Polyurea has a high tensile strength, elongation, and the capability to absorb the energy generated by dynamic and impulsive blast loading. Glass fibers are a reinforcement material for repairing and retrofitting the concrete members. The polyurea provides ductility, and the fibers provide improved stiffness and strength to the composite system. Glass-fiber reinforced polyurea (GFRPU) is a composite of polyurea and fibers and is applied as a reinforcement through a simple spraying method. GFRPU coating has a simple construction, and unlike existing strengthening methods such as fiber-reinforced polymer (FRP) or a steel plate, it prevents a debonding from the concrete surface. Seven beams of one externally nonreinforced concrete beam and six concrete beams with and without a reinforcing bar are tested using the thickness of the spray and the number of coating faces. The applicability of GFRPU was investigated through the experiments, and the test results indicate that the GFRPU strengthening method is feasible for enhancing the load-carrying capacity and flexural ductility.
The structural performance of a reinforced concrete structure can deteriorate owing to an unexpected change in the external load or environment during the service period. Aged and deteriorated members, such as those at the end of their service life or undergoing steel corrosion or concrete spalling, should be repaired to enhance their strength and durability. The structural performance needs to be recovered for continuous and safe occupancy. The building structures need to implement effective and economical repair and strengthening methods.
The structural performance can be recovered using retrofitting techniques corresponding to the increased loading requirements, change in use, and structural deterioration. Structural strengthening techniques contain a section enlargement, reinforced jacketing, externally bonded steel elements, or FRPs, among other factors.
Numerous studies have attempted to introduce repair and retrofitting methods, such as a steel plate or FRP reinforcement method, to enhance the deteriorated strength and ductility. Steel plates have been utilized in the strengthening of concrete beams owing to their economic and ductile characteristics. Barnes et al. [
FRPs have been effectively utilized in structural engineering as internal and external reinforcements. FRPs are made up of high-tensile-strength fibers embedded in an epoxy matrix. An explicit bonding for a sufficient adhesive ability between the FRPs and concrete structures is required for transferring the stress along them. FRPs with a high tensile strength and low weight have failed through a concrete cover separation and interfacial debonding.
Gideon and Alagusundaramoorthy [
One of the weaknesses of an FRP composite is a poor fire performance, leading to an interface debonding between the FRP composites and concrete substrates. There have been many attempts to investigate FRP debonding and intermediate crack bonding. Oehlers et al. [
An elastomer is a class of polymetric materials. Polyurea as an elastomer is an excellent water-proofing material with many mechanical characteristics, such as a high tensile strength, ductility, and high rate of expansion and contraction. It has the capability of flexural and shear reinforcement for structural members rather than blast or impact mitigation [
Glass fiber is divided into two types of chopped and milled glass fibers. The milled glass fibers are created by cutting E-glass fibers into shorter pieces. E-glass fibers have an excellent electrical insulation property and can be processed into various shapes and are mainly applied in the reinforcement of plastics. This is effective in improving not only the strength but also the surface condition and dimensional stability. They are also used as a reinforcement and filler medium in a plastic composite, adhesives, and coatings used to enhance the mechanical properties, increase the modulus, improve the dimensional stability, and minimize the distortion under elevated temperatures. Chopped glass fibers are longer fibers used to increase the tensile and compressive properties of any resin and building materials including concrete.
GFRPU is a composite using a strengthened elastic polyurea material and milled glass fibers. The GFRPU coating systems can yield a multihazard retrofitted material suitable for aging structures and can be used in repair and retrofit applications for strengthening the structural capacity, improving the seismic performance and mitigating the blast and impact damage. Greene and Myers [
In the existing fiber-reinforced polymer sheet (FRPS) methods, the FRF and polyurea are separately constructed on the members. In this study, milled glass fibers are utilized as reinforcing fibers, and the strengthening is completed by simply spraying the GFRPU without the process to bond the fiber on the members. This study was planned for evaluating the strengthening effect of the GFRPU, and the validity was investigated in the concrete beam test. A GFRPU coating is applied by spraying the premixed polyurea and glass fibers. Seven beams, namely, one externally nonreinforced concrete beam and six concrete beams with and without a reinforcing bar, are tested based on the spraying thickness and number of spraying faces. The superiority and applicability of the GFRPU for strengthening concrete members are illustrated in the experiments, and the test results indicate that the GFRPU may enhance the load-carrying capacity and flexural ductility.
GFRPU is a composite manufactured using the polyurea made of a prepolymer and a hardener and milled glass fibers with a length of 300
Chemical constituents of polyurea.
Chemicals | Content (%) | |
---|---|---|
Prepolymer |
|
60∼70 |
ar,ar-Diethyl-ar-methylbenzenediamine | 20∼30 | |
Poly[oxy(methyl-1,2-ethanediyl)], |
1∼10 | |
Titanium dioxide | 1∼2.7 | |
1,4-Benzenedicarboxylic acid, bis (2-ethylhexyl) ester | 1∼10 | |
etc. | 1∼10 | |
|
||
Hardener | Polyurethane resin | 90∼100 |
4-Methyl-1,3-dioxolan-2-one | 1∼10 |
The GFRPU spraying proceeds as follows. First, the primer is painted onto the surface of the concrete member, as shown in Figure
GFRPU coating operation of (a) primer painting; (b) mixing of prepolymer and fibers; (c) GFRPU spraying; (d) primer painting.
