Response of RC columns with transient creep in a natural fire environment

The aim of this study is to investigate the effect of transient creep on the structural response of RC columns subjected to natural or parametric fires using a finite element model developed by the authors. The model, capable of analysing the response of RC columns from pre-fire stages to collapse in fire environment, is specially developed for the analysis of RC structures under severe thermo-mechanical loads, which accounts for transient creep explicitly as an additional component of the total strain of the concrete or implicitly through the use of the materials’ properties recommended by EC2. Through the obtained results, it is shown that the transient creep phenomenon significantly influences the fire response of RC columns. It was also found that the conventional method based on standard fire exposure may not be conservative if the resulting fire has a decay phase similar to the severe fire scenario used in this work

Modeling and influence of shear retention parameter on the response of reinforced concrete structural elements

To obtain the complete solutions describing the balance of a reinforced concrete structure, it is necessary to introduce a behavioral law characterizing the physical properties of material. The goal of this work is to study the response of reinforced concrete elements by taking into account the variation of the shear retention parameter (aggregate interlock) and the mesh density. The concrete was assumed as elastic-plastic material and follow Drucker-Prager failure criterion with associated flow rule, the steel reinforcements were assumed to be elastic-perfectly plastic. The numerical results obtained are compared with other results available in the literature.Для отримання повних розв’язків із метою опису рівноваги залізобетонної конструкції необхідно використати рівняння, що характеризують фізичні властивості матеріалів. Вивчалася поведінка залізобетонних елементів з урахуванням зміни ретенційного параметра зсуву (множинного блокування) і щільності сітки. Постулювалось, що бетон являє собою пружно-пластичний матеріал, для якого правдиві критерій руйнування Друкера-Прагера та закон асоційованої течії, в той час як стальні елементи арматури припускалися пружно-ідеально-пластичними. Отримані результати числових розрахунків порівнювалися з наведеними у літературних джерелах.Для получения полных решений с целью описания равновесия железобетонной конструкции необходимо использовать уравнения, характеризующие физические свойства материала. Изучено поведение железобетонных элементов с учетом изменения ретенционного параметра сдвига (множественной блокировки) и плотности сетки. Постулировалось, что бетон является упругопластическим материалом, для которого справедливы критерий разрушения Друкера-Прагера и закон ассоциированного течения, тогда как стальные элементы арматуры предполагались упруго-идеально-пластичными. Полученные результаты численных расчетов сравнивались с приведенными в литературных источниках

Uniaxial model for concrete under variable temperature and stress

A plasticity model using a strain-rate formulation is presented to describe the uniaxial response of concrete when subjected to combined thermal and mechanical actions. The total strain rate is resolved into three individual components: mechanical strain, thermal strain, and transient creep strain. Each component is formulated individually. The mechanical strain rate is assumed to consist of an elastic strain rate and a plastic strain rate, which itself is taken as temperaturedependent, In describing the plastic strain rate, the nonlinear part of the uniaxial stress-strain curves is assumed to be a quarter of ellipse, defined for a given temperature in the range 20°C-176°C The model was first used to analyze uniaxial (experimental) data under variable load but isothermal conditions, and then to investigate the effect of a sustained load on the deformational response of a concrete specimen under heating. Thirdly, a set of experimental relaxation lesls was studied, in which temperature and stress vary continuously under a zero total strain condition. Here, the importance of a variable temperature and stress, and the significance of the history, is demonstrated

Plasticity models for the biaxial behaviour of concrete at elevated temperatures, Part I: Failure criterion

The mechanical response of concrete at elevated temperatures under a generalized state of stress is extremely nonlinear, with both the plastic behaviour and mechanical properties highly temperature dependent. In this and a companion article the plastic response is modelled as possible. Initially, a general analytical form of the failure surface is developed from the available biaxial test data. The envelope is written in terms of stress invariants as a function of temperature, and shows good agreement with experiment. For a given temperature, the loading surface is found by scaling the failure envelope using a hardening parameter. The model is used in the sequel to develop incremental elastic-plastic relationships, and to study deformations in a range of biaxial tests

Response of RC columns with transient creep in a natural fire environment

The aim of this study is to investigate the effect of transient creep on the structural response of RC columns subjected to natural or parametric fires using a finite element model developed by the authors. The model, capable of analysing the response of RC columns from pre-fire stages to collapse in fire environment, is specially developed for the analysis of RC structures under severe thermo-mechanical loads, which accounts for transient creep explicitly as an additional component of the total strain of the concrete or implicitly through the use of the materials’ properties recommended by EC2. Through the obtained results, it is shown that the transient creep phenomenon significantly influences the fire response of RC columns. It was also found that the conventional method based on standard fire exposure may not be conservative if the resulting fire has a decay phase similar to the severe fire scenario used in this work

Bond behaviour of GFRP reinforcement in alkali activated cement concrete

Bond plays a key role in the performance of reinforced concrete structures. Glass fiber reinforced polymer (GFRP) reinforcing bar and alkali activated cement (AAC) concrete are promising alternative construction materials for steel bars and Ordinary Portland Cement (OPC) concretes respectively. In this study, the bond behaviour between GFRP bars, and AAC and OPC concretes is investigated by using beam-end test specimens. Sand-coated GFRP bars with 12.7 mm and 15.9 mm diameters and embedment lengths of six and nine times the bar diameter were used. The free end and the loaded end bond slip relationships, the failure mode and the average bond stress were used to analyse each of the specimens. Additionally, the distribution of tensile and bond stress along the embedment length was investigated by installing strain gauges along the embedment length. The results of the study indicate that the tensile and bond stress distribution along the embedment length are nonlinear, and the nonlinearity changes with the load. Finite element analysis was also performed to further study the bond stress distribution along the embedment length of the bar. From the finite element analysis, it was found that the bond stress distribution depends on the embedment length of the specimens; approaching uniform distribution as the embedment length decreases

Advances in Cement and Concrete Technology in Africa. Proceedings 2nd International Conference Dar es Salaam, Tanzania January 27 -29 2016

The effects of high temperature on the strength and stress-strain relationship of alkali activated fly-ash concrete were investigated. Stress-strain curves were obtained at the temperatures of 20, 100, 200, 300, 400 and 800oC. At ambient temperature, an analytical model for predicting the stress-strain behaviour of such concretes is presented based on widely accepted models for OPC. High temperature testing only covered one type of temperature-load history; that is: the samples were heated until the test temperature is reached then loaded to failure under displacement control. All the samples tested between 20 to 200oC underwent a decrease in strength. However, samples tested between 200oC to 400oC manifested a moderate to significant gain in strength. Above 400oC all samples underwent a decrease in strength. The initial loss of strength may be attributed to the loss of water from the GPC samples. This hypothesis is supported by Thermogravimetric Analysis (DTG) of fly ash based geopolymer samples where excessive weight loss is associated with this temperature range. Between 200oC and 400oC, the increase in the compressive strength of all tested concrete mixtures is attributed to further geopolymerization occurring at these temperatures. This geopolymerization has been proven by Differential Scanning Calorimetry (DSC) results