Modeling with ANN and effect of pumice aggregate and air entrainment on the freeze–thaw durabilities of HSC

The objective of this work is to calculate the compressive strength, ultrasound pulse velocity (UPV), relative dynamic modulus of elasticity (RDME) and porosity induced into concrete during freezing and thawing. Freeze–thaw durability of concrete is of great importance to hydraulic structures in cold areas. In this paper, freezing of pore solution in concrete exposed to a freeze–thaw cycle is studied by following the change of concrete some mechanical and physical properties with freezing temperatures. The effects of pumice aggregate (PA) ratios on the high strength concrete (HSC) properties were studied at 28 days. PA replacements of fine aggregate (0–2 mm) were used: 10%, 20%, and 30%. The properties examined included compressive strength, UPV and RDME properties of HSC. Results showed that compressive strength, UPV and RDME of samples were decreased with increase in PA ratios. Test results revealed that HSC was still durable after 100, 200 and 300 cycles of freezing and thawing in accordance with ASTM C666. After 300 cycles, HSC showed a reduction in compressive strength between 6% and 21%, and reduction in RDME up to 16%. For 300 cycles, the porosity was increased up to 12% for HSC with PA. In this paper, feed-forward artificial neural networks (ANNs) techniques are used to model the relative change in compressive strength and relative change in UPV in cyclic thermal loading. Then genetic algorithms are applied in order to determine optimum mix proportions subjected to 300 thermal cycling.     

Beam test on bond behavior between high-grade rebar and high-strength concrete after elevated temperatures

In this paper, post-heating bond behavior between high-grade rebar and C80 high-strength concrete (hereafter, HSC) is studied. The high-grade rebar is HRBF500 fine grained steel with a yield strength of 500 MPa and the concrete grade C80 denotes compressive strength not lower than 80 MPa. First, the residual mechanical behavior of both high-grade rebar and HSC were tested after fire exposure. Second, the beam bond test was carried out to study the bond behavior between high-grade rebar and HSC after exposed heating at 200 °C, 400 °C, 500 °C and 600 °C, respectively. During the bond test, the influence of temperature, bond length, and some construction measurements on the bond–slip behavior were compared and evaluated. The investigation demonstrates that (1) the bond strength between high-grade rebar and HSC decreases while the peak slip increases with the elevated temperature, especially when the temperature exceeds 400 °C and (2) the confinement effect of steel wire mesh can help to improve rebar׳s bond behavior. Third, the bond–slip model between high-grade rebar and HSC for post-heating is proposed.     

Strengths and flexural strain of CRC specimens at low temperature

Crumb rubber concrete (CRC) is made by adding rubber crumbs into conventional concrete. This study undertakes an experimental study on the cubic compressive strength, axial compressive strength, flexural strength and splitting tensile strength of CRC specimens at both ambient temperature 20 °C and low temperature ?25 °C. The flexural stress–strain responses were also recorded. The averaged size of rubber crumbs used in the study is about 1.5 mm. Four levels of rubber contents are investigated, which are 0%, 5%, 10% and 15% by volume, respectively. The mix design aimed at 40 MPa of compressive strength and 100 mm of slump for all the CRC specimens. The results show that CRC increases its magnitude in strengths when temperature decreases, which is similar to the case of conventional concrete, but still exhibits ductility in low temperature. The conclusion from this study is that CRC may be more beneficial in its application in low temperature environments than in ambient temperature environments.     

Reliability analysis of reinforced concrete columns exposed to fire

A reliability analysis is conducted on reinforced concrete columns subjected to fire load. From an evaluation of load frequency of occurrence, load random variables are taken to be dead load, sustained live load, and fire temperature. Resistance is developed for axial capacity, with random variables taken as steel yield strength, concrete compressive strength, placement of reinforcement, and section width and height. A rational interaction model based on the Rankine approach is used to estimate column capacity as a function of fire exposure time. Various factors were considered in the analysis such as fire type, load ratio, reinforcement ratio, cover, concrete strength, load eccentricity, and other parameters. Reliability was computed from 0 to 4 h of fire exposure using Monte Carlo simulation. It was found that reliability decreased nonlinearly as a function of time, while the most significant parameters were fire type, load ratio, eccentricity, and reinforcement ratio.     

