Commercial textile reinforcements – performance in green cement and surface treatment

PRODUCTION

POLYMERIC MATERIALS AND

COMPOSITES

Commercial textile reinforcements –

performance in green cement and surface

treatment

Angelika Bachinger, Peter Hellström, Derk de Muinck

Deliverables 2.1 & 2.2 of the project ’Green cement based on blast furnace slag’ funded by Vinnova under their programme ‘Innovationer för ett hållbart samhälle’

Commercial textile reinforcements –

performance in green cement and surface

treatment

Abstract

The present report addresses the Deliverables 2.1 and 2.2 of the project ‘Green cement based on blast-furnace slag’. Deliverable 2.1 aims at the evaluation of the mechanical and thermal performance of commercial reinforcements. Deliverable 2.2 describes the modification of those commercial reinforcements and evaluation.

Two different commercial textile reinforcement grids for concrete were evaluated: (i) a basalt-fibre grid from US Basalt impregnated with epoxy resin and (ii) a carbon fibre grid from V. Fraas that is impregnated with Styrene-butadiene rubber (SBR). The grids were exposed to plasma oxidation to increase their hydrophilicity and create functional groups that can react with the uncured cement. The adhesion to the green cement matrix was then measured of both, the untreated and the plasma treated grids by pull-out testing of fibre bundles.

Key words: textile reinforcements, surface treatment, cement, green cement, slag

RISE Research Institutes of Sweden AB RISE Report 2021:01

ISBN: 978-91-89167-83-4 Mölndal 2021

Content

Abstract ... 1

Content ... 2

Preface ... 3

1 Introduction... 4

2 Materials and Methods ... 4

2.1 Materials ... 4

2.2 Methods ... 4

2.2.1 Plasma treatment ... 4

2.2.2 Sample preparation for pull-out testing ...5

2.2.3 Pull-out testing ... 6

2.2.4 Perimeter characterization ... 6

2.2.5 Calculation of the interfacial shear strength (IFSS) ... 7

3 Results and Discussion ... 7

3.1 Perimeter variations ... 7

3.2 Interfacial shear strength... 8

3.3 Thermal properties ... 10

Preface

The present work was performed at RISE department of polymeric materials and composites, within the project ‘Green cement based on blast-furnace slag’ funded by Vinnova under their program ‘Innovationer för ett hållbart samhälle’ (Diarienummer 2016-03367). The report addresses Deliverables 2.1 and 2.2 of the project (evaluation of the mechanical and thermal performance of commercial reinforcements and

1

Introduction

Textile reinforced concrete (TRC) has been studied extensively in recent years and has been shown to be highly interesting in the development of, for instance, lightweight building applications since cost and resources can be significantly reduced. Thinner structures are possible due to improved mechanical properties and corrosion resistance of textile reinforcements allowing the reinforced structure to be exposed to the environment (as compared to steel reinforcements, which need to be protected by a thick layer of concrete). However, commercial textile reinforcements are impregnated with epoxy resin or other polymers, which leads to two main problems: (i) the adhesion of the hydrophobic polymer to the hydrophilic cement is poor, leading to poor load transfer and thus limited tensile strength. (ii) the thermal stability of the polymer impregnation is limited. Upon exposure of the structure to high temperatures or fire, the softening of the polymeric impregnation will lead to a collapse of the structure. The present report aims to evaluate the adhesion and thermal properties of commercial textile reinforcements (D2.1), along with an evaluation of the potential of improving the adhesion by surface treatment of these commercial reinforcements to enable improved mechanical performance (D2.2).

2

Materials and Methods

2.1

Materials

Commercial reinforcements: SITgrid017 (by V. Fraas): Carbon fibres 48k with SBR coating. US GRD 25-100 (by US Basalt): Basalt fibres with resin coating (assumed to be epoxy resin).

For preparation of the green cement, the following materials were used: The slag was supplied by Merox with a composition as presented in Table 1. COMBIMIX cement, gypsum and sand (fine) were standard grades purchased in a local hardware store. NaOH pellets (EMPLURA®, pure) and colloidal silica (LUDOX® TM-50 colloidal silica, 50 wt.% suspension in H2O) were purchased from SigmaAldrich.

Table 1: Composition of blast-furnace slag.

2.2

Methods

2.2.1

Plasma treatment

Plasma treatment was performed in a vacuum plasma chamber (440 Plasma System, Technics Plasma GmbH, Germany) in the presence of oxygen gas. All plasma

of 320 W for 1 minute.

2.2.2

Sample preparation for pull-out testing

12 Samples of commercial reinforcements (6 basalt fibre samples and 6 CF samples) were prepared in coffee-mugs made from paper and kept in place (straight and ~0.5 cm above the bottom) by a plastic coffee mug lock.

