This study presents a new approach to the utilization of industrial by-products in construction materials by using fly ash (FA) in the production of glass fiber-reinforced polyester (GRP) pipe. The FA was substituted by 10% and 20% (by weight of sand) in the mixtures to produce GRP pipes of 350 mm in diameter and 6 m in length for testing. Stiffness modulus (SM), axial tensile strength (ATS), and hoop tensile strength (HTS) tests were conducted on the produced GRP pipes and their elasticity modulus (EM) values were also calculated. To observe the microstructure of the GRP pipes and the interfacial transition zone of the layers, SEM and microscopic analyses were performed. Furthermore, a strain-corrosion test was conducted to obtain information about long term-performance of samples. The results showed that the FA-filled GRP pipes were found to meet the requirements of the related standards, and that the use of FA in the GRP pipe industry may be an important alternative approach to the utilization of industrial wastes via effective recycling mechanisms.
Fly ash is a by-product constituting about 60–88% of the total combustion residues from coal-fired electric generating plants and is recognized as an environmental pollutant. Since the increase of energy production facilities due to rising energy demands in developing countries, FA is rapidly becoming a very serious environmental problem. From the environmental perspective, the recycling of FA is as important as energy production and thus, the disposal of this by-product has long attracted the attention of researchers (
Research carried out on the reuse of FA as an alternative to landfill disposal covers a wide number of fields, ranging from agriculture to engineering. Bicer (
The recycling/utilization of FA has a number of environmental benefits, including reduced landfill disposal, reduced consumption of virgin resources, and reduced greenhouse gas emissions (
Glass fiber-reinforced polyester (GRP) composite pipes are used in a number of civil engineering applications such as transporting clean and potable water, water for irrigation, for hydroelectric power plants, and for water treatment, water storage, and sewer systems, for storm water and seawater intake and discharge, and for trenchless technologies. Due to this wide area of application, the direct utilization by the GRP pipe sector of a large amount of by-products can be regarded as an industrial-scale solution (
In the experimental stage, GRP pipes were produced using silica sand as a filling material, glass fiber as a reinforcing material, cobalt octoate as an accelerator, methyl-ethyl ketone peroxide as an initiator, and orthophthalic unsaturated polyester resin (with the characteristic of thermoset) as a binder. Two types of resins were used in the production of the inner and outer layers of the GRP pipes. The liner resin, having high chemical resistance, was used in the first 1-mm thickness of the inner surface. The orthophthalic polyester resin was used to produce the other layers of the GRP pipes. Properties of the resins used in this study are presented in
Properties of resins used in this study.
Parameter | Test Method | Orthophthalic Polyester Resin (Body) | Polyester Resin (Liner) |
---|---|---|---|
Viscosity (@25 °C) | ASTM D 2196 | 225 cp | 570 cp |
GelTime (@25 °C) | ASTM D 3056 | 11 min | 12 min. |
Exotherm Peak | ASTM D 2471 | 160 °C | 135.8 °C |
Peak Time/Gel Time | ASTM D 3056 | 2 | 1.8 |
Solid Content (@300cp) | ISO 3344 | 64.09% | 77.04% |
E-glass fiber with a length of ~25‒50 mm was used as a reinforcing material. In the production of the GRP pipes, silica sand was used as a filler material to increase the rigidity.
Properties of silica sand used in this study.
