International Concrete Abstracts Portal

Changes to the New Zealand concrete design standard incorporate requirements for durability, based on a minimum design life of 50 years for structural elements as required under the New Zealand Building Code. Many New Zealand cities are near the coastline. and concrete quality and reinforcement covers are designed to control chloride induced reinforcing steel corrosion. Four exposure classifications in the Standard require increasing protection from chlorides based on increasing exposure to a marine environment. The paper outlines how these exposure classifications were established. The concrete structures standard specifies minimum concrete strength and reinforcement cover based on the use of normal portland cement concretes for each classification. The enhanced durability performance of some blended cement concretes is recognized along with the role of concrete coatings, and alternative combinations are permitted provided the designer establishes equivalent performance. A BRANZ research program is targetted towards developing an assessment methodology in the laboratory for evaluating the durability performance of blended cement concretes against normal portland cement concretes. The first stage of a laboratory program evaluating the performance of normal Portland, slag, silica fume and flyash cement blend concretes is reported. Evaluation methods used included absorption, rapid chloride ion penetration, chloride diffusion and chloride ponding. Further research using site surveys of concrete structures in the near coastal zones is planned.

High Performance Concrete (HPC) has been developed primarily for its strength having a compressive strength in excess of 70 MPa, approximately double the strength of conventional concrete. The porosity of this type of concrete, especially when silica fume is added to the mix, much lower than conventional mixes. It is, therefore, assumed that high performance concrete provides significantly better protection against corrosion of reinforcing steel than conventional concrete, but this has not yet been substantiated. A field exposure program is underway with cast-in-place and pre-cast samples, containing embedded reinforcing steel probes exposed to pulp & paper industry effluent at two locations on Vancouver Island, B.C., Canada. The cast-in-place samples were loaded in three-point bending prior to exposure to initiate cracks in the location of the corrosion probes. Probes were also embedded in uncracked areas of the beams. One lower quality concrete, one industrial standard concrete, one HPC and one HPC containing silica fume were included in the test matrix. Parallel laboratory corrosion studies are being conducted on cast-in-place specimens exposed to a simulated effluent. The initial results of the field exposure showing the difference in corrosion protection of reinforcing steel in the four different types of concrete subjected to accelerated curing and conventionally cured are presented. Laboratory results investigating the effect of cracking on the corrosion protection provided by the four types of concrete are also presented. The reinforcing steel corrosion was evaluated using linear polarization resistance (LPR) and the more recently developed electrochemical noise measurement technology. The suitability of these techniques to measure and monitor the corrosion of reinforcing steel in concrete is also discussed.

In this report, freezing and thawing test, carbonation test and length change test were carried out, determine the effects of differences in cement kind and curing methods on the durability of super-workable concrete, focusing mainly on placing during cold weather. Normal portland cement and three kinds of low heat type cement were used. A w/c of 50% was used for the normal portland cement, and a ratio of between 3O~35% for the low heat type cement. The curing methods of the specimens are standard curing, atmospheric curing, site sealed curing, site water curing and heat curing. During freezing and thawing test and accelerated carbonation tests, it was found that when heat curing is employed to prevent initial frost damage, if due consideration is not given to the temperature and wetness conditions of the curing concrete, there are cases where durability may be worsened instead of improved. With regard to the measurement of length change by the test methods adopted in current standards, there is a distinct possibility that the measurement values are not only due to drying shrinkage, but are also strongly influenced by autogeneous shrinkage of the concrete

Polymer modified cementitious materials are used in construction for applications such as bridge deck overlays and concrete repair. When using this type of material a wet-dry curing regime is usually recommended in order to give optimum mechanical properties. However, such a curing regime is contrary to that which would be expected for a low porosity surface layer, desirable, e.g., for good resistance to chloride ingress. This paper presents the results of investigations into the influence of curing conditions on the surface porosity of polymer modified cements and its influence on chloride diffusivity. Small cement paste prisms were cast and the top faces exposed to: wet, wet-dry, and wet-dry-wet curing regimes. Pore size distributions were then determined for the top, middle and bottom layers using mercury intrusion porosimetry. Results showed that for all mixture proportions the wet-dry curing regime resulted in a surface layer which was more porous and had a coarser pore structure than the deeper layers The extent of this effect depended on: actual curing regime, W/C, and type of polymer latex used. Results were confirmed by determining the effective diffusivity of chloride ions in similar samples.

The effects of a new draining formwork material made from polypropylene fibres on the properties of concretes of different composition were determined. The cements used were portland cement, blast-furnace slag cement, and supersulphated cement. Besides, the addition of air-entraining admixtures, and the use of the draining formwork material were also modified. In addition to the compressive strength other parameters influencing the durability of concrete were examined, i.e. the microstructure of the concrete, gas permeability, imperviousness to water, chloride-ion penetration, depth of carbonation, air void parameters and the frost and frost-deicing salt scaling resistance. In fresh portland cement concrete the effect of the draining formwork material on the water/cement ratio was determined at various depths of the concrete. In some cases the parameters which are important for the evaluation of the durability of concrete structures were considerably improved. A particularly advantageous result is that even concretes containing cements rich in granulated slag and made without air-entraining admixtures may achieve a high frost-deicing salt scaling resistance when this formwork material is used. The use of a draining formwork material is to be recommended when the requirements on the durability of concrete are very high.