Influence of class F fly ash on the abrasion–erosion resistance of high-strength concrete

This study investigates the abrasion–erosion resistance of high-strength concrete (HSC) mixtures in which cement was partially replaced by four kinds of replacements (15%, 20%, 25% and 30%) of class F fly ash. The mixtures containing ordinary Portland cement were designed to have 28 days compressive strength of approximately 40–80 MPa. Specimens were subjected to abrasion–erosion testing in accordance with ASTM C1138. Experimental results show that the abrasion–erosion resistances of fly ash concrete mixtures were improved by increasing compressive strength and decreasing the ratio of water-to-cementitious materials. The abrasion–erosion resistance of concrete with cement replacement up to 15% was comparable to that of control concrete without fly ash. Beyond 15% cement replacement, fly ash concrete showed lower resistance to abrasion–erosion compared to non-fly ash concrete. Equations were established based on effective compressive strengths and effective water-to-cementitious materials ratios, which were modified by cement replacement and developed to predict the 28- and 91-day abrasion–erosion resistance of concretes with compressive strengths ranging from approximately 30–100 MPa. The calculation results are compared favorably with the experimental results.     

Testing of full-scale concrete bridge deck slabs reinforced with fiber-reinforced polymer (FRP) bars

This paper presents an experimental study investigating the behavior of FRP-reinforced concrete bridge deck slabs under concentrated loads. A total of eight full-scale deck slabs measuring 3000-mm long by 2500-mm wide were constructed. The test parameters were: (i) slab thickness (200, 175 and 150 mm); (ii) concrete compressive strength (35–65 MPa); (iii) bottom transverse reinforcement ratio (1.2–0.35%); and (iv) type of reinforcement (GFRP, CFRP, and steel). The slabs were supported on two parallel steel girders and were tested up to failure under monotonic single concentrated load acting on the center of each slab over a contact area of 600 × 250 mm to simulate the footprint of sustained truck wheel load (87.5 kN CL-625 truck). All deck slabs failed in punching shear. The punching capacity of the tested deck slabs ranged from 1.74 to 3.52 times the factored load (P_f) specified by the Canadian Highway Bridge Design Code (CHBDC) CAN/CSA S6-06. Besides, the ACI 440.1R-06 punching strength equation greatly underestimated the capacity of the tested slabs with an average experimental-to-predicted punching capacity ratio (V_exp/V_pred) of 3.17.     

Experimental study of the mechanical behavior of continuous glass and carbon yarn-reinforced mortars

This work aims to evaluate the possibilities of cementitious materials reinforcement by continuous alkaline resistant AR glass or carbon yarns. Bond flexural tests and flexural tests on 7 × 7 × 28-cm specimens were performed at various ages of the mortar and with various layouts and, volume fractions of yarn. The flexural tests showed the capacity of yarn to improve the strength and ductility of the mortar. A definition of the effectiveness of a yarn as reinforcement is given as proportional to the ratio of the post-cracking maximal load on the product of the strength of yarn and the volume fraction of yarns. The effectiveness of a yarn seems to depend on its structure: the one of the carbon yarn, made up of micrometric filaments, is lower than the one of the glass yarn, made up of millimetric strands. Losses of strength and ductility were observed between 28 days and a year for the glass yarn-reinforced mortars. For the carbon yarn-reinforced mortars, post-cracking strength increases with time.     

钢管约束钢筋混凝土压弯构件滞回性能试验研究与分析

进行了4个圆钢管约束钢筋混凝土(CTRC)和4个方钢管约束钢筋混凝土(STRC)压弯构件滞回性能的试验研究,并进行了两个钢筋混凝土(RC)对比试件的试验研究。试验中的主要参数为轴压比(0.34、0.65和0.80)和混凝土强度等级(C30和C60)。试验结果表明,由于钢管对核心混凝土的有效约束,核心高强混凝土柱的承载力、延性和耗能能力得到了显著提高。随轴压比和混凝土强度的提高,CTRC压弯构件的受弯承载力提高;但轴压比和混凝土强度对试件的延性无明显影响。随轴压比和混凝土强度的提高,STRC压弯构件的受弯承载力提高,但延性下降。相同轴压比条件下,CTRC压弯构件的受弯承载力和延性明显优于STRC构件。根据试验结果,建议了钢管约束钢筋混凝土柱截面受弯承载力的计算方法。建立了钢管约束钢筋混凝土压弯构件的纤维模型数值计算方法,计算中采用随荷载的增加而不断增大钢管对核心混凝土的约束效应的方法,数值计算结果与试验结果吻合良好。     

