Influence of corrosion crack patterns on the rate of crack widening of RC beams

Corrosion crack widths are often used by structural engineers in the field to predict level of steel corrosion as well as residual load-bearing capacities of corroding RC structures. This paper presents further work on this matter but with focus on corrosion crack patterns and how they affect rate of crack widening. It is based on results from a research where 17 quasi-full-scale (153 × 254 × 3000 mm) RC beams were corroded under various levels of sustained loads. The rate of widening of corrosion crack widths was found to be very much dependent on crack patterns. Deformation of cover concrete under each crack pattern was discussed. It was found that at maximum crack widths below 0.6 mm, the majority of beams exhibited nearly similar crack patterns as well as rate of widening of corrosion cracks. A mass loss of steel of 1% corresponded to a maximum crack width between 0.14 and 0.22 mm. At large crack widths (>0.6 mm), various beams exhibited very different rates of crack widening. It was shown that at crack widths above 0.6 mm, to be conservative an increase in mass loss of steel of 1% corresponded to corrosion crack widening of 0.02 mm.     

Assessment of the effects of rice husk ash particle size on strength,water permeability and workability of binary blended concrete

This study develops the compressive strength, water permeability and workability of concrete by partial replacement of cement with agro-waste rice husk ash. Two types of rice husk ash with average particle size of 5 micron (ultra fine particles) and 95 micron and with four different contents of 5%, 10%, 15% and 20% by weight were used. Replacement of cement up to maximum of 15% and 20% respectively by 95 and 5 μm rice husk ash, produces concrete with improved strength. However, the ultimate strength of concrete was gained at 10% of cement replacement by ultra fine rice husk ash particles. Also the percentage, velocity and coefficient of water absorption significantly decreased with 10% cement replacement by ultra fine rice husk ash. Moreover, the workability of fresh concrete was remarkably improved by increasing the content of rice husk ash especially in the case of coarser size. It is concluded that partial replacement of cement with rice husk ash improves the compressive strength and workability of concrete and decreases its water permeability. In addition, decreasing rice husk ash average particle size provides a positive effect on the compressive strength and water permeability of hardened concrete but indicates adverse effect on the workability of fresh concrete.     

Damage to cement concrete pavements due to exposure to organic compounds in a cold region

Pop-out and disaggregation of aggregate in a 1-year old cement concrete pavement originally mixed with air-entraining (AE) water-reducing agent was observed after the pavement had been exposed to ethylene glycol based snow-melting agent on the surface in the winter. The study used: gas chromatography–mass spectrometry (GC–MS) tests, ¹H Nuclear Magnetic Resonance (NMR) tests, X-ray fluorescence analysis, emission spectral analysis (ICP), elution tests in anion type surfactant solution conducted for mortar and aggregate taken from the cement concrete where pop-out had occurred, as well as samples made by cement paste in the laboratory. Tests of the tensile strength, thermal-stress, and three-dimensional crack analysis by micro-focus computerized tomography (CT) scanner were conducted for specimens (2.5 × 2.5 × 10 cm) taken from the cement concrete where pop-out had occurred and with cement concrete samples made in the laboratory. Microscope observations and Electron Probe Micro Analyzer (EPMA) analysis were conducted for thin samples (2.5 × 2.5 cm and 20 μm thick) taken from the cement concrete where pop-out had occurred. The tests results showed that organic compounds contained in the cement reacted with the cement during the hardening process, generating cracks and gel in the cement paste. It was established that these caused the pop-out of the aggregate, together with the effects of the ethylene glycol based snow-melting agent that the cement concrete had been exposed to. No pop-out or disaggregation of aggregate were found in cement concrete at a repaired section, at the same location, with aggregate of low absorbing water ratio in this cold region and in place for 2 years.     

Investigation of mixing ratio and restraint condition effects on alkali–silica gel pressure in mortar by using a uniaxial alkali–silica gel pressure measuring device (ASGPM-D)

A pressure of alkali silica gel which is the product of alkali–silica reaction causes pattern crack development and ultimately failure in cement based materials. This paper examines the measurement of alkali–silica gel pressure in cylindrical mortar specimen by a new test device (alkali–silica gel pressure measuring device, ASGPM-D) and investigates also the effects of the test medium on test results. Specimens of various sizes and geometries were tested to develop standard specimen size for ASGPM test method. The most suitable specimen with diameter of 35 mm and 250 mm in length was selected by optimizing the experimental results. Furthermore, the highest alkali–silica pressure value was recorded on a specimen which has 0.45 w/c and 74–150 μm grain size in an 80 °C 1 N NaOH solution. Also, swelling pressures of four reactive aggregates (opal, chert, basalt and chalcedony) were determined under two distinct restraint stages (1.5 kN and 3 kN).     

