CC BY-NC-ND
0Available online at www.sciencedirect.com
ScienceDirect Procedia
Engineering
ELSEVIER Procedia Engineering 108 (2015) 349-354 ;
www.elsevier.com/locate/proeedia
7th Scientific-Technical Conference Material Problems in Civil Engineering (MATBUD'2015)
Fracture energy of foamed concrete based on three-point bending
test on notched beams
• a b b
Marcin Kozlowski Marta Kadela , Alfred Kukielka
aDepartment of Structural Engineering, Silesian University of Technology, Akademicka 5, 44-100 Gliwice, Poland bBuilding Research Institute - Silesian Branch, al. Korfantego 191, 40-153 Katowice, Poland
CrossMar]
Abstract
A series of static loading tests was performed to determine the fracture properties of foamed concrete of varying density. Beams with dimensions of 100x100x840 mm with a central notch were tested in three-point bending. Then, remaining halves of the specimens were tested again as un-notched beams in the same set-up with reduced distance between supports. The tests were performed in a hydraulic displacement controlled testing machine with a load capacity of 5 kN. Apart from measuring the loading and mid-span displacement, a crack mouth opening displacement (CMOD) was monitored. Based on the load -displacement curves of notched beams the values of fracture energy and tensile stress at failure were calculated. Subsequently, the flexural tensile strength was obtained on un-notched beams with dimensions of 100x100x420 mm. Moreover, cube specimens 150x150x150 mm were tested in compression to determine the compressive strength.
© 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-reviewunderresponsibilityof organizing committee of the 7th Scientific-Technical Conference Material Problems in Civil Engineering Keywords: fracture energy; foamed concrete; three-point bending; notched beams
1. Introduction
Foamed concrete can be defined as a cementitious material with minimum of 20% (by volume) [1] of mechanically entrained foam in the mortar suspension in which air-pores are entrapped in the matrix by means
* Corresponding author. Tel.: +48 32 237-11-26; fax: +48 32 237-22-88. E-mail address:marcin.kozlowski@polsl.pl
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 organizing committee of the 7th Scientific-Technical Conference Material Problems in Civil Engineering doi:10.1016/j.proeng.2015.06.157
of a suitable foaming agent. Foamed concrete is widely known for its low self-weight and excellent acoustic and thermal properties [2]. For many years, it has been used worldwide for backfill to retaining walls, insulation to foundations and roof tiles, sound insulation, etc. However, in the last years foamed concrete has become a promising material [3] also for structural purposes e.g. for stabilization of weak soils [4] and as a base layer in a sandwich solution for foundation slabs [5].
Due to favourable properties of foamed concrete, many interests and studies were conducted to investigate its strength, mechanical, acoustic and thermal properties. However, these studies do not cover the investigation of fracture energy which is the main parameter governing the damage and fracture mechanisms. Only limited number of studies can be found in literature. Rahman and Jaini [6] performed studies of fracture energy of foamed concrete on notched beams with the density of 1400-1600 kg/m3. The fracture energy of foamed concrete with compressive strength of 6.4-14 MPa was relatively high around 18-25 N/m. Furthermore, Hengst and Tressler [7] stated that for lightweight concrete, the fracture energy is only a fraction of the normal weight concrete.
The main aim of the research was to determine the fracture energy and mechanical properties of foamed concrete of varying density. The knowledge is required for further numerical simulations of structural behavior and modelling of cracking. This work was supported by the on-going research project "Stabilization of weak soil by application of layer of foamed concrete used in contact with subsoil" (LIDER/022/537/L-4/NCBR/2013) financed by The National Centre for Research and Development within the LIDER Programme.
2. Material and methods
2.1. Materials
An ordinary Portland cement (CEM I 42.5 R) was used in this research. Water used was fresh, clean and drinkable water. Foam was produced by mixing the foaming agent and water in predefined proportions in a foam generator. Subsequently, the foam was mixed with cement in a concrete mixer. After casting in steel molds, the specimens were covered and stored in a curing room at 20±1°C and 95% humidity for the first 24 hours. Subsequently, the beams were removed from the molds and stored in ambient conditions for 28 days before testing. In this study, four different concrete mixes resulting in varying densities were used, see Table 1. All concrete mixes produced had we^c of 0.44.
Table 1. Mix proportions, density and compressive strength of produced specimens.
