Category: Concrete

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Concrete

Usage of self compacting concrete

Usage of self compacting concrete varied from, The certain quality control measures has to be carried out to produce best quality self compacting concrete.

  • Production of self compacting concrete staff should have trained skilled persons
  • Testing of aggregate grading and moisture content should  often carried out because of SCC more sensitive than normal concrete.
  • Workability tests should be carried out for every batch until the results to be consistent & compliant. It also guides to check visibility of each batch before send to the site.
  • The segregation limits during the placement of concrete are, maximum free fall 5 m and distance of flow circled 10 m from shooting point.
  • Curing has to be carried out as soon as practicable after placing in order to minimize the risk of shrinkage cracking.
  • Fiber reinforcement concrete also can be used the self compacting concrete

Precast Concrete Industry

The wide ranges of precast are produced by precast suppliers such as columns, beams, slabs, paving slabs, sewerage tanks, Hume pipes, box culverts etc…  in Sri Lanka. The hardened precast products are installed at the site with combining the in-situ concrete according to structural design.

Usage of self compacting concrete for the precast industry may immensely influenced to precast production rate because SCC can be design to get high early strengths which will help to early de moulding process to continue pre-stressing purpose or next production step.

The SCC is normally required less time to transportation and placing, therefore consistence retention is not necessary, allowing the use of a SCC having a short consistence retention, fast set and early strength development. This faster start of the hardening process is beneficial for the early strength, required to maintain the production cycle.

Another benefit of the usage of Self compacting concrete for precast industry can casts complex shapes as necessity of architectural designs and increasing the life time of the form work or molds just because of absence of the vibration. Nice smooth surfaces are increasing the attraction to the products.

Usage of Self compacting concrete urgently required in presently in Sri Lanka to the precast industry as well as the construction projects and I am suggesting it to use in precast industry as a starting point which will help to aware the peoples and to motivate the investors to use modern technology in Sri Lanka.

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Concrete

Mix design of SCC

Mix design of SCC  is free to design & using single method or combination of methods has achieved universal approval and most of them have adherents. The requirements are

  • Filling ability
  • Passing ability
  • Segregation resistance

Guidelines for the mix design

  • Reducing the volume ratio of aggregate to cementitious material (1:2)
  • Increasing the paste volume and water-cement ratio (w/c)
  • The maximum coarse aggregate size (Normally 16 – 20 mm)
  • Usage of Superplasticizer admixtures
  • Viscosity enhancing admixtures

Initial mix design of SCC

  • The coarse aggregate content in concrete is limited to 50% of solid volume
  • The fine aggregate content in concrete is fixed to 40% of the mortar volume
  • The water/powder ratio is assumed to range of 0.8-1.0,depending on the properties of the powder

The superplasticizer dosage and final water/powder ratio are determined so as to ensure self compactibility.

Testing methods of workability & typical ranges of values.

Adjustments of the mix of SCC

Laboratory trials should be used to verify properties of the initial mix composition. If necessary, adjustments to the mix composition should then be made. Once all requirements are fulfilled, the mix should be tested at full scale at the concrete plant or at site. If the performance cannot obtain, then the fundamental mix design has to be changed.

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Concrete

Objectives of SCC

The Objectives of  SCC should meet the some functions at the plastic stage are different from those on a traditional vibrated fresh concrete. Filling of formwork with a liquid suspension requires workability performance which is recommended to be described as follows:

  • Filling Ability

SCC is filling of entire formwork and encapsulation of reinforcement and other inserts with maintaining homogeneity in both vertical & horizontal directions are essential.

  • Passing Ability

Objectives of SCC is passing through congested area such as narrow sections of the formwork even closely spaced reinforcement etc without blocking caused by interlocking of aggregate particles.

  • Resistance to Segregation

SCC is maintaining of homogeneity throughout mixing and during transportation and casting. The dynamic stability refers to the resistance to segregation during placement. The static stability refers to resistance to bleeding, segregation and surface settlement after casting.

  • Open Time

Open time has to be closely following specified time duration for initial setting. New generations of admixtures offer increased possibilities of setting a specified target open time, e.g. very short for precast applications and very long for in-situ castings with long haulages.

  • Precision & Accuracy

More essential to keep target workability for SCC as the casting is fully relying on the material properties as no tools normally will be available for manual material transport and compaction.

