Premature Cracking from Cement-Treated Base and Treatment to Mitigate Its Effect

A forensic study was conducted to investigate the premature cracking distress on an ongoing construction project on State Highway (SH) 24. Transverse cracks occurred at approximately every 9–15?m (30–50?ft) along the 9.6?km (5.9?mi) project. The field tests involved both destructive tests, including trenching and coring, and nondestructive tests, including falling weight deflectometer (FWD) and ground-penetrating radar (GPR) tests. The laboratory tests mainly included cement-treated base material and asphalt mixture material series tests. By integrating all the test results, it was concluded that the premature cracking was originating from the cement-treated base (CTB). Although CTB is not a new concept in pavement construction, stabilization of base materials is a complex process, which, if not handled properly, may lead to premature failures. The two primary factors that contributed to the premature failure are (1)?an excessive amount of cement in the CTB, and (2)?a high moisture content when the CTB was compacted (almost 2% above optimum moisture). It was believed the possibility of premature failure could be reduced significantly, if the lab verification tests were conducted on the CTB before construction started. Pavement life analysis was conducted to evaluate the current structure adequacy to sustain future traffic. In addition, extensive material design was conducted for the final wearing course asphalt mixture, which was intended to mitigate the effect of existing cracks by resisting crack propagation from the underlying structure.     

Fatigue Evaluation of Rib-to-Deck Welded Joints of Orthotropic Steel Bridge Deck

This study reported fatigue test results of 300-mm-wide specimens with three details: 80% partial joint penetration (80%PJP), weld melt-through (WMT), and both. The specimens were cut out from full-scale orthotropic deck specimens of 16-mm-thick deck plate. In the fatigue test, the deck plate was subjected to cyclic bending loading and the rib was free from loading. The fatigue fracture surfaces showed that the presence of WMT may affect the initiation of fatigue cracks. A propensity to root cracking rather than toe cracking was observed. Plotting fatigue test results in an S-N diagram showed that the specimens with WMT seemed to have slightly lower fatigue strengths than the 80%PJP specimens, but the difference is more likely to be within a usual scatter of test data, which means that both details have comparable fatigue strength. The present test results satisfied the S-N curves of JSSC-E (80?MPa at 2×106 cycles) or AASHTO-C (89?MPa at 2×106 cycles).     

Field Investigations of Cracking on Concrete Pavements

This paper presents the results of several investigations to identify the underlying causes of longitudinal cracking problems in Portland cement concrete (PCC) pavement. Longitudinal cracking is not intended and detrimental to the long-term performance of PCC pavement. Longitudinal cracking problems in five projects were thoroughly investigated and the findings indicate that longitudinal cracking was caused by: (1) late or shallow saw cutting of longitudinal joints; (2) inadequate base support under the concrete slab; and (3) the use of high coefficient of thermal expansion (CTE) aggregates. When the longitudinal cracks were caused by late or shallow saw cutting of longitudinal joints, cracks developed at a very early stage. However, when there was adequate base support, the longitudinal cracks remained relatively tight even after decades of truck trafficking. Another cause of longitudinal cracking was inadequate base support, and cracking due to this mechanism normally progressed to rather wide cracks. Some cracks were as wide as 57?mm. Evaluations of base support by dynamic cone penetrometer in areas where longitudinal cracks were observed indicate quite weak subbase in both full-depth repaired areas and surrounding areas. This implies that the current requirements for the subbase preparation for the full-depth repair are not adequate. Another cause of longitudinal cracking was due to the use of high CTE aggregate in concrete. Large volume changes in concrete when coarse aggregate with high CTE is used could cause excessive stresses in concrete and result in longitudinal cracking. To prevent longitudinal cracking, attention should be exercised to the selection of concrete materials (concrete with low CTE) and the quality of the construction (timely and sufficient saw cutting and proper selection and compaction of subbase material).     

