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Friday, June 11, 2010

HIGH PERFORMANCE CONCRETE


  High performance concrete is a concrete mixture, which possess high durability and high strength when compared to conventional concrete. This concrete contains one or more of cementious materials such as fly ash, Silica fume or ground granulated blast furnace slag and usually a super plasticizer. The term ‘high performance’ is somewhat pretentious because the essential feature of this concrete is that it’s ingredients and proportions are specifically chosen so as to have particularly appropriate properties for the expected use of the structure such as high strength and low permeability. Hence High performance concrete is not a special type of concrete. It comprises of the same materials as that of the conventional cement concrete. The use of some mineral and chemical admixtures like Silica fume and Super plasticizer enhance the strength, durability and workability qualities to a very high extent.

                                 High Performance concrete works out to be economical, even though it’s initial cost is higher than that of conventional concrete because the use of High Performance concrete in construction enhances the service life of the structure and the structure suffers less damage which would reduce overall costs.
Concrete is a durable and versatile construction material. It is not only
 Strong, economical and takes the shape of the form in which it is placed, but it is also aesthetically satisfying. However experience has shown that concrete is vulnerable to deterioration, unless precautionary measures are taken during the design and production. For this we need to understand the influence of components on the behavior of concrete and to produce a concrete mix within closely controlled tolerances.
The conventional Portland cement concrete is found deficient in respect of :
·         Durability in severe environs (shorter service life and frequent maintenance)
·         Time of construction (slower gain of strength)
·         Energy absorption capacity (for earthquake resistant structures)
·         Repair and retrofitting jobs.
                  Hence it has been increasingly realized that besides strength, there are other equally important criteria such as durability, workability and toughness. And hence we talk about ‘High performance concrete’ where performance requirements can be different than high strength and can vary from application to application.
                    High Performance Concrete can be designed to give optimized performance characteristics for a given set of load, usage and exposure conditions consistent with the requirements of cost, service life and durability. The high performance concrete does not require special ingredients or special equipments except careful design and production. High performance concrete has several advantages like improved durability characteristics and much lesser micro cracking than normal strength concrete.
Any concrete which satisfies certain criteria proposed to overcome limitations of conventional concretes may be called High Performance Concrete. It may include concrete, which provides either substantially improved resistance to environmental influences or substantially increased structural capacity while maintaining adequate durability. It may also include concrete, which significantly reduces construction time to permit rapid opening or reopening of roads to traffic, without compromising long-term servicibility. Therefore it is not possible to provide a unique definition of High Performance Concrete without considering the performance requirements of the intended use of the concrete.

                           American Concrete Institute defines High Performance Concrete as
“A concrete which 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 characteristics such as placement and compaction without segregation, long-term mechanical properties, and early age strength or service life in severe environments. Concretes possessing many of these characteristics often achieve High Strength, but High Strength concrete may not necessarily be of High Performance .A classification of High Performance Concrete related to strength is shown below.


Compressive strength (Mpa)
    50
      75
      100
     125
    150
High Performance
Class
      I
      II
       III
      IV
      V


SELECTION OF MATERIALS
                            The production of High Performance Concrete involves the following three important interrelated steps:
·         Selection of suitable ingredients for concrete having the desired rheological properties, strength etc
·         Determination of relative quantities of the ingredients in order to produce durability.
·         Careful quality control of every phase of the concrete making process.
 The main ingredients of High Performance Concrete are

Cement
                     Physical and chemical characteristics of cement play a vital role in developing strength and controlling rheology of fresh concrete. Fineness affects water requirements for consistency. When looking for cement to be used in High Performance Concrete one should choose cement containing as little C3A as possible because the lower amount of C3A, the easier to control the rheology and lesser the problems of cement-super plasticizer compatibility. Finally from strength point of view, this cement should be finally ground and contain a fair amount of  C3S.

Fine aggregate
                     Both river sand and crushed stones may be used. Coarser sand may be preferred as finer sand increases the water demand of concrete and very fine sand may not be essential in High Performance Concrete as it usually has larger content of fine particles in the form of cement and mineral admixtures such as fly ash, etc. The sand particles should also pack to give minimum void ratio as the test results show that higher void content leads to requirement of more mixing water.

