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Al-Qadi, I. L.; Yang, S.-H.; Fini, E.; Masson, J-F.; McGhee, K. K.
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Pe rform a nc e -ba se d spe c ific a t ion guide line s for t he se le c t ion of bit um inous-ba se d hot -poure d c ra c k se a la nt s
N R C C - 5 3 2 5 3
A l Q a d i , I . L . ; Y a n g , S . H . ; F i n i , E . ; M a s s o n , J -F . ; M c G h e e , K . K .
J a n u a r y 2 0 0 9
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Journal of the Association of Asphalt Paving Technologists, 78, pp. 491-532, January 01, 2009
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Performance-Based Specification Guidelines for the Selection of Bituminous-Based Hot-Poured Crack Sealants
Imad L. Al-Qadi1, Sheh-HseinYang2, Elham Fini3, J-F Masson4, and Kevin McGhee5
Abstract
The long-term performance of pavements depends in good part on the quality and frequency of maintenance. Appropriate maintenance protects the pavement from deterioration, corrects deficiencies, and ensures safe and smooth riding. Crack sealing is practiced on a routine basis as preventive maintenance and as part of corrective maintenance prior to an overlay or a greater rehabilitation project. A timely and properly installed sealant adds several years of service life to the pavement at a relatively low cost. As a
consequence, the selection of an appropriate sealant in a maintenance project becomes an important issue. Current sealant selection is based on ASTM standards that consist of quality control tests, not of performance indicators. These standards do not consider the changes in mechanical properties due to aging or the differences in local service temperatures. The main purpose of this study was to develop a systematic process to help users to select appropriate bituminous
1
Founder Professor of Engineering, Department of Civil and Environmental Engineering; Director, Illinois Center for Transportation, University of Illinois at Urbana-Champaign, Urbana, IL
2
Graduate Student, Department of Civil and Environmental Engineering; Illinois Center for Transportation, University of Illinois at Urbana-Champaign, Urbana, IL
3
Graduate Student, Department of Civil and Environmental Engineering; Illinois Center for Transportation, University of Illinois at Urbana-Champaign, Urbana, IL
4
Senior Research Officer, Institute for Research in Construction, National Research Council of Canada, Ottawa, ON, K1A 0R6, Canada
5
Senior Research Scientist, Virginia Transportation Research Council, Charlottesville, VA
hot-poured sealants for pavement cracks and joints. The tests include in this paper covering a summary of four years research project including an accelerated aging test, a viscosity test performed at installation temperatures, a dynamic shear rheometer (DSR) tests to assess tracking resistance in summer temperature, a crack sealant bending beam rheometer (CSBBR) and a crack sealant direct tension test (CSDTT) for cohesive properties at sub-zero temperature, and a blister test for adhesive properties.
Key Words: Crack sealant, Specification, Sealant apparent viscosity, Sealant aging, Sealant cohesive properties, Sealant adhesive properties.
Introduction
ASTM standard D5535 defines a sealant as a material that possesses both adhesive and cohesive properties to form a seal which prevents liquid and solid from penetrating into the pavement system. Crack sealing has been widely accepted as a routine preventative maintenance practice. When proper installation is achieved, crack sealant can extend pavement service life by a period ranging from three to five years (1). Numerous studies also demonstrated the cost effectiveness of crack sealants (2-7).
Crack sealant is produced so that it keeps its shape as applied and hardens through chemical and/or physical processes to form a viscoelastic, rubber-like material that withstands extension or compression (crack movement) and weathering (8). However, in many cases, premature failure of crack sealants may be observed in one of the following scenarios. During the sealant installation, if the viscosity of the sealant is too high, the sealant might not fill the crack properly; hence, it will affect the interface bonding between sealant and pavement substrate. If the viscosity is too low, the sealant might flow out from the cracks.
In the field, a sealant extends at low temperature and compresses at high temperature to accommodate pavement crack openings which increase with decreasing temperature and decreases with rising temperature. At high service temperature, a sealant might fail due to pull-out from the crack by tire passing. At low service
temperature, the crack opening may increase from 10% to more than 90% depending on the environmental location. In these conditions, one of the two mechanisms might be observed: cohesive or adhesive failure. The former occurs in the sealant, while the latter occurs at the sealant-pavement crack wall interface. At low temperature, a sealant becomes more brittle due to physical hardening; and is subjected to short-duration loading due to crack movements associated with stick-slip motions and truck trafficking, as well as long periods of environmental loading.
To introduce a cost-effective crack sealing operation and proper field performance, two factors must be closely controlled: the quality of sealant installation and sealant mechanical and rheological properties (such as viscosity, bulk stiffness, and adhesive bonding). Regardless of sealant quality, improper installation will cause premature failure, and therefore, reduced sealant service life.
Standards and specifications for selecting crack sealant have been established by several organizations, including the American Society for Testing and Materials (ASTM), the American
Association of State Highway and Transportation Officials (AASHTO), and U.S. and Canadian federal, state, provincial, and municipal agencies. The objective of the specifications is to select materials that have the necessary properties to perform adequately in the field. However, these specifications are generally empirical and do not measure sealants’ fundamental properties. Hot-poured bituminous crack sealants are typically selected based on standard empirical tests such as penetration, resilience, flow, and bonding to cement concrete briquettes (ASTM D 6690 [9]). ASTM Standard D5329-04 (Standard Test Methods for Sealants and Fillers, Hot-Applied, for Joints and Cracks in Asphaltic and Portland Cement Concrete Pavements [10]) summarizes most of these tests. These include non-immersed cone penetration, fuel-immersed cone penetration, the flow test, the non-immersed bond test, the water-immersed bond test, the fuel-water-immersed bond test, the resilience test, the oven-aged resilience test, the asphalt compatibility test, the artificial weathering test, the tensile adhesion test, the solubility test, and the flexibility test.
These tests are used by most state highway agencies in selecting their crack sealing materials, but the specification limits may vary from one state to another. These differences create difficulties for crack sealant suppliers because many states with the
same environmental conditions specify different limits for the measured properties. In addition, it is reported that these tests poorly characterize the rheological properties of bituminous-based crack sealants and also poorly predict sealant performance in the field.
