International Concrete Abstracts Portal

The influence of some pozzolanic additions—such as silica fume, fly ash and ultra-fine amorphous colloidal silica (UFACS)—on the performance of superplasticized concrete was studied. Superplasticized mixtures in form of flowing (slump of 230 mm) or self-compacting concretes (slump flow of 735 mm) were manufactured all with a water-cement ratio as low as 0.44, in order to produce high-performance concretes (HPC). They were cured at room temperature (20°C) or steam-cured at 65°C in order to simulate the manufacturing of pre-cast members. Concretes with ternary combinations of silica fume (15-20 kg/m3), fly ash (30-40 kg/m3) and UFACS (5-8 kg/m3) perform better—in terms of strength and durability—than those with fly ash alone (60 kg/m3) and approximately as those with silica fume alone (60 kg/m3). Due to the reduced avail-ability of silica fume on the market, these ternary combinations can reduce by 60-70% the needed amount of silica fume for each pre-cast HPC element at a given performance level. Moreover, at later ages the strength reduction in steam-cured concretes with respect to the corresponding concretes cured at room temperature, is negligible or much lower in mixtures with the ternary combinations of pozzolanic additions.

The development of fly ash based high-strength self-compacting (or self-consolidating) concrete is a positive contribution to sustainable concrete technology. The present work details a mixture proportioning methodology for such concretes based on four steps where simple test procedures are used. Self-compacting concrete with a 90-day compressive strength of about 100 MPa has been obtained. This concrete has been used satisfactorily in a demonstration project involving the manufacture of a prefabricated urban bench with a complex shape. Such applications are promising since they lead to a reduction in energy and labor requirements during the casting and surface finishing, and facilitate the use of complex designs, while improving the factory environment through noise and vibration reduction.

A new environment-friendly block, called "steel slag hydrated matrix", consisting mainly of steelmaking slag, ground granulated blast furnace slag, fly ash, and water was developed. Steel slag hydrated matrix has the following features: 1) Made from 100% recycled resources, 2) same strength performance as ordinary concrete, 3) excel-lent wear resistance, 4) low alkaline dissolution, and 5) excellent growth habitat for biofouling organisms in marine environments. In repair work at Mizushima Port, Okayama Prefecture, Japan, 150,000 tons of steel slag hydrated matrix material were used. The ease of use in construction and low impact on ecological systems of the new material were confirmed in the course of this work.

Chloride ingress into concrete is commonly modeled using Fick's second law of diffusion. The common form of Fick's second law was derived for the boundary condition of one surface being in contact with a reservoir of constant concentration, such as the submerged portion of a marine seawall. However, in structures exposed to cyclic wetting and drying, this constant concentration boundary condition is not satisfied. Essentially, in cyclic exposures the surface concentration is not constant, but increases as a function of time. Chloride ingress occurs through sorption and diffusion in such exposures, but solely through diffusion in saturated environments. This paper compares the difference between chloride ingress in concrete exposed to cyclic wetting and drying environments with that of submerged concrete. This multi-year evaluation consisted of six different concrete mixtures, five of which contained different supplementary cementing materials. In addition, each concrete mixture was tested with and without a surface applied silane sealer. The results of these evaluations illustrate the relative rates of chloride ingress in the different environments and the effectiveness of supplementary cementing materials when combined with a silane sealer to reduce chloride ingress in cyclic exposures.

An investigation of the activation of Class C and Class F fly ash as well as granulated blast furnace slag has established that the amount and nature of the calcium containing phases in these materials significantly affect the activation mechanism. The majority of calcium in blast furnace slag is associated with either Si or with S and it can be activated either by use of a highly alkaline solution or by an alkaline solution containing soluble silicate to produce a solid material having sufficient mechanical strength. Class C fly ash contains calcium that is partly structured with aluminium and silicon and it has been found that this class of fly ash can only be activated using a highly alkaline solution containing soluble silicate. It has been observed that Class F fly ash cannot be successfully activated by either a highly alkaline solution or an alkaline soluble silicate solution, but by a very highly alkaline soluble silicate solution. It is proposed that the poor activation of Class F fly ash is due to this material having significantly lower calcium content than either blast furnace slag or Class C fly ash and that the calcium is in an isolated form.