Coarse aggregate
                      The coarse aggregate is the strongest and least porous component of concrete. Coarse aggregate in cement concrete contributes to the heterogeneity of the cement concrete and there is weak interface between cement matrix and aggregate surface in cement concrete. This results in lower strength of cement concrete by restricting the maximum size of aggregate and also by making the transition zone stronger. By usage of mineral admixtures, the cement concrete becomes more homogeneous and there is marked enhancement in the strength properties as well as durability characteristics of concrete. The strength of High Performance Concrete may be controlled by the strength of the coarse aggregate, which is not normally the case with the conventional cement concrete. Hence, the selection of coarse aggregate would be an important step in High Performance Concrete design mix.


Water
                     Water is an important ingredient of concrete as it actively participates in the chemical reactions with cement. The strength of cement concrete comes mainly from the binding action of the hydrated cement gel. The requirement of water should be reduced to that required for chemical reaction of unhydrated cement as the excess water would end up in only formation of undesirable voids in the hardened cement paste in concrete. From High Performance Concrete mix design considerations, it is important to have the compatibility between the given cement and the chemical/mineral admixtures along with the water used for mixing.

   Chemical Admixtures
                     Chemical admixtures are the essential ingredients in the concrete mix, as they increase the efficiency of cement paste by improving workability of the mix and there by resulting in considerable decrease of water requirement.
Different types of chemical admixtures are
·         Plasticizers
·         Super plasticizers
·         Retarders
·         Air entraining agents
Placticizers and super placticizers help to disperse the cement particles in the mix and promote mobility of the concrete mix. Retarders help in reduction of initial rate of hydration of cement, so that fresh concrete retains its workability for a longer time. Air entraining agents artificially introduce air bubbles that increase workability of the mix and enhance the resistance to deterioration due to freezing and thawing actions.


Mineral admixtures
                 The major difference between conventional cement concrete and High Performance Concrete is essentially the use of mineral admixtures in the latter. Some of the mineral admixtures are
·         Fly ash
·         Silica fumes
·         Carbon black powder
·         Anhydrous gypsum based mineral additives
Mineral admixtures like fly ash and silica fume act as puzzolonic materials as well as fine fillers, thereby the microstructure of the hardened cement matrix becomes denser and stronger. The use of silica fume fills the space between cement particles and between aggregate and cement particles. It is worth while noting that addition of silica fume to the concrete mix does not impart any strength to it, but acts as a rapid catalyst to gain the early age strength.

BEHAVIOUR OF FRESH CONCRETE
     Introduction:
                The behavior of fresh High Performance Concrete is not substantially different from conventional concretes. While many High Performance Concretes exhibits rapid stiffening and early strength gain, other’s may have long set times and low early strengths. Workability is normally better than conventional concretes produced from the same set of raw materials. Curing is not fundamentally different for High Performance Concrete than for conventional concretes although many High Performance Concretes with good early strength characteristics may be less sensitive to curing.

Workability
                     The workability of High Performance Concrete is normally good, even at low slumps, and High Performance Concrete typically pumps very well, due to the ample volume cementitious materials and the presence if chemical admixtures. High Performance Concrete has been successfully pumped even up to 80 storeys. While pumping of concrete, one should have a contingency plan for pump breakdown. Super   workable concretes have the ability to fill the heavily reinforced sections without internal or external vibration, without segregation and without developing large sized voids. These mixtures are intended to be self-leveling and the rate of flow is an important factor in determining the rate of production and placement schedule. It is also a useful tool in assessing the quality of the mixture. Flowing concrete is, of course, not required in all High Performance Concrete and adequate workability is normally not difficult to attain.

Setting time
                Setting time can vary dramatically depending on the application and the presence of set modifying admixtures and percentage of the paste composed of Portland cement. Concretes for applications with early strength requirements can lead to mixtures with rapid slump loss and reduced working time. This is particularly true in warmer construction periods and when the concrete temperature has been kept high to promote rapid strength gain.
                   The use of large quantities of water reducing admixtures can significantly extend setting time and therefore reduce very early strengths even though strengths at more than 24 hours may be relatively high. Dosage has to be monitored closely with mixtures containing substantial quantities of mineral admixtures so as to not overdose the Portland cement if adding the chemical admixture on the basis of total cementitious material.
 

