Simulation model of impact on reinforced concrete

This paper proposes a simple but more effective way to perform finite element analyses of impact on reinforced concrete. The equivalent inclusion method is applied and considers the reinforced concrete as a homogeneous material, simplifying the finite element meshes and greatly reducing the computational cost of analyses. Using Mori-Tanaka's average strain theory, the equivalent stiffness matrix of the homogenized material and the associated equivalent material moduli are derived for finite element analyses. The residual velocity at which a projectile penetrates into an equivalent reinforced concrete slab is studied based on the strength of the equivalent material. Two examples are presented to demonstrate the proposed method. One involves the impact of an ogive-nose projectile on a reinforced concrete slab. The FEM computational results obtained using this method are very close to the test data, implying that the proposed method will be promising in future studies of impact analyses of reinforced concrete structures. Another example of 50° oblique impact is then presented to demonstrate the dependence of projectile ricochet upon the impact velocity.     

Assessment of statistical variations in impact resistance of high-strength concrete and high-strength steel fiber-reinforced concrete

The impact resistance variations of high-strength steel fiber-reinforced concrete (HSFRC), versus those of high-strength concrete (HSC), commanded this research. The impact resistance of the high-strength steel fiber-reinforced concrete improved satisfactorily over that of the high-strength concrete; the failure strength improved most, followed by first-crack strength and percentage increase in the number of post-first-crack blows. The two concretes resembled each other on the coefficient of variation values, respectively, on the two strengths, whereas the high-strength concrete was much higher in the value on the percentage increase. The Kolmogorov-Smirnov test indicates that the high-strength concrete was approximately normally distributed in first-crack and failure strengths, high-strength steel fiber-reinforced concrete was poorly normally distributed in the two strengths, and both concretes were hardly normally distributed in the percentage increase. Finally, for both concretes, failure strength regression models were developed, and then, the accompanying 95% prediction intervals for the strength were established.     

Laboratory assessment of alkali contribution by aggregates to concrete and application to concrete structures affected by alkali-silica reactivity

In Phase I, particles from 17 different aggregates, 1.25-5 mm in size, were immersed in continuously agitated solutions at 38 °C: distilled water, Ca(OH)₂-saturated solution, 0.7 M NaOH (measurement of K supply), and 0.7 M KOH (measurement of Na supply). These solutions were periodically analysed for K and/or Na up to 578 days. More alkalies were released in alkaline solutions than in lime-saturated solution, with lower values in water. After 578 days, the aggregates released between <0.01% and 0.19% Na₂O_e, excluding the nepheline-rich aggregate tested (0.68%). This would correspond to a contribution to concrete from <0.1 to 3.4 kg/m³ Na₂O_e (12.7 for the phonolite), based on an aggregate content of 1850 kg/m³. In general, the feldspar-rich aggregates released significantly more alkalies. In Phase II, the water-soluble alkali content of mass concrete elements from many dams was measured using a hot water extraction method. The values obtained often largely exceed the soluble alkali content expected to be released by the cement used. These results thus also suggest that large amounts of alkalies were supplied with time by the aggregates, particularly by feldspar-rich ones.     

Properties of confined fibre-reinforced concrete under uniaxial compressive impact

The effect of confinement on plain and fibre-reinforced concrete (FRC) prisms subjected to uniaxial compressive impact was studied. It was found that the response of the material changed with the degree of confinement. Confined concrete exhibited more ductile behaviour, with both strength (f_c′) and ultimate strain (ε_ult) increasing with the degree of confinement. However, the elastic modulus (E) of the confined specimens was found to be about the same as or slightly lower than those of unconfined prisms. In addition, the relationship between stress and stress rate (n value) was also determined. It was found that, with confinement, the material became more rate sensitive.     

Energy absorption ability of buckyball C720 at low impact speed: a numerical study based on molecular dynamics

The dynamic impact response of giant buckyball C₇₂₀ is investigated by using molecular dynamics simulations. The non-recoverable deformation of C₇₂₀ makes it an ideal candidate for high-performance energy absorption. Firstly, mechanical behaviors under dynamic impact and low-speed crushing are simulated and modeled, which clarifies the buckling-related energy absorption mechanism. One-dimensional C₇₂₀ arrays (both vertical and horizontal alignments) are studied at various impact speeds, which show that the energy absorption ability is dominated by the impact energy per buckyball and less sensitive to the number and arrangement direction of buckyballs. Three-dimensional stacking of buckyballs in simple cubic, body-centered cubic, hexagonal, and face-centered cubic forms are investigated. Stacking form with higher occupation density yields higher energy absorption. The present study may shed lights on employing C₇₂₀ assembly as an advanced energy absorption system against low-speed impacts.     

