Predicting moisture movement during the drying of concrete floors using finite elements

Predictive methods for determining the point at which it is safe to apply floor coverings to concrete floors can save time and money for the client, contractor and the floor installer. The current standard states that impervious floor coverings should not be applied until the surface of the floor reaches a relative humidity (RH) of 75%, established using a surface hygrometer test. Tests by the authors on drying concrete slabs in natural and forced drying environments show that there is a large variation in the residue of moisture deep in the concrete when the floor covering is applied, particularly so in a forced drying environment. After an impermeable floor covering is applied to the floor surface, this residue of moisture will gradually equilibrate within the slab depth and generate (over a long time) a vapour pressure at the surface that can result in substantial damage to the covering, resulting in expensive repair work.This paper presents a finite element model that predicts the changing moisture content, in terms of the internal RH, during drying and after the application of the floor covering as the internal RH equilibrates over time. The model accounts for the thickness of the slab, w/c ratio, environmental conditions, boundary conditions. It uses non-linear diffusion coefficients and evaporation rates to accurately model the moisture movement in the slab. The results from the model give good correlations with the experimental readings taken at the various depths over time using hand-held humidity probes.     

The influence of soil moisture content variations on heat losses from earth-contact structures: an initial assessment

An initial investigation of the influence of varying ground moisture content beneath buildings on the heat losses through ground floor slabs is presented. A range of finite element analyses have been performed that indicate that soil moisture content changes can have a significant effect on soil thermal conductivity and hence on ground heat transfer. The work is undertaken for conditions of static moisture content distributions. For the particular problems considered, the results obtained show that total heat flux to the ground can increase significantly with decreasing ground water table depth. Steady-state heat conduction analyses were performed for three test problems; a one-dimensional problem, a two-dimensional shallow earth-contact structure and a two-dimensional “deep” earth-sheltered structure. Each problem was analysed for water table depths ranging from 10 m to zero (i.e. the ground surface). The resulting increase in soil moisture content was found to cause a 60% increase in heat flux in the one-dimensional problem, a 20% increase for the two-dimensional shallow structure and 40% for the deep structure. The variation between types of analysis is due to the heat flow path geometry available in each case. The work is viewed as a further step towards a more complete understanding of the influence of ground moisture content on heat transfer problems. The conclusions drawn should be viewed as a first indication of the significance of this aspect of the problem and need to be considered separately from other work on the influence of ground water flow beneath the water table. The results also suggest that transient effects and coupling of heat and moisture transfer processes merit further attention.     

足尺平板无梁楼盖抗火性能试验研究

对2个足尺钢筋混凝土平板无梁楼盖试件进行了抗火试验,测量了火灾作用下板的平面内（水平）位移和平面外（竖向）位移等,考察了板的约束反力变化,分析了混凝土板沿厚度的温度分布以及钢筋的温度变化。结果表明：单面受火的钢筋混凝土板沿厚度的温度呈非线性分布,温度梯度随时间的增长而快速增大,并且毛细孔水和凝胶水等水分的存在使混凝土在100℃时产生了升温水平段;水分不仅影响了混凝土的升温,还使受火混凝土产生爆裂,含水率太高时板将因混凝土的爆裂而导致破坏;柱上板带和跨中板带的约束反力均产生了显著变化,柱上板带的约束反力由第4 min时的8.2 kN/m下降到试验结束时的2.7kN/m,跨中板带则由第4 min时的4.5 kN/m下降到接近0;平板无梁楼盖的板顶裂缝最终形成对角呈双曲线形,与传统塑性铰线理论的柱附近局部屈服线模式一致。     

树脂类整体地坪施工中基层处理设备与工艺的选择及应用

树脂类整体地坪施工中,最为常用的被用作基层处理施工的设备,主要有抛丸机、铣刨机及研磨机三大类,分别对应的基层处理工艺为喷砂、铣刨及研磨。基层处理的设备与工艺的选择,需要结合每个项目的具体实际情况,根据基材的类型、项目环境的适用性以及所需施工的整体地坪系统类型,综合决定选择最佳的基层处理设备与工艺。     