Roadway Heaving Caused by High Organic Matter

A forensic study was conducted to investigate the premature pavement failure of heaving and cracking on the north bound lane of SH6 and to determine (1) the causes of the heaving and cracking; (2) the severity and extent of the problem; and (3) a prevention strategy. Ground penetration radar (GPR), falling weight deflectometer (FWD), dynamic cone penetrometer (DCP), and soil boring and laboratory tests were conducted. Soil maps provided by the Natural Resource Conservation Service (NRCS) and the electrical resistivity tomography (ERT) were utilized to locate areas that may have similar problems for the ongoing project. It was concluded that the heaving was caused by high organic content in soils. The low pH of the lime treated subgrade layer indicated that the lime stabilization was ineffective. This was due to the high concentration of organic matter. FWD and DCP results indicated that the heaved/cracked areas are losing structural load support. Approximately 84% of the bumps/dips detected by the profiler were also detected by the GPR. Based on the GPR results, it was estimated that about 1.2?miles of the roadway may have potential heaving in the future. Although it is not a standard practice to determine the organic content of soil for new construction, it is critical to determine the organic matter through soil boring and laboratory testing in the suspicious areas. It was found that the soil maps provided by the NRCS yielded a reasonable estimate, and can be used as a screening tool. All five locations (O1 to O5) identified by ERT were verified by boring and laboratory tests to have high organic content (1.9–3.3%). Boring results indicated that ERT was able to map the soil strata and could differentiate between sandy and clay soil types. Although ERT was able to identify the anomalies with high organic contents, and the results were confirmed by boring and laboratory testing, additional work is needed to refine the procedure.     

Transverse Thermal Expansion of FRP Bars Embedded in Concrete

Fiber reinforced polymers (FRPs) have a thermal expansion in the transverse direction much higher than in the longitudinal direction and also higher than the thermal expansion of hardened concrete. The difference between the transverse coefficient of thermal expansion of FRP bars and concrete may cause splitting cracks within the concrete under temperature increase and, ultimately, failure of the concrete cover if the confining action of concrete is insufficient. This paper presents the results of an experimental investigation to analyze the effect of the ratio of concrete cover thickness to FRP bar diameter (c/db) on the strain distributions in concrete and FRP bars, using concrete cylindrical specimens reinforced with a glass FRP bar and subjected to thermal loading from ?30?to?+80°C. The experimental results show that the transverse coefficient of thermal expansion of the glass FRP bars tested in this study is found to be equal to 33 (×10?6?mm/mm/°C), on average and the ratio between the transverse and longitudinal coefficients of thermal expansion of these FRP bars is equal to 4. Also, the cracks induced by high temperature start to develop on the surface of concrete cylinders at a temperature varying between +50 and +60°C for specimens having a ratio of concrete cover thickness to bar diameter c/db less than or equal to 1.5. A ratio of concrete cover thickness to glass fiber reinforced polymers (GFRP) bar diameter c/db greater than or equal to 2.0 is sufficient to avoid cracking of concrete under high temperature up to +80°C. The analytical model, presented in this paper, is in good agreement with the experimental results, particularly for negative temperature variations.     

薄板坯角横裂成因分析

薄板坯角横裂对热轧板边裂的影响很大。本文分析和总结了薄板坯角横裂的产生机理及其影响因素,简要地提出可采用提高薄板坯温度和降低氮含量两种措施来防止角横裂的发生。     

Micromechanism of the transition of fibrous cracking to cleavage of C-Mn base and weld steel

In this present work, crack opening displacement (COD), four-point bending (4PB), Charpy V, and three-point bending (3NB) tests of specimens of C-Mn base and weld steel were carried out in the brittle-ductile transition temperature region. Some specimens were fractured and some specimens were unloaded prior to fracture after fibrous cracking occurred and extended to variouslengths. Through detailed observation of the variation in the shapes of microcavities located at the tip or on the two sides of fibrous cracks in unloaded specimens and the variations of shapes of dimples located at various lengths of cracks on fracture surfaces, the micromechanism of the change from fibrous cracking to cleavage was analyzed. It was revealed that no matter whether a specimen was notched or precracked, as long as a fibrous crack initiated and propagated in it, the critical event for cleavage fracture is the unstable extension of a ferrite grainsized crack. The main factor promoting the transition from the fibrous crack to cleavage was the increase of the local tensile stress ahead of the crack which was caused by the increase of the triaxiality of stress and the apparent normal stress in the remaining ligament. The considerable scattering of toughness values in the transition temperature region is due to the random variation of the widths of the tips of the fibrous cracks during their extension and the random distribution of the weakest constituents in the microstructure.     

