The Creep of Cement Paste with Various Content of the Fly Ash in the Age 1 Year

Transcription

1 The Creep of Cement Paste with Various Content of the Fly Ash in the Age 1 Year PAVEL PADEVĚT, PETR BITTNAR Department of Mechanics Czech Technical University in Prague Thákurova 7, , Prague 6 CZECH REPUBLIC pavel.padevet@fsv.cvut.cz petr.bittnar@fsv.cvut.cz Abstract: The paper discusses the creep cement pastes with addition of bulk carrier fly ash. The evolution of the creep was observed in age of one year, for one month. The size of the creep is influenced by the amount the creep physically bound water. The material properties depend on the ratio of components from which the cement paste is composed. The paper presents the results for the ratio of cement and fly ash 70:30, 60:40 and 50:50. Key-Words: - Lever Mechanism, Basic Creep, Creep of Saturated Material, Shrinkage, Fly Ash, Compression Strength, Age of Cement Paste. 1 Introduction The coal- fired power plants produce not only in the Czech Republic many tons of fly ash. Fly ash is generated as the secondary product by burning black and brown coal in the coal-fired power plants. The generated quantity of the fly ash is between 10 % and 30 % of the original volume of a burned coal. Grain size of fly ash from power plants is between 1 and 1000 microns [1]. The grain size of ash is associated with the absorption material. The density of ash from power plants is between 750 and 950 kg/m 3. The fly ash is inert material from perspective of chemistry; the main component is SiO 2 and Al 2 O 3 and CaO and SO 4. Conventional fly ash contains up to 80 % glass phase, as the mail component. On the other hand, the fluidized fly ash contains a significant percentage of SO 3, which can adversely affect processes in the hardened cement paste. For use in concrete, it is preferable to the above described bulk carrier fly ash. Hereinafter described experiments were performed using the bulk carried fly ash. The bulk carried fly ash is a suitable building material for their inert behavior. One possibility of processing of fly ash is its use in concrete or in cement paste. The term fly ash in the following text will be used for bulk carried fly ash. 2 Creep tests and their preparation The cement paste is the basic part of the concrete. Its properties are crucial for the properties of concrete structures. The resulting material properties are improved by adding aggregates into the cement paste. Into concrete is a used aggregate with better mechanical properties than that of the cement paste in most cases. 2.1 Cement paste The experiments were executed by using the cement paste [2] with addition of fly ash. The components of cement paste were: Portland cement CEM I 42.5 R (according to European Standard), fly ash and water [3]. The cylindrical specimens were prepared from components and concreted into the plastic cylindrical moulds. The prism specimens were realized into the steel moulds. These are suitable for preparation the specimens tested in compression. Water/cement ratio (w/c) of the prepared cement paste was % water from weight of solid parts was used for cement paste design. The ratio between the amount of cement and fly ash is defined by c/fa ratio. The coefficient expresses the ratio between the weight of cement and fly ash. Nowadays is replaced up to 30% of cement by fly ash. The higher cement replacement is not documented in detail, and the properties of the final ISBN:

2 product are not precisely defined. The ratios c/fa 70/30, 60/40 and 50/50 were used for the production of the cement pastes with fly ash. The specimens were taking out from moulds one day after production. Prepared material for testing was placed into the water basin. Fig.1 The specimen from cement paste in compression test. The specimens were cut shorted on the lengths near to 70 mm before testing, see Figure 1. A diameter of prepared cylinders was 10 mm. For the experiments were prepared six specimens. First two specimens were determined for a measurement of creep in drying condition. Other two specimens were used for a measurement of shrinkage. The last two specimens were used for a measurement of creep in water saturated condition. Shrinkage of the two cases (for c / f 60/40 and 70/30) was measured at one drained and second water saturated specimen. Shrinkage was measured on two specimens dried on the ratio c / f 50/50. Fig.2 The specimen from cement paste in compression test. The prisms specimens were used for testing the material properties of the cement paste. From the original length of 100 mm was created two blocks with a length of 50mm. The compression test was carried out until failure of the specimens. The modulus of elasticity was measured together with the compression test [4]. Figure 2 shows edges of extensometer for measuring the strain. The specimens were placed in water basin for five month. 2.2 Creep test The age of the tested specimens was 1 year. The measurement of creep was realized in the lever mechanisms [5]. The total number of specimens was six of which two specimens were tested on shrinkage, only. Applied load on specimen achieved size 697N. This force was unchangeable during process of measurement. Fig.3 The test arrangements of creep detail of measuring units. In Figure 3 are shown loading frames. The load is equal to the distance and size of weight from the center of rotation of mechanism. All specimens were covered in the foil due to the guarantee condition of the humidity. The specimens were loaded by a plumbs after their placing in the lever mechanisms. Test time of creep is selected usually 30 days. The specimens that are loaded by plumbs are unloaded before the end of the test. Measurement of shrinkage takes place throughout the test without load. The ambient temperature during the test is maintained at 20 C [6]. In these cases, the compession tests were carried out at 4 with water saturated and 4 dried solids. Measuring strain sensor length was 25 mm, which corresponds to half the length of the body [7]. The cement paste with fly ash is a fine-grained material. For this reason, it is not necessary testing of voluminous specimens. Cross-sectional areas of the prisms were 20 x 20 mm. The dried specimens were placed in a drying oven and heated to the temperature 24 hours before testing in compression. The strain was measured by strain gauge and from adequate strength the modulus of elasticity was calculated. ISBN:

