Widok Tom 67 Nr 3 (2015)

www.ptcer.pl/mccm

1. Introduction

Cold crushing strength is a commonly used parameter for characterizing refractory materials. It is specifi ed in the prod-uct card and is frequently a condition to be fulfi lled by the product to be technically accepted. Compressive strength is determined as the quotient of the maximum force, a

sam-ple is able to withstand without being destroyed, divided by the compressed surface. During a test the sample is sub-jected to the uniformly increasing load until it is destroyed. Due to common application of CCS for refractory materials characterization, it is extremely important that the results obtained in diff erent laboratories are repeatable and repro-ducible. This condition is diffi cult to fulfi l as refractories are

A

G

_{*, J}

_C

_{, J}

_P

_{, E}

_D

1_{Institute of Ceramics and Building Materials, Refractory Materials Division in Gliwice, Poland} 2_{Forschungsgemeinschoft Feuerfest e.V., Höhr-Grenzhausen, Germany}

*e-mail: a.gerle@icimb.pl

An analysis of factors infl uencing the determination

of refractory materials’ compression strength

Abstract

Cold crushing strength (CCS) testing is one of the basic tests conducted in order to characterize a refractory product. Compression strength is frequently a condition that must be fulfi lled by a product to be technically accepted, and the result obtained in the test strongly depends on measuring conditions. For this reason, the applied measuring procedures must guarantee the repeatability and reproducibility of the obtained results. The standards in force specify CCS testing methods depending on the density of a product (dense and insulating products) and its form (shaped and unshaped products). These standards specify the requirements to be fulfi lled by the dimensions of CCS test samples, their perpendicularity and parallelism, the requirements to be met by the machine used for CCS testing, the number of test samples as well as the sample loading rate during the test. However, not all standards suffi ciently defi ne all the requirements. Imprecise testing procedures and diffi cult to test inhomogeneous materials, such as refractories are, make it diffi cult to obtain repeatable and reproducible results and to compare them by diff erent laboratories.

The presented investigations are an initial part of international project ReStaR, aimed at specifying the factors which signifi cantly infl u-ence among others the result of CCS testing. The investigations were based on experimental design methods and variance analysis, which allowed minimizing the number of experiments necessary to identify factors that have a considerable infl uence on CCS testing of refractory materials: dense shaped, insulating and unshaped products. It has been proved that the obtained result of CCS testing is signifi cantly infl uenced by the quality of sample preparation and sample dimensions. In the case of insulating and unshaped products, an important factor turned out to be the rate of loading the sample during the test. In the case of all the types of materials subjected to testing, the use of a cardboard washer had a negative impact on the test result, reducing the value of strength and the repeatability of the obtained results.

Keywords: Cold crushing strength, Sample preparation, Experimental design, Testing standard

ANALIZA CZYNNIKÓW WPŁYWAJĄCYCH NA OZNACZENIE WYTRZYMAŁOŚCI NA ŚCISKANIE MATERIAŁÓW OGNIOTRWAŁYCH

Badanie wytrzymałości na ściskanie w temperaturze otoczenia (CCS) jest jednym z podstawowych badań wykonywanych w celu scharakteryzowania wyrobu ogniotrwałego. Wytrzymałość na ściskanie jest często warunkiem odbioru technicznego wyrobu, a wynik uzyskany w trakcie badania silnie zależy od warunków pomiarowych. Sprawia to, że stosowane procedury pomiarowe muszą gwarantować powtarzalność i odtwarzalność uzyskiwanych wyników. Obowiązujące normy precyzują metody badania CCS w zależności od gęstości wyrobu (wyroby zwarte i izolacyjne) oraz od postaci wyrobu (wyroby formowane i nieformowane). Normy te określają wymagania dotyczące wymiarów próbek do badania CCS, ich prostopadłości i równoległości, wymagań, które musi spełniać maszyna stosowana do oznaczania CCS, ilości badanych prób oraz szybkości obciążania próbki w trakcie badania. Jednak nie wszystkie normy defi niują wszystkie wymaga-nia w wystarczającym stopniu. Nieprecyzyjne procedury badawcze w połączeniu z trudnym do badań, niejednorodnym materiałem, jakim jest materiał ogniotrwały, utrudnia otrzymywanie powtarzalnych i odtwarzalnych wyników oraz ich porównywanie między laboratoriami.

