V. Sirivivatnanon

Effect of different supplementary cementitious materials on mechanical properties of high performance concrete

High performance concrete prepared from general purpose (GP) portland cement and various supplementary cementitious materials are increasingly finding their use in construction worldwide. This study was undertaken to compare mechanical properties as well as fresh concrete properties of concretes containing silica fume, ground granulated blast furnace slag (slag), fly ash and GP portland cement. The aim of the study was to enable evaluation of the suitability of a particular binder system for an application based on fresh concrete properties and mechanical properties. Concrete mixes were prepared with GP portland cement, high slag cement and slag cement, and also mixes were prepared with the addition of silica fume and fly ash. The work focussed on concrete mixes having a fixed waterbinder ratio of 0.35 and a constant total binder content of 430 kg/m3. Apart from measuring fresh concrete properties, the mechanical properties evaluated were development of compressive strength, flexural strength, elastic modulus, and strain due to creep and drying shrinkage. Results indicated that the addition of silica fume to GP portland cement concrete marginally decreased the workability of the concrete but significantly improved the mechanical properties. However the effect of addition of silica fume to high slag cement concrete was less pronounced.

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.

FIELD INDICATOR OF CHLORIDE PENETRATION DEPTH

Chloride-induced corrosion of steel reinforcement is causing serious damage to many concrete structures. A number of methods to evaluate the chloride penetration into concrete have been developed. The most common practice in measuring the chloride profile is very time consuming. A simple colourimetric method of measuring the depth of chloride penetration into concrete by spraying a 0.1-N AgNO3 solution is very attractive. But some questions have been raised about its sensitivity. In this article, the results from colourimetric tests and the corresponding quantity of chloride detected at the colour-change boundary, determined for more than 70 concrete samples, are given. The magnitude and variation of chloride concentrations are compared with those reported in the literature and evaluated in relation to typical chloride threshold values. In addition, a relationship between the colourimetric penetration depth and the charge passed during testing to ASTM C1202 is shown.

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.

Role of permeability in sulphate attack

Abstract The role of permeability in sulphate attack was evaluated in this study. Resistance to sulphate attack was measured by determining the expansion caused in concrete specimens with exposure to 5% Na2SO4 solution. Concrete specimens were prepared from five binders, namely: ordinary Portland cement (OPC), high slag cement (HSC), sulphate-resistant cement (SRC), OPC with 7% silica fume (SF) and HSC with 7% SF. Concrete of grades 35 and 40 were used. The expansions of concrete samples were compared to their permeabilities to establish the role of permeability in controlling the expansion due to sulphate attack. It was found that the relative performance of concretes cannot be explained by either their permeability only or by only the chemical resistance of the binder. However, by combining the information on permeability and the chemical resistance of binder, the relative performance of concretes can be estimated. Thus, both permeability and the type of binder play an important role in sulphate attack.

CHARACTERISTIC SERVICE LIFE FOR CONCRETE EXPOSED TO MARINE ENVIRONMENTS

Abstract A statistical treatment has been applied to a deterministic service life model of concrete structures in marine environments. The chloride ingress model based on Ficks second law of diffusion was assumed. The quality of concrete was quantified in terms of three factors, namely, an apparent diffusion coefficient at 1 year ( D a ), surface chloride concentration ( C s ) and a critical chloride level ( C cr ). The standard deviation of service life can be estimated from standard deviations of the four factors, namely, C s , D a , C cr and cover depth. The effect of the severity of environment on service life was also demonstrated. With data from the literature and an approximation of the inverse error function, sensitivity analyses were carried out. Service life was found to be more sensitive to cover depth than the diffusion coefficient, and more sensitive to surface chloride concentration than the critical chloride level. Characteristic service life of a range of normal Portland cement (NPC) concrete grades was evaluated as a function of 28-day strength and cover depths for a nominated confidence level. Such characteristic service life can be readily used and appreciated by design engineers.

