SELECTION AND DESIGN OF SEMI-FLEXIBLE AND CONVENTIONAL TYPE PAVEMENTS
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PAVEMENTS CONSTRUCTED OF SOIL-CEMENT, LIME-CLAY, ASPHALT STABILIZED BASES, AND MANY OTHER MATERIALS ARE IN A CATEGORY INTERMEDIATE BETWEEN FLEXIBLE AND RIGID IN THAT THEY POSSESS CONSIDERABLE SLAB STRENGTH, AND AT THE SAME TIME DEFLECT SUFFICIENTLY TO TRANSMIT SIZEABLE STRESSES TO THE SUBGRADE. A DESIGN PROCEDURE IS OUTLINED WHEREBY BOTH ELEMENTS ARE CONSIDERED. PHYSICAL PROPERTIES OF VARIOUS TYPES OF PAVEMENT MATERIALS RANGING FROM TRUE RIGID TYPE TO TRUE FLEXIBLE TYPE ARE CORRELATED WITH WATER-CEMENT RATIO FOR EASE IN USING THE VARIOUS DESIGN PROCEDURES.Keywords:
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The main purpose of this paper is to determine and compare the design thicknesses of asphalt pavements constructed from recycled and new asphalt material. For this purpose, preliminary design charts were developed for determining the thicknesses of recycled and new asphalt pavements of similar composition. These design charts were based on the results obtained from a limited number of fatigue tests conducted in the laboratory. In these tests, cylindrical samples were examined at various temperatures, using the indirect tensile test under controlled stress condition. The values of the poisson's ratio and dynamic modulus of elasticity obtained from these tests were used as inputs, together with the properties of subgrades of various strength, into a modified chevron multi layer program to determine the pavement thicknesses. The results obtained from this limited study indicate that, for the same subgrade and equivalent axle load repetitions, the thickness of a recycled full depth asphalt pavement is smaller than that of new asphalt pavements of similar composition (a).
Asphalt pavement
Axle load
Poisson's ratio
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Reflective cracking is one of main distresses for cement/lime/flyash stabilized base in China. Although flexible base can be and has been utilized as sandwich layer to reduce reflecting cracks from stabilized base, the repetitive truck traffic load may cause higher tensile stress at the bottom of asphalt surface course because of large deformation from flexible base. The high tensile stress may lead to fatigue cracking. To minimize the tensile stress at the bottom of the asphalt layer and to establish optimum structures to reduce reflective cracking, nonlinear finite element technique was utilized to support three experimental pavements in Tonghua Highway in Jilin province. Base on the analyses, pavement structures 1 and 2 are recommended. In addition, for pavement structure 1, the optimal design is to include 7–12cm of AM-30, 10–15cm of flexible base, and 30–45cm of semi-rigid subbase (lime-flyash stabilized soil or cement treated base). Furthermore, for pavement structure 2, the optimal design is to include 15–20cm of flexible base, and 25–40cm of semi-rigid subbase (lime-flyash stabilized base or cement treated base).
Subbase
Base course
Base (topology)
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DEFLECTION MEASUREMENTS ARE BEING MADE BY THE BENKELMAN BEAM TO EVALUATE THE STRENGTH AND LOAD-CARRYING CAPACITY OF FLEXIBLE PAVEMENTS. DIFFERENT PAVEMENT DESIGNS WERE TESTED INCLUDING' HOT-MIXED ASPHALTIC CONCRETE OR SAND ASPHALT WITHOUT CEMENT OR LIME STABILIZATION IN UNDERLYING LAYERS, UNTREATED AGGREGATE OR WATER BOUND MACADAM BASES WITH NO STABILIZING IN THE STRUCTURE OR SUBGRADE, AND ASPHALTIC CONCRETE WITH A CEMENT OR LIME STABILIZED SUBGRADE
Wearing course
Asphalt concrete
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This paper describes how heavy loads that are present on airfields aprons, on storage areas at industry facilities and on road sections such as bus lanes and bus terminals need pavement materials that are rut resistant and have a high stiffness modulus, even at low loading frequencies. The traditional selection of materials in these areas has been rigid pavement structures such as cement concrete. However, more flexible materials that can obtain stand deformations without cracking and without joints, both leading to lower maintenance costs, has been asked for. This has resulted in the development of a semi-flexible material for heavy-duty areas. The semi-flexible pavement material described in this paper consists of an open asphalt structure (5/8, 8/11 mm or 11/16 mm aggregates) where a high strength micro silica mortar is penetrated into the air voids (25 - 30 %) of the asphalt mixture. The results are based on the special type of semi-flexible material called Densiphalt. The semi-flexible pavement combines the micro silica mortar's strength and the asphalt material's flexibility. The special designed micro silica mortar from Densit A/S has the ability to penetrate entirely into the open asphalt structure, thus ensuring a homogeneous pavement material. The design stiffness modulus of the semi-flexible pavement has been determined to 8,000 MPa at 25- degrees Centigrade and loading frequency of 33 Hz, 2-3 times higher than for asphalt. The service life of the pavement materials is 15 to 20 years. The material can be constructed in different colours through additives added to the mortar. The finished surface of the material has different appearance depending on the use and requirements to e.g. the friction. In recent years, the construction of Densiphalt has been further improved by introducing steel reinforcement at the bottom of the layer. Semi-flexible pavement materials are normally constructed in thicknesses between 40 and 100 mm depending on the load applied on the areas. The steel reinforcement has shown to further improve the rutting resistance and thus enables the use on areas with traditional asphalt layers that need partial repairs. Furthermore, the short curing time of the pavement (between 24 and 48 hours, depending on the air temperature) makes the materials an excellent choice in areas where the time of repair shall be as short as possible. Semi-flexible pavements have, with excellent results been used for approximately 10-15 years around the world on aprons in airports, storage areas in ports and industrial facilities plus at bus terminals.
