Chloride diffusion in steel fibre reinforced marine concrete

The paper presents some results from a continuing research programme on the marine durability of steel fibre reinforced concrete. A mix of proportions by weight of 1 : 1.5 : 0.86 with a water/cement ratio of 0.4 was reinforced with three types of steel fibres. The cement content of the mix was 590 kg/m³. Uncracked prism specimens were cured under marine splash and tidal zone exposure in the laboratory and at Aberdeen beach. In one batch of prism specimens, flexural cracks of width ranging between 0.07 and 1.08 mm were induced prior to marine exposure. Chloride diffusion characteristics in uncracked and pre-cracked concrete were determined at up to 2000 cycles of marine exposure (1250 days).The results show that Cl⁻ concentrations are significantly greater in laboratory cured specimens relative to those cured on the beach. Most of the Cl⁻ penetration occurs within 150 tidal cycles of exposure at the beach. Cl⁻ concentrations increase with increasing crack widths although the influence of small crack widths of ⩽ 0.2 mm is marginal.     

Pore fluid composition under marine exposure of steel fibre reinforced concrete

The paper presents some results from a continuing study of the marine durability of steel fibre reinforced concrete. Two steel fibre 8 developed which were reinforced with three types of steel fibres. The cement content of the mixes was 430 kg/m³ and 590 kg/m³.Prism specimens of these mixes were cured under marine splash and tidal zone exposure, both in the laboratory and at Aberdeen beach, for up to 2000 wet-dry cycles (1250 days). The paper presents results of chemical analysis of the pore fluid extracted from different depths of these samples. The results show that Cl⁻ concentrations are markedly higher in specimens cured under laboratory marine spray cycles compared with beach cured specimens. The pH of the pore fluid is depressed in the surface zones of specimens and increases gradually with depth to values in excess of 13.     

Diffusion of Cs+ and Cl− through sealing materials

The engineering or sealing materials is an integral part of a repository design system for the disposal by isolation of nuclear wastes in geologic media. Cement, concrete, zeolites and clay minerals are some of the promising candidate borehole plugging or sealing materials. In order to evaluate the efficacy of these sealing materials, Cs⁺ and Cl⁻ diffusion studies were conducted on thin cured discs or cement and cement admixtures with zeolite or montmorillonite or silica fume or blast furnace slag. The cement admixtures are superior to straight cement in retarding the migration of Cs⁺ and Cl⁻. The montmorillonite blended cement performs in an intermediate fashion when compared to the straight cement and zeolite or silica fume bearing cements. The diffusions of Cs⁺ and Cl⁻ appear to be related to the pore structure as well as the crystal structures of the phases added or subsequently formed during curing. These results tend to suggest that the addition of 10–20% silica fume or zeolite phase to the portland cement may be very advantageous in resisting the migration of Cs⁺ and, to some extent, Cl⁻ ions across cement pastes.     

Reproportioning of steel fibre reinforced concrete mixes and their impact resistance

In this experimental investigation, a practical rapid method of proportioning steel fibre reinforced concrete (SFRC) mixes is developed and validated. The basis for developing this is to use the reproportioning method, which has already been developed for proportioning normal density cement concrete mixes, for SFRC mixes. Based on the results of the trial mix, two SFRC mixes having 28 day target strength of 30 and 50 MPa are designed using this technique and examined regarding its validation. In addition, the impact resistance of these reproportioned Plain Concrete (PC) and SFRC is studied at 7 and 28 days. It is observed that the SFRC has developed significant impact resistance even for a small addition of steel fibres. Pulse velocity test is conducted at different ages to assess the quality of concrete. It is found that all concrete specimens could be classified under good quality.     

