3D Finite Element Analysis of Temperature‐Induced Stresses in Dowel Jointed Concrete Pavements

A detailed 3D finite element (3DFE) model is developed to investigate the applicability of Westergaard’s curling stress equations to doweled jointed concrete pavements. The model does not rely on any of Westergaard’s assumptions and is capable of handling nonlinear and/or time‐dependent temperature profiles. However, only linear gradient is applied to facilitate the comparison with Westergaard’s results. The transverse stress calculated using Westergaard’s formula was found to be within 10% of that computed using 3DFE. Westergaard’s longitudinal stress equation required a correction. The 3DFE results confirm Westergaard’s finding that the slab curling stresses are independent of slab length. Thus, curling stress does not explain the field‐observed dependency of mid‐slab cracking on the slab length. Through the examination of the mechanism of dowel‐concrete interaction, it is shown that uniform temperature changes play the major role in mid‐slab transverse cracking of relatively long slabs. Due to built‐in slab curling as well as temperature or moisture curling, the dowel bars bend restricting the slab from free contraction due to uniform temperature drop. This gives rise to a large component of stress that has not been considered in previous investigations. Application of a combined temperature gradient and uniform temperature drop to slabs of different lengths revealed the dependency of mid‐slab transverse cracking on slab length.     

Validation of 3DFE Analysis of Rigid Pavement Dynamic Response to Moving Traffic and Nonlinear Temperature Gradient Effects

The response of dowel jointed concrete pavements to the combined effect of nonlinear thermal gradient and moving axle load is examined using three-dimensional finite-element (3DFE) modeling. The 3DFE-computed response to moving axle load was field validated versus measured concrete slab response to a fully loaded moving dump truck. The 3DFE-predicted slab curling due to nonlinear thermal gradient through the slab thickness was validated versus: (1) corner-dowel bar bending as measured using instrumented dowel embedded in an instrumented rigid pavement section in West Virginia; and (2) Westergaard’s closed-form solution. The effects of slab thickness, slab length, axle loading position, and axle type on slab stresses are examined. It is shown that while a negative temperature gradient reduces the intensity of traffic-induced stresses, positive temperature gradient increases it several fold. Formulas are developed for the computation of the peak principal stresses due to the combined effect of tandem axle load and nonlinear thermal gradient.     

Finite Element Analysis of Concrete Pavements Over Culverts

Accelerated distress of Portland cement concrete pavements (PCCP) over structures such as culverts, pipes, and tunnels beneath roadways is a common occurrence. In this article, finite element analysis is employed to analyze the response of concrete pavements over such structures. The factors that influence the overlying pavement slabs include: (1) cover depth, (2) pavement slab thickness and length, (3) cement concrete elastic modulus, (4) foundation modulus, and (5) backfill soil modulus. The tensile stresses at the bottom and top of the slab induced by wheel loads are predicted. In the traditional pavement design only the tensile stress at the bottom of the slab is considered to be significant. However, this study shows that the tensile stress at the top surface of pavement slabs over culverts may also cause the concrete pavements to fail. A laboratory model was employed to study the mechanical characteristics of Portland cement concrete pavement slabs over culverts and to verify the theoretical analysis.     

Assessment of Retrofit Dowel Benefits in Cracked Portland Cement Concrete Pavements

A parametric study was conducted using the finite-element rigid pavement program ISLAB2000. For cracks that utilize aggregate interlock as the sole means of load transfer, the integrity of the cracks was initially modeled using the aggregate interlock factor. A subsequent analysis was then performed on the same cracks for the case where both dowel bars and aggregate interlock were available for load transfer purposes. The latter scenario represents the case where dowel bar retrofitting (DBR) has been performed on the cracks. In both cases, the deflection load transfer efficiency and critical slab tensile stresses were computed in order to examine the immediate theoretical benefits of the dowel bars. The validity of these theoretical benefits was tested using data from falling-weight deflectometer testing on DBR sites in both Michigan and Washington. It was found that installation of dowel bars did not increase the load transfer efficiency for cracks that had levels greater than 89–95%, depending on pavement parameters. When temperature gradients were not considered, little change in tensile stress due to a load at the crack was exhibited when DBR was performed on cracks that had load transfer efficiency levels less than 70–80%.     

