PERFORMANCE CHARACTERISTICS OF THE OPEN-GRADED ASPHALT CONCRETE FILLED WITH A SPECIAL CEMENT GROUT

audrius.vaitkus@vgtu.lt Abstract. This paper presents a performance of the open-graded asphalt concrete filled with a special cement grout in the road structures, introduces practical examples of the usage of this technology and defines the main properties. In addition, laboratory design of asphalt mixtures filled with special grout is researched. Finally, the results obtained from various laboratory tests are evaluated. The paper contains the open-graded asphalt concrete filled with a special cement grout mix design procedure and results of stiffness measurement, low temperature properties, permanent deformation and skid resistance. Open-graded asphalt concrete filled with a special cement grout mixture is compared to the commonly used asphalt mixes. The open-graded asphalt concrete filled with a special cement grout showed better results than the commonly used asphalt mixes by the stiffness and resistance to the permanent deformation characteristics but behavior at the low temperatures is slightly


Introduction
Hot mix asphalt (HMA) is commonly used material as a wearing layer for pavements of highways (or roads), streets, airports and other industrial areas. During the last decades there have been dramatic changes in traffic volumes, traffic weights and tyre pressures, which have resulted in a significant increase in a permanent deformation of hot mix asphalt pavements. Permanent deformation is a major mode of failure in flexible pavements consisting of both rutting and shoving. This problem is more common in hot climates where the stiffness of the asphalt mix is further decreased with the increase of pavement temperature (Hajj et al. 2011).
The occurrence of such deformations is increasing considerably during the hot period when air temperature rises up to 25-30 °C and asphalt pavement heats up to 40-50 °C (Laurinavičius, Čygas 2003).
According to various scientists, asphalt pavement rutting is affected mainly by three factors: − the asphalt mix factor (eg aggregate gradation, binder performance grade, binder content, air void content, filler); − load factor (eg wheel load, tire pressure, load duration, rest period, load type); − environmental factor (eg moisture, temperature, ageing). Extensive research has been conducted to evaluate the influence of these factors on the performance of pavement structure (Breakah et al. 2011;Collop, Elliot 1999;Ding et al. 2011;Liu, Wang 2003). However, there still remains many problems with the performance of durable pavements. Traffic loading, which directly affects the asphalt pavement and structure, is the most studied and analysed factor. However, it is important to note, that asphalt pavement deteriorate even if there are no traffic at all. This is due to the irreversible physical and chemical processes that change in time, and asphalt pavement surface is also affected by environmental factors such as temperature variations, amount of rainfall and amount of water in soil.
The asphalt pavement is subjected to many distresses during its service life. Recently, rutting in asphalt pavements has become one of the major distress forms with the increase in traffic volume, tyre pressure and axial load.
Depending on the magnitude of the traffic loading and the relative strength of the pavement layers, rutting occur in the asphalt wearing layer, base or subgrade layer or in a combination of these layers (Tarefder et al. 2003). It often happens within the first few years after opening to traffic. The rut is formed because of the shear stress in asphalt layer which causes large plastic deformations, thus the part of the asphalt layer is imprinted into the ridge at the edges of wheel roll zone (Oscarsson 2011;Xu, Huang 2012).
The rutting behaviour of asphalt mix depends not only on the properties of aggregate and binder but also on how they interact with each other in a mixture. There are occasions when binders (eg very stiff binder) and aggregate (eg aggregate with 100% fractured surface) are adequate, but the mixture fails to exhibit low rutting because of other properties (such as incorrect air voids and binder content). Mixture properties alone are not sufficient to ensure low rut because they vary with repeated traffic loads, temperature and moisture etc (Tarefder et al. 2003).
The temperature is one of the most important environmental factors, which affects the stiffness of asphalt pavement. Low temperature cracking is one of the major distresses in asphalt pavements. Cracking is a serious problem in cold regions as well as in areas with large daily temperature fluctuation. Whenever the ambient air temperature drops, the bitumen tends to contract. This continuous contraction phenomenon results in a tensile thermal stress build-up in the mixture. If this thermally induced stress exceeds the tensile strength of the asphalt pavement, a transverse crack will occur at the surface. This thermal cracking develops at a critically low temperature, referred to as the fracture temperature (Bhasin, Motamed 2011;Bražiūnas et al. 2013;Kanerva et al. 1994). Also due to the rise in surface temperature over the temperature of bitumen softening point the bitumen softens and causes formation of waves, inflows and ruts.
