre in the content as mentioned above will cause the decrease of shrinkage of the cement mortar by approx. 15 to 20%.

Literature References
Neville, A. M.: Properties of Concrete, Polski Cement Sp. z o.o., Cracow 2000 Jamroz˙y Zygmunt: Concrete and its properties (Beton I jego technologie), Publishing House for Scientific Works PWN, Warsaw – Cracow 2000
Brandt, Andrzej: Use of fibres as reinforcement in concrete elements (Zastosowanie wlókien jako uzbrojenia w elementach betonowych), Conference Concrete on the verge of the new millennium (Beton na progu nowego milenium), Cracow 2000 Karwacki, Janusz: Concrete reinforced with steel and plastic fibres (Betony zbrojone wlóknami stalowymi i syntetycznymi), Engineering and Construction 2/1995 (Inz˙ynieria i Budownictwo 2/1995) Information materials from Konvers Poland Sp. z o.o.
EN 197-1:2000 “Cement. Composition, specifications and conformity criteria for common cements” ASTM-C 1018-97 “Standard test Method for Flexural Toughness and First-Crack Strength Fibre-Reinforced Concrete (Using Beam Third-Point Loading)

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