УДК 620.179
EVALUATION OF FROST RESISTANCE OF CALCIUM SILICATE MASONRY UNITS WITH ULTRASONIC PULSE AND RESONANCE METHODS
Jiri Brozovsky
Institute of Technology of Building Materials and Components, Faculty of Civil Engineering, Brno University of Technology, CZ-62800Brno, Czech Republic Е-mail: brozovsky.j@fce.vutbr.cz
Abstract. Freeze-thaw resistance is one of the basic parameters of durability of construction materials. Methods of its evaluation vary; most often it is evaluated on the basis of strength of samples before and after the test of frost resistance. To certain extent, standards also codify the use of dynamic non-destructive methods (ultrasonic and pulse method) capable of detecting changes in micro-structure. The decisive factor for evaluation of frost resistance of calcium silicate bricks is compressive strength. Results of the research focused on verification of applicability of ultrasonic pulse method and resonance method proved practical applicability for evaluation of frost resistance of calcium silicate bricks. Difference between parameters of non-destructive testing before and after freezing was -1.1 % to -9.9 %, depending on the number of testing cycles and quality of calcium silicate bricks. Both ultrasonic pulse method and resonance method are applicable for evaluation of frost resistance of calcium silicate bricks. Relevant criteria were determined for evaluation of frost resistance of calcium silicate bricks based on the parameter of non-destructive tests.
Keywords: calcium silicate brick, NDT, freeze-thaw resistance, ultrasonic pulse method, resonance method.
ОЦЕНКА МОРОЗОСТОЙКОСТИ СИЛИКАТНЫХ СТРОИТЕЛЬНЫХ БЛОКОВ УЛЬТРАЗВУКОВЫМ ИМПУЛЬСНЫМ И РЕЗОНАНСНЫМ МЕТОДАМИ
Иржи Брожовски Технический университет Брно, строительный факультет, кафедра технологии строительных материалов и изделий г. Брно, Чехия
Морозостойкость является одним из основных параметров долговечности строительных материалов. Существуют разные методы ее исследования, но чаще всего исследуют образцы на прочность до и после испытания на морозостойкость. Некоторые технические стандарты для определения морозостойкости строительных материалов учитывают применение динамических неразрушающих методов (ультразвукового и резонансного), которыми можно определить изменения в микроструктуре. Основным параметром при исследовании морозостойкости силикатных кирпичей является прочность при сжатии. Результаты исследований, направленных на определение применимости у.з. импульсного и резонансного методов, доказали возможность их применения на практике для исследования морозо стойко сти силикатных кирпичей. Разно сть между параметрами неразрушающего контроля до и после замораживания составила от 1,1 % до 9,9 % в зависимости от количества испытательных циклов и качества силикатных кирпичей. Как ультразвуковой, так и резонансный методы применимы для исследования морозостойкости силикатных кирпичей. Для оценки морозостойкости силикатных кирпичей по параметрам неразрушающего контроля были определены соответствующие критерии.
Ключевые слова: силикатный кирпич, неразрушающий контроль, морозостойкость, у.з. импульсный метод, резонансный метод.
INTRODUCTION
Freeze-thaw resistance is one of the decisive parameters of durability of construction materials and it is usually evaluated on the basis of change of strength of tested material or a product after a frost resistance test by comparison with the strength ofa reference sample. For evaluation offrost resistance ofsome construction materials, non-destructive test methods codified in relevant technical standards are also used. For example, both ultrasonic and resonance methods are used for
evaluation of frost resistance of concrete. In accordance with CSN 731380 [16], internal structure damage of concrete is evaluated on the basis of the change of Dynamic Young's Modulus of Elasticity determined by measurement with ultrasonic pulse method or on the basis of fundamental torcial resonant frequency — by impact resonance method. When tests are carried out in accordance with CSN 731322 [14], frost resistance is evaluated on the basis of tensile and compressive strength as well as on the basis of changes of the observed parameter from the dynamic non-destructive testing method. Use of resonance method is also stated in the standard CSN EN 12371 [18] for evaluation of frost resistance of building stone (the method of testing is stated in CSN EN 14146 [19]).
Use of non-destructive testing methods for evaluation of durability and resistance of construction materials (frost resistance or observation of changes caused by high temperatures) is also stated in many technical publications, but these focus mainly on observation of properties of the materials and nondestructive methods are only a means of evaluation of changes. Use of ultrasonic pulse method for evaluation of physico-mechanical properties, durability and resistance of concrete is stated for example in publications [2, 4, 5, 7—10, 12], for building stone in [11] and use of resonance method for concrete is stated in [3, 6]. Knowledge of testing fired bricks stated in [1] by ultrasonic pulse method imply that use of this method can be problematic or quite limited because there are many defects in internal structure influencing results of measurements by ultrasonic pulse method, however, they are not correlated with the strength.