Full-scale long-term and ultimate experiments of simply-supported composite beams with steel deck

This paper presents the details of an experimental study carried out on two full-scale simply-supported composite steel-concrete beams to investigate their long-term behaviour and how this affects their ultimate response. The specimens were formed by a steel joist and a concrete slab with steel deck, and possessed identical spans and cross-sections. The ribs of the steel deck were orthogonal to the longitudinal axis of the steel joist, representing a secondary beam of a typical composite building flooring system. The beams were designed based on Australian guidelines with a degree of shear connection equal to 0.79. The samples were cast using unpropped construction. One specimen was kept unloaded for the entire duration of the tests. The second one was subjected to a sustained uniformly distributed load for about four months, after which it was loaded to failure. Particular attention was placed in this study on the occurrence of non-uniform shrinkage through the concrete thickness due to the presence of the profiled sheeting which prevented moisture egress to occur from the underside of the slab. Short- and long-term push-out tests were carried out to measure the response of the shear connectors over time. An analytical model was recommended to predict the long-term behaviour of composite beams with steel deck and its adequacy was validated against the reported experiments. For its simplicity, the proposed approach lends itself for design applications.     

竹木复合地板变形有限元模拟

以表层为竹材、芯层为杉木拼板、背层为杨木单板的竹木复合地板为对象,利用ANSYS有限元软件,采用湿热类比法模拟表层和芯层厚度不同的竹木复合地板在干燥环境中的含水率变化,然后进行热结构耦合分析,研究由含水率变化引起的竹木复合地板的翘曲变形.结果表明:胶层能够起到水分屏障的作用,胶层内部木材的含水率变化很小,在使用中受空气湿度变化影响的主要是表层的竹材部分,而且表层竹材越薄的竹木复合地板其含水率越早与环境达到平衡;芯层厚度对竹木复合地板翘曲变形率有较大影响,而表层厚度对其翘曲变形率影响较小.该研究可为竹木复合地板的制造设计提供依据.     

Relationship between tile exfoliation and moisture content

In Japan, tile exfoliation from external walls is a serious problem that must be prevented. In our previous study, a tiled concrete sample was dried at a temperature of 105 °C for preconditioning. Then, tile exfoliation occurred. Since this seemed to indicate a relationship between tile exfoliation and moisture content, it was further pursued in this study. First, drying experiments on concrete samples just after tiling were carried out under different ambient air temperatures. The following results were obtained. ●Tile exfoliation occurs even at the lower drying temperatures. ●Tile exfoliation starts at corner tiles (seen in all experiments). ●When the average moisture content of the tiled concrete sample becomes lower than a certain value, tile exfoliation occurs. Next, the experiments were analyzed using a three-dimensional model of simultaneous heat and moisture transport. The calculated moisture content agreed well with the measured results. By comparing the measurement and simulation results, it is considered that the tiles are exfoliated when the moisture content of the mortar on the back side of the tile becomes lower than a certain value. Therefore, the moisture content can serve as an index for evaluating the occurrence of the tile exfoliation.     

Research information: Superplasticizers and plastic shrinkage in silica fume concrete

Silica fume is an admixture for concrete and it leads to improved engineering properties. However, silica fume in concrete adversely affects the workability of the mix. It also reduces or eliminates bleeding, due to an increase in fineness, thereby resulting in premature drying of the concrete surface, which could lead to plastic shrinkage cracks. Abdulaziz A. Bubshait and Bassam Tahir briefly consider the plastic shrinkage potential of Ordinary Portland Cement (OPC) concrete and silica fume concrete slab panels, both containing superplasticizing and retarding admixtures, and verifies various curing modes.     