BEHAVIOUR OF HARDENED CONCRETE
Introduction:
                    The behavior of hardened concrete can be characterized in terms of it’s short term and long term properties. Short-term properties include strength in compression, tension and bond. The long-term properties include creep, shrinkage, behaviour under fatigue and durability characteristics such as porosity, permeability, freeze-thaw resistance and abrasion resistance.

Strength
             The strength of concrete depends on a number of factors including the properties and proportions of the constituent materials, degree of hydration, rate of loading, method of testing and specimen geometry. The properties of   the constituent materials affect the strength are the quality of fine and coarse aggregate, the cement paste and the bond characteristics. Hence, in order to increase the strength steps must be taken to strengthen these three sources.
                  Testing conditions including age, rate of loading, method of testing and specimen geometry significantly influence the measured strength. The strength of saturated specimens can be 15 to 20 percent lower than that of dry specimens. Under impact loading, strength may be as much as 25 to 35 percent higher than under a normal rate of loading. Cube specimens generally exhibit 20 to 25 percent higher strengths than cylindrical specimens. Larger specimens exhibit lower average strengths.


Strength development
                    The strength development with time is a function of the constituent materials and curing techniques. An adequate amount of moisture is necessary to ensure that hydration is sufficient to reduce the porosity to a level necessary to attain the desired strength. Although cement paste in practice will never completely hydrate, the aim of curing is to ensure sufficient hydration. In general, a higher rate of strength gain is observed for higher strength concrete at early ages. At later ages the difference is not significant.

Compressive strength                          
                     Maximum practically achievable, compressive strengths have increased steadily over the years. Presently,28 days strength of up to 80Mpa are obtainable. However, it has been reported that concrete with 90-day cylinder strength of 130 Mpa has been used in buildings in US. The trend for the future as identified by the ACI committee is to develop concrete with compressive strength in excess of 140 Mpa and identify its appropriate applications.

Tensile strength
                     The tensile strength governs the cracking behavior and affects other properties such as stiffness; damping action, bond to embedded steel and durability of concrete. It is also of importance with regard to the behavior of concrete under shear loads. The tensile strength is determined either by direct tensile tests or by indirect tensile tests such as split cylinder tests.

DURABILITY CHARACTERISTICS
             The most important property of High Performance Concrete, distinguishing it from conventional cement concrete is it’s far higher superior durability. This is due to the refinement of pore structure of microstructure of the cement concrete to achieve a very compact material with very low permeability to ingress of water, air, oxygen, chlorides, sulphates and other deleterious agents. Thus the steel reinforcement embedded in High Performance Concrete is very effectively protected. As far as the resistance to freezing and thawing is concerned, several aspects of High Performance Concrete should be considered. First, the structure of hydrated cement paste is such that very little freezable water is present. Second, entrained air reduces the strength of high performance concrete because the improvement in workability due to the air bubbles cannot be fully compensated by a reduction in the water content in the presence of a superplasticizer. In addition, air entrainment at very low water/cement ratio is difficult. It is, therefore, desirable to establish the maximum value of the water/cement ratio below which alternating cycles of freezing and thawing do not cause damage to the concrete. The abrasion resistance of High Performance Concrete is very good, not only because of high strength of the concrete but also because of the good bond between the coarse aggregate and the matrix which prevents differential wear of the surface. On the other hand, High Performance Concrete has a poor resistance to fire because the very low permeability of High Performance Concrete does not allow the egress of steam formed from water in the hydrated cement paste. The absence of open pores in the structure zone of High Performance Concrete prevents growth of bacteria. Because of all the above- reasons, High Performance Concrete is said to have better durability characteristics when compared to conventional cement concrete.

WHEN TO USE HPC
                     High Performance Concrete can be used in severe exposure conditions where there is a danger to concrete by chlorides or sulphates or other aggressive agents as they ensure very low permeability. High Performance Concrete is mainly   used to increase the durability is not just a problem under extreme conditions of exposure  but  under  normal  circumstances also, because carbon di oxide is always  present in  the air .This results in carbonation of concrete which destroys the reinforcement and leads to corrosion. Aggressive salts are sometimes present in the soil, which may cause abrasion. High Performance Concrete can be used to prevent deterioration of concrete. Deterioration of concrete mostly occurs due to alternate periods of  rapid wetting and prolonged  drying with a frequently alternating temperatures. Since High Performance Concrete has got low permeability it ensures long life of a structure exposed to such conditions.


