Procedia Engineering 114 ( 2015 ) 493 – 499

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1877-7058 © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of INEGI - Institute of Science and Innovation in Mechanical and Industrial Engineeringdoi: 10.1016/j.proeng.2015.08.097

ScienceDirect

1st International Conference on Structural Integrity

Core sampling for fiber concrete constructions – context between quantity of core samples and evaluation of fiber concrete

characteristics Petr Mynarcika*

a VSB – Technical university of Ostrava, Faculty of Civil Engineering, Department of Building Structures, Ludvika Podeste 1875/17, Ostrava, 708 33, Czech Republic, Europe

Abstract

This article is focused on diagnostic methodology for steel fibre concrete slab-on-ground constructions. The number of excepted core samples from construction affected economical, quality and labour intensity aspects of implemented construction diagnostic. Essentially for the quality of diagnostic results is the sample quantity. The described research investigated the influence of core sample quantity to the accuracy and precision of diagnostic results. For this research, a large collection of core samples were taken from real steel fibre concrete construction - parking pavement. The studied material characteristic was the compressive strength. Core samples were sorted to sets with different quantity. Compressive strength of each sample was recalculated to cube´s compressive strength and was statistically evaluated. © 2015 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of INEGI - Institute of Science and Innovation in Mechanical and Industrial Engineering.

Keywords: Fiberconcrete; Core sampling; Diagnostic methodology of concrete; Semi-destructive testing of oncrete.

1. Introduction to the problematic of diagnostic of fiber-concrete constructions

Diagnostic methods of existing concrete structures (destructive, non-destructive or semi-destructive) are described in number of European and national standards. But normative situation for existing fiber-concrete structures is different. In most European countries the specifications of fiber is simply done by fiber type and content. The fiber content is often determined by the supplier fibers in discussion with designer. Agreed fiber type and content are specified for a particular vendor. It is common practice in, for example: Czech Republic, Denmark, Finland, France,

* Corresponding author. Tel.: +420 597 321; fax: +420 59 732 1377.

E-mail address: petr.mynarcik@vsb.cz

© 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of INEGI - Institute of Science and Innovation in Mechanical and Industrial Engineering

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the Netherlands, Norway, Poland, Portugal, Slovakia and United Kingdom [1]. This article is focused on assessment of core sampling from existing fiber-concrete, because special standard for this practice is not formed.

2. Description of experimental measurement

The purpose of the experimental measurements was determination the influence of the number of core samples on the quality of the obtained data . Core samples were drilled from existing parking area with fiber-concrete top layer. Tested parking area had a total surface of 10,000 square meters. Three reference areas were selected from the base parking area. All of these areas had surface of 1,200 square meters. From each reference area were drilled 10 core samples - totally were collected 30 core samples. Core samples were drilled by drilling machine with core drill bit diameter 100mm. Each sample was signed by identification mark.

2.1. Determination of compressive strength

Core samples were removed from parking area to the laboratory for determination of compressive strength. Determination of compressive strength was performed according to European standard EN 12390-3[2]. Measurements were realized on test press machine Beton System with a maximum force of 300kN. Individual samples were inserted between the platens of the press machine, where they were continuously loaded. Recording equipment of test press machine recorded the maximum force for each sample. Slenderness λ of sample (ratio between sample height and sample diameter) was in the range 1<λ<2. Therefore cylinder compressive strength fc,cyl of sample was calculated from the formula:

ccylccylc A

Ff ,, (1)

Where: F - maximal force of test press machine for partial sample [N] Ac - pressed area of test sample [mm2] Χc,cyl - correction factor Correction factor Χc,cyl is necessary for conversion of strength characteristics between cylinder and cube test samples. The correction factor Χc,cyl was calculated from the formula [2]:

(2)

Where: λ - slenderness λ [-]

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The calculated corrected cylinder compressive strength was converted by a factor Χcyl,cu to a cube compressive

strength fc,cube - value for basic size test specimens. The resulting values of cube compressive strength fc,cube for partial samples are shown on Table 2.