Objectives and Scope
Researchers have widely reported that current specifications for selection of hot-poured crack sealants are based on tests whose results showed no correlation with field performance (11-15). In addition, over the past two decades, a new generation of highly modified crack sealants has been introduced to the market (16). These sealants exhibit quite complex behavior compared to those made of traditional sealant materials (12). This requires the development of a new set of specifications.
The most effective way to evaluate the performance of crack sealants would be to perform field tests. However, the results from field tests are sometimes controversial because a sealant can perform well at one site and fail at another simply because of differences in environmental conditions. Thus, the main objectives of this project were to develop laboratory tests that measure bituminous-based crack sealants’ rheological properties, and to develop performance-based guidelines for the selection of hot-poured crack sealants. Meeting these objectives requires the development of new tests to measure the rheological properties of hot-poured crack sealants over a wide range of service temperatures. The developed tests need to be practical, repeatable, and reproducible. Thresholds for each test should be identified to ensure desirable sealant field performance. A special effort was made to make use of the equipment originally developed during the five-year Strategic Highway Research Program (SHRP). These equipment were used to measure binder rheological behavior as part of the performance grading (PG) system.
In general, sealants are composed of asphalt binder with bitumen as the main component, rubbery material such as butadiene copolymer, mineral filler, and processing oil. The styrene-butadiene (SB) copolymer consists of linked blocks of polystyrene (PS) and polybutadiene (PB). The fillers may include crumb rubber block, rubber powder, or fibers. The variety of crack sealants’ chemical composition can significantly influence its rheological properties. Therefore, for this study, 15 laboratory sealants — designated as PP, BB, AD, WW, AE, NN, MM, UU, DD, EE, VV, AB, QQ, YY, and ZZ—and five field sealants—designated A, B, E, G, and J—with varying chemical compositions were selected. The laboratory sealants were obtained from various North American manufactures and were used to investigate the mechanical behavior of crack sealant. There is no field performance data available for the 15 laboratory sealants. The field sealants were five sealants installed in the early 1990s in Montreal, Canada; they have detail performance records. The laboratory sealants represent a wide array of
rheological behaviors, and thus, they are expected to perform in various environments with a low-temperature range of -4°C to -40°C. Variations in the rheological properties can be attributed to various factors, including the source of the origin crude, the refining and modification process, and the content of polymer, filler, and additives.
Although nine types of tests are usually required in the ASTM standard, only three standard tests in accordance with ASTM D 5329 (10) and D 6690 (9) were used. Table 1 summarizes the testing results using non-immersed cone penetration, resilience tests at 25°C, and flow tests at 60°C.
The cone penetration test provides an indication of the consistency of the material. In this test, a special cone weighing 150g penetrates the tested sample for 5s and records the penetration in dmm. The deeper the cone penetrates, the softer the material. The test is typically performed at 25°C. The test itself is empirical and does not provide rheological properties of the tested material.
The resilience test is another empirical test that provides an indication of the elastic characteristics of the sealant. The test is typically performed at 25°C and consists of measuring the material recovery after a 75-g special ball is pushed towards it.
The flow test is performed to indicate whether the material has adequate resistance to softening and flow at high, in-service pavement temperatures. The test measures the flow in mm of a
material placed at a 75° angle with the horizontal surface at a temperature of 60°C. Although this test provides a good indication of the material’s resistance to flow at this temperature, again, it is empirical and does not measure rheological properties.
Table 1 ASTM Standard Sealant Specification Results as Reported by Manufacturers Sealant Cone Penetration 25°C (dmm) Resilience 25°C (%) Flow 60°C (mm) PP 130 44 1 BB 148 80 0 AD 110 80 1 WW <90 >60 <3
AE N/A N/A N/A
NN 75 70 0
MM 120 70 1
UU 62 N/A 1.5
DD 80 50 1.5
VV N/A N/A N/A
EE 47 57 0
AB N/A N/A N/A
YY N/A N/A N/A
ZZ 42 N/A N/A QQ 22 36 0 A 86 57 0.5 B 68 64 0.5 E 104 73 1 G 50 51 0.5 J 66 48 6
N/A: Data is not available.
Force ductility is measured by stretching a standard-sized briquette of sealant to its breaking point. The stretched distance in cm at breaking point is then reported as ductility. This test has limited use, since it is empirical and is conducted at only one temperature, 25°C, which has no correlation to sealant low-temperature properties.
Test Development
To develop performance-based guidelines for the selection of hot-poured crack sealants that meet the previously mentioned
requirements and minimize the long-term cost of investment in new testing equipment, the research group made use of the SuperPave™ binder performance grading (PG) equipment. Modifications to the existing viscosity test, bending beam rheometer, and direct tension test devices, specimen size and preparation, and testing procedures were developed to accommodate the testing of crack sealants. In addition, new tests for sealant aging and sealant evaluation at high service temperatures were introduced. Upon the completion of test validation, test measured performance parameters were
recommended for implementation as part of the newly developed “Sealant Grade” (SG) system.
Installation Workability
Sealant viscosity is among the parameters that affect initial bonding and pouring workability. Therefore, applying a sealant at the appropriate viscosity provides for better crack filling and enhances interface bonding. Several factors affect the measured viscosity of hot-poured crack sealant. Thus, it is essential to identify the material characteristics that influence the rheological behavior of hot-poured crack sealant at the time of installation. These
characteristics need to be set at reasonable limits, to simulate field installation as closely as possible. While standard tests to examine sealant consistency exist, these standard tests have not been proven to predict field performance. As part of an effort to bridge the gap between fundamental properties of sealants and field performance, a test procedure was developed to measure apparent sealant viscosity using the same rotational viscometer equipment used in the
SuperPave™ PG system. The development of this procedure is described in detail elsewhere (18).
