Papers by Keyword: Concrete

Abstract: Rapid freeze–thaw cycle experiments were carried out on concrete specimens with 0.4, 0.5, and 0.6 water–cement (w/c) ratio in 0% (tap water), 1%, and 5% Na₂SO₄ solutions, respectively, to study the performance of ordinary concrete resistance to sulfate freeze–thaw cycle. The specimens underwent visual inspection, and mass losses and relative dynamic elastic modulus (RDEM) were measured regularly. Scanning electron microscope observation and X-ray diffraction analysis were conducted on partial specimens after the freeze–thaw cycle experiment. Research results show that due to the coupling effects of freeze–thaw cycle and sulfate corrosion, freeze–thaw cycles of concrete in Na₂SO₄ solution caused more damages than in tap water. Higher Na₂SO₄ concentration produced severe damages. Concrete with different w/c ratios exhibit different sulfate freeze–thaw cycle resistances, and concrete with lower w/c ratio usually produces stronger resistance. RDEM loss is considered the control index to determine concrete failure. The corrosion products in Na₂SO₄ solution freeze–thaw cycle are mainly ettringite and gypsum. With the increase in Na₂SO₄ concentration, ettringite formation gradually decreases and gypsum formation gradually increases.

Abstract: The main cause of premature deterioration of reinforced concrete structures is the corrosion of steel bars, induced by chloride ions (for example in marine environment) and/or by carbonation (atmospheric CO₂). At the same time, environmental-induced degradations of concrete can also affect the structure, such as sulphate attack. This can lead to the formation of ettringite, inducing expansion inside the materials and finally degradation. Carbonation, chloride and sulphate ingress are usually studied separately in the literature. This is not representative of in-situ conditions since they can occur at the same time and can have an influence on each other. In this paper, chloride ingress are studied for concretes and cement pastes partially carbonated or/and in presence of sulphate in chloride contact solution. The mixtures contain OPC alone or with supplementary cementitious materials (SCM). SCMs are here pozzolanic materials (Fly Ash or Metakaolin) or alkali-activated materials such as ground granulated blast furnace slag (GGBS). The materials, partially carbonated (2 months in chamber at 1.5 % of CO₂) or not, are put in contact with chloride solutions in presence of sulphate. This study focuses on the apparent chloride diffusion coefficients, as well as chloride binding isotherms which are obtained by the profile method. In addition, some aspects of the microstructure and of the pore structure are investigated, by using Mercury Intrusion Porosimetry and ²⁷Al NMR, in order to better understand the results obtained relatively to the apparent chloride diffusion coefficients and to the chloride binding. Chloride ingress increases when sulphates are present in the contact solution for all cement materials tested (partially carbonated or not). In addition, chloride ingress is faster when the material is partially carbonated before contact with chloride solution. It appears that both carbonation or presence of sulphate decrease chloride binding, thus explaining the results. The results show an evolution of the properties as a function of the cement replacement ratio by SCMs.

Abstract: In this paper, we focus on the numerical simulation of degradation of concrete structures subject to External Sulfate Attack (ESA). A diffusion-reaction model is used to describe the diffusion of sulfates inside the material and the reaction with the reactive constituents of the cement paste (calcium aluminates). The mechanical analysis is based on a new bi-phase chemo-elastic model with chemical and mechanical damage. The results obtained with the proposed approach are compared with experimental data on a reduced scale tunnel lining structure subject to ESA.

Abstract: Water-cement (w/c) ratio is an important parameter in concrete mix ratio design, which also plays an important influence on concrete sulfate corrosion rate. In this paper, concrete cubic specimens with w/c of ratio as 0.4, 0.5 and 0.6 were fabricated, respectively, and put into 10% Na₂SO₄ solution and tap water for 240 days. During the submerging process, superficial corrosion phenomena were observed and concrete cubic strengths were measured periodically. The results showed that the lower the w/c ratio, the stronger the concrete sulfate corrosion resistance is. At the same time, concrete with lower w/c ratio always correspond smaller corrosion layer thickness. Based on the degradation of cubic compressive strength of corroded concrete specimens, the development models of corrosion thickness of each w/c ratio concrete are established as sulfate corrosion goes on

Abstract: This study investigates the durability of partially-damaged concrete with the addition of Silica Fume and Ground Granulated Blast Furnace Slag. Portland cement was replaced by 10% SF and 60% of GGBFS as a replacement of Portland cement. Thirty-six concrete cylinders (100 x 200 mm) were subjected to three compressive loading levels (50%, 75%, and 90% of its ultimate strength capacity). After 28 days of curing, the concrete specimens were experimentally tested for electrical resistivity, rapid chloride penetration (RCPT) and chloride migration coefficient (D_nssm) according to NT-BUILD 494. The experimental results showed that the GGBFS improves significantly the durability of concrete with the highest electrical resistivity and lowest chloride permeability compared to control and SF concrete and both SF and GGBFS had significant effect the concrete durability properties even when the concrete was subjected to compressive damage up to 90% of the compressive strength. A correlation between D_nssm and RCPT in partially damaged concrete was observed and an empirical linear relationship was developed to estimate D_nssm.

