Jan Suchorzewski, Miguel Prieto, Urs Mueller
RISE Research Institutes of Sweden, Infrastructure and Concrete Technology, Material Design, Borås, Sweden,
1 Introduction
Within EU project LightCoce (Building an Ecosystem for the upscaling of lightweight multi-functional concrete and ceramic materials and structures), RISE will be running a Pilot Line to allow the design and development of materials and elements of Cellular Lightweight Concrete (CLC) and/or lightweight composite elements with improved functionalities. For this purpose, a test case of a composite CLC/HPC sound-wall element with sound-absorbing, improved durability, self-sensing, and self-cleaning properties will be developed and upscaled. The structural performance and integrity of the sound-wall element will rely on an HPC shell strengthened with FRP textile reinforcement. For this aim, HPC was enhanced starting from a mix design developed in an EU project SESBE. The HPC mix has been upgraded for lowering the overall costs and improving its workability.
2 Self-sensing in concrete
One of the novel concrete functionalities explored in our project is the self-sensing ability gained with the addition of nanoparticles with high conductivity [1]. The recent development of the graphene production process brought novel, economically competitive material called multi-walled carbon nanotubes (MWCNTs), which have super-high-strength (1’000 MPa) and is a perfect electrical conductor. The addition of a small amount of MWCNTs decreases concretes overall electrical resistance enabling material internal structure condition monitoring [2]. A small current must be delivered to analyze the element and the voltage is measured in chosen points of structure (Fig.1). In this way, changes in material resistance are registered over time indicating stress level (Fig.2), cracking or other kinds of damage.
Figure 1: The test setup for resistivity measurements (A) DC current from a power supply (measuring voltage), (B) measuring current.
3 Experimental campaign
A test campaign with different amounts of MWCNT was performed to investigate their effect on mechanical and self-sensing properties. For the investigation of self-sensing several specimens (cubes 100x100x100 mm) and low thickness panels (40x100x700 mm) reinforced with two types of textile FRP (carbon and glass fibers) with different amounts of MWCNT were cast and tested. The textile-reinforced panels were tested (1) in elastic phase under cyclic load, (2) under static continuous deformation or (3) under cyclic load after cracking. Resistivity changes during
22
tests were recorded and compared with the registered load-displacement curves. The FRP reinforcement forced intensive cracking with multiple discrete cracks, which were monitored throughout all the test duration. The test campaign outcomes will help to understand the effect of the amount of MWCNT on strength and self-sensing properties in HPC.
A)
a) b) B)
a) b)
Figure 2: The self-sensing of concrete with the addition of 0.10% MWCNT for panels reinforced with A) GFRP under static loading and B) CFRP under cyclic loading with a) force-displacement curves and b) resistivity to displacement or number of cycles.
4 Conclusions
In our research, we present a novel method for structural health monitoring with no additional sensors. With this method, we were able to measure stresses in uniaxial compression and detect multiple cracks in bending. Moreover, concrete material degradation in cyclic loading (fatigue) could also be observed due to the increase of resistivity. Self-sensing could be used as an inexpensive and easy alternative for monitoring in combination with other methods used nowadays.
References
[1] Konsta-Gdoutos, M.S.; Aza, C.A.: Self-sensing nanotube (CNT) and nanofiber (CNF) cementitious composites for real-time damage assessment in smart structures. Cement&Concrete Composites 53 pp. 162-169, 2014.
[2] Danoglidis, P.A.; Konsta-Gdoutois, M.S.; Gdoutos, E.E; Shah, S.P.: Strength, energy absorption capability and self-sensing properties of multifunctional carbon nanotube reinforced mortars.
Construction and Building Materials 120, pp. 265-274, 2016.
Session C1: Lightweight Concrete and Textile Reinforcement
23
Experimental investigation on the drying shrinkage of structural lightweight aggregate concrete
Mohamed Abd Elrahman 1, Mohamed El Madawy 1, Sang-Yeop Chung 2, Pawel Sikora 3,4, Dietmar Stephan 3
1: Structural Engineering Department, Mansoura University, Egypt
2: Department of Civil and Environmental Engineering, Sejong University, Republic of Korea 3:Building Materials and Construction Chemistry, Technische Universität Berlin, Germany
4: Faculty of Civil Engineering and Architecture, West Pomeranian University of Technology Szczecin, Poland
1 Introduction
Lightweight concrete (LWC) is a material with low density, excellent thermal insulation properties, and reasonable mechanical properties. However, due to replacing normal-weight aggregates with less rigid porous aggregates, LWC suffers high drying shrinkage and micro-cracking. The influence of mineral admixtures on the shrinkage of LWC was studied by Cheng et al. [1]. Grabois et al. [3] found that with increasing lightweight aggregate content compared tom normal-weight aggregate, the shrinkage significantly increases. Chen et al. [2] studied the influence of different types of fibers on mechanical properties and shrinkage of LWC. Results showed that steel, carbon and polypropylene fibers reduced the shrinkage of LWC effectively, while a study by Kayali et al. proves the important role of fibers in improving the tensile strength and reducing the shrinkage of LWC [4].
