State of Practice of Residential Floor Slab Flatness

Forensic engineering investigations of residential structures often include measurements of elevation differences across the slab-on-grade floor as an indicator of movement (settlement or heave) of the underlying soil. Such assessments are predicated on an assumption that the slab was flat or very nearly so at the time of initial placement. Despite the relatively frequent use of such measurements in practice, confirmation of the underlying assumption is scarce in the archival literature. This paper describes a testing program to measure the deviation from level of a large number of residential concrete slab-on-grade floors in Phoenix, Arizona, within a few days of concrete placement. An average maximum elevation change of 13.5 mm was observed. Average and maximum slopes are also presented. The construction of the residential structure atop the slab was found to have no statistically significant effect on the levelness of the slab, nor did the season in which the concrete was placed. Residential slab-on-grade construction methods are reviewed, concluding that this degree of deviation from level is to be expected. Comparison of the data with recommendations in the literature suggests that caution must be taken in evaluation of cosmetic or structural impairment of residential structures based in large part on measurements of interior floor slab levelness.     

Slab-on-Grade versus Framed Slab

This paper presents comprehensive and practical engineering review (not mathematics or computer science paper) of the observed behavior of the two types of slabs that have been used for the bottom floor slabs, at grade and underground, of buildings in the United States and overseas. The typical design of the slab-on-grade (SOG) built in the United States is described. The design requires only nominal reinforcing steel, but modern day design also requires ground preparation and improvement as necessary, including underslab perforated drainage pipe network embedded in the granular subbase, riser pipes, and a series of pumps. Sealing at all slab isolation joints and waterproofing membrane or water barrier system are provided for water tightness. The framed slab is supported directly on the building framing and on the building foundation. The design can accommodate the soil and underground water pressure and in itself is watertight as the slab is cast monolithically with the structural walls and footings. The behavior of the SOG depends so much on the behavior (soil properties) of the soil strata on which the slab is resting on. It is sensitive to the variation of the soil conditions at and around building foundations, leading to uneven bumpy and cracked slab and leaking basement. These were demonstrated in the report of short- and long-term performances of various projects in Thailand and United States in the past 25 years. The paper refers to various analysis and design techniques that may be used to improve the design of both the SOG and the framed slab for serviceability and economy. It is concluded that both types of slabs may be selected to suit the functions and serviceability requirements of the buildings. The SOG requires less concrete and reinforcement than those for the framed slab counterpart, but when all other factors are considered including additional underslab drainage and pump system, operating, and long-term performance and maintenance, the overall costs of both slabs may not be far apart, yet the performance and integrity of the framed slab will certainly be superior.     

Effect of strain rate on high temperature mechanical properties of Q460 continuous cast slab

Gleeble 3500 thermal simulation testing machine and metallographic method were used to test the high temperature mechanical properties of construction steel Q460 continuous cast slab at different strain rates, and the high temperature strength, hot ductility and final room temperature microstructure of Q460 continuous cast slab at different strain rates in the range of 600℃ to 1200℃ were obtained. The results show that when Q460 continuous cast slab is performed at a higher strain rate (10s-1), the reduction of area increases with the increase of the tensile temperature without obvious high temperature brittle zone. But when the strain rate is (10-3s-1), the curves of the reduction of area has two brittle regions. The first one is located at 1100℃ to the melting point temperature, and the second is near 700℃. In general, the reduction of area of this steel grade is greater than 65%, indicating that construction steel Q460 continuous cast slab has good high temperature hot ductility. In addition, under the same strain rate, the tensile strength of Q460 continuous cast slab decreases with the increase of tensile temperature, while the elongation increases with the increase of tensile temperature.     