Prezentowane badania stanowią początkową część międzynarodowego projektu ReStaR, mającą na celu wskazanie czynników is-totnie wpływających między innymi na wynik badania CCS. W badaniach wykorzystano metody planowania eksperymentów i analizy wariancji, co pozwoliło na zminimalizowanie ilości eksperymentów niezbędnych do zidentyfi kowania czynników istotnie wpływających na wynik badania CCS ogniotrwałych wyrobów zwartych formowanych, izolacyjnych i nieformowanych. Wykazano, że istotny wpływ na uzyskiwany wynik badania CCS ma jakość przygotowania próbki oraz jej wymiary. W przypadku wyrobów izolacyjnych i nieformowanych istotna okazała się szybkość obciążania próbki w trakcie badania. W przypadku wszystkich trzech badanych rodzajów materiałów sto-sowanie tekturowej podkładki negatywnie wpływało na wynik badania, obniżając wartość wytrzymałości i zmniejszając powtarzalność uzyskanych rezultatów.

non-homogenous materials and their properties can vary depending on a sample. It is therefore essential that the procedures regarding sample preparation and compressive strength testing are detailed and unambiguous. This allows eliminating discrepancies in the results caused by diff er-ences in testing conditions. Current standards related to the CCS testing methodology concern dense shaped refractory materials (EN 993-5, ISO 10059-1, ISO 10059-2, ASTM C133), insulating shaped refractory materials (EN ISO 8895, ASTM C133), and unshaped refractory materials (EN ISO 1927-6, ASTM C133). These standards specify the shape and dimensions of samples, their dimensional tolerances, sample loading rate during a test as well as requirements to be fulfi lled by a machine for CCS testing. However, not all of the above mentioned standards specify all the param-eters, or parameters are not suffi ciently described. In the presented investigations, experimental design methods [1] and variance analysis have been applied. This allowed minimizing the number of experiments necessary to identify factors which have a signifi cant infl uence on the result of refractories’ CCS testing.

The presented investigations are an initial part of interna-tional project ReStaR, aimed at identifying factors which con-siderably infl uence among others the result of CCS testing.

2. Experimental Part

2.1. Material and test samples

Tests were performed for three types of material: dense shaped refractories, shaped insulating refractory materials and unshaped refractory materials.

2.1.1. Dense shaped refractory materials

Dense shaped refractory materials used for tests includ-ed: high-alumina (> 75% Al2O3) shape bricks in HA75 grade,

characterized by high compressive strength, and carburized magnesium (> 95% MgO, ~10% C) bricks in MC95/10 grade, characterized by low compressive strength. Both grades of material are produced by RHI. To evaluate homogeneity and reject bricks with internal defects, all the shape bricks were tested using a CT1 castable testing machine produced by UNIPAN-ULTRASONIC. Homogeneity was assessed on the basis of velocity of ultrasounds passing through the bricks which was (4,6 ± 0,2)·103 _{m/s and (4,0 ± 0,2)·10}3 _{m/s for}

HA75 and MC95/10 grade products, respectively. Small diff erences in the values of velocity of ultrasounds passed through the bricks prove their homogeneity. Test samples were drilled and cut out of the bricks according to the experi-mental design presented in paragraph 2.2.1. of this article.

2.1.2. Shaped insulating refractory materials

Shaped insulating refractory products used for tests were shapes containing 30% of Al2O3 and 60% of SiO2, in two

heights – 64 mm and 76 mm. Homogeneity of CCS test sam-ples was checked by determining the geometric apparent density of each sample which was 0,536 ± 0,060 g/cm3_{. Small}

diff erences in the samples’ density prove their homogeneity.

Test samples were cut out according to the experimental de-sign presented in paragraph 2.2.2. of this article.

2.1.3. Unshaped refractory materials

Investigations into unshaped refractory products were conducted on a medium-cement bauxite castable (> 75% Al2O3) MCC75 produced by Calderys. According to the

man-ufacturer’s data, it contained 78.0 wt.% of Al2O3, 15.0 wt.%

of SiO2, 3.1 wt.% of CaO and 1.0 wt.% of Fe2O3.

