Study of bond strength between various grade of Ordinary Portland Cement (OPC) and Portland Pozzolane Cement (PPC) mixes and different diameter of TMT bars by using pullout test

Abstract

Since last two decades, the Portland Pozzolane Cement (PPC) is extensively used in structural concrete. But, till to date, a few literature is available on bond strength of concrete using PPC mixes. There are many literatures available on bond strength of concrete mixes using Ordinary Portland Cement (OPC). Hence, a comparative study was conducted on bond strength between OPC and PPC mixes. In the present investigation, total 24 samples consisting of M20, M35 and M50 grades of concrete and 16 and 25 mm diameter of TMT bar were tested for 7 and 28 days. The pullout bond test was conducted on each specimen as per IS: 2770-1967/1997 [1] and the results were observed at 0.25 mm slip at loaded end called as critical bond stress and at maximum bond load called as maximum bond stress. It was observed that the critical bond strength of PPC mixes is 10% higher than OPC mixes. Whereas, marginal improvement was noticed in maximum bond strength of PPC mixes. Hence, based on these findings, it could be concluded that development length for PPC mixes could be reduced by 10% as compared with same grade of OPC mixes.

Keywords

bond strength / Portland Pozzolane Cement (PPC) concrete / Ordinary Portland Cement (OPC) concrete / bond between concrete and steel / pullout test / development length

Introduction

In India, Portland Pozzolana Cement (PPC) contributes the major share (67%) of the total production, followed by Ordinary Portland Cement (OPC) (25%) and Portland Slag Cement (PSC) (8%). A positive trend toward the increasing use of blended cement is noticeable with the share of blended cement increasing to 75% [2]. The extensive use of PPC in concrete was started from just last two decades as compared with the OPC which is being used from early 19 century. Recent studies on concrete using PPC mixes indicate improvement in workability, durability, long-term strength [3], reduction of Water Cement ratio [4] and decrease in heat of hydration at early ages thereby reducing thermal shrinkage and cracking [5] etc. Hence now a days, the PPC cement is widely used for structural concrete work. However, there has been little discussion and literature available on Bond Strength between PPC Concrete and reinforcing steel using Pullout bond test. The Bond strength is an important factor in reinforced concrete design, as the bond strength transfer tensile forces from the concrete to the reinforcing steel. To transfer force adequately, there must be a sufficient embedment length of reinforcing bar, known as the development length, over which the bar force is transferred from the concrete to the reinforcing steel [6]. Few literature indicates improvement of bond strength with use of high volume fly ash in self compacting concrete [7], high performance concrete [8] and geopolymer concrete [9]. However, no experimental evidence is available in respect of bond strength of PPC concrete mixes. Therefore, a study was done on bond strength between OPC and PPC mixes with M20, M35 and M50 grades of concrete in combination with 16 and 25 mm diameters of TMT bars for 7 and 28 days. In this experimental investigation, bond strength test was performed according to IS 2770-1967/1997 [1] for pullout test. The Pullout test is relatively simple test for determination of bond strength because the force in the bar is directly known from the experiment and it is not necessary to determine the bar force from flexural analysis of the beam [10].

Materials and methods

The cement such as OPC 53 grade, PPC grade and TMT bar of 16 and 25 mm diameter are the variable in this investigation. Their physical and chemical properties were determined as given in Tables 1 and 2. The other materials such as locally available river sand, 10 and 20 mm size graded aggregate were used for the mix. All aggregates were tested and confirmed to IS: 383-1970/1997 [11] and their physical properties and combined grading are reported in Table 3 and Fig. 1 respectively. Prior to start the investigation, design mixes for grades of M20, M35 and M50 grade with OPC and PPC were cast as per IS 10262-2009 [12]. Slump, fresh density and 3, 7 and 28 days compressive strength were reviewed. With the number of trials, final mixed design for each grade of concrete was formulated as given in Table 4. In this investigation, 72 numbers of pullout out sample were cast. The casting was done on specially fabricated mold with bar supporting arrangements. Mixing was carried out in laboratory pan mixer and then slump was checked. Once the concrete was obtained with desired level of workability, it was poured into mold and then vibrated. During vibration the verticality of bar was maintained by rechecking of verticality with spirit level and bar was brought in vertical plumb with the help of horizontal locking screws. Then cubes were allowed to set for 24 h. De-molding was carried out after 24 h and then cubes were immediately placed into curing tank. After completion of curing period of cubes for 7 and 28 days, they were capped with gypsum plaster so as to make uniform contact area with base plate. Prior to pullout testing, each cube was kept in the spherical seated bearing block arrangement above the UTM as shown in Fig. 2. The cube was attached with upper yoke and lower yoke consisting of holding arrangement for LVDTs. The top LVDT was pointed to the free end to measure slip at free end while lower two LVDTs were pointed toward the crossbar, which was locked with pulling bar, for measuring the slip at loaded end. Initial reading of LVDTs and Loads were recorded from the pullout digital display unit. The rate of loading of UTM was set to 2250 kg/min. Then loading was started and recording of loads and slips at all three points were recorded at every second in the digital recording channel indicator. Simultaneously, manual recording were also taken.