3 3 Results of creep tests The measurement principle was in the monitoring of the creep increase, of the dry and the watersaturated specimens. Shrinkage of the cement paste with fly ash was measured together with the measurement of creep. The creep was measured by three sensors with a resolution of 0.1 microns. Stress near to 8.82 MPa was originated in the cement paste. The load level corresponds to 22 % of the strength of the cement paste. Unloading of bodies was after 1 month of measurement creep. The following graphs captured this moment at the end of the measurement. Size unloading load corresponds to the power of beginning of the measurement. The cement paste with 50% fly ash admixture reached a size of basic creep of 18 and 48 microns after 25 days as is displayed in the Figure 4 and 5. The average value of the basic creep of the cement paste with 50 % fly ash is 33 microns. The basic creep of the cement paste with content of 40 % fly ash achieved average value of 16.5 microns. Values of basic creep were 20 and 13 microns after 25 days from start of measurement, as it possible see on the Figures 6 and Basic creep Basic creep is defined as the creep of the material without the influence of moisture and shrinkage. This can be achieved by measuring creep dried specimens. Shrinkage is then subtracted from creeping dry solids. Deformation (*0.001mm) Basic creep, w/c 0.4, 50 % fly ash, No Time (Days) Fig.4 Basic creep of cement paste with fly ash and relation C/FA 50/50. Specimen No.1. Fig.6 Basic creep of cement paste with fly ash and relation C/FA 60/40. Specimen No.1. Fig.7 Basic creep of cement paste with fly ash and relation C/FA 60/40. Specimen No.2. Fig.5 Basic creep of cement paste with fly ash and relation C/FA 50/50. Specimen No.2. Fig.8 Basic creep of cement paste with fly ash and relation C/FA 70/30. Specimen No.1. ISBN:

4 Fig.9 Basic creep of cement paste with fly ash and relation C/FA 70/30. Specimen No.2. The creep of saturated cement paste is shown in Figures 10 to 15. Fig.12 Creep of cement paste with fly ash and relation C/FA 60/40. Specimen No.5. Saturated Fig.10 Creep of cement paste with fly ash and relation C/FA 50/50. Specimen No.5. Saturated Fig.13 Creep of cement paste with fly ash and relation C/FA 60/40. Specimen No.6. Saturated Fig.11 Creep of cement paste with fly ash and relation C/FA 50/50. Specimen No.6. Saturated Fig.14 Creep of cement paste with fly ash and relation C/FA 70/30. Specimen No.5. Saturated The creep of cement paste with 30 % fly ash reaches of the average value of 7.5 microns. The creep value was 11 and 4 microns after 25 days, see Figure 8 and 9. The increasing of the amount of fly ash in ISBN:

5 cement paste causes increasing the size of creep. Strength decreases with increasing amount of fly ash in cement. Fig.15 Creep of cement paste with fly ash and relation C/FA 70/30. Specimen No.6. Saturated The development creep of saturated material is diametrically different course from the cement paste with fly ash in the dry state. Shape deformation is increased gradually. The shape of the curve basic creep of cement paste with a lower content of ash is close to the shape of the line. The cement paste with 50% fly ash admixture reached a size of creep of 90 and 85 microns after 25 days as is displayed in the Figure 10 and 11. The average value of the saturated cement paste creep with 50 % fly ash is 87.5 microns. The creep of the cement paste with content of 40 % fly ash achieved average value of microns. Values of creep were 90 and 205 microns after 25 days from start of measurement, as it possible see on the Figures 6 and 7. This material has the largest difference between the results achieved size creep. The creep saturated cement paste with 30% fly ash reaches the smallest deformation after 25 days compared with the other two mixtures. The average value creep of after 25 days is 62 microns. 4 Conclusion As shown in Figure 16 the significant difference is between the dry and saturated cement paste with fly ash. The basic creep reaches values between 7-33 microns for a period of 25 days. The basic creep increases with the content of fly ash in the cement paste. The size of creep the saturated cement paste does not have an upward or downward trend. The results show that the biggest creep arises for the cement paste containing 40 % fly ash. The material properties also depend on the amount of fly ash in the mixture, saturated materials. Fig.16 Comparison of the basic creep and creep of the saturated cement paste. Acknowledgements This work was supported by project GACR under No. P104/11/2285. References: [1] Neville, A.M., Properties of Concrete, John Wiley & Sons, (1997), ISBN [2] Czech building industry in numbers 2011, Czech Statistical Office, ISBN [3] Morsy, M.S., Alsayed, S.H., Aqel, M., Effect of Elevated Temperature on Mechanical Properties and Microstructure of Silica Flour Concrete, International Journal of Civil & Environmental Engineering, IJCEE-IJENS Vol: 10 No: 01, ISSN 2077/1185. [4] Bentz,D.P., Hansen, A.S., Guynn, J.M., Optimization of cement and fly ash particle sizes to produce sustainable concretes, Cement & Concrete Composites 33 (2011), pp , [5] Padevět P., Mechanical Properties of Cement Pastes, International Confernce 70 Yeasr SvF STU. Bratislava, [6] Van Mier, J.G.M., Fracture Processes of Concrete CRC Press (1997), ISBN [7] Bahar Demirel, B.,,Kelestemur, O., Effect of elevated temperature on the mechanical properties of concrete produced with finely ground pumice and silica fume, Fire Safety Journal, 45 (2010) , [8] Padevět, P., Bittnar, P., Zobal, O., Changing the Properties of Cement Paste with Addition of Fly Ash in Time, Experimental Stress Analysis 2012, June 4-7, 2012, Tabor, Czech Republic, ISBN ISBN: