Study the Behavior of High Performance Concrete Circular Short Columns Confined by CFRP

-This paper presents the results of experimental study on reinforced concrete columns rehabilitation with carbon fiber reinforced polymer (CFRP) under concentrated load. Twelve short circular reinforced concrete columns (150 mm diameter and 600 mm height) were tested. Three specimens were unstrengthening and tested until failure as control specimens. Nine specimens were rehabilitation by carbon fiber reinforced polymer after loading about 75% from ultimate axial load capacity of control specimens. The test parameters were the type of concrete are normal strength concrete (NSC), high performance concrete (HPC) and high performance concrete containing engine oil (HPCEO) in additional to effective the ratio CFRP confining (fall wrap (


Introduction
Rehabilitation of existing reinforced concrete columns may be required for a number of reasons. Many older buildings require rehabilitation and structural strengthening to allow for continued service if a change in the use or removal of some adjacent load bearing structural members or when the column is sought to be used in a different manner from previously planned or because it is damaged by external factors during its service. Industrial wastes either solid or liquid based chemical are available in large quantities. Environmental agencies have laws and regulations regarding safe handling of the waste. Hamad et al [1][2][3][4] investigated the effects of used engine oil on properties of fresh and hardened concrete. The main variables included the type and dosage of an air-entraining agent, mixing time, and the water cement ratio of the concrete. The dosage measured as percentage by weight of cement: 0.075, 0.15 or 0.30%. Results showed that used engine oil increased the percentage of entrained air and slump of the fresh concrete mix, without affect the strength properties of hardened concrete. In 2014 Kabashi [5] determined the effect of CFRP wrapped on columns, he used the samples with different shape cross sections: three circular and six rectangular columns, in addition to three circular and three rectangular columns as control specimens. The rectangular column specimens were wrapped in two different ways: partially and fully wrapped (in spacing similar with stirrups and width 5-6 cm). The circular specimens were strengthening (wrapped) fully wrapped. The

Objectives of Research
The objective of this study is to investigate of rehabilitation short concrete columns by CFRP and effect used UEO (used engine oil) in columns as chemical admixture. The main objectives of this research work are: 1-Effect of compressive strength of concrete for columns on rehabilitation. 2-Behavior of rehabilitation of concrete columns when concrete containing used engine oil. 3-Effective the ratio of CFRP confining (fall confined and partial confined).

Experimental Work
The experimental program in the current research work includes testing of A series of reinforced concrete columns normal strength concrete (NSC), high performance concrete (HPC)and high performance concrete containing engine oil (HPCEO) specimens are carried out to illustrate the effect of rehabilitation with carbon fiber reinforced polymer for columns subjected to axial load.

I. Description of Specimens
A total of twelve short column specimens having an overall height of 600 mm with circular crosssection of 150 mm diameter as shown in Figure 1. The experimental parameters were the type of concrete and effective the ratio of CFRP confining [fall wrap (CFRP ratio =100%), 50mm strips wrap 50mm spacing (CFRP ratio=50%), 40mm strips wrap-60mm spacing (CFRP ratio=40% covered), and without CFRP (CFRP ratio=0%)]. The specimens are divided into three groups (NSC, HPC and HPCEO). Each group involves four columns; one-reference columns (control specimen) have not strengthened and three columns strengthened after loaded 75% from ultimate load of control specimen (100% wrapping, 50% wrapping and 40% wrapping). The test program and specimen properties are summarized in Table 1 Deformed steel bars with 6 mm diameter were also used as ties. Steel bars, manufactured by Turkish company. The steel reinforcement meet the ASTM A996M-05 [15]. 9. Carbon Fiber Reinforced Polymers (CFRP) SikaWrap®-230 C Woven carbon fiber fabrics for structural strengthening was used, as shown in Figure 2. This type is classified as mid strength carbon fiber as reported by the manufacturer. Table 2 shows the technical description of Carbon fiber reinforced polymers (CFRP) from SikaWrap®-230 C. 10. Bonding Materials Sikadur®-330 is recommended by CFRP manufacturer to bond CFRP to the concrete. The product data is listed in Table 3.

III. Concrete Mixes
The mix proportions are 1: 1.2: 1.85 by weight with w/c ratio of 0.42 and cement content 500 kg/m3. Several trail mixes were carried out to determine the silica fume content, dosage of super plasticizer and engine oil content. Mixtures details are given in Table 4.

VI. Molds Preparation
Four steel molds are designed for casting all twelve columns in three stages for each type of concrete NSC, HPC and HPCEO. The molds were made of 2 mm thickness steel plate, as shown in Figure 2.

V. Casting and Curing
All column specimens were vertically cast in two layers to simulate the typical construction practice of columns. Each layer was compacted by pendulum concrete vibrator by touch on external surface mold for 1 minute to minimize the air voids and to get well-compacted concrete.
The accompanied control specimens of cubs, cylinders and prisms were also cast and vibrated on vibrating table together with the columns specimens. After two days, the specimens were stripped from the molds and cured in a water bath for curing until age of 28 days.