Parameter | Result |
---|---|
Resin and Sand Mix Gel Time | 07.15 min |
Moisture Content (@105 °C, 2 h) | 0.03% |
Bulk Density | 1.52 g/cm3 |
Particle Size & Distrubution | 0.81% |
Loss on Ignition | 0.13% |
The FA, a by-product obtained from the Çatalağzı thermal power plant (in Turkey), was substituted for silica sand in the mixtures by amounts of 10% and 20% to form the structural layers of the GRP pipes. The FA used in the study contained the following oxides: SiO2 (55.16%), Fe2O3 (5.70%), TiO2 (1.11%), Al2O3 (26.79%), CaO (1.91%), MgO (1.15%), Na2O (0.43%), K2O (4.94%), SO3 (0.001%) and P2O5 (0.24%). Prior to the production process, the silica sand and FA were mixed (135 kg silica sand + 15 kg FA for the 10% FA substitution and 120 kg silica sand + 30 kg FA for the 20% FA substitution) by mechanical mixing methods in a large container and placed into the bulk trailer of the centrifugal casting (CC) system. Three types of GRP composite pipes were produced in this study. One of them was the reference pipe (REF) produced without FA, while the others contained 10% and 20% FA. The GRP pipes were produced on a one-to-one scale under PN 6 bar pressure class and SN 5000 N/m2 stiffness class. All pipes were manufactured by the CC method operating with a PLC-PC fully automatic controlled system (
(a) - (b) General view of pipe production by CC method; (c) GRP pipe containing FA.
The resin was designed to be non-polymerized during the pipe production. For the pipe production, the fibers used were cut in lengths of 25‒50 mm. The 10% FA-substituted GRP pipe produced in this study is given in
10% FA-substituted GRP pipe with cross-sectional details.
After producing the pipes, test samples were prepared according to the ISO 7685 standard for SM, ISO 8521 standard for HTS, and ISO 8513 standard for ATS. Details of all tests applied to the GRP pipes are given in
Description of tests and specifications.
Test /Specification | Sample Dimensions |
---|---|
SM ISO 7685 | 4 pieces: 30-cm wide (2 pieces at one end, 2 pieces at the other end) |
HTS ISO 8521 | 20 pieces: 2.5-cm wide, as circle-shaped strips (10 pieces at one end, 10 at the other end of the pipe after cutting the stiffness samples) |
ATS ISO 8513 | 20 pieces: 2 × 30 cm strips taken along the axis of the pipe (through the circumference of the remaining part of the pipe after cutting the samples, 10 from each end) |
After preparing the samples detailed in
(a) HTS test; (b) ATS test; (c) SM test; (d) details of HTS test samples; (e) details of ATS test samples.
By using the ultimate forces (F) obtained from the test machine, the HTS and ATS were calculated using Equations [
To calculate the stiffness modulus [Sm], Equation [
The SM test was performed to evaluate the rigidity of the GRP pipes and the results are given in
SM results of GRP pipes.
NO | Width | Thickness | Force (F) | SM | Level A |
Level B |
---|---|---|---|---|---|---|
mm | mm | N | N/m2 | |||
298 | 8.74 | 926 | 5465 | No | No | |
292 | 8.58 | 889 | 5352 | No | No | |
305 | 8.86 | 1074 | 6195 | No | No | |
296 | 8.78 | 1010 | 6001 | No | No | |
295 | 9.11 | 1299 | 7752 | No | No | |
298 | 9.17 | 1054 | 6227 | No | No | |
300 | 9.29 | 1105 | 6487 | No | No | |
294 | 9.19 | 1099 | 6582 | No | No | |
297 | 8.76 | 781 | 4625 | No | No | |
297 | 8.67 | 876 | 5186 | No | No | |
300 | 9.22 | 1312 | 7701 | No | No | |
295 | 9.2 | 1127 | 6727 | No | No |
Level A: Visual defects at 11.3% deflection
Level B: Structural cracks at 18.9% deflection
HTS and ATS test results for GRP pipes.
