Innovative test method for the reliable evaluation of joint sealants in concrete pavements

David Ov, Research Assistant, Institute for Building Materials, Ruhr University Bochum, Germany

Rolf Breitenbücher, Full Professor, Institute for Building Materials, Ruhr University Bochum, Germany

Martin Radenberg, Full Professor, Chair of Pavement Engineering, Ruhr University Bochum, Germany

Dominik Twer, Former Research Assistant, Chair of Pavement Engineering, Ruhr University Bochum, Germany

Corresponding author: David.Ov@rub.de

KEYWORDS: concrete pavements, joint sealants, evaluation, test method

Conflict of Interest: None

1. ABSTRACT

Joint sealants as indispensable filling systems in jointed plain concrete pavements (JPCP) are permanently exposed to various stresses during their service life, which often leads to a replacement of the sealing after approx. 7 to 10 years. Aside from seasonal unsteady climatic changes, the cyclical stresses caused by traffic and the ageing of joint sealants are especially significant. Considering the rising number of damages that occur within the overall "joint" system, an increased demand for a durable solution is requested as it is a relevant element for the life cycle costs of concrete pavements.

In this context, a testing and ageing method was developed which comprises of the entire "joint" system, including the saw-cut concrete joint flanks, the primer as well as the joint sealant. This procedure depicts the decisive scenarios of in-situ stresses and allows the characterization of joint sealants. For this purpose, specimens were subjected to horizontal and vertical loads (static/cyclic) as well as to various ageing effects (temperature conditioning, UV-conditioning and freeze-thaw-cycles). After conditioning, a significant influence of the artificial ageing on the residual strength was observed in the tensile/shear tests. By comparing the artificially aged samples tested in the laboratory with extracted and in-situ aged samples, a reliable correlation was determined.

Considering these system tests an initial approach was established which enables the evaluation of joint sealants in both unaged and artificially aged conditions on the basis of scientific parameters and limits.

2. INTRODUCTION

Plain concrete pavements are generally designed in jointed forms to avoid uncontrolled cracking. In order to ensure a long-term behaviour and durability of such concrete pavements, it is essential that the joints are effectively sealed with suitable sealing compounds. Joint sealants prevent in particular the intrusion of dirt, surface water and pollutants into the construction and the subgrade. At the same time, they are exposed to various stresses during their service life, such as seasonal climatic influences, cyclical loads caused by traffic and additionally the ageing of the joint sealant compounds. Under these circumstances, joint sealants and their long-term effectiveness are essential for the life cycle costs of concrete road pavements.

In recent years, the number of damages in such joint sealants has increased, which led in sequence to enhanced demands on the durability of the general joint system. Deficiencies have been found in the hot-applied N2-type joint sealant compounds in particular in the form of leaks caused by separation of the joint sealant compound from the joint surfaces and also in the form of cracks in the joint filler itself (Fig. 1). Both types of damages could be observed during regular joint maintenance and thorough repairs of existing roads as well as during the application of joint sealants in newly constructed roads. Since these deficiencies frequently occur along entire contracted sections, it can be assumed that this is less likely to be due to individual failures such as incorrect application and more likely attributable to systemic faults.

Fig. 1. Detachment (left) and brittle surface (right) of joint sealant compounds

According to the current applicable regulations in Germany, in particular the Additional Technical Contractual Terms and Guidelines for Joint Fillings in Road Surfacing [1], the Technical Testing Standards for Joint Fillers in Road Surfacing [2] and the Technical Delivery Conditions for Joint Fillers in Road Surfacing [3], the joint sealant and its primer are only separately inspected to determine their effectiveness as joint sealants. Joint systems as a complete unit, comprising the saw-cut concrete joint surface, the primer and the joint sealer, have so far not been sufficiently investigated in a holistic concept. Furthermore, not all of the principal stresses that a joint filling system is subjected are realistically reflected in the laboratory tests by now.

In this context, the aim of an extensive research project was firstly to establish the causes for the current problems with hot-applied joint sealant compounds, and secondly to develop a combined testing and ageing method based on scientific principles that enables joint sealant systems to be evaluated as a complete unit, taking into account the main effects that they are exposed to. Such a technically comprehensive evaluation of the joint system is intended to ensure that joints can be planned and produced for a much longer service life in the future, with the result that concrete road pavements can become more robust and cost-effective.

