HIGH PERFORMANCE CONCRETE - HPC

Transcription

1 HIGH PERFORMANCE CONCRETE - HPC Fahim Al-Neshawy & Esko Sistonen Outlines Introduction Materials used in HPC Methods used for making HPC HPC - mix design Properties of HPC Advantages and disadvantages Applications

2 Introduction Based on the compressive strength; concrete is normally classified as normal strength concrete, high strength concrete and ultra strength concrete. High performance concrete is necessary for the construction of high rise building and long span bridges. To achieve high strength, it necessary to use: high cement content with the lowest possible W/C ratio which invariable affect the workability of the mix. Introduction It is important to note the high-strength and highperformance concrete are not synonymous. Concrete is defined as high-strength concrete solely on the basis of its compressive strength measured at a given age. In the 1970 s, any concrete mixtures that showed 40 MPa or more compressive strength at 28-days were designed as high-strength concrete. Later, MPa concrete mixtures were commercially developed world.

3 Definition - HPC Mehta used the term, high- performance concrete (HPC) for 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. P.K. Mehta and P.J.M. Monteiro, Concrete: Microstructure, Properties, and Materials HPC classification Normal Strength High Strength Ultra High Strength Especial MPa MPa MPa > 150 MPa

4 Microstructure MATERIALS USED IN HPC

5 Material Selection The main ingredients of HPC are almost the same as that of conventional concrete. These are: 1) Cement 2) Fine aggregate 3) Coarse aggregate 4) Water 5) Mineral admixtures (fine filler and/or pozzolanic supplementary cementation materials) 6) Chemical admixtures (plasticizers, superplastisizers, retarders, air-entraining agents) Materials Used in High-Performance Concrete Material Portland cement Blended cement Fly ash Slag Silica fume Superplasticizers High-range water reducers Hydration control admixtures Retarders Accelerators Corrosion inhibitors Water reducers Shrinkage reducers ASR inhibitors Polymer/latex modifiers Optimally graded aggregate Primary contribution/desired property Cementing material/durability Cementing material/durability/high strength Flowability Reduce water to cement ratio Control setting Accelerate setting Control steel corrosion Reduce cement and water content Reduce shrinkage Control alkali-silica reactivity Durability Improve workability and reduce paste demand

6 Cements There are two important requirements for any cement: a) Strength development with time Cement with compressive strength up to 60 MPa for HSC b) Facilitating appropriate rheological characteristics for the fresh concrete Experience has shown that low-c3a cements generally produce concrete with improved rheology. Cement contents between 400 and 550 kg/m 3 Cement should compatible with the chemical admixtures Aggregates The higher the targeted compressive strength, the smaller the maximum size of coarse aggregate. 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. Concretes with compressive strengths of over 125 MPa have been produced, with 10 to 14 mm maximum size coarse aggregate.

7 Mineral admixtures GGBS, fly ash and natural pozzolans, not only reduces the production cost of concrete, but also addresses the slump loss problem. While silica fume is usually not necessary for compressive strengths under 70 MPa, most concrete mixtures contain it when higher strengths are specified. Dosage rate 5% to 20% or higher by mass of cementing material. Some specs. silica fume 10% max. Admixtures Use of admixtures is mandatory in high-performance concrete: water reducers, retarders, superplasticizers Air-entraining admixtures not necessary or desirable in protected high-strength concrete. u Air is mandatory, where durability in a freeze-thaw environment is required ie. bridges, piers, parking structures u Recent studies: H H w/cm air required w/cm < no air needed

8 Requirements of ingredients for HPC METHODS USED FOR MAKING HPC

9 Special methods for making HSC 1. Seeding 2. Re-vibration 3. High Speed Slurry 4. Use of admixtures 5. Inhibition of Cracks 6. Sulphur Impregnation 7. Use of Cementitious aggregates Special methods for making HSC 1. Seeding: This involves adding a small percentage of finely ground, fully hydrated Portland cement to the fresh concrete mix. 2. Revibration: Controlled revibration removes all the defects like bleeding, water accumulates, plastic shrinkage, continuous capillary channels and increases the strength of concrete. 3. High speed slurry [liete] mixing: This process involves the advance preparation of cement - water mixture which is then blended with aggregate to produce concrete.