Specimen type Cement (c) Water (w) Foaming agent we]j/c* Density (p) Compressive
[kg/m3] [dm3/m3] [dm3/m3] [kg/dm3] strength [MPa]
A 750 300 30 1024 ± 1.0 5.9 ± 0.2
B 634 241 38 882 ± 0.5 4.4 ± 0.2
C 514 190 36 662 ± 2.0 2.4 ± 0.3
D 386 143 27 488 ± 0.5 0.6 ± 0.2
* — weff!c includes foaming agent in a liquid state
In total 32 specimens were fabricated, five beam and three cube specimens for the same mix. The beams measured 100x 100x840 mm. A 3 mm wide mechanical notch was fabricated with a depth (a) of 42 mm, resulting with a notch to beam depth ratio (a/d) of 0.42. For standardized fracture energy tests [8], the required a/d ratio ranges from 0.45 to 0.55. However, using deep notched beams specimens with foamed concrete may produce undesirable test results, such as large statistical variation and crack initiation under self-weight. The cubes measured 150x150x150 mm. Before testing all manufactured specimens were measured and weighted. Table 1 presents the densities calculated as well as the values of compressive strength determined based on the cube specimens.
2.2. Methods
In this study two testing procedures were performed. In the test I a three-point bending test was performed on notched beams to determine the fracture energy (GF) and maximal tensile stress (ac), according to [8, 9]. Then, in the test II, two halves of the specimens left after test I (after breakage) were tested in a three-point bending to determine the flexural tensile stress (ft) on un-notched beams with respect to [10].
The test set-up I was a three-point bending test on notched beams with dimensions of 100x100x840 mm, see Figure 1. The nominal distance between the supports (/) was 800 mm, while the loading was introduced at mid-span of the beam. The rollers allowed for free horizontal movement. The specimens were loaded at constant displacement rate of 0.1 mm/min. The tests were performed in a displacement controlled hydraulic testing machine with a capacity of 5 kN. Apart from measuring the loading and mid-span displacement, a crack mouth opening displacement (CMOD) was also monitored. To measure the CMOD an extensometer with a gauge length of 5 mm was mounted across the notch.
Specimen -LVDT Stell frame -
400 - Load Load cell
800 840
Clip gauge
Roller support —
Fig. 1. Test set-up I.
Fig. 2. Test set-up II.
Test set-up II was a three-point bending test on un-notched beams with dimensions of 100x100x420 mm, see Figure 2. Two halves of broken beams after test I were used in this test. The nominal distance between the supports was 300 mm and the loading was introduced at mid-span of the beam. The rollers allowed for free horizontal movement. The specimens were loaded at constant displacement rate of 0.1 mm/min. The tests were performed in a displacement controlled hydraulic testing machine with a capacity of 5 kN. In the test the loading and mid-span displacement was measured.
3. Results and discussion
The main objective of this study was to investigate the influence of density of foamed concrete on the fracture energy and mechanical properties based on three-point bending tests on notched and un-notched beams.
3.1. Notched beams
Figure 3 presents typical load vs. CMOD/deflection plots for selected specimens of group A, B, C and D, see Table 1. Typical load-deflection plot (e.g. Figure 3A) comprises three stages of behavior. In the first phase (1) the deflection increases linearly with the load while the notch is opened but does not extend. A fracture process develops during the second phase (2) where microcracks form and slow crack growth is noticeable. In the third phase (3), known as strain softening, rapid crack growth is apparent.
(A) (B)
-CMOD - Deflection
0,2 04 OS 0.8
CMOD/Deflection (mm)
— CMOD
— Deflection
0 0.2 0.4 0.6 0.8
CMOD/Deflection (mm)
Fig. 3. Load vs. CMOD/deflection plots for specimens A, B, C, D.
The fracture energy (GF) was calculated as the total energy consumption divided by the cross-sectional area of the ligament over the notch. The total energy was calculated as a sum of the area of the complete load-deflection curve and work done by the deadweight of the specimen, according to formula:
W0 + mg S (d - a)b
where W0 is the area of the complete load-deflection curve, m is the weight of the beam between the supports, calculated as the beam weight multiplied by l/L, g is the acceleration due to gravity, 5 is the deformation at the final failure of the beam, d is the beam height, b is the beam width, a is the notch depth. Additionally, GF was calculated based on load-CMOD curve.
The maximal tensile stress (ac) was obtained on notched beams after other researchers [11]. In the present work it was calculated using the following equation:
3P l 2b(d - a)2
where Pmax is the maximal load, l is the span of the beam, b is the beam width, d is the beam height, a is the notch depth.
