Mourad Y. Riad

Characteristics of Concrete Contact Stresses in Doweled Transverse Joints

The triaxial state of concrete contact stresses that develop at dowel-concrete interfaces in dowel jointed concrete pavements is examined using detailed nonlinear three dimensional finite element (3DFE) analysis. The results are validated through comparisons with: (a) measured strains from laboratory-loaded doweled joint specimens; (b) computed bearing pressure using closed-form solution; and (c) field-measured dowel bending due to slab curling in an instrumented highway section in West Virginia, USA. A parametric study is conducted to investigate the influence of some design parameters on the magnitudes of concrete contact stresses. It is shown that the triaxial contact stresses include a tensile stress component that develop in concrete on both sides of a loaded dowel bar and may initiate small horizontal cracks that reduce load transfer efficiency of the doweled connection. A modified dowel design that reduces the intensity of concrete contact stresses is developed and its laboratory test results are described. The modified dowel design is being field tested in the newly constructed Robert C. Byrds instrumented Highway section in Elkins, West Virginia, USA.

Early age cracking of reinforced concrete bridge decks

This paper describes the instrumentation and test results of a reinforced concrete bridge deck constructed on three-span continuous steel girders in Evansville, West Virginia. An instrumentation system consisting of 232 sensors is developed and implemented specifically to measure strains and temperature in concrete deck, strains in longitudinal and transverse rebar, the overall contraction and expansion of concrete deck, and crack openings. Data from all sensors are automatically collected every 30 minutes starting at the time of placing the deck. The results indicated that elevated longitudinal stress due to constrained drying shrinkage is the main factor responsible for crack initiation during the first two days after concrete placement. Several factors contribute to the deterioration of the deck: (1) thermal stresses developed in the deck because of the constraining effects of the stay-in-place forms and shear studs; (2) non-uniform curing of the concrete along the deck; (3) in-plane temperature variat...

Nonlinear Temperature Gradient Effects in Dowel Jointed Concrete Slabs

The effect of nonlinearity in Temperature Gradient Profile (TGP) on dowel jointed concrete slabs is examined using nonlinear Three-Dimensional Finite Element (3DFE) modeling. The models thermal response is validated versus field-measured data collected from a heavily instrumented section on the Robert Byrd Highway (Route 33) near Elkins, West Virginia, USA. Results indicate that TGP nonlinearity has its maximum effect on the longitudinal stress 0.28 m away from the transverse joint, i.e. near the end of the embedded dowel bars. A TGP that includes a uniform temperature drop is shown to induce a large magnitude of tensile thermal stress at mid slab. This stress is unaffected by the extent of nonlinearity in TGP and depends on the magnitude of temperature drop and the difference between slab top and bottom temperatures. It is shown that dowel bar bending, due to slab curling, introduces a significant edge restraint to slab contraction and expansion due to ambient temperature changes. Mid-slab longitudinal stress is shown to be minimal for a 4.6 m long slab.

Validation of 3DFE Model of Jointed Concrete Pavement Response to Temperature Variations

The most appropriate method to validate the thermoelastic response of a 3D finite element (3DFE) model of dowel jointed concrete pavements subjected to temperature variation is to compare its results with field-measured data. However, field-measured strains in a concrete slab are not only due to temperature variation but they also include components due to construction curling, shrinkage and moisture. Such nonlinear strain contributors are not simulated in concrete constitutive models currently used in 3DFE; thus, field-measured strains cannot be directly used to validate the thermoelastic response of 3DFE models before some data reduction. This paper presents a data reduction technique that renders strains suitable for 3DFE model response validation. The field-measured data were obtained from an intensively instrumented pavement section in West Virginia whose instrumentation plans are described. Data sets of field-measured strains together with the associated temperature profiles are presented in this study. The data can be used by engineers and researchers to validate the thermoelastic response of any 3DFE model of jointed concrete pavement.

Effect of thermal stresses on mid-slab cracking in dowel jointed concrete pavements

Mid-slab cracking is considered to be one of the main causes of pavement deterioration and enormous funds are spent each year on repairing and maintaining cracked pavements. In this study, a nonlinear three-dimensional finite element (3DFE) analysis, which includes detailed consideration of slab constraints by dowel bars, is used to analyse the problem of premature transverse cracking in jointed concrete pavements. The 3DFE model response to ambient temperature variations is validated versus field-measured data obtained from instrumented concrete slabs constructed in Goshen Road near Morgantown, West Virginia, USA. The modelling results indicate that the combination of ambient temperature drop and slab curling induces slab constraints that lead to the development of mid-slab transverse cracks. The slab length is shown to be a critical parameter that governs the magnitude of the maximum thermal stress induced at maximum mid-slab. It is shown in this paper that 4.57 m is the optimal slab length to avoid mid-slab cracking, a conclusion that agrees with recent observations obtained from the analysis of LTPP (Long Term Pavement Performance).

