Research Article
Evaluation of measurement uncertainty for the determination of the mechanical resistance of the brick samples by using uniaxial compressive strength test
^{1}
Department of Production Management, Faculty of Mechanical Engineering, Polytechnic University of Tirana,
Mother Teresa No. 3, 1001
Tirana, Albania
^{2}
Department of Mathematical Engineering, Faculty of Mathematical & Physical Engineering, Polytechnic University of Tirana,
Mother Teresa No. 3, 1001
Tirana, Albania
^{3}
Department of Metallurgy Engineering, Faculty of Mechanical Engineering, University of Sultan Ageng Tirtayasa,
Jl. Jend Sudirman Km. 3,
Cilegon, Banten
42435, Indonesia
^{*} Corresponding author: kdhoska@upt.al
Received:
8
December
2017
Accepted:
3
October
2018
Clay bricks are the most important engineering materials that can influence the quality of the construction. The quality control of the clay bricks is of prime importance to improve the quality of engineering construction. The qualities of bricks for construction applications should be checked for various types of inspection services. In this article, our inspection service was focused on compression strength test on bricks to determine their mechanical resistance and suitability for construction work. The accurate method for clay brick's production measurements has been elaborated using uniaxial compressive strength testing machine. The method was tested for a rectangular clay brick manufactured with size approximately 250 mm × 250 mm × 200 mm. The measurement method and standard uncertainty estimation of uniaxial compressive strength are briefly described in this article.
Key words: clay brick&apos / s production / inspection services / uniaxial compressive strength / standard uncertainty
© K. Dhoska et al., published by EDP Sciences, 2018
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
1 Introduction
Clay brick materials are the oldest building materials in the world and are still immensely popular [1]. It can be used for a variety of applications such as singlefamily houses or apartment blocks, office or public buildings, and for all architectural styles.
In the last two decades, many manufacturing companies in Albania have been focused on the production and distribution of brick materials made of clay. Furthermore, Edilcentro Ltd. is one of the leading companies in Albania in the field of bricks' production of high quality. In this company, the phases of clay bricks' production are under automatic control and permanent supervision of the technological process [2]. Figure 1 depicts the technological scheme of clay brick's production.
According to this scheme of the automation process, it is clear that inspection services play an important role in ensuring the quality of the clay brick's production. The qualities of bricks for construction applications should be checked for various types of inspection services. Some of the inspection services would be related to absorption test, compression strength test, hardness test, shape and size, color test, soundness test, structure of brick, and presence of soluble salts.
Our research work has been focused on compression strength test on bricks to determine their mechanical resistance and suitability for the construction work. One of the basic and most used methods of testing which is performed on brick samples is the determination of uniaxial compressive strength [3]. Similar testing has been performed successfully on other types of materials such as concrete, rocks, and metals with some differences in testing procedures and minimum measurement equipment properties [4–7]. Furthermore, according to the American Society for Testing and Materials (ASTM) specifications, we have performed compression testing for the determination of the properties of clay brick materials.
In this article, we have briefly described the measurement method and standard uncertainty estimation of measured uniaxial compressive strength which would depend on the force applied to the sample.
Fig. 1 Technological scheme of clay brick's production. 
2 Measurement method
The uniaxial compressive strength machine is located in the laboratory of the manufacturing company Edilcentro Ltd., Albania. The complete range of the uniaxial compression strength machine was designed to meet the demands of our laboratory as shown in Figure 2. Compressive load was applied in displacement control on clay bricks using MTS servohydraulic actuator. Load and displacement measurements were recorded in real time using a computerbased data acquisition system.
The measuring range of the uniaxial compression testing machine varied from 0 to 500 kN. The function of the uniaxial compression testing machine consists in applying the load to the surface area of materials that will be tested. The load frame provides the stability required for accurate test results. A small number of indirect compressive strength tests have been developed, primarily in order to allow in situ quality control testing of materials. The compression testing was performed according to ASTM and ISO specifications [5–8]. In the present test setup, 10 specimens of clay bricks were tested for determining their mechanical resistance. The manufactured clay bricks had approximate length, width, and height of 250, 250, and 200 mm, respectively.
In the measurements, the clay bricks were generally placed inbetween two plates that distribute the applied load across the entire surface area of two opposite faces of the test sample and then the plates are pushed together by a uniaxial compressive testing machine. A compressed clay brick was usually shortened in the direction of the applied forces, and load measurements were recorded in real time using a computerbased data acquisition system. The uniaxial compressive strength of the test specimens was calculated by dividing the maximum compressive load on the specimen by the initial crosssectional area. A compressive strength measurement and the surface area of each clay brick are given in Table 1.
Fig. 2 Test setup for compression testing of clay brick. 
Summary of test results on clay bricks.
3 Uncertainty of uniaxial compressive strength
Uniaxial compressive strength is the quantity that is indirectly determined by the measurement which depends on the force applied to the sample. Sample stress σ of uniaxial compressive strength is evaluated as the ratio of maximum applied force F_{max} and sample crosssectional area A in the beginning of the test according to equation (1). (1)
In accordance with equation (1), the measurement uncertainty of stress will depend uncertainty of the force and the uncertainty of crosssectional area. A Guide to the Expression of Uncertainty in Measurement (GUM) has been used to establish the general rules for identifying sources of uncertainty and evaluating the measurement uncertainty [9]. Before starting an evaluation of uncertainty, the uniaxial compressive strength measurement model has been defined by taking into account influence sources of uncertainty which can be expressed as follows [9–12]: (2) where x is the measurement value of the uniaxial compressive strength, K_{F1} is the correction from calibration of uniaxial compressive strength machine, K_{F2} is the correction that arises from load application rate to sample test, K_{F3} is the correction that arises from the preservation condition effect on compression resistance of a sample, K_{F4} is the correction that arises from the effect of planarity on compression resistance of a sample, K_{F5} is the correction with respect to the angle of the surface plane between the sides, K_{F6} is the correction that comes from sample centering, K_{F7} is the correction from reading of applied load, and K_{A} is the correction that arises from reading of the sample crosssectional area.