Permeability of high-performance concrete subjected to elevated temperature (600 °C)

Permeability is one of the most important parameters to quantify the durability of high-performance concrete. Permeability is closely related with the spalling phenomenon in concrete at elevated temperature. This parameter is commonly measured on non-thermally damaged specimens. This paper presents the results of an experimental investigation carried out to study the effect of elevated temperature on the permeability of high-performance concrete. For this purpose, three types of concrete mixtures were prepared: (i) control high-performance concrete; (ii) high-performance concrete incorporating polypropylene fibres; and (iii) high-performance concrete made with lightweight aggregates. A heating–cooling cycle was applied on 160 × 320 mm, 110 × 220 mm, and 150 × 300 mm cylindrical specimens. The maximum test temperature was kept as either 200 or 600 °C. After the thermal treatment, 65 mm thick slices were cut from each cylinder and dried prior to being subjected to permeability test. Results of thermal gradients in the concrete specimens during the heating–cooling cycles, compressive strength, and splitting tensile strength of concrete mixtures are also presented here. A relationship between the thermal damage indicators and permeability is presented.     

The effect of conditioning to a predetermined weight loss on the permeability of concrete

The durability properties of concrete can be assessed via gas permeability tests. The moisture content of the concrete plays a major role in its permeability to gas, and the conditioning stage of any gas permeability test aims to minimise and standardise the amount of moisture present in the concrete via oven drying, usually at a temperature of 105 °C. However, this is known in some circumstances to take a considerably longer period of time in comparison to the permeability testing itself. This study examines whether a relationship exists between the moisture content of a specimen and its permeability by investigating the permeability of concrete specimens dried by varying amounts. It was found that a simple relationship of the form y = ax^b could be established for both normal and high strength concrete subjected to pressure decay-time permeability testing.     

Heat transfer and pressure drop characteristics of the underfloor air distribution system

This paper reports experimental data for forced convection heat transfer in a heat exchanger, which is a key part of the underfloor distribution system. The experiments were performed to study heat transfer and pressure drop characteristics of tested units with different geometry of flow ducts. The heat exchanger was made of Plexiglas and PVC. The top side of plastic exchanger was covered by a layer of a concrete screed with thickness of 0.035 m. The bottom side was insulated with 0.1 m of polystyrene. The length of the tested units was 1 m and the width 0.5 m. The fluid in the heat exchanger was air. The results of experimental study were presented in a dimensionless form. The average Nusselt number and Fanning friction factor were expressed in terms of the Reynolds number. The experimental results presented in this paper provide useful information for designing of an underfloor air distribution system in the residential housing sector.     

Interfacial interactions in concretes with silica fume and SBR latex

This paper deals with the effect of silica fume and styrene-butadiene latex (SBR) on the microstructure of the interfacial transition zone (ITZ) between Portland cement paste and aggregates (basalt). Scanning electron microscope (SEM) equipped with energy dispersive X-ray analysis system (EDX) was used to determine the ITZ thickness. In the plain concrete a marked ITZ around the aggregate particles (55 μm) was observed, while in concretes with silica fume or latex SBR the ITZ was less pronounced (35–40 μm). However, better results were observed in concretes with silica fume and latex SBR (20–25 μm).     

Rate of placement discussion for the validation of experimental models of fresh concrete lateral pressure in columns

An analysis of experimental models for predicting fresh concrete lateral pressure in columns was carried, raising the issue of a fundamental parameter like the rate of placement.Experimental data obtained from different authors were utilised for comparing the adequacy of the different theories making a division into three zones: rates of placement under 3 m/h, between 3 m/h and 10 m/h and over 10 m/h.The results were established not only in the different zones but also taking into account safety and adequacy. To determine the most suitable model according to rate of placement, level of control, vigilance, planning and inspection of the construction, eight models proposed under different standards or by different authors were compared with respect to the hydrostatic pressure produced by a liquid with concrete density.The influence of concrete yield stress, evaluated from slump test is also analyzed; the study shows a low correlation between concrete yield stress and the maximum pressure vs. hydrostatic pressure ratio.     