Brookfield Rotational Viscometers for Apparent Viscosity Numerous factors affect the measured viscosity of hot-poured crack sealant. Due to the relatively high polymer content, sealant
responses to changes in temperature and loading can be quite
complex. Since hot-poured crack sealants behave as non-Newtonian fluids, variation in the experimental parameters can affect the
measured values; a test setup and testing parameters were thereby identified (18, 23). Laboratory conditions should simulate sealant installation conditions as closely as possible. A critical issue was the shear rate imposed on the material during application. It was
determined that a spindle speed ranging from 115 rpm to 5536 rpm should be used to simulate the shearing of the sealant as it enters the crack during installation. This is calculated based on the shear rate for a Newtonian fluid injected out from a nozzle (41). However, a significant reduction in the shear rate may occur as the sealant exits the applicator wand, due to the sharp temperature drop, as well as the high friction with the crack walls.
Although the Brookfield Thermosel system (as adopted from SuperPave™) is not a high-shear rheometer (its maximum allowable spindle speed is 250 rpm), it was found to be sufficient for the sealant testing. After extensive testing of several viscometers, a Brookfield rotational viscometer was adopted; modification of the test procedure and equipment was implemented (see Figure 1). A SC4-27 spindle at a speed of 60 rpm (shear rate of 20.4s-1) at the recommended installation temperature is used. The spindle is attached to a newly developed rigid rod. The rod is a replacement for the current hook; it prevents a rubber particle from disturbing the spindle rotation, which results in better test repeatability. A conditioning time of 20 min and a waiting time of 30 s before collecting data are also recommended to ensure that the measured apparent viscosity has stabilized.
Figure 1 Brookfield Thermosel System and Rigid Rod Used for Crack Sealant Testing Compared to the Rod Used for Asphalt
In this test, a sealant is cut in small pieces and placed directly in an aluminum chamber; then the sealant is melted inside the chamber. Cutting a sealant without melting it improved test results significantly. The measured apparent viscosity is expected to be an acceptable indication of the sealant’s rheological behavior at installation temperature, assuming that the suggested procedure and equipment are used. Fifteen sealants were tested in accordance with this developed testing procedure; the apparent viscosity of several sealants at various temperatures is presented in Figure 2.
Figure 2 Apparent Viscosity of Several Sealants at Various Temperatures 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 130 140 150 160 170 180 190 200 Temperature (C) V iscosity ( P a.s) VV AD AE WW
Seven laboratories conducted a round-robin test. The repeatability of the measured apparent viscosities was determined through statistical analysis. The average coefficients of variation within and between laboratories were found to be 1.6% and 6%, respectively. Maximum permissible differences within a laboratory and between laboratories are 5.4% (among the best three readings out of four) and 17% (between the test conducted in two different laboratories), respectively. These values are comparable to those of asphalt binder: 3.5% and 14.5% based on ASTM D 4402-06 (24) and 3.5% and 12.1% based on AASHTO 2006 T316 (25), respectively. Because viscosity plays an essential role in predicting field
performance of hot-poured crack sealant, upper and lower viscosity limits are recommended. An upper limit of 3.5 Pa.s ensures that sealant is liquid enough to pour, whereas a lower limit of 1 Pa.s
controls the potential of using excessively fluid sealant. Hence, the sealant apparent viscosity should be between 1.0 and 3.5 Pa.s when measured at the recommended installation temperature.
Accelerated Aging
For an aging test to be effective, it must quickly provide an aging as close as possible to reality. To this end, true aging was determined from the physico-chemical analysis of sealants weathered in Montreal, Canada, for nine years (see Table 2). As expected, sealants with good performance contain components resistant to weathering, whereas sealants with poor performance oxidize quickly. Figure 3 shows an example of sealant stiffening due to weathering.
Because of sealants’ complex mixture, each sealant shows unique aging characteristics. To mimic the effect of weathering on sealants, several accelerated aging methods were compared (alone or in combination) after various aging periods and temperatures,
including small-kettle aging, microwave aging, pressure aging, oven aging, and vacuum oven aging. The results of physico-chemical analysis of sealants weathered in the field were compared to those of sealants aged quickly in the laboratory (21).
Table 2 Physico-chemical Method to Characterize Crack Sealant Aging
Method* Output Use
GPC Separation of bitumen and polymer
Quantify polymer; degradation rate and mechanism
FTIR Fingerprint of composition
Oxidation; identification of polymer and filler; semi-quantitative analysis;
degradation mechanism TG Weight loss upon
heating
Contents of filler and light, medium, and heavy hydrocarbon components DSR Stiffness, relaxation Effect of temperature and aging on
mechanical properties * GPC: Gel permeation chromatography; FTIR: Fourier transform infrared spectroscopy; TG: Thermogravimetric analysis; and DSR: Dynamic shear rheometry .
1.E+3 1.E+4 1.E+5 1.E+6 1.E+7 1.E+8 -40 -20 0 20 40 Temperature, °C C o m p le x v isc o s it y,P a .s Unaged 1-year 9-years
Figure 3 Complex Viscosity Increase for a Field-Aged Sealant at One and Nine Years
Accelerated Aging Test Development
The effect of hot-poured crack sealant oxidation and the change in polymer molecular weight on the sealant complex viscosity between -40°C and 40°C served as the two parameters in examining the applicability of the pressure aging method (Method 1) and the vacuum oven method (Method 2) [see Figure 4]. Microwave heating was found to mimic the aging of sealants that contain mineral filler, but not other sealants. The microwave method thus lacked general application. Pressure aging was also found to be
inappropriate because it often led to insufficient bitumen oxidation and excessive thermo-degradation of the polymer. Vacuum oven aging proved to be the most appropriate method to simulate sealant weathering. In this method, sealants are cut into slices and placed on a stainless steel pan; each pan contains 35 g of sealant. The pan is transferred into a conventional temperature-controlled oven which is maintained at 180°C for approximately five min to allow the sealant to melt and form a film. The sealant is then removed from the oven and cooled to room temperature. Once it cools, the sealant is placed in a vacuum oven preheated at 115°C for 16 hr. After 16 hr, the vacuum is released and the sealant is placed in a conventional oven at 180°C for five min or until the sealant is fluid enough to pour.
