P. S. Mangat

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.

Absorption characteristics of concrete as a function of location relative to casting position

Abstract Three different concrete mixes were prepared, the control mix in which no cement replacement material was added, and mixes where 22% and 9% by weight of cement was replaced with fly ash and silica fume respectively. Mixes were cast in 100mm cube moulds and cured at 20 °C and 45 °C using a variety of different curing regimes with respect to relative humidity and curing time to simulate concretes in hot and temperate climates. After curing, sliced samples were taken from various locations (faces) of the cube to determine their absorption. Two methods were used to study the absorption characteristics, the shallow immersion and the capillary rise. A large variation in absorption values existed between the upper surface during casting of the concrete cubes, the base and the sides. The absorption value of the complete unsectioned cube (100 mm) is similar to that of sectioned side face. A near-linear relationship exists between the two absorption methods.

Influence of superplasticizer and curing on porosity and pore structure of cement paste

Abstract Most concrete produced today contains admixtures. Superplasticizers (SP) are used for the purpose of improving workability and reducing the water to cement ratio; therefore producing more durable concrete. SP cause better dispersion even at high water to cement ratio. Although SP improves the dispersion of particles, it is not quite clear how the addition of SP affect the porosity and pore size distribution of cement paste. The purpose of this study was to examine the influence of one type of SP on porosity and pore size distribution under different curing regimes. Paste specimens with and without SP were prepared at constant water to cement ratio of 0.45. Specimens were cured for 28 days and some for six months. Specimens were exposed to high temperature (45°C) and normal temperature curing (20°C) and also subjected to different relative humidities (∼100%, 55% and 25%). Curing at high temperature was carried out to simulate temperature in hot climates. Tests on porosity and pore size distribution were conducted using mercury intrusion porosimetry. The results show that the inclusion of SP decreases the total intruded pore volume of paste. The dominant pore diameter, however, does not seem to be affected and the percentage of pores smaller than 100 nm increases in the presence of SP.

Influence of elastic modulus on stress redistribution and cracking in repair patches

The paper presents the results of a field investigation of repairs to two highway bridges. The repairs were applied by spraying onto unpropped compression members of the bridges. Two categories of commercial repair materials were used, low stiffness materials relative to the substrate (Erm Esub). The repair materials also represented a range of other properties such as strength, density, shrinkage, and creep. The results show that repairs applied with relatively stiff materials, Erm > Esub, display efficient structural interaction with the structure. High stiffness repairs are effective in redistributing shrinkage strain to the substrate and attracting external loading in the long term. Low stiffness repair materials (Erm < Esub) are much more likely to undergo tensile cracking due to restrained shrinkage. Low stiffness repairs are ineffective in redistributing strain. 2000

Effect of initial curing on chloride diffusion in concrete repair materials

Abstract This paper presents the results of an experimental investigation on the effect of initial curing on chloride diffusion in three generic repair materials and a concrete mix of similar grade. Two field conditions of initial curing were simulated as follows: (1) exposure to a chloride environment after 24 h of casting to simulate repair situations, for example in the tidal zone of marine structures where repairs cannot be protected from tidal exposure for long periods; and (2) 28 days of curing at 20°C, 55% relative humidity to simulate field conditions where the repair patches are exposed to the ambient environment without any prolonged protected curing. For comparison purposes identical specimens were cured in water at 20°C for 28 days before exposure to the chloride environment. Chloride concentration profiles in the materials were determined after 28, 90, and 180 days of exposure. Regression analysis of the experimental data was carried out applying Ficks second law of diffusion to determine the diffusion coefficients (DC) and chloride concentration on the surface (C0). Based on the results obtained, an analytical expression for the long-term prediction of chloride concentration in repair materials is given. The results show a high rate of chloride diffusion in the cementitious repair materials containing polymer latex additives relative to plain concrete. The effect of inadequate initial curing on chloride penetration (e.g., exposure to chlorides after 24 h of casting or 28-day air curing) is more pronounced in concrete mixes than in generic repair materials.

