Fiber-Reinforced-Cementitious-Composites Plate for Anchoring FRP Sheet on Concrete Member

To improve the fiber-reinforced polymer (FRP)/concrete bond capacity, this paper presents a new anchoring approach with the gluing of precast fiber-reinforced cementitious composites (FRCC) plate on top of the FRP sheets. In order to measure the improvement in ultimate load and deformation capacity and to study the failure mechanisms around the anchored area, the direct shear bond test is performed on concrete prisms with bonded FRP. Several sets of tests have been carried out with anchoring plates of different FRCC compositions and lengths. Comparison with the control sample shows that the installation of FRCC plate can significantly increase both the bond and deformation capacities (by up to 100%). On the basis of the shear bond test, two types of FRCC plate materials were found to be particularly effective and were selected for strengthening of beam members to be tested under four-point bending. Comparison with control members (without anchor) and those with conventional U-shaped FRP anchors indicates that both the ultimate load and central deflection can be improved by the new anchoring method.     

Numerical Optimization of a Compact and Reusable Pretensioning Anchorage System for CFRP Tendons

Efficient use of pultruded carbon fiber–reinforced plastic profiles (CFRP tendons) to prestress high-performance concrete (HPC) highly depends on the performance of the anchorage system and on material choice. For current applications, a prestressing degree of approximately 40% of the CFRP material strength is utilized in pretensioned concrete elements. A higher prestress implicates lower costs of fully prestressed concrete elements. The present project aimed to optimize the design of a removable and reusable pretensioning anchorage system for sand-coated CFRP rods. The optimized design was achieved by means of finite-element calculations in which parametric studies were complemented with extensive experimental work for validation. Analytical results demonstrated a reduction up to 25% for the relevant stress peaks in the tendons. The static rupture load under laboratory conditions increased by 25%, and the pretensioning level on-site could be increased by 50%. This improvement in production efficiency can be explained by easier applicability of the new system, i.e., failure tolerant assembly and prestressing process.     

Performance of Tension and Compression Anchors in Weathered Soil

Anchor pull-out tests were performed on seven instrumented full-scale low-pressure grouted anchors installed in weathered soil at the Geotechnical Experimentation Site at Sungkyunkwan University. Anchors were 165 mm in diameter and embedded 9 to 12 m in weathered soil. Four were compression type and three were tension anchors. Performance tests, creep tests, and long term relaxation tests were performed and the results are presented. The characteristics of grout-soil and grout-strand interface were investigated and presented. From the measurements, a load transfer mechanism for tension and compression ground anchors was investigated and evaluated by a simple beam-spring numerical model.     

Tied-Back Excavations in Alluvial Soil of Taipei

In this paper we present monitored results of three tied-back excavations carried out in the alluvial soil of Taipei. All three cases involved large excavations and were supported by a diaphragm wall and multilevel tieback anchors. Based on the anchor loads measured during excavation, the apparent lateral pressure diagrams are calculated following Terzaghi and Peck’s method. It is found that the apparent lateral pressures increase approximately linearly with the depth. As the excavation reaches its final depth, the apparent lateral pressure diagrams tend to converge for the cases studied here. The measured lateral pressure diagrams for these tied-back excavations are close to the summation diagrams of groundwater pressure and lateral pressure calculated from the method of Terzaghi and Peck in 1967 or Tschebotarioff in 1973 for sandy soil but with a minor difference. The dominant sandy soil layer and groundwater pressure played a major role in determining the anchor load diagrams. The measured lateral wall movement for these tied-back excavations is found to be similar to that of cross-lot braced excavations in alluvial soil of Taipei both in magnitude and in profile.     

Numerical Simulation of Vertical Pullout of Plate Anchors in Clay

The behavior of strip and circular plate anchors during vertical pullout in uniform and normally consolidated clays was studied in this paper by means of small strain and large deformation finite-element analyses. Both fully bonded (attached), and “vented” (no suction on rear face), anchors were considered. The current numerical results were compared with existing laboratory test data, finite-element results, and analytical solutions. This study showed that, in small strain analysis, the scatter of existing data was mainly due to the effect of soil stiffness. In large deformation analysis, when soil and anchor base were attached with suction, the pullout capacity factor formed a unique curve independent of the soil strength (su), soil effective unit weight (γ′) and anchor size (B=width of strip anchor and D=diameter of circular anchor). The transitional embedment depth ratio, HSD/B or HSD/D, (where HSD=transition depth between shallow and deep embedment) was 1.4 for a strip anchor and 0.75 for a circular anchor. The ultimate pullout capacity factors (Nc) for deep embedment were 11.6 and 11.7 for smooth and rough strip anchors and 13.1 and 13.7 for smooth and rough circular anchors, respectively. However, when the anchor base was vented, the soil stayed attached to the anchor base for deep embedment, and the pullout capacity was therefore the same as for the attached anchor. The separation depth ratio, Hs/B or Hs/D, (where Hs=embedment depth at which the soil and anchor base separated) was found to increase linearly with the normalized strength ratio, su/γ′B or su/γ′D.     