Influence of high-temperature and low-humidity curing on chloride penetration in blended cement concrete

The influence of high-temperature and low-humidity curing on chloride penetration in concrete containing cement replacement materials was investigated. Three different mixes were studied: a control mix in which no cement replacement materials were added and two mixes where cement was partially replaced by 20% fly ash and 9% silica fume (by weight), respectively, at a constant water-to-binder ratio of 0.45. High-temperature curing was employed to simulate concrete temperature in hot climate. The results show that at early periods of exposure, initial curing has a substantial influence on chloride penetration in concrete. The effect of initial curing is much reduced after a long period of exposure. The chloride penetration at early ages of exposure is directly related to the porosity of the binder phase and the absorption of concrete. Higher chloride penetration resistance was observed when cement is partially replaced with either fly ash or silica fume.     

Alternative blended cement with ceramic residues: Corrosion resistance investigation on reinforced mortar

Blended cements are largely used for concrete: they are usually considered cements with a low environmental impact, as they require less clinker than ordinary Portland cement (OPC). Different constituents can be used as supplementary clinker component usually leading to cement with high resistance to outdoor environment. Polishing residue (PR), coming from porcelain stoneware tiles production, can be successfully used as new constituent for blended cement, however its action for enhancing the durability of cement matrix must be assessed. With this purpose, electrochemical tests (half cell potential, impressed voltage and linear polarization techniques) have been carried out on steel reinforced mortar samples, prepared using a 25% PR based cement and 100% OPC as binder and exposed to a 3.5% NaCl solution. The corrosion resistance results and microstructure analysis highlight better durability performances for PR based cement than those exhibited by OPC, mainly for curing time > 28 days.     

Residual mechanical properties of steel fiber reinforced concrete damaged by alkali silica reaction and subsequent sodium chloride exposure

Infrastructures treated with de-icing salts and those which are in direct contact with sea water are subjected to degradation by chloride ingress. Concrete composed of reactive sources of silica and used near such regions can suffer from both, alkali silica reaction (ASR) and chloride ingress subsequently. This research aims at empirically investigating the residual mechanical properties of plain and steel fiber reinforced concrete damaged by alkali silica reaction (ASR) and subsequent chloride ion ingress. Accelerated degradation tests on three concrete mixes such as plain concrete (PC,control), steel fiber reinforced concrete (SFRC) and high strength fiber reinforced concrete (HSFRC) were done. Specimens were initially damaged by ASR, and then submerged in chloride solution at temperature ranges of 5 ^oC, 25 ^oC and 40 ^oC. 1 mol/L NaOH solution and 3% NaCl solution were used for a period of 20 and 40 weeks. Steel fibers were found to be effective in reducing surface crack widths at 5 ^oC and 25 ^oC. Accelerated mortar bar test showed that steel fibers were able to reduce expansion by 31.5% and 65.3% using single and double hooked fibers. By examining the residual compressive and flexure strengths, it was found that exposure to chloride environment aided in hydration reaction which counter-balanced the damage due to ASR. Fiber-matrix bonding developed over time inducing friction which led to higher ductility and less damage in flexure strength in steel fiber reinforced concrete prisms.     

Compressive behavior of fiber reinforced high-performance concrete subjected to elevated temperatures

In this paper, the effects of elevated temperatures on the compressive strength stress–strain relationship (stiffness) and energy absorption capacities (toughness) of concretes are presented. High-performance concretes (HPCs) were prepared in three series, with different cementitious material constitutions using plain ordinary Portland cement (PC), with and without metakaolin (MK) and silica fume (SF) separate replacements. Each series comprised a concrete mix, prepared without any fibers, and concrete mixes reinforced with either or both steel fibers and polypropylene (PP) fibers. The results showed that after exposure to 600 and 800 °C, the concrete mixes retained, respectively, 45% and 23% of their compressive strength, on average. The results also show that after the concrete was exposed to the elevated temperatures, the loss of stiffness was much quicker than the loss in compressive strength, but the loss of energy absorption capacity was relatively slower. A 20% replacement of the cement by MK resulted in a higher compressive strength but a lower specific toughness, as compared with the concrete prepared with 10% replacement of cement by SF. The MK concrete also showed quicker losses in the compressive strength, elastic modulus and energy absorption capacity after exposure to the elevated temperatures. Steel fibers approximately doubled the energy absorption capacity of the unheated concrete. They were effective in minimizing the degradation of compressive strength for the concrete after exposure to the elevated temperatures. The steel-fiber-reinforced concretes also showed the highest energy absorption capacity after the high-temperature exposure, although they suffered a quick loss of this capacity. In comparison, using PP fibers reduced the energy absorption capacity of the concrete after exposure to 800 °C, although it had a minor beneficial effect on the energy absorption capacity of the concrete before heating.     