1.E+3 1.E+4 1.E+5 1.E+6 1.E+7 1.E+8 -40 -20 0 20 4 Temperature, °C C o m p le x v is c o s it y , P a .s 0 9-years Unaged Method 2 Method 1
Figure 4 Complex Viscosity of a Field-Aged Sealant Compared to that after Accelerated Aging
Sealant Flow and Deformation
Bituminous sealants applied to cracked pavements sometimes fail due to deformation under the combined action of shear stresses and high service temperatures (22). In an attempt to define
performance parameters, 15 sealants were tested with a dynamic shear rheometer (DSR) and subjected to increasing stresses at temperatures between 46°C and 82°C. These conditions were meant to mimic the effects of various traffic levels and maximum
temperature in the field.
Dynamic Shear Rheometry for Tracking Resistance Plots of low-shear viscosity (ηL) versus shear rate ( ), Equation 1, indicated that many sealants were susceptible to shear thinning. Their apparent viscosity decreased with an increase in shear rate and/or temperature (see
•
γ
Figure 5). This indicated that at high service temperatures, high traffic loads or volumes would affect the extent of sealant flow when it is under stress.
σ = C P (1) • γ where, σ = stress; • γ= shear rate;
C = flow coefficient; and P = shear-thinning coefficient Sealant AD 100 1000 10000 100000
1.E-4 1.E-3 1.E-2 1.E-1 1.E+0 1.E+1 Shear rate, 1/s LS V , P a. s 46C 52C 58C 64C 70C 76C 82C Sealant C 100 1000 10000 100000
1.E-4 1.E-3 1.E-2 1.E-1 1.E+0 1.E+1 Shear rate, 1/s L SV, Pa .s 46°C 52°C 58°C 64°C 70°C, 76°C 82°C (a) (b)
Figure 5 Low-shear Viscosity as a Function of Shear Rate; The Stress Doubles for Each Point from Left to Right for (a) Sealant AD,
and (b) Sealant C
Plots of ηL vs were interpreted based on the Ostwald power law model. This model provides two parameters: a flow coefficient (C) and a shear-thinning coefficient (P). These coefficients
correlated well with seal asured
by trac
test, and the open markers s d fail. The semi-log
scale in s
s C
•
γ
ant pseudo-field performance as me gure 6 shows the relationship between t
king (26). Fi hese
factors and performance during the pseudo-field test. The solid markers indicate the sealants that did not fail during the pseudo-field
how those that di
Figure 6 serve to highlight the high-failure regions (open markers). Limiting values for P and C can be established to limit the risk of sealant failure. Each pair of C and P represents performance criteria.
In the absence of effective limiting criteria (area A in Figure 6), the sealant failure rate due to tracking is 39% (see Table 3). A and P limits are raised, the risk of tracking failure is reduced. Area E defines the limits within which no tracking failure is observed. With such demanding criteria, 33% of the sealants are above the pass limits. Any limits in C and P can be used to define the level of sealant performance. However, the most appropriate performance criteria may be that defined by area D, where limits of C = 4000 Pa.s and P = 0.70 provide for a failure risk of only 3% and a sealant acceptance rate greater than 50%. The other limits have greater acceptance rates; but the risk of failure is disproportionately higher.
Table 3 Lower Limits for C and P and their Relationship with Tracking Performance
Areaa C (Pa.s) P Passingb Trackingc A 300 0.46 145 (99%) 39 % failure B 1000 0.64 123 (84%) 24% failure C 2500 0.64 98 (66%) 14% failure D 4000 0.70 76 (52%) 3% failures E 10000 0.80 49 (33%) No failures a Area in Figure 6. b Fr c
From the pseudo-field tes rs within the given plot area.
Figure 6 Semi-log Plot of the Ostwald Parameters and Possible Performance Limits, Areas A to D
Low-temperature Flexural Properties The bending beam rheometer (BBR) is used in most pavement laboratories nowadays to measure binder stiffness at low temperature. A modified BBR test, a crack sealant bending beam
rhe f
crack sealants at evelopment of
this procedure is described in detail in a supporting document (20).
om DSR: Number of samples above C and P limits out of a total of 147. t: ratio of empty to all marke
ometer (CSBBR), was introduced to measure the flexural creep o temperatures as low as -40°C. The d
10 100 1000 10000 a .s 100000 0.4 0.5 0.6 0.7 0.8 0.9 1
Shear thinning exponent (P)
F low c oef fi ci E ent ( C )/ P no tracking 1 sample tracks 2 samples track A B C D
Mo ties
testing of some sealants with the SuperPave™ BBR device. Conse BB the exc , rease. t the ts are 6.35 mm shorter asily
Specimen Supports; and (c) Modified Specimen Supports
dified Bending Beam Rheometry for Flexural Proper The principle underlying the crack sealant bending beam rheometer (CSBBR) test is the application of a constant load of 980 mN (100g) to a sealant beam and then measuring the beam deflection. The crack sealant is a much softer material compared to asphalt binder; therefore, excessive deflection was encountered during the
quently, several modifications were made to the SuperPave™ R test. First, the specimen thickness was doubled to overcome
essive deflection. Second, the device was modified to accommodate the new specimen geometry. In addition, a modified testing procedure, a validated testing period, and stiffness
determination time were introduced. Linearity verification was conducted to verify that bituminous crack sealants behave as linear viscoelastic materials within CSBBR testing range.
The CSBBR beam thickness was doubled from 6.35 to 12.7mm. However, because of the increase in the beam’s thickness the deflection at the center of the beam due to shear would inc The analysis shows that the center deflection contributed by shear force is only 4%, which was deemed acceptable. To adjus
device to accommodate the newly developed specimen geometry, the specimen support and calibration kits of the BBR were modified, as shown in Figure 7. The new design specimen suppor
than the SuperPaveTM BBR specimen supports and can e be replaced. The calibration kits of the system were also modified as shown in Figure 8. The compliance beam for the crack sealant test was modified by adding two footers at each end of the beam (20).