Effects of nanosilica and steel fibers on the impact resistance of slag based self-compacting alkali-activated concrete

In this research, the effects of nanosilica and steel fibers on the impact resistance of ground granulated blast furnace slag based self-compacting alkali-activated concrete were investigated. Nanosilica volume fraction was kept constant at 2%. Two types of hooked-end steel fibers (Kemerix 30/40 and Dramix 60/80) and steel fiber volume contents (0.5% and 1%) were utilized to highlight the combined effects of nanosilica and steel fiber on the impact behavior. The fresh state and mechanical properties such as slump flow, L-box, V-funnel, compressive strength, modulus of elasticity, splitting tensile strength, and flexural strength were evaluated. The microstructure of the samples was examined using a scanning electron microscope. The impact resistance of the specimens was measured by a drop-weight test. Acceleration-time and force-time graphs were plotted and evaluated together with the crack photos of the specimens for the first and failure impactor drops. The incorporations of nanosilica and steel fiber improved splitting tensile strength, flexural strength, impact resistance, and energy absorption capacity, while they decreased compressive strength and modulus of elasticity. For the specimens without nanosilica and with 2% nanosilica, the impact energy improvements were five times and 12.5 times higher for 0.5% short fibrous, 20.5 times and 44.5 times higher for 1% short fibrous, 23.5 times and 31 times higher for 0.5% long fibrous, and 64 times and 144.5 times higher for 1% long fibrous specimens than the specimens without nanosilica and steel fiber, respectively. The long fibers were found more effective in mechanical strength and impact energy than short fibers, and the reinforcing efficiency of fibers enhanced with higher steel fiber volumes. The combined utilization of nanosilica and steel fibers have the potential to delay the crack formation and dissipate energy to the surrounding zones, and this potential increased with higher steel fiber lengths and volume ratios.     

Determination of the energy consumption during the production of various concrete recipes

This article presents a report of the mixing of concrete on the laboratory scale in a single-shaft and twin-shaft mixer. For both mixers we selected five concrete recipes that cover a broad spectrum of concrete mixing techniques. The concrete recipes differ from each other amongst other things by virtue of the aggregate-sized distribution curves, water-cement ratio, flow properties, compressive strength and mixing times. The specifically volume-related application of energy - which is necessary for the homogenization of the particular recipe in the mixer - is an essential influencing variable.The comparison of the specifically volume-related application of energy is possible only if the concrete recipes possess the same homogeneity. The time curve of the homogeneity plotted against the necessary mixing time indicates the mixing efficiency, which in turn is determined by an imaging measurement process. Comprehensive mixing experiments show that the resulting application of energy, measured via the current composition, does not provide sufficient information in order to define the actual homogeneity in the mixture. A method was developed for the purpose of comparing concrete mixtures based on various recipes with the same homogeneity in relation to the specifically volume-related application of energy. The particular application of energy can be determined via the required mixing time and the power output process in terms of time.     

装药弹丸斜侵彻混凝土数值模拟

通过有限元分析软件ANSYSY／LS-DYNA对装药弹丸斜侵彻混凝土过载环境进行数值模拟,分析了弹体侵彻模型及弹载传爆药应力波加载机理,揭示了弹载传爆药应力波加载的加载状态。研究表明,弹载传爆药不同入射角侵彻时,受弹体应力波作用,入射角越小,弹内传爆药所受塑性波幅值越高,对传爆药的损伤有很大的影响。将弹载传爆药过载曲线与弹体过载曲线进行对比,弹体过载与传爆药过载曲线变化规律趋势相近,可以通过弹体的过载特性来估算PBXN-5传爆药的过载特性。此项研究可为侵彻过载中弹载传爆药抗高过载能力评价方案设计提供参考。     