Penetration profile of chloride ion in cracked reinforced concrete

A detailed observation on the penetration profile of chloride ions through and around a crack in reinforced concrete structures was carried out. Electron probe microanalysis (EPMA) and colorimetric tests were conducted on cracked specimens, which were exposed to NaCl solution at a temperature of 20 °C and a humidity of 60% RH, after being conditioned in the same condition for 2 months. Research parameters included water to cement ratio (w/c), single and multicracks, exposed direction, crack width, NaCl solution concentration and cover thickness. Increasing w/c led to a higher ingression rate of Cl⁻ ions, not only from the exposed surface but also around the cracks. It was found that the penetration depth from the surface of the cracks was equal to or slightly higher than that from the exposed surface for higher w/c mixes of 0.45 and 0.65. The transportation of Cl⁻ ion was strongly influenced by the bulk movement of the solution inside the concrete.     

A test method for measuring chloride diffusion coefficients through partially saturated concrete. Part II: The instantaneous plane source diffusion case with chloride binding consideration

A test method is proposed for measuring chloride diffusion coefficients through partially saturated concrete specimens with well characterized water contents. It includes an experimental procedure for supplying a limited amount of Cl⁻ to the tested concrete surface, and two mathematical models for processing the experimental Cl⁻ content profiles obtained at selected diffusion times. The use of the more refined model, taking into account the chloride binding by concrete, allows to increase the reliability of the determined diffusion coefficients. For the two tested Portland cement concretes, (water/cement ratios 0.6 and 0.5), the Cl⁻ diffusion coefficient decreases about two orders of magnitude, from 6 · 10^− 12 to 2 · 10^− 14 m²/s, when the relative humidity of the atmosphere in equilibrium with concrete is lowered from 95% to 54% approximately.     

Comparative evaluation of steel mesh, steel fibre and high-performance polypropylene fibre reinforced shotcrete in panel test

In this study, experimental investigations were performed on steel mesh (SM), steel fibre (SF) and high-performance polypropylene fibre (HPPF) reinforced shotcrete (HPPFRS) panels to evaluate performance characteristics such as toughness, flexural ductility, energy absorption and load capacity. The panel tests, in accordance with European specification for sprayed concrete (EFNARC), were made on 18 prismatic specimens having the same mix designs and were cured for 28 days but reinforced with various fibres. In addition, the rebound characteristics of these mixes were determined to compare the actual in situ fibre contents.Test results show that all reinforcements, including HPPFs that are low-modulus fibres, greatly improved the flexural ductility, toughness, and load-carrying capacity of the brittle matrix. It was seen that there was a positive synergy effect between steel and polypropylene fibre in hybrid fibre usage from a performance point of view. According to results, it can be concluded that a hybrid polypropylene-SF can be used alternatively instead of SM and monosteel fibre as a reinforcement in shotcrete applications to get better efficiency in mechanical properties of composite.     

Calcium release from cement samples of different size

Leach tests on cement samples on the laboratory scale have been carried out to investigate the influence of the geometrical surface on the leach rate of cement constituents. Samples of hydrated Ordinary Portland Cement (OPC) 325¹ were made by blending clinker powder and distilled water at w/c = 0.35. The paste was cured at 60°C and 98% Relative Humidity for 11 days in 2 cm diameter polystyrene containers. Samples 1, 3 and 3.5 cm high were leached in distilled water at 50°C and at a geometrical surface-to-liquid volume ratio SA/V = 0.1 cm⁻¹. For the samples considered, the size had no significant effect on the values of the leach rate L_R in g cm⁻² d⁻¹ vs the leaching time.     