(a) (b) (c)
Figure 8 Calibration Beam: In the Front, Compliance Beam for SuperPaveTM BBR Test; in the Back, Modified Compliance Beam for
CSBBR Test
The test procedure modification was completed in two parts. First, the silicon-based release agent was used to replace the Mylar strip, because the Mylar strip melted at sealant pouring temperature; Second, each specimen was poured from an individual container which has the same weight. Because bituminous-based sealants are co
controlling the pouring weig cimen, the test
repeatability was greatly improved (28). Prior to pouring specimens into the
ith m in hamber
r oading. The load and deflection of the seal
in the mposed of asphalt binder, SBS copolymer, crumb rubber, and various additives, the variability among specimens is high. By
ht of each spe
molds, the sealant was aged in accordance with the aging method developed in this study.
Various sealants were tested at temperatures ranging from -4°C to -40°C. In this test, 35 g of sealant is first heated at its recommended pouring temperature and then poured into an aluminum assembled mold. A rectangular sealant beam is cast w dimensions of 12.7 mm in height, 12.7 mm in width, and 102 m length. The beam is then placed in a fluid environmental c
and the specimen is placed on a two-point support and subjected to a point-creep loading. The specimen is exposed to a creep loading fo 240 s, then followed by 480-s unl
ant beam is recorded during the period of loading and unloading.
Two performance parameters were suggested for use
specification: stiffness at 240 s (see Figure 9) and average creep rate (see Figure 10). It was found that the critical loading time for crack sealant material at low temperature is after five hr of loading. If the
temperature superposition principle is applied, the stiffness at 240 s for a given temperature can be used to predict the stiffness after five hrs of loading at a temperature approximately 12°C higher. Given the variation in sealant response to temperature change, a 6°C deemed appropriat
shift is e. In general, the tests are repeatable, and the
coeffici ,
Pa
Figure 9 Stiffness at 240 s at Various Testing Temperatures for Various Sealants
ent of variation between operators is less than 4%. However a difference in measured values was noted when using devices from different manufacturers (30).
In addition to the 15 laboratory-tested sealants, five sealants that were previously installed in Montreal as part of a long-term field performance evaluation survey were tested using CSBBR. The results were used to establish the selection criteria based on the CSBBR test. The recommended thresholds, which are temperature independent, for the two criteria are a maximum stiffness of 25 M and a minimum average creep rate of 0.31, respectively.
0 50 -4°C -10°C -16°C -22°C -28°C -34°C -40°C 20 30 40 if fn e s s ( M Pa ) 10 QQ ZZ AB UU AE NN PP Sealant St
0 0.1 0.2 0.3 0.4
Figure 10 Average Creep Rate at Various Testing Temperatures for
Low-temperature Extendibility
hen the temperature drops in the winter or at night, the
paveme e
Direct Tension Tester for Extendibility
The pr ) is to slowly pull a c d the Various Sealants W
nt will contract and induce the opening of the crack. In th summer or daytime, the temperature increases, and therefore, the pavement will expand and the crack will close. The crack opening distance during the temperature cycle can varies from 10 to 90% starin. Therefore, to investigate whether a sealant can survive in a particular set of service conditions, the SuperPaveTM Direct Tension Test (DTT) was considered and modified for crack sealants. The development of crack sealant direct tension test (CSDTT) is described in detail elsewhere (19).
inciple of the crack sealant DTT (CSDTT
rack sealant specimen in tension until it breaks. The dog-bone shaped specimen used in the DTT has a rectangular cross section. Its ends are enlarged so that when crack sealant is poure into the mold, it has a large adhesive area between the crack sealant and end tabs. The end tabs are made from Phenolic G-10 material, to provide good bonding. The SuperPaveTM DTT specimen geometry can only extend the sealant specimen up to 32% strain. This is significantly smaller than the expected crack sealant extension in
0.5 QQ ZZ AB UU AE NN PP Sealant Av e rage R a te o f C reep -4°C -10°C -16°C -22°C -28°C -34°C -40°C
field. Thus, for purposes of crack sealant testing, the specimen geometry and preparation procedure were modified.
A finite element (FE) analysis was conducted to determine the opti
.
hat can
presents
Table 4 Comparison of Direct Tension Specimen Dimensions
mized specimen geometry which provides uniform stress distribution within a specimen while allowing sufficient extension This led to a new geometry. The new specimen dimensions are the following: 24-mm long, 6-mm wide and 3-mm thick; and the effective gauge length is 20.3 mm. The maximum extension t be achieved using this specimen is 19 mm, which is equivalent to approximately 94% strain. This meets the extreme service conditions that sealants may experience in the field. Table 4 the geometry comparison of SuperPaveTM binder DTT specimen, transition specimen, and CSDTT specimen (8).
Specimen Width Thickness Nominal Effective Gauge Type (mm) (mm) Length (mm) Length (mm) SuperPaveTM 6 6 40 33.8 Transition 6 3 40 33.8 Crack Sealant 6 3 24 20.2
The effects of geometry and loading rate on sealant tensile behavio e sealant p s ranging from 40 to -g ation r were also investigated (8). The specimen preparation procedure was modified to accommodate various sealant compositions to improve the workability while pouring th
into a mold. The mold was heated to 50°C lower than the sealant pouring temperature prior to pouring the sealant. Right after the sealant was poured into the mold, a spatula was used to slightly ta the sealant to ensure that the sealant filled the mold.
Fifteen sealants were tested at low temperature
4°C. The extendibility of the sealant was measured and used as a performance parameter (see Figure 11). It was found that extendibility is a good criterion for identifying and distinguishin among sealants. In addition, a viscoelastic model was fitted to tensile stress-strain test results to obtain Prony series of crack sealants. The model was used to estimate the stress relaxation modulus for the crack sealant. The fundamental property, relax modulus, can be used to relate to the sealant’s field performance as well (31).