After completion of test, sample was removed from the test setup, physical verification of crack and type of slip was observed as shown in Fig. 4. It was observed that specimen was failed by pullout rather than by splitting of concrete. Then, bond stresses were calculated as per the formula given in Eqs. (1) and (2).

Critical bond stress:

(1){\tau }_{\mathrm{c}}={\displaystyle \frac{P_{\mathrm{0.25}\mathrm{m}\mathrm{m}}}{\mathrm{\pi }{d}_{\mathrm{b}}l}},

maximum bond stress:

(2){\tau }_{\max }={\displaystyle \frac{P_{\mathrm{u}}}{\mathrm{\pi }{d}_{\mathrm{b}}l}},

where, t_c = critical bond stress in MPa at 0.25 mm slip at loaded end; t_u = maximum bond stress in MPa; P_{0.25 mm} = load at slip of 0.25 mm at loaded end; d_b = diameter of bar in mm; l = embedment length of bar in concrete cube of size 150 mm; P_u = maximum pullout bond load.

Results

The test results of all the specimens are presented in Table 5. From these results, graphs were plotted for Critical bond stress, termed as Bond stress at 0.25 mm slip mm loaded end [15], and at maximum bond stress. Hence, graphs were plotted for 16 and 25 mm diameter of TMT bar between Critical bond stress and Grade of concrete as given in Figs. 5 and 6 and maximum bond stress and grade of concrete as given in Figs. 7 and 8 respectively. From the Figs. 5 and 6, of Critical Bond stress, it was observed that the Critical bond strength of concrete with PPC mixes is lowered at 7 days of pullout bond test which may be due to late start of pozzolanic reaction of fly ash in PPC. For 28 days of pullout bond test results it was observed that PPC mixes have higher bond stress about 8 to 10% than OPC mixes for same grade of concrete. Furthermore, from the Figs. 6 and 7, The Maximum bond strength of PPC mixes has marginally bond stress than OPC mixes at 7 and 28 days testing. The generalized critical bond strength equations were derived from Fig. 3 to Fig. 6 for OPC and PPC mixes and for 16 and 25 mm diameter TMT bars. The generalized equation is given in Eq. (3) to (10).

For OPC mixes with 16ϕ,

(3){\tau }_{\mathrm{c}}=\mathrm{0.49}\sqrt[{\displaystyle \frac{\mathrm{2}}{\mathrm{3}}}]{f_{\mathrm{c}\mathrm{k}}},

for PPC mixes with 16ϕ,

(4){\tau }_{\mathrm{c}}=\mathrm{0.54}\sqrt[{\displaystyle \frac{\mathrm{2}}{\mathrm{3}}}]{f_{\mathrm{c}\mathrm{k}}},

for OPC mixes with 25ϕ,

(5){\tau }_{\mathrm{c}}=\mathrm{0.50}\sqrt[{\displaystyle \frac{\mathrm{2}}{\mathrm{3}}}]{f_{\mathrm{c}\mathrm{k}}},

for PPC concrete with 25ϕ,

(6){\tau }_{\mathrm{c}}=\mathrm{0.74}\sqrt[{\displaystyle \frac{\mathrm{2}}{\mathrm{3}}}]{f_{\mathrm{c}\mathrm{k}}}.

For OPC concrete with 16ϕ,

(7){\tau }_{\mathrm{c}}=\mathrm{5.0}\sqrt[{\displaystyle \frac{\mathrm{1}}{\mathrm{3}}}]{f_{\mathrm{c}\mathrm{k}}},

for PPC concrete with 25ϕ,

(8){\tau }_{\mathrm{c}}=\mathrm{5.2}\sqrt[{\displaystyle \frac{\mathrm{1}}{\mathrm{3}}}]{f_{\mathrm{c}\mathrm{k}}},

for OPC concrete with 16ϕ,

(9){\tau }_{\max }=\mathrm{4.9}\sqrt[{\displaystyle \frac{\mathrm{1}}{\mathrm{3}}}]{f_{\mathrm{c}\mathrm{k}}},

for PPC concrete with 25ϕ,

(10){\tau }_{\max }=\mathrm{5.2}\sqrt[{\displaystyle \frac{\mathrm{1}}{\mathrm{3}}}]{f_{\mathrm{c}\mathrm{k}}},

where, t_c = critical bond strength in MPa at 0.25 mm slip at loaded end; t_max = maximum bond strength in MPa; f_ck = grade of concrete in MPa.

