K. Sudhakar Reddy

Artificial neural networks—genetic algorithm based model for backcalculation of pavement layer moduli

Backcalculation of pavement layer moduli refers to the process of evaluating the pavement layers using pavement surface deflections. The genetic algorithm (GA) technique was successfully used in the past for backcalculation. The BACKGA model developed by the Indian Institute of Technology, Kharagpur is one such program used for backcalculation using the GA technique. Though GA-based backcalculation models are considered to be robust due to the search algorithm adopted in the process, they require more computational time due to the large number of times the surface deflections are computed using different sets of layer moduli. In the present work, artificial neural network (ANN) models have been developed for computing surface deflections using elastic moduli and thicknesses of pavement layers as inputs. The ANN models have been used in BACKGA for forward calculation of surface deflections to combine the computational efficiency of ANNs with the robustness of the GAs. The performance of the resulting model, BACKGA–ANN, has been evaluated and found to be satisfactory.

SELECTION OF GENETIC ALGORITHM PARAMETERS FOR BACKCALCULATION OF PAVEMENT MODULI

In the context of pavement evaluation, backcalculation is the process of estimation of elastic properties of pavement layers using measured structural responses. A number of backcalculation programs are available for estimating effective pavement layer moduli from surface deflections. Different optimization techniques have been used in the development of these backcalculation models. Genetic algorithm (GA) has been used successfully in the recent past for backcalculation of pavement layer moduli. Selection of appropriate GA parameters is important for the efficient performance of the GA-based algorithm. However, there are no general guidelines available at present for the selection of GA parameters. This paper presents a study conducted for the selection of optimal GA parameters to be adopted for backcalculation of pavement layer moduli. The parameters have been selected on the basis of the level of accuracy desired and the corresponding computational effort (CE). The performance of the GA-based program with the selected parameters was evaluated using some hypothetical and field problems.

Investigation of cold-in-place recycled mixes in India

Cold-in-place recycling (CIR) is a popular and cost-effective bituminous pavement rehabilitation technique. India has very limited experience with this technique. A few stretches of national highways were rehabilitated in the recent past using CIR technique. In the present investigation, laboratory experiments were conducted on different cold-recycled mixes prepared using the bituminous material milled from in-service bituminous pavement. The effect of compaction effort, curing time, aggregate gradation and choice of additive on the performance of the cold mixes was investigated. Structural evaluation of a cold-in-place recycled layer was carried out using falling weight deflectometer. Laboratory investigation yielded guidelines on appropriate accelerated curing conditions, compaction effort and optimum fluid content to be adopted for the preparing cold-recycled mixes using the milled bituminous material obtained from typical Indian highways. Structural evaluation of the recycled layer suggested significa...Cold-in-place recycling (CIR) is a popular and cost-effective bituminous pavement rehabilitation technique. India has very limited experience with this technique. A few stretches of national highways were rehabilitated in the recent past using CIR technique. In the present investigation, laboratory experiments were conducted on different cold-recycled mixes prepared using the bituminous material milled from in-service bituminous pavement. The effect of compaction effort, curing time, aggregate gradation and choice of additive on the performance of the cold mixes was investigated. Structural evaluation of a cold-in-place recycled layer was carried out using falling weight deflectometer. Laboratory investigation yielded guidelines on appropriate accelerated curing conditions, compaction effort and optimum fluid content to be adopted for the preparing cold-recycled mixes using the milled bituminous material obtained from typical Indian highways. Structural evaluation of the recycled layer suggested significant increase in the elastic modulus of recycled layer.

Evaluation of the sensitivity of different indices to the moisture resistance of bituminous mixes modified by hydrated lime and other modifiers

