Corrosion in Concrete
Structures of the Upper Mahiao ZEDS
Leyte Geothermal Production Field – Characterization and
was initially documented in the concrete spray tower posts of the
Upper Mahiao Zero Effluent Disposal System (ZEDS) in Leyte
Geothermal Production Field (LGPF) during the October 2004 annual
Preventive Maintenance Service (PMS). Field observations showed
deterioration of the cement matrix to a weak and powdery state
leaving the rock aggregates protruding out of the concrete surface.
petrographic examination showed the aggregates to be mainly igneous
rocks; while the vuggy cement matrix showed intense alteration to
finely crystalline calcium carbonate or calcite. Phenolphthalein
tests indicate a depth of carbonation ranging from 4 to 5 mm.
SEM-EDX analysis of the cement matrix revealed the dominance of Ca,
C, and O confirming the CaCO3 composition of secondary
calcite in the matrix.
friable nature of the concrete sample, and the development of
secondary calcite in the cement matrix indicate that the spray tower
posts were attacked by carbonic acid formed by dissolved CO2
in the power plant effluents. Apart from decomposing the cement
paste, carbonation also lowers the pH of concrete hastening
corrosion of reinforcing steel bars.
Mitigating measures were applied in October-November 2005 to protect
the spray tower posts from further carbonic acid attack. These
included using a special high-density cement mix blended with
fly-ash. The spray tower posts were also jacketed up to fluid level,
and the basin was lined with molded high-density polyethylene
plastic that is both durable and resistant to corrosion attack.