Calcium silicate bricks are made from the mix of siliceous sand, common lime and water. The mix is used for forming products by pressing; the products are then placed in autoclave for certain time, where they are hardened at the pressure of water vapor 16 bar and temperature 195 °C (silicon dioxide from the surface of sand grains is released, reacting with dry hydrate and forming a very strong CSH phase). The structure of a calcium silicate brick is formed by conglomeration of sand bonded with products of lime hydration. The structure of the body of calcium silicate bricks is similar to that of fine-grained concrete with water absorbing capacity 12—14 % by weight, therefore it can be assumed that dynamic nondestructive testing method for evaluation of frost resistance could be used. This assumption is confirmed also by dependencies between velocities of propagation of ultrasonic pulse and compressive strength elaborated for calcium silicate bricks by the author [1], which show high level of correlation. The standard for the test of frost resistance of calcium silicate bricks and its evaluation uses only the evaluation based on compressive strength.
The paper gives knowledge from the research of frost resistance of calcium silicate bricks with mentioned methods.
MAIN OBJECTIVE OF THE EXPERIMENTS
The research was focused on observation of damage of internal micro-structure of calcium silicate body caused by cyclical freezing and thaw. Change of parameters of non-destructive tests (ultrasonic pulse velocity, dynamic Young's modulus of elasticity) and strengths depending on the exposition time of testing specimens was observed. Tests were carried out with whole bricks as well as with cut-outs of bricks (CSN 731322 [14] admits testing on cut-outs of masonry units as well).
Testing specimens for measuring with resonance method have standardized dimensions (usually, the proportion of cross-section and length of the specimen is 1:1:3; 1:1:4). The proportions are elaborated for determination of expected fundamental resonant frequencies. Because the dimensions of bricks and cut-outs of masonry units (blocks) do not correspond with mentioned dimensions, this problem was also solved to make it possible to verify correctness of measurements.
METHODS, MATERIALS AND TEST RESULTS Resonance method
The method is based on the fundamental resonance frequencies of a testing specimen, in particular the fundamental transverse, longitudinal and torcial resonant frequencies; measurement and evaluation of test results were carried out in accordance with ASTM C215 [13]. Fundamental resonance frequencies were measured with the accuracy of 0.001 kHz and its correctness is verified by means of proportions between expected frequencies stated in Table 1 (detailed method of calculation of expected frequencies is stated in [3]). Dynamic Young's modulus of elasticity values were calculated from Eq. (1) and (2):
El = 4Z2/L2D; (1)
Ef = 0.0789c1Z4 / D^, (2)
where L — length of brick/speicmens [m]; fL—fundamental longitudinal resonant frequency [kHz]; f— fundamental torcial resonant frequency [kHz]; i — radius of gyration [m]; c — correction factor; D — dry density of speicmens [kg/m3]; ErL — dynamic Young's modulus of elasticity from fundamental longitudinal resonant frequency [MPa]; Ef — dunamic Young's modulus of elasticity from fundamental transverse resonant frequency [MPa].
Table 1
Ratios of frequencies for each type of test samples
Speicmens Brick 290x 140x65 mm Prism 240> 70x70 mm
Ratios of frequencies for v, 0.12 0.14 0.17 0.22 0.12 0.14 0.17 0.20
ff 0.473 0.470 0.464 0.455 0.614 0.609 0.601 0.589
ff 0.399 0.399 0.398 0.397 0.483 0.482 0.480 0.478
ff 0.844 0.849 0.858 0.873 0.787 0.791 0.799 0.812
Where fL — fundamental longitudinal resonant frequency [kHz]; f — fundamental transverse resonant frequency [kHz]; f — fundamental torcial resonant frequency [kHz]; vd — dunamic Poisson ratio.
Ultrasonic pulse method
Measurements were carried out by direct transmission (in 3 measuring points along the length of bricks and 5 measuring points along the width of the brick; in 5 measuring points along the length ofthe block cut out of a brick)—the arrangement of direct transmission is in Fig. 1. Natural frequency of the transducers was 82 kHz, stated frequency of transducers was selected for the reason of achieving the ratio of wave length (X) to the minimum permissible lateral specimen dimension in the direction of transmission (dmin) < 1, to eliminate possible reduction of ultrasonic pulse velocity. Transit time was measured
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