Watertight Concrete FAQs

Pore blockers such as Sika Watertight Concrete Powder and Sika-1+ coat the capillaries with a hydrophobic chemical that impedes or stops the passage of water. Crystalline admixtures, such as Sika WT-200 P include a chemical which reacts with elements of the cement in the presence of water to create new crystals, filling cracks and capillaries in the concrete over time.

Pore blockers react immediately whilst crystalline admixtures only start to grow when moisture or water is present but continue to react over time.

Both are covered by the same Sika guarantee.

• In areas of constant water pressure against the structure, this will fully utilise the crystallising effects of WT-200 P.
• In areas with variable water table, pore blocker may be the preferred option.

Any cost differential depends on several factors, such as:

• Thickness of concrete section.
• Location of site.
• Size of project.
• Access limitations.
• Build schedule of project.
• Joint Sealing

The cost of repairing a leaking basement, which didn't have adequate waterproofing measures at design stage, is usually many times more expensive than making the structure watertight at the start.

Where watertight concrete may be more expensive per/m3 than standard concrete, a membrane will come with an added cost of labour and time in the programme. In most situations reinforced concrete already forms part of the structure, so there is minimal extra labour and construction time required to incorporate watertight concrete admixture.

Please note that for a grade 3 structure, two forms of protection are often required and all projects should be viewed on their own merits based on risk, required internal environment and construction types and limitation.

In addition, when working alongside major warranty providers (NHBC etc), there maybe a requirement to use two forms of waterproofing to provide adequate protection.

Yes, whichever admixture is chosen, it works as part of a system where joint protection and good site practice are equally important to deliver a watertight structure.

There are many types of joint sealing solutions depending on the application and conditions. Generally, for construction joints Hydrophilic Strips (SikaSwell) are popular, but coated metal strips (Sika Metalsheet FBV), Injection hoses (SikaFuko) or PVC Waterbars can also be used as part of the system. All are compatible with watertight concrete and at least one Sika joint protection system should be used in a continuous network in order to receive the Sika Guarantee.

Movement joints must be sealed with either a suitable internal or external waterbar or post-applied flexible strip (Sikadur Combiflex). These allow for differential movement of the adjacent concrete components. Please contact Sika for information on movement joint solutions to ensure the correct one is specified.

Curing is the chemical reaction between cement and water which generates strength and density within the concrete. The curing time of WTC will be similar to plain concrete and should not affect time required to strike formwork, for example. Factors such as cement content, temperature and cement type will have a much greater influence on this aspect.

Drying is the escape of moisture from the concrete element. The rate of evaporation from the watertight concrete surface is slightly slower than conventional concrete due to its higher density and action of the admixtures themselves. It will be greatly affected by factors such as mix design, temperature, ventilation and curing regime. Depending on the requirements of any coatings to be applied to the surface, different levels of moisture may be acceptable.

All concrete should be finished with a suitable curing regime, for example using Sika Antisol MP-10 curing agent.

Yes, generally a liquid admixture (Sika-1+) works best through this type of mixer to allow constant feed during production. Concrete producers operating volumetric trucks should have third party quality accreditation and current calibration certificates for all their vehicles. If a Sika Guarantee is required, an audit of the quality procedures by Sika’s Technical Department will be required prior to starting the project.

The strength class of concrete required will be governed by the engineering requirements of the structure. The mix design should comply with the Sika specification of minimum cement/binder content of 350 kg/m3, maximum water-cement ratio of 0.45, minimum consistency class of S3 / 120mm slump (to enable sufficient compaction when placed).

Cement replacement products can be used with all Sika watertight admixtures, for example up to 75% Ground Granulated Blast-furnace Slag (GGBS).

A pour schedule with joint locations would be normally agreed by the structural engineer and Sika's technical department prior to placing the concrete.

• For vertical watertight concrete, walls should be poured in sections with a maximum aspect ratio (length to height) of 3:1, ending in a construction / day joint, but precise joint locations should be decided by the structural engineer. Dwarf walls with a height of <1 m may be extended to a 5:1 aspect ratio.
• For horizontal watertight concrete slabs, the maximum area without a construction / day joint should in general be 400 m2, assuming the slab shape is close to 1:1 aspect ratio, but precise joint locations should be agreed with the structural engineer.
• For capping beams, precise joint locations should be decided by the structural engineer in agreement with Sika’s technical department.

9 metres in most situations, but precise joint locations should be decided by the structural engineer in agreement with Sika’s technical department. Joint sealing details should be agreed between pours.

Precise joint locations and bay sizes should be decided by the structural engineer in agreement with Sika’s technical department.

Not as a standalone system but it can form part of a gas-resistant waterproofing specification incorporating a suitable gas barrier such as SikaProof or Sikaplan membranes. Overall basement ventilation should be considered, for example in basement car parks, the building may already have appropriate ventilation due to the layout of access ramps for cars whereas a habitable basement may require a forced ventilation system.

In general, the minimum section thickness is 175 mm but aim for 200 mm minimum section thickness to allow for tolerance in waterbar placement location.

The structural concrete element is to be designed in accordance with the flexural and thermal crack limitations stated in the relevant sections of BS EN 1992 Eurocode 2, with maximum crack widths of 0.3 mm or 0.2 mm, depending on design criteria and application, and as stated in relevant product BBA Certification.

Providing the section thickness was compliant and the correct detailing of joints was agreed (eg. Sikadur Combiflex), then Sika Watertight Concrete admixtures can and have been used in precast concrete elements. The concrete would also need to conform to Sika’s mix design requirements.

BS 8102:2022 Code of Practice for Protection of below ground structures against water ingress.

Sika offers a 15 year guarantee for Sika Watertight Concrete system, including joint sealing. This increases to 20 years for certain dual waterproofing systems, where a type A or type C waterproofing solutions are installed in combination with the Sika Watertight Concrete or 25 years where three types are used.

1. A Sika Specification document for the project must be issued by a Sika Specification Manager.
2. A Sika tool-box talk is given to the installing operatives prior to commencement on site.
3. A Sika Technician has performed at least one interim site visit.
4. Any actions and non-conformities identified in the Site Attendance Log have been completed to Sika’s satisfaction.
5. A Final Assessment has been completed prior to commencing internal works, i.e. plastering or covering over the concrete. The areas must be clear, dry and accessible for the Final Assessment.
6. Complete and return the Sika Application for Waterproofing Guarantee form.

The Final Assessment must be carried out with no exceptions. It is the installing contractor’s responsibility to inform Sika they are ready for the assessment. All Sika waterproofing materials must be or have been available for visual check, this includes any repairs carried out by either the Contractor, Sika Limited, or their appointed specialist contractor.

Any areas of waterproofing that were / are not available for assessment may be excluded from any guarantee. When all parties are satisfied that a successful job has been achieved, the Sika Application for Waterproofing Guarantee form must be completed by the installing contractor and returned to Sika within 12 months of final assessment.

Yes, as long as our normal installation and guarantee processes are followed.

Mix designs are produced by the ready-mixed concrete supplier then checked and approved by the Sika Technical Services team.

This would be reviewed on a case by case basis in conjunction with Sika Technical.

Yes, we have a full suite of standard details for all waterproof systems, also available on our waterproofing website.

If the project requires a Sika guarantee, then site visits by Sikae technicians are required and supplied free of charge. Otherwise, site support would be on a project-by-project basis.