Factors Affecting Initial Roughness of Concrete Pavement

Past studies have shown that initial pavement roughness greatly affects future pavement roughness and roughness progression rate. Initial pavement roughness is also an important input to the roughness prediction model in mechanistic-empirical design guide. This study analyzed the design and construction factors affecting initial pavement roughness. Initial international roughness index of 90 concrete pavements constructed in Wisconsin from 2000 to 2004 were analyzed using multiple regression method. The factors considered in this study included concrete pavement slab thickness, project location, dowel bar placement, joint spacing, base type, and pavement length. The factors affecting initial pavement roughness were identified.     

Performance of Dowel Bar Retrofit Projects in Texas

In Texas, many miles of plain jointed concrete pavement (JCP) were constructed without proper load transfer devices such as dowels. After a number of years of service, some JCP sections without dowels showed distresses in the form of faulting at transverse joints. Some of the sections were designed in accordance with the AASHTO 1986 Guide, which required 50–75?mm thicker slabs in exchange for not using dowels. This pavement design did not work, with faulting at transverse joints that cause poor ride. Dowel bar retrofit (DBR) was performed on four projects to restore the pavement condition. Overall, DBR restored load transfer efficiency and resulted in improvement of ride quality. Even where the subbase stiffness is 5–10 times less than the minimum value required for proper performance of JCPs, properly installed DBR effectively restored pavement condition with minimum faulting after decades of service. Therefore, it indicated that DBR is able to minimize the faulting even where there is poor base/subgrade support. This is significant in that there are no effective and practical methods to improve subbase conditions in existing concrete pavement, whereas DBR can restore pavement conditions at a reasonable cost. However, not all DBR projects were successful. In one DBR project, faulting in the range of 6.4–9.5?mm occurred after less than 2 years of treatment. Forensic investigation revealed voids under the dowel bars, which indicates poor consolidation of the grout material. Efforts are currently underway in TxDOT to improve specifications for grout materials and DBR construction.     

基于有限元的水泥混凝土路面裂缝加装传力杆维修分析

使用三维有限元软件对传力杆在水泥混凝土路面裂缝维修中的应用进行了分析.主要内容包括:1)加装传力杆作用分析;2)不同位置横向裂缝加装传力杆的效果分析.通过加装传力杆对裂缝传荷能力及板内应力的影响分析,得出水泥混凝土路面裂缝加装传力杆是可行的.     

Lessons Learned from Field and Laboratory Testing of a DBR Project

Where faulting takes place due to the absence of dowel bars and inadequate subbase support in jointed concrete pavement (JCP), dowel bar retrofit (DBR) is used to improve load transfer efficiency (LTE) and to prevent further faulting of slabs at transverse joints. Even though DBR generally improves LTEs and overall performance of JCPs, not all DBR projects have been successful. Faulting reoccurred within 2?years after DBR treatment on US59 in Texas. An investigation from the cores taken in the project revealed excessive voids under a dowel bar due to poor consolidation of the grouting material. A laboratory investigation was performed to determine the most critical factors for adequate consolidation of grouting materials in DBR. Typical rapid-setting grout materials widely used in DBR were selected and full-scale specimens were made for evaluations. Four testing variables for consolidation performance were investigated: time of placement after mixing, vibration time, slot width, and maximum aggregate size. Maximum aggregate size and slot width were not critical factors for consolidation performance of grout. The most significant factor was vibration time. Twenty s of vibration is recommended. Placement time was also an important factor, with grout materials placed after initial set performing poorly. Delayed placement of grout materials without vibration led to the most voids under the dowel bars.     

Forces on Plunge Pool Slabs: Influence of Joints Location and Width

The hydrodynamic pressures due to jets impinging on plunge pools must be taken into account in the stability design of pool floor concrete slabs. The contraction joints between slabs are normally sealed with waterstops, which prevent the transmission to the foundation of the pressures applied on the upper faces of the slabs. However, a waterstop failure will allow pressure transmission to the foundation, inducing uplift forces on the slabs. The use of open joints might also become a feasible solution for the lining of plunge pool floors if the pressure field that develops around each slab could be adequately evaluated. This paper presents an analytical model and experimental research developed to assess the forces on plunge pool slabs, considering either open or closed. The influence of the relative width of the contraction joints and the joint between the slabs and the foundation is analyzed. The mean value and standard deviation of the hydrodynamic vertical force are determined based on point pressure measurements, and their relative importance is discussed.     

