Transfer Length of Strands in Prestressed Concrete Piles

A top bar effect has been identified in prestressed concrete piles. The effect that this top bar effect has on the development of the prestressing strand is investigated. Strand transfer length is found to be proportional to the observed end slip. While the average transfer length of all strands in a section may satisfy the assumptions inherent in the ACI transfer length equation, due to the top bar effect, top-cast strand transfer lengths are considerably in excess of the ACI-calculated value. The flexural behavior of the pile, accounting for varying transfer lengths through its section, is investigated. Finally, recommendations for in-plant testing and acceptance criteria for prestressed strand bond quality are proposed

Repair of Wood Piles with Fiber Reinforced Composites

Piles made of treated wood have been traditionally used for the construction of piers and other waterfront structures. The main concern related to wood piles is deterioration due to marine borers, which limits the lifespan and requires frequent repair and replacement. Furthermore, since the use of preservative treatments for wood piles has been reduced due to environmental concerns, there is a current need for efficient methods for wood pile protection. Marine borer activity in Maine coastal waters was assessed through a survey directed to harbor masters correlated with historic data. In order to illustrate the type and extent of wood pile deterioration, two case studies in Maine harbors are presented. A special prefabricated Fiber Reinforced Polymer (FRP) composite shield or jacket was developed to repair wood piles in the field. FRP composite shells or sleeves are bonded with an underwater curing adhesive to form a shield. The main concern for durability of the adhesive bond is the resistance to freeze-thaw cycles. To assess adhesive bond durability, single lap shear tests were performed after exposure to freeze-thaw cycles. Two types of load-transfer mechanisms between the wood pile and the FRP composite shield were developed and tested: (1) cement-based structural grout; and (2) steel shear connectors with an expanding polyurethane chemical grout. Push-out tests by compression loading were performed to characterize the interfaces and discriminate the effect of the design parameters. The outcome of the push-out tests was the evaluation of the shear force-slip non-linear response and the progressive failure mechanism. The structural response of full-size pre-damaged wood piles repaired with the FRP composite shield system was characterized. A three-point bending test procedure was used to simulate the response of a pile subjected to lateral loads. The loaddeformation response, deflected shape profile, relative longitudinal displacements (slip), strain distribution, ultimate bending moment capacity and mode of failure were evaluated. Wood piles were pre-damaged by reducing approximately 60% of the crosssection over a portion of the pile. It was found that a pre-damaged wood pile repaired using the FRP composite shield with cement-based grout exceeded the bending capacity of a reference wood pile. The repair system using the FRP composite shield with steel shear connectors and polyurethane grout did not fully restore the bending capacity of a reference wood pile; however it can be used for marine borer protection when wood damage is not critical. A beam structural model to predict stiffness and strength properties of wood piles restored with FRP composite shells was developed. The model accounts for different pile dimensional properties and various amounts of pre-damage. The structural model was successfully correlated with experimental data from three-point bending tests of wood piles

Top Bar Effects in Prestressed Concrete Piles

The top bar effect in reinforced concrete is a widely recognized phenomenon. Currently, the ACI Building Code prescribes a 30% increase in the development length of top cast reinforcing bars. No such provision is required for strands in prestressed concrete members. In this paper, the top bar effect for prestressing strands is introduced. Parameters affecting top bar phenomena in prestressed concrete piles are identified, and strategies for reducing this effect are presented. Finally, for the first time, the application of a top bar effect factor for prestressed concrete development length calculations, similar to the one applied in reinforced concrete structural elements, is proposed

Excessive Strand End Slip in Prestressed Piles

This paper presents the results of a research project that investigated excessive strand end slip observed recently in some prestressed piles. From measurements taken in the field, it is apparent that the problem o excessive initial strand slip is independent of pile shape and size. Strand end slip is evident in piles of different manufacturers in different states in the Southeast. Excessive strand end slip was found in both the top and bottom of the cross section of the piles, although the top portion of the cross section generally exhibited much higher initial slip. Several preventive measures can be adopted to reduce the excessive strand end slip. These preventive measures include: a) proper concrete mixture proportioning to reduce top bar effect; b) use of higher-strength concrete with the lowest possible slump and setting time; c) assessment of the condition of the strands prior to installation to insure excellent bond characteristics; d) gradual release of prestress, with an optimal release sequence; and e) use of adequate vibration to ensure consolidation. The strand end slip measured at five prestressing plants in the Southeast is considerably higher than the allowable end slip and is expected to affect the pile performance. If the strand slip theory is adopted, the strand development length increases substantially due to the excessive strand end slip. A top bar effect factor similar to the one used in reinforced concrete design is recommended. To maintain the excellent quality of precast and prestressed concrete products, manufacturers should adopt a dynamic quality control process that follows the rapid changes in the industry. More tests are necessary to ensure excellent quality, such as the Moustafa or an equivalent test, to assess the bond capabilities of the strands, end slip measurements, and direct measurement of the transfer length. Installation of piles should proceed in a manner to alleviate the top bar effects by placing piles alternately in their best and worst directions