3 samples of each commercial reinforcement were embedded in green cement directly, 3 samples of each reinforcement were exposed to plasma treatment according to above parameters before embedding in cement. The plasma treated samples were embedded in green cement within 30 minutes after treatment and stored in a closed container until then. The different samples are summarized in Table 2.

Green cement mixture, sand and waterglass were mixed and then added to the coffee mugs before closing the mugs again and embedding the reinforcements.

The samples were then stored in closed boxes. The paper mugs were placed on a wet cloth, in order to keep them moist and additional water containers were placed inside the boxes in order to ensure a high moisture environment to avoid cracking during curing (see Figure 1).

Figure 1: Samples prepared in paper-based coffee-mugs and stored in closed containers at high humidity.

After 24h, water was added to the cloth for increased moisture content. After 7 days, the wet cloth was taken away and the samples were freed from the coffee mugs. The samples were continuously stored in a box with water containers. After additional 2 days, the samples were taken out and stored at environmental conditions until pull-out testing after 1 week.

Fibre grid Sample Treatment Basalt TRGC1 No TRGC2 No TRGC3 Plasma TRGC4 Plasma TRGC5 plasma Carbon TRGC6 Plasma TRGC7 Plasma TRGC8 Plasma Basalt TRGC9 No Carbon TRGC10 No TRGC11 No TRGC12 No Basalt TRGC13 No

2.2.3

Pull-out testing

The pull-out strength of the fibre bundles was tested using an AG-XK plus Shimadzu with a 50 kN grip at a speed of 1 mm/min. The set up used to place the samples in the machine is shown in Figure 2. A rubber material was placed between the grips and the cement to prevent contact stress. To reduce slippage, the fibre bundles were held between two tabs with P800 sandpaper glued to them when placed into the grip.

Figure 2: Set-up for pull-out testing and sample

2.2.4

Perimeter characterization

The perimeter of the bundles was initially characterized by microscopy, using the following procedure: The impregnated fibre bundles were cut and cast into a mix a Epofix Resin and EpoFix Hardener. The resin was mixed by hand for 1 minute, the bundles were then placed vertically in the mixture and each mixture was placed in vacuum for 20 minutes before being left to cure for 24h in ambient conditions.

grinding papers were used in decreasing grain size P500, P1200, P2000, and P4000. Finally the samples were polished in the same machine using a MD Dur polishing paper with a Diapro Dac 3 suspension. The samples were then examined in an Olympus BH2-UMA Universal Vertical Ilumminator Microscope. The perimeter was measured from the microscopy images at a magnification of 5 times. The method is described in more detail in SICOMP-report TR17-001.

Due to the limited potential of this method to detect variations in perimeter, 3D scanning was used to evaluate the perimeter at different locations of the sample.

2.2.5

Calculation of the interfacial shear strength (IFSS)

The interfacial shear strength was calculated from the pull-out force (F) measured during pull-out testing above, the perimeter of the bundles (P) and the embedded length (L) according to the following equation:

Equation 1

The shear strength is presented in stress-displacement graphs and the maximum shear strength for each sample was identified along with the pull-out behaviour.

3

Results and Discussion

3.1

Perimeter variations

By 3D scanning, the perimeter of 3 different bundles was measured for each of the four cases. For each bundle, the perimeter was measured at 9 different locations. The perimeter variation from 3D scanning and the measured perimeter from microscopy are presented in Figure 3.

Figure 3: Perimeter variations depending on characterization method. 0 2 4 6 8 10

BF untreated BF plasma CF untreated CF plasma

Peri

m

eter,

m

m

Perimeter variations depending on characterization method

It is concluded from these results that the perimeter of the commercial bundles is rather uniform.

3.2

Interfacial shear strength

The maximum interfacial shear strength (IFSS) was calculated according to Equation 1 for the perimeter detected by microscopy as well as the average, maximum and minimum perimeters detected by 3D scanning. Figure 4 shows the resulting IFSS and its variations between different samples and different perimeters.

Figure 4: Maximum IFSS for the different experiments and variations depending on variations between samples and on perimeter variations.

It is seen from Figure 4 that the IFSS could be improved by plasma treatment of the grids. However, independent of the perimeter characterization method, great variations were found for the IFSS. These variations are due to variation of the measured

maximum forces between the different specimens. A significant influence of eventual perimeter variations could be excluded (as discussed above). The variations are therefore most likely assigned to the quality of the resin impregnation of the grids. However, it can also be seen from Figure 4 that the variation further increased with plasma treatment for the basalt fibre grids. It is concluded that the plasma treatment process would require optimization to ensure uniform quality if such approach would be further pursued. It is further seen from Figure 4 that the IFSS of the basalt fibre is higher compared to the carbon fibre grid. At the same time, the increase of IFSS due to plasma treatment is higher for the carbon fibre grids. This observation is assigned to the fact that a more hydrophobic SBR coating was used for carbon grids (leading to lower adhesion to the hydrophilic cement). The formation of polar groups on the grids shows thus a more pronounced effect on the IFSS for the more hydrophobic carbon fibre grids.

