Corrosion of steel reinforcement in concrete of different compressive strengths

Corrosion of steel bars embedded in concrete having compressive strengths of 20, 30 and 46 MPa was investigated. Reinforced concrete specimens were immersed in a 3% NaCl solution by weight for 1, 7 and 15 days. In order to accelerate the chemical reactions, an external current of 0.4 A was applied using portable power supply. Corrosion rate was measured by retrieving electrochemical information of polarization technique. Pull-out tests of reinforced concrete specimens were then conducted to assess the corroded steel/concrete bond characteristics.Experimental results showed that corrosion rate of steel bars and bond strength between corroded steel/concrete were dependent on concrete strength and accelerated corrosion period. As concrete strength increased from 20 to 46 MPa, corrosion rate of embedded steel decreased. First day of corrosion acceleration caused a slight increase in steel/concrete bond strength, whereas sever corrosion after 7 and 15 days of corrosion acceleration significantly reduced steel/concrete bond strength. Visual and metallographic observation of steel bars removed from concrete samples after testing revealed that the severity of corrosion reactions and reduction of steel bar diameter increased as the corrosion acceleration period increased. Presence of localized corrosion pits as well as severe corrosion grooves of steel bars was confirmed after 7 and 15 days of corrosion acceleration, respectively.     

Experimental evaluation of the flexural behavior of corroded P/C beams

Corrosion phenomena and related effects, such as size reduction in both rebars and strands, bond decay at steel–concrete interface, and cracking in the surrounding concrete, are particularly critical in prestressed-concrete members, not only for safety reasons, but also for their huge potential socio-economic effects. As a matter of fact, this technique has been used for the last 50 years in the majority of viaducts and bridges built in many countries like Italy.In order to evaluate the influence of the corrosion on prestressed pretensioned beams, a number of tests has been carried out in the Laboratory of the University of Rome “Tor Vergata”.Nine prestressed beams (section size 200 × 300; total length 3000 mm; clear span 2700 mm) were first subjected to artificial corrosion, to obtain different damage levels, and then were tested in four-point bending.The results clearly show the sizable effects that corrosion has on the ultimate capacity (that is significantly reduced), on the failure mode and on the structural response, that turns from ductile to brittle.     

A study on improvement of durability of reinforced concrete structures mixed with Natural Inorganic Minerals

As a fundamental study on the corrosion resistance of reinforced concrete structures using Natural Inorganic Minerals exposed to carbonation environment. The test specimens were concrete(W/C = 60%) with Natural Inorganic Minerals content of 0% and 10%. Accelerated arbonation and autoclave corrosion accelerated curing were then conducted with them. The corrosion resistance of steel in concrete with Natural Inorganic Minerals content of 0% and 10% was examined by corrosion form, half-cell potential, polarization resistance, corrosion area and weight loss after 24 h of autoclave corrosion accelerated curing.The results of the study showed that as for steel in concrete with Natural Inorganic Minerals content of 10%, the corrosion resistance was more excellent than steel in concrete with Natural Inorganic Minerals content of 0%.     

Use of tribometer to estimate interface friction and concrete boundary layer composition during the fluid concrete pumping

The friction at the steel–concrete interface (on the boundary layer) which occurs when concrete flows in a drain plays an important role in its pumpability. This friction is directly related to the composition of the formed boundary layer, which depends on the concrete composition parameters. In order to highlight the relationship between concrete composition and interface friction, the authors initially developed and validated an apparatus (called “tribometer”) allowing steel–concrete interface friction measurements and deduce interface parameters. It is simple to use at building sites. The paper highlights the rigorous relationship between the boundary layer (BL) composition and the concrete composition. The results show that the BL is formed by water, cement and fine sand particles of concrete with a diameter lower than 0.25 mm. It was also shown that the water–cement ratio of the BL and that of the concrete are almost the same. The relative enrichment of fine sand in the BL decreases with the increase of the fine sand voluminal proportion in the concrete.     