Test samples made of refractory castable were prepared in the following way: 8.8% of tap water having a temperature of 20 ºC was added to castable placed in a Hobart mixer and, next, cast into moulds with the following dimensions: 230 mm × 114 mm × 64 mm (format A), 230 mm × 64 mm × 54 mm (format B), 230 mm × 64 mm × 64 mm (format C) and 160 mm × 40 mm × 40 mm (format D). After the casta-ble was cast into moulds, it was vibrated for 30 s. Next the samples were placed in a climatic chamber (humidity: 90%, temperature: 20 °C) for 48 h, and, after 24 hours, the moulds were dismantled. Immediately after cooling, homogeneity of all the samples was checked with a CT1 castable test-ing machine. Ultrasounds passed through the samples had a velocity of (4,6 ± 0,3)·103 _m/s.

Small diff erences in the values of ultrasounds velocity prove the examined samples’ homogeneity. Immediately af-ter the ultrasound test, the samples were prepared for testing and compressive strength was determined according to the experimental design presented in paragraph 2.2.3. of this article.

2.2. Experimental design and compressive

strength determination

To minimize the number of experiments necessary to determine factors considerably infl uencing the result of compressive strength determination, experimental design according to Placket-Burmann was applied. Each of the in-vestigated factors was tested on two levels: high (+) and low (-).

2.2.1. Dense shaped refractory materials

In the case of dense shaped refractory materials, 14 fac-tors were tested which together with their corresponding val-ues for the high (+) and low (-) level have been presented in Table 1.

To check the infl uence of compression plates’ smooth-ness (x11) and hardness (x12), four sets of plates were pre-pared: with 3 μm smoothness and 60 HRC hardness, 3 μm smoothness and 52 HRC hardness, 0.5 μm smoothness and 60 HRC hardness as well as 0.5 μm smoothness and 52 HRC hardness that were next used according to the ex-perimental design (Table 2).

As a washer (x13), a 7-mm thick cardboard was used. In-vestigating 14 factors on two levels required 16 experiments to be conducted. During each experiment, 3 samples were tested, i.e. 48 samples of dense shaped refractory materials were examined. Table 2 presents the experimental design matrix and the results of compressive strength testing

ob-The experimental design matrix and the results of com-pressive strength determination in particular experiments have been presented in Table 4.

2.2.3. Unshaped refractory materials

Unshaped refractory materials were investigated in two stages. In the fi rst stage, the infl uence of the size and sample preparation on the obtained results was examined. For this purpose, castable beams in the A, B and D format were made from which samples for CCS determination were prepared by three-point bending and cutting into two even pieces with tained in particular experiments for dense shaped refractory

products.

2.2.2. Shaped insulating refractory materials

In the case of shaped insulating refractory materials, 6 factors were examined on two levels which gives a total of 8 experiments. The investigated factors and their cor-responding levels have been presented in Table 3. In each experiment, 3 samples were measured, i.e. the total number of the tested samples was 24.

Table 1. Investigated factors and their corresponding levels for dense shaped refractory products.

Tabela 1. Badane czynniki i odpowiadające im poziomy dla zwartych, formowanych wyrobów ogniotrwałych.

Symbol Factor Level

High (+) Low (-)

x1 Shape cube cylinder

x2 Sampling site corner middle

x3 Direction of forming // ┴

x4 Loading rate 1.0 MPa 0.2 MPa

x5 Initial load 2000 Pa No x6 Grinding Yes No x7 Height 36 mm 50 mm x8 Dimensions 36 mm 50 mm x9 Parallelism // Δh = 0,5 mm x10 Perpendicularity ┴ ΔI = 2 mm

x11 Smoothness of plates 3.0-3.2 μm polished

x12 Hardness of plates 60 HRC 50 HRC

x13 Washer No 7 mm

x14 Material A C

Table 2. Experimental design matrix and the results of compressive strength determination for dense shaped refractory products. The CCS result is the average of three measurements.

Tabela 2. Macierz planu eksperymentu oraz wyniki oznaczania wytrzymałości na ściskanie zwartych formowanych wyrobów ogniotrwałych. Wynik wytrzymałości na ściskanie (CCS) stanowi średnia z trzech pomiarów.