3. INVESTIGATION PROGRAM

Development of a test method

In order to develop a suitable test method that also can consider adequate accelerated ageing scenarios, parameter studies were initially carried out involving a total of five hot-applied and two cold-applied joint sealants. For the hot-applied joint sealant compounds, rheological and other physical parameters were determined beforehand. Besides the detailed collection of data on the condition of the joints themselves, the studies also focused on systematic laboratory tests regarding ageing behaviour and possible damage mechanisms on the entire joint system. Furthermore, on three representative road sections (A2, A40 and A48 as federal autobahns in Germany), in-situ core samples from the joint areas were additionally extracted for further investigations alongside the laboratory tests. The combination of laboratory and in-situ obtained results provided the fundamentals for the development of an adequate combined test method.

Thus, the comprehensive test method needed to incorporate vertical as well as horizontal joining movements. In order to simulate both movements in a test, a specific system test sample with a joint inclined at an angle of less than 70° to the vertical axis was developed (Fig. 2). This designed system test specimen can optionally be extracted from core samples.

Fig. 2. System test samples for pressure/tension/shear tests (static and cyclic)

Under consideration of the relevant ageing influences on the entire joint system during its service life, an ageing scenario was created with which a selection of the specimens (produced in the laboratory) were artificially aged. This ageing process referred as V1, in which pressure and temperature stresses (PAV conditioning), UV exposure and freeze-thaw stresses (CDF test) are used, depicts the main ageing factors realistically (Tab. 1).

Tab. 1. Overview of the basic conditions and sequence of artificial ageing V1

Artificial laboratory ageing (V1) of joint sealant compounds

Basic conditions and sequence

1) Pressure and temperature stresses (65 h; 85 °C; 2.1 MPa)2) UV exposure (11 days)3) Freeze & thaw stresses (14 days)

In order to conduct the experimental investigations, a test setup has also been developed which allows the simultaneous testing of a total of three specimens (Fig. 3). Therefore, it was possible to perform static (tension/shear tests) as well as cyclic stresses (pressure/tension/shear tests) with controlled deformation on the system test samples. Due to the integration of the test setup in a climatic cabinet, the tests could be conducted at different temperatures of -20 °C, +20 °C and +60 °C.

Fig. 3. Test setup for static and cyclic pressure/tension/shear tests of three specimens

Test results

For the static tension/shear tests and the cyclic compression/tension/shear tests, which were the focus here, a test temperature of -20 °C proved to be the most relevant for the hot-applied joint sealants. This is because the stiffness and thus the stresses increase due to the behaviour of the thermoplastic material at this temperature, so that a sufficient differentiation between different materials is possible.

Within the scope of the investigation program, the following joint sealants were tested:

FM 1 – hot-applied joint sealant (type N1)

FM 2 – hot-applied joint sealant (type N2)

FM 3 – hot-applied joint sealant (type N2)

FM 4 – hot-applied joint sealant (type N2)

FM 5 – cold-applied joint sealant (class 25)

FM 6 – hot-applied joint sealant (type N2+)

FM 7 – cold-applied joint sealant (class 35)

A comparison of study results on non-aged reference samples (REF) and artificially aged samples (V1) showed that the conventional hot-applied joint sealants developed much higher maximum tension values with significantly reduced expansion capacity after artificial ageing (Fig. 4). On the other hand, a modified variant of this joint sealant compound (N2+-type, FM6) demonstrated much better stress-strain behaviour even at the lower test temperature.

Fig. 4. Tension curve for hot-applied joint sealant compounds FM1 (N1-type), FM2-4 (N2-type) and FM6 (N2+-type) as REF and V1 in static tension/shear test at -20 °C

The artificial ageing and the involved pressure and temperature stresses (PAV conditioning), which is primarily designed for bituminous cements, turned to be less reliable with cold-applied joint sealant compounds (FM5 and FM7). This was expressed in the form of significantly reduced tension values in the aged samples (V1) in comparison to the non-aged reference samples (REF), see Fig. 5. The cause of this is presumed to be the breakdown of the chemically networked polymers into shorter polymers or monomers as a result of the effect of pressure and temperature. Therefore, the artificial ageing process for cold-applied joint sealant compounds must be modified accordingly.