Experimental Study on the Performance of Approach Slabs under Deteriorating Soil Washout Conditions

In the U.S. bridge design practice, an approach slab is commonly provided to facilitate a smooth transition from the highway pavement to the bridge deck. Maintenance of bridge approaches often necessitates the repair or replacement of approach slabs owing to damage from heavy traffic loads, washout of fill materials, and settlement of the approach embankment. Approach slab damage because of embankment settlement is considered a more common problem and has been extensively investigated in the literature. In this paper, performance of the approach slab degraded by void formation underneath the slab is examined by load testing. Full-size approach-slab specimens were tested under increasing magnitude up to four times AASHTO HS20-44 design truck loads. The test matrix included four slab specimens with the following details: (1)?conventional steel reinforcement representative of current California design; (2)?steel reinforcement replaced by a double-layer pultruded fiber-reinforced polymer grating; (3)?steel reinforcement replaced by glass fiber-reinforced polymer rebars; and (4)?incorporation of steel and polyvinyl alcohol fibers in the concrete mix and removal of top longitudinal and transverse steel. Results indicated that the slabs show satisfactory performance under standard HS20-44 design truck load. Tests also revealed that these slabs exhibited similar performance in terms of stiffness, deformation, and crack pattern when fully supported, but registered noticeable difference in performance under deteriorating soil washout conditions. The fiber-reinforced concrete slab in general showed the best crack control and the smallest deflection and end rotation among the four slabs.     

Utilizing Advanced Characterization Tools to Prevent Reflective Cracking

To prevent premature failures of rehabilitated concrete pavements, transportation authorities need tools to characterize the prerehab pavement condition of its load carrying capacity, and to determine the resistance of the overlay material to underlying crack/joint movements. Two quantitative methods, the rolling dynamic deflectometer (RDD) and overlay tester (OT), along with field performance data were employed in rehabilitation studies involving reflective cracks. The RDD is able to continuously assess vertical differential movements at joints/cracks that represent the potential for reflective cracks on existing pavements. The OT has the ability to determine the resistance of the overlay material to underlying crack/joint movements. The RDD W1?W3 deflections were used to determine areas that have a high potential for reflective cracking due to poor load transfer across joints and cracks. This paper documents results from the RDD and OT on the following five rehabilitation projects: (1) SH225; (2) US96; (3) SH12; (4) SH342; and (5) IH35W. Based on the available test results from these five projects, it was observed that the W1?W3 threshold values of 5.5 mils (0.140 mm) for exposed concrete pavement and 6.5 mils (0.165 mm) for composite pavement with existing hot mix asphalt overlay and an OT threshold value of 700 cycles correlated well with the field performance. Ignoring either of these critical factors may lead to premature reflective cracking.     

Influence of Reinforcement on the Behavior of Concrete Bridge Deck Slabs Reinforced with FRP Bars

This paper presents the results of an experimental study to investigate the role of each layer of reinforcement on the behavior of concrete bridge deck slabs reinforced with fiber-reinforced polymer (FRP) bars. Four full-scale concrete deck slabs of 3,000?mm length by 2,500?mm width and 200?mm depth were constructed and tested in the laboratory. One deck slab was reinforced with top and bottom mats of glass FRP bars. Two deck slabs had only a bottom reinforcement mat with different reinforcement ratios in the longitudinal direction, while the remaining deck slab was constructed with plain concrete without any reinforcement. The deck slabs were supported on two steel girders spaced at 2,000?mm center to center and were tested to failure under a central concentrated load. The three reinforced concrete slabs had very similar behavior and failed in punching shear mode at relatively high load levels, whereas the unreinforced slab behaved differently and failed at a very low load level. The experimental punching capacities of the reinforced slabs were compared to the theoretical predictions provided by ACI 318-05, ACI 440.1R-06, and a model proposed by the writers. The tests on the four deck slabs showed that the bottom transverse reinforcement layer has the major influence on the behavior and capacity of the tested slabs. In addition, the ACI 318-05 design method slightly overestimated the punching shear strength of the tested slabs. The ACI 440.1R-06 design method yielded very conservative predictions whereas the proposed method provided reasonable yet conservative predictions.     