Experiment x1 x2 x3 x4 x5 x6 x7 x8 x9 x10 x11 x12 x13 x14 CCS ± SD [MPa] 1 + + + - - - + - - - + + + - 39.7 ± 7.1 2 + + + + - - - + - - - + + + 84.1 ± 19.7 3 - + + + + - - - + - - - + + 90.9 ± 15.3 4 + - + + + + - - - + - - - + 53.5 ± 11.3 5 + + - + + + + - - - + - - - 35.5 ± 8.3 6 + + + - + + + + - - - + - - 21.5 ± 4.3 7 - + + + - + + + + - - - + - 40.1 ± 6.1 8 - - + + + - + + + + - - - + 52.1 ± 11.8 9 - - - + + + - + + + + - - - 33.9 ± 2.2 10 + - - - + + + - + + + + - - 41.8 ± 2.8 11 - + - - - + + + - + + + + - 34.2 ± 6.9 12 - - + - - - + + + - + + + + 94.4 ± 21.3 13 - - - + - - - + + + - + + + 102.9 ± 25.6 14 + - - - + - - - + + + - + + 85.0 ± 17.6 15 + + - - - + - - - + + + - + 66.0 ± 17.6 16 - - - 30.9 ± 4.6

a saw (Table 5). Compressive strength was tested using the remaining parameters, fulfi lling the requirements of EN ISO 1927-6 standard, i.e. pressure perpendicular to the direction of casting, a loading rate of 1.0 MPa, samples without grind-ing, without initial loading and without a cardboard washer. Six CCS determinations were carried out for each level, i.e. a total number of 36 test samples was prepared.

In the second stage, one C format was used. Six factors on the levels specified in Table 6 were

se-lected, and Plackett-Burmann experiments were de-signed for tests.

Tests in the second stage were performed in 6 cycles, with 3 repetitions. The total number of experiments reached 48. The results obtained in the fi rst stage have been given in Table 7.

Experimental design matrix and the results of compres-sive strength determination in particular experiments have been presented in Table 8.

Table 3. Investigated factors and their corresponding levels for shaped insulating refractory materials. Tabela 3. Badane czynniki i odpowiadające im poziomy dla izolacyjnych formowanych wyrobów ogniotrwałych.

Symbol Factor Level

High (+) Low (-)

x1 Loading rate 0.20 MPa 0.05 MPa

x2 Initial load 2000 Pa No

x3 Height 76 mm 64 m

x4 Dimensions 114 mm ×114 mm 230 mm × 114 mm

x5 Washer No 7 mm

x6 Operator A B

Table 4. Experimental design matrix and the results of compressive strength determination for shaped insulating refractory products. The CCS result is the average of three measurements.

Tabela 4. Macierz planu eksperymentu oraz wyniki oznaczania wytrzymałości na ściskanie izolacyjnych formowanych wyrobów ogniotrwałych. Wynik wytrzymałości na ściskanie (CCS) stanowi średnia z trzech pomiarów.

Experiment x1 x2 x3 x4 x5 x6 CCS ± SD [MPa] 1 + - - + + + 2.1 ± 0.5 2 - + - - + + 1.8 ± 0.3 3 + - + - - + 2.4 ± 0.3 4 + + - + - - 1.6 ± 0.2 5 + + + - + - 2.2 ± 0.6 6 - + + + - + 0.7 ± 0.1 7 - - + + + - 2.0 ± 0.0 8 - - - 1.3 ± 0.1

Table 5. Format and preparation of samples for the fi rst stage of CCS tests (stage 1). Tabela 5. Format i przygotowanie próbek do pierwszego etapu badań CCS (etap 1).

Format A, B, D

Level High (+) Low (-)

Preparation 3-point bending Cutting into two even pieces

Table 6. Investigated factors and their corresponding levels for unshaped refractory products (stage 2). Tabela 6. Badane czynniki i odpowiadające im poziomy dla nieformowanych wyrobów ogniotrwałych (etap 2).

Symbol Factor Level

High (+) Low (-)

x1 Preparation 3-point bending cutting

x2 Casting direction ^ ¤ ¤

x3 Loading rate 1.0 MP/s 0.2 MPa/s

x4 Initial loading No 2000 Pa

x5 Grinding No Yes

3. Discussion of Results

Based on the obtained results of compressive strength determination, the following regression equation has been determined:

y = b0x0 + b1x1 + b2x2 + …. + bkxk (1) by calculating the coeffi cient of regression bj:

N y x b N i i ji j

∑

where: i – number of experiment, j – number of factor, xji – value of jth _{factor in i}th _{experiment, y}

i – result of ith experi-ment.

The signifi cance of regression coeffi cients was evaluated using Student’s t-test. tobl was calculated from the following equations: j b j obl

_S

b

t

N

S

S

where Srep is the average standard deviation from all the determinations.

Next the value of calculated tobl was compared to the critical value of tkrytin the tables of Student’s t-tests; if

tobl tkryt for coeffi cient bj , this means that factor xj con-siderably infl uences the result of compressive strength determination.