The study recommends using the DTT as a standard test to evaluate the bituminous-based, hot-poured crack sealant at low temperature. The performance parameter of extendibility is recommended for use in the specification. The thresholds for the extendibility depend on the sealants’ lowest application temperatures and are presented in Table 5. In addition, because the test is
conducted under a relatively higher deformation rate compared to real crack movement, the research team recommends a +6°C shift in the crack sealant grading system. For instance, if the lowest service temperature is determined as -16°C, the test would then be conducted at -10°C. If the extendibility of such a sealant is over 25%, the sealant passes the criteria and may be used.
0 10 20 30 40 50 60 70 80 90 DD MM WW NN AD PP BB Sealant Extendi bil ity ( % ) -22 -28 -34 -40
Figure 11 Extendibility of Various Sealants at 22, 28, 34, and -40°C
Table 5 Thresholds for Crack Sealant Extendibility at Various Temperatures
Temperature (°C) -4 -10 -16 -22 -28 -34 -40 Extendibility (%), > 10 25 40 55 70 85 85
Low-temperature Adhesive Properties
The adhesion capability of hot-poured bituminous sealants is usually evaluated using a standard test of an empirical nature (ASTM D 5329 [10]). There is, however, no indication that the results of this test are relevant to field performance. In addition, this test examines
adhesion of sealant to Portland cement concrete, and the test result does not account for aggregate composition, which is the main component of HMA. Thus, a reliable test method which is based on sealant rheology, that accounts for aggregate composition, and that correlates with field performance is urgently needed. This study proposes three laboratory tests to predict interface bonding of crack sealant to aggregate at service temperatures ranging from 4ºC to -40ºC. The three tests are designed to address the needs of,
respectively, manufacturers, transportation organizations—including contractors, transportation agencies, and consultants—and
researchers.
The first laboratory approach addresses the compatibility of sealant with a specific substrate, by measuring the free energy of the bond, work of adhesion. The second test makes use of the direct tension test (DTT) device. The third test is a fracture-type test that uses a fracture mechanics approach to derive a fundamental property of the bond, interfacial fracture energy (IFE).
Work of Adhesion
Even a quality sealant may fail if used with an incompatible aggregate. A compatibility test can be performed using the Sessile drop method (see Figure 12) which is used to determine both the surface energy of the hot-poured crack sealant and its wettability (contact angle) with respect to the aggregate. If the contact angle is less than 90°, it demonstrates that the aggregate is wetted by the sealant and a good adhesion bond is formed between the aggregate and sealant. For this test, the sealant is heated and mixed at the manufacturer’s recommended installation temperature and poured onto an aluminum sheet to form a thin, smooth surface. The sealant is cooled at room temperature to solidify and make thin plates. A five-micrometer pipette is used to manually apply liquid drops from three probe liquids (water, formamide, and glycerol) onto the sealant plate. The image of each drop is captured by microscope within 15 s after it is applied. The resulting contact angle is used to determine the work of adhesion between sealant and substrate; hence, sealant-substrate compatibility.
(a) (b)
Figure 12 Surface Energy Method: (a) Sessile Drop Equipment and Microscope; and (b) Contact Angle of a Liquid Drop on a Solid
Due to the high variation among aggregate properties, replacing aggregate substrate with a standard material would be beneficial. Various potential reference materials were examined and aluminum was selected because it has compatible thermal expansion, the relatively smoothest surface, and a surface chemistry similar to natural aggregate (32). Figure 13 presents the calculated work of adhesion between sealant and four substrates: aluminum, granite, quartzite, and limestone.
0 2 4 6 8 10 12 14 16 DD QQ NN VV EE ZZ YY W o rk of A dhe sion (m J /m 2) Limestone Quartzite Granite Aluminum
Figure 13 Work of Adhesion between Sealants and Limestone, Quartzite, Granite and Aluminum for Sealants DD, QQ, NN, VV,
EE, ZZ, YY
Direct Tension for Bond Strength
The premise of the DTT for adhesion is to detach sealant from its aggregate counterpart by applying a tensile force. A new
test fixture that simulates sealant pouring condition and loading mechanism in the field was developed. The briquette assembly consists of two aluminum half cylinders of 25 mm in diameter and 12 mm in thickness; aluminum is conservatively selected as a substrate reference material. Each aluminum briquette is confined within an aluminum grip designed to work with the DTT setting posts. The assembly has a half-cylinder mold, open at the upper part. The mold is placed between the two aluminum half cylinders on an even surface. To ensure that adhesive failure occurs, and to define a failure’s location, a notch is made at one side of the sealant–
aggregate interface. A 12.5x2 mm shim is placed at one aggregate– sealant interface. The assembly then is placed in the DT machine so the notch is placed at the non-moving side.
To conduct the test, the sealant is heated at its recommended installation temperature and poured into the half-cylinder mold. After one hour of annealing at room temperature, the specimen is trimmed and placed in the cooling bath for 15 min. The specimen is then removed from the bath, demolded, and placed back in the bath for another 45 min before testing. Using the DT device, the end pieces are pulled apart by moving one of the end pieces at a speed of 0.05 mm/s, strain rate of 0.005 mm/mm/s (see Figure 14). The results of the test are the maximum load and calculated energy, which is the area under the load-displacement curve up to failure, reported as indications of bond strength (17).
Figure 14 Pulling End Piece Apart at a Constant Displacement Rate Using DT Device
Error! Reference source not found. presents the maximum load required to break the bond between various sealants and the aluminum. The maximum load showed good repeatability, and it was able to clearly distinguish among various sealant–aggregate pairs. Variation between operators and setups were checked, and no significant variations were found. The maximum load was selected as the test performance parameter (17). Error! Reference source
not found. presents this parameter for several pairs of aged sealant-aggregate. Using comparisons between test results of laboratory-aged specimens and field data, a minimum 50 N at tested
temperature was selected as the performance threshold.
