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%.     

Modeling Cracking in Shell-Type Reinforced Concrete Structures

A three-dimensional (3D) hypoelastic material model for modeling material properties of cracked reinforced concrete is proposed. Material properties of multidirectionally cracked reinforced concrete are represented by the material properties of intact concrete and a number of uniaxially cracked concrete with their coupling solids. Cracking effects due to multiple nonorthogonal cracks are traced in each uniaxially cracked concrete. Tension softening and aggregate interlock occurring at the crack interface as well as tension stiffening and compression softening initiated in concrete between cracks due to multiple nonorthogonal cracks are all incorporated explicitly. RC panels under in-plane loading and RC slab under pure torsion have been analyzed. The developed 3D hypoelastic material model has been proved to be efficient and effective in modeling the material behaviors of cracked reinforced concrete in shell-type RC structures. The deformational response, the ultimate strength, and failure mode can be captured reasonably well.     

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.     

Microstructural Viscoplastic Continuum Model for Permanent Deformation in Asphalt Pavements

Permanent deformation is one of the major distresses in asphalt pavements. It is caused mainly by high traffic loads associated with high field temperatures. An anisotropic viscoplastic continuum damage model is developed in this study to describe permanent deformation of asphalt pavements. The model is based on Perzyna’s formulation with Drucker–Prager yield function modified to account for material anisotropy and microstructure damage. The material anisotropy is captured through microstructural analysis of aggregate distribution on two-dimensional sections of hot mix asphalt. A damage parameter is included in the model to quantify the nucleation of cracks and growth of air voids and cracks. A parametric study was conducted to demonstrate the sensitivity of the model to strain rate, aggregate distribution, and microstructure damage. Triaxial strength and static creep measurements obtained from the Federal Highway Administration Accelerated Loading Facility were used to determine the model parameters.     

Shear Transfer across Cracks in FRP Strengthened RC Members

The shear capacity of unplated reinforced concrete (RC) beams depends on the transverse shear to form the critical diagonal crack (CDC) as well as the transverse shear capacity across the CDC. The latter depends on the reinforcing bars crossing the CDC as they provide forces normal to the CDC that allow the shear to be transferred by aggregate interlock. For steel reinforcing bars, these normal forces can be assumed to depend on the ductile yield capacity of the reinforcing bar. However, the problem is more complicated when dealing with fiber reinforced polymer (FRP) plated RC beams, as the normal force now depends on the brittle intermediate crack debonding resistance of the plate as well as the brittle nature of the FRP material. In this paper, eight push tests have been used to directly determine the contribution of externally bonded (EB) and near surface mounted (NSM) FRP plates to the shear capacity, and these are compared with further six EB and NSM steel plated members. It is shown that plate reinforcement can substantially increase the shear capacity and, surprisingly, that the brittle FRP plates can provide a more ductile shear mechanism than the ductile steel plates.     

Fatigue of Concrete Beams Strengthened with Glass-Fiber Composite under Flexure

A detailed investigation on the fatigue performance of concrete beams strengthened with glass-fiber composite (GFC) is performed in this study. Cyclic load tests were conducted on reinforced concrete beam specimens strengthened with two layers of GFC bonded to the beams’ bottom surface using a special epoxy resin. Midspan-deflection and cracks were measured at different numbers of load cycles and varying fatigue loading levels during the tests. Investigated parameters include total midspan-deflection, residual midspan-deflection after unloading, crack width, crack length, and crack distribution at different loading stages. The fatigue performance of concrete beams strengthened with GFC was evaluated by comparing the deflections, crack sizes, and crack distributions with unstrengthened beams. The concrete beams strengthened with GFC investigated in this study showed significant improvement on fatigue performance.     

PCC Airfield Pavement Response during Thaw-Weakening Periods

A field study was performed at two regional airports in Wisconsin during spring thaw to determine its effects on portland cement concrete (PCC) airport pavements. This study was part of a research program to model the performance of airfield pavements for the Federal Aviation Administration. Subsurface temperature and falling weight deflection measurements of the pavement structures were taken at both airports and used to calculate the frost penetration depths, the changes in bearing capacity, and the joint and load transfer efficiencies. This paper summarizes the findings of this study and includes several relationships between various engineering properties of the subsurface layers below the PCC layer, along with a procedure for evaluating pavement performance using falling weight deflection data for PCC pavements during spring thaw.     

Transverse Cracking of Concrete Bridge Decks: Effects of Design Factors

Early transverse cracking is one of the dominant forms of bridge deck defects experienced by a large number of transportation agencies. These cracks often initiate soon after the bridge deck is constructed, and they are caused by restrained shrinkage of concrete. Transverse cracks increase the maintenance cost of a bridge structure and reduce its life span. Most of the past efforts addressing transverse bridge deck cracking have focused on changes over the years in concrete material properties and construction practices. However, recent studies have shown the importance of design factors on transverse bridge deck cracking. This paper presents results of a comprehensive finite-element (FE) study of deck and girder bridge systems to understand and evaluate crack patterns, stress histories, as well as the relative effect of different design factors such as structural stiffness on transverse deck cracking. The results of this study demonstrate the development of transverse deck cracking and emphasize the importance of these design factors. They also recommend preventive measures that can be adopted during the design stage in order to minimize the probability of transverse deck cracking.     

