International Concrete Abstracts Portal

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.

This paper reports results of an investigation in which the effect of moisture condition of concrete at the time of application of silane on permeation and durability characteristics of concrete was determined. T h e permeation properties (i.e. air permeability and sorptivity) were measured with the Autoclam permeability system, an apparatus developed at Queen’s University of Belfast. The durability characteristics were determined in terms of chloride penetration and carbonation. Also, a comparison of the performance of silane was made with that of a controlled permeability formwork system. The results indicated that the moisture condition of concrete at the time of application of silane does not affect its protective properties to any significant degree. Silane’s performance was comparable with, or better than, that of the controlled permeability formwork in terms of sorptivity and chloride penetration at three moisture conditions included in the investigation. However, silane was found to have relatively little benefit in terms of carbonation resistance in accelerated tests, surface tensile strength and air permeability of the near surface concrete. The use of controlled permeability formwork resulted in improvements in these properties as well.

The results on the relatively high level of protection provided to the top steel in a reinforced concrete slab when the bottom steel is just adequately protected are presented in this paper. The steel reinforcement corrosion was accelerated with the use of chloride contaminated concrete. The chloride gradient within the specimen was created by inserting a relatively higher chloride bearing macrocell. Current reversal technique was used to monitor the corrosion protection level. The results showed that the presence of a higher chloride gradient requires a higher protection current density and potential. An instant-off potential of 632 mV CSE and a 4-hour decay potential of 100 mV were sufficient to protect steel reinforcement against corrosion in chloride rich concrete (19 kg/m3). The protection level at the top steel is significantly high when the bottom steel is just adequately protected from the anode source located above the top steel. In a chloride bearing concrete with the chloride gradients of 3.0 and 1.5, the top steel received 103 and 59 mA/m2 cathodic protection current densities compared to the current densities of 42 and 31 mA/m2, respectively required to protect the bottom steel. The instant-off potential of 730 mV CSE and the decay potential of 128 mV for the top steel also indicate a relatively high level of protection of the top steel. In a two-layer steel reinforced concrete slabs, it is necessary to develop a mechanism, specially in new construction, to facilitate the flow of current to the bottom steel. The current reversal technique used in the test was effective in determining the protection level against corrosion.

The electrical chloride test was developed some years ago and is carried out by driving chlorides through concrete samples using electric fields with high voltages up to 60 V. In the test, the total current passing through the sample in a few hours is used as an estimate of the transport properties of the chlorides in the concrete. This test has major advantages that it is rapid, and can be used insitu; and, it has been accepted by the ASTM. It has been criticised in the literature and has, for example, been found to give misleading results when pozzolanic materials are present. It has been pointed out by the author that if silica fume is added to the concrete, the current falls during the test. However, in plain concretes it normally rises. This paper is based on the premise that the current test procedure only makes use of part of the available data . Therefore, it provides only part of the possible results for evaluation and analysis. By analys in the shape of the current-time curve, mu’c h more information about the constituents and properties of the concrete may be obtained. For the data presented in this paper, a large number of samples were tested and computer analysis of the shape of the current-time transients was used to identify the causes of the different attributes of the current-time transients. This analysis is of particular importance in Europe where new Eurocodes for cement permit addition of pozzolanic materials to almost all cements.