Lateral deformation of RC beams under simultaneous load and steel corrosion

Cracking of cover concrete due to steel corrosion is one of the clear physical indicators of loss of service life of corroding RC structures. Its prediction is therefore very important for service life modelling of these structures. Models developed to predict the time to cover cracking assume that stresses due to steel corrosion follow the principles of a thick-walled cylinder under internal pressure. Considering the errors in the models, this paper contests the applicability of the thick-walled cylinder approach to model the time to cover cracking as well as the rate of lateral expansion of concrete after cover cracking using experimental results from 12 RC beams (153 × 254 × 3000 mm) corroded under a sustained load. It is shown in the paper that, contrary to the assumptions of uniform expansion made in the thick-walled cylinder approach, before cracking of the cover concrete, tensile strains are applied on the face of beams where corrosion agents are drawn whilst other faces are in compression. Corroded steel coupons are used to verify that this variation of strains is caused by the corrosion process not being uniformly distributed around the steel bar. It is also shown in the paper how cracking and location of cracks affects the rate of lateral deformation of concrete due to steel corrosion.     

The importance of chloride content at the concrete surface in assessing the time to corrosion of steel in concrete structures

A literature review in the present study has shown that the chloride ions on the concrete surface exposed to a marine environment including tidal, splash and aerated zones can be accumulated then to increase with time. In order to take this effect into account in a model prediction of the rate of chloride ingresses in concrete, the surface chloride was expressed as a function of time such that (1) constant, (2) linear build-up, (3) square root build-up and (4) square root build-up with an initial set of the surface chloride (a refined model), at a given diffusion coefficient (2 × 10^?12 m²/s). As a result, the constant surface chloride model produced the greatest level of chloride penetration, as an initial set of the surface chloride content was overestimated. The rate of chloride ingresses derived from the highest surface chloride led to the greatest risk of chloride-induced corrosion of steel in concrete, whereas the linear and square root build-up models indicated 2–3 times longer duration of corrosion-free service life. The refinement of a surface chloride build-up suggested a more realistic prediction of chloride profile and corrosion risk, thereby imposing the rate of chloride ingresses between the constant surface chloride, and linear or square root build-up.     

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.     

Frost attack resistance and steel bar corrosion of antiwashout underwater concrete containing mineral admixtures

This study aims to evaluate frost durability and steel-bar corrosion in antiwashout-underwater concrete, which has been neglected to date. To achieve this goal, repeated freezing and thawing and accelerated steel-bar corrosion tests have been performed for three types of antiwashout-underwater concrete specimens.The results of repeated freezing and thawing test reveal that adding mineral admixtures has little effects on frost durability because of the large and uneven entrapped-air imprisoned by the cellulose-type antiwashout-underwater admixture. Slight improvement of frost durability was observed through the action of air-entrained (AE) agent in the case of SG50 which presented an air content of 6 ± 0.5%.Measurement results using the half-cell potential showed that, among the entire specimens, steel-bar in Control specimen manufactured under artificial seawater was the first one that exceeded the threshold value, −350 mV proposed by ASTM C 876, at 14 cycles, where the corresponding corrosion current density and concentration of water soluble chloride were measured as 0.3 μA/cm² and 0.258%, respectively. For the other specimens, potential values became below −350 mV later than 18 cycles.     

Effect of metakaolin and foundry sand on the near surface characteristics of concrete

This paper deals with the effect of foundry sand (FS) and metakaolin (MK) on the near surface characteristics of concrete. A control concrete having cement content 450 kg/m³ and w/c of 0.45 was designed. Cement was replaced with three percentages (5%, 10%, and 15%) of metakaolin weight, and fine aggregates were replaced with 20% foundry sand. Tests were conducted for initial surface absorption, sorptivity, water absorption and compressive strength at the ages of 35, 56, and 84 days.Test results indicated that with the increase in MK content from 5% to 15%, there was a decrease in the initial surface absorption, decrease in the sorptivity till 10% metakaolin replacement. But at 15% MK replacement an increase in sorptivity was observed. All mixtures showed low water absorption characteristic i.e. less than 10%. Compressive strength shared an inverse relation with sorptivity. Higher MK replacements of 15% are not helpful in improving inner core durability, even though it helps in improving surface durability characteristics. Inclusion of foundry sand resulted in reduction in compressive strength. This aspect cements the findings that addition of FS causes permeability of concrete to increase causing in an increase in sorptivity and water absorption of concrete.     