10 Special methods for making HSC 4. Use of admixtures: Use of water reducing agents are known to produce increased compressive strength. 5. Inhibition of cracks: If the propagation of cracks is inhibited, the strength will be higher. Concrete cubes made this way have yielded strength up to 105MPa. Special methods for making HSC 6. Sulphur Impregnation: Satisfactory high strength concrete have been produced by impregnating low strength porous concrete by sulphur. The sulphur infiltrated concrete has given strength up to 58 MPa. 7. Use of Cementitious aggregates: Cement fondu is kind of clinker. Using a slag as aggregate, strength up to 25 MPa has been obtained with water cement ratio 0.32.

11 Air-cooled blast furnace slag (ABS), coarse aggregate Granulated blast furnace slag (GBS), fine aggregate HPC MIX DESIGN

12 Mix design A generalized systematic approach to the selection of mix proportions of HPCs has not yet been developed Specific requirements to considered: Water content should be chosen on the basis of the required w/c ratio (from strength considerations) Excessive content of cementitious material should be avoided (to control shrinkage) Compatibility between Portland cement and the superplasticizer If air entrainment is to be used, mix proportions have to be modified by trial and error Mix design 1. Define proportioning strength Average strength + effect of deviation ex. f cr = f c + 1.3s where σ is the standard deviation in MPa The equation ensures that there is a 99% probability that the average of all sets of three consecutive compressive strength tests must be equal to or greater than f c. 2. Define the amount of binder C = cement Si = Silica fume Lt = Fly ash Mk = Blastfurnace slag From the report: Korkealujuuksisten betonien suhteitus ; Penttala V. et. al. (1990). Publication 108. Figure from page 40.

13 Mix design 3. Calculate the amount of cement and additional binders Binder amount (C+2,5 Si+0,3 Lt+Mk) 4. Define the amount of (super)plasticizer as a percentage of binder Mix design 5. Define the water amount

14 Mix design 6. Calculate the amount of aggregate with the basic equation of concrete. The amount of air is assumed to be 10 dm 3 /m 3 7. Define the components of the batch 8. Combine the aggregate 9. Make a trial batch C = cement Si = Silica fume Lt = Fly ash Mk = Blastfurnace slag I = air content Nt = Plasticizer W = Water R = Aggregates

15 Example of HPC mix design PROPERTIES OF HPC

16 Properties of HPC High modulus of elasticity High abrasion resistance High durability and long life in severe environments Low permeability and diffusion Resistance to chemical attack High resistance to frost and deicer scaling damage Toughness and impact resistance Ease of placement Chemical Attack Carbonation High modulus of elasticity

17 Low permeability and diffusion The durability and service life of concrete exposed to weather is related to the permeability of the cover concrete protecting the reinforcement. HPC typically has very low permeability to air, water, and chloride ions. The dense pore structure of high-performance concrete makes it so impermeable HSC is considerably more brittle than NSC. HSC behaves linearly up to a stress level which is about 90% of the peak stress, whereas lower strength concrete shows nearly no linear part at all When the peak stress has been reached, the stress decays rapidly in high strength concrete.

