Holdfast Training Services Trial Blog

I have learnt today that this week that the term “Honey Combing” on a construction site has nothing to do with the thick sweet substance that Ryan pours on his porridge in the morning. Honey Combing is in fact the term given to poorly Lid concrete that results in voids in the finished surface of concrete.

Removing the form work during the excavation revealed a large area of steel rebar that the concrete had failed to cover. The cause was determined to be number of factors:
1. The operatives were instructed not to compact, vibrate and tamp the concrete excessively during the pour as the beam in question contained Geo Thermal pipes that were at risk of being damaged.
2. The concrete mix was not loose enough to allow it to flow around the steel to fill the void. In fact some of the slump test during the pour revelled stiffer then required concrete. The slump had been determined at an average diameter of 550mm and we had loads that varied between 650 and 400mm. The result was the expected dismissal of the concrete and a reshow for the concrete contractor it was a case of working the concrete a little harder. This was principally due to the fact that the concrete was already being pumped by the time a slump had been completed. Poor practice in my mind.
3. However ultimately I feel that pour steel fixing was the principal cause, although it is not a theory accepted among the entire site and principally not by the steel fixers. The pictures below clearly show that steel bars are packed too tightly and have prevented any concrete from passing between the bars to fill the void. The steel should in fact be lapped one bar on top of the other with the use of a cranked bar. In this case the bars were allowed to lie next to each other.
The upshot of all of this has been not and engineering problem more of a contractual problem. The making good of this void would seem fairly straight forward. The use of a fast setting cementitous mortar although it must reach the same strength or better then the surrounding concrete. My fag packet engineering behind why I feel this is a minor engineering issue is that the missing area of concrete is in fact only cover concrete and that the concrete within this region is concrete in tension with and in fact it is the steel that is carrying the load. Given that the beam is a rectangular section the area of concrete that could be removed would be half the depth of the concrete as all below the centroid would be in tension. The greater issue is that this section is typical in terms of steel across the site. A more worrying concern is that there may be more voids that can’t be seen. If a void is within the compressive section of the beam, typically here it would be the upper half of the beams, this would significantly reduce the load that could be carried across the beams. The quality control during the steel fixing was fairly stringent and while most of the team on site are confident that there would not be any further voids I looked into how one might determine if there were any further voids . The solution that I had come up with was the use of a ground penetrating radar. These appear to be fairly available for hire by many companies on line and are a very simple non invasive method of detection. Unfortunately the confidence of the team and the reluctance of the expense and delays to the programme means that we won’t be playing with any GPR systems and a non compliance report has been raised, once the making good has been completed the case will be closed.