The effects of fiber reinforcenent; on the response of high strength fiber reinforced concrete in conmpression, tension and flexure are reviewed. Tradeoffs between the use of high fiber contents to achieve comiposite strength and ductility, and a reduction in matrix porosity to achieve high matrix strength are discussed. Methods used to increase composite strength include application of pressure after casting, addition of fly-ash, silica fumes and or sulerplasticizers to the cement matrix, and mixing high volurite contents of high strength stiff fibers.

An attempt is made to provide a prospective on new very high strength cement based materials. The mechanical properties of concrete and of conventional cement pastes are considered, and limitations on the behavior of paste set by the Griffith concept of a critical crack explored. Evidence is cited confirming that spherical air voids do indeed act as critical Griffith flaws in undried pastes; however it is suggested that shrinkage cracks dominate the behavior of cement paste exposed to drying. Very high strength systems.must avoid both large air voids and other pores and also be resistant to shrinkage cracking. Streams of development leading to “DSP” (Aalborg) and “MDF” (ICI) systems are described, and details of the functioning of the two classes of product are described. Current and potential commercial developments are briefly noted, and possible interrelations with the emerging areas of “high technology ceramics” mentioned. Finally, a brief summary of relevant mechanical properties is provided.

High strength mortars have been prepared utilizing optimized particle packing, reactive substituents to modify the chemistry- and addition of superplasticizers. Otherwise the processing techniques were conventional. The compressive strengths of one prototype material after curing at temperatures from 38 C to 250 C were above 70 MPa. The strengths were particularly high at 175 C (195 MPa) where excellent bonding had developed; one chemically modified material reached 245 MPa. The specimens cured at 175 and 250 C (after a lower temperature precure) developed their strengths rapidly, having reached essentially full strength by 7 days. At lower curing temperatures the strength increased with time, apparently still increasing at 56 days (106 MPa) for the materials cured at 38 C. Modified mixtures were prepared using different proportions of silica fume, MgO, different ratios of sand to fine components, and different sand mineralogy and other admixture proportions for rheological optimization. Microhardness, dynamic Young's modulus, density, and permeability were measured in addition to strength. Matrix chemistry and sand mineralogy and proportions affected the strength. Matrix-aggregate bond was very important. The above types of cementitious materials have potential importance for applications where they may be exposed to extreme conditions and to temperature cycling.

The composition and fabrication of a high strength cementbased composite containing graded stainless steel aggregate and fine silica are described. The microstructure, physical properties and temperature cycling durability of this material are examined and some examples are given of technological applications as a molding/tooling material.

The properties of Macro-Defect-Free (MDF) cements are reviewed. Combinations of these properties place MDF cements in a unique position in the materials field, a position which makes direct replacement of current engineering materials such as metals plastics and ceramics a viable proposition. Perhaps of more importance is the invention of devices and components hitherto unavailable because of the property-restricted nature of available materials. Examples are given of unusual uses for cement. These include applications in the fields of cryogenics, ballistic protection, electromagnetic radiation screening and acoustics.

Steel fiber reinforced concretes (SFRC) are typically prepared by adding the fiber along with the other concrete ingredients in the mixing operation. At fiber contents in excess of 2 volume percent, the SFRC becomes difficult or impossible to mix and place. Inasmuch as improvements in concrete properties attributed to the fibers increase as a function of increasing fiber content, this situation places a limit on the ultimate property development in SFRC prepared using the premix approach. Recently, a procedure has been developed wherein steel fiber contents up to 20 volume percent have been provided. Slurry Infiltrated Fiber Concrete (SIFCCN) composites possess outstanding strength, ductility, and crack/spall resistant properties. Equally:important, the ability to construct with SIFCON has been demonstrated.

Well compacted, silica fume-modified ordinary Portland cement pastes are well known for their low permeability. This quality can be maintained to temperatures slightly in excess of 200°C. Above this temperature, however, a continuous pore structure develops, making the material permeable and therefore unsuitable for high temperature applications (250–400°C) where vacuum integrity is necessary. To overcome this obstacle the permeability can be exploited for impregnating the concrete with an organic polymer. Organic polymers traditionally employed for impregnated concretes are not thermally stable enough to be used at high temperatures. Therefore, a high temperature-resistant organic resin has been studied for this application. The optimum conditions for impregnating this viscous resin into a high strength mortar have been investigated.

A number of new materials are presently being introduced for application in high temperature environments. Though some of these are old materials fabricated with new methods or processes, many are actually new. This paper focuses on the use of high strength concretes or mortars for application to high temperature environments, such as furnaces or boilers for coal gasification and similar processes requiring high pressure and temperature. This paper reports the results of an investigation on the high.temperature performance of mortar of different strengths incorporating 0%, 8% and 16% of condensed silica fume (CSF). The experiments described have been carried out to compare the performance of different strength level mortars with different CSF contents. Generally speaking, it was found that low strength mortar with CSF is less susceptible to strength loss after high temperature exposure than mortar without CSF. On the other hand, high strength mortar with CSF seems to be more sensitive to high temperature than high strength mortar without CSF since it retains only 65% of its room temperature strength after heating to 3200C as compared to regular high strength mortar which retains 95% of its room temperature strength.

Though there may be a possible practical problem in using high strength CSF concrete, this shown loss of strength in high strength CSF concrete upon heating may prove very important in giving us insight into the fundamental mechanisms of strengthening concrete with CSF. An explanation is hypothesized on the observed behavior of high strength CSF concrete, which will produce a theory for one of the ways in which CSF strengthens portland cement paste. It is theorized that the CSF touohens the portlandite in high strength paste, but after heating to 320 C, the CSF particles which have been converted to C-S-H have lost their ability to toughen the portlandite.

With the advent of microsilica concrete, a new generation of high to ultra high strength and high durability concretes have become commercially feasible and are now being specified and used internationally. Microsilica concrete is produced by incorporating microsilica (beneficiated condensed silica fume) additives in conventional concrete mixes, using conventional materials and equipment. Flowing microsilica concretes with strengths as high as 17000 psi have become field realizable, also benefiting by durability improvements expressible by factors, not percents.

Properties of microsilica concrete are reviewed, and property improvements qualitatively linked to a much refined micropore structure of the binder phase.

Some recent and rapidly developing field and laboratory experience both in the U.S. and overseas are presented.