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Nano-Clay (NC) is reported as a candidate partial replacement for cement, due to its abundance and relatively low cost - beside reported promotion of different concrete properties. On the other hand, Steel Fibres (SF) has proven to have a positive effect on post fire exposure residual strength of concrete. This paper aims to present the outcomes of a comprehensive research program assessing a hybrid mix between NC and SF in concrete mixtures (NCSF-CRETE).

Physical chemical and physical characterization of NC is performed using different tools as XRF spectrometer, and TEM micrograph. Fresh concrete properties of NSCF-CRETE as slump and air content are investigated. Enhancement in permeability using NSCF is verified by comparing its resistance to the penetration of chlorides resistance with regular concrete mix. Besides, the proposed NCSF-CRETE compressive strength is evaluated compared to mixes with NC and SF each used separately at different curing ages. Besides, NSCF and compared mixes are exposed to an indirect fire testing program – two hours exposure – for: 300, 450 and 600°C. Degradation in compressive strength was investigated after exposure to different temperatures and percentage of residual strength is reported.

Results indicated an improved performance of NCSF -CRETE of about 40% compared to regular concrete in compressive strength at normal conditions. This improvement extended to its behavior when subjected to indirect fire exposure NSCF also maintained 40% more strength than the residual in regular concrete mix – which suffered severe damage – after 2 h exposure to 600°C.

Using NCSF-Crete allows retrofitting the structure after exposure to such drastic conditions.

This paper presents the second part of the investigation on resistance to elevated temperatures of a proposed hybrid composite concrete (NCSF-Crete) mix. The composite including nano metakaolin (NC) and steel fibers (SF) in addition to regular concrete components has proven -in the first published part-earlier promoted fresh concrete behavior, and to have reduced loss in compressive strength after exposure to a wide range of elevated temperatures. This presented work evaluates another two critical mechanical characteristics for the proposed composite -namely- splitting and bond strengths.

A modified formula correlating splitting and compressive strength (28 days) based on experiments results for NCSF is proposed and compared to formulas derived for regular concrete in different design codes. Finally, both spitting and bond strengths are evaluated pre- and post-exposure to elevated temperatures reaching 600 °C for two hours.

The proposed NCSF-Crete shows remarkable fire endurance, especially in promoting bond strength as after 600 °C heat exposure tests, it maintained strength equivalent to 70% of a regular concrete control mix at room temperature. Improving residual splitting strength was very significant up to 450 °C exposure.

Obvious deterioration is monitored in splitting resistance for all concretes at 600 °C.

This proposed composite improved elevated heats resistance of the most significant concrete mechanical properties.

Using a more green and sustainable constituents in the composite.

The proposed composite gathers the merits of using NC and SF, each has been investigated separately as an addition to concrete mixes.

Even though it is widely used, reinforced concrete (RC) is susceptible to damage from various environmental factors. The hazard of a fire attack is particularly severe because it may cause the whole structure to collapse. Furthermore, repairing and strengthening existing structures with high-performance concrete (HPC) has become essential from both technical and financial points of view. In particular, studying the postfire behavior of HPC with normal strength concrete substrate requires experimental and numerical investigations. Accordingly, this study aims to numerically investigate the post-fire behavior of reinforced composite RC slabs.

Consequently, in this study, a numerical analysis was carried out to ascertain the flexural behavior of simply supported RC slabs strengthened with HPC and exposed to a particularly high temperature of 600°C for 2 h. This behavior was investigated and analyzed in the presence of a number of parameters, such as HPC types (fiber-reinforced, 0.5% steel, polypropylene fibers [PPF], hybrid fibers), strengthening side (tension or compression), strengthening layer thickness, slab thickness, boundary conditions, reinforcement ratio and yield strength of reinforcement.

The results showed that traction-separation and full-bond models can achieve accuracy compared with experimental results. Also, the fiber type significantly affects the postfire performance of RC slab strengthened with HPC, where the inclusion of hybrid fiber recorded the highest ultimate load. While adding PPF to HPC showed a rapid decrease in the load-deflection curve after reaching the ultimate load.

The proposed model accurately predicted the thermomechanical behavior of RC slabs strengthened with HPC after being exposed to the fire regarding load-deflection response, crack pattern and failure mode. Moreover, the considered independent parametric variables significantly affect the composite slabs’ behavior.

This study aims to investigate and compare the influence of various fiber types (polypropylene, steel and glass) on the workability, mechanical properties, ductility, impact resistance, durability and microscopic properties of geopolymer concrete (GPC) with conventional concrete (CC).

The CC and GPC of M40 grade were incorporated with an optimum 1% of fibers and superplasticizers were added in a ratio of 2% by weight of the geopolymer binder. The slump cone and compaction factor tests were performed to analyze the workability. To evaluate the mechanical performance of GPC, the compressive strength (CS), split tensile strength (STS), flexural strength (FS) and modulus of elasticity (MOE) tests were performed. A falling weight impact test was performed to determine the impact energy (IE) absorbed, the number of blows for initial cracking, the number of blows for complete failure and the ductility aspect.

Fibers and superplasticizers significantly improve GPC properties. The study found that fibers reduce the brittleness of concrete, improving the impact and mechanical strength compared to similar-grade CC. The steel fibers-reinforced GPC has a 15.42% higher CS than CC after three days, showing a faster CS gain. After 28 days, GPC and CC have MOE in the range of 23.9–25.5 GPa and 28.8–30.9 GPa, respectively. The ultimate IE of the GPC with fibers was found to be 5.43% to 21.17% higher than GPC without fibers.

The findings of the study can be used to explore different combinations of raw materials and mix designs to optimize the performance of GPC.