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.     

Effects of Fabrication Procedures on Fatigue Resistance of Welded Joints in Steel Orthotropic Decks

A common practice for the fabrication of steel orthotropic bridge decks in the United States is to use 80% partial joint penetration (PJP) groove welds between the closed ribs and deck plate. However, it is difficult to eliminate weld melt-through with the thin rib plates. Heat straightening after welding, sometimes combined with precambering, is used to meet the deck plate flatness requirement. To study the effects of both weld melt-through and distortion control measures on the fatigue resistance of the rib-to-deck plate welded joint, six full-scale two-span orthotropic deck specimens were subjected to laboratory testing. Specimens, 10 m long and 3 m wide with four closed ribs, were fabricated with and without weld melt-through and were heat straightened; three specimens were also precambered. To simulate the effect of repetitive truck traffic, each specimen was tested up to 8 million cycles. Test results showed that six cracks initiated from the weld toe outside the rib. Only one crack developed at the weld root inside the rib; this crack initiated from a location transitioning from the 80% PJP to 100% penetration weld. None of the cracks propagated through the deck plate thickness. Precambering was beneficial in fatigue resistance as two effectively precambered specimens did not experience cracking in the PJP welds.     

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。     

Use of an Infrared Joint Heater to Improve Longitudinal Joint Performance in Hot Mix Asphalt Pavements

Longitudinal joint cracking is one of the most prevalent forms of distress in asphalt concrete pavements. The joint area does not achieve the same density as the mat due to an unconfined edge on the initial pass and a cold joint during the second pass. The lower density allows water to penetrate and the material cracks, usually within one?year of construction. There are many techniques for constructing longitudinal joints, one being to preheat the joint prior to paving the second lane. This paper describes a field study conducted in New Hampshire using an infrared joint heater. Thermocouples were embedded in the pavement to determine the extent of heat penetration from the infrared heaters. Cores were taken along the joint and in the travel lanes for both the control and test sections. Density and strength measurements were taken on the cores. Permeability measurements along the control and test joints were performed. A cracking survey performed one?year after construction showed that the section of pavement where the infrared heater was used had significantly less cracking than the control section.     

加载方向对Al—Zn—Mg合金型材应力腐蚀开裂行为的影响

采用恒载荷拉伸应力腐蚀试验和电化学试验研究取向对Al-Zn-Mg合金型材的应力腐蚀(SCC) 开裂的影响, 腐蚀介质采用质量分数3. 5%的Na Cl溶液, 容器温度维持在50±2℃, 并通过光学金相显微镜(OM)、扫描电子显微镜(SEM)、电子背散射衍射(EBSD) 等研究不同取向试样应力腐蚀前、后的微观形貌.结果表明横向试样在315 h时断裂, 而纵向试样在整个加载过程中未发生断裂, 纵向试样有更好的抗应力腐蚀开裂性能; 纵截面(L-S面) 的腐蚀电流密度为0. 980 m A·cm-2, 约为横截面(T-S面) 的5倍, 腐蚀倾向于沿挤压方向发展; 相比T-S面, L-S面晶粒间取向差较大, 大角度晶界多, 容易被腐蚀产生裂纹; 在应力腐蚀加载过程中, 试样先发生阳极溶解, 形成腐蚀坑, 聚集的腐蚀产物所产生的楔入力和恒定载荷的共同作用促使裂纹在腐蚀介质中加速扩展, 两种取向试样均发生了明显的晶间腐蚀, 存在应力腐蚀开裂的倾向.      