Seven concrete beams, one without an external reinforcement as a control, three without reinforcing bars, and three fully reinforced concrete beams were prepared for testing. The six specimens other than the control beam were strengthened using polyurea or GFRPU. Previous experiments have indicated that the optimum weight ratio of milled glass fibers of 300
(a) Experiment of coated beam (unit: mm) of specimen under two-point loading, (b) strengthening at three faces without reinforcing bar, and (c) strengthening at a single face with reinforcing bar.
Specimen sign.
The four-week compressive strength of concrete after concrete casting was shown to be 43.07 MPa. AD10 reinforcing bar with a yield strength of 493 MPa is utilized. The test beams are strengthened using polyurea or GFRPU applied three days in advance. The polyurea and GFRPU act as a lateral reinforcement and lead to an enhanced strength and ductility.
Steel forms of 150 mm × 150 mm × 550 mm for testing the flexural strength of concrete were prepared, the concrete was casted into the required forms, and the concrete was cured after demolding during three days. The loading of the specimen was applied at two points, as shown in Figure
Setting of the specimen in UTM.
The initial crack began at the bottom face of the midspan section because of the low tensile strength of the concrete. The failure modes of the U-series specimens are shown in Figure
Failure modes of U-series specimens of (a) U31, (b) U51, and (c) U53.
The GFRPU exhibits a similar mechanical behavior as the tensile reinforcing bar. The increase in tension-resisting region and capacity leads to an enhanced peak load-carrying capacity. The specimens reaching the second peak load carry the tension force through the polyurea or GFRPU. The development of diagonal cracks can rarely be observed by the naked eye. The additional cracks and their widths are gradually propagated within the neighborhood of the midspan region with an increase in load. The polyurea or GFRPU controls the development without peeling from the concrete surface. Eventually, the strengthened specimens exhibit flexural failure modes.
The R-series specimens also represent a similar mechanical behavior as the U-series. The specimens ultimately fail through a flexure because they retain a sufficient concrete-carrying shear strength. The R51 specimen shown in Figure
Failure modes of R-series specimens of (a) RP, (b) R33, and (c) R51.
Tables
Summary of test results (without reinforcing bar).
Specimen | Load-carrying capacity | Flexural tensile strength ratio | Flexural ductility | ||
---|---|---|---|---|---|
Peak (kN) | Ratio | Ductility ( |
Ratio | ||
U | 29.17 | — | 1 | ||
U31 | 43.01 | 1 | 1.47 | 248.7 | 1 |
U51 | 42.37 | 0.99 | 1.45 | 249.35 | 1.00 |
U53 | 45.94 | 1.07 | 1.57 | 474.37 | 1.91 |
Summary of test results (with reinforcing bar).
Specimen | Load-carrying capacity (exp.) | Load-carrying capacity (anal.) | Flexural ductility | |||
---|---|---|---|---|---|---|
Peak (kN) | Ratio | Peak (kN) | Anal./exp. | Ductility ( |
Ratio | |
RP | 61.65 | 1 | 69.8 | 1.13 | 298.2 | 1 |
R33 | 77.17 | 1.25 | 80.8 | 1.05 | 1,085.7 | 3.64 |
R51 | 73.65 | 1.19 | 76.6 | 1.04 | 999.6 | 3.35 |
The R-series specimens exhibit a flexural failure mode by flexural cracks in the pure bending region. The flexural strength of the GFRPU-reinforced concrete beams corresponds to 1.19-and 1.25-times that of the control beam without the GFRPU reinforcement. The increase in flexural strength among the RP, R33, and R51 specimens shown in Table
The analytical flexural capacity of the GFRPU-reinforced beam shown in Table
GFRPU-reinforced concrete beam.
The area of the GFRPU in tension is calculated using
Substituting equation (
Taking the moment at the centroid of the reinforcing bar, the nominal flexural strength can be derived by
If the mechanical properties of the GFRPU are given, the nominal flexural strength of the specimens can be calculated, as listed in Table
Figure
Load-deflection curves of (a) U-series and (b) R-series.
Figure
Figure
Peak load ratio of (a) U-series and (c) R-series; ductility ratio of (b) U-series and (d) R-series.
The structural performance of the GFRPU composite of polyurea and glass fibers was evaluated in this study. The GFRPU reinforcement can prevent an abrupt concrete spalling and failure by debonding. In addition, it is simply completed through a spraying method. The GFRPU reinforcement leads to an enhanced load-carrying capacity as well as an improved flexural ductility. Assuming the use of GFRPU as a tensile reinforcing bar, the analyzed flexural strength can properly predict the strength found experimentally. The experimental results on the flexural strength are more sensitive to the number of spraying faces than the spraying thickness because the side-reinforcement provides a greater flexure-resisting capacity. In addition, it has been estimated from the analytical results that the GFRPU reinforcement retains the shear-resistance capacity.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This study was supported by the 2018 Research Grant (PoINT) from Kangwon National University.