Received date: 30 May, 2017; Accepted date: 26 July, 2017
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Retrofitting is the strengthening of existing structures to enable them in carrying increased external loads. The need for retrofitting arises due to change in utility, expansion, damages due to accidents and environmental conditions. This paper presents an investigation of flexural behavior of plain cement concrete (PCC) beam retrofitted with ultrahigh strength concrete (UHSC) overlay. The proposed study consists of applying UHSC as a strengthening material on the bottom surface of the plain cement concrete beam in order to increase its stiffness and flexural strength. The beam of size 150 mm × 150 mm × 650 mm is tested for the failure load using M25 grade of concrete. Experiments were carried out under displacement controlled loading method.
Retrofitting, Ultra-high strength concrete overlays
Plants Concrete structures can become deficient during their service and require strengthening and repairing. This need arise as a result of design or functional errors, construction changes, design code update, damage accumulated over time, or caused by accidental overloading, fires or earthquakes. Since replacement of deficient structures requires huge investments, strengthening has become suitable way for improving the load carrying capacity and prolonging there service life. Retrofitting can be applied on old structures and structures in seismic zone to resist there structural behavior. Retrofitting can be achieved by using composite materials. To improve the load carrying capacity of this damaged structure retrofitting is the most widely used techniques now a day. There are many composite materials such as FRP, CFRP, GFRP and external steel plates etc. but FRP currently in use has some drawbacks such as difficulty of application at low temperature, moisture incompatibility, and emission of toxic fumes, etc.
There were also considerable disadvantages such as huge cost of methods labours required. To overcome these disadvantages, cementious composites have been come into industry before some decades. The Ultra High Strength Concrete overlay consists of cementious binders with steel fibers which provides greater degree of ductility and crack width resting property. Hence this material can be used as an effective retrofit material (Alaee and Kariholoo, 2003).
In the present paper, the flexure performance of the PCC beams strengthened with UHSC overlays has been studied. Four point bending test has been carried out on all beams. The comparative study has been done on the on the performance of conventional PCC beams and strengthened beams (Xu, et al., 2012).
Cement: The cement used in this study is OPC 53 grade cement.
Fine aggregate: The sand used for the experimental work was locally procured and confirmed to zone II. The sand is sieved through 4.75 mm sieve and to remove any particles greater than 4.75mm. The fine aggregates were tested as per Indian standard specification IS 383 – 1970.
Coarse aggregate: Locally available coarse aggregates were used in these work . Aggregates passing through 20 mm sieve is retained in 4.75 mm were sieved and tested as per Indian standard specification IS – 383 – 1970 (IS, 12269, 1987).
Water: Fresh and clean water is used for the experiments. Water is free from organic matters, sulphates, chlorides, oil and slits.
UHSC materials: Two UHSC ductile material has Portland cement, silica fume, quartz sand, steel fibers, and super plasticizer.
Cement: Cement dosage of about 800 to 1000 kg/m3. A large amount of cement has negative impacts such as heat of hydration and hydration problems.
Fine aggregate: The fine aggregate used in these experiment was natural siliceous clean and has a maximum reactivity during heat treating.
Steel fibers: Use of steel fibers improves ductility. Volume of only 2.5% to 10% is only used in overlays. Steel fibers of diameter 0.3 mm are used. Use of these fibers increases the flexural strength of concrete of about 50 Mpa (Table 1).
|Properties of steel fibers|
Table 1. Properties of steel fibers
Silica fume: Silica fume imparts improvement in rheological, mechanical and chemical properties. It improves durability of concrete b reinforcing the microstructure through filler effect thus reduces segregation and bleeding. Silica fumes of specific gravity 2.34 used in powder form in these studies (Tables 2 and 3).
Table 2. Chemical properties of silica fume
|Appearance||White color powder|
|Particle size||10nm – 20nm|
|Density||2.2 – 2.6g/cm3 at 25c|
Table 3. Physical properties of silica fume
Quartz powder: Maximum reactivity during heat treating.
Super plasticizer: The super plasticizer used in these projects confirm to the guide lined confirm to the guideline drawn by IS 9103-79. Water reducing admixture is used only in the ultra-high strength concrete overlays. Physical properties of super plasticizer are as follows.