Fundamental phenomena governing heat transfer during rolling

To quantify the effect of roll chilling on the thermal history of a slab during hot rolling, tests were conducted at the Canada Center for Mineral and Energy Technology (CANMET) and at the University of British Columbia (UBC). In these tests, the surface and the interior temperatures of specimens were recorded during rolling using a data acquisition system. The corresponding heat-transfer coefficients in the roll bite were back-calculated by a trial-and-error method using a heat-transfer model. The heat-transfer coefficient was found to increase along the arc of contact and reach a maximum, followed by a decrease, until the exit of the roll bite. Its value was influenced by rolling parameters, such as percent reduction, rolling speed, rolling temperature, material type,etc. It was shown that the heat-transfer coefficient in the roll gap was strongly dependent on the roll pressure, and the effect of different variables on the interfacial heat-transfer coefficient can be related to their influence on pressure. At low mean roll pressure, such as in the case of rolling plain carbon steels at elevated temperature, the maximum heat-transfer coefficient in the roll bite was in the 25 to 35 kW/m² °C range. As the roll pressure increased with lower rolling temperature and higher deformation resistance of stainless steel and microalloyed grades, the maximum heat-transfer coefficient reached a value of 620 kW/m² °C. Obviously, the high pressure improved the contact between the roll and the slab surface, thereby reducing the resistance to heat flow. The mean roll-gap heat-transfer coefficient at the interface was shown to be linearly related to mean roll pressure. This finding is important because it permitted a determination of heat-transfer coefficients applicable to industrial rolling from pilot mill data. Thus, the thermal history of a slab during rough rolling was computed using a model in which the mean heat-transfer coefficient between the roll and the slab was determined from an estimate of the rolling load. It was found that the heat loss of a slab to the roll was 33 pet of the total, which emphasizes the importance of accurately characterizing the heat-transfer coefficient in the roll bite during hot rolling.     

Creep deformation and damage evaluation of service exposed reformer tube

Abstract

This paper aims at studying the creep deformation behaviour and quantifying creep damage of ～11 years service exposed primary hydrogen reformer tube made of HP40 grade of steel in a petrochemical industry, in terms of Kachanov’s continuum damage mechanics model (K model) and Bogdanoff model (B model) based on Markov process. Hot tensile, conventional creep deformation under identical test conditions, optical microscopy and fractography were extensively carried out. The as received tubes did not possess any sign of degradation including voids or creep cavitations and decarburisation. There was indeed loss of tensile strength from room temperature to 870°C for the bottom portion of the tube due to aging and overheating. Accumulation of damage due to creep has been quantified through microstructural studies in terms of two damage parameters A and A*. Experimental scatter observed in creep deformation and creep strain rate curves of the material at 870°C and at various stress conditions, is probably due to scatter in creep cavitations/voids and also due to variation in the mode of fracture in top as well as bottom portion of the tube. From statistical point of view, Weibull distribution pattern for analysing probability of rupture due to void area, shifts with increase in true strain towards the higher population of void. The estimation of average time to reach a specific damage state from K model and B model is in close agreement with that of experimental data and can describe the sudden changes of the creep damage in the tertiary region as well.     

石墨形态和铬含量对铸铁干摩擦学性能的影响

使用ＭＭ—２００磨损试验机研究了石墨形态和铬含量对铸铁与石棉基摩擦材料配副时的干摩擦学性能。结果表明,蠕墨铸铁的磨损率最低;摩擦系数由高到低依次为：片墨铸铁、蠕墨铸铁、球墨铸铁。蠕墨铸铁中,随着铬含量的增加,耐磨性能明显提高,摩擦系数略有降低。在高速、高载荷条件下,铬含量的质量分数达到１．５％时,耐磨性可达普通蠕墨铸铁的４倍以上;随着载荷的增加,蠕墨铸铁的摩擦系数降低,磨损率提高。在高速条件下,摩擦系数降低的幅度比在低速条件下小。铬改善蠕墨铸铁干摩擦学性能的效果在高速使用条件下更为突出。     

Tensile fracture of coarse-Grained cast austenitic manganese steels

Tensile fracture of coarse-grained (0.25 to 1 mm) cast austenitic manganese (Hadfield) steels has been investigated. Numerous surface discontinuities nucleate in coarse slip bands, on the heavily deformed surface of tensile specimens. These discontinuities do not propagate radially and final fracture results from central specimen cracking at higher strains. On the microscopic scale, bulk voids nucleate during the entire plastic deformation and they do not coalesce by shear localization(e.g., void-sheet) mechanism. Close voids coalesce by internal necking, whereas distant voids are bridged by means of small voids which nucleate at later stages of the plastic deformation. The high toughness of Hadfield steels is due to their high strain-hardening capacity which stabilizes the plastic deformation, and avoids shear localization and loss of load-bearing capacity. The observed dependence of measured mechanical properties on the specimen’s geometry results from the development of a surface layer which charac-terizes the deformation of this coarse-grained material.     