0 20 40 60 80 100 120 140 -4 -10 -16 -22 -28 -34 -40 Temperature (ºC) Pma x (N) NN WW PP AE UU LL DD VV
Figure 15 Maximum Load Required to Break the Bond between Unaged Sealant and Aluminum at Various Temperatures
0 50 100 150 200 250 300 350 400 PP A-34 NNA -34 NNA -28 AE A-28 AE A-22 LLA -22 LLA -16 VVA -22 VV A-16 VV A-10 UUA -16 UU A-10 ZZ A-10 ZZ A-4 QQA -4 QQA +2 P ma x (N ) Aluminum Limestone Granite Quartzite
Figure 16 Maximum Load Measured for Bonding between Sealants and Substrates
Interfacial Fracture Energy
The third test for identifying low-temperature adhesive properties of sealants is a fracture-type test that uses fracture mechanics to derive a fundamental property of the bond. The fundamental property is the interfacial fracture energy (IFE). A geometry-independent, pressure blister test was developed (33). The intrinsically stable interface de-bonding process makes this test attractive and allows calculation of fundamental properties of the interface (34-36). The blister test, which is an enclosed system, exposes the interface to simultaneous loading and environmental conditions.
In this test, a servo-hydraulic pump displaces a piston at a constant rate. The upward movement of the piston injects a liquid medium (alcohol) at a constant rate of 0.1 L/hr through a channel that is connected to the specimen (see Figure 17). The specimen is composed of an annular (doughnut-shaped) substrate plate
(aggregate or a standard material) covered with binder or sealant on one side. Alcohol pushes the adhesive (binder or sealant) away from the substrate, creating a blister which continues to grow until the adhesive separates from the substrate. The blister height and the pressure are recorded during the test, and they are used to calculate the IFE. In simplified form, IFE can be calculated as half of the product of the maximum pressure and the corresponding blister height.
Figure 18 shows the IFE values for several sealant-substrates. It clearly shows that IFE can differentiate among sealants at various temperatures. The crack sealant blister test was further used to study the effect of sealant aging, temperature, loading rate, viscosity, and curing time on interface bonding. In addition, within-laboratory variation was checked; and no significant difference was found between operators (33).
0 200 400 600 800 1000 1200 -22 -34 -40 IF E (J /m 2 ) AD BB PP NN LL AE MM
Figure 18 IFE for Various Sealants at Temperatures Ranging from -22 to -40ºC
Preliminary Field Validation
Montreal Field Site
Five sealants (A, B, E, G and J) with previously documented field performance were installed in Montreal, Quebec, Canada in fall 1990 and were used in this study to validate the crack sealant
performance criteria. Field samples were collected during the visual survey at Years 1, 3, 5, and 9. A long-term sealant performance survey was conducted four years after installation (see Table 6). The long-term performance is mainly affected by the sealant weathering and stiffening. A performance index (PI) was suggested based on the level of de-bonding and pull-out. The PI is calculated as follows (13):
PI = 100-(D+nP) (2)
where,
PI = sealant performance index;
D = percent de-bonded length of the sealant; P = percent pull-out length; and
n = an integral that accounts for the effect of pull-out over de-bonding on performance.
The n value was assigned 4 in the Masson et al. study (13); the suggested value is based on the reasoning that the loss of 1m of sealant may allow the intrusion of sand and stone into the pavement, which can damage the pavement during its expansion and
contraction (40). Loss of sealant also allows more water to penetrate into the pavement. This damage is more critical than de-bonding over the same length. The higher the PI value, the better the sealant performance is.
Table 6 Four-year Sealant Performance (13)
Sealant De-bonding (%) Pull-out (%) PI (%)
A 11 14 33
B 22 1 74
E 20 2 72
G 36 14 8
J 13 12 39
ASTM Specification versus Performance-Based Specification The test results of the five Montreal field sealants were tested in the laboratory in accordance with the ASTM D 6690 Type II test specification; the results of the five sealants are reported in Table 7 (13). The results reveal that only Sealant B passes the specifications. Sealants A, E, G, and J do not pass the specifications. Compared to the field performance, Sealant B and E exhibit performance that is superior to the rest of sealants.
Table 7 Montreal Sealant ASTM Test Results (13) Sealant Cone Penetration (25°C) Flow 60°C Resilience (25°C) Bond (-29°C) Result (<90 dmm)*, † (<3 mm)* (>60%)* (3 cycles)* A 86 0.5 57 F Fail B 68 0.5 64 P Pass E‡ 104 1 73 P Fail G 50 0.5 51 F Fail J 66 6 48 P Fail
*ASTM D 3405 requirements. Levels beyond acceptable limits are underlined. †1dmm = 0.1mm.
The five Montreal field sealants were also tested in the laboratory in accordance with the newly developed crack sealant performance tests; the results of the five sealants are reported in Table 8. The sealants were tested under the service conditions of Montreal, which has a high summer temperature of 58°C and a low winter temperature close to -40°C. Therefore, the DSR test was conducted at 58°C and the low-temperature cohesive and adhesive properties tests were conducted at -34°C. According to the
specifications, the low-temperature tests are conducted at 6°C higher than service temperature. The superior sealant was identified as Sealant E, which shows good tracking resistance at summer temperature and low stiffness; high average creep rate and no de-bonding were observed. Sealant B also passes the DSR, CSBBR and CSAT tests; but it does not pass the CSDTT test at -34°C. However, if the tests were shifted to the higher grade (-34°C) such that the sealant would be tested at -28°C, the sealant could pass the
specification. Sealant A did not pass the tracking resistance test for summer temperature, average creep rate, extendibility, and adhesion tests at low temperature. Sealants G and J show poor laboratory performance at low temperature. They demonstrate high stiffness, low average creep rate and little extendibility, and weak bonding.
Table 8 Montreal Sealant Performance-based Test Results
Test DSR CSBBR CSDTT CSAT Result Performance Parameter C (kPa) P Stiffness (MPa) Avg. Creep Rate λ (%) Max Load (N) Criteria >4.0 >0.70 <25 >0.31 >85 >50 Temp (C) 58 58 -34 -34 -34 -34 A 3.0 0.67 21 0.30 11 59 Fail B 38.3 0.98 22 0.31 92 157 Pass E 19.3 0.94 3 0.44 93 No de-bond Pass G 7.0 0.94 126 0.24 0.4 50 Fail J 7.6 0.94 602 0.16 0.7 N/A Fail
New sealant tests were developed based on the performance of sealants tested in the field and on the characterization of other sealants widely used in North America. The newly developed procedures provide fundamental sealant properties that include apparent viscosity at the recommended installation temperature, vacuum oven aging to simulate sealant weathering in the field, a DSR test to assess sealant’s tracking resistance at high service
temperatures, the CSBBR test to evaluate a sealant’s creep properties at low temperatures, the CSDTT to characterize a sealant’s low-temperature extendibility, and low-low-temperature adhesion (surface energy, direct adhesion, and blister) tests to evaluate the bonding between sealant and its substrate.
