John B. Metcalf

Characterization of Asphalt Concrete Layer Interfaces

A new constitutive model for the asphalt concrete layer interface is proposed. Direct shear tests at four levels of normal load and three temperatures were performed on two types of asphalt concrete layer interface: with and without a tack coat. The shear stress-displacement curves determined in these tests were used to derive the constitutive model, as the tangential and normal stresses at the interface are decoupled. In the proposed model, the shear stress and displacement are proportional until the shear stress equals the shear strength and the interface fails. After failure, a friction model may be used to represent the interface condition. Three parameters were considered to completely describe the interface behavior: the interface reaction modulus K, which is the slope of the shear stress-displacement curve; the shear strength Smax; and the friction coefficient after failure μ. For the interface with a tack coat, K and Smax are not affected by the normal stress level, but they are affected for the interface without a tack coat. All three parameters of the constitutive model are temperature dependent. A testing configuration for determining the shear fatigue behavior of the interface is also described. The fatigue tests indicated a linear increase of the permanent shear displacement with the number of load repetitions, the rate of increase being higher for higher stresses. The fatigue test can be used for a comparative evaluation of the durability of different types of interfaces.

Simple Approach to Estimation of Pavement Structural Capacity

The use of the falling weight deflectometer (FWD) as a tool for investigating the structural strength of flexible and rigid pavements is now common. The backcalculation of layer moduli is the primary technique for data interpretation. This technique is cumbersome and expensive to apply to low-volume roads. A simple way of relating the FWD surface deflections to the performance of pavement structures, expressed by the structural number (SN), is presented. The technique is proposed as an alternative to existing methods in Louisiana using Dynaflect data. The relationship was deduced by statistical analysis, using the data collected at the Louisiana Transportation Research Center Pavement Research Facility. The study found that the correlation between the SN and the FWD deflections is satisfactory. Two different relationships are recommended for flexible and semirigid pavements. Both relationships can be applied in a pavement management environment because they allow evaluation of the required thickness for the asphalt overlay. It was found that the structural layer coefficients must be assigned functions of the laboratory moduli determined by the indirect tensile test, not functions of the backcalculated moduli.

STABILIZATION OF SULFATE-CONTAINING SOIL BY CEMENTITIOUS MIXTURES MECHANICAL PROPERTIES

Winn Rock (CaSO4) gravel from a quarry in Winn Parish in north Louisiana was used extensively as a surface course for local parish roads. Stabilization of these roads with Type I portland cement followed by an overlay of asphaltic concrete resulted in heaving. A study was undertaken to investigate the cause or causes of the expansion as well as to identify an alternate means of stabilization. Specimens of representative soil from the affected area were stabilized in the laboratory using various cementitious materials and were cured using a variety of methods. The mix contained 5% to 20% cementitious material. The cementitious materials were Type I portland cement, lime, and supplementary cementing materials such as granulated blast furnace slag (BFS), Class C fly ash (CFA), silica fume, and an amorphous silica (AS). The unconfined compressive strength of the stabilized soil was determined. The effect of size fractions other than the gravel on the expansion was assessed, and the expansion of the specimens over time was monitored. The cement and BFS mixtures almost doubled the compressive strength of the specimens compared with portland cement alone. The finer size fractions were responsible for expansion. The magnitude of expansion was directly proportional to the amount of Type I portland cement, the amount of available moisture, and the curing temperature. Replacement of a part of the portland cement by BFS significantly reduced the amount of expansion even at the highest moisture content. No expansion was detected when CFA and AS partially replaced the cement.

Effects of interface condition and horizontal wheel loads on the life of flexible pavement structures

The effects of interface condition and horizontal wheel loads on the life of flexible and semirigid pavements were determined. The methodology consisted of implementing a previously derived interface constitutive model into the ABAQUS finite element program to compute the stresses and strains in typical flexible and semirigid road structures. The Shell transfer functions for fatigue cracking and terminal serviceability were used to estimate the life of the two pavements. The study revealed that the horizontal loads acting at the pavement surface lead to dramatically increased tensile strains at the top and bottom of the wearing course and at the top of the binder course. This may justify the initiation of cracking at the surface of the pavement and not at the bottom of the asphalt layer, as generally assumed. For semirigid pavements, the condition of the wearing-binder course interface affects the strains in the wearing course, whereas the condition of the binder-base interface affects the horizontal strain field in the binder layer more as well as the vertical strains at the top of the subgrade. For flexible pavements, the condition of the interface between the wearing and binder courses dramatically changes the strain field in the wearing and binder layers and increases the vertical strains at the top of the granular base and subgrade layers. The cumulative effect of the interface condition and horizontal forces acting at pavement surface is expressed by a dramatic reduction in pavement life, especially for the semirigid pavement.

The influence of admixtures on the strength and linear expansion of cement-stabilized phosphogypsum

Abstract The effect of admixture content, dry density and curing condition on linear expansion of cement-stabilized phosphogypsum (CSPG) was studied over a ninety-day period. The phosphogypsum was stabilized using 8% Type I portland cement. Cylindrical CSPG specimens (51mm x 102 mm) were fabricated by static compaction (ASTM D 698) at three density levels: standard Proctor maximum dry density (13.7 kN/m3) and 5% on either side of this density with a moisture content (20%) corresponding to the maximum standard Proctor dry density. CaCl2 (1% and 2 %) and Daraset (0.05% and 0.15%) as a percentage of the amount of cement, were added to CSPG. Curing conditions were (at ambient temperature): open to air, moisture-controlled and soaked. Selected specimens were analyzed by derivative thermogravimetry and scanning electron microscopy. When cured under moisture-controlled environment, CSPG had a short initial period of expansion irrespective of the dry density or admixture content. Increasing dry density led to a period of contraction following expansion. At the same dry density, the additon of CaCl2 led to a period of no length change while the addition of Daraset led to more initial expansion. The length change, over time, of air-cured CSPG specimens was negligible. The correlation between ettringite content and expansion was crude. For soaked specimens, ettringite growth was widespread and unusually high. Compacted at the lowest density (13.0 kN/m3) and cured in moisture-rich environments, CSPG deteriorated significantly.

