High-Strength versus High Performance Concrete
High strength concrete and high-performance concrete are not synonymous because strength and performance of concrete are different properties of concrete. High-strength concrete is defined based on its compressive strength at a given age.
During 1970s, any concrete mixtures which showed 40 MPa or more compressive strength at 28 days were designated as high strength concrete. As the time passed, more and more high strength concrete such as 60 – 100 MPa, were developed which were used for the construction of long-span bridges, skyscrapers etc.
While high strength concrete is defined purely on the basis of its compressive strength, Mehta and Aitcin defined the high-performance concrete (HPC) as concrete mixtures possessing high workability, high durability and high ultimate strength.
ACI defined high-performance concrete as a concrete meeting special combinations of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing, and curing practice.
TYPICAL CLASSIFICATION OF CONCRETE:
|Concrete Types||Concrete Strength|
|Normal strength concrete||20 – 50 MPa|
|High Strength Concrete||50 – 100 MPa|
|Ultra High Strength Concrete||100 – 150 MPa|
|Especial Concrete||> 150 MPa|
High strength of concrete is achieved by reducing porosity, in-homogeneity, and micro-cracks in the hydrated cement paste and the transition zone. Consequently, there is a reduction of the thickness of the interfacial transition zone in high-strength concrete. The densification of the interfacial transition zone allows for efficient load transfer between the cement mortar and the coarse aggregate, contributing to the strength of the concrete. For very high-strength concrete where the matrix is extremely dense, a weak aggregate may become the weak link in concrete strength.
Materials for High-Strength Concrete:
Cement composition and fineness play an important role in achieving high strength of concrete. It is also required that the cement is compatible with chemical admixtures to obtain the high-strength. Experience has shown that low-C3A cements generally produce concrete with improved rheology.
Selection of right aggregates plays an important role for the design of high-strength concrete mix. The low-water to cement ratio used in high-strength concrete makes the concrete denser and the aggregate may become the weak link in the development of the mechanical strength. Extreme care is necessary, therefore, in the selection of aggregate to be used in very high-strength concrete.
The particle size distribution of the fine aggregates plays an important role in the high strength concrete. The particle size distribution of fine aggregate that meets the ASTM specifications is adequate for high-strength concrete mixtures.
Aitcin recommends using fine aggregates with higher fineness modulus (around 3.0). His reasoning is as follows:
- High-strength concrete mixtures already have large amounts of small particles of cement and pozzolan, therefore fine particles of aggregate will not improve the workability of the mix;
- The use of coarser fine aggregates requires less water to obtain the same workability; and
- During the mixing process, the coarser fine aggregates will generate higher shearing stresses that can help prevent flocculation of the cement paste.
Guidelines for the selection of materials:
- For the higher target compressive strength of concrete, the maximum size of concrete selected should be small, so that the concrete can become more dense and compact and less void ratio.
- Up to 70 MPa compressive strength can be produced with a good coarse aggregate of a maximum size ranging from 20 to 28 mm.
- To produce 100 MPa compressive strength aggregate with a maximum size of 10 to 20 mm should be used.
- To date, concretes with compressive strengths of over 125 MPa have been produced, with 10 to 14 mm maximum size coarse aggregate.
- Using supplementary cementitious materials, such as blast-furnace slag, fly ash and natural pozzolans, not only reduces the production cost of concrete, but also addresses the slump loss problem.
- The optimum substitution level is often determined by the loss in 12- or 24-hour strength that is considered acceptable, given climatic conditions or the minimum strength required.
- While silica fume is usually not really necessary for compressive strengths under 70 MPa, most concrete mixtures contain it when higher strengths are specified.
Differences between Normal Strength Concrete and High Strength Concrete:
Micro-cracks are developed in the normal strength concrete when its compressive strength reaches 40% of the strength. The cracks interconnect when the stress reaches 80-90% of the strength.
For High Strength Concrete, Iravani and MacGregor reported linearity of the stress-strain diagram at 65 to 70, 75 to 80 and above 85% of the peak load for concrete with compressive strengths of 65, 95, and 105 MPa.
The fracture surface in NSC is rough. The fracture develops along the transition zone between the matrix and aggregates. Fewer aggregate particles are broken. The fracture surface in HSC is smooth. The cracks move without discontinuities between the matrix and aggregates.