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Watertight Concrete

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Watertight concrete offers a simple, cost effective alternative to traditional watertight construction methods.

There’s no need for additional finishes or external membranes, so it’s very quick and easy to install. It’s also highly reliable, requires minimal maintenance and provided the concrete has been placed, compacted and cured correctly it will not leak.

Ordinary concrete contains fine pores that allow moisture to penetrate the surface and pass through. The size and volume of these pores is dictated by the water/cement ratio; the higher the ratio, the more porous the concrete.

Roadstone employ a two-step process to achieve a watertight concrete:

  • Firstly, the water/cement ratio is lowered to a maximum of 0.5 reducing the volume of capillary pores in the concrete matrix. This increases the density of the mix and minimises the size of pores.
  • Secondly, a Sika pore blocking admixture is used to create a hydrophobic layer within the remaining pores, thus blocking the passage of water through the remaining capillary pores.


The minimum requirements for watertight concrete are:

  • A minimum cement content of 360kg/m³
  • A maximum water cement ratio of 0.5
  • The use of a Sika pore blocker

Depending on the location and the scale of the project Roadstone use one of two pore blocking admixtures. At larger locations or for the supply of larger projects, Sika 1+, a liquid water reducing admixture is typically used. For smaller projects Sika Watertight Concrete Powder is commonly used which allows for the production process to be simplified.

In conjunction with the watertight concrete admixture a plasticer such as Sika 15RM is commonly used to allow for high water reduction which in turn results in low water/cement ratio concrete being achieved with high density and reduced permeability. In addition, the effect of plasticisers results in improved placing and compaction characteristics of the concrete.


Key Features & Benefits

  • Watertight Construction (keeps water out/in)
  • Reduces shrinkage of concrete
  • Increased durability
  • Prevents corrosion of reinforcement
  • No impact on setting time
  • Can be used and placed like normal concrete



  • Water retaining structures
  • Basements
  • Electrical and plant rooms
  • Reservoirs
  • Lift shafts
  • Swimming pools
  • Underground car parks
  • Tunnels

Technical Specification

  • I.S. EN 206:2013 – Concrete – Specification, performance, production and conformity

Product Specific Specifications

  • I.S. EN 1992 : Part 3 – Design of concrete structures for retaining aqueous liquids
  • BS 8102: 2009 Code of practice for protection of below ground structures against water from the ground

Case Studies & Resources

Case Studies

watertight concrete, concrete

Case study: Innovative concrete use at Páirc Uí Chaoimh