Author: Nari K. Vaswani Publisher: ISBN: Category : Pavements Languages : en Pages : 31
Book Description
The use of a weak sandwich layer in a four-layer system is common in the construction of flexible pavements, but the use of a sandwich layer in a three-layer system is in the experimental stage in Virginia. Theoretical and field studies have been carried out to determine how sandwiched layers affect the design and performance of pavement systems. It has been determined that a flexible sandwiched layer can be economically used in a four-layer system by providing an optimum thickness of the sandwiched material. The optimum thickness as determined in this investigation is the minimum thickness that will: (1) act as a cushion to prevent cracking in the soil cement subbase from reflecting to the surface, and (2) permit compliance with the density specifications. For crushed stone this thickness is 4". Use of this thickness should increase pavement life .and reduce construction costs. It has also been shown that the four-layer system pavements can be evaluated through elastic layered theory. A three-layer sandwich system of economical design and based on traffic requirements is recommended for low traffic volumes. In this case it has been determined that the optimum thickness is that which will (1) prevent reflection cracking through the untreated aggregate from the 6-inch soil cement layer, and (2) satisfy the density specification. These requirements can be met with a 3" to 4" layer of crushed stone with a prime and double seal. The evaluation of the four-and three-layer systems has shown that the strains and the resulting pavement life can be predicted from dynaflect deflections.
Author: Kenneth H. McGhee Publisher: ISBN: Category : Languages : en Pages : 23
Book Description
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. 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. 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. 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. 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. 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.
Author: Kenneth H. McGhee Publisher: ISBN: Category : Pavements, Flexible Languages : en Pages : 60
Book Description
The construction and performance of seven Virginia flexible pavements containing at least some experimental features were evaluated. The objective was to evaluate the performance of the pavements incorporating new or timely design concepts and to assess the flexibility of these concepts for further use.
Author: Nari K. Vaswani Publisher: ISBN: Category : Pavements Languages : en Pages : 40
Book Description
Studies were conducted to relate the deflection of flexible pavements to such environmental factors as temperature and moisture content of the pavements and their subgrade soils. Also considered were the thickness and the relative positions of the different components making up the pavement systems. Seven pavement designs were studied with respect to the above factors. The major conclusions of the study are: 1. The effect of a weak sandwiched layer in reducing pavement strength needs to be considered during pavement design and evaluation. 2. The air temperature considerably affects the pavement modulus. There is a great need for correcting dynaflect deflections for temperature in Virginia. 3. Another factor that affects the value of the pavement modulus is the rigidity of the support to the asphaltic concrete; the greater the rigidity, the, higher the pavement modulus. 4. The temperature sensitivity of the pavement modulus is directly proportional to the pavement modulus and the thickness of the asphaltic concrete layer. 5. The primary factor that affects the subgrade modulus appears to be the relative density of the subgrade soil. Low density soils cause high subgrade moisture and low subgrade modulus and high variations in both the moisture and modulus. The reverse is also true.
Author: National Research Council (U.S.). Highway Research Information Service Publisher: ISBN: Category : Highway engineering Languages : en Pages : 984
Author: Publisher: ISBN: Category : Pavements, Asphalt Languages : en Pages : 43
Book Description
Advances in pavement design technology in recent years have led to more dependence on mechanistic approaches and less reliance on subjective design criteria. In Virginia, the tendency is toward a pavement design and evaluation methodology based on elastic design theories. Underlying this design approach is a need to determine the elastic properties of paving materials. These properties had been evaluated for materials other than subgrade soils in earlier research. Thus the purpose of the present research was to determine the elastic moduli of Virginia subgrade soils and to provide designers with a range of moduli values that might be used in design evaluations. The project was only partially successful because of unidentified factors that appear to have significant effects on the elastic moduli of subgrade soils. Nevertheless, the study showed that within the statistical limitations set forth in the report, the elastic moduli of subgrade soils can be estimated from information routinely collected at the time the preliminary engineering soil survey is. conducted on a proposed highway construction project. The factors having statistically significant impacts on subgrade elastic moduli were determined to be the dry density and the gradation, particularly the percentage passing the No. 200 sieve, of the soil. Contrary to expectations, there was no significant relationship between the California bearing ratio and subgrade modulus. Because of the possibility of large errors in estimating the elastic modulus from the prediction equations developed, the reader is cautioned to use those predicted values as guidelines only in the absence of further data.
Author: Nari K. Vaswani
Author: Kenneth H. McGhee
Author: Kenneth H. McGhee
Author: Nari K. Vaswani
Author: 
Author: National Research Council (U.S.). Highway Research Information Service
Author: Harry T. Craft
Author: 
Author: Nari K. Vaswani
Author: