Elsevier

Construction and Building Materials

Volume 197, 10 February 2019, Pages 474-488
Construction and Building Materials

Utilization of magnetic water in cementitious adhesive for near-surface mounted CFRP strengthening system

https://doi.org/10.1016/j.conbuildmat.2018.11.219Get rights and content

Highlights

  • Modifying cement based adhesive for bond between CFRP and concrete using magnetized water.

  • Studying mechanical properties of magnetized cement based adhesive.

  • Prediction of magnetized mortar properties using genetic programming.

Abstract

Cement-based adhesive (CBA) is used as a bonding agent in Carbon Fibre Reinforced Polymer (CFRP) applications as an alternative to epoxy-based adhesive due to the drawbacks of the epoxy system under severe service conditions which negatively affect the bond between the CFRP and strengthened elements. This paper reports the results of, an investigation carried out to develop two types of CBA using magnetized water (MW) for mixing and curing. Two magnetic devices (MD-I and MD-II), with different magnetic field strengths (9000 and 6000 Gauss) respectively, were employed for water magnetization. Different water flows with different water circulation times in the magnetizer were used for each device. Compressive and splitting tensile strength tests of the magnetized CBA (MCBA) were conducted for different curing periods (3. 7, 14, 21 and 28 days) using MW. It was found that MW treatment increases the strength of CBA. The highest strength was obtained for MCBA samples when MD-I was used at a low flow rate (F = 0.1 m3/hr) for 15 mins of circulation time (T). The latter was found to positively affect MCBA properties when T was increased from 15 min to 60 mins. Prediction of the compressive and tensile strength values are also studied in this paper using genetic programming, the models showed good correlation with the experimental data.

Introduction

The drawbacks of the epoxy system associated with the application of FRP strengthening systems under severe service conditions [1], [2] have led many researchers to explore either a modification of the epoxy system [3] or to replace the epoxy with other bonding agents between CFRP and concrete [4]. Cement-based adhesive (CBA) is a commonly-used alternative to epoxy in near-surface mounted (NSM) CFRP strengthening systems [5]. Most reported attempts to modify CBA have focused on cement-based materials to improve the properties of CBA in order to enhance the bond between CFRP and the concrete substrate [6]. However, there is a gap in knowledge which needs to be explored in terms of modifying CBA by changing the properties of the water used either for mixing or curing CBA.

Magnetic water treatment is a technique used in different fields [7], [8] and it has been shown to be effective when used to enhance the properties of cement [9], [10] and concrete. Workability and strength of concrete were conducted using magnetized water [11]. Normal strength concrete specimens were fabricated using three different concrete mixes (mix A, mix B and mix C) with various water-to-cement ratios using two types of water (tap water and magnetized water). The mix proportions (cement: sand: gravel) for these three mixes were: (1:1.87:3.37) (mix A), (1:1.5:3) (mix B); and (1:1.7:2.54) (mix C). The water-to-cement ratios were 0.43, o.448 and 0.55 for mix A, mix B and mix C respectively. The specimens were cured for various curing periods (7, 14 and 28 days) for compression test while other samples were cured for 28 days only for indirect tension test. The mixing water in the above mixes was tap water for control samples. Other set of samples (MW samples) was prepared using magnetized water. The latter was magnetized using magnetic fields with different intensities (6000 and 9000 Gauss). The circulation time of water in the magnetizer was 5 mins and the water velocity was 1000 mm/sec. Both the time of circulation and the velocity of the water were kept constant. Strengths and workability of concrete samples were conducted. The workability of control samples as well as MW samples was evaluated by means of slump test. Higher slump was reported for MW samples in comparison with control specimens for concrete mixes (mix A, mix B and mix C). The reported results showed that the higher the intensity of the magnetic field, the high the slump. For example, within Mix C, an increment in the slump (71% and 90%) was achieved for MW samples in comparison with control samples using magnetic field of (6000 and 9000 Gauss) respectively. In terms of strength tests, an increment in the compressive strength was obtained for all concrete mixes when MW was used for mixing concrete samples for all curing periods. The highest enhancement (16% and 22%) was for MW samples of mix B of (14 days and 28 days) curing periods respectively when magnetic field of 9000 Gauss was employed for magnetization. The splitting tensile strength of MW specimens was higher than those of tap water samples. No significant differences in the splitting tensile strength were recorded with the MW samples for mix A and B when two different intensities of the magnetic field was used. The enhancement in the reported strengths of MW samples is due to continuous hydration process which produces more hydration products in comparison with tap water samples.

