Research on the performance of pavement crack repair materials based on foaming polyurethane

In this paper, Pucs, a new two-component foaming polyurethane material, is selected to carry out a feasibility study on its application in pavement crack repair. Based on the two common crack repair materials in current projects, tests of pavement performance of repair materials are carried out, such as bonding performance, water damage resistance performance, high-temperature performance, and aging resistance performance. The comparative analysis of the results reveals that Pucs have better pavement performance than conventional crack repair materials. This indicates that Pucs is a new kind of material with excellent performance in pavement crack repair and can be widely used in the field of crack repair.


Introduction
Crack is one of the most common diseases of asphalt pavement.If not treated as soon as possible, it will further extend after repeated application of traffic loads and rainwater, and lead to mud-pumping, void, breakup, potholes, and other damages [1][2][3] , which can seriously affect driving comfort and the service life of the road [4][5] .The SHRP points out that if pavement cracks can be repaired in time, the life of asphalt pavement can be extended by 10 to 20 years, and the maintenance costs can be saved by more than 50%.
At present, crack maintenance mainly focuses on crack pouring and surface filling processes for repair.Major repair materials are ordinary asphalt, modified asphalt, fiber asphalt, emulsified asphalt, rubber asphalt sealant, silicone jointing sealing compound, sand-gravel hot mix asphalt, etc, with common shortcomings of insufficient bonding strength with the original pavement cracks.Perhaps due to their poor elasticity and toughness as well as inconsistent deformation of the original pavement, they are prone to make the repaired pavement re-cracking and fail [6] .Thus, traditional crack repair materials cannot fundamentally solve the existing problems of pavement cracks.In recent years, polymer materials represented by polymer grouting technology as a new type of crack repair materials are gradually attracting attention, such as epoxy resin adhesives, modified polyurethane materials, polyurethane/epoxy resin interpenetrating polymer networks, and so on [7][8][9] .Studies have shown that these materials are effective in pavement crack repair.In this paper, Pucs [10] , a two-component foaming polyurethane material, is selected as a crack repair material.And the pavement performance of Pucs is evaluated through a comparative study of repair materials for conventional asphalt pavement cracks, such as bonding, water damage resistance, high temperature, and aging resistance, to study the application prospects of Pucs as a crack repair material.

Raw materials of pucs
In this paper, a solvent-free two-component polyurethane is used as a crack repair material, which is a kind of chemically reactive foaming polyurethane material.Its indexes before and after the reaction are shown in Table 1 and Table 2. Component A is a mixture of polyether polyols, which is a light yellow transparent liquid; component B is a mixture of isocyanates, which is a brown liquid and needs to be preserved in a lucifugal and dry environment.The material is environmentally friendly, non-toxic and harmless, and easy to operate.When in use, component A and component B are mixed in a volume ratio of 1:1, and 2% water is added as a foam initiator.Then, after being fully mixed, the material can be cured and formed naturally.The whole low-carbon and energy-saving process is carried out at ambient temperature without heating.

Experimental preparation
In this paper, the performance of Pucs is studied to repair asphalt pavement cracks.Therefore, all specimens are asphalt mixtures and an AC-13 gradation is adopted.The wheel track plate is cut into 5 cm×5 cm×5 cm cube specimens and 5 cm×5 cm×10 cm cuboid specimens for the pull-out test.The bonding area S is calculated based on the smaller cube specimen (S = 25 cm 2 ).The bonded pull-out test specimen is shown in Figure 1.In addition, the surface of the Marshall specimen is cut into a crack with a depth of about 15 mm and a width of about 5 mm to stimulate pavement cracks.The cut specimen is shown in Figure 2.After filling various repair materials in the crack, relevant performance tests can be carried out.

Performance of materials
At present, polymer-modified asphalt is a frequently used crack repair material in the market, and matrix asphalt is also used in low-temperature areas.In this paper, SBS-modified asphalt and matrix asphalt No. 70 are selected as the control group to verify the performance of Pucs.Also, the following types of tests are carried out: (1) pull-out test is used to examine the performance of Pucs as crack repair materials; (2) freeze-thaw splitting test is used to study its water damage resistance performance; (3) pull-out test at high temperature is used to study its high-temperature stability; (4) aging test is used to study its aging resistance.In these tests, 3 parallel data are tested for each group and the results are analyzed by taking the average value.

