According to the Egyptian standard specifications for concrete aggregates [7] , sand with fineness modulus equal 2.59 and specific gravity of 2.57 was used, whereas the coarse aggregate used was dolomite maximum normal size (5 - 14 mm) and specific gravity of 2.74. Cement (CEM I-52.5N) used was fresh product from Sina factory and complies to E.S.S 4756/2013 [8] . CKD was obtained from the El Nahda cement factory in Qena, Egypt. SF was obtained from Egyptian Company for Ferroalloys (Alferrosilicon), Edfo factory, Aswan, Egypt. SF was confirming to ACI 234R-96 [6] . The chemical composition of Portland cement, CKD and SF were shown in Table 1. Potable water free from salts, oils, acids, sugars and other harmful substances was used for mixing and curing of specimens. Sikament-163M was used as super plasticizer (SP), for highly effective water reducing agent, density (at 20˚C) 1.200 ± 0.005 kg/liter (ASTM C494),

*Note: These average values are supplied by the manufacturer.

Appearance/Colour is brown liquid and recommended dosage is 0.6% - 2.5% by weight of cement. The experimental dosage was 2.0% by weight of cement.

In this experimental investigation, control concrete mix to achieve compressive strength of 50 N/mm² at the age of 28 days was done according to ECP 203-2007 [9] . The use CKD with different percentages 5%, 10%, 15%, 20% and 25% and a fixed proportion of silica fume (SF) 15% as partial replacement by the weight of (OPC). The mixtures proportions for 1 m³ in all concrete mixtures were shown in Table 2. Slump test was done on the fresh concrete to find its consistency and workability. For every mix, cubes (150 mm × 150 mm × 150 mm) were cast for the compressive strength tests, cylinders (Diameter; 150 mm and height; 300 mm) were cast for the split tensile strength and modulus of elasticity tests and prisms (100 mm × 100 mm × 500 mm) were cast for the flexural strength tests. The Bond test for reinforcing steel describes by pull-out specimen as a steel reinforcing bar 12 mm diameter embedded in concrete cylinder (Diameter; 150 mm and height; 300 mm), An anchorage length equal to 300 mm. After casting, all specimens were left in the forms for 24 hours before demoulding. The curing of specimens was done by placing all specimens in water basins until the ages of testing.

The concrete slump flow test was used according to Egyptian Code for the design and implementation of concrete structures, tests guide C203 to determine fresh concrete workability [10] . Table 3 shows slump test results for all mixtures, in general a decrease in slump were observed with increase ratio of CKD

*Mix designation is (Mix-CKD%-SF%).

with SF. The results indicate that CKD with SF leads to decrease the fresh concrete workability. The increase of (CKD) amount, reduces the fresh mixes workability and also the strength of hardened mortars and concretes in generally [2] . The workability of fresh concrete decreases with increase of silica fume amount in concrete. For getting equal workability, the concrete containing of silica fume will tend to shows less slump height than conventional concrete [11] . For each level of w/b ratio, the measured slump was the same or it decreased as the amount of CKD replacement level increased. As w/b ratio increased, the impact of CKD replacement of Portland cement on slump loss became more significant [12] . For obtained ordinary consistency with increase CKD amount, this required to increasing water, slump loss may be to high amounts of lime, sulfates, alkalis and volatile salts that require more water in concrete [13] .

The test is conducted in Compression Testing Machine with a capacity of 2000 kN and loading rate 0.6 N/mm²/sec, according to Egyptian Code for the design and implementation of concrete structures, tests guide C203 [11] . Figure 1 shows the variation of compressive strength in CKD with SF mixtures when

