DECORATIVE APPLICATION OF STRAIN-HARDENING
CEMENTITIOUS COMPOSITES
Vahid Ibrović (1), Mladena Luković (1) and Erik Schlangen (1) (1) Microlab, Delft University of Technology, The Netherlands
Abstract
Strain hardening cementitious composites (SHCC) have been used in variety of structural applications. Apart from this, they are also suitable for non-structural applications. In this work, the application of SHCC for producing cover plates for light switches and power sockets is presented. For design and decorative industry, cement based materials are appealing and look more natural compared to variety of plastics usually used. However, their inherent brittleness makes it difficult to be produced as very thin products, mainly due to issues with demoulding. With the addition of fibres, thin elements could be successfully demoulded. They were further subjected to standard mechanical and durability tests for this type of product. These tests included exposure to high temperatures (70°C and 95°C), varying RH (from 95% to 50%), and mechanical impact hardness tests. After these tests, samples were tested in four point bending in order to check how different exposure conditions and mechanical preloading influence the strain hardening capacity. Two different mixtures were designed, with a plate thickness of 3mm and 5mm. Slender samples cast in thinner moulds resulted in enhanced ductility due to preferential fibre orientation in plane. Samples that were exposed to high temperatures exhibited lower ductility compared to samples cured at room temperature. In addition, mechanical impact hardness test reduced the initial SHCC stiffness and maximum load capacity which might be the consequence of inducing distributed microcracking prior to bending test. Overall, SHCC showed to be a promising material for this application.
1. INTRODUCTION
The use of SHCC in a wide variety of applications proved to be very promising. Over the last couple of years, SHCC has been used for repair and strengthening of damaged structures, as bridge deck link, in high rise buildings, in pipes for installations, as joint between structural elements, etc. In recent years, strain hardening cementitious composites (SHCC) have also been used in a variety of non-structural applications: for example, water canoes made in Michigan and Delft [1], and a sculpture made in Denmark, are both made of SHCC. This enables that the objects are quite slender, and yet have sufficient load bearing capacity and ductility.
Recent trends in interior design show an increased demand for “natural” looking products. Therefore, wood and stone are replacing plastics in many decorative applications. Concrete, as a man-made stone material, is also promising in this aspect, as it is easily shaped in various forms. One of the products that still cannot be made of concrete are cover plates for light switches. Due to their small thickness and the way of installation and use, they cannot be made of conventional cement based materials. In this research, the use of SHCC for producing thin covers for light switches is presented [2].
Several hurdles needed to be crossed to achieve this goal: first, the cover plate needs to be quite thin (2-5mm), presenting challenges in the production phase, especially with their demoulding; second, a number of application-specific demands need to be satisfied to prove that the product is both durable and safe for use. Therefore, a number of tests, standard for the industry, are performed to investigate if the product is suitable for use. These tests included the demoulding test, ageing test, impact test, ball pressure test and four-point bending test. The four point bending test was conducted in order to observe the influence of preloading and different exposure conditions on the ductility and strength performance of SHCC. Based on the findings from these tests, some conclusions regarding the achieved strength and strain hardening capacity of very thin SHCC plates are given.
2. MATERIALS AND METHODS
2.1 Mix design
For development of decorative cover plates, two different SHCC mixtures were tested. These two SHCC mixtures are referred to as M1 and M6 and more about their mechanical properties can be found in [3]. Both mixtures were optimized in order to easily demould the samples and withstand the safety tests successfully. M1 has characteristics for a higher early age strength (lower w/binder ratio and more Portland cement as a binding component) whilst M6 has the characteristics of higher ductility (higher w/binder ratio, more limestone powder and less Portland cement as a binding component). In addition, specimens that have the same composition as M6, but without fibres, were made. Table 1 depicts the compositions of tested specimens.
Table 1: SHCC mixtures used in the study (weight %)
Materials M1 M6 M6 without fibres
Portland Cement (PC) 1 1 1
Limestone Powder (LP) 0.8 3.33 3.33
Blast-Furnace Slag (BFS) 1.2 2.33 2.33
Water/binder ratio 0.35 0.53 0.53
Super Plasticizer 0.025 0.033 0.033
PVA fiber (by volume, %) 2 2 0
2.2 Specimen fabrication
After mixing, the samples were casted in wooden moulds, as shown in figure 1a. Afterwards, the moulds were sealed in special foil and placed in a curing room with relative humidity of 95%. The next day the samples were demoulded and placed back in the curing room for additional 6 days. Specimens (Figure 1b) with three different thicknesses were tested: 2mm, 3mm, and 5mm (Figure 1b).
a)
b)
c)
Figure 1: a) Specimen preparation procedure (a) and specimen dimensions (b, c) 2.3 Experimental testing
Demoulding test, ageing test, impact test, ball pressure test and four point bending test were conducted. All the tests, except the demoulding test, were performed at the age of 7 days. For every mixture, two samples were tested in each test.
