N. Gowripalan

Chloride diffusivity of concrete cracked in flexure

Abstract It has been recognized that corrosion of steel in cracked concrete is affected by both the surface crack width and the concrete cover thickness. The crack width/cover ratio (Wcr/C) can be a suitable parameter to consider in relation to the durability performance of a cracked reinforced concrete. A linear relationship was observed when plotting the chloride threshold level against C/Wcr. It appears that the threshold level can be related to Wcr/C by a hyperbolic relationship. The effect of Wcr/C on the chloride threshold level appears to be more pronounced as this ratio is decreased. The Australian Standard, AS 3600, does not give any guidance on the allowable crack width at serviceability for reinforced concrete structures, except for the ‘deemed to comply’ rules. From the viewpoint of durability, a crack width limitation in AS 3600 is necessary in addition to the cover thickness, to minimize Wcr/C. Using Wcr/C=0.01, in this study, the effect of tensile steel area on the chloride diffusivity in the tension and compression zones of concrete cracked in flexure was investigated. The apparent chloride diffusion coefficient (Da) in the tension zone was found to be higher than in the compression zone. When the tensile steel area was doubled, a significant decrease in the Da of the compression zone was observed. This could be attributed to the reduction in the porosity of the concrete in compression, which impedes diffusion process. In contrast, a marginal increase in the Da of the tension zone was observed.

Microcracking and chloride permeability of concrete under uniaxial compression

Abstract In the previous studies on microcracks and rapid chloride permeability tests, microcracks were quantified in terms of total crack length. This was carried out by examining concrete slices after compression tests. No attempts have been made to characterise the microcracks during the compression test prior to the chloride permeability test. In the present study, concrete cylinders were loaded under uniaxial compression between 30% and 95% of the ultimate strength. A non-destructive method of microcrack evaluation was used to study the progressive microcracking in concrete cylinders during compression tests. After the compression test, a rapid chloride permeability test (RCPT) (ASTM C1202) was carried out on a specimen cut from the same cylinder. The total crack length was also determined from the same specimen to compare with the observed microcracking behaviour, assessed by the non-destructive testing. The characteristics of the microcracks in terms of the specific crack area are different when a concrete is under a load and when it is completely unloaded. The chloride permeability of a concrete (after it was unloaded) appears to be influenced by the occurrence of a certain stress level known as the critical stress. When the critical stress is exceeded in a concrete specimen, a comparatively large chloride permeability was measured. Where the critical stress in a specimen is not exceeded, the increase in the permeability is marginal in spite of the large increase in microcracks.

Chloride-ion induced corrosion of galvanized and ordinary steel reinforcement in high-performance concrete

An experimental investigation has been carried out to assess the effectiveness of the use of high-performance concrete (HPC) and galvanized steel in reducing reinforcement corrosion. Two normal strength concrete (NSC) mixtures with 28-day compressive strengths of 30 and 40 MPa and two high strength concrete (HSC) mixtures with compressive strengths of 50 and 80 MPa were used for this study. The rapid chloride ion penetration test was used to study the ion penetration and the results are compared with the results of long-term immersion tests in 4% NaCl solution over a period of 1 year. No correlation between the results of these two tests could be established. Half-cell potential measurements were used to monitor the initiation of corrosion. The pH of HPC pastes and mortars were monitored for 90 days to study the effect of silica fume on pH of concrete and corrosion initiation. The results showed that HPC reduced chloride ion penetration significantly. Silica fume at 10% replacement level reduced the pH of concrete from 14.00 to 12.8 over a period of 90 days.

DESIGN OF DISTURBED REGIONS IN REACTIVE POWDER CONCRETE BRIDGE GIRDERS

This paper details the benefits of using reactive powder concrete (RPC) to carry bursting forces in prestressed bridge girders. Tests were conducted on 3 RPC, 150 MPa, deep beams with results reported herein. A finite element analysis of a 35-m prestressed RPC bridge girder is also provided. The most important potential advantages of using RPC in bridge engineering are described.

Experimental Investigation of Drying Shrinkage and Creep of Concrete Using Fibre-Optic Sensors:

Drying shrinkage and creep are two of the most important time-dependent properties of concrete, and the health monitoring of any large-scale concrete structure is practically the monitoring of the combined effects of these two physical properties. We present a fibre-optic sensing technique using fibre Bragg grating (FBG) sensors for the experimental investigation of drying shrinkage and creep of structural grade (40 MPa) concrete. It offers many advantages over the conventional electrical and mechanical methods for both structural health monitoring and standard tests of the mechanical properties. The FBG sensors are directly embedded into concrete specimens and the data are obtained by an optical measurement system. Standard mechanical method, as specified by the Australian Standards, is also used for comparison. Good agreements between the two methods are achieved for both drying shrinkage and creep. The long-term behaviours are predicted from the experimental data, and the accuracies of prediction are discussed. This optical technique can yield a strain resolution of better than 3 μɛ.

STRENGTH PROPERTIES OF RAFFIA BAMBOO

The results of tensile and bending tests carried out on specimens of raffia bamboo are presented. Raffia bamboo are natural materials commonly used for the construction of buildings and household furniture in southeastern Nigeria. The test results show that the material is capable of attaining an ultimate tensile strength of about 150 n/sq mm and a modulus of elasticity in tension of about 24 kn/sq mm. These strength properties compare favourably with those of most natural timber species and also some oriented polymers used in fibre reinforced concrete. (Author/TRRL)

Modified polymer bar reinforcement for concrete elements

Abstract High strength polymer bars produced from thermoplastics were investigated with a view to using these as reinforcement, mainly for semi-structural concrete elements. This paper describes some of the properties of these bars and their possible applications.

Elasto-plastic damage modelling of beams and columns with mechanical degradation

Within the context of continuum mechanics, inelastic behaviours of constitutive responses are usually modelled by using phenomenological approaches. Elasto-plastic damage modelling is extensively used for concrete material in the case of progressive strength and stiffness deterioration. In this paper, a review of the main features of elasto-plastic damage modelling is presented for uniaxial stress-strain relationship. It has been reported in literature that the influence of Alkali-Silica Reaction (ASR) can lead to severe degradations in the modulus of elasticity and compression strength of the concrete material. In order to incorporate the effects of ASR related degradation, in this paper the constitutive model of concrete is based on the coupled damage-plasticity approach where degradation in concrete properties can be captured by adjusting the yield and damage criteria as well as the hardening moduli related parameters within the model. These parameters are adjusted according to results of concrete behaviour from the literature. The effect of ASR on the dynamic behaviour of a beam and a column are illustrated under moving load and cyclic load cases.