Tensile bonding strength of epoxy coatings to concrete substrate

In this study, four commercially available epoxy coatings were selected to investigate their tensile bonding characteristics to dry and wet concrete with and without 105-kPa backwater pressure. The CIGMAT CT-2 (modified ASTM D4541-95e1) and CIGMAT CT-3 (modified ASTM C321-00) tests were used to determine the tensile bonding strength of the coatings with curing time. The tests were conducted over a period of 2 years. Fifty-three CIGMAT CT-2 tests and 26 CT-3 tests were performed for the four epoxy coatings. Five failure types were identified for both the test methods based on the failure mechanisms observed during the tests. In situ bonding tests (CIGMAT CT-2, full-scale test with back pressure on) were also performed on coatings that were tested under a hydrostatic (back) pressure of 105 kPa (simulating 10 m of groundwater) for at least 6 months (full-scale test). All the coatings investigated were epoxy based, but their bonding strength and failure modes to dry and wet concrete surfaces were different. For all the coatings investigated, in situ bonding strength with dry concrete was higher than wet concrete. Epoxy-A and Epoxy-D (fiber-mat-reinforced epoxy) had good bonding strength (>1.3 MPa, 190 psi) to both dry and wet concrete surfaces during the 2 years of testing period. The bonding strength of Epoxy-B and Epoxy-C varied with time for both dry and wet concrete surfaces. Although for coatings with higher bonding strength, better correlations between the two test methods were observed, in general, the bonding strength from CIGMAT CT-3 test was higher than that from CIGMAT CT-2 test for the coatings tested.     

The use of migrating corrosion inhibitors to repair motorways' concrete structures contaminated by chlorides

The effectiveness of migrating corrosion inhibitors (MCIs) and repair mortars against rebar corrosion was studied in concrete specimens made by ordinary Portland cement with w/c ratio equal to 0.6 and containing 1 wt.% of chlorides. An alkanolamine-based inhibitor was tested in addition with a common mortar and two repair mortars. Electrochemical techniques, measurements of corrosion potential and electrochemical impedance spectroscopy (EIS) were used to determine the corrosion behaviour of the specimens when a cell containing a 3.5% NaCl solution was applied on the rehabilitation mortar. Mercury intrusion porosimetry (MIP) was also used for the characterisation of repair mortars' total porosity and a chemical analysis was made to determine the amount of chlorides penetrated in the mortar layers and in the concrete substrate. Results demonstrate that the simultaneous use of the alkanolamine-based inhibitor with a good barrier coating offers protection against rebar corrosion and allows rehabilitation of deteriorated concrete structures.     

Performance of recycled aggregate concrete monitored by durability indexes

The report of an investigation into the performance of concrete manufactured with recycled aggregate (RA) using durability indexes as indicators is presented in this paper. Durability indexes, such as chloride conductivity, oxygen permeability and water sorptivity, of three different concrete mixes containing 0%, 50% and 100% RA were monitored at ages 3, 7, 28 and 56 days. The results show that durability quality reduced with increase in the quantities of RA included in a mix; however, as expected, the quality improved with the age of curing. At the age of 56 days, increases in index value of a concrete mix made with 100% RA over that made with 100% natural aggregate were 86.5% and 28.8%, respectively, for chloride conductivity and water sorptivity. The corresponding value of oxygen permeability index (OPI) for the same concrete mixes was a reduction of 10.0%. For 50% RA concrete, the reductions in chloride conductivity and water sorptivity indexes at the curing age of 56 days compared to 3 days were 62.7% and 42.7%, respectively. The corresponding figure for OPI was an increase of 37.6%. The poor performance of the RA concrete is associated with the cracks and fissures, which were formed in RA during processing, thereby rendering the aggregate susceptible to permeation, diffusion and absorption of fluids.     

Study on impact response of laminated woven fabric rubber composite armour against shaped charge jet

The performance of different types of laminated woven fabric rubber composite armours (WFRCAs) against a 56?mm diameter shaped charge at 68° NATO angle is investigated. WFRCA is a laminated armour element consisting of the following layers: steel, woven fabric, rubber and steel. Carbon, glass, Kevlar and poly(phenylene benzobizoxazole) (PBO) woven fabrics are used as reinforcement materials. The effects of the four types of woven fabric on the efficiency factors were studied. Several X-ray tests made the interaction between the shaped charge jet and the laminated WFRCAs visible. The effect of the interaction region between the jet and the armour plate was also discussed. A microscopic study of the edge of the fabric, rubber and steel plates after shaped charge jet impact was conducted to evaluate the structure and composition. The defeat mechanism in the PBO WFRCA target is different from the mechanisms of other composite armour types.     