Bonding performance.
In this paper, the test method for the bonding strength, which is described in Appendix E of the Technical Guide for Design and Construction of Highway Steel Box Girder Bridge Deck Pavement issued by the Ministry of Communications, is imitated to test the bonding performance of Pucs.The test device and method for bonding strength are shown in Figure 3.The water damage resistance performance of the materials is evaluated by the tensile strength ratio (TSR) of the specimens.

3.2.3
High-temperature performance.In this paper, the high-temperature performance of the materials is evaluated by testing the bonding performance of the specimens at high temperatures, so the test method is almost the same as that of the bonding strength in 2.2.1, except that the test temperature is raised to 60C.

Aging resistance performance.
The UV aging test chamber is used to simulate the aging process of the materials under natural light irradiation.The aging condition is that UVA340 lamps continuously illuminate for 100 h with a radiation intensity of 200 w/m 2 .Then, according to the T0709-2011 Marshall stability test method of asphalt mixtures in JTG E20-2011 Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering, the Marshall stability of the specimens before and after aging is tested respectively.And the aging resistance of the materials is evaluated by analyzing the percentage decrease in stability before and after aging.

Bonding performance
The primary task of examining the availability of materials as crack repair materials is to examine their firm bonding to the original crack interfaces, otherwise, it is prone to cause interracial peeling and then repair failure.In this paper, Pucs is applied evenly on the bottom surface of the 5 cm×5 cm×5 cm cube specimen which then is bonded to the middle part of the 5 cm×5 cm×10 cm cuboid specimen.After curing at room temperature for 8 h, performance tests are carried out.The asphalt specimens of the control group are also formed with this method, and the binder dosage of all specimens is 4 g.The results of the bonding strength of each group of specimens at 25℃ are shown in Table 3.  3, it can be seen that among the three crack repair materials, Pucs has the highest bonding strength which is more than two times higher than that of SBS-modified asphalt and matrix asphalt No. 70, two conventional crack repair materials.This fully shows the excellent bonding performance of Pucs with asphalt concrete interfaces and also proves that it can be used as a crack repair material for asphalt pavement.

Water damage resistance performance
Water damage is the most common and the most serious disease that causes early damage to asphalt pavement, so the water damage resistance of a certain material can directly affect the application prospect of the material.In this paper, freeze-thaw splitting tests are used to evaluate the water damage resistance performance of Pucs.The crack simulation specimen in Figure 2 is filled with different repair materials after being tested, the splitting tensile strength of the repair specimens before and after freeze-thaw cycles is tested respectively, and then the TSR is calculated.The test results are shown in Table 4 and Figure 4.   4 and Figure 4, it can be seen that the order of the splitting tensile strength of the three crack repair specimens before and after freeze-thaw cycles is Pucs＞SBS modified asphalt＞matrix asphalt No. 70, which indicates that Pucs shows better structural strength before and after water damage.Based on TRS, it can be seen that after 2 freeze-thaw cycles, the splitting tensile strength of Pucs is almost unchanged (TRS=99%), the splitting tensile strength of SBS-modified asphalt decreases slightly (TRS=95%), and the splitting tensile strength of matrix asphalt No. 70 shows an obvious reduction (TRS=90%).Thus, Pucs has good water damage resistance which is better than conventional asphalt crack repair materials.

High-temperature performance
Asphalt is an amorphous polymer.Its fluidity increases with temperature, so the bonding performance of conventional asphalt crack repair materials are susceptible to decline due to softening at high temperature, which will cause repair failure.In this paper, pull-out tests at high temperatures are designed to examine the high-temperature performance of Pucs.The fabrication and testing devices of the specimens are the same as the pull-out tests at ambient temperature, and the testing temperature is raised to 60℃.The high-temperature performance of the materials is evaluated by the percentage decrease in their bonding strength at high temperatures.The test results are shown in Table 5.
Table 5 5, it shows that the bonding strength of Pucs remains the highest among the three crack repair materials at a high temperature of 60℃.Based on the test results of the bonding strength at ambient temperature in Table 3, it can be seen that the bonding strength of Pucs at 60℃ is still higher than that of SBS-modified asphalt at ambient temperature, showing good bonding performance and high-temperature performance.Through further comparative analysis of the data in Table 3 and Table 5, it can be seen that the bonding strength of the three crack repair materials is in a downward trend with increasing temperature.The bonding strength of Pucs, SBS-modified asphalt, and matrix asphalt No. 70 decreases by 11%, 21%, and 35%, respectively, among which the decent degree of Pucs is the smallest, which means that the high-temperature performance of Pucs is better than that of the conventional asphalt crack repair materials.