Failure Analysis of a Bridge Embankment with Cracked Approach Slabs and Leaking Sand

This paper describes a successful failure analysis to determine the causes of loss of backfill sand from a mechanically stabilized earth (MSE) wall, and cracks on the concrete approach slabs on top of it. The Texas Department of Transportation was concerned that the cracks on the approach slabs may be related to the excessive loss of backfill from behind the MSE walls, and that the embankment structure may be unsafe due to potential voids under the concrete slab. Several cubic meters of sugar sand had washed out of the wall and deposited adjacent to the paneled walls. A series of destructive and nondestructive tests were carried out to determine the causes of the problems. It was found that the cracking of the approach slab and the loss of backfill were unrelated. Suggestions for resolving both problems were made based on this study.     

Performance of Methods for Analysis of Relative Floor Elevation Measurements in Residential Structures

Forensic evaluations of residential structures often make use of relative elevation surveys of the floor slab. The objective for the analysis of these data is to estimate the postconstruction distortions which have occurred to the floor slab, in order to determine if such distortions might result from excessive movements of the supporting soils. However, such estimates are complicated by the findings of several recent studies that residential slab-on-grade floors exhibit substantial deviations from level at the time of construction, and by the expectation of some postconstruction movements in response to structural loads. In this study, commonly used methods were used to back-analyze relative elevation data from slabs with known distortions as a means to check the performance of these methods. The results strongly suggest that relative elevation surveys may be overemphasized in current practice, as (1) maximum elevation differences significantly overestimate distortions; (2) maximum slopes are quite insensitive to distortions; and (3) pattern recognition approaches are unreliable as an indicator that distortions have occurred.     

载荷对接触热导影响的实验研究

接触界面热导是工程热设计中的一个重要参数，目前国内外普遍采用稳态法进行测量，但该法不能测试大塑性变形下的接触热导．在许多工程应用如双辊连续铸轧中，材料发生大应变塑性变形，其变形率达到50％以上．本文利用瞬态法对大变形条件下的接触热导进行了测试，测试结果表明当接触压力远远超过接触试样中硬度较小的试样的屈服极限以后接触热导变化趋于平缓，接触热导值相对于小压力条件有显著提高。     

应变速率对Q460连铸坯高温力学性能的影响

摘要：采用Gleeble-3500热模拟试验机和金相法测试了不同应变速率下建筑用钢Q460连铸坯的高温力学性能,获得了600~1200℃范围内Q460连铸坯的高温强度、热塑性和最终室温组织随拉伸温度和应变速率的变化规律。结果表明,当Q460连铸坯在较高的应变速率（10s-1）下进行高温拉伸时,试样的断面收缩率随着拉伸温度的升高而升高,没有出现高温脆性区;在较低的应变速率（10-3s-1）下进行高温拉伸时,试样的断面收缩率出现了2个脆性区,第1个在1100℃至熔点温度,第2个脆性区间在700℃附近。总体来说,实验钢种的高温断面收缩率均大于65%,表明建筑用钢Q460连铸坯具有较好的高温热塑性。此外,同一应变速率下,Q460连铸坯的抗拉强度随着拉伸温度的升高而降低,而伸长率随着拉伸温度的升高而升高。     

连铸辊断裂失效分析研究

采用成分分析和金相分析等方法,对连铸辊断裂原因进行了分析。结果表明:堆焊层存在较多夹杂物,并在高温下形成尖锐裂纹源;连铸机发生"夹辊",造成局部温度剧烈升高,加快裂纹的扩展;坯辊锻造后的晶粒较粗大,S局部沿晶界分布,降低了坯辊的韧性,也促进裂纹的扩展。     