Influence of Mortar Rheology on Aggregate Settlement

The influence of the rheology of fresh concrete on the settlement of aggregate is examined. Fresh concrete exhibits a yield stress that, under certain conditions, prevents the settlement of coarse aggregate, although its density is larger than that of the suspending mortar. Calculations, based on estimates of the yield stress obtained from slump tests, predict that aggregate normally used in concrete should not sink. To test this prediction, the settlement of a stone in fresh mortar is monitored. The stone does not sink in the undisturbed mortar (which has a high yield stress), but sinks when the mortar is vibrated, presumably due to a large reduction in its yield stress. This implies that during placement of concrete, the aggregate settles only while the concrete is being vibrated. A unique experimental method for measuring aggregate settlement is also introduced and demonstrated

Assessing the quality of concrete – reinforcement interface in Self Compacting Concrete

© 2019 Elsevier Ltd. This manuscript is made available under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International licence (CC BY-NC-ND 4.0). For further details please see: https://creativecommons.org/licenses/by-nc-nd/4.0/Research has shown that even self-compacting concrete (SCC) mixtures can exhibit the so-called “top-bar effect” which impacts bond and anchorage. Several instances of conflicting results have nevertheless been published regarding interfacial bond between self-compacting concrete and steel reinforcement. The scope of this paper is to present an experimental methodology for assessing the quality of the interface between self-compacting concrete and ribbed reinforcement. For this purpose, seven different self-compacting and four normally vibrated concrete (NVC) mixtures with diverse rheological characteristics were examined. Digital Image Analysis of cut sections containing reinforcing bars at different cast-heights was used as a diagnostic tool. The study illustrates that the quality of the interface is strongly affected by the viscosity of the SCC mixtures and by the slump values in NVC. Self-compacting concrete mixtures show greater inherent robustness and cohesion at the steel–concrete interface compared to conventionally vibrated concretes.Peer reviewe

The influence of light and tidal exposure on primary production in the tropical seagrass Zostera capricorni and Halophila ovalis

The growth, survival and depth penetration of seagrass is directly related to light availability, which drives photosynthesis. The amount of light reaching seagrass beds is highly variable and can be easily disrupted by human activities, such as dredging. Dredging results in increased turbidity and decreased light penetration to the seagrass beds, invariably influencing overall productivity and seagrass health. To better understand seagrass light requirements and resilience to environmental stressors such as dredging requires knowledge on seagrass photophysiology and the impact air exposure during a tidal cycle has on photosynthesis. Oxygen, fluorescence and bio-optical properties were measured over a tidal cycle in seagrass beds of Zostera capricorni and Halophila ovalis in Gladstone Harbour to provide insight into the variability in carbon production in intertidal seagrass meadows. Both species showed an increase in photosynthetic activity with increased irradiance as the tide receded. However, sensitivity to desiccation was observed during air-exposure with a significant decline in photosynthesis irrespective of increased light availability. Understanding the complex dynamics of seagrass photosynthesis over a tidal cycle will help in the mitigation of dredging-related light loss to Gladstone seagrass meadows

Multivariate generalised linear mixed-effects models for analysis of clinical trial-based cost-effectiveness data

Economic evaluations conducted alongside randomized controlled trials are a popular vehicle for generating high-quality evidence on the incremental cost-effectiveness of competing health care interventions. Typically, in these studies, resource use (and by extension, economic costs) and clinical (or preference-based health) outcomes data are collected prospectively for trial participants to estimate the joint distribution of incremental costs and incremental benefits associated with the intervention. In this article, we extend the generalized linear mixed-model framework to enable simultaneous modeling of multiple outcomes of mixed data types, such as those typically encountered in trial-based economic evaluations, taking into account correlation of outcomes due to repeated measurements on the same individual and other clustering effects. We provide new wrapper functions to estimate the models in Stata and R by maximum and restricted maximum quasi-likelihood and compare the performance of the new routines with alternative implementations across a range of statistical programming packages. Empirical applications using observed and simulated data from clinical trials suggest that the new methods produce broadly similar results as compared with Stata’s merlin and gsem commands and a Bayesian implementation in WinBUGS. We highlight that, although these empirical applications primarily focus on trial-based economic evaluations, the new methods presented can be generalized to other health economic investigations characterized by multivariate hierarchical data structures

Consolidated health economic evaluation reporting standards (CHEERS) statement

<p>Economic evaluations of health interventions pose a particular challenge for reporting. There is also a need to consolidate and update existing guidelines and promote their use in a user friendly manner. The Consolidated Health Economic Evaluation Reporting Standards (CHEERS) statement is an attempt to consolidate and update previous health economic evaluation guidelines efforts into one current, useful reporting guidance. The primary audiences for the CHEERS statement are researchers reporting economic evaluations and the editors and peer reviewers assessing them for publication.</p> <p>The need for new reporting guidance was identified by a survey of medical editors. A list of possible items based on a systematic review was created. A two round, modified Delphi panel consisting of representatives from academia, clinical practice, industry, government, and the editorial community was conducted. Out of 44 candidate items, 24 items and accompanying recommendations were developed. The recommendations are contained in a user friendly, 24 item checklist. A copy of the statement, accompanying checklist, and this report can be found on the ISPOR Health Economic Evaluations Publication Guidelines Task Force website (www.ispor.org/TaskForces/EconomicPubGuidelines.asp).</p> <p>We hope CHEERS will lead to better reporting, and ultimately, better health decisions. To facilitate dissemination and uptake, the CHEERS statement is being co-published across 10 health economics and medical journals. We encourage other journals and groups, to endorse CHEERS. The author team plans to review the checklist for an update in five years.</p&gt