An apparent viscosity at installation temperature of between 1 and 3.5 Pa.s provides for good crack filling while not being
excessively fluid. Note that this test is the only test performed on un-aged material.
Resistance to tracking at high service temperatures can be controlled using a minimum flow coefficient of 4k Pa.s and a shear thinning exponent of 0.7, as determined using a dynamic shear rheometer (DSR).
Using the modified BBR test (CSBBR), a maximum stiffness at 240 s of 25 MPa and a minimum average creep rate of 0.31 will promote good field performance of sealant materials that must withstand low-temperature service conditions.
Extendibility, as measured with the CSDTT, is another good measure of expected low-temperature performance of crack sealants. The threshold for good expected performance is tied to the lowest application temperature (+ a 6°C shift), which is reported in Table 5.
At this time, of the three adhesion tests that were developed, the direct adhesion test is best suited for use in practical
performance-based guidelines. A minimum load of 50 N at tested temperature coincides with good field performance for sealant adhesion.
Recommendations
The performance-based guidelines for crack sealant grading (SC) are presented in Table 9. For example, SG 52-34 means the
sealant can be used at a high service temperature of 52°C and a low temperature of -34°C. The apparent viscosity test (SC-2) helps to ensure smooth sealant installation. The DSR test (SC-4) sets the sealant’s high temperature grade to prevent tracking. If a sealant does not meet the performance criteria at a selected temperature, the test is repeated at a lower temperature until it does. At low
temperatures, the CSBBR (SC-5) and CSDTT (SC-6) tests predict cohesive performance of sealants. The direct adhesion test (SC-7) addresses the expected bond performance. The sealant is first tested using CSBBR and CSDTT tests at 6°C higher than its lowest service temperature to examine its low-temperature cohesive property. If the sealant passes the cohesive property test, it is tested using the direct adhesion test to predict its bonding capability at the same test temperature as CSBBR and CSDTT. The low-temperature grading is determined if the sealant passes three low-temperature tests. If sealant does not pass the bond test but the extendibility is still appropriate at a lower grade, then the low-temperature grade is determined by the cohesive test (CSBBR and CSDTT). Otherwise, the sealant is rejected for use at the testing temperature. The low-temperature cohesive grade is selected at -6°C below the low testing temperature. The reliability approach used by SuperPave™ may be applied.
Acknowledgments
This study is based on work supported by the Federal Highway Administration and the U.S.–Canadian Crack Sealant Consortium under pool fund award # TPF5 (045). Virginia serves as the leading state; the Virginia Transportation Research Council of the Virginia Department of Transportation managed the project. The contribution of the participating states, industries, and provinces is greatly appreciated. The contents of this paper reflect the views of the authors, who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the pool fund-participating departments of transportation or the Federal Highway Administration. This paper does not constitute a standard, specification, or regulation. Special
thanks are extended to Amara Loulizi, Mostafa Elseifi, William Hobbs, Samer Dessouky, and Hector Figueroa Hernandez.
Table 9 Crack Sealant Perform a nce Grade Crack Sealant Perform ance Grade SG 46 SG 52 SG 58 SG 64 SG 70 SG 76 SG 82 -46 -40 -34 -28 -22 -16 -10 -46 -40 -34 -28 -22 -16 -10 -46 -40 -34 -28 -22 -16 -10 -46 -40 -34 -28 -22 -16 -10 -46 -40 -34 -28 -22 -16 -10 -46 -40 -34 -28 -22 -16 -10 -46 -40 -34 Apparent Viscosity, SC-2 Installation Tem p erature Maxim u m Viscosity (Pa.s) 3.5 Minim u m Viscosity (Pa.s) 1 Vacuum Oven Residue (SC-3) Dyna m ic Shear, SC-4 46 52 58 64 70 76 82 Minim u m Fl ow Coeff. (kPa.s) 4 M in imu m S h ea r Thinning 0.7 Crack Sealant BBR, SC-5 -40 -34 -28 -22 -16 -10-4 -40 -34 -28 -22 -16 -10 -4 -40 -34 -28 -22 -16 -10-4 -40 -34 -28 -22 -16 -10-4 -40 -34 -28 -22 -16 -10-4 -40 -34 -28 -22 -16 -10-4 -40 -34 -28 Maxim u m Stiffness (MPa) 25 Minim u m
Avg. Creep Rate
0.31 Crack Sealant DTT, SC-6 -40 -34 -28 -22 -16 -10-4 -40 -34 -28 -22 -16 -10 -4 -40 -34 -28 -22 -16 -10-4 -40 -34 -28 -22 -16 -10-4 -40 -34 -28 -22 -16 -10-4 -40 -34 -28 -22 -16 -10-4 -40 -34 -28 Minim u m Extendib ility (%) 85 85 70 55 40 25 10 85 85 70 55 40 25 10 85 85 70 55 40 25 10 85 85 70 55 40 25 10 85 85 70 55 40 25 10 85 85 70 55 40 25 10 85 85 70
Crack Sealant AT, SC-7
-40 -34 -28 -22 -16 -10 -4 -40 -34 -28 -22 -16 -10 -4 -40 -34 -28 -22 -16 -10 -4 -40 -34 -28 -22 -16 -10 -4 -40 -34 -28 -22 -16 -10 -4 -40 -34 -28 -22 -16 -10 -4 -40 -34 -28 M in imu m L o ad ( N ) 50 Not e: Cr
ack sealant sur
face ener gy is provide d by ma nufact ure r.
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