Erosion corrosion of low-alloy wear-resistant steels in alkaline slurry

Erosion corrosion causes significant problems in various industrial environments through a synergistic effect which results in much greater weight loss than the sum of the weight losses in the individual processes. The erosion-corrosion behavior of three low-alloy steels was investigated in a simulated concrete slurry using the rotation method. The key influencing factors and mechanism of material degradation were analyzed. The experimental results indicate that the weight loss increases with the linear velocity according to a nearly exponential relationship (W = KVⁿ), where n is 1. 40–2. 14. This weight loss is mainly caused by erosion in the alkaline slurry, and steels with higher tensile strengths show higher erosion-corrosion resistance. The formation of many platelets and ring cracks and their removal from the sample surface during erosion corrosion in the slurry are thought to constitute the mechanism responsible for this weight loss. These platelets and ring cracks are formed by solid particles striking the sample surface. Craters are initially produced and subsequently disappear as they grow and come in contact with each other. Fewer craters were observed on the surfaces of samples that exhibited higher weight loss. The surface of the material became work-hardened because of the effect of the particles striking and scratching, and a deformed layer was produced on the surface for steels of lower strengths, leading to deeper and more abundant gouges.     

Tensile Mechanical Behavior and Spall Response of a Selective Laser Melted 17-4 PH Stainless Steel

The tensile mechanical behavior and spall response of a selective laser melted (SLM) 17-4 precipitation-hardening (PH) stainless steel were studied comprehensively through tensile test, plate impact experiment and microstructure characterization in the present study. The results reveal a steel with significant strain rate dependence on the tensile mechanical behavior and spall response. As the strain rate increases, the tensile yield stress increases, but there is no monotonic variation trend for the peak stress; grain structure remains unchanged first and then becomes fine; high-angle grain boundaries (HAGBs) increase; the martensite phase decreases at first and then increases. There is a close correlation among impact velocity, strain rate, peak stress, Hugoniot elastic limit (HEL) and spall strength. Strain rate, peak stress and HEL increase, while spall strength remains almost constant with the increase of impact velocity. As impact velocity increases, grain structure becomes fine, HAGBs increase and the martensite phase increases. The significant phase transformation is responsible for the tensile mechanical behavior and spall response, and the temperature rise was calculated to analyze its effect on phase transformation. Whether the preferred orientations are along the building direction or tensile direction is dependent on strain rate. Tensile and spallation specimens exhibit the ductile fracture mode and the damage originates from voids. It is interesting that the voids always tend to nucleate at melt pool boundaries. A spall damage evolution model is illustrated to describe the damage process.

     

Experimental Study of the Behavior of a Lumpy Fill of Soft Clay

Land reclamation is a major civil engineering activity in Singapore. Due to depletion of suitable local fills and the cost of imported sand, dredged and excavated clay fills, in spite of their poor engineering properties, are being evaluated as a fill material. To reduce double handling, it is desirable for the clay to be used directly in a lump form, instead of the more conventional slurry fill. While the performance of a slurry fill is relatively well understood, the behavior of lumpy fill is not. This paper reports the results of a laboratory study carried out on lumpy fill made of cubical clay lumps of size ranging from 12.5?to?50?mm. The study showed that the interlump voids are substantially closed at a consolidation pressure much lower than the preconsolidation pressure of the lumps. The study also shows that at a consolidation pressure of about 100?kPa, the permeability of a lumpy fill is reduced to an order similar to that for homogeneous clay. However, the shear strength profile obtained using the cone penetration test indicates that the fill is still highly heterogeneous under a pressure of 100?kPa. When the preconsolidation pressure of the lumps is exceeded, the strength profile becomes uniform. The degree of swelling of the lumps plays a significant role. For fully swollen lumps, the consolidation pressure required to close the interlump voids is considerably less than that if the lumps were not allowed to swell. The coefficient of secondary compression of the lumpy fill is comparable to the homogeneous clay indicating that secondary compression is not a serious issue.     

