Pavement Parameters Research Articles

Curvilinear Behavior of Base Layer Moduli from Deflection and Seismic Methods

The reality of curvilinear relationships of stiffness versus deformation is usually neglected when moduli values from seismic methods are compared with those of deflection methods. On the basis of extensive field testing, results showed that moduli values for the base layers from deflection methods did not conform to those of seismic methods. Deflection testing techniques were signified by the falling weight deflectometer (FWD) and the Dynaflect methods. Seismic testing was carried out by use of the seismic pavement analyzer (SPA) method. The SPA test results yielded moduli values higher than those obtained from the deflection methods. Utilizing pavement parameters obtained from the SPA data, researchers determined surface deflections by use of frequency response functions of signals from the two groups of sensors used in the testing setup. Because of the types of hammers in the SPA testing, two different deflection basins were obtained at each testing point. Comparison of surface deflections from these methods indicated that deflection amplitudes from the FWD method were about 100 times higher than those obtained from the high-frequency hammer of the SPA. At certain pavement sections, deflections from the Dynaflect method were comparable to those obtained with the SPA low-frequency hammer. Accordingly, curvilinear relationships between surface deformation versus stiffness values were derived. These relationships can be used to determine moduli values at all surface deflections, including those from service loads.

Relating In Situ Properties of Cement-Bound Bases to Their Performance

A rationale for explanation of the in situ layer modulus expected from a cementitious base is presented. It is assumed that all such bases crack to various degrees, depending on material strength and other pavement parameters, under the influence of shrinkage, temperature changes, and trafficking. The layer modulus is shown to be a function of hinge and slip effects at cracks, as well as the intrinsic material modulus, and simple, practical, but fundamental-based equations are given for its prediction. The point that any given layer modulus could, in principle, be due to any of several different combinations of material modulus, crack spacing, and condition is made, and so careful interpretation is vital. The use of the predictive equations is then demonstrated in six selected cases, covering materials ranging from cracked pavement-quality concrete to lime- and cement-stabilized clay, and the equations are seen to enable the measured layer moduli to be interpreted sensibly. Finally, two examples are given. In these examples similar layer moduli relate to two quite different materials and conditions, which have given rise to different pavement performances. The weaker material resulted in a better combination of crack spacing and condition and a longer-lasting pavement, whereas the stronger material resulted in a pavement very susceptible to reflective cracking. The conclusion is drawn that, for optimum pavement performance, the key is to limit the amount of slip at cracks by ensuring good interlock as well as adequate intrinsic material strength and stiffness. A slowly curing material is ideal.

Back-Calculation of Parameters for Slab on Two-Layer Foundation System

Most highway and airport rigid pavements constructed today consist of a subbase layer, giving rise to a system of a concrete slab supported on a two-layer foundation. This paper makes use of the solution of a slab on a two-layer system to derive a closed-form procedure for back-calculation analysis of pavement parameters. A computer program called NUS-DEF3 to solve for surface deflections is developed as a subroutine in the back-calculation analysis. The algorithm for the closed-form back-calculation of this system is presented. This algorithm reduces a system of three equations with three unknowns to one single nonlinear equation. It differs from the conventional approach of searching for the best solution iteratively by means of the least-squares method, and gives unique closed-form solutions without the need to select an initial modulus to start with. Solutions to numerical examples of three typical slab-on-two-layer-foundation problems are presented.

Assessment of Concrete Pavement Blowups

Blowups of concrete pavements have been a thorn in the sides of highway and airport engineers for years. The author of this paper conducts an assessment based on the analyses of pavement blowups. He briefly reviews the various early attempts to explain the blowup phenomenon, then describes the blowup mechanism. Next, he presents the adopted criterion of the safe temperature increase and outlines the analytical methodology used in the assessment. The author provides analytical solutions for three typical pavement problems and the steps for their numerical evaluation. Pavement parameters and a graphical presentation of the results are given. The author also discusses the practical implications of the analytical results for concrete pavement design and for assessing pavement blowups after years of service. Several specific pavement problems are identified and suggestions of full-scale tests for the determination of the pavement parameters are made.

Tire Tread Temperatures in Actual Contact Areas

Abstract This report treats the temperature in an actual contact area between the pavement and the tread of a cornering tire. A method to compute the temperature by using Jaeger's formula for a moving heat source is proposed. Additionally, a temperature measurement was conducted. The computed value is consistent with the measured value. The computed results for various pavement parameters show that the actual contact area size and pavement thermal diffusivity significantly affect the temperature, and that the distribution of tire tread temperature around the actual contact area varies considerably along the slip direction of the tread.

Loading Mode Effects on Pavement Deflections

Pavement deflection data collected using static (the Benketman Beam), vibratory (the Road Rater), impulse (the Falling Weight Deflectometer), and vehicular loading have been used to identify and explain the factors affecting pavement response to different loading modes relevant to structural pavement evaluation. Pavement parameters derived using the different loading mode devices are shown to be loading mode and device dependent. Comparisons among different testing devices and methods indicate that the Falling Weight Deflectometer best simulates the pavement response under moving trucks. The Road Rater induces a stiffened response of the pavement‐subgrade system due to the steady‐state harmonic loading and static preload, and the quasi‐static loading with the Benkelman Beam induces the highest deflections in most pavements.

Pavement Strength Rating Methods as Viewed by Airframes Manufacturers

The effects of allowable aircraft payload of variations in tire pressure, axle spacing, and wheel spacing were explored using four important pavement design/rating methods. The methods used were the California Bearing Ratio (CBR) (SEFL165A variation) method (flexible pavement), the Load Classification Number (LCN) method (flexible and rigid pavement) and the Portland Cement Association method (rigid pavement). The effects on payload were determined for both dual and dual tandem type loading gear on pavements of an assumed strength. This analysis was followed by a study of the payload effects of variations on pavement parameters such as subgrade strength or pavement thickness in allowable aircraft payload given a particular aircraft configuration. Based upon the foregoing analysis five airframe manufacturer's objectives for new pavement design methods were stated.