Study on the cause of pop-out defects on the concrete wall and repair method

Concrete pop-outs are a common occurrence in foreign countries; however, this has not been the case in Korea, until recently when pop-outs were observed on the concrete walls of an apartment. Our investigation revealed that the electric arc furnace (EAF) slag, as fine aggregates in the concrete caused the pop-out.This study discusses the cause that EAF slag as fine aggregates for concrete create the pop-outs, and an appropriate repair depth for deteriorated concrete as suggested by finite element (FE) analysis.Results show that the free CaO and free MgO in the EAF slag were primarily responsible for creating the pop-outs. These findings led us to conclude that the defect could occur to the depth of 34 mm in the concrete with 21 MPa strength as the EAF slag size was up to 5 mm. As an effective repair method for pop-outs, replacing concrete up to 23 mm in depth from the surface with polymer mortar of 50 MPa design strength was suggested, and the proposed method is verified through experimentation.     

Probabilistic assessment to predict the time to corrosion of steel in reinforced concrete tunnel box exposed to sea water

This study concerned the service life prediction, in terms of chloride-induced steel corrosion, of a concrete tunnel-box structure placed on seabed. To calculate the time to steel corrosion, the rate of chloride transport in the identical concrete mix to the structure was tested, and the chloride threshold level for corrosion was assumed by a literature review from 65 published data. Then, the Monte Carlo Simulation was used to calculate the probability of steel corrosion and its corresponding service life, assuming that the corrosion initiates at 10% of the probability of corrosion. As a result, the service life depended on the time dependency of chloride transport. The service life was equated to 31, 51, 85, and 147 years, at 0.1, 0.2, 0.3 and 0.4 of the age factors, respectively, while the time independent model indicated only 27 years of the service life. Finally, methods to raise the service life were discussed.     

Service life prediction of a concrete bridge structure subjected to carbonation

Carbonation-induced corrosion in concrete may often occur in a high carbon dioxide environment. In this study, the risk of carbonation of a concrete bridge in an urban area was evaluated by measuring the carbonation rate and concrete cover depth in three different parts: the sound, cracked and construction joint parts of cover concrete. The average carbonation rate was ordered by the sound > joint > cracked parts, and the concrete cover depth measured by an ultrasonic detector indicated the slightly greater value than the designed one (50.0 mm). Then, the carbonation-free service life at the depth of the steel was calculated, based on in situ information, by the safety factor method and the Monte Carlo simulation. The service life calculated by the two methods was mostly identical. The sensitivity of the carbonation rate and concrete cover depth to the time to carbonation at the depth of the steel was mathematically determined.     

The effect of thiosemicarbazide on corrosion resistance of steel reinforcement in concrete

This study describes a laboratory investigation of the influence of thiosemicarbazide (TSC) on the corrosion of reinforcing steel and the compressive strength of concrete. The effect of TSC on the corrosion resistance of steel reinforced concrete was evaluated by carrying out electrochemical tests in NaCl and NaCl + TSC solutions for 60 days. Polarisation resistance (R_p) values of TSC added reinforced concrete were much higher than those without TSC. Similarly, AC impedance spectra revealed that the resistance of TSC mixed electrodes were also quite higher than those without. The compressive strength of concrete specimens containing TSC was measured and an increase of 20–25% was observed.     

The inter-particle coefficient of friction at the contacts of Leighton Buzzard sand quartz minerals

A series of micro-mechanical tests was carried out in order to investigate the inter-particle coefficient of friction at the contacts of quartz minerals of Leighton Buzzard sand. For this purpose, a custom-built inter-particle loading apparatus was designed and constructed, the main features of which are described briefly in this paper. This apparatus is capable of performing shearing tests at the contacts of soil minerals of a particle–particle type in the range of very small displacements, from less than 1 μm to about 300 μm, and very small normal loads, between about less than 1 N and 15 N. The laboratory data showed that the effects of the normal force and the sliding velocity on the coefficient of dynamic friction are not significant, while dry and saturated surfaces had similar frictional characteristics. The steady state sliding was mobilized within a range of 0.5–3.0 μm of horizontal displacement, and the coefficient of static friction was very similar to the corresponding coefficient during constant shearing. Repeating the inter-particle shearing tests on the same particles and following the same shearing track indicated a small reduction in the inter-particle coefficient of friction after the first shearing, which is possibly related to plastic deformation and damage to the asperities.     