Fig. 5. Tension curve for cold-applied joint sealant compounds FM5 (class 25) and FM7 (class 35) as REF and V1 in static tension/shear test at -20°C

The stiffening of the hot-applied joint sealant compounds caused by the accelerated ageing under pressure and temperature stresses (PAV conditioning) was also demonstrated by the corresponding changes in the phase angles and complex shear moduli calculated in the dynamic shear rheometer (Fig. 6).

Fig. 6. Complex shear modulus and phase angle for REF (left) and PAV-conditioning (right) at 0.1 Hz

In the case of cyclic loads after artificial ageing, the maximum stress in the pressure/tension/shear test with the N1-type joint sealant (FM1) was not increased, but the expansion capacity was further reduced (Fig. 7). In contrast, the additional cyclic stress after artificial ageing caused a more or less significant rise in maximum tension with the N2-type joint sealant compounds. The modified hot-applied N2+-type joint sealant compound (FM6) was least affected under these conditions, thus clearly underlining the efficiency of this modification.

Fig. 7. Tension curve for hot-applied joint sealant compounds FM1 (N1-type), FM2-4 (N2-type) and FM6 (N2+-type) as V1 and V1 cyclic at -20°C

Since the applied method of artificial ageing using pressure and temperature stresses (PAV conditioning) for cold-applied joint sealant compounds has proven to be relatively ineffective, further static tension/shear tests and cyclic pressure/tension/shear tests were carried out for these joint sealant compounds (FM7) without PAV conditioning, simply in order to provide a rough guide. This revealed that the remaining ageing steps (UV exposure and freeze & thaw stresses (CDF)) have practically no effect on the relation between stress and strain of this cold-applied joint sealant compound (Fig. 8).

Fig. 8. Tension curve for cold-applied joint sealant compounds FM7 (class 35) in static tension/shear test at -20°C

Validation of test results

In order to validate the developed test method, various joint sealant compounds and preparatory measures, which were also used in the laboratory tests, were applied in four different test areas on the A23 federal autobahn and subjected to the usual practical conditions. These four test areas differ firstly in terms of how the joint surfaces are pre-treated (brushing of the joints or recutting and brushing of the joint surfaces) and secondly in terms of the joint sealant compounds used (one hot-applied joint sealant compound (FM6) and one cold-applied joint sealant compound (FM7)).

In addition to the supervision of the repair measure itself, a validation section in each test area of 300 m or 500 m length was intensively observed for a period of 21 months. Measurement markers were installed on 10 joints in each of these validation sections in order to monitor the changes in the joint gap widths. A thermal element was also installed in each test area. As a result, a good linear correlation could be noticed between the measured temperatures and the associated joint widths (transverse contraction joints) (Fig. 9).

y = -0,2741x + 6,3737R² = 0,72

y = -0,2973x + 9,6721R² = 0,89

y = -0,2644x + 8,8154R² = 0,81

y = -0,3311x + 9,4084R² = 0,93

y = -0,2913x + 9,2500R² = 0,87

0,0

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10,0

15,0

20,0

25,0

-10,0 0,0 10,0 20,0 30,0 40,0

Fuge

nspa

ltdeh

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Mittelwert Testfeld 1.1

Mittelwert Testfeld 1.2

Mittelwert Testfeld 2.1

Mittelwert Testfeld 2.2

Linear (ohne TF 1.1)

join

tgap

exte

nsio

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]

surface temperature [� C]

mean value test field 1.1

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20.0

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y = -0.2741x + 6.3737R2 = 0.72

y = -0.2644x + 8.8154R2 = 0.81

y = -0.2973x + 9.6721R2 = 0.89

y = -0.2913x + 9.2500R2 = 0.87

y = -0.3311x + 9.4084R2 = 0.93

25.0

Fig. 9. Linear correlation between the surface temperature and the change in joint gap width

At the same time, the leak-tightness of the joint filler systems was also examined during the in-situ tests by using a vacuum suction cup and evaluated on the basis of semi-quantitative criteria. No changes in leak-tightness were identified during the 21-month observation period.

For the actual validation of the developed test method, core samples were taken from the joint areas after 12 months and implemented to the corresponding laboratory tests. For the modified hot-applied N2+-type joint sealant compound (FM6), the dynamic shear rheometer showed only a slightly increased complex shear modulus. Thereby, no significant change in the viscoelasticity of the material (defined by a change in the phase angle) was detected.