Rates of placement discussion for the validation of experimental models of fresh concrete lateral pressure in walls

An analysis of experimental models for predicting fresh concrete lateral pressure in walls and bases was carried out, including a comparison of three rate of placements.Experimental data obtained from the bibliography were used for comparing the adequacy of the models, making a division into three placement rates ranges: under 2 m/h, between 2 m/h and 10 m/h and over 10 m/h.The comparisons were established not only in the different ranges but also taking into account the level of construction control, vigilance and inspection, considering in all cases safety and adequacy for better formwork design.The influence of concrete yield stress, evaluated from slump test, is also analyzed; the study shows a low correlation between concrete yield stress and the ratio maximum pressure vs. hydrostatic pressure for low rates of placement. On the other hand for higher rates of placement, the correlation increases.     

Water absorption,permeability, and resistance to chloride-ion penetration of lightweight aggregate concrete

This paper presents an experimental study to evaluate effect of cumulative lightweight aggregate (LWA) content (including lightweight sand) in concrete [water/cement ratio (w/c) = 0.38] on its water absorption, water permeability, and resistance to chloride-ion penetration. Rapid chloride penetrability test (ASTM C 1202), rapid migration test (NT Build 492), and salt ponding test (AASHTO T 259) were conducted to evaluate the concrete resistance to chloride-ion penetration. The results were compared with those of a cement paste and a control normal weight aggregate concrete (NWAC) with the same w/c and a NWAC (w/c = 0.54) with 28-day compressive strength similar to some of the lightweight aggregate concrete (LWAC). Results indicate that although the total charge passed, migration coefficient, and diffusion coefficient of the LWAC were not significantly different from those of NWAC with the same w/c of 0.38, resistance of the LWAC to chloride penetration decreased with increase in the cumulative LWA content in the concretes. The water penetration depth under pressure and water sorptivity showed, in general, similar trends. The LWAC with only coarse LWA had similar water sorptivity, water permeability coefficient, and resistance to chloride-ion penetration compared to NWAC with similar w/c. The LWAC had lower water sorptivity, water permeability and higher resistance to chloride-ion penetration than the NWAC with similar 28-day strength but higher w/c. Both the NWAC and LWAC had lower sorptivity and higher resistance to chloride-ion penetration than the cement paste with similar w/c.     

Parametrical static analysis on group studs with typical push-out tests

Group studs are known as shear connectors in steel and concrete composite structures. By now, many composite bridges have been characterized by long lateral cantilevers. The shear studs are actually under biaxial action consisting of shear force and action in light of lateral bending moment on concrete slab induced by long cantilever and passing by moving loads. Moreover, lateral bending moment may even lead to the initiation of bending-induced concrete cracks. These two situations can both affect mechanical performance of group studs. Thus, a parametrical FEM analysis was carried out, in which damage plasticity was introduced to simulate material nonlinear behavior. In the analysis, lateral bending moments respectively inducing maximum concrete crack widths of 0.1 mm and 0.2 mm, shank diameters of 13 mm, 16 mm, 19 mm and 22 mm and stud heights including 80 mm and 100 mm were parameters. It was found that mechanical behavior of group studs with large shank diameter would be less affected by biaxial action and initial bending-induced concrete cracks seemed unfavorable to stud shear stiffness. On the other hand, typical push-out tests were executed to investigate reductions of shear stiffness and shear capacity of group studs. The reliability of FEM analysis was also verified based on the tests. In addition, stud shear capacity evaluations according to several design specifications were presented. It indicated shear capacity evaluation of Eurocode 4 got a relatively large safety factor. Moreover, the applicability of these specifications for group studs on shear capacity evaluation was also discussed.     

A comparative experimental investigation of concrete,reinforced-concrete and steel-fibre concrete pipes under three-edge-bearing test

Three-edge-bearing and crack size measurement tests were carried out on plain concrete, reinforced-concrete, and steel-fibre concrete pipes of 500 mm diameters. The average three-edge-bearing strength and crack size of steel-fibre concrete pipes having steel fibres of RC80/60-BN type at a dosage of 25 kg/m³ turned out to be 82% greater and 47% smaller than those of plain concrete pipes, and 6% greater and 15% smaller than those of reinforced-concrete pipes, respectively. Tests on those steel-fibre concrete pipes having a steel fibres dosage of 40 kg/m³ revealed that a steel fibres dosage of 25 kg/m³ seems to be close to optimum because a 60% increase in the amount of steel fibres engenders only minor improvements. By these findings, steel-fibre concrete pipes are more economical than and mechanically and physically superior to reinforced-concrete pipes.     

