Author: John McCarron CEng MIEI, Donegal County Council The catalyst for this project began when residents of the 40-unit social housing scheme, constructed in 2001, at Long Lane, Letterkenny, reported a problem of dampness on the inside walls of their homes to the local authority. As the recently appointed housing engineer for the area, I was charged with finding a solution to this problem which had been progressively worsening.

Survey

I set about inspecting all 40 units located on high ground in an exposed area with my camera and dampmeter. Using the dampmeter I was able to plot the extent of the dampness accurately and with confidence. On completion of the survey I was able to establish a number of issues:
  • The 300mm thick rendered block work cavity walls were filled with a fibrous insulation material which soaked water like a sponge when exposed to moisture. This was the medium which played a large part in drawing the moisture through to the internal walls;
  • Poor construction was evident at penetrations, utility boxes, window reveals etc;
  • There was a theme of horizontal and vertical cracking in the external render;
  • The most interesting finding was that the dampness was confined to the southwest gables only, which of course are the faces exposed to the prevailing winds and storms commonly encountered in the west and southwest of Ireland. There were no damp issues on any of the other faces in all of the houses.

Specification

[caption id="attachment_28063" align="alignright" width="300"]View of affected houses View of affected houses[/caption] I drew up an initial specification for remedial works which involved extracting the contaminated insulation from the effected gables, picking off the existing render, allowing a drying period, which also allowed for a thorough examination of the exposed external block work. The 100mm wide cavities were to be re-pumped with an SEAI approved bonded 'eco-bead' filling. A waterproof render system would then be applied including painting. [caption id="attachment_28068" align="alignright" width="300"]Internal Dampness Internal dampness[/caption] The internal walls would then be dehumidified prior to redecoration. Due to the resources available I prioritised the five worst effected houses and was lucky in the tendering process to procure a reputable insulation specialist who had a keen interest and experience in this particular area. It had developed a technique of effectively removing the insulation with high pressured air through strategically placed 50mm diameter cores which also served as inspection holes to verify everything had been removed.

Works on site

[caption id="attachment_28070" align="alignright" width="300"]External cracking External cracking[/caption] The works commenced on site and within a short space of time the external render had been removed and the walls were exposed for examination. Due to the general appearance of the block work when exposed - including evidence of some cracking - I decided to carry out a laboratory test programme on the five houses. Following a consultation process with a number of experienced professionals in the field, the following tests were carried out: Queens University Belfast (QUB) [caption id="attachment_28073" align="alignright" width="300"]Test Results - Compressive Strength N/mm2 Test results - compressive strength N/mm2[/caption] Compressive Strength test to BS EN 7721:2011 - 18 Blocks Cement Content Test - two Blocks Mineralogy XRD Test (Chemical Composition) - two Blocks Material Test Centre (MTC), Termon Co Donegal Compressive Strength 'Quick Dirty' Test - 10 Blocks

Analysis

[caption id="attachment_28075" align="alignright" width="300"]Test Results: Mineralogy Test results: mineralogy[/caption] From the results as shown the following assumptions were made:
  • The compressive strength from both test facilities was up to acceptable standards. This after 14 years in exposed conditions including the two most severe winters in over 50 years in 2010;
  • Whilst the QUB BS/EN test procedure is more accurate and was said to produce a higher result, the considerably cheaper and quicker turnaround of the MTC test gave comparable readings;
  • There was also no notable difference in strength between the internal “control” blocks when compared to the external blocks;
  • The cement content test, whilst yielding results on the low side, was within acceptable industry norm limits, for non load bearing masonry block work;
  • The mineralogy test produced a very high 'Mica' content in both the coarse aggregate and possibly more significantly in the fine aggregate. QUB made the statement that “this may have a detrimental effect on the long term durability and strength of the block work”.

Decision

This statement from QUB is a very relevant point that many would reason with. In making a final decision on whether or not to take down the existing block work walls I had to take into account the proven existing overall strength of the block work walls from the lab tests, this after a 14-year life span in exposed conditions. I also carried out a cost benefit analysis which showed a €2,000 saving per house to retain the existing wall as opposed to demolition and rebuild. I finally made the decision to preserve and protect the existing walls as this was the most reasonable course of action when all the relevant factors were considered. Whilst the test results were pending, the contractor and I revised the initial specification to improve the external finish to the wall by introducing a latex based admixture which increased the durability and strength of the plaster. [caption id="attachment_28077" align="alignright" width="300"]Reinforced scratch coat Reinforced scratch coat[/caption] The final specification included for a high concentration bonding coat which was applied to the block work following a crack repair application with a high strength water proof product. A cement 'scud/key' coat then preceded a reinforced 'scratch' coat applied in two layers with a polymer mesh sandwiched in between to enhance the crack resistance properties of the plaster. This was effectively a reinforced render. See picture below A wet dash 'rough cast' final layer was selected for its superior water repellent surface texture. Its origins from the west coast of Scotland in exposed conditions, for good reason as we see it now. A finish which had gone out of fashion in more recent times but may make a comeback in certain areas in the future. This high specification finish offered the best that could have reasonably been done to prevent the dampness recurring as well as protecting the block work from any further deterioration and provided a practical engineering solution to the problems.

Considerations

The following points may also be considered with regard to this case study:
  • In certain situations where the problem with the block work is not chronic, the option to carry out the quick and inexpensive compressive test results, allows a more informed decision to be made to either retain and treat the existing walls or demolish and rebuild;
  • This was a case study involving multiple units owned by a local authority, which has a different dynamic from a situation with say an owner occupied dwelling;
  • The decision taken in this case to protect and preserve the existing walls allows for a follow up monitoring process to determine the success or otherwise of the project;
  • Each case is unique and will have its own unique situations with regard to, the extent and seriousness of problem, homeowners expectations and individual circumstances;
  • The external block work leaf in cavity wall construction is non load bearing and its engineering function is to protect the building from weathering. Defects which carry through to the internal leaf are more serious with regard to the structural integrity of the building;
  • The matter of defective block work is currently under review by an expert group which has recently been set up by the Minister for Environment, Community and Local Government.
John McCarron is a graduate of the University of Ulster at Jordanstown and currently works as a senior executive engineer with Water Services, Donegal County Council. Up until recently he was the area housing engineer in Letterkenny Municipal District Council. He was the last town engineer with Letterkenny Town Council between 2007 and 2014. He joined Donegal County Council in 2005 and prior to this worked as a structural engineer with a number of private practices on a wide range of medium sized commercial, retail and residential projects. He was the project manager on the first eight Aldi stores in Ireland.