compared with control mixture Mix-00-00. At all mixtures that have CKD with SF, The presence of SF with CKD lead to improve the strength up to 20% CKD. In mixtures Mix-05-15, Mix-10-15, Mix-15-15 and Mix-20-15, the maximum ratio of increase at different ratios of CKD with 15% SF are 18.66%, 10.28%, 7.22% and 4.24% respectively after 7 days, 18.34%, 11.48%, 6.17% and 3.55% respectively after 28 days and 18.02%, 9.63%, 5.42% and 2.81% respectively after 56 days. In mixture Mix-25-15 the strength values decrease with 4.93% after 7 days, 3.69% after 28 days and 6.05% after 56 days. Compressive strength of concrete mixtures were decreased with the increase in CKD percentage at the age 28 days, the reduction in strength at 10% and 40% CKD are 15% and 44% respectively [14] . The optimum replacement percentage of SF varies from 10% to 15%. Compressive strength of concrete decreases when the cement replacing is above 15% of SF [11] .

Indirect tensile strength was described by split tensile strength, the test is conducted in Compression Testing Machine with a capacity of 2000 kN and loading rate 0.04 N/mm²/sec, according to Egyptian Code for the design and implementation of concrete structures, tests guide C203 [11] . Figure 2 shows the variation of split tensile strength in CKD with SF mixtures when compared with

control mixture Mix-00-00. At all mixtures that have CKD with SF, the presence of SF with CKD lead to improve the strength up to 20% CKD. In mixtures Mix-05-15, Mix-10-15, Mix-15-15 and Mix-20-15, the maximum ratio of increase at different ratios of CKD with 15% SF are 18.82%, 11.53%, 6.60%, 3.71% respectively after 7 days, 18.15%, 11.14%, 6.18% and 3.96% respectively after 28 days and 17.63%, 11.71%, 7.61% and 5.59% respectively after 56 days. In mixture Mix-25-15 the strength values decrease with 4.12% after 7days, 4.91% after 28 days and 7.69% after 56 days. By the increase in CKD content, compressive strength, split tensile strength and also modulus of rupture was decreased [15] . Beyond 15% silica fume when replacement with cement in concrete, the increased in split tensile strength is almost insignificant. But get sizeable gains in the flexural strength was occurred even up to 25% silica fume replacement [16] .

By analysis of the present test results, the relationship between the split tensile and compressive strengths of (CKD with SF) concrete mixtures after 28 days, it was found within range of 10.6% to 10.76%, whereas 10.73% for control mixture.

All prism specimens were tested under four point static load according to Egyptian Code for the design and implementation of concrete structures, tests guide C203 [11] , by using a 1000 kN capacity (loading rate 0.06 N/mm²/sec) hydraulic jack mounted on a steel frame in the R.C laboratory of Al-Azhar university. Figure 3 shows the variation of flexural strength in CKD with SF mixtures when compared with control mixture Mix-00-00. At all mixtures that have CKD with SF, the presence of SF with CKD lead to improve the strength up to 20% CKD. In mixtures Mix-05-15, Mix-10-15, Mix-15-15 and Mix-20-15, the maximum ratio of increase at different ratios of CKD with 15% SF are 17.27%, 11.36%, 7.27%, 3.64% respectively after 7 days, 17.03%, 11.15%, 7.35% and 4.41% respectively after 28 days and 18.19%, 10.92%, 5.63% and 2.82% respectively after 56 days. In mixture Mix-25-15 the strength values decrease with 5.45% after 7days, 4.41% after 28 days and 5.63% after 56 days. It was observed both of flexural strength and toughness values are decreased with the increase in CKD replacement ratios, but at 5% and 10% replacement ratios did not have an appreciable adverse effect, it is especially clear at low w/b ratios [12] . The reduction in flexural strength and toughness values attributed to a reduction in the cement amount in the blends as the content of CKD increased [11] . Content of 15% silica fume as partial replacement of cement were the optimum amount to significantly flexural strength enhancement [17] .

By analysis of the present test results, the relationship between flexural and compressive strengths of (CKD with SF) concrete mixtures after 28 days, it was found within range 14.55% to 14.88%, whereas 14.71% for control mixture.