Abstract: With the aim to test the applicability of the commonly used maturity concept introduced by Freiesleben et al [1] to modern concrete and to investigate the impact of the curing history on the compressive strength of laboratory samples cured at elevated temperatures, four concretes with different binder compositions (a pure CEM I 42.5N, CEM I 42.5N with fly ash, CEM I 52.5N with fly ash and a CEM III/B) were cured and tested at temperatures ranging from 5 to 60 °C.To test the maturity concept, the development of the compressive strength of samples cured at temperatures ranging from 5 to 60 °C were tested at maturities ranging from 1 to 28 days.To test the impact of curing history at elevated temperatures on the compressive strength, concrete samples were cured at 60°C using two different temperature scenarios: (1) at a constant temperature of 60 °C and (2) at gradually increasing temperature from the casting temperature to the maximum temperature of 60 °C.It was found that the commonly used maturity concept is still applicable to modern concrete although the activation energy is dependent on the binder composition. Concerning the impact of curing history it was found that at 28 days of maturity, the strength of concrete cured at constant temperature of 60 °C was significantly lower than that of concrete cured at 20 °C. For the concrete exposed to gradually increasing temperature up to 60 °C, only a slight decrease in strength was observed for the pure cement concretes while the strength of the binder systems with fly ash increased.

Abstract: The design of concrete structures exposed to environmental attack requires serious attention for concrete durability. Early age cracking due to autogenous deformations should be avoided.In this work the study of the structural effects of hydration heat and rheological behaviour of a massive concrete casting is presented. The object of the study is a skyscraper foundation slab. Aim of the work is the numerical simulation of what occurs to the structure during the hardening, in order to avoid unforeseen autogenous cracking and therefore the choice of a tailor-made concrete mixture able to fulfil the performance criteria.Non-linear finite element coupled thermal and mechanical analyses have been performed taking into account: hydration heat generation and dispersion, dimension and sequence of the casting, evolution of concrete mechanical properties in time during the hardening reaction, creep and differential shrinkage.

Abstract: To evaluate the vulnerability of ultra-high performance fiber reinforced concrete (UHPFRC) infrastructure to rigid projectile penetration, CEA-Gramat has led since few years an experimental and numerical research program in collaboration with French universities. During the penetration process, concrete is subjected to extreme conditions of pressure and strain-rate. Plasticity mechanisms as well as dynamic tensile and/or shear damages are activated during the tunneling phase and the cratering of the concrete target. Each mechanism has been investigated independently at the laboratory scale and the role of steel fibers has been specially analyzed to understand their influence on the macroscopic behavior. In parallel, some improvements have been introduced into the concrete model developed by Pontiroli, Rouquand and Mazars (PRM model), especially to take into account the fibers contribution in the tensile fracture process. The capabilities of the PRM model have been illustrated by performing numerical simulations of material characterization experiments. Next step will be to assess the concrete model to simulate projectile penetration into UHPFRC concrete structures.

Abstract: A new type of concrete, which contains volcanic aggregate and well-graded river sand, having both high strength and ultra-high modulus of elasticity has been developed for tall buildings under severe conditions, and its properties were characterized. The structural deformation of high-rise buildings can be substantially reduced by using this type of concrete, thereby decreasing the cross-section area of the structural members and increasing the economic benefits, not to mention alleviating shrinkage and creep problems of concrete. Uniaxial compression test, static elastic modulus test and scanning electron microscope test were conducted to characterize the properties of concrete specimens with different mix proportions. The experimental results showed that volcanic rock as coarse aggregate, well-graded river sand as fine aggregate, sand ratio around 43%, silica fume content about 10% as well as some mineral admixtures such as metakaolin and nanosilica led to higher modulus of elasticity of concrete. A particular type of concrete with the compressive strength of 146 MPa and modulus of elasticity of 53.5 GPa was developed, which is much stiffer than normal concrete in the code of practice. Four concrete mixtures including C45, C80, high modulus and ultra-high modulus concrete were applied in producing structural members, verifying the advantages of utilization of high stiffness concrete.

Abstract: Properties of water transport and depth of chloride penetration into the Ultra High Performance Concrete (hereafter as UHPC) with mild steel fibres are presented in this paper. The main aim of this experimental part of work is to obtain sufficiently accurate input data for the evaluation of long-term durability of architectural concrete which are connected with water transport and its accompanying effects such as biological degradation or chloride transport. The article also presents the one dimensional chloride diffusion into UHPC which can be potentially dangerous particularly for durability of reinforced concrete structures. For the simulation of aggressive environments the concrete samples were exposed to chloride solution for one year. Measured data were examined in relation to the depth of penetration of chloride ions into the UHPC structure. Comparative measurements with normal strength concrete (hereafter as NSC) are done as well. An about five-time lower value of moisture absorption of UHPC compared to the NSC was observed and further the curve of chloride penetration into the structure is significantly steeper for UHPC samples.