Forensic Investigation of a Sulfate-Heaved Project in Texas

This paper focuses on the cause, possible solutions, and future prevention of pavement heave at a new construction project. We speculated that heaving on the east side of the project was caused by a reaction between the lime stabilizer and minerals in the soil. Because of a difference in soil chemistry, the west side of the project (which was still under construction) did not show evidence of heaving. A forensic investigation was initiated to test our hypothesis. The findings of the investigation concluded that the cause of the heaving on the east side of the road was related to the formation of the expansive mineral, ettringite. Ettringite formed due to the reaction of the lime stabilizer with seams of high sulfate soil on the east side. Laboratory testing did not find any effective stabilizer for the high-sulfate soils on the east side. Therefore, reconstruction would involve removing and replacing the treated layer with a select material that has less than 2,000?ppm sulfates. Test results indicate that there was no threat of sulfate heave on the west side. District personnel had performed the field conductivity tests to evaluate and monitor the concentration of the sulfate content on the remaining project. The treatment of 3%/72-h mellowing period/3% lime treatment was employed on the west side. The whole project has been completed for 1?year and no heave has been observed.     

Simulation of transverse crack formation on continuously cast peritectic medium carbon steel slabs

Reliable data are limited to the critical strain for the formation of transverse cracks on the slabs, owing to experimental difficulty to simulate temperature gradient in solidified shell in continuous casting mold. The present study is to determine the critical strain, ?_c, for the formation of transverse cracks on continuously cast slabs. A convenient and simple hot tensile test using rectangular test pieces with either V-notch or semi-circle notch or oscillation marks has been developed by placing the specimen under similar temperature gradient to that in solidified shell in the mold. The ?_c has been determined at a better accuracy and reproducibility, and the ?_c at a strain rate of 5?10^?4s^?1 is found to be a high 35% for test pieces without notch. It sharply decreases, however, to 10% for those with V- and semi-circle-notches, slightly decreases with increasing notch depth, and further decreases for those with oscillation marks that accompany solute segregation. Reduction of the oscillation mark depth is shown to be important measure to prevent the occurrence of transverse cracking of continuously cast slabs.     

Surface cracking identified in polished and self-glazed dental porcelain

PURPOSE: Polishing and glazing porcelain surfaces of restorations is thought to help reduce the wear on the opposing occlusion because of reduced roughness. MATERIALS AND METHODS: To test this theory, samples of a variety of dental porcelains were prepared and subjected to various polishing and self-glazing treatments commonly used in dentistry and then viewed using scanning electron microscope (SEM). Fine cracks were discovered in the surface of the samples that had been polished and then self-glazed. These cracks were typically greater than 50 microns in length with submicron surface openings and crack depths less than 20 microns. To establish the treatments responsible for the formation of these cracks, a more controlled study was performed. Samples of Vita VMK 68 no. 559 incisal porcelain (Vita Zahnfabrik, Bad S?chingen, Germany) were prepared and subjected to six treatments (sample size = 3): treatment no. 1, as-fired condition; treatment no. 2, self-glazed (32 degrees C per minute to 960 degrees C, then cooled); treatments no. 3 and 4, wet ground followed by 1 micron diamond polish (no. 4 was then self-glazed); treatments no. 5 and 6, ground and polished using a Shofu porcelain adjustment kit (Shofu Inc, Kyoto, Japan) (no. 6 was then self-glazed). The samples were observed using SEM. RESULTS: Treatments no. 1, 2, 3, and 5 showed no cracking; however, treatments no. 4 and 6 showed cracking in all specimens (approximate levels 5,100 microns/mm2 and 3,600 microns/mm2, respectively). CONCLUSIONS: Polishing followed by self-glazing produces fine surface cracks. The rougher surface resulting from the surface cracks and subsequent porcelain chipping may cause wear of opposing occlusal surfaces.     

含铌钢板坯角横裂纹的控制

采用金相显微镜和扫描电镜分析了含铌微合金化钢铸坯角部横裂纹的形成原因.发现矫直区铸坯角部温度位于该钢种第Ⅲ脆性温度区间内;拉速波动导致角部温度变化较大;二冷喷嘴状况不好导致两边角部温度相差较大.采用提高拉速、矫直区铸坯角部喷嘴遮挡等"热行"方法后,铸坯角部温度明显提高,铸坯的角横裂纹的发生率大大降低,但铸坯中心偏析恶化.采用增加角部喷嘴的"冷行"方法后,铸坯角部温度明显降低,铸坯的角横裂纹和中心偏析大大改善.     