The first test that was performed was the demoulding test. Essentially, it was tested if the cover plate can be successfully demoulded without breaking, as this was a major issue when regular cement based materials (e.g. cement paste and mortar) were initially tested for this application. It was expected that SHCC will be able to easily pass this test due to its high ductility.
The second test that was conducted was the ageing test. The specimens were placed in a preheated oven of 70°C for 5 days. Afterwards, they were stored in a curing room with a temperature of 25°C and a relative humidity of 95% for 2 days. The requirement of this test was that the samples may not show any deformations or visible cracks.
The third test was the impact test. The impact of a rubber tip bullet with a weight of 160 grams and a fall height of 10 cm was analysed at several locations of the cover plate samples.
Requirements stated that the cover plate may not show any visible cracks or damage. Figure 2 shows the setup of the impact test and the test locations of the samples.
The fourth test that was performed was the ball pressure test. In this test a constant load of 20N, under a temperature of 95°C, was applied on the cover plate samples. The load was transferred through a metal pin with a diameter of 2mm. The constant load acted for one hour after which the impression made in the sample was measured. Figure 3 shows the setup of the ball pressure test.
The final test that was performed was the four point bending test (Figure 4). The four point bending test can be, in this application, seen as mimicking the decommissioning of the cover plate with a screwdriver. The load is applied through two small metal cylinders, which are taped on the samples. Tape is used in order to prevent the cylinders from rolling off. The test was conducted under deformation control with a loading speed of 0.05mm/s.
This test was performed both on samples at the age of 7 days but also on the specimens used in the other tests. This was done in order to investigate the influence of different curing and preloading conditions on the achieved ductility and strength. Four point bending tests at later age included samples from the ageing test (tested at the age of 14 days), ball pressure test and impact test (tested at the age of 10 days).
Figure 2: Setup for the impact test (left) and the impacted locations on the plate (right)
Figure 4: Setup for four point bending test
3. RESULTS AND DISCUSSION
3.1 Demoulding Test
After a curing time of 24 hours, the samples were demoulded. The requirement for this test was that the samples may not show any visible cracks or damage during and after demoulding. As a trial, 5mm thick samples without PVA fibres were demoulded. Due to their brittleness, none of the samples was demoulded successfully. In contrast, all SHCC mixtures were demoulded without visible damage. The demoulding process is shown in figure 5 for both the brittle (i.e. without PVA fibres) and SHCC specimens.
a) b)
Figure 5: Demoulding of specimens without fibres (a) and SHCC (M1) specimens (b) 3.2 Ageing Test
The requirement for this test was that the samples may not show any visible damage or deformations after the exposure. The ageing test was conducted on four samples of the two mixtures M1 and M6 (3 and 5 mm thick). No cracking was observed.
3.3 Impact Test
The requirement for the impact test was that the samples may not show any visible cracks. This is a test where samples were subjected to impact loading (see figure 2). The impact test was conducted on four samples of the two mixtures M1 and M6 (3 and 5 mm thick). No visible damage occurred in the samples due to the impact test.
3.4 Ball pressure test
The requirement for this test was that the impression of the bullet point (shown in figure 3) was not allowed to exceed 2mm. The static load that acts on the samples can be related to the hardness of the mixtures. Again both mixtures (with 3 and 5mm thickness ) underwent this test.
The Figure 6 shows a 3mm sample of the M1 mixture.
Figure 6: Result of the ball pressure test for a 3mm specimen of M1 mix
It was found that the ball pressure had hardly any impact on the samples. The impression is not noticeable. It seems that the applied static load was simply too low in comparison with the hardness and compressive strength of both mixtures. This applies for both mixtures and both 3 mm and 5 mm samples.
3.5 Four point bending test
With four point bending test the flexural strength and deflection capacities of the samples were analysed. In addition, this text shows how different mechanical and durability pre-tests influence on achieved strain-hardening capacity and microcracking of different SHCC mixtures. A representative damage pattern is shown in Figure 7.
Figure 7: Damaged induced by four point testing: crack spacing (a) and crack width (b) The figures below show the load-deflection graphs of the M1 and M6 mixtures at the age of 7 days. At least three specimens per mixture for thickness of 5mm and 3mm were tested while only one sample for 2mm thickness. The 5mm, 3mm and 2mm samples are for mixture M1 and M6 are all depicted in Figure 8 and Figure 9 respectively.