  • Pumping Ability

More essential to keep target workability for SCC as the casting is fully relying on the material properties as no tools normally will be available for manual material transport and compaction.

  • Finishing Ability

SCC normally has less bleeding tendency. In many cases the finishing will be made on a coarser and stickier surface. Appropriate materials on the finishing tools are thus to be recommended.

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Concrete

Development of Self Compacting Concrete

Origination of self compacting concrete is in 1983 from Japan as a solution for the shortage of the skilled labor who is required for the compaction purposes which is adequately required for constructing of durable concrete structures. However, one solution for the achievement of durable concrete structures is the employment of the self-compacting concrete that is able to flow under its own weight and completely fill the formwork, even in the presence of dense reinforcement, without the need of any vibration, whilst  maintaining homogeneity. The necessity of this kind of concrete was proposed by Okamura in 1986.

Research have been carried out to development of self compacting concrete, including a fundamental study on the work- ability of concrete, by Ozawa and Maekawa at the University of Tokyo and first the  prototype of  self compacting concrete was completed using materials already in the market in 1988.

The prototype performed satisfactorily with regard to drying and hardening shrinkage, heat of hydration, denseness after hardening, and other properties. This concrete was named “High Performance Concrete”.

The method for achieving self-compacting ability involves

  • Limited aggregate content
  • Low water-powder ratio
  • Employment  of superplasticizer

The required material constituent to development of self compacting concrete are cement, Fine aggregates, Coarse aggregate, admixtures, additions, fiber etc…

The each material should meet certain characteristics to develop required concrete such as strength, durability, etc…The typical cement content is 350-450kg/m3, fine aggregate which are used to normal concrete both crushed and rounded sand can be used, normally coarse aggregate range may be 16-20mm, superplasticizer admixture is acting as water reducing and high mobility agent, viscosity modifying agent used to controlling the segregation when the amount of powder usage limited.

Remember, each material & constituent should be selected according to British standards (BS), ASTM or Euro codes to meet standard specification of project or body specified and mix design has to be done to calculate each material composition.

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Concrete

Advantages of the Self Compacting Concrete

Why Self compacting concrete should used

Advantages of the self compacting concrete that is able to flow under its own weight and completely fill the form work, even in the presence of dense reinforcement, without the need of any vibration, whilst maintaining homogeneity.

Financial & Environmental Benefits

  • Minimal labor involved
  • Rapid construction without mechanical vibration
  • Low noise-level in the plants and construction sites
  • Overcome problems arise with vibration.
  • Safer working environment
  • Accelerated project schedules
  • Reduced equipment wear
  • Allows for innovative architectural features
  • Greater Range of Precast Productions

Engineering advantage of the self compacting concrete

  • Better surface finishes
  • Easier placing
  • Improved durability
  • Greater freedom in design
  • Thinner concrete sections
  • Ease of filling restricted sections and hard to reach areas
  • Encapsulate congested reinforcement
  • Allows for innovative architectural features
  • Homogeneous and uniform concrete
  • Better reinforcement bonding

Categories
Concrete

Self Compacting Concrete

Self compacting concrete Introduction

Inception of self compacting concrete (SCC), which flows under its own weight and does the compaction as well without mechanical vibration, was in 1988 to achieve durable concrete by Japanese researches. Since then various investigations have been carried out and practical structures such as wider bridges using in situ concrete in transportation, carried out in Japan and European countries, The SCC having advantages of high early strength ,durable structures, easy placement and many other advantage rather than traditional concrete, is widely used to precast concrete industry to form architectural designs.

SCC is highly workable concrete that can flow under its own weight through highly congested reinforcement sections without segregation and bleeding. Self compacting concrete should also have properties of rheology such as relatively low yield value to ensure high flow ability, a moderate viscosity to resist segregation and bleeding, and must maintain its homogeneity during transportation, placing and curing to ensure adequate structural performance and long term durability. The successful development of SCC offers beneficial of economically and wide engineering varieties.

The two main purposes of SCC during the plastic stage are flowability which can be achieved using the high dosage of superplasticizer and stability or resistance to segregation of the plastic stage mixture that is attained by increasing the total quantity of the fines or otherwise using the viscosity agent admixture