Any sign of deterioration in the structural concrete of multi-level strata buildings should be concerning to unit owners. Underlying problems could be more or less severe than suggested by superficial appearances. Astute property owners will engage a qualified and suitably experienced structural engineer to inspect the building for signs of concrete spalling and oversee any remedial works critical to maintaining building value. Strata Engineering Solutions takes a new approach to remediation which minimises the cost and impact of works.
CASE STUDY EXAMPLE
Strata Engineering Solutions was engaged by the owners of a residential property at Edgecliff to provide structural engineering advice regarding concrete spalling to reinforced concrete slabs throughout the building.
The property is an 11-storey concrete-framed building with flat slab construction and masonry infill walls. Constructed in the 1980s, magnesite floor-levelling compound had been used throughout the building.
How concrete cancer develops
When magnesite is exposed to moisture ingress over prolonged periods of time, chloride ions from the magnesite diffuse into the pores of the underlying concrete slab eventually reaching the reinforcing steel. As chloride levels in the slab reach the chloride threshold (Cth ≈0.2% – 0.4% of cement mass), corrosion of the steel reinforcement will begin to take place. The longer the exposure-time to the moisture source, the more aggressive the corrosion of the steel reinforcement. Corrosion of this nature results in pitting of the steel reinforcement where it loses its cross-sectional area and strength. Iron oxide deposits also occur, causing expansive forces within the concrete. These forces cause cracking and spalling of the concrete around the corroding steel. A common name for this type of concrete spalling is concrete cancer.
During initial inspections of the building at Edgecliff, signs of concrete spalling were noted throughout various units. This had been caused by water leaking from various sources, saturating the magnesite for prolonged periods of time.
Concrete spalling appeared to be affecting the top surface of the slab only, limiting the reinforcement corrosion to the top layer of steel. This observation was later confirmed by testing the depth of chloride penetration into the slab which indicated that chloride levels dropped significantly at a depth of approximately 50mm.
Although the corrosion was limited to the top layer of steel, the quantity of concrete spalling was observed to be significant. Concrete spalling repairs are generally carried out on a $/litre rate basis. It was clear that the remediation works would be widespread throughout the building and therefore costly.
Conventional concrete spalling repairs generally involve removal of cracked or spalled concrete to sufficient depth to allow the steel reinforcement to be cleaned of all corrosion. Suitable priming materials are then applied to protect against further corrosion, and finally the removed concrete is reinstated with high-strength concrete repair mortar.
More efficient approach to repairs by SES
When SES inspected the concrete slab in the building at Edgecliff it became clear that carrying out conventional repairs to all areas of corroded steel would result in excessive quantities and costs. A new approach was required to mitigate against excessive costs.
SES devised a measured approach to the required repairs. This commenced with the production of analytical computer design models of the existing concrete slabs, based on the original structural design drawings.
Using extensive knowledge of reinforced concrete design, a detailed review of the original structural design of the concrete floor slabs was carried out which allowing areas of critical steel reinforcement and areas where non-critical shrinkage reinforcement was located to be determined.
This knowledge of the building’s structural design allowed SES to provide directions to the contractor during the repair works regarding areas where only cosmetic repairs were required. Wherever repair areas were non-critical, excessively corroded steel reinforcement could be removed and not replaced without impacting the structural adequacy of the concrete floor slab.
Repairs in areas of steel reinforcement that were deemed critical to the structural adequacy of the concrete floor slab were carried out as a traditional concrete spalling repair with reinforcement replacement as required.
The overall result of the measured approach to the concrete spalling repairs allowed for an overall reduction in the concrete spalling repair quantities without any impact on the structural adequacy of the concrete floor slabs and the building as a whole. The cost to building owners was significantly reduced and the repair process was made more efficient.