EVALUATION OF PROBABILITY DISTRIBUTION FUNCTION FOR THE LIFE OF PAVEMENT STRUCTURES

Determination of the probability distribution function for the time to failure is essential for the development of pavement life models, because the probability distribution function reflects the variability in pavement degradation. The pavement life and failure time are associated with the number of equivalent standard axle load applications for which the degradations reach a critical level. When the critical degradation level is reached, maintenance and rehabilitation work needs to be done to improve pavement condition. Research was undertaken to identify the appropriate statistical models for determination of the probability distribution function for the time to failure of pavement structures. The study used the rutting data collected on a test lane at the first full-scale accelerated pavement test in Louisiana. The research indicated that closed-form solutions or Monte Carlo algorithms can be used when the degradation models have a known form. The bootstrap algorithm can be used to determine the confidence intervals for probability of failure at a given time. If the form of the degradation model is not known, the survival analysis method based on censored observations must be used. The methods can be used not only for rutting life models but also for other pavement life models: cracking initiation time, cracking life, roughness, and serviceability lives.

Performance and Failure Modes of Louisiana Asphalt Pavements with Soil-Cement Bases Under Full-Scale Accelerated Loading

The performance and failure modes of conventional soil-cement base asphalt pavements and alternative pavements have been investigated under accelerated loading. Surface cracking was evaluated in terms both of the crack rate (in meters per square meter) and the AASHTO Class 1, 2, and 3 classifications. Pavement structural capacity was evaluated in terms of falling weight deflectometer (FWD) deflection-based back-calculated moduli of asphalt layer and soil-cement bases. Analysis showed that there was no significant difference in pavement performance among the soil-cement base pavements, although the cement content, strength, and construction procedures for the soil-cement bases were different. A significant improvement in pavement fatigue life was found for the “inverted” pavement structure, in which a stone crack relief layer was placed between the soil-cement base and the asphalt surfacing. The fatigue life of the inverted pavement was five to six times longer than that of pavements without the stone layer. Different failure modes were found between the inverted pavement and the others, and the corresponding failure mechanisms were analyzed on the basis of the observations from the pavement postmortem, when loading was terminated. Analysis of the FWD deflection data indicated that under accelerated loading there was a significant decrease in asphalt and soil-cement moduli for the inverted pavement, although the same trend was not observed on the other soil-cement bases. The crack data were used to evaluate two pavement fatigue life prediction models. It was found that the current models are not applicable to the soil-cement base pavement with a stone crack relief layer.

ASSESSMENT OF PAVEMENT LIFE AT FIRST FULL-SCALE ACCELERATED PAVEMENT TEST IN LOUISIANA

The first full-scale accelerated pavement testing experiment in Louisiana began in February 1996. The purpose was to evaluate the historically prevalent flexible crushed-stone and in-place soil cement-stabilized base construction in comparison with several alternative base construction materials and construction processes for pavements designed for a semitropical climate. More than 6 million equivalent single-axle loads (ESALs) were applied to nine test lanes in the three phases of the project. The full-scale loading was provided by an accelerated loading facility (ALF) machine, the second of its type in the United States. Surface deflection, longitudinal and transverse profiles, surface cracking, stresses and strains in the pavement structure, as well as environmental conditions were monitored during testing. The initial findings are presented in relation to rutting, roughness, cracking, layer modulus, and stress and strain evolution. In order to account for the localized deterioration of some lanes, a method based on statistical survival analysis theory was used to assess the pavement life. Reasonable agreement was observed between the life of the tested structures and the life predicted by the current procedure, on the basis of the 1993 pavement design guide from AASHTO, for the crushed-stone base pavements. The observed lives of the pavements with a soil cement base were shorter than the predicted lives.

Practical Approach to Criteria for the Use of Lime–Fly Ash Stabilization in Base Courses

In October 2000 the Mississippi Department of Transportation (MDOT) initiated a study to evaluate the long-term performance of lime-fly ash (LFA) stabilized soil as a base course material. This study entailed performing falling weight deflectometer (FWD) tests on both newer and older pavements and coring pavement at each FWD location to observe the condition of the layers, to obtain pavement thicknesses, and to perform unconfined compressive strength (UCS) testing. Visual observation, backcalculated modulus, and in situ structural layer coefficient values showed that MDOT LFA-stabilized soil base courses have highly variable material properties and thicknesses. Recommendations were made to increase the average LFA material property values and to reduce the spread in these values by increasing the required compaction of the LFA-stabilized soil base layer to 100% standard Proctor effort, setting the required in situ Proctor UCS at 400 psi, and reducing variability by either improving the current method of f...

AN INITIAL INVESTIGATION OF THE USE OF A RUBBER WASTE (EPDM) IN ASPHALT CONCRETE MIXTURES

The need to improve the performance of asphalt concrete mixes for heavier traffic loads has led to several developments which include the use of polymers to improve asphalt cement (AC) properties resulting in Hot Mix Asphalt Concrete (HMAC) mixes with decreased temperature susceptibility, and increased resistance to rutting and load-associated. Polymer additives can also improve adhesion and cohesion and resistance to moisture-induced damage. A large quantity (300 Kilotons) of Ethylene Propylene Diene Monomer (EPDM) is produced by several industries in Louisiana each year. Of this, 1 to 5 percent represent residual waste and is being landfilled at a cost of