PERFORMANCE OF AFRP CONFINED RC DRIVEN PILES SUBJECTED TO VERTICAL LOADS

Transcription

1 International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 4, April 2018, pp , Article ID: IJCIET_09_04_146 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed PERFORMANCE OF AFRP CONFINED RC DRIVEN PILES SUBJECTED TO VERTICAL LOADS J. Prakash Arul Jose Research Scholar, Department of Civil Engineering, Bharath IHER, Chennai P. Rajesh Prasanna Professor, Department of Civil Engineering, Anna University, Tiruchirappalli-24 ABSTRACT Soil-Structure Interaction is the mechanism that governs the pile response behaviour and ultimate capacity of the structure to applied loads. The analysis of the interaction between structural foundations and supporting soil media is of fundamental importance to both structural and geotechnical engineering. This experimental study finds the performance of AFRP confined RC driven piles subjected to vertical loads. Load carrying capacity of AFRP confined and unconfined piles were found out using the dynamic formulae and pile load test. Safe load carrying capacity of piles determined from piles load test was slightly higher than the dynamic formulae. The experimental result also shows that surface roughness of specimen is significantly changes the interface friction angle. The shear strength at the interface increases with the increase in surface roughness of the specimens. Keywords: Strengthening, RC piles, FRP, AFRP, vertical loads, driven piles. Cite this Article: J. Prakash Arul Jose and P. Rajesh Prasanna, Performance of AFRP Confined RC Driven Piles Subjected to Vertical Loads, International Journal of Civil Engineering and Technology, 9(4), 2018, pp INTRODUCTION Pile foundation is one of the most popular form of deep foundations and is widely used for both land based and water front structures. Piles are generally adopted for structures in weak soils characterized by low shear strength and high compressibility and also in good soil formations if structures are subjected to heavy loadings and moments. Piles embedded in cohesive soils may experience a negative skin friction which may in turn reduce the load carrying capacity of the pile. The swelling potential of clay may also be a disturbing factor when we employ pile foundations. As far as the uplift load is concerned which is caused due to the swelling of the soil, the end bearing will not contribute to resistance of these forces. The uplift forces are only resisted by skin friction or cohesion and to battle these forces these editor@iaeme.com

2 J. Prakash Arul Jose and P. Rajesh Prasanna properties of the soil must be enhanced. The response of an individual pile to externally applied load is one of the most complex soil-structure interaction problems in the field of foundation engineering. An extensive amount of theoretical and analytical work has been reported on the behavior of single and group piles under vertical and lateral loads. The main focus of this research is to determine the potential of AFRP materials in strengthening of RC piles subjected to vertical loads. To establish the feasibility of using AFRP materials in strengthening of RC piles, more information and performance data were gathered in critical areas, interface behavior between AFRP materials & soil and soil-pile load transfer interactions. Frost and Han (1999) conducted experiments to investigate the behaviour of sand-frp interfaces and concluded that interface shear behaviour between FRP composites and granular materials depend on the relative roughness (surface roughness/particle mean size), the normal stress level, the initial density of the soil mass and the angularity of the particles. Wang and Richwien (2002) studied the important design parameter of reinforced soil structures, i.e. the pullout friction. The direct shear test gives much smaller values than the pullout test. A simple approach was proposed by them, from which the pullout friction can be estimated from the coefficient of friction between soil and reinforcement determined by the direct shear test. Meera et al. (2007) developed the generalized procedure to analyze and predict the flexural behavior of axially and laterally loaded pile foundations under liquefied soil conditions. The response of the piles as found by using the proposed model are found to be in excellent agreement with theoretical and experimental values reported in literature. Sen and Mullins (2007) studied the application of FRP composites for underwater piles repair. They suggested that the bi-directional material should be preferred over uni-directional material and carbon fibre over glass fibre. Ilki et al. (2008) studied the axial behaviour of RC columns retrofitted with FRP composites. The study concluded that the external confinement of columns with CFRP sheets resulted in an increase in the strength and ductility. Dai et al. (2010) investigated the influence of moisture on the bond behavior of FRP to concrete interfaces. The study showed that all FRP-to-concrete bonded joints failed at the interface between the primer and concrete after exposure while those not exposed usually failed within the concrete substrate. They suggested that the effect of the concrete substrate moisture content on short-term interfacial bond performance could be eliminated by using an appropriate primer. 2. INTERFACE BEHAVIOR BETWEEN SOIL AND AFRP COMPOSITES It is essential to determine the interface strength between soil and geotechnical structures to make a good estimation of load transfer between structures and soils. FRP has limited use in geotechnical engineering applications to date, due to lack of information regarding the behavior of systems that include these materials. So it is important to study the interface behaviour between FRP and soil. An experimental study was performed to evaluate the importance of various factors. The behavior of the AFRP-soil interfaces was also compared with the concrete-soil interfaces. The interface frictional strength study was carried out in a conventional direct shear test apparatus. Concrete specimens of size 6 cm x 6 cm x 1.4 cm were prepared for interface frictional study. The specimens were tested without any AFRP wrapping and wrapped with AFRP sheet. The specimens were placed in the lower half of the direct shear box and the upper half of the shear box was filled with soil at predetermined density as per Tables 1. The direct shear test setup is schematically shown in Fig editor@iaeme.com