Mechanical properties of natural fibers reinforced sustainable masonry

This research evaluates the physical and mechanical properties of Portland cement masonry blocks reinforced with lechuguilla natural fibers, that were lightened with 2-l bottles of polyethylene terephthalate.A concrete mix was designed for a target compressive strength of 16 MPa at 28 days, and slump of 70 mm. Masonry concrete blocks with dimensions of 730 × 340 × 130 mm were produced for two different fiber lengths (25 and 50 mm) and with fiber contents of 0.25%, 0.50%, 0.75% and 1.0%.Based on the obtained results, it was found that as the aspect ratio decreases the compressive strength increases and that the use of natural fiber (Vf = 0.5–0.75%) improves masonry post-cracking features, showing a ductile behavior and generating a uniform cracking pattern in the longitudinal sides of the blocks.     

Effects of steel fiber addition on mechanical properties of concrete and RC beams

C20 and C30 classes of concrete are produced each with addition of Dramix RC-80/0.60-BN type of steel fibers (SFs) at dosages of 0, 30, 60 kg/m³, and their compressive strengths, split tensile strength, moduli of elasticity and toughnesses are measured. Nine reinforced concrete (RC) beams of 300 × 300 × 2000 mm outer dimensions, designed as tension failure and all having the same steel reinforcement, having SFs at dosages of 0, 30, 60 kg/m³ with C20 class concrete, and nine other RC beams of the same peculiarities with C30 class concrete again designed as tension failure and all having the same reinforcement are produced and tested under simple bending. The load versus mid-span deflection relationships of all these RC and steel-fiber-added RC (SFARC) beams under simple bending are recorded. First, the mechanical properties of C20 and C30 classes of concrete with no SFs and with SFs at dosages of 30 and 60 kg/m³ are determined in a comparative way. The flexural behaviours and toughnesses of RC and SFARC beams for C20 and C30 classes of concrete are also determined in a comparative way. The experimentally determined (mid-section load)–(SFs dosage) and (toughness)–(SFs dosage) relationships are given to reveal the quantitative effects of concrete class and SFs dosage on these crucial properties.     

Experimental study of hybrid composite beams

In this study, a new type of hybrid confining device, a perforated steel tube that is externally protected by a thin fiber reinforced polymer skin is proposed and experimentally investigated. Hybrid composite beams were fabricated by filling fresh concrete into the hybrid composite tube. Fifteen scaled-down square beams, which had varying numbers of perforated steel faces or ‘steel grids’ and a dimension of length 55.9 cm, height 10.1 cm, and width 10.1 cm, were prepared. Four-point bending tests were conducted on all the specimens. In addition to the load–displacement curves obtained from the tests, strain gages were installed to monitor the local strain distributions. Test results show that the grid tube encased specimens lead to higher specific strength and ductility than the solid steel tube encased counterparts. Compared to other configurations, the specific strength and ductility are the highest when all the four faces are made of steel grids.     

Strength and deformability of waste tyre rubber-filled reinforced concrete columns

This study aims to investigate the efficiency of waste tyre rubber-filled concrete to improve the deformability and energy absorption capacity of RC columns by considering different concrete compressive strength, size of waste tyre rubber particles and rubber content. Twelve column specimens were tested using concrete of compressive strength 24 and 28 MPa mixed with 0.6 and 1 mm tyre rubber particles. For each concrete batch, 27 control specimens were prepared to examine the concrete properties. Using waste tyre rubber-filled concrete leads to a slightly lower compressive strength and modulus of elasticity, but the curvature ductility can increase up to 90%. It is concluded that this type of concrete can offer good energy dissipation capacity and ductility, which makes it suitable for seismic applications.     