II. Preparing Columns for CFRP Application
After loading each group of columns 75% from ultimate load. The surface of the columns must be grounded by using electric hand grinder to remove any loose and weak materials and then washed with water to obtain a clean surface. That clean surface ensures a good bond between the concrete surface and CFRP.

III. Application of CFRP
After the preparing of the surfaces and making sure that, they are clean and dry; type A and type B epoxy impregnation resin had been mixed to the recommended ratio (4:1) as directed by the manufacturer until the color be homogenous. Resin mixing was in quantities sufficiently small to ensure that all mixed resin can be used within the resin's pot life. Applying epoxy on column with thickness about 1mm. After that, setting CFRPsheet on column surface on the coated region by epoxy. The CFRP was then coated with another layer of epoxy resin. Figure 4 shows application of CFRP. Finally, the column be ready to test after curing for 7 days at laboratory temperature 35oC.

I. Slump Test
In this study, all mixes exhibited the desired workability (slump of 100 ± 5 mm) for all types of concrete (NSC, HPC, and HPCEO). The w/c ratio for high performance concrete mix was adjusted to have the same workability of normal strength concrete where reduced from 0.42 to 0.27. The significance of the study was to check the hypo thesis that adding used engine oil to the fresh concrete mix could be similar to adding chemical admixture, thus enhancing some properties of fresh and hardened concrete while serving as technique of disposing the Iraqi oil waste. Results of high performance concrete with used engine oil showed that performance of used engine oil acted as a chemical plasticizer by improving the fluidity and the slump of the concrete mix.

II. Compressive Strength Test
The compressive strength test results for all types of concrete mixes are presented in Table 5. It can be observed that the compressive strength results of the high performance concrete containing superplasticizer only (HPC) is significantly increased relative to normal strength concrete (NSC). The increase in strength was due to adding 10% of silica fume (by weight of cement) and reducing w/c ratio from 0.42 to 0.27 due to adding superplasticizer. The compressive strength of the concrete containing superplasticizer and used engine oil (HPCEO) is increased relative to (NSC) but less than the compressive strength of (HPC). The decreasing in compressive strength of (HPCEO) in comparison with the (HPC) is due to reduce the dosage of superplasticizer from 1.3 to 1 (liter/100kg of cement) and used engine oil is acted as a chemical plasticizer only. In additional to used engine, oil could slightly decrease in compressive strength (16). Based on results of this experimental work can considered concrete containing used engine oil with another chemical and mineral admixture (HPCEO) as high performance concrete.

IV. Flexural tensile strength test
The flexural strength test results for all type of concrete mixes are presented in Table 5. The results show that the flexural tensile strength of high performance concrete with superplasticizer and used engine oil (HPCEO) is higher than normal strength concrete (NSC) and less than high performance concrete with superplasticizer only (HPC).

V. Experimental axial load capacity of the tested columns
Most application strengthening of CFRP for element structure is applied after a period from constructed the structure. The structure through this time is exhibited to service load or any external factors during structure service. That cause to damages and deformation of structure. Therefore, experimental programs of this investigation work to simulates applied reality where loaded the columns about 75% from ultimate load of reference columns before strengthening. Then three columns for each group was loaded about 75% from ultimate loaded of control specimen. The NSC columns was appearing first cracks in this range of load while HPCEO and HPC was did not appearing any cracks. Table 6 and Figure 5 show results of investigation work. The results of ultimate load for columns shows that compressive strength of concrete has significantly effect on ultimate strength of unconfined concrete columns and behavior of confined concrete columns.   Figure 9 shows the effect Load-longitudinal displacement behavior of NSC, HPC and HPCEO unconfined columns it can be seen there is little difference between this unconfined columns but improving in ultimate load of columns with increasing in compressive strength of concrete. Figures 10 to 12 show the effect Loadlongitudinal displacement behavior on confinement columns with CFRP for each type of columns (NSC, HPC and HPCEO columns) it can be see significantly improving in reducing of longitudinal displacement. In addition a part from this reducing of longitudinal displacement may be because loading before strengthening columns.  respectively. For the same strengthening, the ratio of increasing enhancement in ultimate axial load capacity of rehabilitation HPCEO is (37%, 26% and 17%) respectively from control specimen, while for HPC this ratio becomes (12%, 5% and 2%) from control specimen for these strengthening(100%, 50% and 40% wrapping). 5. The ratios of increasing in ultimate axial load capacity of rehabilitation RC columns with 100% and 50% wrapping in comparison with 40%wrapping are 20% and 4% respectively for NSC, while these ratios become 15% and 5% respectively for HPCEO and for HPC , these ratios are 10%and 3% respectively. 6. Significantly reducing in rehabilitation using wrapping ties in comparison with fully confined columns. 7. Slightly different between wrapping ties (50-50) and wrapping ties (40-60). Therefore, prefers using wrapping that width of CFRP is equal to spacing between ties of CFRP. 8. When rehabilitation of columns by wrapping ties of CFRP prefer application-wrapping ties between ties of reinforced columns. 9. Failure of rehabilitation columns by CFRP was mainly by rupture of CFRP.