Properties of high-volume fly ash concrete incorporating nano-SiO2

This paper presents a laboratory study on the properties of high-volume fly ash high-strength concrete incorporating nano-SiO₂ (SHFAC). The results were compared with those of control Portland cement concrete (PCC) and of high-volume fly ash high-strength concrete (HFAC). Assessments of these concrete mixes were based on short- and long-term performance. These included compressive strength and pore size distribution. Significant strength increases of SHFAC compared to the high-volume fly ash high-strength were observed as early as after 3 days curing, and improvements in the pore size distribution of SHFAC were also observed. In this work, the hydration heat of nano-SiO₂ fly ash cement systems was also studied in comparison to the fly ash-cement systems and to the pure cement systems. In addition, the weight change of fly ash incorporating nano-SiO₂, fly ash, and nano-SiO₂ alone after immersed in saturated lime solution was also studied.     

Corrosion resistance and chloride diffusivity of volcanic ash blended cement mortar

This article reports the results of an investigation on the chloride diffusivity and corrosion resistance of volcanic ash (VA) blended cement mortars with varying curing times of up to 1 year. The mortars had 20% and 40% VA as cement replacement and water/binder ratio of 0.55. The accelerated chloride ion diffusion (ACID) test was used to calculate the chloride ion (Cl⁻) diffusion coefficient (D_i) of the mortars using the Nernst-Plank equation for steady state condition. In addition, electrical resistivity, mercury intrusion porosimetry and differential scanning calorimetry (DSC) tests were conducted. Electrochemical measurement such as linear polarization resistance was used to monitor the corrosive behavior of the embedded steel bars. The chloride ingress into the mortars was also studied. Good correlations were found among D_i, total pore volume (TPV) and electrical resistivity of the mortars. It was also found that blending cement with VA significantly reduced the long-term D_i and hence increased the long-term corrosion resistance of mortars. This fact was also supported by the presence of lower quantity of Ca(OH)₂ and higher quantity of Friedel's salt in the VA blended mortars as observed from the DSC tests. Mortars with 40% VA showed better performance in terms of Cl⁻ diffusivity, chloride ingress and passivation period of embedded steel compared with the control mortar with 0% VA.     

Effect of short fibers on residual permeability and mechanical properties of hybrid fibre reinforced high strength concrete after heat exposition

Mechanical and permeability performance of fibre reinforced high strength concrete after heat exposition were evaluated in the experimental study. Cylindrical concrete specimens were exposed to heat with the rate of 10 °C/min of up to 400 °C. In order to study the effect of short fibres on residual performance of heated high strength concrete, polypropylene and steel fibres had been added into the concrete mix. The melting and vaporization of its fibre constituents were found to be responsible for the significant reduction in residual properties of polypropylene fibre reinforced high strength concrete. In terms of non-destructive measurement, UPV test was proposed as a promising initial inspection method for fire damaged concrete structure. Furthermore, the effect of hybrid fibre on the residual properties of heated fibre reinforced high strength concrete was also presented.     

Influence of initial curing on sulphate resistance of blended cement concrete

The paper presents results of an investigation on the effect of initial curing conditions on the sulphate resistance of concrete made with ordinary portland cement and using pfa, silica fume and ground granulated blast furnace slag for partial replacement of cement. In addition, porosity and pore structure analysis of representative pastes was carried out to examine the relationship between these properties and sulphate resistance of concrete. The depth of carbonation in specimens of pastes was also determined.

