Effects of Silica Fume and Carbon Fibers on Rubberized Concrete
Rubberized concrete is a type of concrete that utilizes crumb rubber particles as aggregate. Apart from reducing rubber pollution, studies have shown that an appropriate amount of rubber mixing into concrete enhances the elasticity, toughness, seismic performance, frost resistance, acoustic performance of concrete. But adding rubber reduces the strength of concrete, which limits the application of rubberized concrete.
Some have studied the effect of five kinds of concrete fibers on the strength of concrete. The results show that steel fibers and carbon fibers have the most obvious improvement in the mechanical properties of concrete. Take carbon fibers for an example, when the dosage is 0.1%, the compressive strength and splitting tensile strength of concrete are increased by 5 %, 9.6%, respectively. Moreover, the addition of silica fume also can improve the compactness, durability, impermeability, and mechanical strength of concrete.
The test below is to study the effects of carbon fibers and silica fume with different dosages on the performance of rubberized concrete.
- Cement: PO 42.5 portland cement;
- Coarse aggregate: 5 – 20 mm well-graded crushed stone;
- Fine aggregate: sand with a fineness modulus of 2.6;
- Rubber particles: 16 mesh;
- Carbon fibers: 7 mm;
- Silica fume: SiO2 content 95.7%.
The fine aggregate is replaced by 10% of the rubber particles in equal volume. Add carbon fibers with a dosage of 0.2%, 0.4%, 0.6%, or 0.8% to prepare carbon fiber reinforced rubberized concrete. Then mix silica fume with 3%, 5%, 7%, 9% of cement mass ratio into the 0.2% fiber reinforced rubberized concrete.
The test results show that the compressive strength of carbon fiber reinforced rubberized concrete increases first and then decreases with the dosage of carbon fibers. That with 0.2% carbon fiber dosage has the highest compressive strength at 7 days, which is 22.13 MPa and 4.4% higher than that of normal rubberized concrete. Those with dosages from 0.4% to 0.8% all have lower compressive strength than normal rubberized concrete. For 28-day compressive strength, that with 0.4% dosage has the highest value, which is 1.52 MPa higher than the 29.65 MPa of the normal one. Those with a dosage of 0.6% and 0.8% have lower compressive strength than the normal one.
For concrete samples also adding silica fume, Both the 7-day and 28-day compressive strength increase as the increase of silica fume dosage from 0% to 7%, then slightly decrease when the dosage is 9%. The 7-day and 28-day compressive strength of that with 7% silica fume are 18.3%, 13.2% higher than that without silica fume. All those adding silica fume have higher compressive strength than that without silica fume, indicating that silica fume has a good effect on improving the compressive strength of 0.2% carbon fiber reinforced rubberized concrete.
The effect of carbon fibers and silica fume on the splitting tensile strength of rubberized concrete
According to the test results, the splitting tensile strength of rubberized concrete first increases then decreases with the increase of carbon fiber dosage. That with a 0.2% dosage has the highest value, a 24.14% increase compared with the normal one. And none of the four carbon fiber reinforced rubberized concrete samples has a lower value than the normal rubberized concrete, indicating that carbon fibers can improve the splitting tensile strength of rubberized concrete, and a 0.2% dosage is recommended.
With the increase of silica fume dosage, the splitting tensile strength of rubberized concrete first increases and reaches the peak value at 7%, and decreases when the dosage is 9%. It can be seen from the results that silica fume also improves the splitting tensile strength, and among the tested dosages, the 7% dosage has the best effect.
The effect of carbon fibers and silica fume on the flexural strength of rubberized concrete
Comparing the four carbon fiber reinforced rubberized concrete samples with the normal one, carbon fibers increase the flexural strength by 20% – 40%. And that with 0.4% dosage has the highest flexural strength ratio. As the silica fume dosage increases, the flexural strength ratio first increases then decreases. The flexural strength of samples with 3%, 5%, 7%, 9% silica fume dosage are 4.83%, 9.94%, 16.48%, 11.08% higher than that without silica fume.