HTS Results (N/mm) | ATS Results (N/mm) | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
1 | 2 | 3 | 4 | 5 | 1 | 2 | 3 | 4 | 5 | |
1226.9 | 1145.6 | 1067.3 | 1070.2 | 1145.1 | 146.67 | 135.02 | 150.5 | 159.18 | 138.86 | |
1076.7 | 1201 | 1150.5 | 1009.6 | 1106.1 | 149.15 | 154.06 | 136.86 | 145.93 | 142.7 | |
933.3 | 943.4 | 1326.9 | 1183.7 | 1006.2 | 165.81 | 158.89 | 132.64 | 134.58 | 164.63 | |
1037.4 | 1260.6 | 1235.5 | 966.9 | 974.6 | 132.07 | 153.51 | 137.92 | 162.97 | 144.53 | |
1024.4 | 1205.4 | 1094.6 | 1062.3 | 1198.1 | 151.75 | 175 | 159.42 | 172.96 | 172.83 | |
1043.2 | 1098.8 | 1400 | 1131.7 | 763.1 | 146.6 | 151.14 | 164.11 | 140.64 | 136.27 | |
1137.46 | 1011.61 | 1083.31 | 774.5 | 828.13 | 177.03 | 186.89 | 164.46 | 165.01 | 138.88 | |
1142.92 | 940.34 | 972.89 | 931.8 | 1164.26 | 195.18 | 162.46 | 152.45 | 163.52 | 171.38 | |
947.08 | 1080.89 | 1075.64 | 1066.74 | 1162.24 | 158.36 | 149.78 | 157.84 | 126.21 | 170.08 | |
1160.92 | 1168.09 | 1042.04 | 1014 | 1040.65 | 154.85 | 139.65 | 137.5 | 145.77 | 154.31 | |
632.91 | 961.57 | 1039.56 | 836.53 | 1149.43 | 152.2 | 186.36 | 157.98 | 158.36 | 170.57 | |
1029.38 | 1040.02 | 815.43 | 977.93 | 1038.34 | 156.92 | 178.18 | 173.93 | 149.67 | 156.9 |
When the HTS and ATS results were compared with the lower limits of the standards (443 N/mm for HTS in AWWA C950 and 102 N/mm for ATS in EN 1796), it was observed that the results for all pipes were higher than the corresponding lower limits of the standards.
When the SM test results were evaluated in terms of compatibility with the EN 1796 standard, it was seen that the GRP pipes produced with FA substitution met the 5000 N/m2 requirement given in the EN 1796 standard. The SM results indicated that the use of FA in the production of GRP pipe is possible.
It was expected that the test results of samples taken from the same GRP pipe would be close to each other. When the HTS, ATS, SM, and EM results presented above were evaluated, it was observed that the results for samples taken from the same pipe exhibited a wide range of values. For example, in the HTS tests performed on samples containing 10% FA, the results varied between 763.1 N/mm and 1198.1 N/mm. To analyze the reason for the standard deviation of the test results, a section was taken from each type of pipe, as shown in
Cross-section and microscopic images showing cross-section details.
The microscope images in
Within the scope of the microstructural analysis, scanning electron microscopic (SEM) images of the FA-filled GRP pipes are given in
SEM images of the microstructure of the FA-filled GRP pipe.
The microstructures of all interfacial transition zones (ITZs) in a GRP pipe cross-section are given in
Optical microscopic images of ITZs in GRP pipe layers.
As seen in
The samples of reference GRP pipe and 20% FA GRP pipe were tested according to ASTM D3681. A schematic illustration of the test is given in
Schematic illustration of strain-corrosion test.
In the experiments, the samples were placed in the test apparatus with the measured wall thicknesses at the bottom and the force was applied to the apparatus to deflect the samples while keeping the top and bottom plates of the apparatus as nearly parallel as possible. When the desired deflection was obtained, the apparatus was locked to maintain the sample in the deflected condition. An addition of 1N sulfuric acid (H2SO4) was introduced into the test frame. The vertical deflection degree applied to the samples in order to achieve a failure at between 100 h and 600 h was calculated as 21.5%, which, through experience, was seen as an expected value.
Application of the strain-corrosion test on the GRP samples.