Behaviour of RC beams corroded under sustained service loads

This paper presents the results of an experimental study conducted to characterize the structural behaviour of reinforced concrete beams corroded whilst subjected to constant sustained service loads. Corrosion of tensile steel bars was induced by an accelerated corrosion process using a 5% solution of NaCl and a constant impressed current. Four RC beams were tested, each with a width of 153 mm, a depth of 254 mm and a length of 3000 mm. Beams were tested whilst under a load equivalent to 1%, 8% and 12% of the ultimate load. Longitudinal tensile and compressive strains were monitored during the corrosion process and used to determine the variation of the depth of the neutral axis, the curvature and the second moment of area of beams with the time of electrolysis. The results indicate that the longitudinal strains, the depth of the neutral axis and the curvature of beams depend on both the level of corrosion and the applied service load whilst the second moment of area is mostly influenced by the level of corrosion.     

The properties of concrete made with fine dune sand

This paper presents the results of a study that investigated the properties of concrete made with dune sand. Different control concrete mixtures using ordinary Portland cement (OPC) with a minimum design compressive strength of 40 N/mm² were prepared. The amount of fine aggregates constituted about 36% by weight of all the aggregates. The workability ranged from low of 16 mm to a high of 122 mm. For each control mix, other mixtures were prepared in which the fine aggregates were replaced by different percentages of dune sand ranging from 10% to 100%. The effect of dune sand on the workability, compressive strength, tensile strength, modulus of elasticity and initial surface absorption test (ISAT) was studied. Experimental results show an improvement in the workability of concrete when fine aggregates were partially replaced by dune sand. An increase in slump was measured with increase in dune sand content. However, at high dune sand contents (above 50%); the slump starts to decrease with an increase in dune sand. Generally, the strength values decrease with increase in dune sand replacement. The strength loss was not found considerable as the maximum reduction was less than 25% when fine aggregates were fully replaced by dune sand. The absorption characteristics of concrete made with OPC as measured by the (ISAT) generally increased with higher dune sand contents.     

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.     

Performance of generic and proprietary corrosion inhibitors in chloride-contaminated silica fume cement concrete

This paper presents the results of a study conducted to evaluate the improvement in corrosion-resistance of chloride-contaminated silica fume cement concrete due to the use of corrosion inhibitors. Three proprietary and one generic corrosion inhibitors were evaluated for their performance in inhibiting reinforcement corrosion in the silica fume cement concrete specimens contaminated with 0.4%, 1%, and 2% chloride concentration, by weight of cement. Some of the specimens were subjected to wetting and drying cycles and reinforcement corrosion was monitored by measuring corrosion potentials and corrosion current density. Another batch of concrete specimens was partially immersed in the chloride solution and reinforcement corrosion was accelerated by impressing an anodic potential of 2 V. The extent of corrosion increased with increasing chloride contamination in the concrete specimens. Incorporation of inhibitor generally decreased the rate of reinforcement corrosion. The rate of reinforcement corrosion in the concrete specimens incorporating an organic inhibitor that was added to the concrete during mixing was the least followed by that in the concrete specimens on which a penetrating corrosion inhibitor was applied. Further, the accelerated impressed current technique was noted to be suitable for quickly screening the performance of corrosion inhibitors.     

Placing conditions,mesostructural characteristics and post-cracking response of fibre reinforced self-compacting concretes

The benefits of adding fibres to concrete, evidenced in the post-cracking behaviour, are strongly influenced not only by the type and content of fibres but also by their orientation. The objective of this study is to evaluate the influence of the casting/placing procedure on the post-peak behaviour of fibre reinforced self-compacting concrete, and its relationship with the mesostructural characteristics of the material (type, distribution and orientation of fibres). Three concretes were prepared using two types of steel fibres of different lengths (50 mm and 30 mm) and a structural type polymer fibre. Beams of 150 × 150 × 600 mm were cast in three different ways: filling the moulds from the centre in accordance with the EN 14651 Standard, pouring concrete from one end of the mould after a flowing along a 5 m length and 150 mm diameter pipe, and finally, filling the moulds vertically. Flexural tests according to the European Standard indicate that the three types of fibres achieve a preferential orientation along horizontal planes, like in conventional vibrated fibre reinforced concrete. The mechanical response of beams cast with longer steel fibres was strongly affected by the casting procedure while the flexural performance of the other two fibre concretes, was less affected. Such results are well in accordance with the density of fibres measured by fibre counting in different cut planes.