18 Durability parameters 1. Water/ (cement + mineral admixture) ratio 2. Strength 3. Densification of cement paste 4. Elimination of bleeding 5. Homogeneity of the mix 6. Particle size distribution 7. Dispersion of cement in the fresh mix 8. Stronger transition zone 9. Low free lime content 10.Very little free water in hardened concrete

19 High abrasion resistance Abrasion resistance is directly related to the strength of concrete. This makes high strength HPC ideal for abrasive environments. The abrasion resistance of HPC incorporating silica fume is especially high. This makes silica fume concrete particularly useful for spillways and stilling basins, and concrete pavements or concrete pavement overlays subjected to heavy or abrasive traffic. High durability and long life in severe environments Durability problems of NSC can be associated with the severity of the environment and the use of inappropriate high water/binder ratios. HPC that have a water/binder ratio between 0.30 and 0.40 are usually more durable than NSC not only because they are less porous, but also because their capillary and pore networks are somewhat disconnected due to the development of self-desiccation. In high-performance concrete (HPC), the penetration of aggressive agents is quite difficult and only superficial

20 High resistance to frost and deicer scaling damage Because of its very low W/C ratio (<0.30), HPC should be highly resistant to both scaling and physical breakup due to freezing and thawing. Resistance to chemical attack For resistance to chemical attack on most structures, HPC offers a much improved performance. Resistance to various sulfates is achieved primarily by the use of a dense, strong concrete of very low permeability and low water-to-cementing materials ratio; these are all characteristics of HPC. Similarly resistance to acid from wastes is also much improved.

21 Carbonation HPC has a very good resistance to carbonation due to its low permeability. In practical terms, non-cracked HPC cover concrete is immune to carbonation to a depth that would Properties - summary Flowability/pumpability Workability/compactability Bleeding Finishing Setting time Early strength (up to 7-day) Ultimate strength- 90day + Crack resistance Plastic shrinkage Thermal shrinkage Drying shrinkage Resistance to penetration of chloride ions Electrical resistivity Durability Resistance to sulfate attack Resistance to alkali-silica expansion Resistance to reinforcement corrosion Environmental benefits (reduced CO2 emission) Easier Easier None or negligible Quicker Slower up to 2 h Lower but can be accelerated Higher Higher Higher (if unprotected) Lower Lower Very high after 3 months Very high after 3 months Very high Very high High Very high

22 ADVANTAGES / DISADVANTAGES Advantages of using HPC Reduction in member size, resulting in increase in plinth area/useable area and direct savings in the concrete volume saved. Reduction in the self-weight and super-imposed DL with the accompanying saving due to smaller foundations. Reduction in form-work area and cost with the accompanying reduction in shoring and stripping time due to high early-age gain in strength. Construction of High rise buildings with the accompanying savings in real-estate costs in congested areas.

23 Advantages of using HPC Longer spans and fewer beams for the same magnitude of loading. Reduced axial shortening of compression supporting members. Reduction in the number of supports and the supporting foundations due to the increase in spans. Reduction in the thickness of floor slabs and supporting beam sections- which are a major component of the weight and cost of the majority of structures. Advantages of using HPC Superior long-term service performance under static, dynamic and fatigue loading. Low creep and shrinkage. Greater stiffness as a result of a higher modulus,e c Higher resistance to freezing and thawing, chemical attack, and significantly improved longterm durability and crack propagation. Reduced maintenance and repairs. Smaller loss in value as a fixed cost.

24 Disadvantages The current disadvantages of HPC pointed out by some engineers include: the initially higher construction bid prices to be expected with the use of any new technology quality control concerns related to various material selection, testing methods in use and the number of tests Instabilities concerns that could result from reduced stiffness Fire resistance concerns HSC APPLICATIONS

25 The East Bridge of the Great Belt Link in Denmark, (1994) Concrete structures à 100 years 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 The Confederation Bridge in Canada (1997)

26 The Confederation Bridge in Canada (1997) high_performance_concrete.pdf The Normandie Bridge in France (1993)

27 The Normandie Bridge in France (1993) high_performance_concrete.pdf 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. 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 11 L/m3 of melamine-type superplasticizer. Water Tower Place: Shopping mall in Chicago (1975 )

28 Water Tower Place: Shopping mall in Chicago (1975) Summary Definition of HPS, HSC and NSC Materials used in HPC (cement, mineral and chemical admixtures, aggregates) Methods used for making HPC HPC - mix design Advantages and disadvantages of the use of HPS HPC Applications