This particular blog was one of the last pieces of work that Andy did before he sadly passed away recently. Andy was an extremely knowledgeable member of the Sika team and was always keen to share his knowledge and experience, so we feel its appropriate to share his last blog with you.
First used in the UK more than 60 years ago, the Sika® Watertight Concrete system is as flexible as it ever was. Not only does the system allow for the use of cement replacements (GGBS, PFA), but also a wide range of aggregate types can be used with the system. Due to Sika® Watertight Concrete system’s impressive benefits, it is now the most widely used system of its type in the UK.
Even good quality concrete will allow water to permeate the surface. Capillarity is a natural act occurring when there is water on one side of the structure and air on the other. The volume of capillary pores in the concrete matrix is normally proportional to the water/cement ratio. Understanding and controlling water/cement ratio is the basis of concrete technology and, as well as permeability, affects the strength and durability of concrete.
The Sika® Watertight Concrete System comprises: Sika Watertight Concrete Powder or Sika-1+, in conjunction with Sika® high range water reducing admixtures. The Sika system consists of a dual admixture system:
- A powerful superplasticiser to provide a workable concrete for ease of placement and compaction at a low water cement ratio (max 0.45).
- A hydrophobic pore blocker or crystalline system to physically block the passage of water through any remaining capillary pores
The admixtures can be supplied in the liquid form Sika 1+ (dosed at specific rates with an additional superplasticiser) or as a combined powder in soluble bags in the form of Sika Watertight Concrete powder with an integral superplasticiser. This product, WT100P, is the traditional powder and pore blocking system, and WT200P is a crystalline technology system. There are no set rules on which system should be used in any particular application as individual consultants tend to have their own preferences
With Sika Watertight Concrete, the addition of water resisting admixtures, plus the optimised water/ cement ratio, creates a dense micro-structure that permanently seals the concrete
Using Sika® Watertight Concrete
The practical implementation of Sika® Watertight Concrete as the primary means of waterproofing requires little more than proper placement, compaction and curing in line with standard good concrete practice applicable to any concrete.
To assist the contractor and to ensure that observance of the specified requirements is met, Sika personnel attend site at regular intervals outlined by the Sika® Waterproofing Technician to monitor the works. Special details are required on occasions where construction joints will occur or where Sika® Watertight Concrete may be used as part of a composite with ordinary concrete, for example in raft slabs. On the other hand, standard details are provided to ensure proper and guaranteed performance of the overall waterproofing function. Practical guidance is given to the contractor regarding: optimum bay size and layout; formwork ties; joint detailing; service entry details and any other special considerations arising on a particular site.
The base concrete mix, regardless of compressive strength requirements, is required to have no less than 350 kg per m3 of cementitious material and a maximum free water cement ratio of 0.45. Cement replacement materials (GGBS, PFA) are acceptable where, for example, an exposure class to BS 8500-2:2006 dictates.
The base concrete should be designed with the mix designed for a standard S3 consistence class, unless otherwise stated. Higher consistence classes are allowable providing that the water cement ratio is protected.
The base mix should be modified to allow for the relevant water reduction from normal due to the Sika® Superplasticiser which forms one part of the admixture package. Adjustments should be made to ensure correct yield.
All Sika watertight systems carry a 15-year guarantee backed by the company. Combined with the easy use of each system and the technical-site-backup available, Sika watertight concrete is the UK’s market leader for all of your watertight concrete needs.