Thermal Stresses in Steel Girder Bridges with Integral Abutments

This paper demonstrates the use of field measurements to evaluate the performance of integral abutment bridges and to check the validity of the design assumptions. A newly constructed integral abutment bridge was heavily instrumented to monitor its long-term performance under the effects of environmental conditions and traffic loading. The collected data indicate that integral abutments resist the expansion of the bridge superstructure during summer time, leading to excessive axial compressive forces in the steel girders. Under such a condition, the 2002 AASHTO criteria for stability and yield of steel girders are barely satisfied under the effect of dead loads and temperature variations and are not satisfied when considering the effect of HS20-44 live load.

Effect of skewed joints on the performance of jointed concrete pavement through 3D dynamic finite element analysis

The effect of skewing the transverse joints in concrete rigid pavements is studied through 3D finite element modelling. Models of multi-layered systems consisting of Portland cement concrete slabs, base and subgrade were developed. These consisted of slabs having straight as well as skewed joints. In order to study the state of stresses and strains in both straight and skewed slabs, the models were subjected to moving axle loads as well as non-linear temperature gradients. More refined models were developed and subjected to falling weight deflectometer dynamic loads to study the state of stresses at the dowels–concrete interfaces. The study, shows that skewing the joints did not reduce the level of stresses and strains in critical locations in the concrete slab. Also a close study of the induced stresses and strains showed that fatigue relations adapted for concrete pavements needed to be refined. The distribution of stresses around the dowel sockets indicates the formation of a tensile stress component, which initiates a tensile crack before any failure of concrete material in compression.

Curling of concrete slabs on grade: experimental versus theoretical analysis

Based on a mathematical approach, Westergaard (1926, 1927, 1943, 1948) derived formulae that predicted the state of stresses and strains in concrete slabs on grade subjected to temperature variations. Although Westergaard made various assumptions, his approach remained a basic tool for the design of concrete pavements. This paper presents a close experimental examination of the deformation of concrete pavement slabs subjected to temperature variations. Full-scale instrumented slabs provided data from key-performance parameters within the scope of a long-term monitoring program where concrete pavement slabs equipped with over 200 sensors are subjected to weather conditions in the absence of traffic loading. A quantitative validation of Westergaards predictions is presented and a modification to Westergaards curling strain formula is suggested based on temperature profiles recorded across the slab cross section. Time histories of temperature profiles showed a temperature change at the mid-slab thickness accompanied by a gradient profile (in contradiction of Westergaards assumption). The measured response and data analysis are further verified through a three dimensional finite element study.

Enhancement of bridge live loads based on West Virginia weigh-in-motion data

This paper presents the development of two weigh-in-motion (WIM) systems in West Virginia to provide site-specific traffic data that can be employed for bridge design and evaluation. The paper discusses the traffic spectra measured in both sites to evaluate the design of live load trucks and discusses the possible enhancement of bridge live loads using the WIM data. The data indicated that the current truck loads are heavier than the AASHTO specified loads. A fatigue design truck model has been developed based on the WIM data. The WIM enhanced fatigue design truck loading was found to be 31% heavier than the HL-93 AASHTO design truck. The data can also be used by bridge engineers and researchers to build a nationwide traffic spectra that would yield a new live load model in future AASHTO editions.

Effect of FWD Testing Position on Modulus of Subgrade Reaction

This paper discusses the variation of the Modulus of subgrade reaction (k) backcalculated from slab deflection basins, interactive with the location of the Falling Weight Deflectometer (FWD) load pulse, and curling of slabs due to daily temperature variations. The k-value was calculated following the AASHTO design guides procedures, while deflection basins were recorded at an interval of 3 to 4 hours along the day on an instrumented concrete pavement test section in West Virginia. The state of deformation of the slabs are continuously monitored, through dowel bar bending measurements and records of the temperature gradient profiles through the slab thickness, as well as joint openings every 20 minutes. The results indicated that the backcalculated k-values are greatly affected by the positive temperature gradient, and the least variation in (k) was found in the slab center. In order to minimize errors in back-calculations of k-values, it is recommended to perform the FWD test for recording deflection basins in the interior of the slab during late evening or in the early morning.