Figure 8: Four point bending test of mixture M1
Figure 9: Four point bending test of mixture M6
The deflection capacity of the M6 mixture is higher than that of the M1 mixture. For M1 the maximum deflection is around 10mm where for M6 it is between 15 and 20mm. This is due to the proportion of materials that is used for mixtures (Table 1). For M6 the water/binder ratio is higher (0.53) than for M1 (0.35). Also the amount of limestone powder (LP) is higher. Due to lower hardness of limestone powder, toughness of the matrix is decreased and this is beneficial for higher strain hardening capacity of the material. Thus the increase of the water/binder ratio and an increase of the amount of LP leads to an increase of the deflection capacity. Therefore, the ductility of the M6 mixture is higher. Overall, both 5mm and 3mm thickness samples show very high deflection capacity, which is attributed to the small thickness of the samples. In this case, fibres are preferentially orientated in the direction of the principal stresses and once the crack is opened they activate and take over stresses at the location of the crack. This further enables that a new crack opens at a different location, finally resulting in tightly spaced cracks crossing the specimen.
The graphs also show that the average first cracking strength of M1 is higher than that of M6. Stress is calculated in the critical cross section of the sample (with the lowest resistance moment). Again, this is related to the water/binder ratio and the content of limestone powder in both mixtures. A lower water/binder ratio of the M1 mixture leads to a higher strength, and therefore a higher first cracking stress. In addition. the limestone powder content of the M6
mixture is by a factor 1.85 higher than that of the M1 mixture. Since limestone powder is an inert filler material, its addition results in lower strength of the cement matrix. The reduction of binder content and the addition of limestone powder therefore lead to a decrease of the first cracking stress and peak flexural strength. One exception is observed in the specimens M6 with 2mm thickness. This specimen was tested at the age of 8 days and this might be one of the reasons that higher flexural strength is observed compared to other specimens.
In general, M1 has a higher maximum flexural strength and a higher first cracking stress. On the other hand, M6 has a higher deflection capacity. This is in accordance to previously observed results [3].
In figures 10 and 11 load displacement diagrams for M1 and M6 specimens (both 3mm and 5mm), that were exposed to different durability and mechanical tests, are shown. As previously explained, samples from the three tests are tested on different days, due to the different duration time of the pretesting. From the graphs (Figure 10 and 11) it can be observed that tendencies are the same, regardless of the mixture or thickness of the sample.
Figure 10: Stress deflection digarams in four point bending test after normal curing (7 days), after impact test (10 days), ageing test (14 days) and ball pressure test (10 days) for mixture
M1: 5mm thickness (left) and 3mm thickness (right)
Figure 11: Load deflection diagrams in four point bending test after normal curing (7 days), after impact test (10days), ageing test (14 days) and ball pressure test (10days) for
mixture M6: 5mm thickness (left) and 3mm thickness (right)
The ball pressure test leads to a small increase in maximum load capacity compared to the specimens tested at 7 days. This may be due to the older age of the specimens when tested. During the test, specimens were kept for 24 hours at a relative humidity of 50% and a temperature of 20°C. Besides these 24 hours, the samples were cured for 2 more days, because the bending test was performed after 10 days in order to compare them with the
specimens tested in ageing test. Longer curing time resulted in higher first cracking stress and peak flexural strength. However, the ductility remained the same as that of the specimens tested after 7 days.
Bending after ageing tests results in a lower deflection capacity compared to the bending of the specimens pretested in ball pressure test. In addition, for mixture M6, higher first cracking stress is observed. Therefore, samples that were exposed to high temperatures exhibited lower ductility and higher first cracking strength compared to samples cured at room temperature. At present, few results are available for durability of SHCC under elevated temperatures [4]. These results can be probably attributed to the softening of the fibres or faster hydration of the paste cured at higher temperature. This resulted in higher matrix toughness leading to lower ductility of SHCC. However, since in this case only one sample was tested per mixture, more research is needed to explain it.
The impact test shows the most critical influence on mechanical properties of two mixtures. It results in a decrease in maximum load capacity and first cracking load. This can be explained by the fact that, although no visible damage was observed, the impacts probably did cause some microcracking. This further led to a decrease in the elastic stiffness and first cracking strength of the samples. From the load-deflection diagrams, it seems that this initial microcracking did not affect the strain-hardening capacity of the material. However, further research is needed to investigate the maximum crack widths and crack distribution in the samples.
4. CONCLUSIONS
Based on presented results, some conclusions can be stated:
SHCC can be successfully made for very slender and crack sensitive applications With the amount of raw components it can be tailored to satisfy either high strength
criteria or higher deflection capacity, depending on desired application.
Different specimen preconditioning and preloading can significantly influence its flexural response.
Static loading prior to flexural testing, applied in these experiments, did not have a significant influence on the resulting flexural response. This is attributed to the magnitude of the applied static load, which was significantly lower than strength of the tested material.
High temperature resulted in the reduction of deflection capacity while in M6 mixture also higher first cracking strength was observed.
Impact test had the most critical influence on the first cracking strength and initial stiffness of the specimens. This is probably caused by microcracking which was induced during impact tests.
ACKNOWLEDGEMENTS