3 Performance of AFRP Confined RC Driven Piles Subjected to Vertical Loads Figure 1 Schematic view of interface friction measurement set up Engineering and Index properties of Soil: Engineering and index properties of soil were determined by conducting experiments in order to understand the interface frictional resistance between AFRP wrapped concrete and soil. The index and engineering properties of the soil is presented in Table 1. Table 1 Engineering and Index properties of clayey sand % Passing Atterberg Limits Dry unit weight (kn/m 3 ) Type of soil LL PL Ip γ (IS 1498) mm μm μm (%) (%) (%) d (max) γ d (min) γ d (test) SC Surface Roughness of Specimens: Surface roughness of the material is one of the important factors that influence the shear strength parameters. Generally, Absolute roughness (R a ) is considered for calculating interface friction between two different materials. The absolute surface roughness of the specimens was estimated using Surface Roughness Meter. The absolute surface roughness values of specimens are presented in Table 2. Table 2 Surface roughness of specimens Specimens Surface roughness, R a (μm) Concrete Bi-AFRP wrapped specimen The obtained interface friction angle between clayey sand and AFRP composites is presented in Table 3. Table 3 Interface friction angle between soil and AFRP composites Type of interaction Angle of internal/interface friction (degree) Soil-Soil Soil Concrete Soil Bi-AFRP wrapped specimen editor@iaeme.com

4 J. Prakash Arul Jose and P. Rajesh Prasanna The experimental results show that surface roughness of specimen is significantly changes the interface friction angle. The shear strength at the interface increases with the increase in surface roughness of the specimens. 3. PREPARATION OF AFRP CONFINED RC DRIVEN PILES Piles are generally used to transmit vertical loads to the surrounding soil media. A relatively new trend in deep foundation industry is to use a fiber reinforced polymer composite materials as a substitute in piling system. This experimental study presents the performance of AFRP confined and unconfined piles subjected to vertical loads. RC pile of length 750 mm with 50 mm diameter was used for this study. The following steps are followed to cast the AFRP confined and unconfined precast RC piles. Concrete: The characteristic compressive strength of concrete used for the study was 30 N/mm 2. The mix ratio adopted for casting the piles was 1: 1.204: (Cement: Fine aggregate: Coarse aggregate) with water-cement ratio of Reinforcement: The yield strength of steel used for the study was 415 N/mm 2. Six numbers of 6 mm diameter bars were used as longitudinal reinforcement with a cover thickness of 10 mm. Bars of 6 mm diameter at 100 mm spacing were used as ties. Fiber Reinforced Polymer (FRP): Aramid fiber reinforced polymer was used in the study. Properties of AFRP material are given in Table 4. Table 4 Properties of AFRP Materials Properties Aramid Bi- directional Mass of fiber (g/m 2 ) Fiber thickness (mm) 0.33 Nominal thickness per layer (mm) 0.55 Fiber tensile vertical load carrying capacity (N/mm 2 ) 3900 Tensile modulus (N/mm 2 ) Primer Coating: The mixed material of Nitowrap 30 primer was applied over the prepared and cleaned surface and it was allowed for drying about 24 hours before the application of Nitowrap 410 saturant. Properties of Nitowrap 30 primer is given in Table 5. Table 5 Properties of Nitowrap 30 primer Density 1.14 g/cc Pot life 25 27⁰ C Full cure 7 days Saturant Coating: The Nitowrap 410 saturant system used in this work was made of two parts, resin and hardener. The components were thoroughly hand mixed for 3 minutes before application. Properties of Nitowrap 410 saturant is given in Table 6. Fig.2 shows the AFRP confined and unconfined RC piles editor@iaeme.com

