SUPERHYDROPHOBIC CONCRETE

Mechanical Behavior of Superhydrophobic

Engineered Cementitious Composites

The micromechanical design of engineered cementitious composites dictates that the strength of the cementitious matrix must not exceed the maximum fiber bridging strength (Yang EH. et al., 2009) in order to achieve the strain hardening behavior. To achieve this, the strength of the cementitious matrix has been reduced. However, with superhydrophobic engineered cementitious composites, the strength of the cementitious matrix has remained high while the addition of superhydrophobic admixtures act as artificial flaws to initiate cracking which leads to multi-cracking and strain hardening behavior.

To demonstrate this behavior, five samples were produced, one reference (R) with no admixtures, two with a single dose of hydrophobic admixture, and two with a double dose of hydrophobic admixture (0.25 g/liter and 0.50 g/liter, respectively. The admixtures were tested with and without the addition of metakaolin (H_MK and H, respectively). The addition of metakaolin to the admixture improves the hydrophobicity of the void while at the same time allowing for a better dispersion of the voids, allowing for better multi-cracking behavior.

Table 1. Contents of Superhydrophobic Emulsions (Muzenski et. al, 2013)
Table 2. Experimental Matrix for ECC/SECC Mechanical Tests (Muzenski et. al, 2013)


Fig 1. Compressive Strength of ECC/SECC (Muzenski et. al, 2013)


Fig 2. Flexural Behavior of ECC/SECC (Muzenski et. al, 2013)

The addition of hydrophobic admixtures to ECC improves the flexural behavior by introducing a “tailored” air void structure acting as artificial flaws to initiate cracking. This, in turn, allows for multi-cracking and strain hardening behavior. Moreover, the addition of air voids in SECC displays only a minimal reduction in compressive strength; whereas typical air entrainment is used there is a significant reduction in compressive strength. The use of metakaolin in the admixtures produces a better distribution of air voids, which not only leads to better flexural behavior but very little reduction in compressive strength. A double dose of admixture displayed reductions in performance which is an indication of high air generation. Knowing this, the addition of metakaolin to hydrophobic admixtures at a single dosage proves to be the ideal product based on the improved mechanical behavior (Muzenski et. al., 2013).

Fig 3. Cracking pattern of ECC/SECC samples under 4-point Flexural Bending (Muzenski et. al, 2013)

References:

  • Muzenski S., Flores-Vivian I., and Sobolev K., 2013.
    • “Hydrophobic engineered cementitious composites for highway applications”. Submitted for publication in Cement and Concrete Composites.

  • Yang EH., Yang Y., Li VC., 2009
    • "Use of high volumes of fly ash to improve ECC mechanical properties and material greenness". ACI Materials Journal; 104(6):620-628.

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