Study on early-age cracking of cement-based materials with superplasticizers

The addition of superplasticizers is an important approach to prepare high performance cement-based materials. The effect of polynaphthalene series superplasticizer (PNS) and polycarboxylate type superplasticizer (PC) on early-age cracking and volume stability of cement-based materials was investigated by means of multi-channel ellipse ring shrinkage cracking test, free shrinkage and strength test. The general effect of PNS and PC is to increase initial cracking time of mortars, and decrease cracking sensitivity of mortars. As for decreasing cracking sensitivity of mortars, PC > H-UNF (high-thickness-type PNS) > C-UNF (common-thickness-type PNS). To incorporate superplasticizers is apparently to increases free shrinkage of mortars when keeping the constant W/B ratio and the content of cement pastes. As for the effect of controlling volume stability of mortars, PC > C-UNF > H-UNF. Maximum crack width of mortars with PC is lower, but the development rate of maximum crack width of mortars with H-UNF is faster in comparison with control mortars. Flexural and compressive strength of mortars and concretes at 28 days increased with increasing superplasticizer dosages under drying conditions. C-UNF was approximate to H-UNF, but PC was superior to PNS in the aspect of increasing strength of cement-based materials.     

Effects of steam curing on strength and porous structure of concrete with low water/binder ratio

A research program was carried out to investigate the effects of duration of initial steam curing at atmosphere pressure on compressive strength of concrete with low water/binder ratio. The results showed that the compressive strength of samples steam cured for 5, 10, 14 h increased, while it decreased distinctively for sample steam cured for 24 h. Mercury intrusion porosimetry (MIP) method and scanning electron microscope-backscattered electron (SEM-BSE) image analysis technique were adopted to measure the corresponding variation of porous characteristics caused by the increasing duration of steam curing. The changes in coarse porosity and total porosity calculated by SEM-BSE image analysis and MIP method respectively could indicate the relationship between porosity and mechanical properties of the concrete subjected to different duration of steam curing. Compared with total porosity obtained by MIP method, the coarse porosity by SEM-BSE image analysis was in better accord with the compressive strength because the coarse pores measured by SEM-BSE image analysis were larger than 0.5 μm and included not only the interconnected pores but also the closed ones. An empirical model was developed to evaluate the influence of duration of initial steam curing on the compressive strength of concrete. By comparison, the measured compressive strength was in great accordance with the compressive strength calculated by the proposed model.     

Effect of micro-silica loading on the mechanical and acoustic properties of cement pastes

This study aimed at investigating the role of ultra fine sand (UFS) in enhancing the mechanical and acoustic properties of cementitious pastes. The microstructural origin of these properties was also identified and compared to the conventional materials. The maximum particle size of the UFS used was 100 μm (100% passing) while 50% of the UFS had less than 20 μm in diameter. Ordinary Portland cement (OPC) was partially substituted by UFS at 1%, 2%, 3%, 4%, 5%, 7.5% and 10% by weight of binder. The blended compounds were prepared using the standard water of consistency. Test samples with dimension of 20 × 20 × 20 mm and 40 × 40 × 160 mm were cast for compression and bending strengths tests, respectively. Circular samples with diameters of about 100 and 29 mm and average thickness of about 30 mm were used for sound absorption tests. All samples were kept in molds for 24 h, and then de-molded and allowed to cure in water for 28 days. The specimens were dried at a temperature of 105 °C for 24 h in an oven before testing. It was found that as the loading of UFS increases both the compressive and bending strength increase up to about 5% UFS loading, then a decrease in these properties was observed. This can be attributed to the pozzolanic effect of UFS resulting in enhancing the chemical reaction between free lime in cement and silica producing more hydration products that makes the paste more homogeneous and dense. In addition, the dispersed UFS has improved the filling effect allowing denser packing of the paste. These dense microstructural features were captured by scanning electron microscope (SEM) examination of the 5% UFS modified compound. The results also showed that, the sound absorption and noise reduction coefficient (NRC) for modified cement paste decreases with the increase of UFS up to 5% and this may be due to the decrease in porosity. However, the NRC began to increase at UFS loadings of 7.5% and 10% due to the increase in the porosity of the compounds.     

