Civil Engineering Design最新文献

An analytical model is presented in this paper which, based on the maximum crack velocity, provides a hypothesis for one of the reasons of the increase in tensile strength of concrete under high loading rates. Due to the fact that the formation of cracks needs a certain time to pass through the cross section and not happens suddenly, stresses can still be transmitted over the remaining uncracked cross section during this time. The hypothesis is that at high loading rates, the increase in externally induced stresses can be greater than the decrease in the load-bearing cross-sectional area due to limited crack propagation velocity, which results in an externally measurable increase in strength. This measured strength increase depends on the stress distribution in the crack plane. In this paper two variants of this stress distribution during the failure process are described, and their effect on the increase in strength is mathematically evaluated.

The damage analysis of reinforced concrete (RC) is of high interest for reasons of effective maintenance and structural safety of buildings. The damage structures of RC plates loaded by an impact were investigated, applying X-ray planar tomography and finite element method (FEM). Planar tomography allows getting three-dimensional information of the RC elements and the damage including crack, spalling, and scabbing. The FEM model validated on the tomography data justifies the application for further predictions of the damage description. In this study, we investigated concrete plates of three different thickness subjected to impacts at different low- and medium-velocity, whereby the used impactor had a flat tip, which resulted in small penetrations on the front side and scabbing on the rear side. In order to quantify the damage, the damage volume and its distribution through the plate were computed and the correlations between degree of damage and impact velocity were found out.

Carbon reinforcements enjoy increasing popularity both in building reinforcement and in new construction. The use of yarns with more than 50 000 filaments per roving and finenesses of up to 3300 tex, so-called heavy tows, enables greater permissible stress and thus increases the performance of the textile reinforced concrete structures. However, high yarn tensile forces with an almost constant roving surface lead to an extension of the required end anchorage and overlap areas. In the project, it was investigated whether a modified loop-shaped yarn arrangement at the selvages could guarantee force transmission over shorter lengths and thus enables a more economic design of this type of construction. This paper presents the results generated within the investigations, proving the potential of the applied method. Manufacturing possibilities, force transmission mechanisms, material properties, and failure mechanisms were analyzed.

In ranges with negative moments, crack formation influences the structural behavior of statically indeterminate composite beams under service conditions. Composite beams with partial-depth precast concrete units show some particularities, which cannot be described accurately using the established methods of calculation. This article presents methods of calculation, which can be used to cover the specific structural behavior and describe crack formation, crack widths and deformations realistically.

In dissipative seismic design concepts, capacity design rules must be applied to control the plastic mechanism. For this purpose, nondissipative structural elements require an overstrength with respect to dissipative zones. The scattering of the yield strength of structural steel is considered in DIN EN 1998-1 by the material overstrength factor γ _ov with a recommended value of 1.25. However, current measurement data often show significantly higher yield strengths, especially for steel grades of a low nominal strength. In this article, new yield strength data of European steel grades are evaluated and compared with statistical values from the literature. A steel grade-dependent over strength factor was derived from a new probabilistic model for the yield strength, which has been included in the new German National Annex DIN EN 1998-1/NA:2018.

The particularity of highway tunnel location and the limitation of space result in the closure of its environment, which makes evacuation, rescue, and firefighting activities more difficult. In this study, a fire dynamics simulator was used for numerical simulation to study the effect of jamming on the critical velocity of slope tunnel fire. This experiment carried out two sets of working conditions simulations: (1) In the straight tunnel, the heat release rate and the blockage ratio are considered and numerically simulated. (2) Under the heat release rate of 20 MW fire source, the blockage ratio and the gradient of the tunnel are considered and numerically simulated. The vehicle blockage is set to distribute in two rows and two columns upstream of the fire source, accounting for 6.6%-22.0% of the cross-section of the tunnel. The slopes were selected in nine cases of 0%, ±1%, ±2%, ±3%, and ±4%. The six heat release rates were 5 MW, 10 MW, 15 MW, 20 MW, 25 MW, and 30 MW, respectively. The fire occurred on the tunnel centerline. The results show that in flat tunnels, the variation trend of critical velocity with heat release rate is basically the same, and it increases with the increase of heat release rate. However, when the heat release rate is the same, the larger the vehicle blockage ratio is, the smaller the critical velocity is. It can be seen from the second experimental case that the influence of the tunnel slope on the critical velocity is different. In the downhill tunnel, the critical velocity increases with the increase of slope. In the uphill tunnel, the critical velocity decreases with the increase of slope.