The problem of rutting has been relevant and yet unresolved in Lithuania as well as in other countries. Ruts become deeper as heavy vehicles drive on highways and other country roads. With no timely corrections to the pavement in a few years rutting would exceed the limit depth of 20 mm and reach a critical depth of 40 mm, what would inevitably increase the number of traffic accidents (Sivilevičius, Vansauskas 2013).
According to Lenfant (2012), there are several ways to solve rutting problem: − to use a harder binder grade, − to reduce the thickness of wearing courses, − to increase stiffness modulus, − to use a high penetration index binder, − a higher quality of aggregates in asphalt mixes. Generally, pavement structures are divided into two groups: flexible and rigid. While flexible asphalt pavement stiffness depends heavily on the temperature, the temperature has no effect on rigid concrete pavements. Asphalt pavements consist of several relatively thin asphalt layers, which are connected and interact with each other. They are laid on the base layer, which lies on the subgrade. Loads from traffic are distributed through all the layers from the surface to the subgrade and by approximately triangular spreading of forces. Concrete pavements consist of concrete slabs and base layers which lie on the subgrade. The layer has a thickness of approximately 200 mm to 300 mm (depending on traffic load) and it is necessary to use longitudinal and construction joints reinforced with dowels and tie bars. The advantages and disadvantages of the flexible and rigid pavements are given in Table 1.
Semi-rigid or semi-flexible pavements was developed in France in early 1960's as a cost effective alternative to Portland cement concrete pavement (Gawedzinki 2008). This type of pavement has the advantages of the both parts: performs like concrete and is flexible like asphalt mixtures. As the construction of grouted macadam has developed, so have the applications. Today it is used for bus stations, port pavements, industrial and warehouse floors, airport platforms, taxiways and runways, brake and acceleration strips at traffic lights and bridge deck overlays (Al-Qadi et al. 1994;Oliveira et al. 2006;Van de Ven, Molenaar 2004).
Semi-rigid pavements due to high performance have become the prevailing structure type of base layers used on major highways in China (Sun 2007;Zhuang et al. 2003). Vaitkus and Paliukaitė (2013) determined that after 5 years of road exploitation the rut depth of open graded asphalt filed with cement grout is the lowest one of the tested 27 different pavement structures. Anderton (2000) made an inventory of four USA Air Force facilities where grouted macadam had been used. The overall condition of these sites was very good. The city of Stockholm has been using grouted macadam at bus stops since 2001. Most of them needed no maintenance.
Mississippi Dept of Transportation performed a project investigating grouted macadam at two signalized, The base course must be of a higher quality; Difficult repairs and recycling; Higher initial costs.
heavily trafficked intersections (Battey, Whittington 2007). The project included construction, testing and a five-year performance evaluation. In the performance evaluation the grouted macadam was the only pavement material that had no rutting. It was also otherwise in a good condition but there were some problems with skid resistance. Jacobsen (2012) concluded that grouted macadam has mechanical properties similar to those of asphalt concrete, the difference is that it is stronger, stiffer and not prone to rutting. Thus it is considered suitable as a pavement material alternative for intersections experiencing problems with severe rutting. The downside of this material is that it is expensive and takes more time and precision to construct compared to asphalt concrete and that a special license is often required.
From the analysis of the experience of semi flexible asphalt pavement with a special cement grout, it is necessary to further develop this technology. The aim of this paper is to evaluate the determined characteristics of designed opengraded asphalt concrete filled with a special cement grout (AC o-g CG) and compare to common asphalt mixes. Choice of binder type is also prescribed (50/70, 70/100 and PMB 25/55-55). Content of fibers is at least 0.2%. The quality of cement mortar is controlled visually as in Czech requirements. Taking into account the functional characteristics, AC o-g CG has usually higher values of stiffness modulus compared to conventional asphalt mixtures.