大型工业厂房承重耐磨混凝土地面设计与施工

大型工业厂房承重耐磨混凝土地面的设计和施工,关键在于如何避免混凝土产生无规则裂缝和控制地坪表面的平整度。混凝土施工控制和地坪缝的设置决定了地坪表面质量好坏。本文采用实际工程,阐述了如何形成高质量地面,对大面积承重耐磨混凝土地面的设计与施工要点进行了详细的介绍。     

矿粉混凝土干燥收缩性能

主要研究了矿粉对混凝土干燥收缩性能的影响。试验结果表明:①在同水胶比条件下,矿粉掺量越多,混凝土干缩值越大;②在同标号条件下,普通掺量(20% ~35% )的矿粉混凝土的干缩值与基准混凝土相比增幅不大;③在同标号(R28)条件下,矿粉掺量≥35%时,混凝土的早期干缩值(7d)的增幅都较大。     

The tensile membrane action of unrestrained composite slabs simulated under fire conditions

Following a major fire test programme on a full-scale, steel-framed building it was found that the composite flooring system, comprising lightweight concrete, anti-crack mesh reinforcement and steel deck, had a greater inherent fire resistance than suggested by current codified design methods. It was felt that this was due to tensile membrane action occurring in the slab at large displacements. This led to an independent test being conducted at the Building Research Establishment where a 9.5 m×6.5 m composite slab, with nominal horizontal restraint to its edges, was tested to failure. To simulate the behaviour of the slab in fire, the steel deck was removed, leaving the concrete and anti-crack reinforcement, before load was applied. Tensile membrane action was shown to occur, with the failure load being approximately double that calculated using the classic yield line theory.     

三聚氰胺浸渍纸层压阻燃地板的燃烧性能

在纤维拌胶的同时加入阻燃剂,并优化生产工艺,生产阻燃高密度纤维板作为地板基材,开发三聚氰胺浸渍纸层压阻燃地板。根据GB/T 17657-1999《人造板及饰面人造板理化性能试验方法》规定的方法,检测所生产强化地板的密度、含水率、吸水厚度膨胀率、内结合强度、静曲强度和甲醛释放量,并采用锥形量热仪测试和对比地板的燃烧性能。结果表明:生产的阻燃地板符合GB/T 18102-2007《浸渍纸层压木质地板》中规定要求,与普通强化木地板相比阻燃地板点燃时间延长了8 s,pkHRR下降了62%,Av-HRR下降了31.7%,EHC下降了11.5%,THR值下降了21.8,阻燃效果明显。     

Foundation heat loss from heated concrete slab-on-grade floors

In recent years, there has been a renewed interest in heated concrete slab floors to provide for space heating in both residential and commercial buildings. The existing design procedures for these heating systems are based on simplified thermal models with several assumptions. In particular, the simplified models assume that both the upper and the lower concrete slab surface are isothermal and that the heat transfer through the bottom of the slab surfaces is uniformly distributed over the entire surface. In this paper, a more realistic and flexible model for heated or cooled concrete slab floors is considered to determine the heat transfer between the concrete slab and the ground. In particular, steady-state and steady-periodic semi-analytical solutions are developed to determine the temperature field within the ground medium and within the concrete slab-on-grade floor where hot or chilled water pipes are embedded. The solution presented in this paper is applied to determine the ground heat loss/gain for a heated or cooled floor under various design conditions including the level of floor insulation, and the temperature of the water pipes. These solutions are obtained using the interzone temperature profile estimation (ITPE) procedure. Detailed analysis is presented to determine the effect of the slab insulation configuration on soil and slab temperature field and on the monthly variation of the total slab heat loss.     

Immersion test of loess in ground fissures in Shuanghuaishu,Shaanxi Province,China

The ground fissure of Shuanghuaishu in Shaanxi Province, China, is a tectonic ground fissure developed in the northern Loess Plateau of the Weihe Basin. Remarkably, it exhibits multiple activities under heavy rainfall conditions. To explore how heavy rainfall infiltration can induce ground fissures to reactivate, we conducted an in situ large-scale water immersion test on the ground fissure zone. Through measurement of soil moisture content and observation of subsidence and deformation of surface and soil layers at different depths before and after water immersion, we determined the variation law of soil moisture content and the humidification deformation of the ground fissure zone after rain infiltration. Results show that ground fissures destroy the structure of soil layers and form a certain width of influence zone, which allows surface water to seep through the fissure zone into the deep soil below the relative aquifuge. After water immersion, the deformation of the soil in the ground fissure zone is obviously larger than that in the non-fracture zone, the influence zone width of hanging wall of the ground fissure is larger than that of the footwall, and the influence range on the upper soil layer is larger than that on the lower soil layer.

