Study on the cohesion and adhesion of hot-poured crack sealants

Abstract

Filling crack sealant is a main method to repair cracking of pavement. The cohesion and adhesion of crack sealant directly determine its service performance and durability. However, the competitive mechanism of cohesion and adhesion failure modes is not clear currently. This research proposed two methods to evaluate cohesion and adhesion of crack sealant, and analyzed the influence of temperature on cohesion and adhesion. The effect of moisture on low-temperature performance of crack sealant was also be evaluated by conducting a soaking test. Results show that with the decrease of temperature, the cohesion force of crack sealant increases significantly, while the adhesion force changes little. There is a critical temperature at which the cohesion force equals the adhesion force. When the temperature is higher, the adhesion force will be greater than cohesion force, and the cohesion failure will happen more easily. In contrast, the adhesion failure will happen more easily when the temperature is lower than the critical value. Soaking in 25 °C water for 24–48 hours will slightly improve the low-temperature tension performance of crack sealant. However, soaking in 60 °C water for 24 hours will decrease the failure energy of low-temperature tension and damage the durability of crack sealant.

Keywords

crack sealant / concrete pavement / cohesion / adhesion / moisture damage

Introduction

In the long term use of roads, factors such as volume of traffic, overloading, water damage and air temperature can cause pavement cracks and may result in more severe damage if it is not repaired in time. Some of the cracking is visual, but more of the cracking is hard to be found. The latter cracking occurs in the interior and base courses of road structure and finally results in pot holes or even damage of a subgrade. If crack repair is conducted before virtual damage, the service life of a road will be extended. Filling crack sealant is the most common method to repair pavement cracks [1–3].

The crack sealant applied in cement or asphalt pavement is usually composed of polymer modified asphalt. A good crack sealant should have been possessed with the basic performances including the strong bonding property with the concrete surface, and to ensure low fluidity at high temperature and low brittleness at low temperature.It should also have good durability to extend the service life of the crack sealant. In addition, it should have good anti-embedded, flexibility, adhesion, water resistance, aging resistance, temperature stability and so on. The cost of crack sealant should not be too high and it should have the advantage of easy construction. Two of the most common forms of crack sealant damage are cohesion and adhesion damage destruction. How to accurately evaluate crack sealant cohesion and adhesion performances based on test method is the key to determining the crack sealant durability [4–12].

In 1996, Zanzotto [13] proposed a test draft used for evaluating adhesion and stress relaxation ability of crack sealant at a low temperature (−30 °C). In the draft, stress relaxation was measured by the following method: a cylinder sample was stretched by 50% of its original length at a speed of one millimeter per minute and then the stretched length was maintained for one hour. The load size after finishing a test was recorded. The method to determine adhesion is: to pull crack sealant away from a concrete surface at a constant speed of 10 millimeters per minute. 14 kinds of crack sealant were tested and analyzed indoors and then a correlation analysis between the results and their conditions in practical application was carried out.

In 1997, Abd El Halim et al. [14] evaluated tensile and compressive properties of crack sealant in heating construction and normal-temperature construction at different temperatures with displacement control tests. In 1999, Al-Qadi et al. [15] evaluated crack sealant properties of rigid pavement with periphery-restrained cyclic shear tests. In 2005, Worms and Shalaby [16] estimated heating-construction-type crack sealant at three temperatures with tension-compression cycling tests and it showed that the results of the tension-compression cycling tests had a good correlation with the properties of crack sealant in practical engineering.

In 2005, Al-Qadi et al. [17] found out that a standard BBR test was not applicable to flexible crack sealant since the test error was out of the permitted range of instrument and suggested thickening samples to overcome the problem. In 2005, Al-Qadi et al. [18] assessed asphalt crack sealant with a modified BBR test and graded the crack sealant with creep stiffness, the change rate of stiffness (value m), the constant creep rate and the average creep rate in second 240. In addition, they found that there was a good consistency among different index parameters.