Another study was carried out to explore the effect of using MW in mixing and curing concrete [12]. A concrete mix of (cement:sand:gravel) was prepared to obtain a concrete samples with compressive strength of 26.6 MPa. The concrete mix was (1:1.5:3) with water-to-cement ration of 0.55. For magnetizing the water, a beaker was filled with water and placed over a circular magnet of 985 Gauss for 24 h. The magnetized water that used for mixing concrete was mixed pole water which consists of equal amount of North pole water and South pole water. Mechanical properties of concrete samples with and without magnetized water were evaluated by means of compressive strength, splitting tensile and flexural strength. Compressive strength at various curing periods (7, 14, 21, 28, 60, 90, 120, 180 and 360 days) was obtained for concrete samples with and without magnetized water. All samples that prepared using magnetized water (MW samples) showed higher compressive strength when compared with control samples (without magnetized water (NW samples)) for all curing ages. The percentage enhancement in the compressive strength was reported at curing ages up to 28 days and the highest (59.1%) was recorded at 7 days. Splitting tensile strength of concrete of various curing ages (28, 60, 90,120,180 and 360 days) was observed to be increased when MW was used for mixing concrete. Additionally, all MW samples showed higher flexural strength than control samples (NW samples) for different curing periods (7, 28, 60, 90, 120, 180 and 360 days). The reported enhancements in the strengths was attributed to the increment in the hydration process between cement gains and water due to the increment in the surface area of water that exposes to the magnetic field. Such increment means more hydration products which fill voids and pores in the microstructure of the concrete which increase the strength.

Superior durability of concrete was reported when magnetic water (MW) was used for mixing concrete [13]. Concrete samples that prepared with and without MW were subjected to different conditions such as acid immersion as well as freezing and thawing. The water was magnetized using three round magnets obtained from a scientific store with an average magnetic strength of 965 Gauss. Various magnetized water were prepared (North pole water, South pole water and mixed poles (North and South) water). The latter was used in the study and it was obtain by mixing equal amounts of North and South Pole waters. Set of cubic concrete specimens with MW and without MW (normal water (NW)) were immersed for 28 days in different concentrations (5%, 10% and 15%) of Hydrochloric Acid (HCl). Strength of samples before and after immersion in HCl as well as before each cycle of freezing and thawing were obtained. Higher strength were reported for MW samples that immersed in different concentrations of HCl when compared with NW samples. The highest strength (44.92 MPa) was for MW samples that subjected to 5% of HCl in comparison with (36.47 MPa) for NW samples. This was attributed to the influence of MW on the enhancing the microstructure of concrete (make it denser) which leads the less amount of scaling of surface of the samples. In terms of exposing the samples to cycles of freezing and thawing, another set of concrete samples (with and without MW) were subjected to various cycles (0, 3, 6, 9,12 and 15 cycles) of freezing and thawing. Samples were placed inside a freezer at −14 °C for 24 h. This was followed by placing the specimens in an open atmosphere for 24 h. The weight of samples before and after exposure was obtained in order to calculate the percentage of weight loss (% mass loss) for MW and NW samples. The latter samples showed higher percentages of weight loss in comparison to MW samples and for all cycles of freezing and thawing.

The explanation for such enhancements in the properties of cement and concrete may be clarified when the effect of the magnetic field on the structure of water is understood. In general, a non-magnetized water molecule consists of two positive hydrogen atom and one negative oxygen atom. The attraction of a positive hydrogen atom of one molecule to the negative oxygen atom of another leads to the formation of water clusters. The structure and distribution of water clusters is affected when it passes through a magnetic field [14]. The water molecule clusters break down due to change in surface tension [15]. This breakdown into single molecules or small clusters increases the activity of water. Many factors may affect the degree of magnetization, such as the quantity of water in contact with the magnet, the magnetic field strength and the exposure time of water to the magnetic field [16]. During the hydration process of cement, water starts to react with cement particles. A thin layer of hydration products forms on the surface of the cement grains at the beginning of the reaction which slows the hydration. However, when MW is used, the cement grains are easily penetrated by the MW due to its smaller molecular clusters. Therefore, more water is involved in the hydration. In addition, MW has lower surface tension than non-magnetized water (NW), which makes the cement more active in hydration. This leads to denser hydration products and a more complete hydration process [10] and therefore the final strength of cement mortar increases with the usage of MW.

