High Performance Concrete

High-Performance Concrete

High Performance Concrete

P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials

A Brief History of Development

Products that last longer are called, high-performance products. Properly proportioned and cured mixtures of superplasticized concrete, with 0.4 or less w/cm, show a little or no permeability. In fact, superplasticized concrete mixtures made with blended portland cements containing mineral additives exhibit unusually low permeability ratings in the ASTM C 1202, Rapid Chloride Penetration Test.



Introduction

Mehta and Aitcin suggested the term high-performance concrete (HPC) for concrete mixtures that possess the following three properties: high-workability, high-strength, andhigh durability.



ACI Definition

HPC is defined as a concrete meeting special combination of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing, and curing practices.



Characteristics that may be considered critical for particular application

Ease of placement Compaction without segregation Early age strength Long-term strength and mechanical properties Permeability Density Heat of hydration Toughness Volume stability Long life in severe environments Note: according to the ACI definition, durability under severe environmental conditions is an optional, not a mandatory requirement for HPC.



Field Experience

The Strategic Highway Research Program (SHRP) in the United States defined HPC for highway structures by three requirements, namely a maximum w/cm, a minimum durability factor to cycles of freezing and thawing (ASTM C 666, Method A), and a minimum early-age or ultimate compressive strength.



SHRP

Four types of HPC were subsequently developed:Very Early Strength (14 MPa in 6 hours), High Early Strength (34 MPa in 24 hours), Very High Strength (69 MPa in 28 days), High Early Strength with Fiber-reinforcement.



Field Experience

U.S. Federal Highway Administration (FHWA) sponsored a national program of field testing HPC bridge decks. The assumption that “stronger concrete mixtures would be more durable” did not turn out to be true in the case of many cast-in-place, exposed, concrete structures, therefore FHWA has revised the definition of HPC for highway structures.



New definition

HPC is a concrete that has been designed to be more durable and if necessary, stronger than conventional concrete. HPC mixtures are essentially composed of the same materials as conventional concrete mixtures. But the proportions are designed or engineered to provide the strength and durabilityneeded for the structural and environmental requirements of the project.



Off-shore, Oil Drilling Platforms

Since 1970s some 20 concrete platforms have been installed in the British and Norwegian sector of the North Sea. The construction process involves the fabrication of massive oil-storage cells and prestressedconcrete shafts on a dry dock under rigorous quality control conditions, and then towing the structural components to the job site.




The concrete is required to withstand the corrosive action of sea water, and impacts and erosion from high tidal waves. Earlier North Sea platforms were designed for a service life of approximately 30 years and have performed satisfactorily. For the platforms completed in 1993-95, Troll A and Heidrun, the design life has been extended to 50-70 years.




Extra precautions were taken to prevent thermal cracking: cooling the concrete constitutes, partly replacing the mixing water with crushed ice, increasing the silica fume content from 3 to 9%, and decreasing the w/cm ratio from about 0.40 to 0.36. The mix proportions of the lightweight (1940 kg/m3), high-strength (70 MPa specified cube strength at 28 days) and pumpable (220-250 mm slump) concrete for the Heidrunoffshore platform.



Long Span Bridges

HPC is being extensively used now for the fabrication of precast pylons, piers, and girders of many long span bridges in the world:The Normandie Bridge in France (1993), the East Bridge of the Great Belt Link in Denmark (1994), and the Confederation Bridge in Canada (1997)



Port de Normandie

the Port de Normandie cable-stay bridge was the longest in the world when it was built in 1993. It has 2141m overall length and a center span of 856m. Approximately 35,000 m3 of 60-Grade HPC (60 MPa specified strength at 28 days) were used in the construction of pylons and cantilever beams.



Port de Normandie

The concrete mixture was composed of 425 kg/m3 blended portland cement containing 8% silica fume, 770 kg/m3 fine aggregate, 1065 kg/m3 coarse aggregate of 20mm max. size, 153 kg/m3 water (w/cm=0.36), and 11L/m3 of melamine-type superplasticizer. Measures to prevent thermal cracking included the use of insulated forms, and protecting the deck slab with hot air for 18 hours followed by a 6-hour waiting period before exposing the concrete to ambient temperature.



Great Belt Link in Denmark

At a cost of 4 billion USD , the Great Belt fixed link in Denmark provided a major improvement to the Northern European transportation system. The island of Sprogo divides the 18-km Great Belt into two parts.



Great Belt link

The Great Belt link has a railway tunnel, a high level motorway bridge across the East Channel and a low level bridge for rail and motorway across the West Channel. High-quality precast concrete segments were fabricated on dry docks under controlled environment. Even for the 50,000-tonne precast concrete units, construction tolerances were within a few centimeters.



Confederation Bridge

the 12.9 km long Confederation Bridge between Prince Edward Island and the mainland of Canada (across the Northumberland Strait), completed in 1997, consists of 44 main spans of 250 m length each and massive main pier shaft and foundation elements fabricated on land Approach pier foundations and some mass concrete sections, requiring control of thermal cracking, were built with Class C concrete which contained approximately 32% fly ash as a cement replacement material. Both concrete mixtures contained 7.5% silica fume by mass of the total cementitious materials.