Calculations of the signifi cance of infl uence exerted by particular factors on the value of CCS were extended with ANNOVA variance analysis using OptiVAL statistical soft-ware, version V.3.3.2.4, produced by Quodata.

3.1. Dense shaped refractory materials

Fourteen factors contained in Table 1 were examined; on the basis of the obtained results the following linear equations of regression were calculated:

y = 56.9 · x0 – 3.1 · x1 – 4.9 · x2 + 3.3 · x3 + + 4.5 · x4 – 4.8 · x5 – 15.6 · x6 – 11.6 · x7 + + 1.6 · x8 + 10.8 · x9 + 1.9 · x10 – 3.2 · x11 +

+ 4.4 · x12 + 14.6 · x13 + 21.8 · x14 (5) The value of tkryt read out from the tables is 2.03. The calculated values of tobl for particular coeffi cients bj at a sig-nifi cance level α = 0.05 and with the number of degrees of freedom f = 33 have been presented in Fig. 1.

The obtained results presented in Fig. 1 indicate that: 1. A strong infl uence on the results of compressive strength testing is exerted by the kind of material (x14), grind-ing of the compressed samples’ surfaces (x6), cardboard washer (x13), sample height (x7) and parallelism of the loaded surfaces (x9).

Table 7. Results of compressive strength determination depending on the format of samples and the manner of their preparation. The CCS result is the average of six measurements (stage 1).

Tabela 7. Wyniki oznaczania wytrzymałości na ściskanie w zależności od formatu i sposobu przygotowania próbek. Wynik wytrzymałości na ściskanie (CCS) stanowi średnia z sześciu pomiarów (etap 1).

Experiment Format/level (preparation) CCS ± SD_[MPa]

1 A/+ 93.7 ± 2.4 2 A/- 75.7 ± 2.6 3 B/+ 92.1 ± 2.0 4 B/- 86.9 ± 7.9 5 D/+ 90.6 ± 7.4 6 D/- 92.0 ± 4.9

Table 8. Experimental design matrix and the results of compressive strength determination for unshaped refractory products. The CCS result is the average of three measurements (stage 2).

Tabela 8. Macierz planu eksperymentu oraz wyniki oznaczania wytrzymałości na ściskanie nieformowanych wyrobów ogniotrwałych. Wynik wytrzymałości na ściskanie (CCS) stanowi średnia z trzech pomiarów (etap 2).

Experiment x1 x2 x3 x4 x5 x6 CCS± SD [MPa] 1 + - - + + + 82.4 ± 5.7 2 - + - - + + 79.1 ± 2.2 3 + - + - - + 73.0 ± 7.9 4 + + - + - - 64.4 ± 2.7 5 + + + - + - 70.0 ± 2.3 6 - + + + - + 73.3 ± 1.9 7 - - + + + - 42.9 ± 4.9 8 - - - 61.0 ± 8.8

A slight influence was observed for sampling site (x2), loading rate (x4), initial loading (x5) and plate hardness (x12).

2. The values of tobl were very similar to tkryt for the fol-lowing factors: shape (x1), forming direction (x3) and plate smoothness (x11); these factors have a slight infl uence on the result of compressive strength determination.

3. Dimensions (x8) and perpendicularity (x10) do not have a considerable infl uence on the result of compressive strength determination.

At the same time a statistical analysis of experimental data was conducted using OptiVAL programme. Table 9 shows infl uence exerted by the investigated factors on the result of CCS testing.

According to the obtained mathematical model, CCS will have a maximum value when the tested material is of type A, no cardboard washer is applied, the sample has a height of 50 mm and the compressed surfaces are parallel to each other. Among the examined factors no mutual infl uences have been observed.

Fig. 1. Graphic illustration of the signifi cance of infl uence exerted by the examined factors on the compressive strength of dense shaped refractory materials.

Rys. 1. Grafi czne zilustrowanie istotności wpływu badanych czynników na wytrzymałość na ściskanie zwartych formowanych materiałów ogniotrwałych.

Table 9. Infl uence of particular factors on the value of CCS result for dense shaped refractory materials (the level for which higher CCS values were obtained has been marked with a square).

Tabela 9. Wpływ poszczególnych czynników na wartość wyniku CCS dla zwartych formowanych materiałów ogniotrwałych (kwadratem oznaczono poziom, dla którego otrzymano wyższe wartości CCS).