Flexure of Two-Way Slabs Strengthened with Prestressed or Nonprestressed CFRP Sheets

This paper presents a study on the flexural behavior of two-way reinforced concrete slabs externally strengthened with prestressed or nonprestressed carbon fiber-reinforced polymer (CFRP) sheets. Four large-scale flat plate slabs (3,000?mm×3,000?mm×90?mm) are tested and a nonlinear three-dimensional finite-element analysis is conducted to predict the flexural behaviors of the tested slabs, including the load-deflection response, strain distribution, crack propagation, and crack mouth opening displacement. An increase in the load-carrying capacity of 25 and 72% is achieved for the slabs strengthened with nonprestressed and prestressed CFRP sheets, respectively, in comparison to the unstrengthened slab. A reduction of the deflections up to 32% in service is noted for the strengthened slabs. The unstrengthened slab shows very ductile behavior, whereas, progressive failure is observed for the strengthened slabs, exhibiting pseudoductility in postpeak behavior. Stress redistribution between the internal and external reinforcement is significant in the slab strengthened with prestressed CFRP sheets.     

Moving-Wheel Fatigue for Bridge Decks Strengthened with CFRP Strips Subject to Negative Bending

This paper presents the negative bending of reinforced concrete slabs strengthened with near-surface mounted (NSM) carbon fiber-reinforced polymer (CFRP) strips. Six slab specimens, three of which are strengthened with CFRP strips, are tested in static and fatigue loads. A wheel-running fatigue test machine is used to simulate vehicular loads on a bridge deck. The effectiveness of CFRP strengthening for bridge decks in cantilever and pseudonegative bending is examined based on moment-carrying capacity and cyclic behavior under the wheel-running fatigue loads, including crack patterns and damage accumulation. The moment-carrying capacity (static) of the cantilever slab strengthened with the NSM CFRP strips is improved by 68.4% when compared to that of an unstrengthened slab. The damage accumulation rate of the strengthened cantilever slab owing to the fatigue load is significantly lower than that of the unstrengthened slab. The damage accumulation of the strengthened slab gradually increases and is irreversible when the fatigue cycles increase. The fatigue-induced flexural cracks of the slabs develop along the wheel-running direction. A simple predictive model is presented to estimate the fatigue life of the test slabs.     

Efficient Strengthening Technique to Increase the Flexural Resistance of Existing RC Slabs

Composite materials are being used with notable effectiveness to increase and upgrade the flexural load carrying capacity of reinforced concrete (RC) members. Near-surface mounted (NSM) is one of the most promising strengthening techniques, based on the use of carbon fiber-reinforced polymer (CFRP) laminates. According to NSM, the laminates are fixed with epoxy based adhesive into slits opened into the concrete cover on the tension face of the elements to strength. Laboratory tests have shown that the NSM technique is an adequate strengthening strategy to increase the flexural resistance of RC slabs. However, in RC slabs of low concrete strength, the increase of the flexural resistance that NSM can provide is limited by the maximum allowable compressive strain in the compressed part of the slab, in order to avoid concrete crushing. This restriction reduces the effectiveness of the strengthening, thus limiting the use of the NSM technique. A new thin layer of concrete bonded to the existing concrete at the compressed region is suitable to overcome this limitation. Volumetric contraction due to shrinkage and thermal effects can induce uncontrolled cracking in the concrete of this thin layer. Adding steel fibers to concrete [steel fiber-reinforced concrete (SFRC)], the postcracking residual stress can be increased in order to prevent the formation of uncontrolled crack patterns. In the present work, the combined strengthening strategy, a SFRC overlay and NSM CFRP laminates, was applied to significantly increase the flexural resistance of existing RC slabs. Experimental results of four-point bending tests, carried out in unstrengthened and strengthened concrete slab strips, are presented and analyzed.     