The concept of a semi-rigid layer
This type of layers is used in parking areas, bus stops, airport runways, terminals, or areas ahead of crossroads, where they effectively prevent formation of permanent deformations. Furthermore, the layer is also used in container depots, port trans-shipments and in industrial storage areas, where it is possible to take the advantage of its resistance against local puncture loads. They are also used on surfaces of petrol stations and oil depots, where the subgrade or soil is not polluted due to impermeability of the layer. Semi-rigid layers are capable of resisting the effects of chemical defrosting substances and are resistant even to large variations in temperature. Due to fast strengthening, it is possible to put the surfaces into operation already after 12-24 h.
AC o-g CG is a semi-flexible wearing course composed of a flexible asphalt mixture with a gradation up to 8 mm or 11 mm and grouted mortar. The semi-flexible properties are achieved by filling the open-graded asphalt mixture (air void content between 25% and 30%) with a self-levelling grouted mortar. After mixing, mortar is spread evenly or pumped on to the open-graded asphalt and spread evenly by using a rubber scrapers and thereby penetrates into the voids without requiring vibration. The mortar is applied only on the cooled down open-graded asphalt mixture. Usual dosage of mortar is 10 l/m 2 −15 l/m 2 at 50 mm thickness of AC o-g CG layer.
In the Czech Republic, this technology is fully approved and a valid standard ČSN 73 6127-3:2008 exists since 2008. Semi-rigid layer is now used quite commonly, eg in many industrial areas and on bus stops in Prague. In Lithuania, semi-rigid pavements have started being used in 1999. Since then this pavement type was laid on the Vilnius International Airport, on bus stops, roundabouts, and in 2007 also on the experimental pavement structure road in the Vilnius City (Vaitkus et al. 2012). An example of a semi-rigid layer is shown in Fig. 1.

Design of filling grout
Filling grout forms an essential part of semi-rigid layers and has a significant effect on its major properties. It is therefore important to pay close attention to its design and to find a suitable composition. The accuracy of the composition was assessed using the Marshall specimens. After cutting, the quality of the filling of gaps was inspected visually. The initial composition was determined experimentally and gradually improved until reaching the final version given in Table 4.
Czech standard ČSN 73 6127-3:2008 specifies the requirements of filling grout compression (at least 25 MPa) and flexural strength (at least 4 MPa). To verify that both parameters are within the standard requirements, two sets of the test specimens of grout final composition were made. The first specimen strength was determined after 7 days and the second − after 28 days. The test was carried out according to the standard EN 196-1:2005 Methods of Testing Cement -Part 1: Determination of Strength, which specifies the specifications of the compressive and flexural strength determination test with specimens, dimensions of which are 40×40×160 mm.
The results of the final composition filling grout strength after 28 days were very promising and with sufficient margins. The determined value of compressive strength was 29.9 MPa and the flexural strength − 8.7 MPa. This composition of filling grout was used for further tests.

Methods of the experimental research
The research scheme of AC o-g CG and common asphalt mixtures characteristics is shown in Fig. 2.
Mechanical properties of the AC o-g CG, which were carried out during the research are given in Table 5. All laboratory tests were performed in road laboratory of Brno University of Technology. For the purposes of the stiffness modulus comparison, five trapezoidal shaped specimens were produced from each mixture. Each measured value of low temperature characteristics is an average of the results from five measurements (five test specimens).
For all tests of the AC o-g CG the test slabs were made. Specimens of stiffness modulus and low temperatures characteristics determination test were cut using a circular saw with a diamond blade.   Fig. 3.
The test results showed, that the stiffness modulus of the AC o-g CG at the range of natural pavement condition is 7980 MPa. This value complies with the requirements of the asphalt mixture stiffness modulus according to the Czech National Annex (Performance Approach) of the standard EN 13108-1:2006. In comparison with other mixtures, which are commonly used in the Czech Republic in urban areas for wearing courses and binder courses, the stiffness modulus of the AC o-g CG is the highest at all frequencies.
The relationship between stiffness modulus of the AC o-g CG and temperature is presented in Fig. 4. For the purpose of more extensive evaluation of the mixtures, selected parameters of the common asphalt mixtures with different binders are also included in Fig. 4.
Stiffness modulus of AC o-g CG at 40 °C was 4012 MPa and it was the highest of all compared mixtures. Fig. 3 shows that the AC o-g CG does not exhibit such a high temperature influence as in the case of asphalt mixtures. This is due to the presence of the cement mortar. Such property is advantageous especially at the higher temperatures when the AC o-g CG exhibits high stiffness and therefore the lowest tendency towards formation of the permanent deformations (rutting). Table 6 shows a comparison of low temperature characteristics of the AC o-g CG and the selected commonly used asphalt mixtures with different binders.