Three different initial curing conditions immediately after casting of specimens were adopted, namely: WET/AIR CURED at 45°C, 25% RH; AIR CURED at 45°C, 25% RH; AIR CURED at 20°C, 55% RH. The results show that pore volume and pore structure of the paste bear no direct relationship with the sulphate resistance of concrete. The presence of a carbonated layer on the surface is generally accompanied by superior sulphate resistance—there are, however, important exceptions. Low humidity curing at high temperature (45°C) results in higher depths of carbonation but lower sulphate resistance than similar curing at 20°C.

The sulphate resistance of concrete increases with the replacement of cement with 22% pfa, 9% silica fume and 80% ggb slag. The sulphate resistance also increases due to drying out of concrete during early curing at low relative humidity and due to carbonation. The possible common factor which leads to this improved sulphate resistance is the reduced Ca(OH)₂ content which leads to smaller volume of the expansive reaction products with sulphate ions. The effect of initial curing at high temperature (45°C) is significantly harmful to the sulphate resistance of plain concrete but much less so to the blended cement concretes.     

PFA in structural precast concrete: Engineering properties

The effects are investigated of including pulverized-fuel ash (pfa) on the strength development (compressive and tensile) and deformation behaviour (elastic, creep and shrinkage) of concretes subjected to initial accelerated curing and tested at ages ranging from 18 hours to 1 year. It is shown that (for the test conditions and materials used) concretes containing pfa perform as well as, or better than, concretes containing rapid-hardening portland cement (RHPC) alone and it is suggested that suitable blends of pfa and RHPC can be confidently specified for use in precast prestressed concrete.     

A test method for measuring chloride diffusion coefficients through nonsaturated concrete: Part I. The instantaneous plane source diffusion case

A test method is proposed for measuring chloride diffusion coefficients through nonsaturated concrete specimens with controlled water contents. The experimental setup used allows one to supply an initial limited amount of Cl⁻ to the tested concrete surface. The procedure consists of submitting the surface of concrete specimens to interaction with the products of combustion of PVC, which contain mainly gaseous hydrogen chloride. This interaction yields a limited Cl⁻ contamination of the concrete surface. After returning the specimens to their controlled humidity exposure conditions, the kinetics of Cl⁻ transport from the surface inwards may be studied. The experimental Cl⁻ concentration profiles determined at selected time intervals have been adjusted to a diffusion model of “instantaneous plane source,” which takes into account the particular initial and boundary conditions of the experimental procedure, for obtaining the corresponding diffusion coefficients.     

The combined action of Mg2+ and Cl− ions in cement pastes

In order to study the phenomenon of seawater attack on hydrated cement components, we focused our interest on the combined action of Mg²⁺ and Cl⁻ ions on hydrated cement pastes. Thus, cement pastes were prepared from portland cement and its mixture with 30% pozzolan (Santorin Earth). These pastes were cured in baths of varied concentrations of Mg²⁺ and Cl⁻ ions and stored in 18 ± 2°C. The hydration phenomena were studied in these cement pastes, by XRD and SEM.     

Combined effects of mineral admixtures and curing conditions on the sorptivity coefficient of concrete

The effect of mineral admixture and curing condition on the sorptivity of concrete are investigated. In the present work, the maximum particle size and the grading of coarse aggregate, the cement content and water/cement ratio of the concrete are kept constant. Then, in the ordinary Portland cement (OPC) 42.5 concrete, a portion of the sand is replaced by a mineral admixture such as fly ash (FA), limestone filler, sandstone filler or silica fume (SF). This paper presents the results of both the sorptivity coefficient and the compressive strength of OPC 42.5 concretes with these mineral admixtures, and concretes with OPC 32.5, blended cement (BC) or trass cement (TC). The results obtained indicate that the sorptivity coefficient of concrete decreases as the compressive strength of concrete increases. It is also shown that the sorptivity coefficient of concrete is very sensitive to the curing condition. The effect of curing condition on the sorptivity coefficient of concrete seems to be higher in low-strength concretes.