How the results are determined;
The four classes generally considered relevant to hardcore used in house construction are as follows, expressed in both gramme and milligrams per litre of sulphate determined as SO4 :-
Class 1 <.10g SO4/Litre to <0.50g SO4/Litre (<100mg SO4/Litre to 500mg SO4/Litre)
Class 2 0.50g SO4/Litre to 1.50g SO4/Litre (500mg SO4/Litre to 1500mg SO4/Litre)
Class 3 1.60g SO4/Litre to 3.00g SO4/Litre (1600mg SO4/Litre to 3000mg SO4/Litre)
Class 4 3.10g SO4/Litre to 6.00g SO4/Litre (3100mg SO4/Litre to 6000mg SO4/Litre)
In most properties tested the result will fall into Classes 1,2 or 3.
Class 1 result is generally acceptable to Surveyors and mortgage lenders in the Midlands as the level of sulphate is not considered to be high enough to cause damage to the floors and walls of the property in the future.
Class 2 result generally poses the most problems for Surveyors and Mortgage Lenders. Generally a low Class 2 result is acceptable especially if there is no evidence of damage and the hardcore is dry. The presence of a polythene membrane will provide additional protection. A high Class 2 result increases the potential for damage although if the hardcore sample is dry and there is no evidence of damage due to sulphate action then the chances of any damage occurring in the future are still very small. Any decision however concerning the removal of the floors and hardcore infill are at the discretion of the Surveyor and/or Mortgage Lender.
Class 3 result is generally not acceptable especially if there is no polythene membrane present between the hardcore infill and the concrete floor slab.
The above method will apply to all properties built with traditional foundations.The property may however have been built on a raft foundation due to poor ground conditions, mining subsidence etc..
Where a raft foundation is found it is usually not necessary to take and test a sample of the hardcore infill as the amount of hardcore infill under the raft is likely to be very small, the raft will be usually minimum 200mm thick, will be a stronger concrete mix than in a normal traditional floor and will be reinforced with steel mesh reinforcement.
Taking all these factors into consideration we interpret current BRE guidance as indicating the Building Research Establishment say that even if sulphate containing industrial fill is present under the raft foundation, the rate of attack on the concrete is unlikely to exceed 1mm per year making the risk of damage minimal.