Influence of PFA, slag and microsilica on chloride induced corrosion of reinforcement in concrete

Abstract Steel reinforcement electrodes embedded in different matrices of concrete were exposed to simulated marine splash zone exposure for about 600 days (1200 cycles) after initial curing in air for 14 days. The corrosion potentials and polarisation resistance were monitored at regular intervals to determine the state and rate of corrosion. The concrete matrices included different cement replacement levels of pfa, ground granulated blast furnace slag (BFS) and microsilica, at a water/(cement+blend) ratio of 0.58. The Cl− and OH− concentration in pore fluid and the acid soluble Cl− in concrete were also determined. The results show that maximum protection against rebar corrosion is provided at 60% replacement of cement by BFS and at 10 and 15% replacement by microsilica. The corrosion rates are more sensitive to Cl− concentration in the matrix than to the Cl−/OH− ratio.

Corrosion resistance of steel fibres in concrete under marine exposure

This is the final paper of a series (1, 2, 3) which have reported different aspects of a long term study on the marine durability of steel fibre reinforced concrete (sfrc). Two mixes, one with and one without pfa were reinforced with three types of steel fibres. The cement content of the mixes was 430 and 590 kg/m3 respectively. Prism specimens of these mixes were cured under marine exposure, both in the laboratory and at Aberdeen beach, for up to 2000 wet-dry cycles (1200 days). The state of corrosion of the steel fibres was investigated visually and by electrochemical analysis of fibres exposed at fractured surfaces of specimens after flexural testing. The results show that the generally accepted activation level of 0.4% Cl− by weight of cement does not apply to sfrc. Similarly, the threshold value of 0.61 for the (Cl−)(OH−) ratio, as proposed by Hausemann for initiation of corrosion, is not valid to steel fibre reinforced concrete. No corrosic of fibres embedded in concrete was evident at Cl− and (Cl−)(OH−) levels greatly exceeding the above values.

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.

REPAIR MATERIAL PROPERTIES WHICH INFLUENCE LONG-TERM PERFORMANCE OF CONCRETE STRUCTURES

The paper presents the results of an experimental investigation to determine some typical properties of three commercially available generic repair materials, which are of significance to the subsequent structural behaviour of repaired concrete members. It also compares the performance of these materials with plain concrete mixes of similar strength and stiffness, which are also suitable for repair applications. Data on strength, stiffness, shrinkage and creep under different temperature and humidity exposure are presented. In addition, water permeability coefficients of the materials have been determined. The results show that shrinkage of the repair materials is significantly greater than shrinkage of normal concrete. The shrinkage of the specially formulated repair mortars, especially those modified with a polymer admixture, is very sensitive to relative humidity of exposure compared with normal concrete. The permeability of normal concrete is similar to that of both the high performance non-shrinkable repair material and the polymer modified repair material. The other repair material based on a mineral cement binder is more permeable than normal concrete. The inclusion of aggregates improves the mechanical properties and dimensional stability of repair materials.

Chloride diffusion in steel fibre reinforced concrete containing PFA

Abstract The paper presents chloride diffusion characteristics of a steel fibre reinforced OPC-pfa concrete mix which was manufactured by replacing 26 per cent of ordinary Portland cement with pfa. Steel fibre reinforced marine mixes generally require high cement contents of the order of 590 kg/m 3 and use of pfa results in equivalent practical mixes of a more reasonable cement content. A mix of proportions by weight of 0.26 (pfa) : 0.74 (OPC) : 1.51 : 0.84 with a water/(OPC+pfa) ratio of 0.4 was reinforced with three types of steel fibres. Uncracked and pre-cracked prism specimens were cured under simulated splash zone exposure in the laboratory and at Aberdeen beach, after initial dry curing in the laboratory. The results show that the period of initial dry curing has an insignificant effect on Cl − diffusion in concrete. Cl − concentrations are higher in the OPC-pfa mix in comparison with the marine mix based only on OPC. Most of the Cl − penetration occrs within 150 marine cycles (110 days) of exposure and Cl − concentrations increase significantly in the vicinity of wider cracks.