Low temperature cracking and properties by Thermal Stress Restrained Specimen Test
The Thermal Stress Restrained Specimen Test showed that AC o-g CG temperature of failure (frost crack) was -11.2 °C. In comparison with other types of asphalt mixtures, the temperature of failure was 36% higher than the average of all mixtures. The highest stress at failure was 5.03 MPa lower than the average of all compared mixtures. Probably a low resistance to frost is caused by low stress relaxation of cement grout in low temperatures.

Wheel tracking test
The wheel tracking tests were performed according to the standard EN 12697-22:2003+A1:2007 at 50 °C temperature using small device (air conditioning). Two parameters were evaluated according to the standard: WTS AIR = the increment of the rut depth, which is calculated as an average value at which the rut depth increases as a result of repeated movement of loading wheel in a small test device, in mm/10 3 cycles.
PRD AIR = relative depth of the rut of the tested asphalt mixtures after N load cycles in a small test device, in %.
The results of the AC o-g CG wheel tracking test and a comparison with the requirements of the standard valid in the Czech Republic are given in Table 7. The measured value is an average of the results determined on two slabs.
The test results showed that the AC o-g CG performs extremely well and complies with the requirements of the standard − PRD AIR value is 6 times lower and the WTS AIR value is 17.5 times lower than the requirement for asphalt wearing course.

Skid resistance
Skid resistance is influenced especially by macrotexture and microtexture of pavement surface. When the macrotexture and microtexture increases, the skid resistance also increases.
Macrotexture is a deviation from the ideal flat surface with specific dimensions of 0.5 mm to 50 mm corresponding to the wavelength of the third-octave bands with an average wavelength of 0.5 mm to 50 mm. Macrotexture consists of coarse and fine fractions of aggregates or surface treatment of concrete pavements.   were used to evaluate skid resistance by measuring the mean texture depth. The MTD value was 0.89 mm, which falls into the first level of classification from five-stage classification (the best classification). This level must be obtained on all roads for the surface of pavements in order to be accepted for the service of the road section.
Other MTD values of the different commonly used surfaces, taken from literature, were used for comparison in Fig. 5.
These values clearly show that the designed semi-rigid layers perform quite well in comparison with the other surfaces.
Microtexture is a deviation from the ideal flat surface with characteristic dimensions of less than 0.5 mm corresponding to the wavelengths of the third-octave bands with an average wavelength of 0.5 mm. The microtexture is determined by the size and shape of the individual grains of the aggregate.
Pendulum Test Value (PTV) = loss of energy as the standard rubber coated slider assembly slides across the test surface and provides a standardized value of skid resistance, test procedure according to the standard EN 13036-4:2011.
The value of microtexture was determined using the British pendulum test, where the PTV value is deducted from a scale. To achieve higher accuracy, the test was repeated five times and the resulting value was given as the average value rounded to the nearest integer. Because the test was performed in a room with a controlled temperature (20±2) °C, no corrections of the value were necessary. Technical Guidelines TP 87 and the standard ČSN 73 6177:2009 were used to evaluate skid resistance using the pendulum test. The determined PTV value was 63, which falls into the second level of classification from five-stage classification (the second best classification).

Conclusions
The open-graded asphalt concrete filled with a special cement grout showed better results than the commonly used asphalt mixtures by stiffness and resistance to permanent deformation characteristics. Also, the specimen of this material showed excellent skid resistance results.
The open-graded asphalt concrete filled with a special cement grout showed better results than common asphalt mixtures by these characteristics: − Stiffness Modulus at 15 °C and 10 Hz − from 2.8% to 41.2% higher; − Stiffness Modulus at 40 °C and 10 Hz − from 1.35 times to 3.33 times higher; − Proportional Rut Depth − from 3.75 times to 6.25 times lower; − Wheel Tracking Slope − from 12.5 times to 17.5 times lower. Slightly problematic is the open-graded asphalt concrete filled with a special cement grout behaviour at low temperatures, which could be due to the actual composition of the filling grout (eg the usage of cement of a lower strength or the addition of resins). The Thermal Stress Restrained Specimen Test showed that open-graded asphalt concrete filled with a special cement grout temperature of failure (frost crack) was 36% higher than the average of all the mixtures.
Based on the evaluation of the experimental research result, the authors maintain that the proposed open-graded asphalt concrete filled with a special cement grout complies with all the standard mechanical properties requirements. In addition, other selected properties (mostly functional) were also assessed in order to better understand the behaviour of the layer.