The specifications for Class A concrete included a minimum of 55 MPa compressive strength and a maximum of 1000 coulombs chloride permeability (ASTM C 1202 test) at 28 days. Some piers, with an abrasion resistant ice shield, were made with 80 MPa concrete. The requirements for Class C concrete were 30 MPaand 40 MPa minimum compressive strength at 28 and 90 days, respectively.




Mix Proportions, kg/m3

Class A Concrete for Main Piers and T-beams

Class C Concrete for Massive Foundations

Abrasion-resistant Ice Shield Concrete

Portland cement 416 285 478 Silica fume 34 22 42 Fly ash, Class F - 133 60 Fine aggregate 737 744 650 Coarse aggregate 1030 1054 980 Water 153 159 142 Superplasticizer 3 2 6 W/cm 0.34 0.37 0.25 Properties Entrained air, % 6.1 7.0 - Slump, mm 200 185 - Compressive Strength, MPa

1-day 35 9.7 - 3 days 52 27.4 -

28 days 82 50.0 100 91 days - 76.0 -

Rapid chloride permeability, Coulombs (AASHTO T277)

28 days 300 420 - 90 days - - -



Bridge-decks, Pavements, and Parking Structures

Concrete mixtures containing low cementitiousmaterials content ( 300 kg/m3) and w/cm 0.5 are prone to show premature deterioration when exposed to corrosive conditions, such as sea water or de-icing salts. The advent of superplasticizers provided an impetus for the development of very high-strength concrete mixtures that found their way quickly into cast-in-place structures designed for long-term durability under severe environmental conditions.



Cast-in-place bridge decks, pavements

The total cementitious material content does not exceed 400 kg/m and, typically, 30 to 40% portland cement is being replaced by fly ash or granulated blast-furnace slag. A low chloride permeability rating, e.g., max. 1500 or 2000 coulombs at 56 days (ASTM C1202 Test Method) is specified. A very low chloride permeability ( 1000 coulombs) can be readily achieved even at the early age of 28 days by incorporation of 7-10% silica fume, metakaolin, or rice husk ash in the concrete mixture.



High-Performance, High-Volume Fly Ash Concrete

The high-volume fly ash (HVFA) concrete system has emerged as a powerful tool to build concrete structures in the future that would be far more durable and resource-efficient than those made of conventional portland-cement concrete. Whether used as a component of blended cements or as a mineral admixture added to concrete batch during mixing, the fly ash content of concrete must be above 50 percent by mass of the total cementitious material.



HVFA

In the past, the HVFA concrete mixtures generally did not perform well with respect to strength development, drying shrinkage, and durability. This is because the fly ash produced by old thermal power plants was coarser and usually contained high carbon. Laboratory and field experience have shown that fly ash from the modern thermal power plants, generally characterized by low carbon content and high fineness, when used in a large volume, is able to impart excellent workability to concrete at a water content that is 15 to 20 percent lower than without the fly ash.



HVFA

Further reductions in the mixing water content can be achieved with better aggregate grading and with the help of a superplasticizingadmixture.



MIXTURE PROPORTIONS FOR 25-MPa CONCRETE

(0.252)25.0%-(0.296)

29.6%-Paste volume:Percent:

-0.38-0.58w/cm

1.00024131.0002350Total

0.2987750.319825Fine Aggregate

0.45012100.3851040Coarse aggregate

0.020-0.020-Entrapped air (2%)

0.1201200.178178Water

0.064154--Fly ash

0.0481540.098307Cement

By volume(m3/m3)

By mass(kg/m3)

By volume(m3/m3)

By mass(kg/m3)

HVFA ConcreteConventional Concrete

Materials



HVFA

Compared to the conventional concrete mixture, the HVFA system contains one-third less mixing water. Also the total volume of the cement paste is nearly 16 percent less. Consequently, the drying shrinkage is much reduced. HVFA concrete generates nearly 40 percent less heat of hydration at early age and, therefore, in massive structural members the potential for thermal shrinkage and cracking is also greatly reduced.



Durability of HVFA

For frost resistance, the use of an air-entraining admixture is mandatory. The setting time is somewhat longer and the early strength development rate of HVFA concrete is slower. However, under warm weather conditions, the strength at 24 to 48-hour should be adequate for formwork removal. When possible the forms may be kept in place for 7 days or even longer to maintain the moist-curing environment. The long-term strength and impermeability of HVFA concrete is, generally, far superior to ordinary concrete with no fly ash or a small quantity of fly ash. Also, from laboratory test data on the corrosion of steel reinforcement, alkali-aggregate expansion, and sulfate attack.



Summary

Flowability/Pumpability Easier Workability/Compactability Easier Bleeding None or Negligible Finishing Quicker Setting Time Slower up to 2 h Early Strength (up to 7-day) Lower but can be accelerated Ultimate Strength- 90day + Higher Crack Resistance Higher

• Plastic Shrinkage Higher (If unprotected) • Thermal Shrinkage Lower • Drying Shrinkage Lower

Resistance to Penetration of Chloride Ions Very high after 3 months Electrical Resistivity Very high after 3 months Durability

• Resistance to Sulfate Attack Very high • Resistance to Alkali-Silica Expansion Very high • Resistance to Reinforcement Corrosion High

Cost • Materials Lower • Labor Similar • Life Cycle Very low

Environmental Benefits ( Reduced CO2 emission) Very high

High Performance Concrete

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