The beneficial effect of hydrated lime added as filler to bituminous mixes in improving the moisture damage resistance of mixes has been known for a long time. However, the effect of binder modification using hydrated lime is less explored. Similarly, very little information is available on the influence of hydrated lime on the moisture resistance of bituminous mixes under critical conditions of high temperature and slow rate of loading. Extensive experimental investigations were conducted in the present study to quantify the relative beneficial effect of addition of hydrated lime in dry and wet methods on the moisture resistance of bituminous mixes. In the dry method, 1.5% and 2% lime (by weight of dry aggregate) was added as filler whereas in the wet method, VG30 and VG40 bitumen were modified using 20% and 30% lime (by weight of virgin binder). The moisture damage resistance of these mixes was evaluated by conducting different tests such as indirect tensile strength, dynamic modulus and dynamic creep tests to assess the sensitivity of different mix parameters in estimating the moisture resistance and to measure the influence of moisture damage on the mechanical properties and rutting performance. The performance of the conventional and lime-modified mixes was also compared with that of polymer and crumb rubber-modified bituminous mixes since lime-modified mixes have been evaluated in this study as possible alternatives to PMB and CRMB mixes, which are popularly used for roads with high traffic volumes. Lime-modified mixes were found to have the best moisture resistance compared to unmodified mixes and PMB- and CRMB-modified mixes. Evaluation of moisture damage in terms of dynamic modulus and dynamic creep tests was found to be significantly more sensitive to moisture damage compared to tensile strength ratio. The beneficial effect of lime has been observed to be significantly more under more severe conditions of high temperature and slow rate of loading compared to lower temperature and high frequency conditions.

Performance Criterion for Thin-Surface Low-Volume Roads

About 80% of the total road network in India is under the rural roads category, and these roads usually carry low traffic volumes. These roads are generally built as granular pavements (with or without a thin bituminous surface) and are designed according to Indian Roads Congress guidelines for the design of rural roads, which do not include mechanistic principles. Considering the size of the rural road network in the country, rationalization of the design approach can result in the optimal use of resources. Gathering experience on the performance of pavements under different climatic and traffic conditions is crucial for developing rational pavement design approaches. Because granular material forms the main structural layers of these pavements, accurate modeling of the granular layers is essential for analysis of low-volume (rural) road pavements. Under application of a load, granular material behaves nonlinearly and shows stress hardening characteristics (i.e., the resilient modulus is stress dependent). In this study, an attempt has been made to develop a rational performance criterion for thin-surface low-volume roads by use of performance data collected from selected road sections in the eastern part of India and consideration of the nonlinearity of the granular layers in the mechanistic analysis of the pavements.

A predictive model for estimating the viscosity of short-term-aged bitumen

For accurate prediction of pavement performance, it is necessary that the effect of ageing on different mix parameters is accounted for. The short-term- and long-term-aged bitumen viscosities estimated using Global Ageing System (GAS) models are used in the Mechanistic Empirical Pavement Design Guide for the estimation of dynamic modulus. Comparison of the viscosities of the bitumens extracted from field- and laboratory-aged asphalt mixes with the viscosities predicted using the GAS model suggests over-prediction for high temperature and lower bitumen film thickness (FT) combinations and under-prediction for low temperatures and high film thicknesses. That the ageing codes used in the GAS model are not sensitive to FT and temperature also has been established in this study. The new statistical model proposed in this study based on fixed effect regression modelling approach for estimating the short-term-aged bitumen viscosity as a function of bitumen type, FT and temperature has been found to predict the short-term-aged viscosity more accurately compared to the GAS model. The predictions with the new model are particularly better for certain combinations of temperature and FT for which large errors are observed in the predictions made with GAS models. The paper also proposes short-term-ageing conditions (sample weight and exposure time) to be adopted for bitumen in a Rolling Thin Film Oven to simulate the field ageing occurring in asphalt mixes having different film thicknesses.

Evaluation of rutting potential of asphalts using resilient modulus test parameters

Rutting potential of asphalts can be evaluated in the laboratory by different tests. Any visco-elastic parameter of the mix evaluated at a high pavement temperature can be considered to be a possible parameter to distinguish mixes in terms of their rutting susceptibility. In this study, the potential of the time lag (between load and deformation) observed in a resilient modulus test as an appropriate parameter to explain the rutting resistance of different mixes has been examined. Resilient modulus is a parameter routinely evaluated by many agencies and is used as an input for design and evaluation of pavements. Time lag values were extracted from the resilient modulus test conducted at 35°C and 50°C on asphalt mixes prepared with (a) nine different types of binder and one aggregate gradation and (b) nine different aggregate gradations and with VG30 binder. A wheel tracking test was conducted on the mixes at 60°C. Time lag has been found to be sensitive to bitumen type and aggregate gradation. A strong correlation was observed between the time lag and rut depth measured in the wheel tracking test. It is evident from the present study that time lag measured from the resilient modulus test, which is conducted routinely by many agencies, has the potential to be used as a mix rutting parameter.