Influence of Secondary Elements and Deck Cracking on the Lateral Load Distribution of Steel Girder Bridges

The AASHTO LRFD load distribution factor equation was developed based on elastic finite element analysis considering only primary members, i.e., the effects of secondary elements such as lateral bracing and parapets were not considered. Meanwhile, many bridges have been identified as having significant cracking in the concrete deck. Even though deck cracking is a well-known phenomenon, the significance of pre-existing cracks on the live load distribution has not yet been assessed. The purpose of this research is to investigate the effect of secondary elements and deck cracking on the lateral load distribution of girder bridges. First, secondary elements such as diaphragms and parapets were modeled using the finite element method, and the calculated load distribution factors were compared with the code-specified values. Second, the effects of typical deck cracking and crack types that have a major effect on load distribution were identified through a number of nonlinear finite element analyses. It was established that the presence of secondary elements may produce load distribution factors up to 40% lower than the AASHTO LRFD values. Longitudinal cracking was found to increase the load distribution factor by up to 17% when compared to the LRFD value while the transverse cracking was found to not significantly influence the transverse distribution of moment.     

Relation between microstructure, composition, and hot cracking in Ti-stabilized austenitic stainless steel weldments

A stabilized, fully austenitic alloy D9, a 15Cr-15Ni-2Mo stainless steel with a titanium addition corresponding to UNS 38660, is a candidate material for the fuel-clad and wrapper applications of the Prototype Fast Breeder Reactor (PFBR). The fully austenitic microstructure and the presence of titanium in this alloy lead to high susceptibility to hot cracking during welding. The fusion-zone and the heat-affected zone (HAZ) cracking susceptibility of alloy D9 was studied at three titanium levels, 0.22, 0.32, and 0.42 pct, all other elements remaining constant. The longitudinal and transverse Varestraint (Transvarestraint) hot-cracking tests were used to evaluate fusion-zone and HAZ cracking. The results showed that titanium increases cracking in the fusion zone by 15 to 20 pct in the range of Ti levels studied. The microanalysis of fusion-zone hot cracks using electron probe microanalysis (EPMA) showed an enrichment of Ti, C, N, and S along cracks and in the interdendritic regions. The corresponding phases were identified as TiC, TiC_0.3N_0.7, and the carbosulfides Ti₂CS and Ti₄C₂S₂, which are believed to form eutectics with austenite to produce cracking. The amounts of these phases increased with increasing Ti content. In the HAZ, a similar relation between titanium level and cracking was found. The comparison of the weldability of the D9 with an FA mode type 321 revealed that Ti-bearing eutectics were responsible for a high degree of cracking irrespective of the solidification mode. The results show that in the D9, the ratio of Ti to C and N must be controlled to minimize cracking.     

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.     

Crystallographic features of intergranular crack initiation in fatigued copper polycrystals

In order to study the mechanism of grain boundary (GB) cracking in fatigued polycrystalline copper, specimens were fatigued in symmetrical push-pull at an intermediate constant plastic strain range at room temperature in dry air. The intergranular cracks were examined under the scanning electron microscope. Many GB cracks were found to have been formed by the impingement of persistent slip bands (PSBs) against the grain boundaries (PSB-GB cracks). The orientations of the grains adjacent to the cracks were determined by electron back-scattering patterns. The misorientations of the cracked boundaries were calculated and the boundary plane orientations were also determined. High-energy grain boundaries were found to be preferred sites for cracking. The activated slip systems in the component grains adjacent to the cracks were determined and analyzed. With these data, the cracking stresses due to the interaction between the PSBs and the boundaries were calculated for the observed PSB-GB cracks in a pile-up type dislocation model in a three-dimensional analysis. The results confirmed that, with reasonable assumptions, the estimated minimum theoretical shear stresses which are required to act in the PSBs for causing PSB-GB cracks were always smaller than the real shear stresses operating in the PSBs.