International Concrete Abstracts Portal

This is the second in the series of Q&A articles attempting to clarify some common questions and misconceptions about glass fiber-reinforced polymer (GFRP) reinforcing bars. Discussed topics include traceability, hybrid reinforcement, sustainability, elevated temperature and fire, seismic provisions, limitations, and splices and headed bars.

Developments in the prestressed concrete industry evolved to incorporate innovative design materials and strategies driven towards a more sustainable and durable infrastructure. With steel corrosion being the biggest durability issue for concrete bridges, FRP tendons have been gaining acceptance in modern prestressed technologies, as bonded or unbonded reinforcement, or as part of a “hybrid” system that combines unbonded CFRP tendons and bonded steel strands. Assessments of the efficacy of hybrid-steel beams, combining bonded and unbonded steel tendons. in the construction of segmental bridges and in retrofitting damaged members has been established by several researchers. However, limited research has been conducted on comparable hybrid prestressed beams combining CFRP and steel tendons (hybrid steel-cfrp beams). This paper provides an insight on the flexural behaviour of eighteen prestressed beams tested under third-point loading until failure with the emphasis on the tendon materials (i.e., CFRP and steel) and bonding condition (i.e., bonded, unbonded). In addition, a comprehensive finite element analysis of the beams’ overall behaviour following the trussed-beam methodology is conducted and compared with the experimental results. Results show that hybrid beams, utilizing CFRP as the unbonded element maintained comparable performance when compared to hybrid steel beams. The results presented in this paper aim to expand the use of hybrid tendons and facilitate their incorporation into standard design provisions and guidelines.

The current market of utility poles is growing rapidly. The dominant materials that are used for this purpose are generally wood, steel, concrete, and fiber-reinforced polymers (FRP). FRP poles are gaining wide acceptance for what they provide in terms of strength and durability, lack of maintenance and a high strength to weight ratio. Hybrid structures can combine the best properties of the materials used, where each part enhances the structure to provide a balanced structure. This study evaluates a hybrid structure composed of three main layers, an outer FRP shell, a hollow concrete core and an inner hollow steel tube, this whole system is to be utilized as a tapered utility pole. The outer FRP shell provides protection and enhances the strength of the pole, the concrete core provides stiffness, and the inner steel tube enhances the flexural performance while reducing the volume in consequence the weight of the structure compared to a fully filled pole. A new design for a 12-feet long hybrid FRP pole using finite element is presented in this paper. The design was based on a parametric study evaluating the effect of key-design parameters (i.e., the thickness of FRP, the volume and strength of the concrete, the thickness and diameter of the steel tube). Concrete strength affected the general performance of the pole, the decrease in concrete strength due to utilizing lightweight concrete was compensated with increasing the FRP pole thickness. For the same pole configuration, with incremental variation of the FRP thickness values from 3 mm to 7 mm up to the initial concrete cracking load, no significant variation of the pole top deflection was observed. However, at failure load the increase of FRP thickness from 3 mm to 7 mm decreased the ultimate tip deflection by 50%. New hybrid utility poles have the potential to be an interesting alternative solution to the conventional poles as they can provide better durability and mechanical performances.

In this study, the cyclic axial compressive behavior of newly proposed concrete-filled fiber-reinforced polymer (FRP) tubes (CFFT) was examined. Different types of FRP, including small rupture strain FRP (SRSFRP) and recyclable large rupture strain FRP (LRS-FRP), were used to reinforce the concrete columns. LRS-FRP is made of polyethylene naphthalate (PEN) and polyethylene terephthalate (PET) and has a high tensile rupture strain (usually greater than 5%), while SRS-FRP has a lower tensile rupture strain of 1-2%. A total of twelve CFFTs were tested to assess the strength, confinement ratio, ductility, ultimate strain, and energy dissipation of the columns. The results showed that the use of LRS-FRP led to improved ductility and ultimate strength in the confined concrete columns compared to the use of SRS-FRP. The study also compared the experimental results to existing analytical models available in the literature and proposed two new models for predicting the ultimate stress-strain behavior of hybrid LRS-FRP.

Supplementary cementitious materials (SCMs) can be effective levers to reduce the environmental impact of concrete. One of the major limitations for high substitution of clinker by SCMs in Portland cement is the loss of strength of the resulting concrete, particularly at early age, compared with similar concrete made with equivalent dosage of pure clinker Portland cement. Furthermore, the possibility to use larger fractions of SCMs requires strongly alkaline activators which pose environmental and safety problems and are not compatible with commercial PCE superplasticizers. A new hybrid additive was synthesized which works, at the same time, as seed for the clinker phase and as an activator for alkali-activated binders. The new product consists of an alkaline suspension of micro-sized particles containing nano-structured CSH seeds, amorphous portlandite, and AFm phases embedded in a polymeric matrix. The structure and the mechanism of action of the new product on slag cements have been investigated by Environmental Scanning Electron Microscopy, X-ray Powder Diffraction, Thermal Analysis, and Isothermal Calorimetry. The new product is fully compatible with PCE superplasticizers and can be used to produce alkali-activated concretes characterized by low-carbon impact.