2.2. Statistical evaluation of core sets samples

Statistical evaluation of core sets samples was performed according to international standard ISO 13822:2010 [3]. The first set of samples consisted of three samples (samples V1 , V2 , V3) . the number of samples in the following sets has been increased by one sample per set (V1, V2, V3,...V30). 28 sample sets were created for statistical evaluations. The last set contained all 30 core samples. Results of this set were used for affiliation to strength class of concrete. Also, this sample set was used as a reference set for comparison deviations between the resulting characteristic compressive strength of partial core sample sets. Determination of the minimum number of test core samples in each set was the target of statistical evaluation. Acceptable limits in comparison with the reference sample set were the criterion of assessment [4,5]. Characteristic compressive strength was calculated for each partial sample set X3-X29. Percentage differences of characteristic compressive strength of partial sample sets X3-X29 were calculated from comparison with a characteristic compressive strength of reference set X30. Influence of core samples quantity was found in this statistical analysis. Core sample sets with the number of samples 3 - 9 piece had a difference in characteristic compressive strength greater than 10% in comparison with reference set X30. Differences of compressive strength of core sample sets with the number of samples greater than 10 pieces were no longer so pronounced. Graphical representations of statistical analysis are visualized in Fig. 2 and Fig. 3. Differences of characteristic compressive strength are presented on Fig. 2. Fig. 3 presented the significant difference in strength mainly in the group with a lower number of core samples [6, 7, 8, 9].

Table 1. Values of correction factor Χc,cyl [2]

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Table 2. Dimensions and strength characteristics for partial core samples

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Fig. 1. Differences of characteristics compressive strength.

Fig. 2. Characteristic compressive strength of concrete – partial sets of samples.

3. Magnetic separation of steel fibers

Magnetic separation of steel fibre was realized after destructive compressive strength testing [10, 11, 12,]. Core samples were crushed in a mill on concrete. Steel fibres from each core sample were separated with magnetic equipment and considered. Quantity and quality of steel fibres distribution were compared from weight of separated steel fibres. Graphical representation of magnetic separation is visualized in Fig. 3.

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Fig. 3. Closer relations between partial characteristics of tested fiber-concrete core samples

4. Conclusion of statistical analysis

Characteristic compressive strength of fiber concrete X30=33.44 MPa was calculated from a set of 30 core samples, calculated according to the international standard ISO 13822:2010. Large number of core samples allowed statistical evaluation of partial core sample sets. Core samples were assorted to 28 partial sets. Core samples V1, V2 and V3 were in first sample set X3. This sample set has a minimum quantity of samples for statistical evaluation. Maximum quantity of available core samples - 30 samples (V1, V2, V3,..., V30) were in last sample set X30. Sample set X30 was elected as a reference sample set. A maximum deviation of 68,5 % in the observed values of compressive strength between partial sets and reference set was detected during this statistical analysis. A minimal number of core samples - 10 pieces is recommended for an objective evaluation of the existing fiber concrete structures. Fiber concrete constructions are becoming a very important part of the modern construction industry. Today there are many types and technical solutions for use of this material. These assertions are supported by the existence of specialized companies with their own sophisticated technology [13, 14]. The paper presents the process of a statistical analysis of large volume of core samples from real construction. The described analysis was part of a series of experiments focusing on the problematic of fiber concrete constructions and was realized at the Faculty of Civil Engineering, VŠB –Technical university of Ostrava [14, 15, 16, 17, 18, 19,]. The described analysis is only an initial study of the problems and it is necessary to make a number of similar statistical evaluations for the creation of a comprehensive normative standards.

Acknowledgements

This paper has been achieved with the financial support of the Ministry of Education, specifically by the Student Research Grant Competition of the Technical University of Ostrava under identification number SP2015/52.

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