Dynamic Fracture and Strain Rate Behavior of Aggregates Used in Transportation

The static and dynamic uniaxial compressive strengths of coarse aggregate materials used in portland cement concrete (PCC) were determined under dry and saturated conditions for three blast furnace slags, three limestones, four dolomites, and two mafic igneous rocks. The slag aggregates exhibited the lowest compressive strength, followed by the carbonates (limestones and dolomites), and the mafic igneous rocks. Both the dry and saturated aggregates revealed a higher compressive strength under dynamic loads compared to the static loads. Based on the experimental data, a rate sensitivity parameter was defined to describe the increase in compressive strength as a function of strain rate. This parameter is deemed to have considerable relevance in evaluating the ability of a specific aggregate to resist dynamic loads such as in aggregate interlock in PCC cracks and joints, friction in asphalt, and also in the development of microfracture during rock blasting. Comparison of the compressive strength data to density and LA abrasion values revealed that the dynamic data have a better correlation to the above properties than the static data.     

轨道车辆抗侧滚扭杆轴表面裂纹研究

表面裂纹是影响轨道车辆抗侧滚扭杆轴疲劳性能的关键因素,研究发现扭杆轴表面裂纹的形式主要有横向裂纹和纵向裂纹两种,并采用化学成分、低倍组织及金相检验等方法对这两种表面裂纹进行了分析.结果表明,在原材料质量得到很好的控制的情况下,热处理工艺是导致扭杆轴表面裂纹产生的重要因素,并提出了预防和解决措施.     

Tripartite Cohesive Crack Model

A new multicomponent cohesive crack model for concrete is presented. The model, which is directly applicable to interface finite elements, has three main components termed undamaged, bridging, and fully debonded. The relative sizes of these components, each of which simulates a proportion of a representative material volume, change according to evolution functions that are developed from data from uniaxial cyclic tests on notched concrete specimens. The undamaged component is treated as elastic damaging, the bridging component has two subcomponents, which are elastoplastic and elastic with contact, and the fully debonded component is elastoplastic with contact. The relationships governing the normal-shear interactions are developed from experimental data on combined shear-tension tests on cracked concrete specimens. Comparisons with experimental data illustrate that the model is able to represent the cyclic behavior of cracked concrete in tension, full crack closure, the interaction between shear and normal behavior, and aggregate interlock behavior.     

亚包晶微合金钢连铸板坯角部横裂纹研究进展

亚包晶微合金钢由于具有较强的裂纹敏感性,在生产中角部横裂纹缺陷频发,为了揭示亚包晶微合金钢角部横裂纹形成原因,从产生机理及生产条件出发,综述了亚包晶微合金钢连铸坯角部横裂纹的影响因素及相应的控制措施,得出铸坯角部横裂纹的内因在于钢中化学成分及凝固组织演变特点,外因是实际生产过程中的结晶器参数、二冷参数及设备条件等。未来控制角部横裂纹技术的发展方向主要在倒角结晶器的优化以及铸坯角部控冷双相变技术的精确控制及理论完善上。     

Theoretical Modeling of Signal Loss versus Crack Opening for a Novel Crack Sensor

The degradation of concrete structures is always accompanied by the formation of cracks. Crack monitoring is hence useful in assessing the “health condition” of the structure. In our previous investigation on an optical-fiber crack sensor, we have successfully demonstrated the possibility of crack detection and monitoring without requiring prior knowledge of crack locations. Also, a single fiber can be employed to monitor a number of cracks. In practical applications, the signal loss versus crack opening relationship of the sensor needs to be properly “designed” to satisfy performance requirements on sensitivity to small cracks and/or the total number of detectable cracks (which is limited by the maximum loss at each crack). In this paper, a theoretical model for the signal loss versus crack opening relationship is developed through a combination of mechanical and optical analyses. Using concrete beams with embedded sensors, crack monitoring experiments are conducted. Test results are found to be in reasonably good agreement with predictions from the model. With the verified model, simulations are carried out to study the effect of various parameters on sensor performance. The potential application of the theoretical model to generate sensor design guidelines is hence demonstrated.     

Evaluation of Thermal Cracking at Elmendorf AFB Using SHRP Technology

An evaluation of runway and taxiway pavements was conducted using technology developed or utilized during the Strategic Highway Research Program (SHRP) to determine the effectiveness for identifying thermal cracking propensity of asphalt pavements. SHRP performance grades (PG) of PG52-28 and PG58-28 were measured for the 3 and 6% (weight-to-weight ratio) styrene-butadiene-styrene copolymer-modified asphalt binders employed in taxiway and runway construction. The high temperature SHRP performance grades were above that required by SHRP for the Anchorage, Alaska area according to the SHRP weather database. The low temperature SHRP PG of the binders were found to be insufficient for the area. No rutting has been observed; however, the pavements developed transverse cracks after the first winter following construction of both the runway and taxiway pavements in 1994 and 1996, respectively. The SHRP thermal cracking model failed to predict any cracking within a 10-year period for both pavements. No obvious cause for the model failure could be ascertained. The thermal stress restrained specimen test revealed no significant difference between cracking temperatures for the 3 and 6% styrene-butadiene-styrene-modified binders.     