On the analysis of delamination fractures in high-strength steels

The influence of the stress state on the occurrence of delamination during tensile testing has been examined in two high-strength steels. The stress state was varied by either machining notches into cylindrical tensile samples or by conducting the tensile tests under superimposed hydrostatic pressures up to 690 MPa. In addition, the magnitude of the transverse stresses was modified by varying the yield stress of the material due to the tempering treatments. In all cases, fracture occurred either by ductile fracture or by ductile fracture accompanied by extensive delamination along the tangential planes. The mechanism of delamination on the planes involved nucleation of voids at carbides followed by ductile rupture of the surrounding matrix. The results suggest a criterion for delamination which involves both the level of plastic strain and the normal stress acting on the delamination plane. The competition between the failure modes is illustrated using fracture mechanism maps constructed in stress space. F. ZOK, formerly with the Department of Materials Science and Engineering, McMaster University.     

Analysis of Impact Response of Composite Laminates under Prestress

An analytical solution for the impact response is obtained for the central impact of mass on a simply supported laminated composite plate under prestresses based on the Fourier series expansion and Laplace transform technique. A linearized version of the elastoplastic contact law proposed was used in the analytical formulation to consider permanent indentation during the impact. Permanent indentation including damage effects was included in the elastoplastic contact law. The effects of initial stresses on the contact force, plate center displacement, as well as strain time histories are presented. It is shown that higher initial stresses increase the maximum value of the contact force but reduce the plate central displacement. Effects of impactor velocity, mass, interlaminar shear strength of the laminates, and plate thickness on the contact force and dynamic response of the plate under tensile prestresses are also discussed.     

7055铝合金恒载荷应力腐蚀性能及机理研究

采用室温拉伸、光学显微镜和扫描电镜研究了不同恒载荷条件下7055铝合金型材的应力腐蚀性能。结果表明,在腐蚀周期内,随着应力腐蚀载荷的提高,试样的强度并未出现明显的下降,而塑性下降40%。应力腐蚀载荷越接近材料的屈服强度,塑性下降就越明显,抗拉应变和延伸率均有降低。应力腐蚀试样表层部分区域有明显的点蚀、沿晶腐蚀空洞和裂纹;拉伸时,腐蚀缺口处会产生明显的应力集中而成为起裂源,拉伸断口边缘被腐蚀区域为明显的沿晶断裂。     

The influence of tensile stress states on the failure of HY-100 steel

The failure of an HY-100 steel plate has been examined as a function of stress state using notched and un-notched axisymmetric tensile specimens. The results show that increasing stress triaxiality leads to a rapid decrease in failure strains in a manner that is exponentially dependent on the degree of triaxiality. Two ductile failure mechanisms are identified: a void coalescence process, in which relatively equiaxed voids grow to impingement, and a void-sheet process, which links by a shear instability process large, elongated inclusion-initiated voids. The void-sheet mechanism intervenes and limits ductility at high-stress triaxialities in transversely oriented HY steel plate material, whereas the former process controls failure in longitudinally oriented material. These orientation effects are related to the morphology and alignment of the nonmetallic inclusion stringers that act as the primary void nucleation sites. Calcium treatments for inclusion-shape control improve ductility, especially at intermediate-stress triaxialities, primarily by suppressing the local conditions which give rise to the void-sheet instability process.     

CSP热轧过程中夹杂物周围孔洞的演变

借助于动态显式有限元软件ABAQUS/Explicit,模拟了CSP热轧过程中,板坯厚度方向上不同位置处直径为50μm的Al2O3夹杂物周围孔洞的形成和演变过程。结果表明,对于不同位置处的Al2O3夹杂物,在它们沿轧制方向的前、后部位都形成了孔洞,但前孔洞面积大于后孔洞面积;夹杂物越靠近板坯表面,形成的孔洞越大;热轧过程中,孔洞的演变是一个"愈合—长大—愈合"的动态过程;随着道次的增加,夹杂物前、后孔洞在轧制方向上的投影长度变化与面积变化趋势相一致,而与孔洞尖端夹角的变化趋势恰好相反;孔洞在热轧过程中发生转动,导致前孔洞尖端朝着带钢表面扩展。模拟结果有助于揭示夹杂物周围孔洞与表面缺陷的关系,并为实际生产提供理论指导。