The use of USPV to anticipate failure in concrete under compression

The use of the ultrasonic pulse velocity tester is introduced as a tool to monitor basic initial cracking of concrete structures and hence to introduce a threshold limit for possible failure of the structures. Experiments using ultrasonic pulse velocity tester have been carried out, under laboratory conditions, on various concrete specimens loaded in compression up to failure. Special plots, showing the relation between the velocity through concrete and the stress during loading, have been introduced. Also, stress–strain measurements have been carried out in order to obtain the corresponding strains. Results showed that severe cracking occurred at a stress level of about 85% of the rupture load. The average velocity at this critical limit was about 94% of the initial velocity and the corresponding strain was in the range of 0.0015 to 0.0021. The sum of the crack widths has been estimated using special relations and measurements. This value that corresponds to the 94% relative velocity was between 5.2 and 6.8 mm.     

Durability of metakaolin concrete to sulfate attack

This study investigates the effect of metakaolin (MK) replacement of cement on the durability of concrete to sulfate attack. Three MK replacement levels were considered in the study: 5%, 10%, and 15% by weight of cement. The other experimental parameters investigated in the study were: water to binder ratio (0.5 and 0.6), initial moist curing period (3, 7, and 28 days), curing type (moist and autoclaving), and air content (1.5% and 5%). After the specified initial moist curing period, concrete specimens were immersed in 5% sodium sulfate solution for a total period of 18 month. The degree of sulfate attack was evaluated by measuring expansion of concrete prisms, compressive strength reduction of concrete cubes, and visual inspection of concrete specimens to cracks. The study showed that MK replacement of cement increased the sulfate resistance of concrete. The sulfate resistance of MK concrete increased with increasing the MK replacement level. The sulfate resistance of MK concrete at w/b ratio of 0.5 was found higher than that at w/b ratio of 0.6. Autoclaved MK concrete specimens showed superior sulfate resistance compared to moist cured ones. The pore volume of autoclaved MK concrete was found less than that of moist cured MK concrete. The air entrained MK concrete showed higher improvement in the sulfate resistance than the non-air entrained MK concrete. However, the air entrained plain concrete showed lower improvement in the sulfate resistance than the non-air entrained concrete.     

Adsorption of Pb2+, Cu2+ and Co2+ by polypropylene fabric and polyethylene hollow fiber modified by radiation-induced graft copolymerization

Cation-exchange adsorbents were prepared by radiation-induced grafting of glycidyl methacrylate (GMA) onto polypropylene (PP) fabric and polyethylene (PE) hollow fiber and subsequent phosphonation of epoxy groups of poly(GMA) graft chains. The adsorption characteristics of Pb²⁺, Cu²⁺ and Co²⁺ for the two cation-exchange adsorbents were studied. In the grafting of GMA onto PP fabric, the degree of grafting (%) increased with an increase in reaction time, reaction temperature, and pre-irradiation dose. The maximum grafting yield was observed around 60% GMA concentration. In 50, 130 and 250% GMA-grafted PP fabric, the content of phosphoric acid was 1.52, 3.40 and 4.50 mmol/g at 80 °C in the 85 % phosphoric acid aqueous solution for 24 h, respectively. The adsorption of Pb²⁺, Cu²⁺ and Co²⁺ by PP fabric adsorbent was enhanced with an increased phosphoric acid content The order of adsorption capacity of the PP fabric adsorbent was Pb²⁺>Co²⁺>Cu²⁺. In adsorption of Pb²⁺, Cu²⁺ and Co²⁺ by PE hollow fiber, the amount of Pb²⁺ adsorbed by the PE hollow fiber adsorbent containing 1.21 mmol/g of -PO₃H wasca. 54.4 g per kg. The adsorption amount of Cu²⁺ and Co²⁺ in the same PE hollow fiber wasca. 21.0 g per kg andca. 32.1 g per kg, respectively. The order of adsorption of the PE hollow fiber adsorbent was Pb²⁺>Co²⁺>Cu²⁺.     