The modulus of elasticity was determined by subjecting a cylinder specimen (Diameter; 150 mm and height; 300 mm) to uniaxial compression by using Universal Testing Machine of capacity 1000 Kn, according to Egyptian Code for the design and implementation of concrete structures, tests guide C203 [10] . A Keithley-500 A Data Acquisition System consists of 4 channels for strain measurement and corresponding load. The deformations were measuring by dial gauges which fixed between a certain gauge length. Modulus of elasticity was found out with reference to the initial tangent drawn at the origin of the stress strain curves Figure 4. The test of modulus of elasticity was carried out for control mixture and CKD with SF mixtures. Table 4 shown the value of the modulus of elasticity after 28 days, the role of CKD with SF are observed for the increase in modulus of elasticity of mixtures; (Mix-05-15, Mix-10-15, Mix-15-15 and Mix-20-15), with ratio 43.82%, 28.25%, 17.29% and 11.32% respectively, while in mixture (Mix-25-15) modulus of elasticity are decrease with 3.13% when compared with control mixture (Mix-00-00). The modulus of elasticity is decreasing with the increasing of CKD content from 10% - 50%, the reduction of modulus of elasticity within range between (5.31% to 77.87%) respectively [14] .

By analyzing test results statistically, the relationship between modulus of elasticity and compressive strengths after 28 days for control mixture and (CKD with SF) mixtures has been obtained as the following equations in Table 5. For (CKD with SF) mixtures the direct proportional relationship between modulus of elasticity and compressive strength are observed.

Bond strength was describes by pull-out test, as a steel reinforcing bar 12 mm diameter embedded in a concrete cylinder (Diameter; 150 mm and height; 300 mm), An anchorage length equal to 300 mm, according to Egyptian Code for the design and implementation of concrete structures, tests guide C203 to [10] . The test is conducted in a tensile testing machine (TTM) with a capacity of 800 kN for the direct pull out test of specimens. Specimen were inverted and positioned through the bottom platform of test machine load frame. Table 6 shows the value of bond strength and failure mode for all mixtures after 28 days. Figure 5 shows the variation of bond strength in CKD with SF mixtures when compared with control mixture. The increasing in bond strength of mixtures; (Mix-05-15, Mix-10-15, Mix-15-15 and Mix-20-15), with ratio 7.20%, 6.21%, 0.69% and 0.69% respectively, while in mixture (Mix-25-15) bond strength were decreases with 7.58% when compared with control mixture (Mix-00-00). Whereas by increasing ratios over (10% CKD with 15% SF) the failure mode changed from rebar cutting to concrete splitting.

The analysis of the present test results to determined relationship between bond and compressive strengths of (CKD with SF) concrete mixtures after 28 days, it was found within range 10.28% to 11.04%, whereas 11.34% for control mixture.

With control mixture after 28 days. The test results are shown in Table 7. It is observed that the amount of SO₃ oxide within the range 1.89% to 2.2% for all CKD with SF mixtures, and not significant when compared with control mixture 2.13%. In general the use of CKD with SF in concrete mixtures lead to

*E denote modulus of elasticity. **Fcu denote compressive strength.

increase ratios of sulfate SO₃, chloride CL, potassium K₂O and silicon SiO₂. Where aluminum Al₂O₃, iron Fe₂O₃ and calcium CaO ratios are decreased but sodium Na₂O ratio still constant when compared with control mixture. The total content of sulfates in concrete in the form of SO₃, shall not exceed 4% of the weight of cement with a hardened concrete at 28 days according to ECP 203-2007 [10] . The total content of chloride ions in the form of Cl in non-chloride-resistant of hardened concrete, shall not exceed 0.3% of the cement weight at the 28 days [10] . The amount of Cl oxide in mixture (Mix-05-15) are 0.3%, so that this mixture can be used in steel reinforced concrete, but the amount of Cl oxide in mixtures; (Mix-10-15, Mix-15-15, Mix-20-15 and Mix-25-15) within the range 0.35%, this mixtures can be used in plain concrete or in FRP reinforced concrete.