The same applies to the static and cyclic pressure/tension/shear tests on the extracted samples. This could be verified in system tests on the in-situ samples from the validation sections (A23), as they had even greater expansion capacity at -20 °C than the non-aged reference specimens (Fig. 10).

Incidentally, a positive effect on the adhesion between the joint sealant compound and joint surface was also achieved by additional recutting of the joint surfaces (and subsequent brushing, see A23 T1.2), especially for the cold-applied joint sealant compound (Fig. 11).

Fig. 10. Tension curve for the test samples from the A23 and the laboratory test samples for the hot-applied joint sealant compound FM6 (N2+-type) in static tension/shear test at -20 °C

Fig. 11. Tension curve for the test samples from the A23 and the laboratory test samples for the cold-applied joint sealant compound FM7 (class 35) in static tension/shear test at -20 °C

Evaluation approach

Based on the performed system tests on laboratory and in-situ samples, a first evaluation approach was developed with which joint sealants in their original reference state as well as in their artificially aged state can be evaluated on the basis of scientifically established parameters and limits (test temperature: -20 °C). The characteristic parameter was defined as the expansion at 80 % of the maximum stress (ε80) in the declining stress branch. For categorization, additional test results from samples of a hot-applied joint sealant compound (FM2) extracted from the federal autobahns A48 (usage duration 1.5 years) and A2 (usage duration 3 years) were also included.

The comparison of the static tension/shear tests at -20 °C on the laboratory test samples with and without artificial ageing (REF and V1) against the in-situ test samples from the representative road sections (A48 and A2) illustrates that the stress-strain curves of the in-situ test samples tend to approach those of the artificially aged test samples (V1) as their usage time increases (Fig. 12). As shown in Fig. 13, it was also possible to observe a steady decrease in the characteristic expansion (ε80) with increasing age of the joint sealants. Assuming an exponential progression of the ageing of hot-applied joint sealants, a simulated usage time of approx. 6 years with a coefficient of determination R = 0.9976 could be estimated for the artificially aged sample (V1).

Fig. 12. Tension curve of in-situ test samples (A48 and A2) and laboratory test samples (REF and V1) for the hot-applied joint sealants (FM2, N2-type)

Fig. 13. Calculation of simulated usage time of hot-applied joint sealants (N2-type)

4. CONCLUSIONS

By following a quantitative evaluation approach based on scientifically derivated limit values from in-situ investigations, it is expected that a reliable evaluation of hot-applied joint sealants will be possible in the future. With regard to the durability of these applied joint sealants, both the stress curves determined in various laboratory investigations and the developed evaluation approach have demonstrated that ageing has a significant influence on the entire joint system, so it is essential to accommodate this parameter in the system as a whole. Since the hot-applied joint sealing compounds of type N2 revealed several deficiencies in recent years, the manufacturers have also further developed these joint sealing compounds. This has led to significantly improved performance results for the FM6 joint sealant (type N2+) regarding its expansion capacity and ageing characteristics.

To summarize, it has been established that the test method developed in this research project can be used to characterize and evaluate hot-applied joint sealant compounds with the inclusion of artificial laboratory ageing. For cold applied joint sealants, the artificial ageing method still needs to be modified (e.g. by eliminating PAV conditioning) before proper evaluation criteria can also be defined for these joint sealants.

5. ACKNOWLEDGEMENTS

This paper is based on parts of the research project commissioned by the Federal Ministry of Transport, Building and Urban Development, represented by the Federal Highway Research Institute and conducted under the identifier FM 08.0228/2013/BRB. The authors are grateful for the support of this research project.

6. REFERENCES

[1] German Road and Transportation Research Association (FGSV) (2015) “ZTV Fug-StB 15 – Additional Technical Contractual Terms and Guidelines for Joint Fillings in Road Surfacing”

[2] German Road and Transportation Research Association (FGSV) (2015) “TP Fug-StB 15 – Technical Testing Standards for Joint Fillers in Road Surfacing”

[3] German Road and Transportation Research Association (FGSV) (2015) “TL Fug-StB 15 – Technical Delivery Conditions for Joint Fillers in Road Surfacing”