     

深圳俊园超高层建筑结构工程施工

深圳俊园工程地下３层，地上４７层，高１６１ｍ。施工中采用了人工挖孔桩加预应力错杆深基坑支护，大体积混凝土三掺技术、桩墙合一、颈性钢筋混凝土结构、梁式了板式转换层等技术，效果良好，经济效益明显。     

青藏直流输变电工程基础冻结强度试验研究

土与基础接触面间的冻结强度是评价冻土区基础工程抗拔性能,分析构筑物与冻土相互作用的基础和关键。通过不同含水率和不同温度青藏粉土与混凝土接触面的直剪试验,研究了冻结强度变化规律。结果表明,温度、含水率和法向应力均对冻结强度和冻结强度恢复有显著的影响。冻土沿接触面的剪切滑移会导致冻结强度损失,冰胶结能力的破坏导致接触面黏聚力减小,表面粗糙程度的增加导致内摩擦角增大,内摩擦角增加对冻结强度增加的贡献远远小于黏聚力减小导致的冻结强度降低。由于含水率、温度和法向应力对冻结强度有显著的影响,冻结强度对于评估基础所受的切向冻胀力以及评价基础的冻拔安全性具有重要的意义。但是,现行规范中冻土区基础切向冻胀力设计值主要依据土体冻胀敏感性确定,并未考虑含水率、温度和法向应力的影响,可能导致切向冻胀力设计值与基础实际受到的冻胀力存在较大的差异,引起基础冻胀失稳或经济浪费。     

浅谈商品混凝土现浇楼板裂缝的防治

针对商品混凝土现浇楼板在混凝土施工过程中形成的裂缝，提出了对混凝土施工过程环节控制的具体措施，以有效避免商品混凝土现浇楼板裂缝的产生，指出裂缝的防治应以环节控制为主，辅以技术措施及补救措施。     

预应力混凝土简支板抗火性能试验与分析

完成了9块无粘结和1块有粘结预应力混凝土单向简支板的抗火试验。各试验板内的热电偶在火灾下的实测结果表明,温度由迎火面至背火面逐渐递减,由迎火面至背火面的温度变化梯度随离迎火面距离的增大而减小。布置于无粘结预应力筋锚固端的力传感器和试验板上的位移传感器的测试结果表明,受火初期试验板挠曲变形增长较快,无粘结预应力筋应力呈增长趋势;受火一段时间之后试验板变形增长速度有所减缓,无粘结预应力筋应力呈减小趋势,且这种趋势一直延续到停火后一段时间;停火一段时间之后,试验板有变形恢复趋势,无粘结预应力筋应力水平又有所恢复。在受火过程中试验板背火面经历了水分溢出、水蒸气形成和发展、溢出水干涸及水蒸气消失的过程。在试验炉两侧壁的观察孔观测到有6块试验板发生了爆裂,爆裂发生在预应力度较高、荷载作用水平较低的试验板的邻近支座区域。基于试验结果,试算确定了对流换热系数、辐射换热相当发射率系数、混凝土导热系数和比热的取值,应用ANSYS     

工业废渣混凝土多孔砖墙体干缩性能实验研究

阐述了工业废渣混凝土多孔砖由于含水率降低引起的干燥收缩,是其建筑裂缝的主要成因之一,通过对混凝土多孔砖在自然温、湿度条件下从饱和含水率到趋于平衡含水率的失水及干缩情况及其砌体在中等湿度和干燥条件下干缩值的持续观测,得出其失水规律和干缩率。