Innovation in asphalt covering over cement concrete pavement
I. Project overview
1. Background
The North Second Ring Expressway is paved alternatively with cement concrete and asphalt concrete. After nine years of operation, it has contributed greatly to the economic and traffic development of Guangzhou City, but with the growth of traffic and the action of heavy-loaded vehicles, its existing cement concrete pavement has been damaged to varying degrees, affecting driving safety and comfort. Therefore, we proposed to pave asphalt covering on the cement pavement (“White & Black” for short).
2. Construction priorities and difficulties
1) In order not to affect operation and toll collection, this project should be constructed while the expressway is open. The high traffic of the expressway with a high proportion of heavy-loaded vehicles affected construction greatly. In order to ensure smooth traffic in the construction area, we developed a diversion-based traffic organization program through discussion and research.
2) The height above the pavement at the crash wall of the median strip on the North Second Ring Expressway is 90cm, higher than the value of 81cm specified in the Guidelines for Design of Highway Safety Facilities (JTG/TD 81-2006). Since the thickness of the asphalt covering pavement layer is up to 8cm, the only way is to improve the technical indicators of the asphalt mixture in order that the thin-layer structure can withstand heavy loads.
II. Innovation of new material and process
1. White & Black new technology for reflective crack inhibition
The White & Black pavement structure was damaged, and reflective cracks generated by horizontal tensile stresses caused by temperature contraction and vehicular vertical load on cracks and joints, and great shearing stresses caused by vertical displacements when vehicle loads acted on one side of cracks and joints, where the latter contributed more to pavement damage.
The new material used in the preliminary design – GSOG high-viscosity asphalt gap semi-open gradation asphalt mixture – is a both structural and functional asphalt mixture, featuring high porosity, high stiffness modulus and other excellent properties, where high porosity ensures stress absorption and dissipation, while high stiffness modulus enhances the mixture’s resistance to vertical shear, thereby preventing the generation of reflective cracks in the “White & Black” pavement structure.
2. Review of new material and process
After knowing this GSOG new material, our leaders paid a site visit in Shanghai together with technicians. It was learned that Yanggao Road in Shanghai had a high traffic with a high proportion of heavy-loaded vehicles and a limited pavement level, and after 5 years of operation with GSOG asphalt mixture, the pavement developed no reflective crack. Since Yanggao Road was very similar to the North Second Ring Expressway in condition, we thought that the solution used was of great reference value for White & Black. However, RST modified GSOG asphalt mixture was a patented product invented by Shanghai Pudong Road & Bridge Construction Co., Ltd., and was of uniqueness in source.
3. Design finalization
The White & Black project was designed by CCCC First Highway Consultants Co., Ltd. – an authority in the highway industry. Through consultation with the design agency, we recommended this new material. After a survey with Shanghai Pudong Road & Bridge Construction Co., Ltd., the design agency thought that this design was feasible for the current traffic and pavement condition of the North Second Ring Expressway, in which the combination of 5cm GSOG-20 modified asphalt mixture + 3cm SMA-10 modified asphalt mixture was chosen preliminarily.
4. Asphalt and asphalt mixture testing
We found from the testing of asphalt and asphalt mixture that GSOG used RST directly paved modified asphalt mixture without finished RST modified asphalt. Since this mixture is uncontrollable in production and patented, it was of uniqueness in source. Since the White & Black project involved the extensive construction of the main line of the North Second Ring Expressway, it was proposed at the technical briefing to test five mixtures, and choose the best option based on test results in order to find an option more suited to the North Second Ring Expressway and with better quality.
5. Option comparison and selection
The five options were as follows: Option 1—GAC-20 high-viscosity modified asphalt mixture, Option 2—GSOG-20 high-viscosity modified asphalt mixture, Option 3—GSOG-20 RST modified asphalt mixture, Option 4—GAC-20 RST modified asphalt mixture, and Option 5—GAC-20 SBS modified asphalt mixture. Since vertical shear stresses were the main cause of reflective cracks, shear effect-simulating dynamic viscous stiffness modulus reflecting shear resistance, indirect tensile stiffness modulus difference reflecting temperature contraction stability and bending fatigue life reflecting bending fatigue property were to be tested in addition to high temperature stability and water stability as required by the standard, in which shear effect-simulating dynamic viscous stiffness modulus and bending fatigue life were the primary indicators of reflective crack generation.
The test results showed that the bending fatigue property values of Options 4 and 5 were nonconforming, while those of the other three options were conforming and close to the target value; the shear resistance values of Options 1, 2 and 3 were conforming, in which Option 1 had the highest shear resistance; Option 1 performed best in high temperature stability, water stability and temperature contraction stability. Therefore, GAC-20 high-viscosity modified asphalt mixture was finally chosen for the White & Black project after appraisal.
In the White & Black project, we introduced a new material and a new process boldly, and chose the best option through comparison resolutely with quality in mind. The chosen option attained the required reflective crack resistance while reducing construction costs and controlling construction quality effectively without being restricted in material source, thereby accelerating construction progress, shortening construction period and generating practical benefits to the Company.
When completed, this project will ensure the safe operation of the expressway, extend its service life, improve driving safety and comfort greatly, and play an important political and economic role in promoting the economic development of Guangzhou City.
III. Innovative traffic organization program
The traffic organization program of this project was finalized after review and many revisions, and involved a combination of different traffic organization modes based on traffic volumes of different road segments and detours of toll station ramps. It ensured smooth traffic in the construction area, and reduced impacts on operation and toll collection on the North Second Ring Expressway with a high traffic with a high proportion of heavy-loaded vehicles.