Sensitivity of rutting and moisture resistance of asphalt mixes to gradation and design air void content

ABSTRACT Bituminous mixes are generally designed with gradation and air void content ranges adopted by different agencies. The selected gradation and binder content are permitted to vary within specified tolerance limits. This leads to the possibility of a wide range of mixes with varying mix performance. Thus, it is essential to understand the influence of variation in these parameters on the performance of mixes. The present study aims to examine the sensitivity of the mix performance to variation within the specified gradation and air void ranges. Five aggregate gradations were considered for designing mixes for the volumetric design criterion of 4% air voids. For three of the five gradations, mixes were also designed for two additional design air void contents, 3% and 5%. Rutting resistance of the mixes was measured using a rut tester and moisture resistance was evaluated in terms of Tensile Strength Ratio (TSR). The densification characteristics represented by voids in mineral aggregate (VMA) values of the mixes prepared using the five aggregate gradations are similar to those of the corresponding aggregate packing characteristics (dry aggregate air voids, DAAV). A good relationship was observed between mix rutting and minimum voids in mineral aggregate and DAAV. Sensitivity analysis carried out with different aggregate gradations, design air void contents and tolerances, indicated that rut depth as well as TSR vary over a wide range for the combinations of mixes considered (all within the gradation, air void content and tolerance limits).Compared to the design air void content, aggregate gradation is found to have significantly larger effect on rut depth whereas both gradation and design air void content have similar effect on the TSR value. Using the results obtained from the present investigation, the Indian Marshall mix design criteria (MoRTH 2013) for stability and flow were revisited and new criteria are proposed.

Evaluation of wheel tracking and field rutting susceptibility of dense bituminous mixes

Rutting in bituminous layers is a major mode of distress, especially in pavements with thick bituminous layers. Different binder and mix rutting specifications are adopted by agencies worldwide to address the issue of rutting in bituminous mixes. The wheel tracking test is also widely used to evaluate the rutting potential of bituminous layers which depends on the aggregate gradation, binder type and also on the method of mix design adopted. The present study was conducted to examine the relationship between laboratory wheel track rutting and field rutting of different mixes prepared with different aggregate gradations, binder types and binder contents selected using different compaction efforts. Fifteen pavement sections with different surface mixes were constructed on a national highway. Loose bituminous mixes collected from the field were compacted to prepare specimens for wheel tracking tests. Two sets of specimens were prepared to have (a) 7% air voids and (b) an air void content similar to that observed in the field 1 day after construction. The rut depths measured using a wheel tracking machine correlated very well with field rut depths measured after 1 year when the laboratory specimens were prepared at the same initial air void content as achieved in the field. As expected, the rut depth measured in the laboratory wheel tracker for specimens prepared with 7% air voids did not correlate well with 1 year rut depths of field mixes which had different initial air void contents. Limiting wheel tracking test rut depths have been identified on the basis of comparison of the field rutting performance of different mixes and the corresponding laboratory rutting performance of the mixes. Limiting criteria have been proposed for two limiting rut depths of 10 and 20 mm and for different traffic levels.

Evaluation of rehabilitated urban recycled asphalt pavement

Recycling of pavements is a relatively new technique for India and is gaining popularity in recent times due to several merits the technique has over conventional pavement rehabilitation techniques. The first recycling work of flexible pavements in India was undertaken a decade ago and since then the option of milling and recycling of bituminous layers has been adopted in some parts of urban and high-volume roads in India. However, no research studies were conducted on these works. Foamed bitumen recycled pavement work, which was undertaken on some urban roads of Kolkata city in India, has been reported in this paper. Detailed investigations conducted on the milling and foamed bitumen recycling work have been presented in this paper. Laboratory investigation was carried out on the milled and recycled material. Field evaluation of the pavements was done before and after rehabilitation using falling weight deflectometer. Results indicate that foamed bitumen recycling is a very promising alternative to rehabilitate bituminous pavements and to address the concern of conservation of energy and natural resources.