5 Performance of AFRP Confined RC Driven Piles Subjected to Vertical Loads Table 6 Properties of Nitowrap 410 saturant Color Application temperature Viscosity Density Pot life Full cure Pale yellow to amber 15⁰C - 40⁰C Thixotropic g/cc 2 30⁰ C 5 30⁰ C a) Unconfined b) Bi AFRP Figure 2 AFRP Confined and unconfined RC piles 4. SAFE LOAD BASED ON PILE DRIVEN FORMULAE The allowable load Q a is the safe load which the pile can safely and is determined on the basis of (i) ultimate bearing resistance divided by suitable factor of safety, (ii) the permissible settlement, and (iii) overall stability of the pile foundation. The load carrying capacity of a pile can be determined by the following methods: 1. Dynamic formulae 2. Static formulae 3. Pile load tests 4. Penetration tests Dynamic formulae: When a pile hammer hits the pile, the total driving energy is equal to the weight of hammer times the height of drop or stroke. In addition to this, in the case of double acting hammers, some energy is also imparted by the steam pressure during the returns stroke. The total downward energy is consumed by the work done in penetrating the pile and by certain losses. The various dynamic formulae are essentially based on this assumption. It is also assumed that soil resistance of dynamic penetration of pile is the same as to the penetration of pile under static or sustained loading. Following are some the commonly used dynamic formulae. Engineering news formulae: The Engineering News formula was proposed by A. M. Wellington (1818) in the following general form editor@iaeme.com

6 J. Prakash Arul Jose and P. Rajesh Prasanna Where, = Allowable load C = Empirical constant C= 2.5 cm for drop hammer and C = 0.25 (single and double acting hammer) Single acting hammer: If the hammer is raised by steam, compressed air or internal combustion but is allowed to fall by gravity alone, it is called a single acting hammer. The energy of such of hammer is equal to the weight of the ram times the height of fall. Double acting hammer: The double acting hammer employs steam or air for lifting the ram and for accelerating the downward stroke.it operates with succession of rapid blows. Drop hammer: If a hammer is raised by winch and allowed to fall by gravity on the top of a pile, it is called a drop hammer. F = Factor of safety = 6 S = Final set (penetration) per blow, usually taken as average penetration cm per blow for the last 5 blows of a hammer, or 20 blows of a steam hammer. W = Weight of hammer (W = 4.90 kg) H = Height of fall (H = 45 cm) Piles are commonly driven by means of a hammer supported by a crane or by a special device known as pile driver. During pile driving, heads, helmets, or caps are placed on the top of the pile to receive the blows of the hammer and to prevent damage to the head of the pile. A cushion, consisting of a pad of resilient materials, hard wood or rope, is placed between the drive cap and the top of pile to protect the pile head. Single acting hammers are advantageous when driving heavy piles in compact or hard soil, while double acting hammers are generally used to drive piles of light or moderate in soils of average resistance against driving. Piles are ordinarily driven to a resistance measured by the number of blows required for the last 1 cm of penetration. Resistance of 3 to 5 blows per cm is commonly specified for concrete pile. Final set penetration per blow, usually taken as average penetration per blow for the last 5 blows of a drop hammer, or 20 blows of a steam hammer. It is denoted as S. When a pile hammer hits the pile, the total driving energy is equal to the weight of hammer times the height of drop or stroke. In addition to this, in the case of double acting hammers, some energy is also imparted by the steam pressure during the returns stroke. Driving of AFRP confined and unconfined RC piles by single acting hammer are shown in Fig. 3. Table 7 presents the safe load based on pile driven formulae. a) Unconfined b) Bi AFRP Figure 3 Driving of AFRP confined and unconfined RC piles editor@iaeme.com