Control of transverse cracking in special quality slabs

Abstract

Rolled plates from continuously cast slabs at Bhilai Steel Plant have been frequently rejected due to the formation of irregular shaped fine surface cracks. These cracks originate from fine transverse cracks located below oscillation marks/surface depressions of the cast slabs. The random occurrence of this defect from heat to heat was of major concern to Bhilai Steel Plant where slabs are cast of special grades such as boiler, microalloying, and high tensile quality. The mechanism of formation of this defect was identified through a detailed metallographic study of defect bearing slabs and plates and analysis of rejection patterns with respect to various important process parameters. The crack formation mechanism was verified through a process perturbation trial. A defect potential index was developed to explain the random occurrence of these cracks. The index incorporated parameters which influence crack formation in the Bhilai casters. A significant reduction in this type of cracking was observed, based on the recommendations made in this study     

Modeling Nonlinear Anisotropic Elastic Properties of Unbound Granular Bases Using Microstructure Distribution Tensors

The resilient properties of unbound aggregate bases are important parameters in the design of asphalt pavements. Previous studies have shown that these resilient properties exhibit nonlinear and transverse anisotropic characteristics. The paper in hand presents a micromechanics-based approach to model the nonlinear and anisotropic properties of unbound aggregate bases. The anisotropic behavior is captured using two microstructure parameters representing the preferred orientation of aggregate particles, and the ratio of the normal contact stiffness to shear contact stiffness among particles. The nonlinear response is modeled using a relationship that relates the shear modulus to particle packing, material properties, particle size, and confining pressure. The micromechanics model is used to represent the resilient properties for a total of 18 different combinations of material conditions with different aggregate types, moisture contents, and gradation characteristics. Anisotropic and nonlinear resilient properties were measured at ten different stress states for each of the material conditions. The results presented in this paper show that the micromechanics model is capable of successfully representing the experimental measurements.     

Cracking phenomena in twin roll strip casting of steel

In thin strip casting of steel surface cracks may occur; it was analysed which influencing parameters are important and how the cracks can be avoided. The studies, mainly with C60‐steel, lead to the following results: the cracks are formed during the casting process under high temperatures; on the strip surface hot and colder areas occur, and the cracks have to be studied in both areas separately; mostly transverse cracks are formed; the crack density depends on roll material, ‐ force, ‐ roughness, ‐ surface layers; the cracks are initiated by a temperature break down and by rolling within the pool range.     

水泥砼旧路面加铺沥青砼路面层中防反射裂缝的主要措施简析

水泥混凝土路面是我国城市道路的主要类型，在旧水泥混凝土路面上加铺沥青混凝土面层是改善路面使用性能、提高路面使用寿命的主要措施之一。本文简要分析了反射裂缝产生的机理，以及各主要技术措施能防止或延缓反射裂缝的机理。     

Water Migration Phenomenon Model in Cracked Concrete.?I: Formulation

A mathematical model for water migration in concrete that changes from a homogeneous to a nonhomogeneous material due to its loading history is developed in detail. In the proposed model, concrete is assumed to be composed of aggregate, cement paste, water, a crack band, and an interfacial crack between the aggregate and cement paste. The discontinuities for displacement and hydraulic gradient on cracks are considered on the microlevel. Then the governing equation for water migration in concrete as a homogeneous and a nonhomogeneous material is developed by coupling the mass conservation law and the force equilibrium. Therefore, it is possible to apply the developed model not only to cement-based material but also to porous permeable material like soil material and rock. However, the applicability of the developed model must be confirmed by comparing the experimental results.     

Microstructural Mechanisms of Early Age Cracking Behavior of Concrete: Fracture Energy Approach

This paper presents findings from a study directed at identifying key properties of ingredient materials that are influential on the early age cracking behavior of concrete, with an emphasis on the effects of aggregate size, aggregate morphologies, and water-cement ratio (w/c). Fracture energy (GF) was determined using a wedge-split test configuration for concrete samples at the age of 12?h. Based on image analysis, three signature morphologies of aggregate particles, i.e., the angularity, surface texture, and surface area, were quantitatively determined in terms of developed angularity index (AI), surface texture (ST) index, and surface area (SA) measurement, respectively. The high consistency between GF and aggregate SA of the concrete samples suggests that the interfacial transition zone (ITZ) at the cement paste-aggregate interface is the critical location that primarily accommodates the 12?h cracking of concrete. The critical role of ITZ in the early age cracking of concrete was further confirmed by its microstructural and chemical features under scanning electron microscopy/energy dispersive X-ray spectroscopy.