SUPERHYDROPHOBIC CONCRETE

Durability Aspects

Engineered Cementitious Composites

The durability of superhydrophobic engineered cementitious composites proves to be vastly superior to conventional concrete used on roadways and bridges today. The high cement content and high strengths lead to lower permeability and thus better durability. The addition of fiber reinforcement to the material allows strain-hardening response and improved strain performance, which is the downfall of conventional materials used in roadways. Moreover, the addition of superhydrophobic admixtures restricts water from entering capillary voids, thus greatly improving the durability. The durability of engineered cementitious composites is very good; however the addition of superhydrophobic admixtures improves the durability even further by allowing a strong cementitious matrix while at the same time having water repellant features.

To test the durability of superhydrophobic engineered cementitious composites, four different mixes were designed. The mixes had water to cementitious material ratios of 0.30 and 0.45. A single dose of superhydrophobic admixture was tested and compared with reference ECC. All mixes incorporated 50% of slag cement (ASTM Grade 100 ground granulated blast furnace slag; GGBFS) as a replacement of portland cement.

Table 1. Experimental Program for Durability of ECC/SECC (Muzenski et. al, 2013)

*single dose of emulsion is equivalent to an admixture of 0.25 g of siloxane to 1 liter of SECC


    The ECC/SECC mixtures were composed of the following materials and tested as follows:
  • 2.75% by volume of PVA fibers
  • Superplasticizer (polycarboxylate ether) content of 0.125% for w/cm of 0.30
  • Superplasticizer (polycarboxylate ether) content of 0.05% for w/cm of 0.45
  • Compressive strength tests were performed on 50.8 x 50.8 x 50.8 mm cubes in at a loading
    rate of 0.9 kN/sec in accordance with ASTM C109
  • Flexural tests were performed on 160 mm long x 40 mm wide x 14 mm tall beams under four-point bending at a deflection controlled rate of 1.2 mm/min
  • Absorption after immersion was tested in accordance with ASTM C642
  • Rate of absorption was tested in accordance with ASTM C1585
  • Rapid chloride permeability tests were performed in accordance with ASTM C1202
  • Freeze–thaw testing was performed in accelerated method (temperatures oscillating between –50°C and 20°C) in both fresh water and salt water (5% NaCl solution) medium
    • A freeze-thaw cycle multiplier factor of 5 can be used as comparison to ASTM C666 standard freeze-thaw testing for the above mentioned accelerated method (Sobolev and Bartrakov, 2007)
    • Freeze-thaw performance was monitored by calculating the durability factor, which is based on standard calculations dynamic modulus of elasticity from ASTM C215


Fig 1. Durability Properties of ECC/SECC (Muzenski et. al, 2013)

The use of superhydrophobic admixtures in engineered cementitious composites provides improved durability. The use of a lower water to cementitious materials ratio improves the durability properties in all tests. When considering absorption, rate of absorption, rapid chloride permeability, and flexural behavior, the addition of superhydrophobic admixtures proves to be beneficial. When considering the flexural behavior, the addition of the admixtures creates artificial flaws that initiate cracking and lead to multi-cracking and strain hardening behavior. The water repellant nature of the engineered air voids reduces the permeability and absorption of the material leading to better durability properties.

The results of freeze-thaw testing prove that using a lower water to cementitous material ratio is beneficial. Samples with a lower water to cementitious material ratio survived 700 accelerated freeze-thaw cycles at -50° C (-58° F) by maintaining a durability factor of at least 100 meaning that stiffness of the material is the same if not higher then when it was placed in the freeze-thaw chamber. The 700 freeze-thaw cycles would be equivalent to 3500 freeze-thaw cycles tested by standard freeze-thaw testing procedures at -20° C (-4° F).

At higher w/cm, the addition of superhydrophobic admixtures improves performance in salt water cycling. It can be assumed that as lower w/cm ratio samples become more deteriorated, the addition of superhydrophobic admixtures will provide extended durability (Muzenski et al., 2013).

References:

  • Muzenski S., Flores-Vivian I., and Sobolev K., 2013
    • “Freeze-Thaw Resistance of Fiber Reinforced Composites with Superhydrophobic Admixtures”, Submitted for publication in Proceedings of Concreep9, Cambridge Mass.

  • Sobolev K. and Bartrakov V., 2007
    • "The effect of a PEHSO on the durability of concrete with supplementary cementitious materials", ASCE Journal of Materials in Civil Engineering, pp. 809-819.

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