Thursday, October 19, 2017

MAT-VANTAGE

New Age Retrofitter

 


Sifcon laminate, a new version of fibre reinforced concrete, has demonstrated considerable potential when it comes to strengthening structures. DR GS THIRUGNANAM, HEAD, DEPARTMENT OF CIVIL ENGINEERING, INSTITUTE OF ROAD AND TRANSPORT TECHNOLOGY and S BALAJI, RESEARCH SCHOLAR, DEPARTMENT OF CIVIL ENGINEERING, KS RANGASAMY COLLEGE OF TECHNOLOGY tell us how.
 

The cost of civil infrastructure constitutes a major portion of the national wealth. The rapid deterioration of RC structures has created an urgent need for the development of long-lasting and cost-effective methods for repair, retrofit and new construction. Besides the well known problems of seismic retrofitting, the strengthening of structures can be also required by the degradation of structural materials or by the increase in design loads. Moreover, there are important infrastructures, such as bridges or tunnels, which have to be repaired to avoid the social costs related to the demolition and the reconstruction of new structures. Complete replacement is likely to be uneconomical and might certainly be a waste of natural recourses if upgrading or strengthening is a viable option.

 

Strengthening is defined as a modification of the structure, either damaged or new, with the purpose to increase its load-carrying capacity or stability with respect to its previous condition. As reinforced concrete attain ages, a variety of detrimental effects can occur. These include spalling, flaking, or cracking of the concrete and subsequent corrosion of the reinforcing steel. These occurrences can significantly affects the strength of structural members. Many existing structures were designed and built before the development of current seismic codes or on the basis of earlier codes before ductile reinforcement detailing was required. Hence it becomes necessary to strengthen these structures to enhance the ductility of the structure under seismic loading.

 

Various techniques used for the strengthening of existing reinforced concrete (RC) structural members includes external pre-stresssing (by means of post-tensioned bars), epoxy resin injection, plate bonding technique (in which steel plates and ferro cement laminates were bonded to the structure, to enhance the strength of the structure), composite material such as steel plate - concrete composites, jacketing (to improve the load carrying capacity).

 

New FRC options

 

In recent times there has been a growing interest in the use of high performance fibere reinforced concretes (FRC) like SIFCON and SIMCON (Slurry Infiltrated Mat Concrete) for seismic repair and retrofit of concrete structural elements. Use of these high performance materials for strengthening techniques would lead to substantially higher strengths, seismic resistance, ductility, corrosion resistance and durability while also being faster and more cost-effective method to construct than conventional methods.

 

Composition of SIFCON 

 

Slurry Infiltrated Fibrous Concrete (SIFCON), an exceedingly improved version of conventional fiber reinforced concrete, is a unique high performance concrete having unique properties in areas of both strength and ductility. The primary constituents of SIFCON are steel fibers and cement-based slurry. The matrix was different from normal FRC in the sense that in FRC, fiber volume usually varies from 1-3 per cent by volume whereas in SIFCON, fibere contents may range from 4-20 per cent using special manufacturing techniques. SIFCON is cast using a replacing technique in which fiberes are placed in the mold or on a substratum and infiltrated with cement based slurry. The fiberes can be sprinkled by hand or by using fibere-dispensing units. The amount of fiberes that can be incorporated depends on fibere aspect ratio, fibere geometry, and placement technique. More fiberes can be incorporated if the aspect ratios are low. The volume fraction can also be increased by mild vibration. The SIFCON matrix will not have coarse aggregates; however, it may contain fine sand and additives such as fly ash, micro silica, quartz powder and latex emulsions.

 

Tried and tested

 

Researchers have indicated that the use of SIFCON jackets as external shear reinforcement eliminated the brittle shear failure and increased the ultimate shear strength of the repaired beams from 25-50 per cent (Shannag et al, 2001). SIFCON shows significant improvement in the behavior of the compression as well as tension zone of reinforced concrete structural members (Naaman et al, 1992). Balasubramanian et al (1997) investigated the behavior of SIFCON under pure torsion and found that SIFCON possess high torsional strength when compared with FRC specimens. SIFCON specimens show greater ductility and greater resistance to cracking and spalling (Raghuprasad et al, 2003).

 

The experiment

 

Laboratory experiments have shown that SIFCON is an innovative construction material possessing both high strength and large ductility. An experimental investigation was carried out to study the behaviour of conventional RC beams and beam-column joints with precast SIFCON laminates. Beams and beam-column joints were cast using M30 grade concrete, 20 mm thick precast SIFCON laminates were used to strengthen the beams and beam-column joints. Round crimpled fibres of 0.5 mm diameter and aspect ratio of 60 were used to cast SIFON specimens.  Fibere volume fraction was 9 per cent. Cement, micro silica, fly ash and quartz powder were used for making cement slurry for SIFCON with the mix proportion 1:0.1:0.5:0.5. Water binder ratio was about 0.45. Super plasticizer of 1.5 per cent was used to increase the workability of the cement based slurry.

 

The behaviour of RC beams with laminate confinement at the bottom face, side face, and on three faces (bottom and side faces) where studied under two point flexural cyclic loading system. Results indicate that the strengthening of RC beams with SIFCON laminates has significantly improved the cracking behavior in terms of significant increase in first crack load and the formation of larger number of finer cracks. The stiffness, ductility and energy absorption are found to be increased to a great extent when the beams are strengthened by three face confinement (bottom and side faces).

 

The behaviour of exterior RC beam-column joints strengthened with precast SIFCON laminates were investigated under static reverse cyclic loading. The specimens were tested under cyclic loading system (forward and reverse cyclic loading) similar to the load experienced during earthquakes. The study reveals that the precast SIFCON laminates can be effectively used for the strengthening of beam-column joints which increases the load carrying capacity, ductility and energy absorption capacity of the joints.

 

Ideal seismic retrofitter

 

In general it is concluded that the use of SIFCON in structural element increases the load carrying capacity, ductility and energy absorption capacity of beam and beam-column joints. This type of behavior is desirable for the structures located in earthquake prone areas. SIFCON laminates proves to be an effective material for the seismic retrofitting of structures. Out of various existing strengthening techniques such as jacketing, plate bonding technique, epoxy resin injection, ferro cement laminates etc; strengthening of RC structural elements with precast SIFCON laminates enhances the overall performance of the structure. Moreover the application of precast SIFCON laminates over the existing structure involves least complexity and hence it can be easily adopted under any circumstances.

 

 

The cost of SIFCON laminates of size 1m x 1m x 0.2 m is approx Rs 1,500. The steel fibere and the cement matrix (slurry) play an important role in SIFCON.  Hence the production of SIFCON laminates can be commercialiszed by means of fibre dispersion units and by pumping units (like shotcreting). So far no records were found regarding the practical application of SIFCON laminate as strengthening technique however in future the study can be extended for the practical implementation of precast SIFCON laminates for the strengthening/retrofitting of existing structure.




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