Effect of rice husk ash on the strength and durability characteristics of concrete

This work investigates the effects of adding residual rice husk ash (RHA) from South Vietnam, generated when burning rice husk pellets in the boiler, to cement. To improve pozzolanic reactivity, RHA was ground for 1 h. The non-ground RHA and ground RHA were used to test strength activity index according to ASTM C311. The properties of the concrete were investigated, including compressive strength, concrete electrical resistivity, and ultrasonic pulse velocity. Results show that the non-ground RHA can be applied as a pozzolanic material. Decreasing the non-ground RHA average particle size provides a positive effect on the compressive strength of mortar. Compressive strength of cylindrical concrete in the 47–66 MPa range was obtained in this study. The results also indicate that up to 20% of ground RHA could be advantageously blended with cement without adversely affecting the strength and durability properties of concrete.     

Composite tubes as an alternative to steel spirals for concrete members in bending and shear

Glass fibre-reinforced polymer (GFRP) tubes are compared to steel spiral reinforcement in circular concrete members with longitudinal reinforcement and prestressing, using six beam tests. Two 324 mm diameter and 4.2 m long prestressed specimens were tested in bending. Four 219 mm diameter reinforced specimens were also tested, including two 2.43 m long beams tested in bending and two 0.6 m long beams tested in shear. In each set, one specimen was essentially a concrete-filled GFRP tube, while the other control specimen included steel spiral reinforcement of comparable hoop stiffness to that of GFRP tube. The strength of control specimens was governed by crushing and spalling of concrete cover. Unlike spiral reinforcement, GFRP tubes confined larger concrete areas and also contributed as longitudinal reinforcement, leading to increases in flexural and shear strengths, up to 113% and 69%, respectively.     

Correlations among mechanical properties of steel fiber reinforced concrete

In this paper, applicability of previously published empirical relations among compressive strength, splitting tensile strength and flexural strength of normal concrete, polypropylene fiber reinforced concrete (PFRC) and glass fiber reinforced concrete (GFRC) to steel fiber reinforced concrete (SFRC) was evaluated; moreover, correlations among these mechanical properties of SFRC were analyzed. For the investigation, a large number of experimental data were collected from published literature, where water/binder ratio (w/b), steel fiber aspect ratio and volume fraction were reported in the general range of 0.25–0.5, 55–80 and 0.5–2.0%, respectively, and specimens were cylinders with size of Φ 150 × 300 mm and prisms with size of 150 × 150 × 500 mm. Results of evaluation on these published empirical relations indicate the inapplicability to SFRC, also confirm the necessity of determination on correlations among mechanical properties of SFRC. Through the regression analysis on the experimental data collected, power relations with coefficients of determination of 0.94 and 0.90 are obtained for SFRC between compressive strength and splitting tensile strength, and between splitting tensile strength and flexural strength, respectively.     

Experimental investigation of ultimate capacity of wired mesh-reinforced cementitious slabs

Experimental tests conducted on 27 square cementitious slabs of 490 × 490 mm simply supported on four edges and subjected to patch load are presented. The slabs had a clear span of 400 × 400 mm and provided with a 445 × 445 mm closed frame of 8 mm diameter steel bar to hold the reinforcement in place and to act as a line support. The test variables were the wire mesh volume fraction: four expanded and two square types; slab thickness: 40, 45, 50 and 60 mm; and the patch load pattern: square and rectangular. The test results showed that as the volume fraction increased the punching strength of the slabs was also increased. Adding a wire mesh to ordinary reinforcement increases significantly the punching resistance at column stub. Moreover, as the loaded area size increases both ductility and stiffness increases and the bridging effect due to the difference in the reinforcement ratio in orthogonal directions was clearly noticed. More research was needed to identify the volume fraction ratio at which the mode of failure alter from flexure to punching.     