In 2008, Haithem et al. [19] attempted to evaluate the low temperature performance of eight kinds of heating construction type crack sealant based on a Dynamic Shear Rheometer (DSR) and Bending Beam Rheometer (BBR). The evaluation indexes adopt creep stiffness, the change rate of creep stiffness, the change rate of composite modulus temperature and crack sealant index. The results demonstrate that the crack sealant index can be used in the proposed evaluation standard and it can quickly make a cost-effective decision for the practical engineering. In 2009, Al-Hadidy and Tan [20] developed a heating construction type crack sealant, made by the process that the 30% of the starch was added to a 20% CA (citric acid) modified bitumen and simply stirring them above 160 °C. In accordance with ASTM procedures, a range of different test methods can be adopted to evaluate the crack sealant performance in the asphalt industry. In addition, according to the US Strategic Highway Research Program (SHRP), researchers had selected three pilot sites in Northeast China’s Heilongjiang province and evaluated the crack sealant performance after using the crack sealant for 12 months and 18 months respectively. The studies have proved that crack sealant could meet the requirements of ASTM D6690. And practical application has demonstrated that crack sealant was a good material for pavement crack repair which is also considered as the most common material for road maintenance. The results show that the storage and construction temperature will be decreased by 12-12.5% because of the modified starch crack sealant, and it has good resistance to oil and possesses chemical solvents capacity compared with the rubber powder modified asphalt crack sealant.

In 2006, Zhai and Salomon [21] evaluated low temperature properties of crack sealant by applying a direct tensile test (DTT) and the evaluation indexes were breakdown stress and temperature variation stress at-29 °C. The temperature variation stress was the both ends of fixed DDT samples and it was induced by gradual temperature reduction from 5 °C to-30 °C. In this way, it simulated the temperature stress put on constraint samples. Hou et al. also conducted researches on asphalt-like materials [22–25].

This study mainly focuses on the cohesion and adhesion performance of crack. The mechanism of competition failure of crack sealant and the effect of moisture on cohesive performance were also analyzed.

Materials and test methods

Materials

Materials used in this research mainly include sealant and cement concrete. The sealant used in this research was made by the authors in lab [10]. The compressive strength of cement concrete used to evaluate the adhesion property of sealant is higher than 30 MPa.

Laboratory tests

The crack sealant tensility is restricted by various aspects of factors. Due to the lack of cohesion, crack sealant will cracking itself. Besides, as the lack of bond force between the crack sealant and the concrete wall will cause both detachment (Fig.1), the cohesion and the adhesive force are becoming as two extremely important indexes for crack sealant. Therefore, this paper studied the influence of temperature change on the cohesion and the adhesive force, and analyzed the reason about the crack sealant two of the most common failure modes (the adhesion failure and the cohesion failure). At last, the energy method was adopted to conduct preliminary studies on the influence of water on the crack sealant tensility performance.

Cohesion test

Through high precision professional pull-off machinery, the testing instrument could be applied to steel deck pavement test on the evaluation methods of adhesion capability between pouring asphalt concrete and steel deck, which was used in determining cohesion of crack sealant. The specific test is described as follows:

(1) The test device

The test device is shown in Fig. 2.

(2) The test method

① Test temperature:

The same crack sealant was tested at 25 °C, 5 °C, -4 °C, -10 °C, respectively.

② The test instruments:

Pull-off test instrument, freezer, thermometer, brush, 4.5×9 rectangle steel plate test mould, self-made cement concrete block;

③ The basic steps of the test:

– Put self-made concrete block in a horizontal position and fix it. Wipe the surface of steel plate and concrete block cleanly. Test the same kind of crack sealant at 25 °C, 5 °C, -4 °C, -10 °C, respectively;

– Put the crack sealant under the test. Heat to the flow state, and brush a layer on the surface of steel plate and concrete respectively with taking proper crack sealant;

– Press the rectangle steel plate test mould on the surface of self-made concrete block hard which had been brushed crack sealant. After cooling down, scrape off the redundant crack sealant around the rectangle steel plate, and it should stand at the room temperature for 24 hours;

– Open the pull-off test instrument for preheating and start the testing after reset. Shake screw slowly to put the upward pull-off effect on the contact surface, and pay attention to the readings on the digital displays at the same time. Then, write down the reading after the reading becoming stable, and the value was the maximum pull-off force F_max for the material;

– The resistance pull-off strength index σ_s was calculated and analysed according to the following formula (1).