For a more detailed evaluation of the pull-out behaviour of the different experiments and samples, the stress-displacement graphs are presented below. Figure 5 presents the

0,0 0,5 1,0 1,5 2,0 2,5 3,0

BF untreated BF plasma CF untreated CF plasma

Interfacial Shear Strength depending on method of

Perimeter Characterization

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 0 1 2 3 4 5 6 7 8 9 10 Sh ear St re ss , N /m m 2 Displacement, mm

Carbon fibre grids

CF plasma 1 CF plasma 2 CF plasma 3 CF untreated 1 CF untreated 2 CF untreated 3 0 0,2 0,4 0,6 0,8 1 1,2 0 0,1 0,2 0,3 0,4 Sh ear Str es s, N /mm 2 Displacement, mm the graphs for the basalt fibre grids.

Figure 5: Stress-displacement diagrams for the carbon fibre grids. Left: global overview, right insert: zoom into the low displacement area.

From the stress-displacement diagrams of treated (black) and untreated (grey) CF grids in Figure 5, it is obvious that the plasma treatment has a positive effect on the IFSS, especially regarding the retention of the bundles after an initial debonding. An enlarged view of low displacements (Figure 5 right) reveals that one of the untreated samples exhibits a higher shear stress at the initial debonding than all of the plasma treated samples. However, the shear stress drops to low levels for all untreated samples after initial debonding. Therefore, the main effect of the plasma treatment is on the shear stress at larger displacement, after initial debonding. Such treatment of grids may therefore be utilized to provide increased ductility to cements, especially after failure initiation. The sample CF plasma 1 reveals that the potential for such increased ductility is enormous. However, it is obvious from the poorer performance of the other treated samples that the treatment process would require optimization to ensure uniform quality.

Figure 6: Stress-displacement diagram of basalt fibre samples.

The stress-displacement diagram for the basalt fibres in Figure 6 shows that the IFSS before as well as after plasma treatment is not uniform. While it seems that on average the pull-out stress is increased by plasma treatment, there is one sample with plasma treatment which performs worse than all of the untreated samples. At the same time, one untreated sample exhibits a similarly high IFSS as the best sample with plasma treatment.

One trend, however, can be observed: the failure is shifted to higher displacement after plasma treatment, indicating increased ductility of such reinforced material.

3.3

Thermal properties

Epoxy resins and SBR do not exhibit high thermal stability due to their organic nature. Even under inert atmosphere, typical epoxy resins have lost 50% of their mass at temperatures of about 360°C [1]. This is an unfortunate drawback of textile reinforcements compared to steel reinforcements, which remain stable until about 500°C, thus allowing for important evacuation time upon fire. Moreover, the rather high thermal stability of the fibre reinforcements (carbon fibres: above 500°C, 1000°C if protected from oxidation [2], basalt fibres: mass loss only 3% at 1000°C [3]) would in principle allow for increased fire safety compared to steel-reinforced concrete. However, the high thermal stability of the fibres cannot be exploited if combined with a polymeric impregnation with low thermal stability. Upon exposure to high temperatures, the polymeric material will soften and/or degrade, leaving the fibres without adhesion to the concrete. Fibre reinforcements without adhesion do not allow

1 _{Salasinska K. et al. Thermal Stability, Fire and Smoke Behaviour of Epoxy Composites}

Modified with Plant Waste Fillers. Polymers 2019, 11, p.1234. doi:10.3390/polym11081234.

2 _{S. Feih, A.P. Mouritz. Tensile properties of carbon fibres and carbon fibre–polymer composites}

in fire. Composites: Part A 43 (2012) 765–772. doi:10.1016/j.compositesa.2011.06.016.

3 _{Tang C. et al. Combustion Performance and Thermal Stability of Basalt Fiber-Reinforced}

Polypropylene Composites. Polymers 2019, 11, p. 1826. 10.3390/polym11111826. 0 0,5 1 1,5 2 2,5 0 1 2 3 4 5 Sh ear St re ss N /m m 2 Displacement, mm BF untreated 1 BF untreated 2 BF untreated 3 BF untreated 4 BF plasma 1 BF plasma 2 BF plasma 3

structure. Therefore, we refer to our Deliverable 2.3, where we investigate the possibility of applying inorganic coatings and impregnation to carbon fibre reinforcements for concrete and green cement.

4

Conclusions

The present studies indicate that plasma treatment may provide an effective tool to provide increased ductility and ability to take loads after initial cracking to textile-reinforced green cement. An optimization of the treatment parameters may enable a more uniform behaviour of the reinforcements.

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Polymeric materials and Composites

RISE Report 2021:01 ISBN: 978-91-89167-83-4