Service life
Fatigue cracking
Asphalt pavement
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This report describes the physical requirements of the five most commonly used subbase materials and suggests how these materials may be used most effectively for varying traffic conditions. Minimum thicknesses of subbases for both rigid and flexible pavements are obtained by use of a combination K-values, potential vertical rise, and conventional pavement thickness design methods. The problem of treating soils that have high volume changes at great depths is discussed, and its solution is used to remedy the subbase problem. Examples for both rigid and flexible pavements are presented, as is a procedure for determining the desired moisture contents of swelling soils at various depths below the pavement.
Subbase
Pavement Engineering
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Heavy static loading that is present on airfields aprons, on storage areas at industry facilities and on road sections such as bus lanes and bus terminals needs pavement materials that are rutting resistant and have a high stiffness modulus, even at low loading frequencies. The traditional selection of materials for this type of loading has been rigid pavements such as cement concrete. However the need for more flexible materials that can obtain deformations without cracking and without joints both leading to lower maintenance cost has been asked for. This has resulted in the development of a semi-flexible material for heavy loading areas. The semi-flexible pavement material is defined as a composite material consisting of an open graded asphalt material where micro silica mortar is penetrated into the air voids.
Heavy duty
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THREE ASPHALT PAVEMENT STRUCTURES ARE CURRENTLY AVAILABLE TO HIGHWAY ENGINEERS: CONVENTIONAL, DEEP STRENGTH, AND FULL DEPTH. SINCE THE COST PER TON OF ASPHALT CONCRETE IS SUBSTANTIALLY HIGHER THAN THAT OF AGGREGATE, A NUMBER OF FACTORS ARE LISTED TO INDICATE POSSIBLE SOURCES OF ECONOMY IN THE USE OF FULL DEPTH OR DEEP STRENGTH ASPHALT PAVEMENTS. IT IS SHOWN THAT THE USE OF DENSE GRADED PAVING MIXTURES, MADE WITH AGGREGATES OF LARGER NOMINAL MAXIMUM PAVING MIXTURES, MADE WITH AGGREGATES OF LARGER NOMINAL MAXIMUM PARTICLE SIZE, COMPACTION DURING CONSTRUCTION TO 100 PER CENT OF LABORATORY COMPACTED DENSITY, AND THE USE OF THICK LAYERS DURING CONSRUCTION, WOULD MAKE DEEP STRENGTH AND FULL DEPTH PAVEMENTS LESS EXPENSIVE AND MORE COMPETITIVE. THE LAYER EQUIVALENCY VALUES PROVIDED BY A NUMBER OF TEST ROADS ARE EXAMINED. PROGRESS TOWARD THE DEVELOPMENT OF A RATIONAL METHOD OF ASPHALT PAVEMENT STRUCTURAL DESIGN IS REVIEWED. THE LOSS OF SMOOTHNESS OF RIDING QUALITY BECAUSE OF ENVIRONMENTAL FACTORS IS LITTLE UNDERSTOOD, POSING A PROBLEM EVEN AFTER A RATIONAL DESIGN METHOD FOR ASPHALT PAVEMENT DESIGN COMES INTO GENERAL USE. /AUTHOR/
Smoothness
Asphalt pavement
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This study investigates de-bonding of asphalt concrete (AC) layers in flexible pavements and determines how de-bonding affects the performance of these pavements using mechanistic models. At present, all flexible pavement design methods assume that there is complete bonding between all AC layers. In complete bonding conditions, the strain at the base of the asphalt layer are transferred to the asphalt layer below, and the displacement at these locations is equal. These assumptions are made to facilitate the modeling of AC pavements. However, in reality, these assumptions are not completely satisfied. Materials in each layer are not identical so the response to traffic loading is different. Using complete bonding conditions in flexible pavement design thus reduces design reliability due to increasing traffic volumes, traffic speed, and tire pressure. Current mechanistic-empirical analysis shows that 90% of the pavements presented in this study fail by top-down cracking if de-bonding occurs between the AC layers. Bottom-up cracking and AC rutting also increase significantly in de-bonded environments. This study recommends the development of a model that accounts for partial bonding/de-bonding in flexible pavement design.