We studied the effects of superplasticizer (SP) (PCE1 and PCE2), air-entraining admixtures (AEA), and supplementary cementitious material (silica fume and fly ash) on the mechanical and durability properties of concrete samples. Eight concrete mix designs were prepared. The first six concrete mix design contained similar aggregates, PCE2 SP, AEA, 350 kg/m³ cement, and water to cement ratio equal to 0.38, and one of the mentioned mix design was selected as control samples with water to cement ratio equal to 0.57, without PCE2 SP and AEA. We used a different quantity of PCE2 SP and AEA and replaced the fly ash or silica fume as part of cement in two of the mentioned mix designs. The last two concrete mix designs studied the effect of PCE1 SP and AEA on freezing and thawing of concrete mix design. Adding PCE2 SP in concrete mix design increased compressive strength at age 11, 42, and 90 days sharply and reduced the depth of water penetration at the age of 28 and 90 days compared to the control sample. Using simultaneous PCE2 and AEA in concrete mix design did not improve compressive strength significantly and increased slightly depth of water penetration compared to only using PCE2. However, increasing the quantity of AEA to 5% improved both compressive strength and reduced depth of water penetration. In the second group of concrete mix design, adding both PCE1 and AEA increased the number of resistible freezing and thawing cycles of concrete. Side chain length and molecular weight of PCE1 and PCE2 SPs had no important effect on the compressive strength and performance properties of concrete.

This investigation aims to study the effect of varying superplasticizer dosage and water-to-cement ratio (w/c) on the bond strength between aramid fiber reinforced polymers (AFRPs) bar and self-consolidating concrete (SCC). A total of 15 compression tests and 36 pullout tests are conducted on AFRP and steel bars within five different groups of concrete mixtures. Results show a significant effect of superplasticizer and water-to-cement ratio on the bond strength of AFRP bars embedded within SCC. The bond-slip behavior between the AFRP bars and SCC was quantified and is presented in this research.

The mechanical behavior of carbon fiber-reinforced polymer (CFRP) fabric-reinforced cementitious matrix (FRCM) overlay on clay brick masonry was characterized by means of double-shear bond, tensile, and out-of-plane tests. Different bond lengths in the range 55-250 mm were analyzed during the double-shear bond tests. The failure mechanism was slippage of the CFRM mesh, with peak stresses in the fabric around 500 N/mm² per 100 mm bond length. Tensile tests were performed following AC434.13 with FRCM coupons of 10 and 15 mm thickness, where the mechanical behavior was divided into three stages. The four-point bending experiments on FRCM-reinforced prisms showed that the CFRP mesh provided significant added value in both moment capacity (>110%) and deformation capacity (>2800%), when compared to specimens reinforced with solely a cementitious matrix. Additionally, no significant difference was observed between the envelope of the cyclically tested specimen and the statically loaded specimens. A model was proposed where, in contrast to existing design models, the influence of the cementitious matrix layer also was considered. Using the modified tensile test results as input parameters for the model, a good estimation of the experimental results was obtained.

This article presents results of tests on full-scale wall models with two window openings. Tested walls were made of autoclaved aerated concrete (AAC) units and calcium-silicate (C-S) units. They were erected on thin unfilled perpend joints. This article strictly corresponds to previous tests performed in the spandrel area. The aim of the research was to verify results obtained for smaller models. Stress-strain analysis of the spandrel area in full-scale models was performed and results were compared with results obtained for smaller models. According to the analysis of the stress-strain graphs, it was found that the horizontal stress in the spandrel area was ca. 0.2 N/mm². The software based on the finite element method was used to perform calculations for tested walls. Results from laboratory tests and numerical simulations were analyzed to determine the level of stress and strain in tested walls.