   APPLICATIONS:
     High strength and superior durability characteristics of High Performance Concrete have already been utilized in many structural applications in various countries. Some of the applications of High Performance Concrete are:

Bridges –Joigny (France), Greatbelt (Denmark), Akkegawa (Japan), Willows (Canada)
High rise buildings-Water tower plaza (US), Nova Scotia (Canada)
Tunnels-La Bauma and Villejust (France), Manche (UK)
Pavements-Valerenga (Norway), Highway 86,Paris airport (France)
Nuclear structures-Civeaux (France)

CASE STUDY
   Joigny Bridge:
                              After extensive research and development in French laboratories, the French ministry of public works and the national project on New concretes team agreed to build an experimental bridge using High Performance Concrete. The organizations wanted to demonstrate the feasibility of building a typical prestressed bridge with High Performance Concrete, using means and materials that could be found throughout France. The bridge was built crossing the river Yonne near the town of Joigny, approximately 150 km southeast of Paris. Aesthetical and economical considerations led to the classical design of a balanced continuous three span bridge, which span lengths of 34.00m, 46.00mand 34.00m, a height of 220m and overall width of 15.80m.
                           The bridge was designed according to the French codes BPEL (Beton Precontraint aux Etats Limites ie. Limit State Design Of Prestressed Concrete) and BAEL (Beton Arme aux Etats Limites i.e.. Limit State Design Of Reinforced concrete). These codes have been upgraded to incorporate 60Mpa concretes since they previously dealt only with concrete strength up to 40Mpa. The use of these codes which include different safety factors, led to an actual maximum compressive strength of 30Mpa in the lower fibre to the central span’s mid sections during the last stages of prestressing.
                 It should be emphasized that comparison carried out during the preliminary design of the bridge showed that the concrete quantities could be reduced from 1395m3 when using ordinary 35Mpa concrete to 985m3 with 60Mpa high strength concrete. This 30 percent reduction in concrete volume led to a 24 percent load reduction on the pier, abutments and foundations.
                   Laboratory tests were run to define a mix design allowing the production of ready mix concrete with
·         A 28 day mean strength of about 70Mpa.
·         The ability to be transported 30km on a boat from the concrete plants and deliver fresh to the construction site
·         The ability to be pumped through 120m long pipes.
·         A high workability and a sufficient setting time
  As the concrete plant was producing concrete fir the bridge, tests were run on every batch. The water cement ratio remained between 0.36 and 0.38.The entrained air contents were within 0.5 and 12 percent. The slumps, measured at the site were 200mm for more than 2 hours. The concrete strength was measured according to French standards using 160mmx320mm test cylinders cast in metallic mould. At 28 days the minimum and maximum strength values were 65.5Mpa and 91.7Mpa. The tensile strength was measured on cylinders where the average tensile strength was 5.1Mpa on 28-day sample.
The first French prestressed concrete bridge designed and built with a 60Mpa characteristic strength and following the French building codes was successfully completed in early 1989.

ADVANTAGES
·         Reduction in size of the columns
·         Speed of construction
·         More economical than steel concrete composite columns
·         Workability and pumpability
·         Most economical material in terms of time and money 
·         Increased rentable\useful floor space
·         Reduced depth of floor system and decrease in overall building height
·         Higher seismic resistance, lower wind sway and drift
·         Improved durability in aggressive environment 
·         Wearing resistance, abrasion resistance
·         Durability against chloride attack
·         Increased durability in marine environment
·         Low shrinkage and high strength
·         Service life more than 100 years
·         High tensile strength
·         Reduced maintenance cost
  

       LIMITATIONS

·         High Performance Concrete has to be manufactured and placed much more carefully than normal concrete.
·         An extended quality control is required
·         In concrete plant and at delivery site, additional tests are required. This increases the cost
·         Some special constituents are required which may not be available in the ready mix concrete plants.


 



2 comments:

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