Factor Symbol Level _{CCS result}■ Higher ■ Reduced scatters of CCS results

Shape x1 Cube ■ cylinder ■ Sampling site x2 corner ■ middle ■ Forming direction x3 // ■ ■ ┴ Loading rate x4 1.0 MPa 0.2 MPa ■ ■ Initial loading x5 200 Pa No ■ ■ Grinding x6 Yes No ■ ■ Height x7 36 mm ■ 50 mm ■ Dimensions x8 36 mm 50 mm ■ ■ Parallelism x9 // ■ Δh = 0.5 mm ■ Perpendicularity x10 ┴ ■ ■ Δl = 2 mm Plate smoothness x11 3.0-3.2 μm ■ polished ■ Plate hardness x12 60 HRC ■ 50 HRC ■ Washer x13 No ■ 7 mm ■ Material x14 A ■ C ■

3.2. Shaped insulating refractory materials

Based on the obtained results of tests conducted accord-ing to the factorial design presented in Table 3, which takes into account six independent variables, a regression equa-tion has been determined as follows:

y = 1.8 · x0 + 0.3 · x1 – 0.2 · x2 +

+ 0.06 · x3 – 0.2 · x4 + 0.3 · x5 – 0.01 · x6 (6) The signifi cance of coeffi cients of equation tobl has been calculated.The table value of tkryt is 2.11. The assumed level of signifi cance α = 0.05, the number of degrees of freedom

f = 17.

Fig. 2 presents the calculated values of tobl compared to the value of critical coeffi cient tkryt which allow evaluating the infl uence of a particular factor on the result of compressive strength determination.

The results presented in Fig. 2 indicate that:

1. There is evidently no infl uence of operator (x6) and sample height (x3) on the results of compressive strength determination.

2. Considerable infl uence is exerted by initial loading (x2) and dimensions (x4).

3. The strongest infl uence observed is that exerted by loading rate (x1) and washer (x5).

At the same time an analysis of experimental data was conducted using OptiVAL programme. Table 10 presents the investigated factors’ infl uence on the result of CCS.

According to the obtained mathematical model, CCS will have a maximum value, when the sample is loaded at a rate of 0.20 MPa, a cardboard washer is not applied, the sam-ple is not initially loaded and its dimensions are 230 mm × 114 mm. Among the examined factors, no mutual infl uences have been identifi ed.

3.3. Unshaped refractory materials

The results obtained in the fi rst stage of investigations (Table 7) reveal that the lowest results of compressive strength were noted for the biggest samples in format A, when these samples were cut into two even halves. In the case of samples which were halves that remained after bending strength testing, the obtained results were similar to each other irrespective of the format. Based on the investiga-tions conducted in the second stage (Table 8), a regression equation has been determined:

y = 68.5 · x0 + 3.8 · x1 + 3.8 · x2 +

+ 3.1 · x3 – 2.1 · x4 – 0.04 · x5 – 9.1 · x6 (7) The values of tobl for regression equation coeffi cients were calculated and compared to tkrytread out from the tables, which reached 2.11 at the level of signifi cance α = 0.05 and for the number of degrees of freedom f = 17, which has been presented in Fig. 3.

The results presented in Fig. 3 show the following: 1. A very strong infl uence on the result of compressive strength determination is exerted by cardboard washer (x6).

Table 10. Infl uence of particular factors on the value and scatter of CCS result for shaped insulating refractory materials (the level for which higher CCS values were obtained has been marked with a square).

Tabela 10. Wpływ poszczególnych czynników na wartość i rozrzut wyniku CCS dla formowanych izolacyjnych materiałów ogniotrwałych (kwadratem oznaczono poziom, dla którego otrzymano wyższe wartości CCS).

Factor Symbol Level

■ Higher CCS result

■ Reduced scatters of CCS results

Loading rate x1 0.20 MPa ■ ■ 0.05 MPa Initial loading x2 2000 Pa No ■ ■ Height x3 76 mm ■ 64 mm ■ Dimensions x4 114 mm × 114 mm ■ 230 mm × 114 mm ■ Washer x5 No ■ ■ 7 mm Operator x6 A ■ B ■

Fig. 2. Graphic illustration of the signifi cance of infl uence exerted by the examined factors on the compressive strength of shaped insulating refractory materials.