(1){\sigma }_{\mathrm{s}}={\displaystyle \frac{F_{\max }}{S}}

The failure area S=4.5×9.5=42.75 cm².

– Parallel test four averaged under the same conditions.

Adhesion test

Adhesion failure was one of the most common damage modes of crack sealant. Therefore, it must have a good adhesion property between the crack sealant and the concrete wall to adapt to all kinds of load and the cyclic change of the temperature and would not depart from the concrete wall. At the current stage, there were less test methods for the self-adhesion property evaluation of crack sealant,and they had not been formed uniformly. And in the specification,it just mentioned the comprehensive evaluation of the low-temperature tension testing of crack sealant concerning its low-temperature property. There was no description of the evaluation methods about adhesion capability for the crack sealant and the concrete wall. Therefore, this paper attempted to use PosiTest to test the crack sealant.

(1) The test device

The test device is shown in Fig. 3:

(2) Test method:

① The operating principle of PosiTest is to test adhesion force of coating on surface of substrate by using the hydraulic pressure according to ASTM D4541-95 and ISO 4624 standards.

② Test temperature:

The same crack sealant was tested at 25 °C, 10 °C, -5 °C, -10 °C, -15 °C, respectively.

③ The test instruments:

PosiTest, freezers, thermometers, spindle, cutter, polishing pads, self-made cement concrete block.

④ The basic steps of the experiment:

– Remove oxides and dirt at the bottom of the spindle. The spindle to the polishing pad friction 4-5 times and then wipe it with a dry cloth or tissue ;

– Use a polishing pad to wipe the surface coating gently. To make sure the spindle stick to coating, wipe the grease and dirt on the surface coating using alcohol or acetone.

– Put the same amount of two kinds of adhesion agents on the glue board, and stir them with a stirring rod.

– Apply Stir adhesive to the spindle flat bottom smoothly, and the thickness of the adhesive is about 50-100 microns. Then, the spindle was stuck to the test coating region and pressed down gently. The excess adhesive will be extruded and the adhesive around the spindle edge will be wiped by the a cotton swab carefully. Finally, wait for the adhesive solidification (at least 24 hours).

– Use cutter to cut the coating along the edge of the spindle,and clean the clastic which was produced by the cut process;

– Make sure the pressure valve of hydraulic pump is opening, press the pull-off device handle into the pull-off device fuselage totally, put the sleeve on the top of the spindle and then close the pressure valve of hydraulic pump;

– Start up, choose the size of the spindle,measurement unit and reset. Use hydraulic pump handle to pressure with the speed of 1MPa per second until the spindle was pulled up from the surface of coating;

– Write down the reading, opening the pressure valve of hydraulic pump, and remove the spindle from the fast plug.

Low-temperature tension test

(1) Test machine

Tensile testing machine: this machine is electric and its distance velocity is 0.05 mm per minute. the MTS-810 materials testing system is used in this paper.

Cryogenic equipment: thermostat can control temperature at-10 °C ~ (30±3) °C. MTS-810 material test system comes with the thermostat that is used in this research and can automatically maintain a temperature of −10 °C.

Cement mortar block: there are several pieces of cement mortar blocks that the size is 120 × 60 × 34 mm and the compressive strength is greater than 30MPa.

Joint slab: its size is 15 × 20 × 100 mm.

Others- stopwatch, thermometer.

(2) Test process

A joint slab and two baffle (size 10×15×70mm) were put between two cement mortar blocks and fixed by fixture so that the groove (size 15×40×100mm) between two cement blocks was formed (As shown in Fig. 4).

