040 029 035 035

Source: Zia et al. Mechanical Behavior of high-programme concreter, vol. 1, National Research Council, Washington DC, 1993.

2. Definition from FHWA. According to Federal Highway Administration (FHWA), HPC is defined as ''a concrete that has been designed to be more durable and, if necessary, stronger than conventional concrete.'' Concrete so designated should meet significantly more stringent criteria than those required for normal structural concrete. It should give optimized performance characteristics and should have high workability, very high fluidity, and minimum or negligible permeability. Serviceability as determined by crack control and deflection control, as well as long-term environmental effects, is equally important as durability parameters. In 1996, the FHWA proposed additional criteria for four different performance grades of HPC as shown in Table 15.2.

3. Definition from ACI. In 1993, the American Concrete Institute (ACI) published a broad definition for HPC:

High-performance concrete (HPC) is defined as concrete which meets special performance and uniformity requirements that cannot always be achieved by using only the conventional materials and mixing, placing and curing practices. The performance requirements may involve enhancements of placement and compaction without segregation, long-term mechanical properties, early age strength, toughness, volume stability, or service life in severe environments.

Based on the HPC definitions, it can be seen that HPC performance characteristics consist of two categories: durability and strength. The durability and strength parameters divide field conditions into three categories: climate, exposure effects, and load. To achieve the strength and durability requirements, a lower w/b ratio is necessary. For most application of HPCs, the w/b ratio is less than 0.4. Furthermore, some admixtures (both chemical and mineral) are usually needed to produce HPC.

In general, HPC is not one product but includes a range of materials with special properties beyond conventional concrete and routine construction methods at that time. It only can be made by appropriate materials, suitable mix design and properly mixed, transported, placed, consolidated, and cured so that the resulting concrete will give excellent performance in the structure in which it will be placed, in the environment to which it will be exposed, and with loads to which it will be subjected to during its design life. Presently, the primary characteristics of HPC can be summarized as easy placement, high early age strength, toughness, superior long-term mechanical properties, and prolonged service life in severe environment. Of course, some HPC applied in special fields and occasions possess some other chacteristics, and these characteristics of HPC will change with time.

Herein, an important point must be emphasized. HPC is different from high-strength concrete: a high-strength concrete is usually a HPC, but a HPC is not always a high-strength concrete because HPC stresses concrete durability behavior, not excessively showing indication of high strength. A high-strength concrete does not ensure that a durable concrete will be achieved. Given that the required durability characteristics are more difficult to define than strength characteristics, specifications often use a combination of performance and prescriptive requirements, such as permeability and a maximum water-cementitious material ratio to achieve a durable concrete. The end result may

TABLE 15.2 Definition of HPC According to Federal Highway Administration [6]

FHWA HPC performance grade

TABLE 15.2 Definition of HPC According to Federal Highway Administration [6]

FHWA HPC performance grade

Performance characteristics

Standard test method

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