2 Materials and tests
This investigation studies the shrinkage behavior of lightweight concrete with a dry density of 800 ± 50 kg/m³. The influence of silica fume and fly ash on mechanical properties and shrinkage was investigated. The effect of two types of polypropylene (PP, density 0.91 g/cm³) fibers; 12 and 6 mm (0.031 mm diameter) on LWC characteristics were examined. Six different mixes have been prepared and tested (Table 1). Dry density, compressive and flexural strength, and drying shrinkage were measured. Chemical admixtures were adopted to produce stable LWC mixes with consistency class of F3/F4 (acc. to EN 206-1). As a lightweight aggregate, expanded glass (Liaver®) was used. Drying shrinkage has been measured as specified in DIN 52450 using the Graf-Kaufmann method.
Table 1: Composition of LWC mixes
Mix Liaver Cement Silica fume Fly ash Water Short fibers Long fibers
[kg/mix] [vol.-%]
Ref 275 410 50 - 240 - -
C S 275 410 50 - 240 0.2 -
C L 275 410 50 - 240 - 0.4
C F 275 410 - 50 240 - -
C FS 275 410 - 50 240 0.2 -
C FL 275 410 - 50 240 - 0.4
3 Results and Discussion
The experimental results showed that all LWC mixes have dry densities in the range of 750 - 850 kg/m³ as planned. Compressive strength is correlated directly to the dry density of LWC (Fig. 1a). Similarly, the thermal conductivity of lightweight concrete is proportioned directly to the dry density of concrete. However, flexural strength depends mainly on the PP fiber type and dry density. It is clear that addition of short fibers is more effective in improving the flexural strength of concrete (Fig. 1b). On the other hand, the addition of PP fibers has a negative influence on compressive strength but does not have any direct effect on thermal conductivity.
The experimental results of drying shrinkage at different ages are presented in Fig. 1c. It is clear that in all mixes shrinkage is increasing with age. The addition of PP fibers reduces the shrinkage significantly. Both short and long fibers have similar effects on drying shrinkage. On the other hand, fly ash is more effective in hindering the micro-cracking and reducing the shrinkage compared with silica fume which increases the shrinkage at all replacement levels as
24
concluded by Nadesan et al. [5]. The addition of fly ash reduces the shrinkage by about 35%
compared to control mix with silica fume. However, combination of fly ash and PP fibers resulted in significant reduction in shrinkage by about 70% after 28 days of curing.
Figure 1: Dry density and compressive strength (a), flexural strength, and thermal conductivity (b), shrinkage (c) and micro-CT imaging process for lightweight aggregate concrete (d).
Concrete microstructure
X-ray micro-CT was applied to examine the microstructure characteristics of LWC specimens non-destructively. Fig. 1d presents the imaging process for classifying pores, aggregates, and solid matrix examination in order to obtain the material characteristics of higher accuracy. In this procedure, the original 8-bit image described in grayscale (Fig. 1d) was converted to the binary image, as shown in the 2nd image. The 3D image was then obtained by stacking a series of binary images, and the detailed pore size distribution of the specimen was evaluated.
The obtained results from the micro-CT data showed that the general trends of the wall-thickness and the pore structures are affected by its constituents. In particular, in each set of the specimens with silica fume and fly ash, the porosity and the wall thickness level within the specimen is almost the same regardless of the use of the fibers. However, as shown in Figs.
1a-1c, the material properties are enhanced by following the use of the fibers, and this result confirmed the effectiveness of the fibers to improve the material properties.
4 Conclusions
This investigation studied the influences of supplementary materials and polypropylene type fibers on mechanical properties as well as shrinkage of lightweight concrete. The experimental results indicated that polypropylene fibers significantly reduces the drying shrinkage. In addition, mixes prepared with silica fume suffer higher shrinkage compared to fly ash mixes.
Moreover, X-ray micro-CT can be used to characterize the microstructure of lightweight concrete.
References
[1] Cheng, S., Shui, Z., et al.: Properties, microstructure and hydration products of lightweight aggregate concrete with metakaolin and slag addition, Construction and Building Materials 127 (2016) 59-67.
[2] Chen, B., Liu, J.: Contribution of hybrid fibers on the properties of the high-strength lightweight concrete having good workability, Cement and Concrete Research 35 (2005) 913-917.
[3] Grabois, T., Cordeiro, G., Filho, R.: Fresh and hardened-state properties of self-compacting lightweight concrete reinforced with steel fibers, Construction and Building Materials 104 (2016) 284-292.
[4] Kayali, O., Haque, M., Zhu, B.: Drying shrinkage of fibre-reinforced lightweight aggregate concrete containing fly ash, Cement and Concrete Research 29 (1999) 1835-1840.
[5] Nadesan, M., Dinakar, P.: Influence of type of binder on high performance sintered fly ash lightweight aggregate concrete, Construction and Building Materials 176 (2018) 665-675.
Session C1: Lightweight Concrete and Textile Reinforcement
25