Discussion

The main objective of this investigation was to determine Bond Strength of concrete with PPC mixes. Hence comparative study was done between OPC and PPC mixes because bond strength of OPC mixes has been already established. From the above Figs. 5 and 6, The experimental result of Critical bond strength of OPC mixes and PPC mixes are higher than limiting bond strength as per IS 456-2000 [17] and Euro-code. It is also observed that 7 days Critical bond strength of OPC mixes is higher than PPC mixes whereas 28 days results indicates that the critical bond strength of PPC mixes is higher than OPC mixes. It could be due to start of pozzolanic action of flyash in PPC cement after 7 days. Further from the Figs. 7 and 8, The Maximum bond strength of PPC mixes is similar to OPC mixes. Therefore, it could be inferred that the fly ash in PPC is responsible for higher critical bond strength however, it failed to support higher Maximum bond strength in PPC mixes. The Maximum bond strength of OPC and PPC mixes is also found to be higher than Maximum bond strength curve recommended by CEB-FIB MC90 [16] code as shown in Figs. 5 and 6. Therefore, PPC is improving 28 days critical bond strength in the PPC mixes.

The generalized equations for limiting bond stress as per IS 456-2000 [17] and Eurocode-2 are as given in Eqs. (11) and (12) respectively.

Limiting bond stress as per IS 456-2000,

(11){\tau }_{\max }=\mathrm{0.42}\sqrt[{\displaystyle \frac{\mathrm{2}}{\mathrm{3}}}]{f_{\mathrm{c}\mathrm{k}}},

permissible bond stress as per Euro code,

(12){\tau }_{\max }=\mathrm{0.296}\sqrt[{\displaystyle \frac{\mathrm{2}}{\mathrm{3}}}]{f_{\mathrm{c}\mathrm{k}}}.

It could be observed form the Eqs. (11) and (12) that limiting bond strength as per IS 456-2000 is proportional to the square root of compressive strength whereas permissible bond stress as per Euro code is proportional to the 2/3rd power of compressive strength of concrete. However, the generalized equations for Critical bond stress of OPC and PPC mixes are observed as proportional the 2/3rd power of compressive strength of concrete as given in Eq. (3) to (6) which is similar to permissible bond stress as per Euro code given in Eq. (12).

Conclusions

Based upon the present investigation and observed results following conclusions are presented as below.

1) The comparison over given range of grades of concrete viz. M20, M35 and M50, between OPC and PPC with 16 and 25 mm diameter of bars, it could be concluded that the critical bond strength with PCC mixes could develop 8 to 10% higher bond strength as compared with OPC mixes.

2) There were no significant difference on Maximum bond strength between OPC and PPC mixes.

3) For the standard grade of concrete ranges from M25 to M55, the Eqs. (4) and (6) may be used for determination of Critical bond strength of PPC mixes which is formulated from the research work.

References

[1] IS: 2770–1967/1997, Part-1. Methods of Testing Bond in Reinforcement Concrete, Pullout Test, Bureau of Indian Standards, New Delhi, India

[2] 95th Report on Performance of Cement Industry. Department Related Parliamentary Standing Committee On Commerce, Rajya Sabha Secretariat New Delhi, February, Phalguna, 2011, 1932

[3] Mindess S, Young J, Darwin D. Concrete. 2nd Ed. Upper Saddle River, NJ: Pearson Education Inc, 2004, Ch.5, 106-111

[4] Ravina D, Mehta P. Properties of Fresh Concrete Containing Large Amounts of Fly Ash. Cement and Concrete Research, 1986, 16(2): 227-238

[5] Mehta P, Monteiro P. Concrete: Microstructure, Properties, and Materials, 3rd edition. The McGraw Hill Companies, Inc, 2006, 485-491

[6] Soltani A. Bond and serviceability characterization of concrete reinforced with high strength steel. Dissertation for the Doctoral Degree, Pittsburgh: University of Pittsburgh, 2010, 13

[7] Hossain K M A, Lachemi M. Bond behavior of self consolidating concrete with mineral and chemical admixtures. Journal of Materials in Civil Engineering, 2008, 20(9): 608-616

[8] Nadim Hassoun M. Akthem Al- Manaseer. Structural concrete, Therory and Design, Forth Edition, John Wiley and Sons, Canada

[9] Sarker P K, 0. Bond Strength of reinforcing steel embedded in fly ash- based geopolymer concrete. Materials and Structures, 2011, 44(5): 1021-1030

[10] ACI 408R–03. Bond and Development of Straight Reinforcing Bar in Tension, ACI Committee 408

[11] IS: 383–1970. Specifications for Coarse and Fine Aggregates from Natural Sources for Concrete, Bureau of Indian Standards, New Delhi, India

[12] IS: 10262–2009. Recommended Guidelines for Concrete Mix Design, Bureau of Indian Standards, New Delhi, India

[13] IS: 12269–1987. Specification for 53 grades of Ordinary Portland Cement, Bureau of Indian Standards, and New Delhi, India

[14] IS 1786–2008. High Strength Deformed Steel bars and Wires for Concrete Reinforcement-Specification, Bureau of Indian Standards, New Delhi, India

[15] Chan Y W, Chen Y S, Liu Y S. Development of bond strength of reinforcement steel in self-consolidating concrete. ACI Structural Journal, 2003, 100(4): 490-498

[16] CEB/FIB Model Code. Structural Concrete. Volume-1, International Federation of structural concrete, Switzerland. 1990

[17] IS 456–2000. Plain and Reinforcement Concrete-Code of Practice, Bureau of Indian Standards, New Delhi, India

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