Asphalt pavement
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A program of construction and performance evaluation of seven Virginia flexible pavements containing at least some experimental features is reported. The objective of the program is to evaluate the performance of the pavements incorporating new or timely design concepts and to assess the flexibility of these concepts for further use. Among the major findings of the study to this point are the following. 1. Pavements having equivalent design thickness indices are not necessarily equivalent in construction cost or in early structural strength. 2. Very early deflection tests do not give good indications of the ultimate strength characteristics of pavements having cement stabilized layers. 3. Full-depth asphaltic concrete pavements can give excellent performance in very poor soil areas, especially when the design is modified through the provision of a cement stabilized subgrade. 4. An unstabilized sandwich layer placed between a cement stabilized layer and asphaltic concrete layers is effective in significantly delaying the reflection of transverse cracking from the cement stabilized layer through the asphaltic concrete layers. There is some evidence that reflective cracks may develop after many years under heavy truck traffic. 5. Such a sandwich layer is weaker than either of the two layers it contacts and can cause a net reduction in pavement strength as compared with the situation where the weaker layer is on the bottom. 6. Transverse shrinkage cracks reflect from a cement treated stone subbase through 3 inches (75 mm) of bituminous concrete in as little as 18 months and through 7 inches (175 mm) of bituminous concrete in less than 5 years. 7. Cement treatment of stone subbases can be omitted in passing lanes with no detriment to performance. (This may not be true with traffic volumes near capacity because of the change in distribution of truck usage as that point is approached.) The two following recommendations for consideration by administrators of the Highway and Transportation Department seem appropriate at this time. 1. The Department is encouraged to consider the full depth asphalt concept as a desirable alternative in flexible pavement design. In poor soil areas the designs should be modified to provide cement (or lime) stabilization of the native subgrade soil. Although full depth design may be considerably more expensive than many alternatives, there is strong evidence reported herein that the full-depth pavements can provide performance somewhat better than most of these alternatives. 2. In cases where it is deemed appropriate to stabilize aggregate base materials on divided highways with four or more lanes where truck traffic is normally channeled into the outer lanes, it is structurally feasible to omit such stabilization from the inner or passing lanes. While in many cases there may be no economic advantage in such a practice because of construction difficulties, the concept is recommended for cases where it may be practically feasible.
Subbase
Shrinkage
Wearing course
Fatigue cracking
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NEW MEXICO'S EXPERIMENTAL PROJECT WAS CONSTRUCTED TO COMPARE 'UPSIDE DOWN' STABIILIZATION WITH OTHER BASE CONSTRUCTION. THE TERM WAS APPLIED TO THE DESIGN BECAUSE IT CALLED FOR THE SUBBASE MATERIAL TO BE TREATED WITH CEMENT. NINE EXPERIMENTAL SECTIONS WERE CONDUCTED. THE OBJECTIVE WAS TO DETERMINE THE EFFECT OF SUBBASE STABILIZATION COMPARED TO BASE COURSE STABILIZATION AND THE EFFECT OF A LOWER CEMENT CONTENT IN THE BASE. OF PARTICULAR INTEREST IS POSSIBLE DEGRADATION OF THE MINERAL AGGREGATES IN ALL SECTIONS. THE TREATED SUBBASE SECTIONS SHOULD ELIMINATE INTRUSION OF SUBGRADE SOILS INTO THE BASE. THROUGH PERIODIC INSPECTIONS AND CHECK TESTING IT IS HOPED THAT BETTER KNOWLEDGE CAN BE OBTAINED TO DETERMINE WHICH DESIGN PROVIDES THE BEST PROTECTION FOR FUTURE DISTORTION AND ROUGHNESS. AN ATTEMPT WILL BE MADE TO EVALUATE THE VARIOUS DESIGNS RELATIVE TO COSTS AND SERVICEABILITY. /AUTHOR/
Subbase
Serviceability (structure)
Crushed stone
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