METHODS FOR THE DETERMINATION OF WATER PERMEABILITY OF CONCRETE

Two test methods have been successfully used to determine the water permeability of different concretes. The methods are based on the determination of coefficient of permeability using either a constant flow or a depth of penetration technique. The flow method has generally been found to suit concretes with higher permeability, while the penetration method is used for concretes with very low permeability. Presently no clear guidelines exist for the selection of the appropriate method for a particular type of concrete. This study was carried out to examine the correlation between the two methods. A broad guideline has also been established for the selection of the appropriate method for a particular concrete with respect to its binder composition, 28-day compressive strength, and age. The concretes examined were prepared from five types of binders and with a grade range of 35-50 MPa.

CHLORIDE-INDUCED STEEL CORROSION IN CONCRETE: PART 1--CORROSION RATES, CORROSION ACTIVITY, AND ATTACK AREAS

This paper presents results from a major long-term study on chloride-induced steel corrosion in concrete. The performance of a set of 50 reinforced concrete slabs made with a range of portland and blended cement binders was evaluated. Two portland cements, a high C3A and a low C3A, a blended fly ash cement, and a blended blast furnace slag cement were used. All reinforced concrete slabs were exposed to high chloride conditions by partial immersion in a 3% NaCl solution modeling seawater conditions. The reinforcement was cleaned and weighed prior to inclusion into the concrete slabs. Periodic nondestructive measurements of concrete performance included half cell potential monitoring, concrete resistivity, and electrochemical measurements of rates of corrosion of steel in concrete using potentiodynamic anodic procedures. Individual slabs were broken for reinforcement recovery at predetermined times during the study. Measurements were made of the area of corrosion and the weight loss of steel through corrosion. This information was related to the nondestructive data acquired on the slabs. In Part 1, estimated corrosion current (Ic) data and areas under the Ic versus time envelope for reinforcement within concrete slabs is presented. These data were obtained using potentiodynamic anodic polarization techniques. Measurements of the reinforcement corroded area in concrete are also presented and related to the electrochemical measurements taken. It was found that concrete water:binder ratio significantly influenced the corrosion rate of steel in concrete. Relationships were established between corrosion activity and the area of chloride-induced steel corrosion. Observed relationships were different for reinforcement within portland cement concretes and blended cement concretes considered.

Corrosion of steel in concrete with and without silica fume

Abstract A corrosion cell based on constant impressed voltage of 400 mV was developed to study corrosion rate of steel in concrete of different strength levels and in concrete with different silica fume contents. The results indicate that corrosion rate of steel embedded in saturated concrete can be reduced by increasing concrete strength, particularly in chloride contaminated environment. The use of 10 percent silica fume as cement replacement material in making 70 MPa concrete can have beneficial effect in terms of reduced corrosion rate. Adverse effect may results from using silica fume at higher replacement level of 20 percent.

CHLORIDE-INDUCED STEEL CORROSION IN CONCRETE: PART 2--GRAVIMETRIC AND ELECTROCHEMICAL COMPARISONS

This is the second of a two-part paper covering research into chloride-induced steel corrosion in concrete. The work focused on relationships between electrochemical data on chloride-induced reinforcement corrosion and gravimetric steel weight losses. Data were collected on a series of reinforced concrete slabs that were partially immersed in 3% NaCl solution for a period of 5 years. Slabs were made with a range of water-binder ratios (w/b) with portland cements having high and low C3A contents, slag-blended cement, or fly ash-blended cement. Data focused on measurements over time of concrete resistivity, corrosion rates, and gravimetric weight losses of steel taken at the end of the exposure period. Rates of corrosion of steel in concrete were measured using potentiodynamic anodic procedures. An analysis of estimated corrosion currents (I sub c) and the area under the I sub c versus time envelope for reinforcement within the concrete slabs is described. Measurements of weight loss of steel through corrosion in concrete are analyzed and related back to the electrochemical measurements taken. It was found that concrete w/b highly influenced the corrosion rate of steel in concrete. Quantitative links between steel weight loss, the electrochemical data, and concrete resistivity have been found. Under high-chloride conditions, the blended cement concretes having low w/b were found to perform better than other concretes investigated. The data suggest that such concretes had higher resistivity characteristics, had lower corrosion rate characteristics, and were likely to result in lower reinforcement weight losses when compared with equivalent portland cement concretes. Reinforced concrete performance under high-chloride conditions did not reflect concrete strength data from the materials considered. Results provide some guidelines for the design of durable concrete structures.