High Performance and High-Strength Concretes Coursework (Coursework Sample)

High Performance and High-Strength Concretes
High-performance concrete is a term used to describe concrete with special properties not attributed to normal concrete. High-performance means that the concrete has one or more of the following properties: low shrinkage, low permeability, a high modulus of elasticity, or high strength. Further, high performance concrete is "concrete that meets special performance and uniformity requirements that cannot always be achieved routinely by using only conventional materials and normal mixing, placing, and curing practices. The 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.
On the other hand, high-strength concrete is typically recognized as concrete with a 28-day cylinder compressive strength greater than 6000 psi or 42 KN/M (Mpa). More generally, concrete with a uniaxial compressive strength greater than that typically obtained in a given geographical region is considered high-strength, although the preceding values are widely recognized. Strengths of up to 20,000 psi (140 Mpa) have been used in different applications. Laboratories have produced strengths approaching 60,000 psi (480 Mpa) at 28 days for high strength concrete.
High-strength concrete can resist loads that normal-strength concrete cannot. Several distinct advantages and disadvantages can be analysed. It is important to consider all peripheral results of selecting high-strength concrete since special considerations must be addressed beyond strength properties.
Once it is decided to use high-strength, high-performance concrete, the mix design and production process can begin. The materials used and concepts involved in increasing the strength of concrete must be clearly understood in order to obtain the desired properties. Testing is an integral step in the production process, since quality control studies show that slight changes in mixture proportions can lead to large changes in the compressive strength of concrete. When the design proportioning is complete, mixing can commence with extra consideration for workability and related properties of the mix.
The primary difference between high-strength concrete and normal-strength concrete relates to the compressive strength that refers to the maximum resistance of a concrete sample to applied pressure. Although there is no precise point of separation between high-strength concrete and normal-strength concrete, the American Concrete Institute defines high-strength concrete as concrete with a compressive strength greater than 6,000 psi.  Manufacture of high-strength concrete involves making optimal use of the basic ingredients that constitute normal-strength concrete. Producers of high-strength concrete know what factors affect compressive strength and know how to manipulate those factors to achieve the required strength. In addition to selecting a high-quality Portland cement, producers optimize aggregates, then optimize the combination of materials by varying the proportions of cement, water, aggregates, and admixtures.
When selecting aggregates for high-strength concrete, producers consider the strength of the aggregate, the optimum size of the aggregate, the bond between the cement paste and the aggregate, and the surface characteristics of the aggregate. Any of these properties could limit the ultimate strength of high-strength concrete.
Materials for High Strength Concrete
Cement
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 (high strength).
Aggregate
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. The recommended fine modulus of aggregate for high strength concrete is around 3.0 due to the following reasons:
* 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 denser and compacted with 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.
* 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 (NSC). This is not applicable to the high strength concrete (HSC).
* 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.
* 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. The strength of high performance concrete may not be more than 25 MPa at 28 days.
* While high strength concrete is defined purely on the basis of its compressive strength, high-performance concrete (HPC) is defined as concrete mixtures possessing high workability, high durability and high ultimate strength at 28 days. It is also a concrete that meets special combinations of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing, and curing practice.
Main Ingredient of High Strength Concrete
Pozzolans, such as fly ash and silica fume, are the most commonly used mineral admixtures in high-strength concrete. These materials impart additional strength to the concrete by reacting with Portland cement hydration products to create additional C-S-H gel, the part of the paste responsible for concrete strength.
It would be difficult to produce high-strength concrete mixtures without using chemical admixtures. A common practice is to use a superplasticizer in combination with a water-reducing retarder. The superplasticizer gives the concrete adequate workability at low water-cement ratios, leading to concrete with greater strength. The water-reducing retarder slows the hydration of the cement and allows workers more time to place the concrete.
High-strength concrete is specified where reduced weight is important or where architectural considerations call for small support elements. By carrying loads more efficiently than normal-strength concrete, high-strength concrete also reduces the total amount of material placed and lowers the overall cost of the structure.
The most common use of high-strength concrete is for construction of high-rise buildings. At 969 feet, Chicago's 311 South Wacker Drive uses concrete with compressive strengths up to 12,000 psi and is one of the tallest concrete buildings in the United States.
High strength concrete can also be 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.
High Density Concrete
High density concrete is a concrete having a density in the range of 6000 to 6400 kg. High density concrete is also known as Heavy weight concrete.  High density concrete is mainly used for the purpose of radiation shielding, for counterweights and other uses where high density is required. The high density concrete has a better shielding...