7 Performance of AFRP Confined RC Driven Piles Subjected to Vertical Loads Table 7 Safe load based on Engineering News formula Types of Confinement No. of blows Safe load (N) Unconfined pile Pile confined with bidirectional AFRP mat (Bi-AFRP) SAFE LOAD BASED ON PILE LOAD TEST The pile load test is carried out in clayey sand. The pile test can be performed either on a working pile which forms the foundations of the structure or on a test pile. The pile load test setup is shown in Fig.4. a) Schematic diagram b) Photograph Figure 4 Test setup Loading Procedure: The vertical load to the pile was applied through steel discs of known weights. The load is applied equal increments of about one fifth of the estimated allowable load. The settlement was recorded with the help of dial gauge of sensitivity Each load increment is kept for sufficient time till the rate of settlement becomes less than 0.02 mm per hour. The test piles are loaded until the ultimate load is reached. Safe load based on Settlement criteria: Experimental investigations have been conducted on AFRP confined and unconfined piles. Fig. 5 shows the pile load test on AFRP confined and unconfined RC piles in the field. The vertical load test was performed in accordance with IS: 2911 (Part 4) At each stage of loading, settlement of piles were measured. Vertical load corresponds to 5 mm and 12 mm settlement is tabulated in Table 8. As per IS : 2911(Part 4) , safe load of the pile was taken as 50% of final load at which the total settlement increases to 12 mm or final load at which the total settlement corresponds to 5 mm at ground level. However, the safe load of the pile is taken as the minimum of the above two. In all the cases it was observed that the first condition is the minimum editor@iaeme.com

8 J. Prakash Arul Jose and P. Rajesh Prasanna Types of Confinement a) Unconfined b) Bi AFRP Figure 5 Pile load test on AFRP confined and unconfined piles Table 8 Safe load based on Settlement criteria Load corresponding to 5 mm settlement at GL (N) Load corresponding to 12 mm settlement at GL (N) Safe load (N) Unconfined pile Pile confined with bidirectional AFRP mat (Bi- AFRP) CONCLUSION The strength and stability of geotechnical structures depends on soil-solid interface behaviour. Direct shear tests were conducted to investigate the interface friction angle between AFRP wrapped concrete specimens with soil. The experimental results show that surface roughness of specimen is significantly changes the interface friction angle. The shear strength at the interface increases with the increase in surface roughness of the specimens. To check the capability of AFRP materials, RC piles were cast with the same reinforcement details to study the behaviour of RC driven piles confined with AFRP under static vertical loads. Experimental results indicate that surface roughness of pile significantly affects the load carrying capacity of driven piles. The load carrying capacity of driven piles increases with the increase in surface roughness of the pile. REFERENCES [1] D i. J.G, Yokot. H, Iw n mi. M nd K to. E (2010) Experiment l investig tion of the influence of moisture on the bond behavior of FRP to concrete interf ces, Journ l of Composites for Construction, 14, [2] Frost. J.D nd H n. J (1999) Beh viour of interf ces between fibre-reinforced polymers nd s nds, Journ l of Geotechnic l nd Geo-Environmental Engineering, 125, [3] Ilki. A, Peker. O, K r muk. E, Demir. C nd Kumb s r. N (2008) FRP retrofit of low nd medium strength circul r nd rect ngul r reinforced concrete columns, Journ l of Materials in Civil Engineering, 20, editor@iaeme.com

9 Performance of AFRP Confined RC Driven Piles Subjected to Vertical Loads [4] IS: Classification and identification of soils for general engineering purposes. Bureau of Indian standards. [5] Meer. R.S, Sh nker. K And B sudh r. P. K (2007) Flexur l response of piles under liquefied soil conditions, Geotechnic l nd Geologic l Engineering, 25, [6] Sen. R nd Mullins. G (2007) Applic tion of FRP composites for underw ter piles rep ir, Journ l of Composites, 38, [7] W ng. Z nd Richwien, W (2002) A study of soil-reinforcement interf ce friction, Journal of Geotechnical and Geo-Environmental Engineering, 128, [8] Siddhan Sivakumar, S. Suresh, P. Pradeep, S. Rajesh and R. S. Kumar, Finite Element Analysis of landing grid Ship Assembly for 6.5 Tonnes in Vertical Loading conditions, International Journal of Civil Engineering and Technology, 8(9), 2017, pp editor@iaeme.com