Based on the law of propagation of uncertainty and uncertainty of input quantities, we can evaluate combined uncertainty as expressed below: (3) where u(x_{i}) is the standard uncertainty of the input quantities and c_{i} = df/dx is the sensitivity coefficient. According to the input quantities, the combined uncertainty could be expressed as follows: (4)where u_{F1} is the calibration uncertainty of uniaxial compressive strength machine, u_{F2} is the uncertainty of the load application rate to sample test, u_{F3} is the uncertainty of the preservation condition effect on compression resistance of a sample, u_{F4} is the uncertainty of the effect of planarity on compression resistance of a sample, u_{F5} is the uncertainty with respect to the angle of the surface plane between the sides, u_{F6} is the uncertainty of sample centering, u_{F7} is the standard uncertainty of the reading of applied load, and u_{F8} is the standard uncertainty of the reading of sample crosssectional area.
As the calibration uncertainty contribution, the specifications of the uniaxial compressive strength machine are used [8,9,13]: (5) where U_{F1} is the expanded calibration uncertainty of the machine which corresponds to 250 N with a coverage factor k = 2 and F_{a} is the test device scale which corresponds to 1000 N.
Since it was difficult to quantify more accurately the effect of load application rate on the sample test, we have assumed for simplification of the technical documents that evaluation of the uncertainty will be expressed as follows [6,11–14]: (6) where F_{max} is the maximum applied force on the clay brick materials. We have used similar assumptions for the evaluation of the other sources of uncertainty which can be expressed as follows: (7) (8) (9) (10)
The standard uncertainty due to the reading of indications of applied load is calculated as the standard deviation of the readings of applied loads S_{F7}, expressed as follows [8,9]: (11) where n is a complete set of measurement values, x_{i} is the individual measurement value, and x_{m} is the arithmetic mean of individual measurements.
The standard uncertainty due to the reading of indications of the sample crosssectional area u_{A} is expressed as follows [15]: (12) where x_{W} and x_{L} are the reading values of the width and length for the sample crosssectional area which can be evaluated as follows: (13) (14)where l_{m} is the mean value of length, w_{m} is the mean value of the width, Δ_{l} is the correction from readings of length, and Δ_{w} is the correction from readings of width which are Δ_{l} = Δ_{w}= 0.
The standard uncertainties of the length u_{L} and width u_{W} are evaluated as follows: (15) (16) where u_{l} and u_{w} are the standard deviations of the readings of length and width, respectively, and u_{Δl} and u_{Δw} are the relative uncertainties of length and width, respectively, which are u_{Δl} = u_{Δw} = 0.05 mm.
The final evaluation of equation (12) can be expressed by equation (17): (17)
The sensitivity coefficient of the applied force and crosssectional area were calculated from equation (1).
In the end, we can evaluate combined standard uncertainties according to equation (6), and expanded uncertainty U is obtained through multiplication by coverage factor k = 2, as expressed in the following equation: (18)
Furthermore, using the above equations, we can summarize in Table 2 all the sources of uncertainties associated with their values.
According to equations (1) and (18), the estimated results of the sample stress of uniaxial compressive strength have shown that expanded uncertainty of 2.23 N/mm^{2} (estimated value) was 0.28 N/mm^{2}.
As can be seen, the larger uncertainty components arise from load application rate to sample test, preservation condition effect on compression resistance of a sample, effect of planarity on compression resistance of a sample, and sample centering.
The difficulties in quantifying more accurately the effect of a load application rate to sample test, preservation condition effect on compression resistance of a sample, and effect of planarity on compression resistance of a sample have influenced the uncertainty measurement results. The alignment in manual way of the sample centering has also influenced the uncertainty measurement results. Therefore, further research work will be focused on the improvement of the largest uncertainty components.
Sources of uncertainties associated with their values.
4 Conclusions
This article describes measurement method and contributions to uncertainty of results obtained by testing of clay brick's production. The accuracy of clay brick's production results can be extracted through evaluation of measurement uncertainty. Our measurement model can be used for most of the rectangular types of clay brick's production measured by uniaxial compressive testing machine at good accuracy. The method was tested for a rectangular clay brick manufactured with size approximately 250 mm × 250 mm × 200 mm. The expanded uncertainty of 2.23 N/mm^{2} was 0.28 N/mm^{2}. The thorough uncertainty analysis has shown that larger uncertainty arises from load application rate to sample test, preservation condition effect on compression resistance of a sample, effect of planarity on compression resistance of a sample, and sample centering.
Acknowledgments
The authors gratefully acknowledge the Edilcentro Ltd. for providing the clay bricks used in this work and Polytechnic University of Tirana for partial funding of this work.
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Cite this article as: Klodian Dhoska, Saimir Tola, Agus Pramono, Indrit Vozga, Evaluation of measurement uncertainty for the determination of the mechanical resistance of the brick samples by using uniaxial compressive strength test, Int. J. Metrol. Qual. Eng. 9, 12 (2018)
All Tables
All Figures
Fig. 1 Technological scheme of clay brick's production. 

In the text 
Fig. 2 Test setup for compression testing of clay brick. 

In the text 