In the present study, the main objective was to employ MW treatment to develop cement-based adhesive (CBA) to be used for NSM CFRP strengthening. Two types of permanent magnets were used to magnetize the water used in mixing the CBA. The properties of MCBA were investigated in compression and indirect tension for different periods of curing. The results were then compared with control specimens (NCBA). The experimental parameters explored were: magnetic field strength, water flow rate in the magnetizer, and exposure time to the magnetic field. A genetic programming was also employed in this work to predict the compressive and tensile strength.

Section snippets

Cement-based adhesive

Two types of cement-based adhesive (CBA) were used in this work: CBA-A and CBA-B. Each type of CBA consisted of: Ordinary Portland Cement (OPC), micro-cement (MC), water (W), filler (F), silica fume (SF) and super-plasticizer (SP). However, only CBA-A contains primer (P) in its composition. The mix proportion was 1:0.250:1.063:0.125 for OPC:MC:F:SF respectively, with a water-to- binder ratio (w/b) of 0.365. The SP was added at the rate of 2.5% by weight of OPC. In the case of CBA-A, the primer

Water magnetization

The process of magnetization of water was carried out using two set-ups with two magnetic devices (MD-I and MD-II) with magnetic field strengths (MFSs) of 9000 and 6000 Gauss respectively. Each set-up included a magnetic device (MD), a flow meter (FM) to control the water flow through the MD, a pump to circulate the water in the MD, a plastic container with the capacity of 15 L as a source for water and as a collector for magnetic water, pipes and valves (Fig. 1). Different water flows (0.1,

Preparation of magnetized cement-based adhesive

A total of 600 cement-based adhesive (CBA) samples were fabricated. A laboratory mortar mixer was used for mixing the ingredients (refer to Section 2.1). First, all powder state materials (OPC, MC, F and SF) were mixed at slow speed for approximately 2 mins. After the addition of water (with and without magnetization), the SP was added and mixed at low speed for 2 min followed by high speed for 2 min. For the preparation of CBA-A, the above procedure was used and then the MP was added and the

Compressive strength

A total of 180 CBA cylindrical samples were tested. Half of these specimens were of cement- based adhesive CBA-A while the others were of CBA-B. An average of three samples was tested for each type of CBA and for each age of curing, flow rate, MD and time of circulation of water. All CBA cylindrical specimens were tested in accordance with the Australian Standard 2000 (AS 1012.11) [22]. A Technotest® compression machine (see Fig. 4) with a load cell capacity of 300 kN was used to apply the load

Effect of magnetic field strength (MFS)

Table 1 and Table 2 summarize the results of testing CBA samples using NW and MW in compression and indirect tensile strength, respectively. Each value in Table 1 and Table 2 is an average of testing three samples. As illustrated in Fig. 6a, Fig. 6b, both CBA samples (CBA-A and CBA-B) showed higher compressive and indirect tensile strengths when MW was used for mixing, in comparison with control samples (NW) for all curing periods and for the same circulation time (T = 15 mins). This higher

Genetic programming

Genetic programming is a method of processing large number of data and assign fitness value depending on the performance of data analysis. The basic of this method is genetic algorithm which was proposed by John Holland in 1975 [25]. Understanding the Darwinian theory was the main motivation to develop the genetic algorithm theory by John Holland. The basic concept of this GP is by giving a fitness value to individual objects and develop a new generation, and then a procedure of genetic

Conclusions

This paper has reported the possibility of enhancing the properties of CBA adhesives used to bond CFRP and concrete in NSM strengthening systems. Different MDs were used to magnetize the water for mixing and curing CBA samples. The following observations were made:

  • 1)

    MCBA samples showed higher compressive and splitting tensile strengths compared with NCBA. The greatest improvement was for MCBA samples prepared with MW using MD-I (MSF = 9000 Gauss) with a flow rate of 0.1 m3/hr for T = 15 mins.

  • 2)

    The

Acknowledgements

Deep gratitude is due to the technical staff in the Smart Structures Laboratory at Swinburne University of Technology, Melbourne, Australia for their support and technical assistance in carrying out the experimental program of this project. The first author would like to thank the third author for his valuable guidance and financial support of this project.

Conflicts of interest

None.

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