Esb?nderup Hospital successfully treated by natural drying of the concrete

The concrete structure had suffered from various types of damage caused by alkali-silica reaction and freeze/thaw action, such as comprehensive cracking to the bottoms of balcony slabs and to some of the protruding cantilever beams, as well as to the concrete surface of the balconies. Moreover, consequential

damage in the form of leaching with the formation of stalactites, stalagmites and curtains was also found. In addition to this, the parpaets of the balconies were suffering from spoiling due to rusting of the reinforcement bars, caused by the low concrete cover. If the balconies were to survive, action had to be taken against the moisture that was slowly but surely destroying the concrete.

Identifying a solution

The County of Frederiksborg, which is responsible for the maintenance of Esb?nderup Hospital, had been monitoring the deterioration in the condition of the balconies for a number of years. The technicians and engineers agreed that the damage was accelerating, but that there was no clear solution as to how it could be remedied. Visual inspection of the concrete balconies at the hospital revealed clear signs of water ingress into the concrete caused by the cracks. Sporadic repairs involving the surface treatment of the balcony floors had been attempted, but this failed to stop the formation of cracks. It was concluded that comprehensive renovation was required if the balconies were to be saved - if they were repairable at all.

Alkali-silica reaction

The serious cracking was the result of alkali-silica reaction, which had created tensile stresses in excess of those that the concrete floor could withstand. Esb?nderup Hospital was designed in 1970 when the 1949 concrete Standard was still in force. This Standard did not consider alkali-silica reaction and therefore there were no requirements aimed at preventing damage caused by this phenomenon. Moreover, the joint sealing designed to absorb the temperature deformations in the balconies was not sound, allowing rainwater to enter, dissolving some of the calcium hydroxide in the concrete.

Renovation strategy

While it is not difficult to prevent water entering the concrete and causing corrosion of the reinforcement, repairing cracks in the concrete caused by alkali-silica reactions is a different matter. Normally, such repairs are temporary because it is not possible to restore the original condition of the concrete. However, it is possible to prevent alkali-silica reaction so that new cracks are avoided and old cracks do not develop further.

The problem was identified as water ingress and therefore water had to be excluded. This meant removing the moisture and inserting an efficient seal. To this end, the floor was replaced, a watertight membrane was laid on the 50m-long balcony tops and a water repellent protection was offered to the lower sides. The renovation of the balconies provided an opportunity to demonstrate new methods and technologies, in that although the drying method had been used before in Denmark, it had never been used consistently. The method was therefore suggested for the balconies at the hospital.

Data analysis

The notable point in this renovation strategy was whether the moisture in the concrete could be removed to the extent needed to prevent any further alkali-silica reaction. It was therefore essential to obtain valid data which could document the drying-out process. FORCE Technology carried out tests to analyse the damaged concrete structures and contributed to identifying the causes of the problem.

Having become familiar with the extent of the damage and the renovation strategy, the company commenced the task by taking moisture measurements for three months prior to the actual renovation work.

Using six different moisture measurement probes. FORCE Technology was able to maintain accurate readings of the level of water saturation. The measurements were made both with portable and permanently mounted probes and gave both a detailed and varied picture of the drying-out process. After three months of measurements, the moisture content of the concrete had stabilised to a level where renovation work could begin. However, in order to completely document the process, measurements were performed over a four-year period.

The detailed measurements were the determining factor for the entire process. No remedial steps were taken until measurements documented that the moisture level was within the acceptable range.

Concluding remarks

Renovating hospital buildings presents many challenges because many factors must be considered - not least, the hospital's patients. The damaged balconies were part of a ward at Esbonderup Hospital and the County had stipulated that it should remain in full operation throughout the renovation work and this was successfully achieved.

It is believed that the methods used on this project will set a precedent which can be followed on other county buildings requiring renovation as a result of similar damage.