Bond–slip Behavior of Fiber-reinforced Polymer/concrete Interface in Single Shear Pull-out and Beam Tests

It has been assumed that the fiber-reinforced polymer/concrete interface is subjected to in-plane shear condition when intermediate crack debonding failure occurs. Therefore, the single shear pull-out test results are often used to predict the intermediate crack debonding failure in beams. In this study, the behavior of fiber-reinforced polymer-strengthened concrete beams and single shear pull-out specimens were studied experimentally and numerically. The bond–slip behavior of the fiber-reinforced polymer/concrete interface was obtained by single shear pull-out and beam tests. In all beam specimens, a concrete wedge located at the edge of the notch detached with the fiber-reinforced polymer debonding failure. This phenomenon shows that the initiation of debonding is due to a diagonal crack formation close to the major flexural/shear crack inside the concrete. The diagonal crack formation is due to a local moment at the tip of the notch. This causes the different stress state and slip of the fiber-reinforced polymer/concrete interface of beam specimens from that of the pull-out specimens. It is found that the bond–slip relation obtained from the pull-out test does not represent the bond–slip relation of the fiber-reinforced polymer/concrete interface in the fiber-reinforced polymer-strengthened concrete beams, and it cannot be directly used for predicting the load capacity of the fiber-reinforced polymer-strengthened concrete beams

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

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

Finite element guidelines for simulation of fibre-tension dominated failures in composite materials validated by case studies

This paper presents a finite element modelling methodology to predict the initiation and damage progression in notched composite laminated plates subjected to increasing in-plane tension load. An important feature of the methodology is it does not rely on customized user-subroutines but solely on the analysis capabilities of the general purpose software Abaqus; thus ensuring that the numerical results can be universally reproduced. The methodology presented copes with intralaminar failure modes and uses the Hashin failure criterion to predict the onset of failure (cracking). To account for damage progression after crack initiation there is a fracture energy calculated for each of four failure modes. Four open-hole laminated plates taken from the literature are used for benchmark examples. The predicted ultimate strength based on the analytically-obtained stress-displacement curve was found to be within 10% of the experimental observations. To study the influence of the interaction of having two or three holes across the mid-plane of a pultruded open-hole tension specimen, a parametric study was carried out. The paper ends giving guidelines for the generalized modelling methodology using Abaqus without user-subroutines

Bolted connections of pultruded GFRP : implications of geometric characteristics on net section failure

The results of a three-dimensional finite element study of the net section dominated failure behaviour of pultruded open-hole specimens are presented. Computer models are developed using the general-purpose software Abaqus. Several issues are addressed in the study with respect to the notched plate geometry: (i) thickness of plate, (ii) transverse centre-to-centre spacing of holes (gauge), and (iii) distance from the centre of the hole to the nearest edge. The analytical results provide information on basic performance and the effects of these parameters on strength and damage tolerance performance, thereby furthering the current understanding of pultruded plate-to-plate connection behaviour under static loading. Based on the results, design recommendations for minimum edge distance and gauge spacing for bolts are given

Geometry, material properties and bond performance of prototype titanium reinforcing bars

The use of titanium as a concrete reinforcing bar material has been proposed. This study summarizes measured geometric and experimentally determined material properties of 6Al-4V titanium reinforcing bars and comparable properties of ASTM A615 steel. All bars are nominally #5 bars. Bond characteristics of the titanium bars were assessed through ASTM D7913 pull-out tests, ASTM A944 beam-end tests and four concrete prism tension tests. The nature of reinforcing bar bond to concrete is such that deformed bars exhibit very similar patterns of bond stress-slip behaviour. Provided adequate deformations are provided, the bond-slip relationship is dominated by concrete behaviour. The bond performance of the 6Al-4V titanium bars was similar to that of A615 steel bars and, as expected, clearly affected by the rib ratio. The results presented reinforce the ASTM A615-implied lower limit for the rib ratio, Rr &gt; 0.05. The implication of a similar bond-stress behaviour is that existing bond relationships for steel-reinforced concrete likely apply to titanium bars provided they meet the deformation requirements of ASTM A615 – the standard for which steel reinforcing bars, and therefore their bond characterisation – is calibrated. Both the pull-out and beam-end test results reinforce the conclusion that bond behaviour of titanium bars is essentially the same as that steel bars. The bond stresses, normalised to account for variation in concrete strength, are similar and the calculated development lengths are essentially in the ratio of yield strengths of the materials. The prism tension tests demonstrated that concrete crack width is proportional to modular ratio of the reinforcing material, while spacing is inversely proportional to the stiffness of the initial bond-slip response.</p

Flange local buckling of pultruded GFRP box beams

An experimental program investigating the flange local buckling (FLB) behavior of pGFRP box-sections is reported. The commonly accepted design equation based on plate theory was validated although importance of accurate assessment of the rotational stiffness of the web-flange junctions was identified. It is concluded that the lower bound solution, assuming the flange is a simply-supported plate subject to uniform compressive stress, results in uniformly conservative predictions of the critical FLB moments. The theoretical solution accounting for flange plate edge support stiffness based only on web stiffness, material and geometric properties of the cross section over predicts the support stiffness resulting in unconservative predictions of FLB behavior. The rotational stiffness of flange-web junction of the pGFRP box-section is also investigated experimentally. It is found that the actual rotational stiffness of flange-web junction is relatively low, closer to the simply-supported boundary condition. The role of fiber architecture at the web-flange junction is identified as affecting this behavior. The conclusions of this study support the use of the lower bound solution for design of pGFRP box-sections.</p