Structural Evaluation of Cement Concrete Roads in Mumbai City

Plain jointed concrete pavements laid in Mumbai City (India) during the early 1990s were structurally evaluated using a falling weight deflectometer (FWD) and testing of concrete cores extracted from the pavement slabs. The ultrasonic pulse velocity (UPV) of the concrete in the cores was determined first and then the cores were crushed under compression. The pavement deflections were found to be within the limits as suggested in the Indian codes and the international literature. The joint conditions were also found to be satisfactory. The design strength of the concrete was back-calculated from the compressive strength of the cores and was found to conform to the design specifications. However, the construction quality was found wanting as the thickness of pavement slabs at a few locations was lower than that specified and it has resulted in cracking of the slabs. The dynamic modulus of elasticity of concrete as determined by the FWD was found to correspond well with that computed from the UPV of cores and from the compressive strength of concrete. A method is suggested to estimate the structural parameters of uncracked pavement slabs from the dynamic modulus of elasticity obtained through the indirect method of UPV testing which is less expensive compared to evaluation by the FWD.     

Study on Mitigating Center Macro‐Segregation During Steel Continuous Casting Process

In the current paper, Methods of enlarging the area for the distribution of segregation solutes were introduced to mitigate center macro‐segregation in steel billets and steel slabs during continuous casting process, which cost less and have significant effect. The location of center macro‐segregation is relative to the shape of liquid‐core at the solidification end during steel continuous casting. A method of dissymmetrical cooling on different surfaces, by which the area for the precipitation of segregation solutes was enlarged, was introduced to mitigate the center macro‐segregation in billets during continuous casting process. Method of optimizing the uniformity of solidified shell in the transverse direction was introduced to mitigate the center macro‐segregation in steel slabs. The uniform cooling intensity along the transverse direction guaranteed a regular solidification end in the continuous casting slab, which aided in the effective application of dynamic soft reduction technology. A relevant 2‐D heat transfer model was developed for the optimization of uniform solidification. The current method was applied to the industrial slab continuous casting using the heat transfer model. The results indicated a better industrial slab quality with much less center macro‐segregation after the use of the method.     

Curling of Concrete Floor Slabs on Grade — Causes and Repairs

Many industrial floors are required to have high-quality flat surfaces for the operation of specialized equipment, particularly high-reach stackers operating from the surfaces of warehouse floors. For new floors, the essence of floor flatness lies in the manner of finishing and the systematic monitoring of the flatness achieved immediately following construction. Achieving such surfaces in floor slabs however, is quite difficult because of the moisture and temperature gradients that cause them to curl at the joints. Such curling seriously affects the operation of an industrial facility. Floors subjected to heavily loaded forklift traffic may rapidly deteriorate, causing safety problems. Curling is also exacerbated in industrial floors by the use of power-troweled surface hardeners to produce the dense high-strength top surface required for high wear resistance. Repair of curled floors in industrial locations involves grinding, patching, installation of dowels, and grouting underneath the curled slab. The timing and appropriateness of the method used are of vital importance to the durability of the repair. Aspects of design and construction to minimize curling of new industrial floors, the factors that contribute to cracking and curling, measures to minimize curling, and the repair of curled floors are discussed in this paper.     

连铸板坯辊道输送和堆冷过程热力分析

对连铸板坯辊道输送和堆冷过程的变形问题进行了有限元热力分析。分析结果表明,冷却初期辊道输送和堆冷过程铸坯都会发生两端翘曲变形,冷却后期辊道输送过程铸坯会恢复平直而堆冷过程铸坯会发生反向弯曲。堆冷过程中加盖保温罩能使铸坯在冷却后期恢复平直而不发生反向弯曲,并且铸坯内残余应力也变小了。     

Investigation of non-uniform slag films and air gap during slab continuous casting

Based on an inverse algorithm and a full-scale heat transfer model of slab/mould, the non-uniform temperature fields of mould and slab are obtained. Considering the formation mechanism and heat transfer characteristics of the slag films and air gap, a mathematical model of the heat transfer between the liquid/solid slag films and air gap is further developed. According to the model, the non-uniform distributions and evolution of liquid/solid slag films and air gap are comprehensively revealed, which proposes a theoretical foundation for exploring the complex heat transfer of mould/slab and provides a helpful tool for further improving the casting parameters and operations.