This study aims to investigate the impact of moderate and elevated temperatures on compressive strength, mass loss, ultrasonic pulse velocity (UPV), and gas permeability of mortars made using metakaolin (MK) or Ordinary Portland cement (OPC). The geopolymer mortar comprises MK, activated by a solution of sodium hydroxide (SH) and sodium silicate (SS) with a weight ratio of SS/SH equal to 2.5. For most of the tests, the MK and OPC mortar specimens were cured for 7 and 28 days before exposure to elevated temperatures, ranging from 100 °C to 900 °C in increments of 100 °C. In the permeability tests, conducted at temperatures ranging from 100 °C to 300 °C in 50 °C increments, the results revealed significant findings. When exposed to 200 °C, MK geopolymer mortar demonstrated an increase in compressive strength by 83% and 37% for specimens initially cured for 7 and 28 days, respectively. A strong polynomial correlation between UPV and compressive strength in MK mortar was observed. Prior to heat exposure, the permeability of MK mortar was found to be four times lower than that of OPC mortar, and this difference persisted even after exposure to 250 °C. However, at 300 °C, the intrinsic permeability of MK mortar was measured at 0.96 mD, while OPC mortar exhibited 0.44 mD.

The steel-concrete interface (SCI) is extensively acknowledged to affect the durability of reinforced concrete. The main objective of this paper is to conduct a state-of-the-art review that contributes to sufficient knowledge on the determination of the SCI properties and its effect on the overall performance of reinforced concrete elements. The physical characteristics at the SCI are influenced by segregation, flow, hydration, and drying shrinkage of concrete, hence affecting the presence of voids and cracks within this interface. The bond strength is one of the measures of the SCI and this is conducted through pull-out, push-in, and tie-beam testing. It was shown that the rebar shape and diameter, the anchorage length, the concrete grade strength, binder type (geopolymer concrete), and the distribution of aggregates have a significant effect on the interface properties and behavior, where geopolymer concrete offered improved bond behavior over conventional concrete. Various studies have demonstrated that the presence of the steel-concrete interface and the application of mechanical stresses contribute to the flow transfer (inflow/outflow) through the reinforced concrete structure. Some of these studies focused on the initial state of the SCI within the structure, and some conducted tests with shear loading on the SCI. Regarding the transfer properties at the SCI, it was shown that the presence of steel rebar, crack dimensions, degree of saturation of concrete, and the concrete mix design, influence the permeability of the concrete, specifically at the vicinity of the SCI, because of the development of micro-cracks at the interface. In other studies, the shear stresses were also found to affect the transfer properties through the SCI. Researchers have implemented several software solutions such as finite element models on ABAQUS and mesoscale numerical simulations and have used machine learning models that predict and verify the effects of bond failure behavior at the SCI. Good agreement was established between the numerical and actual experimental results. The influence of different exposure conditions on the steel-concrete interface that change throughout time needs to be dealt with, which includes moisture-related environmental conditions, variation in temperature, and chemical exposure. Furthermore, the influence of structural loading, such as “creep effect”, deterioration (ageing) of material must be studied at the interface. The studies were limited to short-term behavior.

Summary: This contribution aims at improving the finite element modelling of concrete's damage‐permeability coupling using continuous and elastic‐based damage approaches. With this aim in view, a review of existing local and nonlocal damage formulations is performed showing how they differ in terms of damage estimation, crack propagation, and spatial discretization restrictions (when the energy‐based Hillerborg criterion is applied). Another critical review of damage‐based permeability laws is achieved to pinpoint their limitations. Indeed, the use of a bounded and strictly increasing damage variable does not allow (1) an accurate description of permeability in the presence of macrocracks that are physically unbounded and (2) a proper description of the permeability decrease as those macrocracks are closed. Accordingly, a new strain‐based permeability law is suggested. Its analytical validation is based on the BIPEDE tensile test for one loading‐unloading cycle. This new formulation intends to enhance the predictiveness of damage‐permeability coupling models leading to a better assessment of large structure's durability and serviceability.

This paper assesses the feasibility of metakaolin (MK)-based geopolymer (GP) composite as an environmentally friendly substitute for cement-based composite in repair applications. The Taguchi orthogonal array method was used to find the optimum GP mix in terms of mechanical properties and adhesion to concrete substrates. Four key parameters, each with three levels, are investigated including the alkaline activator-to-MK ratio (A/M: 1, 1.2, 1.4), the sodium silicate-to-sodium hydroxide ratio (S/H: 2.0, 2.5, 3.0), sodium hydroxide (SH) molarity (12, 14, 16), and curing temperature (30, 45, 60 °C). The evaluated properties include flowability, compressive strength, splitting tensile strength, flexural strength, ultrasonic pulse velocity, and bond strength under various interface configurations. Experimental results demonstrated that the performance of MK-based GP composite was primarily governed by the A/M ratio and sodium hydroxide molarity. The Taguchi optimization method revealed that the mix design featuring A/M of 1.4, SS/SH of 2, 16 M sodium hydroxide, and curing at 60 °C yielded notable improvements in compressive and bond strengths compared to conventional cement-based composites.

Purpose: This paper aims to investigate the bond strength of metakaolin-based geopolymer mortar with cement mortar.Design/methodology/approach: The mortar-mortar bond strength is assessed by slant shear and split tensile tests; pure shear strength is evaluated by Mohr’s criterion for result validation. Metakaolin-based geopolymer mortar is cast over the cured cement mortar specimen with two levels of surface roughness: smooth or grooved interface. The influence of the alkaline solution to metakaolin ratio on geopolymer bond strength is studied. Compressive strength, ultrasonic pulse velocity, permeability and flow table tests are also performed.Findings: The paper’s findings are highlighted as follows: (1) strong mortar-mortar bond properties achieved for geopolymer mortar in all tests and conditions and validated by Mohr’s criterion and pure shear, (2) a lower alkaline solution to metakaolin ratio achieves higher bond strength to Portland cement mortar and (3) geopolymer mortar has higher compressive strength and ultrasonic pulse velocity than cement mortar at all curing ages; additionally, it is more flowable and less permeable.Practical implications: The full replacement of Portland cement with metakaolin, a more sustainable cementitious material, will contribute to the decarbonization of the construction industry.Originality/value: Limited research has been carried out on the bond strength of metakaolin-based geopolymer mortar to Portland cement mortar. Also, computing the pure shear using Mohr’s circle criterion of metakaolin-based geopolymer to validate the results can be considered original.