Three-Dimensional Lower Bound Solutions for Stability of Plate Anchors in Clay

Soil anchors are commonly used as foundation systems for structures that require uplift or lateral resistance. These types of structures include transmission towers, sheet pile walls, and buried pipelines. Although anchors are typically complex in shape (e.g., drag or helical anchors), many previous analyses idealize the anchor as a continuous strip under plane strain conditions. This assumption provides numerical advantages and the problem can be solved in two dimensions. In contrast to recent numerical studies, this paper applies three-dimensional numerical limit analysis to evaluate the effect of anchor shape on the pullout capacity of horizontal anchors in undrained clay. The anchor is idealized as either square, circular, or rectangular in shape. Estimates of the ultimate pullout load are obtained by using a newly developed three-dimensional numerical procedure based on a finite-element formulation of the lower bound theorem of limit analysis. This formulation assumes a perfectly plastic soil model with a Tresca yield criterion. Results are presented in the familiar form of break-out factors based on various anchor shapes and embedment depths, and are also compared with existing numerical and empirical solutions.     

Penetration Resistance of Offshore Skirted Foundations and Anchors in Dense Sand

Penetration of skirts is an essential design issue for offshore skirted foundations and anchors in sand. Skirts may not penetrate far enough into dense sand by the available submerged weight alone. It may therefore be necessary to apply underpressure inside the skirt compartment to produce an increased driving force and to reduce the penetration resistance. This paper recommends procedures to calculate penetration resistance and required underpressure for skirts penetrated in dense sand with and without interbedded clay layers. The recommendations are based on interpretation of skirt penetration data from prototypes, field model tests, and laboratory model tests in dense sand. The paper first presents a model to calculate the penetration resistance of skirts penetrated by weight, or other external vertical load that does not cause flow of water in the sand. Two models are considered; one based on bearing capacity equations with friction angles from laboratory tests, and the other one based on empirical correlations with CPT tip resistance. The bearing capacity model gives more consistent correlations with the empirical data than the CPT model. Thereafter, a model to account for the effect of underpressure applied inside the skirt compartment is proposed. This model is developed based on interpretation of available prototype and model test data from skirts penetrated by underpressure. The results show that underpressure facilitates skirt penetration in sand considerably by providing both an additional penetration force and a reduced penetration resistance. It is also shown that interbedded clay layers can prevent flow of water through the sand and eliminate the beneficial reduction in penetration resistance.     

关于预应力筋锚固区的荷载传递试验——预应力锚固区安全探讨之二

预应力筋锚固区的承载能力如何确定,是中外业界长久以来十分关注的问题,国外的技术标准要求采用对试件加压力的试验方法进行验证,中国的惯例是混凝土结构以内的埋入件及抗裂钢筋由结构设计者负责设计,结构以外的锚具由锚具制造者负责达到标准。但整套的夹片式锚具却包含了结构体外的锚具和用于结构体内的配套零件,形成了分管不清的局面。从2010年10月起,中国的行业标准JGJ85-2010《预应力筋用锚具、夹具和连接器应用技术规程》首先提出在进场验收锚具时,如果锚垫板是铸造型的,锚具生产厂应进行锚固区传力性能试验,结构设计方认为锚固区传力性能不能满足要求时也可能进行这项试验,该标准与美国后张预应力学会(PTI)的验收标准基本一致。2011年3月中国交通部发布了《公路桥梁预应力钢绞线用锚具、夹具和连接器》(JT/T329-2010)行业标准,要求锚下垫板的构造尺寸应能将预应力可靠地从锚具传递到混凝土构件中。该标准的规定与FIP1993关于荷载传递试验要求基本相同。但是中国的业界对"荷载传递试验"比较陌生,不太了解如何进行这项试验。今后的国内外工程(如高铁、高速),特别是国外用户可能要求锚具生产厂提供此项试验证明,锚具生产方和结构设计方都需要直面应对。本文就是在这种形势下汇总了国内外的试验概念、试件制作、试验方法和要求,给业内人员提供一个概括的资讯。     

大型堆载边坡前缘锚拉支挡结构设计

某滑体前缘地形平坦部位被利用作为大型砂石骨料系统堆场,从而形成大型堆载边坡。上部堆载有可能导致墙后坡体挤压变形和挡墙剪切破坏。采用锚拉抗滑挡墙进行支挡,并辅以排水、监测措施,稳定计算和运行监测成果表明,抗滑及抗倾稳定满足要求,各监测数据处于收敛状态,前缘锚拉抗滑挡墙稳定可靠。     

Use of Mixed-Mode Fracture Interfaces for the Modeling of Large-Scale FRP-Strengthened Beams

In this study, numerical models of fiber-reinforced polymer (FRP)-strengthened beams were developed using nonlinear fracture mechanics for the modeling of the concrete-FRP (longitudinal and U-wrap) interfaces. Mode 1, Mode 2, and mixed-mode interfacial behaviors were considered. Results from the finite-element models were compared with experimental tests of large-scale strengthened beams using FRP U-wraps as anchors. The numerical program assessed the effect of the interfacial modeling in the global and local responses. A parametric study was conducted to determine the effect of additional longitudinal FRP sheets in strengthened beams with and without FRP U-wraps. Results from this study indicate that the use of a mixed-mode concrete-FRP interface is a robust numerical approach for the prediction of the global and local responses of large-scale FRP-strengthened beams. The parametric study shows that the use of FRP U-wraps could improve the strength and ductility of the FRP-strengthened beams by changing their failure mode and deflection response. Appropriate modeling of the concrete-FRP interfaces is needed to successfully predict these effects.