Rys. 2. Grafi czne zilustrowanie istotności wpływu badanych czyn-ników na wytrzymałość na ściskanie izolacyjnych formowanych materiałów ogniotrwałych.

2. A considerable infl uence is exerted by sample prepa-ration (x1), loading direction in relation to the direction of casting (x2) and loading rate (x3).

3. The value of tobl for initial loading (x4) is close to the value of tkryt, therefore this factor has a minor infl uence.

4. Grinding (x5) does not have a signifi cant eff ect. OptiVAL programme used for analysis of experimentally obtained data showed a mutual infl uence of such factors as cardboard washer and initial loading. Mutual infl uence of these two factors can be caused by cellulose fi bre crush-ing durcrush-ing the process of initial loadcrush-ing. Table 11 presents infl uence exerted by the investigated factors on CCS deter-mination result.

According to this model, maximum values of CCS de-termination can be expected when the test sample is cut, loading is perpendicular to the direction of casting, loading rate is 0.2 MPa/s and no cardboard washer or initial loading is applied.

4. Summary

Application of experimental design methods and AN-NOVA variance analysis allowed identifying factors which

considerably infl uence the result of CCS determination for three kinds of refractory materials.

In the case of dense shaped refractory materials a con-siderable infl uence on CCS determination results is ex-erted by the use of cardboard washer, sample height, grinding of compressed surfaces and their parallelism in relation to each other. Factors, which have the most sig-nifi cant infl uence on the result of CCS of shaped insu-lating refractory materials, include sample loading rate, cardboard washer, sample dimensions, and initial loading. In the case of unshaped refractory materials the result of CCS determination is considerably dependent on such fac-tors as cardboard washer, manner of sample preparation (cutting, bending), sample loading direction in relation to the direction of its forming as well as loading rate. It has been confi rmed that in the case of dense shaped refrac-tory materials a signifi cant factor infl uencing the result is the quality of sample preparation. Non-parallelism of com-pressed surfaces lowers the obtained result. In the case of all three types of refractory materials the use of cardboard washer considerably reduces the obtained result which is confi rmed by literature data [2]. Application of cardboard washers during CCS determination should be avoided. Similarly to the cardboard washer, initial sample loading before tests also causes CCS results’ lowering and their considerable scatter.

Sample dimensions also considerably influence the obtained result of CCS irrespective of the kind of refrac-tory material. Loading rate has a strong impact on the measured value of CCS. In the case of dense shaped and unshaped products, higher values of CCS were obtained at a lower loading rate, whereas in the case of insulating products a higher loading rate led to higher CCS results.

Information obtained in this stage of ReStaR project al-lowed preparing guidelines for further investigations which will be conducted by diff erent laboratories and will enable evaluating the repeatability and reproducibility of the ob-tained results.

Table 11. Infl uence of particular factors on the value of CCS results for unshaped refractory materials (the level for which higher CCS values were obtained has been marked with a square).

Tabela 11. Wpływ poszczególnych czynników na wartość wyników CCS dla nieformowanych materiałów ogniotrwałych (kwadratem za-znaczono poziom, dla którego otrzymano wyższe wartości CCS).

Factor Symbol Level ■ Higher CCS _result ■ Reduced scatters of CCS results

Preparation x1 bending ■ cutting ■ Casting direction x2 ┴ ■ ■ // Loading rate x3 1.0 MPa/s 0.2 MPa/s ■ ■ Initial loading x4 No ■ ■ 2000 Pa Grinding x5 No ■ Yes ■ Washer x6 No ■ ■ 7 mm

Fig. 3. Graphic illustration of the signifi cance of infl uence exerted by the investigated factors on compressive strength determination of unshaped refractory materials.

Rys. 3. Grafi czne zilustrowanie istotności wpływu badanych czyn-ników na wytrzymałość na ściskanie nie formowanych materiałów ogniotrwałych.

Acknowledgements

The work was completed within the framework of ReStaR project funded from the resources of the Seventh Frame-work Programme of the European Union, in accordance with agreement number 314884.

References

[1] Plackett, P., Burman, J. P.: The design of optimum multifacto-rial experiments, Biometrika, 37, (1946), 305-325.

[2] Majdic, A., Hagemann, L., Lichomski, H.: Einfl uss der gute der probekörperdruckfl ächen und der Druckplattenrauheit auf Mittlwer und Streubreite der Klatdruckfestigkeit Steine, Tonind. Zeit., 97, 9, (1973), 237-243

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