The pipe samples were checked and controlled at every 6 h. After exceeding the expected time to failure (100‒600 h), the strain-corrosion test was terminated after 1029 h. Initial observations showed that the FA content did not have a negative effect on the durability of the GRP pipe.
In addition to the above-mentioned application, after terminating the strain-corrosion test at 1029 h, the vertical deflection was applied to the samples until visual cracks and structural failure occurred in order to make a relative comparison between the mechanical performances of the pipes following the effect of an acidic environment.
In order to determine the visual crack deflection level (%) and structural failure deflection level (%), additional deflections were applied with a 1% increase in addition to the 21.5% initial deflection on the samples. Sample images of the deflection application with specific increases are seen in
Sample images of the deflection application.
The visual crack deflection level of the 20% GRP pipe sample was found to be 41.5%. Later, the number of cracks increased rapidly and the deflection level of the failure was observed as 46.5%.
Crack propagation and failure in the samples after acid effect.
The results obtained seemed to indicate that the performance of the GRP pipes containing FA after the effect of the acidic environment was acceptable, and similar to that of the reference GRP pipe. The slightly higher deflection values found in the GRP pipe containing FA compared to the reference GRP pipe were not sufficient enough to conclude that the performance of the FA-filled GRP pipe was better than the reference GRP pipe. This difference may have been related to the effect of the pipe layer thickness, as explained in section 3.2.
Since natural raw material resources are limited, the use of recycled materials in industrial production is very important for sustainability. Various types of recycled materials are currently being used in many branches of engineering. Civil engineering is one of the leading engineering fields in the utilization of large amounts of solid wastes. According to this experimental investigation carried out to evaluate the potential reusability of FA in the production of GRP pipes, the following conclusions can be drawn:
For all GRP pipes (including FA-filled GRP pipes), no visible damage was observed at a deflection level of 11.3% and no structural cracks at a deflection level of 18.9%.
The 10% FA substitution increased the SM results by 17.53%, while the 20% FA substitution increased them by 5.33%.
The 10% FA substitution decreased the HTS results by 3%, while the 20% FA substitution decreased them by 8%.
The 10% FA substitution increased the ATS results by 6.8% while the 20% FA substitution increased them by 1.47%.
When the results of SM, HTS, and ATS were evaluated together, it was difficult to conclude whether or not the FA substitution had increased the mechanical properties. However, all results showed that both 10% and 20% substitution ratios of FA were suitable for the production of GRP pipes that meet the related standards. Determining the adequacy of mechanical performance of FA-filled GRP pipes within the scope of related standards is more meaningful than comparing their performance with a reference pipe.
The SEM images showed that it was possible to achieve the homogeneous distribution of FA particles within the resin matrix without any agglomeration.
According to the microstructural analysis, sufficient interfacial bonding was observed between the resin matrix and the FA.
According to the ITZ analysis, in both in the REF and the FA-filled GRP pipes, the resin had penetrated into the rough surfaces and formed a good interfacial bond. It can be concluded that the use of FA does not adversely affect the bond between the layers.
The results of the strain-corrosion test indicated that the performance of the FA-filled GRP pipes after the effect of the acidic environment was acceptable, and similar to that of the reference GRP pipe.
The use of FA in GRP pipes reduces the demand on raw materials such as silica sand.
In addition to well-known conventional studies related to the use of FA in cement and concrete, the use of FA in GRP production presented in this study can open a new path for the consumption of FA in a useful way.
All findings of this experimental study indicate that the use of FA in GRP pipe production is possible. Low-lime FA (Class F, having CaO<10% according to ASTM C618) was used in this study. Because of its high pozzolanic activity, low-lime FA is commonly used rather than high-lime FA (Class C, having CaO>10% according to ASTM C618). It is recommended that future research should focus on the potential use of high-lime FA, which has a relatively low pozzolanic activity, in GRP pipe production.
The authors would like to thank the management of Superlit Pipe Industry Inc. for their support in producing and testing all GRP pipes.