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Automatization and additive manufacturing provide a platform to move away from traditional monolithic concrete forms towards more material efficient functionally-graded or structural composite elements. This has a great potential to tailor the element’s mechanical or thermal performance which allows to efficiently use material and furthermore safe costs and reduce the environmental footprint of an element. However, the deposition of different wet-on-wet and weton- hardened concrete layers introduces the risk of a weak interlayer zone (ILZ). Furthermore, functionally layered concrete made of multi-mixes introduces a new dimension whereby the mix compositions and the deposition delay times potentially interact to influence the interlayer behaviour. To provide a better insight on the interlayer zone behaviour of functionally graded concrete, a detailed investigation of the interlayer fracture properties of multi-mix concrete elements cast in layers and the effect of the pour delay between layers is presented. Wedge-splitting-tests are used to infer the fracture behaviour of layered specimens fabricated using three different concrete mixes in single and multi-mix combinations. The influence of a pour delay between layers of up to 4.0 h is also interrogated. The results show that the multi-mix fracture behaviour depends on not only the mix compositions but also on the pour delay time. Cases where the fracture properties were at least as good as, or even better, than those of the comparator weaker single mix suggest that judicious mix designs and controlled fabrication processes for functionally layered concrete can mitigate the formation of weak ILZs.

The bond between Carbon Fiber Reinforced Polymer (CFRP) tendons and concrete depends on the resin mechanical properties that can degrade under environmental conditions. An analytical model to predict the combined effects of temperature and humidity fluctuations on the bond behaviour of CFRP tendons embedded in concrete is presented. The interaction between a CFRP tendon and concrete is studied by considering different competing mechanisms such as tendon swelling and plasticisation effects due to the concrete internal humidity, differential thermal expansion, concrete autogeneous shrinkage, Poisson’s ratio effects and radial bond stresses generated during the pulling out of a tendon from concrete. The analytical bond failure predictions correlate reasonably well with experimentally observed radial-cracking failure modes of CFRP tendons in high strength concrete. Matrix plasticisation appears to influence the long-term bond strength of FRP rods in normal or high strength concrete. Swelling and shrinkage effects become significant in uncoated tendons due to their impact on the frictional bond component. Deviations between measured and predicted bond strength values are attributed to a possible overestimate of the moisture-induced swelling expansion of CFRP tendons. The research findings suggest the introduction of environmental bond factors to consider durability effects in the design of FRP prestressed concrete structures.

Due to the relatively low tensile strength of concrete, cracks are inherent in reinforced concrete structures. A common cause of cracking is the corrosion of internal steel reinforcement, a deterioration process that can affect the bond behaviour and anchorage capacity of reinforcing bars. Corrosion leads to a reduction of the reinforcing bar diameter, the formation of a weak layer of corrosion products around the bar and expansive forces on the surrounding concrete (that can lead to cracking). In the past, the impact of corrosion on bond has been investigated by means of accelerated corrosion tests. However, the more fundamental impact of cracking as distinct from corrosion products on the bond reduction is still not fully understood.This study applies a novel test method to investigate the bond behaviour of reinforcing bars in cylindrical cracked reinforced concrete specimens. The influence of the number of cracks, crack orientation, confinement and concrete cover are investigated.The results indicate a significant loss of bond strength for single cracked specimens. This reduction becomes as high as 65% for double cracked specimens in the absence of confinement. It is shown that the crack orientation with respect to the rib pattern is of minor influence, but the concrete cover and confinement play a significant role in the obtained bond characteristics.

The corrosion resistance of glass-fibre reinforced polymers (GFRPs) is a key advantage in pipeline applications. Adhesive bonded joints are often used in tensile resistant GFRP pipe systems and the behaviour of small-diameter GFRP tubular pipe-sections joined by using an adhesive bonded coupler is of interest. In particular, the performance of the joint when subjected to a combination of high tensile axial loads and relatively low internal pressure is investigated. Experiments are conducted where a self-equilibrating pressurisation system is used to control the internal pressure and the coupled sections are tested in tension until failure. The cases of no pressure and a 32 bar working pressure are considered. A preliminary one-dimensional linear-elastic model is developed to predict the behaviour in the early stages of loading and to quantify the influence of internal pressure on the joint performance. Based on the experimental and analytical findings, it is concluded that the most critical case for the joint is likely to be the presence of large tensile loads in absence of internal pressure.

In fibre reinforced polymer (FRP) prestressed concrete applications, an FRP tendon must sustain high axial tensile stresses and, if cracks occur, additional dowel forces. The tendon may also be exposed to solutions and so the combined axial-shear stress performance after long-term environmental exposure is important. Experiments were conducted to investigate the combined axial-shear stress failure envelope for unidirectional carbon FRP tendons which had been exposed to either water, salt water or concrete pore solution at 60 °C for approximately 18 months. The underlying load resisting mechanisms were found to depend on the loading configuration, restraint effects and the initial stress state. When saturated, CFRP tendons are likely to exhibit a reduced shear stiffness. However, the ultimate limit state appeared to be fibre-dominated and was therefore less susceptible to reductions due to solution uptake effects.

Fibre reinforced plastics describes a group of materials composed of inorganic or organic fibres embedded in a resin matrix. frps are strong, non-magnetic, light-weight and for the most part, non-corrodable. There is great scope for the use of frps as concrete reinforcement and the high strength of the materials is conducive to prestressed applications. However, finding a suitable method of anchoring the tendons without inducing stress concentrations in the fibres has been identified as a problem. The current paper investigates the potential for the use of expansive cement couplers as a means of pretensioning frp tendons. An experimental study was carried out on couplers to join steel reinforcing bars and then extended to include the coupling of frp materials to steel prestress wire.