Specific gravity: 1.170 – 1.190
All the beams were casted and retrofitted and loaded for four points or three points bending whose beam with a span of 650 mm and loaded at the span of 125 mm from both the ends. Length of beam of 650 mm, width of beam was 50 mm and depth is 150 mm. in these study PCC beam has been designed by using M 25 grade of concrete. The beams were retrofitted using mechanical anchors. Mechanical expansion anchors work by applying frictional force on either side of the predrilled holes. There are three types of mechanical anchors are stud, drop-in and selfdrilling anchors.
In these experiments we are going to use stud type mechanical anchors. The stud mechanical anchor consists of threaded rod with an expansive device on one end and threaded portion with a nut on the other end. This anchor is installed by sliding the expansive end of the anchors into the pre drilled holes. It is done by tightening the nut to the recommended torque. Stud anchors protrude above the concrete after installation should be used when the object being attached requires washer and a nut of specified size and diameter (Karihaloo, et al., 2012).
Advantages of using stud anchors
The biggest advantage of using these mechanical expansion anchors over adhesive anchors for immediate loading. Mechanical expansion anchors can accept the immediate load after installing. This may be advantageous that it can be used that they have limited installation time (Figure 1).
Concrete mix design is the process by which the proportions of various raw materials of concrete are determined in an aim to achieve a certain minimum strength and durability as possible. The concrete mix design is designed as per IS 10262–2009 for M25 grade of concrete (Table 4).
|Cement||Coarse Aggregate||Fine Aggregate||Water|
Table 4. Mix proportions
Casting of beams
For the study M25 grade of concrete is used. Beams or prisms are of size 650 mm × 150 mm × 50 mm used for testing for flexural strength of concrete. All the materials were placed in the rotary type drum mixer and water is added during rotation of drum. Cast iron mold’s of standard size were used for the casting of beams. The specimens were de-moulded after 24 hours and submerged in water till testing is done at 28 days.
All beams have been tested under simply supported end conditions. Two point loading condition were adopted for testing. Testing of beam has been done with help of hydraulic jack connected with load cell and the value was obtained from data causation system. Load has been applied with the help of the beam cell. Five dial gauges has been fixed, one at the center and other two at the support conditions and at the compression and tension face of the beams. The deflections value of the beam has been obtained through this load cells (Figure 2).
Strengthening of beams
The beam has been loaded up to the ultimate load of 80% before strengthening. The strengthening has been done with UHSC overlays, attached beneath the tension surface of beams. The thickness of overlay was kept as 30 mm and 60 mm overlays. The bonding between the beam and UHSC layer was done by the mechanical anchors. The overlay is provided throughout the span of the beam (Table 5).
|PCC _ CONT||Control beam|
|PCC _ UHSC30||Beams retrofitted with UHSC overlays|
|PCC _ UHSC60|
Table 5: Characteristics of test specimen
PCC _ CON: The intinal crack was seen on the center point on control beam. These cracks were occurred at the load of 14.13 KN. The crack was predominant at the center and it is constant at bending moment zone. The crushing occurs on the compression face. Only center line cracking was seen. No other cracks were seen on the control beam (Figure 3).
The intinal crack was occurred at the load of 13.07 KN for UHSC30 and at 14.22 KN. Several cracks has been seen in beam (Figure 4).
Load v/s displacement
The load v/s deflection of the beams was compared as shown in the (Figure 5). All the beams were tested for four – point bending test under constant displacement loading. As mentioned in the table the PCC _ UHSC is decreased by the increase in load carrying capacity is due to steel fibers which is ductile and takes mare load and there by widening of cracks is seen in Table 6.
|PCC _ CON||14.13||0.68|
|PCC _ UHSC30||13.07||0.89|
|PCC _ UHSC60||14.22||1.34|
Table 6. Test results of beam.
The flexural behavior of the plain cement concrete beam and the strengthened beam using UHSC overlays is compared in this paper. Four point bending test were carried out in the plain cement concrete beams and PCC beams with UHSC overlay. It is reveled that there is an improved in ductility of performance of PCC beam was seen when strengthen with UHSC overlays.
Alaee, F.J. and Kariholoo, B.L. (2003). Retrofitting of Reinforced concrete beams with CARDIFIC. Journal of Composites for Construction. 7.
IS 12269. (1987). Specification for 53 grade ordinary Portland cement (OPC).
Karihaloo, B.L., Alee, F.J. and Benson, S.D.P. (2012). A new technique for retrofitting of damaged concrete structures. Structures and Buildings. 152.
Xu, S.L., Wang, N. and Shang, X.F. (2012). Flexural behavior of plain cement concrete beam strengthened with ultra-high strength cementations composites layers. Materials and Structures. 45.