The results obtained on the notched beams are presented in Table 2. As expected, the increase of density results in an increase of fracture energy and maximal tensile stress of tested beams. The fracture energy was calculated based on the area under load-deflection and load-CMOD curves, resulting in GF(u) and GF(CMOD). The GF(CMOD) to GF(u) ratio for all tested beams was constant and equals 0.37 ± 0.01, see Table 2 and Figure 4a. This confirms the finding made by others [12] that the factor representing the ratio between deflection u and CMOD can be calculated as auCMOD=l/4(D-(d-a)/2). For the tested beams the factor was 0.36.
Table 2. Test results of notched beams. The mean values of fracture energy and maximal tensile stress.
Specimen Gf (u) GF(CMOD) GF(CMOD) /Gf (u) oc
type [N/m] [N/m] [MPa]
A 12.54 ± 0.21 4.94 ± 0.09 0.39 0.555 ± 0.01
B 6.55 ± 0.42 2.53 ± 0.12 0.38 0.350 ± 0.01
C 4.95 ± 0.08 1.81 ± 0.10 0.36 0.278 ± 0.01
D 1.39 ± 0.12 0.48 ± 0.03 0.34 0.112 ± 0.01
3.2. Un-notched beams
Results of flexural strength of the un-notched beams are presented in Table 3. As expected, the increase of density causes the increase of flexural strength of foamed concrete, see Figure 4b.
Table 3. Test results of un-notched beams: mean values of flexural strength.
Specimen type_ft [MPa]_
A 0.585 ± 0.37
B 0.550 ± 0.22
C 0.362 ± 0.24
D 0.163 ± 0.09
4. Conclusions
Experiments were conducted to determine the fracture and mechanical properties of foamed concrete of varying density in three-point bending on notched and un-notched beams. The main conclusions that can be drawn from this study are the following:
• An increase of the density of foamed concrete results in an increase of fracture energy (GF) and maximal tensile stress (ac). For the notched beams of density of 488-1024 kg/m3 the mean values (based on five specimens per
mix) of Gf (based on the load-deflection relationship) and ac obtained were of 1.39-12.54 N/m and 0.112-0.555 MPa, respectively. The GF(CMOD) to GF(u) ratio for all tested beams was constant and equals 0.37± 0.01;
• The mean value of flexural tensile strength (based on ten specimens per mix) obtained for the un-notched beams of density of 488-1024 kg/m3 was 0.163-0.585 MPa.
(a) (b)
Fig. 4. Density versus fracture energy (a) and tensile stress (b) plots for tested specimens.
References
[1] Van Deijk S. Foamed Concrete. A Dutch View; 1992; p. 2-8.
[2] Ramamurthy R, Kunhanandan Nambiar EK. Indu Siva Ranjani G. A classification of studies on properties of foam concrete. Cement and Concrete Composites 31(6) 2009; p. 388-396.
[3] Kearsley EP, Wainwright PJ. The effect of porosity on the strength of foamed concrete. Cement and Concrete Research 32(2) 2002; p. 233-239.
[4] Drusa M, Fedorowicz L, Kadela M, Scherfel W. Application of geotechnical models in the description of composite foamed concrete used in contact layer with the subsoil. Proceedings of the 10th Slovak Geotechnical Conference "Geotechnical problems of engineering constructions", May 30-31; 2011.
[5] Hulimka J, Knoppik-Wrobel A, Krzywon R, Rudisin R. Possibilities of the structural use of foamed concrete on the example of slab foundation. Proceedings of the 9th Central European Congress on Concrete Engineering. 2013; p. 67-74.
[6] Rahman N, Jaini M. An experimental study on the fracture energy of foamed concrete using v-notched beams. Proceedings of the International Civil and Infrastructure Engineering Conference, September 28 - October 1, Kota Kinabalu, Malaysia; 2014.
[7] Hengst R, Tressler R. Fracture of foamed portland cements. Cement and Concrete Research 13(1) 1983; p. 127-134.
[8] RILEM FMC-50: Determination of the fracture energy of mortar and concrete by means of threepoint bend tests on notched beams. Materials and Structures 18(4) 1985; p. 287-290.
[9] RILEM TC 187-SOC: Experimental Determination of the Stress-Crack Opening Curve for Concrete in Tension. Final report of RILEM Technical Committee TC 187-SOC; 2007.
[10] PN-EN 12390-5:2011. Testing hardened concrete. Flexural strength of test specimens.
[11] Paj^k M, Ponikiewski T. Flexural behavior of self-compacting concrete reinforced with different types of steel fibers. Construction and Building Materials 47 2013; p. 397-408.
[12] Brokenshire D, Barr B. A comparative study of gf test results. In: Wittman, F. AEDIFICATIO Publishers, Freiburg, Germany, 1995; pp. 3-16.
