Effects of Length and Dosage of PVA Fibers on ECC
PVA fiber reinforced engineered cementitious composite (ECC) is a new type of cement-based material that has characteristics of enhanced crack resistance, toughness, and strength, and is unlike ordinary cement materials which are brittle, and of low elongation. In this article, the effects of length and dosage of PVA fibers on the fluidity, flexural strength, compressive strength of ECC are discussed.
Materials and mix ratio design for engineered cementitious composite
- Cement:Fly Ash (mass) = 56:44; Water/binder ratio: 0.4.
- PVA Fibers: Density 1.3 g/cm3; Tensile strength 1620 MPa; Modulus of elasticity 43 GPa; Diameter 12μm; Length 3mm, 6mm, 9mm, 12mm; Dosage 0.5%, 1.0%, 1.5%, 2.0%, 2.5% (by volume).
- Superplasticizer: Dosage 1% (by mass of cement).
- Aggregate: river sand.
First mix the aggregate, fly ash and cement for 2 minutes. Then evenly add PVA fibers in the mixture and stir for 5 minutes. After adding water and superplasticizer to the mixture and stirring for 2 minutes, test the flowability and slump of ECC. Afterward, prepare 40mm x 40mm x 160mm specimens and test their flexural strength and compressive strength after curing for 28 days.
Effect of PVA fibers on the fluidity of ECC
The test results show that with the increase of the dosage of PVA fibers, the flowability of ECC gradually decreases. This is due to the random distribution of fibers forming a spatial network structure, where the mixture is agglomerated and not easy to flow. As the fiber content increases, the space of the structure in a unit volume becomes smaller, and the distribution density becomes larger.
Different lengths of PVA fibers all play a role in reducing the flowability of ECC, but the effect of the length on the flowability of ECC is not obvious.
The results show that the slump of ECC decreases significantly with the increase of the dosage of PVA fibers. This is because the addition of PVA fibers increases the surface area of the specimens. The diameter of PVA fibers is 12 μm, which is in the range of that of cement particles, and more mortar is required to wrap the fiber surface, resulting in a lower slump.
The increase of the content of PVA fibers of various lengths will reduce the slump of ECC. At all the tested dosage rates, the slump of ECC blended with 3mm PVA fibers is greater than that of ECC blended with fibers of other sizes. Moreover, the effect of adding other sizes of PVA fibers on the ECC slump is not obvious.
With the same diameter, the longer the fiber length is, the easier the fibers to form agglomerates during stirring, i.e., the shorter the fibers, the easier they are to disperse. Especially when the fiber dosage rate exceeds 2%, there will be a certain degree of agglomeration which will not be dispersed even if the stirring time is increased.
Effect of PVA fibers on flexural strength of ECC
The addition of PVA fibers can significantly improve the flexural strength of ECC. Within the selected dosage range (0.5%-2.5%), the flexural strength of ECC increases with the increase of fiber dosage rate, indicating that PVA fibers can well improve ECC strain hardening and crack resistance. Taking the ECC specimen with 9mm PVA fibers as an example, when the dosage is increased from 0.5% to 2.5%, the flexural strength increases from 5.75 MPa to 9.35 MPa.
Generally, the longer the fibers, the greater the flexural strength of ECC. For ECC specimens with 2% PVA fibers, the flexural strength of ECC with 12mm PVA fibers is 8.35 MPa, and that of ECC with 3mm PVA fibers is 5.74 MPa.
Effect of PVA fibers on compressive strength of ECC
Since ECC does not use coarse aggregates, while considering to improve the toughness, the strength requirements must also be considered. From the test results, the addition of PVA fibers of various lengths will slightly reduce the compressive strength of ECC, and the larger the dosage of fibers, the higher the reduction in compressive strength. This is because adding fibers reduces the density of ECC, thereby weakening the ability of ECC to resist pressure.
Meanwhile, the compressive strength of ECC has no obvious correlation with fiber length.