The prepared crack sealant was poured into the test mode. After curing 24 hours under 15−30 °C temperature, the baffle and joint slab were removed. The different samples were cured for 24 hours (or 48 hours) by immersed in water which the temperature at 25 °C and 60 °C. Then the samples were taken out of the water and drying their surface. Again the samples were cured for 24 hours at 25 °C, and finally the MTS tester was used for tensile test to collect and analyze the response curves under the 0 °C test temperature and 50 mm/min tensile rate.

Results and discussion

Cohesion property of sealants

The crack sealant cohesion results of this study are shown in Table 1 and Fig. 5.

As shown in Table 1 and Fig. 6, pull-off strength of crack sealant will change with the temperature, the change rate of pull-off strength is relatively small when the temperature is more than 4 °C, however, the pull-off strength will increase rapidly as the temperature decreases when the temperature is less than 4 °C, which means that the pull-off strength of crack sealant cohesion is improved.

Adhesion property of sealants

The crack sealant adhesion results of this study are shown in Table 2 and Fig. 6.

The test results can be seen from Table 2 and Fig. 6, the adhesion of crack sealant and concrete wall is not sensitive to temperature. It will increase slightly as the temperature decreases.

Analysis of failure mode of low-temperature tension property of sealant

According to the results of Sections 3.1 and 3.2, the conclusions are as follows:

An internal stress may appear inside a material because of various factors. In addition, internal stresses caused by temperature decrease and contracting and stretching crack sealant by concrete are the main factor resulting in failure of crack sealant in winter. Since these internal stresses increase with the temperature decrease, they are called the temperature stress.

With the decrease in temperature, the temperature stress increases gradually. Mark the changes of the temperature stress showed as yellow, green and blue curves in Fig. 7, if the temperature stress first intersects the cohesion curve (yellow), it indicates that the temperature stress first exceeds the cohesion permitted by the material and the fracture failure of the crack sealant occurs in the meantime; if the temperature stress first intersects the adhesion force curve (green), it shows that the temperature stress first exceeds the adhesion force between the crack sealant and the concrete wall and failure, caused by separation of the crack sealant from the concrete wall, appears at the same time; if the temperature stress never intersects the two curves (blue), it manifests that the temperature stress is within the tolerance limit of crack sealant and the crack sealant will not fail.

Effect of moisture on low-temperature tension property of sealant

The low temperature tensile results of the crack sealant at different soaking times are shown in Fig. 8. Each area surrounded by the X-axis and the each curve represents the work done by this procedure. The area is defined as failure energy in this paper. The pink area w in Fig.8 means the work done of crack sealant which is immersed in 60 °C water. The failure energy in four situations is shown in Fig. 9.

Fig. 9 shows that soaking the crack sealant for 24 hours or 48 hours at 25 °C water has little or no effect on the tensile properties of crack sealant, however, the crack sealant tensile properties will be weak significantly after soaking 24 hours at 60 °C water, which reflects that the entire curve moves down and the area surrounded by the curve decreases. The slight increase of soaking in 25 °C water for 24-48 hours may be due to that the soaking can increase the stiffness of crack sealant by the interaction between water and sealant molecules.

Conclusions

A steel plate pull-out tester was used to determine the cohesion of crack sealant at different temperatures. The test results showed that the cohesion was sensitive to temperature variation and the cohesion increased rapidly with the decrease of temperature under 4 °C. An adhesion tester with a pulling and separating method was utilized to determine adhesion forces between the crack sealant and concrete walls at different temperatures. The test results indicated that adhesion forces were not sensitive to temperature variation. In combination with causes of tensile failure modes of crack sealant obtained above, it can be seen that if a temperature stress exceeds the cohesion or the adhesion force, a corresponding failure will occur. Immersion for 24- 48 hours at 25 °C slightly enhances tensile ability of crack sealant, while immersion for 24 hours at 60 °C damages tensile ability of crack sealant.

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