Aging resistance performance
Sunlight exposure is one of the important factors causing a decline in the durability of asphalt pavement, with ultraviolet rays having the greatest influence on the durability of the materials.So in this paper, the ultraviolet aging process is stimulated through UV aging chamber tests to evaluate the aging resistance of the materials.Marshall specimens repaired with different materials are subjected to ultraviolet aging under the set conditions, and then their Marshall stability is tested.The test results are shown in Table 6 and Figure 5.   6, the Marshall stability of the specimens is decreased after ultraviolet aging, and they are all subjected to ultraviolet aging to a certain extent.Among them, the Marshall stability of Pucs is the highest, and even the Pucs specimens after ultraviolet aging have higher stability than asphalt repair specimens before aging.According to the comparison of the Marshall stability before and after aging, it can be seen that the stability of the three materials decreases in the following order: Pucs＜SBS modified asphalt＜matrix asphalt No. 70.The results show that Pucs is a better crack repair material than asphalt and that Pucs has better aging-resistance performance.Pucs material is a kind of synthetic material.The addition of a UV absorber to component A raw materials of Pucs can effectively absorb some UV light, thereby delaying aging and exhibiting a better aging resistance.This also shows the variability and controllability of synthetic materials in terms of performance.

Conclusion
In this paper, Pucs, a new crack repair material based on foaming polyurethane is introduced, which is formed by on-spot mixing and reaction of polyether polyol components and isocyanate components at room temperature.Based on SBS-modified asphalt and matrix asphalt No.70, the two common crack repair materials, the pavement performance of Pucs is evaluated in terms of bonding strength, water damage resistance, high-temperature stability, and aging resistance.The conclusions are as follows: (1) With simple preparation and easy operation, Pucs material is safe, environmentally friendly, low-carbon, energy-saving, non-toxic, and harmless.
(2) Pucs material is cured and formed through a chemical reaction.With a fast curing time, it can be used as a rapid crack repair material.According to different temperatures, traffic can be opened within 2-6 hours, greatly shortening the road occupying time for repair.
(3) Under the same test conditions set in this paper, Pucs material shows better bonding performance than conventional crack repair materials.
(4) Under the same test conditions set in this paper, Pucs material demonstrates better water damage resistance and high-temperature stability than conventional crack repair materials.
(5) Under the same test conditions set in this paper, Pucs material exhibits better-aging resistance than conventional crack repair materials and can extend the service life of the road.

Figure 3 .
Figure 3. Test device and method for bonding strength.

3. 2 . 2
Water damage resistance performance.According to the T0729-2000 freeze-thaw splitting test methods for asphalt mixtures in JTG E20-2011 Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering, the frost and thaw condition is changed in the test to keep the specimens in -18C freezer for 12 h, and then keep them in 60C constant temperature water baths for 12 h.In this process, each specimen is freeze-thawed twice, and other operations refer to T0729-2000.

Figure 4 .
Figure 4. Test results of the freeze-thaw splitting test.According to the test results of freeze-thaw splitting tests in Table4and Figure4, it can be seen that the order of the splitting tensile strength of the three crack repair specimens before and after freeze-thaw cycles is Pucs＞SBS modified asphalt＞matrix asphalt No. 70, which indicates that Pucs shows better structural strength before and after water damage.Based on TRS, it can be seen that after 2 freeze-thaw cycles, the splitting tensile strength of Pucs is almost unchanged (TRS=99%), the splitting tensile strength of SBS-modified asphalt decreases slightly (TRS=95%), and the splitting tensile strength of matrix asphalt No. 70 shows an obvious reduction (TRS=90%).Thus, Pucs has good water damage resistance which is better than conventional asphalt crack repair materials.

5 .
(a) Marshall stability before and after aging (b) Percentage decrease in stability Figure Test results of aging resistance performance.According to the test results of aging resistance performance in Table

Table 1 .
Indexes of physical parameters of two-component polyurethane cementitious materials.

Table 2 .
Chemical reaction parameters and main performance indexes of foaming polyurethane.

Table 3 .
Test results of bonding strength at ambient temperatures.

Table 4 .
Test results of freeze-thaw splitting test.

.
Test results of the bonding strength of different repair materials at high temperatures.

Table 6 .
Test results of aging resistance performance.