Contribution of granular interactions to self compacting concrete stability: Development of a new device

Understanding the SCC behaviour is essential for resolving placement and consolidation problems in the field. As far as segregation is concerned, one of the main remaining obstacles is the design of a concrete mixture suitable for given casting conditions. Physical approaches which consist in studying the sedimentation of a single particle in a yield stress fluid failed to describe SCC static segregation. Segregation is a more complex phenomenon and the interactions between coarse aggregates have to be taken into account. They contribute to the stability of fresh SCC and this contribution should only depend on the solid fraction of the granular skeleton. A new experimental device has been developed in order to highlight and quantify the combined effects of coarse aggregates. This device allows studying lattices of particles. These latter are organised according to a cubic centred pattern and are immersed in a yield stress fluid. The experimental device and the test procedure are described in this paper. The validity of measurements has been demonstrated by performing a first series of tests and numerical simulations. Repeatability is quite satisfactory and “wall effects” can be limited.     

Degradation of concrete by flue gases from coal combustion

The effect of flue gases from coal combustion on the concrete shell of a power plant stack was analyzed and the damage to the concrete was evaluated. The compressive and tensile strengths of concrete were determined by rebound hammer test and pull-off test on the site. Samples of concrete taken from various zones of the stack shell were analyzed in detail. The examination techniques used included chemical analysis, water suspension analyses, XRD, thermal analysis, SEM and EDS. It was found that the flue gases and the acid condensate, acted very aggressively at an elevated temperature and caused severe degradation of the inside surface zone of the concrete shell. The binding material in this zone was converted into sulfur-bearing compounds. Gypsum was identified as the dominant compound in the degraded zone of concrete in the upper parts, while anhydrite in the lower parts of the stack. Carbonated zone was located below the clearly delimitated sulfated zone of the concrete. The aggressive environment in the stack did not affect the internal zones in the concrete.     

From single particle impact behaviour to modelling of impact mills

An approach to quantify the impact grinding performance of different materials is presented. Based on a dimensional analysis and on fracture mechanical considerations two material parameters, f_Mat. and W_m,min, are derived from theoretical considerations. f_Mat. characterises the resistance of particulate material against fracture in impact comminution. W_m,min gives the mass specific energy which a particle can absorb without fracture. Using this approach various materials in a wide size range, e.g. different polymers, crystalline substances, glass and limestone, can be characterised quantitatively. The derived material parameters are applied to the systematic and multi-scale modelling of grinding in impact mills. A population balance model is presented and the results of the simulation for two different impact mills are shown. The developed model allows for a clear separation of the influence of material properties, mill specific features and operating conditions, thus enabling a deeper understanding of the impact grinding process.     

Simulation of crack propagation in fiber-reinforced concrete by fracture mechanics

Mode I crack propagation in fiber-reinforced concrete (FRC) is simulated by a fracture mechanics approach. A superposition method is applied to calculate the crack tip stress intensity factor. The model relies on the fracture toughness of hardened cement paste (K_IC) and the crack bridging law, so-called stress-crack width (σ-δ) relationship of the material, as the fundamental material parameters for model input. As two examples, experimental data from steel FRC beams under three-point bending load are analyzed with the present fracture mechanics model. A good agreement has been found between model predictions and experimental results in terms of flexural stress-crack mouth opening displacement (CMOD) diagrams. These analyses and comparisons confirm that the structural performance of concrete and FRC elements, such as beams in bending, can be predicted by the simple fracture mechanics model as long as the related material properties, K_IC and (σ-δ) relationship, are known.     

Resistance of metallic substrates protected by an organic coating containing aluminum powder

The corrosion behavior of an epoxy primer containing aluminum powder (10 vol.%) applied on carbon steel and on galvanized steel was examined by electrochemical impedance spectroscopy (EIS). The data show that this coating is more protective when applied onto carbon steel substrates, and that on galvanized steel thicker coatings allow to achieve similar protection levels as those obtained for carbon steel. These effects are probably due to aluminum pigments providing a cathodic protection of the substrate, and to the resulting products precipitating inside the pores of the polymeric coating. Three stages can be distinguished during exposure of the coated specimens. Upon immersion of the coated samples in the test solution, a pre-saturated stage is observed. After a certain period of immersion, which strongly depends on the thickness of the applied coating, a saturation stage is reached in which an effective protection of the metallic substrate against corrosion is achieved. Finally, at sufficiently long exposure times, swelling through the coating eventually leads to the detachment of the coating.