Effect of concrete mixing parameters on propagation of ultrasonic waves

An experimental study was conducted to evaluate the effect of concrete aggregate gradation, water–cement ratio, and curing time on measured ultrasonic wave velocity (UPV). 30 × 30 × 10 cm Portland cement concrete slabs were cast for ultrasonic evaluation, while 10 cm diameter by 20 cm height cylinders were cast for compressive strength evaluation The slabs and cylinders were prepared using Portland cement and limestone aggregate. Two slabs were cast from each combination of coarse aggregate gradations and water cement ratio (0.40, 0.45, 0.50, and 0.55). Four ASTM gradations were considered, ASTM No: 8, 67, 56, and 4. These gradations have nominal maximum aggregate size 25, 4.75, 19.3, and 12.5 mm, respectively.The ultrasonic equipment used in this study was the portable ultrasonic non-destructive digital indicating tester (PUNDIT) with a generator having an amplitude of 500 V producing 54 kHz waves. The time needed to transfer the signal between the transducers was recorded and used to calculate the signal velocity, which was used as a parameter in the evaluation. Ultrasonic measurements were performed at 3, 7, 28, and 90 days after concrete casting.The results of the analysis indicated that water–cement ratio was found to have a significant effect on UPV. The UPV was found to decrease with the increase of water cement ratio. Aggregate gradation was also found to have significant effect on UPV. In general, the larger the aggregate size used in preparing Portland cement concrete, the higher the measured velocity of ultrasonic waves. Also, UPV was found to be increased as concrete curing time increased. Concrete compressive strength was found to be significantly affected by water–cement ratio and coarse aggregate gradation. Lower water–cement ratio produced higher concrete strength. Also, the concrete compressive strength increased as maximum aggregate size decreased.     

火灾下钢筋混凝土墙计算模型及影响因素分析

采用有限元软件ABAQUS对火灾下钢筋混凝土墙的变形全过程进行了计算,计算结果与以往实验结果吻合较好.在此基础上,系统分析了轴压比、侧向荷载比、高厚比、墙厚度、混凝土抗压强度、钢筋屈服强度、配筋率和混凝土保护层厚度对钢筋混凝土墙变形和耐火极限的影响规律.研究结果表明,受火过程中,钢筋混凝土墙在无侧向荷载且轴压比或墙厚度...     

Influence of supplementary cementitious materials on engineering properties of high strength concrete

The influence of supplementary cementitious materials (SCMs), namely silica fume, metakaolin, fly ash and ground granulated blast-furnace slag, on the engineering properties of high strength concrete (HSC) has been investigated in this study. Workability, compressive strength, elastic modulus, porosity and pore size distribution were assessed in order to quantify the effects of the different materials. The results show that the inclusion of the different SCMs has considerable influence on the workability of HSC. Silica fume and metakaolin significantly enhanced the strength of HSC. Fly ash reduced the early-age strength; however, it enhanced the long-term strength of the HSC. Likewise, ground granulated blast-furnace slag impaired the early-age strength, but marginally improved the long-term strength at low replacement levels. The general effect of the different SCMs on the elastic modulus of HSC is rather small compared to their effect on strength. There are good correlations between both static and dynamic moduli and compressive strength. The EC 2 and ACI 209 provide a good estimate of static modulus of elasticity from compressive strength, while the BS8110 gives a good estimate of static modulus of elasticity from dynamic modulus of HSC containing the different SCMs. Porosity and pore size were reduced with the addition of the different SCMs. The volume of mesopores in the ranges of <15 nm and 15 – 30 nm was notably increased for HSC containing SCMs, whereas the percentage of macropores was significantly reduced.     

Compressive strength loss and reinforcement degradations of reinforced concrete structure due to long-term exposure

Degradations due to long-term weathering actions on a reinforced concrete structure were investigated. Compressive strength and reinforcement corrosion developments of a prototype RC structure were monitored for 6 years using destructive and non-destructive tests which include periodic coring, compressive strength, rebound hammer, ultrasonic pulse velocity, carbonation, half-cell and tensile strength tests. Eventually, results have shown that more than a quarter of peak compressive strength can be lost within 5 years of continuous exposure. Corrosion of the exposed bars within the range of the testing period was also observed to be quite alarming. Thus, defects caused by prolonged actions of environmental factors may pose serious threats on the integrity of partially completed structures especially abandoned projects.