Most of the previous studies on reinforced concrete columns with elastically restrained thermal elongation were carried out on square, rectangular or ‘+’-shaped cross sections. The number of fire resistance tests on circular reinforced concrete columns with elastically restrained thermal elongation is still very small. In order to examine the influence of several parameters on the behaviour in fire of this type of columns several fire resistance tests were carried out. The parameters tested were the load and restraint level, slenderness of the column and longitudinal reinforcement ratio. In the fire resistance tests the specimens were exposed to the ISO 834 standard fire curve and the critical time (fire resistance) and temperature and failure modes were determined. The test results showed that the spalling phenomenon may occur in circular columns and so reducing its fire resistance. The increasing of the load level led to a reduction while the increasing of the longitudinal reinforcement ratio or the decreasing of the slenderness of the columns led to an increasing of their fire resistance. The restraint level might not be much relevant on the fire resistance of circular reinforced concrete columns.
This paper describes the results of laboratory tests with samples of a new generation self-compacting fine-grained re-fractory concrete (self-flow castables – SFC refractory equivalent to self-compacting concrete – SCC) and tests of the properties of products made of the developed compositions. The tests were conducted with cube specimens 100x100x100 mm in size for developed SFC and prism specimens 40x40x160 mm in size for cement pastes. Laboratory tests were conducted to determine the residual strength after heating to 800℃ and the ability of specimens to withstand sharp changes in temperature from 800℃ to 20℃ in water. A new method for selecting the composition by using main structural-rheological criteria (see section 3.1) was applied: ratio in volume of water-concrete dispersive mixture (rheological matrix of the first kind) over the absolute volume of fine sand SFSCD = VCD / VFS and ratio in volume of dis-persive fine-grained mixture (rheological matrix of the second kind) over the absolute volume in sand-aggregate SSACDFS = (VCDFS+VFS / VSA. It is shown that one of the ways to reduce the negative impact of high temperatures on the physical-mechanical properties of refractory concrete that uses Portland cement is the addition of the three slags com-ponents, that have equal thermal-expansion coefficient, amorphous silica fume of 10-15% of the cement mass. The pro-ject was conducted from February 2012 to February 2013.
This study aims to propose a theoretical calculation formula to predict crack width of reinforced concrete members both for steel and FRP reinforcement. The proposed formula is led by solving the force equilibrium and compatibility conditions in the bond problem under the assumption of bond constitutive law (bond stress - slip relationship). A pullout bond test is conducted for the specimens with one reinforcing bar arranged in the simple concrete block. The bond stress - slip relationship of test result is used to confirm the adaptability of the proposed formula by predicting crack width of the specimen for a tensile bond test, which is also conducted in this study. The strain - crack width curve predicted by the proposed formula shows a good adaptability with the experimental result.
Micromechanical interface properties in a concrete repair system determine the performance and reliability of a repaired structure. In order to characterize these properties, nanoindentation technique was applied. Three different repair material mixtures, based on Portland cement or partial replacement of Portland cement with blast furnace slag, were tested. Hardness and elastic modulus values obtained from the nanoindentation were used directly as input for simulated direct tension test. This way, the fracture behaviour of original microstructure, “mimicked” by Delft lattice model, is analysed. Backscattered electron (BSE) image analysis is utilized to estimate the average size of the interface zone which is distinguished as locally more porous area. This, together with simulation results, is further used for calculation of the interface stiffness. Simulation results enable prediction and better insight into fracture propagation and micromechanical response of the tested zone. They also indicate the ratio between interface and bulk material fracture properties when different types of repair materials are used. Load displacement diagrams of interface, old and new material can serve as an input for numerical modelling and understanding of fracture behaviour of the repair systems at the higher scale. Potentially, this approach may develop as a tool for verifying and engineering interface properties such that desired performance of the repaired structure is achieved.
This paper reports on corrosion of reinforcing steel bars in strain hardening cement-based composites (SHCC) under chloride attack. As part of a continued research project on durability of SHCC, the main focus here is on flexurally induced cracks in reinforced SHCC (R/SHCC) specimens with different cover depths. After unloading from the flexural test, these specimens are subjected to chloride attack, while monitoring steel corrosion. The R/SHCC specimens comprised series manufactured with two different types of sand, reinforced with single and double tensile steel bars with three different cover depths, in order to relate the crack patterns, cover depth and rebar corrosion. Crack widths of below 50 m are found to allow chloride penetration to the rebar within hours. Corrosion potential and corrosion rate measurements, following a Coulostatic method, are reported. By removal of rebars from a number of specimens after more than a year of chloride exposure, corrosion damage is studied by visual observation, pitting depth measurements and tensile testing of the rebars. Chloride profiles are also determined through XRF in cracked and uncracked regions of the specimens. Relation of corrosion damage calculated from corrosion rate measurements, with observed corrosion damage is complicated by localised corrosion.