Stress-dependence of the permeability and porosity of sandstone and shale from TCDP Hole-A

We utilize an integrated permeability and porosity measurement system to measure the stress dependent permeability and porosity of Pliocene to Pleistocene sedimentary rocks from a 2000 m borehole. Experiments were conducted by first gradually increasing the confining pressure from 3 to 120 MPa and then subsequently reducing it back to 3 MPa. The permeability of the sandstone remained within a narrow range (10^?14–10^?13 m²). The permeability of the shale was more sensitive to the effective confining pressure (varying by two to three orders of magnitude) than the sandstone, possibly due to the existence of microcracks in the shale. Meanwhile, the sandstone and shale showed a similar sensitivity of porosity to effective pressure, whereby porosity was reduced by about 10–20% when the confining pressure was increased from 3 to 120 MPa. The experimental results indicate that the fit of the models to the data points can be improved by using a power law instead of an exponential relationship. To extrapolate the permeability or porosity under larger confining pressure (e.g. 300 MPa) using a straight line in a log–log plot might induce unreasonable error, but might be adequate to predict the stress dependent permeability or porosity within the experimental stress range. Part of the permeability and porosity decrease observed during loading is irreversible during unloading.     

Impact resistance of steel fibrous concrete containing fibres of mixed aspect ratio

The paper presents results of an investigation conducted to study the impact resistance of steel fibre reinforced concrete containing fibres of mixed aspect ratio. An experimental investigation was planned in which 108 plain concrete and SFRC beam specimens of size 100 × 100 × 500 mm were tested under impact loading. The specimen incorporated three different volume fractions i.e. 1.0%, 1.5% and 2.0% of corrugated steel fibres. Each volume fraction incorporated mixed steel fibres of size 0.6 × 2.0 × 25 mm and 0.6 × 2.0 × 50 mm in different proportions. The drop weight type impact tests were conducted on the test specimens and the number of blows of the hammer required to induce first visible crack and ultimate failure of the specimen were recorded. The results are presented in terms of number of blows required as well as impact energy at first crack and ultimate failure. It has been observed that concrete containing 100% long fibres at 2.0% volume fraction gave the best performance under impact loading.     

Inhibition of counterflow methane/air diffusion flame by water mist with varying mist diameter

In the present study, the effect of fine water mist on extinguishment of a methane–air counterflow diffusion flame was investigated to understand the underlying physics of fire extinguishment of highly stretched diffusion flame by water mist. Twin-fluid atomizers were used to generate polydisperse water mist of which Sauter mean diameters were 10, 20, 40, and 60 μm. When water mist is not added, the critical stretch rate at extinguishment is 439 s⁻¹ as compared to the theoretical value of 460 s⁻¹. For the case with water mist addition, when the stretch rate is small enough, almost all the water mist evaporates within the flame zone. On the other hand, for high stretch rate case, large mist droplets pass through the flame zone and can reach the stagnation plane. However, no oscillatory motion was found around the stagnation plane. Critical stretch rate at extinguishment decreases monotonously with the mass fraction of water mist independently of the mist diameter within the range of D₃₂ from 10 μm to 60 μm. On the other hand, with increase in the surface area parameter, the critical stretch rate at extinguishment decreases rapidly and becomes less sensitive at large surface area parameter, of which tendency is qualitatively in good agreement with theoretical predictions. For a constant surface area parameter, the critical stretch rate decreases with mist diameter because the mass fraction of water mist should increase in proportion to the mist diameter to keep the surface area parameter constant. When the water mist evaporates completely in the flame zone as in the present study, the mass fraction of the water mist is the dominant factor for fire extinguishment, rather than the surface area parameter. Therefore, an appropriate combination of stretch rate and water mist mass fraction should be provided to suppress effectively a given fire with a small amount of water mist.     

Effect of fiber type and content on bleeding of steel fiber reinforced concrete

In general, the addition of fibers in concrete mix significantly improves many of the engineering properties of concrete. On the other hand, steel fibers reduce the workability of concrete. This paper presents the effect of steel-fiber length (aspect ratio) and content on bleeding of steel fiber reinforced concrete (SFRC). Two different steel fiber types (both is hooked-end) were used at a ratio of 0% (control), 0.3%, 0.64%, 1% and 1.3% by volume. Slump, Ve-Be test, air content and unit weight were determined experimentally. Specimens were poured in the standard moulds and the bleeding water content was measured 30 min, 60 min, 90 min, 120 min, 150 min and 180 min after starting the test. The results indicated that the workability of concrete significantly reduced as the fiber dosage rate increases. This was assessed through standard slump test and Ve-Be consistometer test. The bleeding water content was increased by increase of the fiber volume fraction and fiber aspect ratio according to experimental results. Also, a bleeding coefficient value for SFRC made with and without steel fiber was proposed as a result of this study.