Structural Repair
Concrete repair
in waste water
environments
Adrian Couper, Technical Manager of Structural
Concrete Alliance member company Balvac Ltd,
explains the reasons for concrete degradation in
waste water environments and reviews possible repair
solutions
Concrete structures in waste water environments are susceptible
to hydrogen sulphide attack, as well as the normal mechanisms
of degradation of concrete, known as carbonation and chloride
contamination.
The chemical attack, caused primarily by hydrogen sulphide in sewage,
will lead to the degradation of cementitious materials, including concrete.
Furthermore, chemicals in some trade effluents (for example, acid alkalis
and some solvents) can also attack concrete material. Chemical attack of
metal pipes and steel reinforcement contained in reinforced concrete will
also occur.
The degradation of the outer concrete layer of waste water treatment
consolidation tanks. The degradation of the concrete matrix can clearly be
seen with the remaining strong, smooth, quartzose lithic type aggregate
remaining in places
THE OCCURRENCE OF SULPHIDE IN SEWAGE
Sewage contains all of the elements required for sulphide generation,
namely bacteria, sulphate, and organic matter. Under anaerobic conditions
sulphates can be reduced to sulphides. This is a state that can develop in
sewage, because many kinds of bacteria are present that rapidly consume
dissolved oxygen and ‘oxygen’ from nitrate.
Complete loss of cover
concrete & degradation/
corrosion of steel
reinforcement in a
consolidation tank, as a result
of sulphuric acid attack
The sulphide corrosion cycle is primarily dependent on the quantity of
total sulphur within the wastewater. Under strictly anaerobic conditions,
sulphate reducing bacteria, present within the wastewater or slime layer
on the tank sides (the variation of liquid level or ‘tide mark’ on the tanks
increases this ‘slime layer’), convert the sulphates to form dissolved
hydrogen sulphide and hydrogen sulphide ions. Hydrogen sulphide is also
formed as a result of bacterial action on organic sulphor compounds.
Under aerobic conditions hydrogen sulphide released from the wastewater
is oxidised by bacteria in the presence of moisture to form sulphuric acid,
which attacks cement-based materials.
Extensive corrosion and
loss of section to steel
reinforcement within a waste
water treatment tank
Sulphuric acid acts upon the concrete (and metal) surface, degrading the
material matrix from the external surface towards the internal zone and
changing the composition and microstructure of the concrete. The effect
of these changes is an expansive process which will degrade the cement
matrix and lead to an overall loss of concrete strength.
REPAIR & PROTECTION TECHNIQUES
Large scale areas/volumes of concrete reinstatement is best carried
out using the sprayed concrete technique, which allows relatively large
volumes to be applied in an economical manner. Smaller patch repairs
can be undertaken using a hand applied technique. The repair of concrete
structures should be undertaken in accordance with BS EN 1504: 2008:
Products and systems for the protection and repair of concrete structures.
There are several measures that can be introduced to reduce hydrogen
sulphide attack including: reducing sedimentation through flow
management; reducing turbulence; increasing ventilation; protecting the
tank structure from chemical attack; and chemical dosing of the tank
contents.
Following the repair/replacement of concrete, further protection options
include resin or mineral (cementitious) based protective coatings. Resinbased systems include epoxy resin coatings, which provide chemical
resistance, durability, low porosity and strong bond strength applied
to concrete tank surfaces. However, resin-based systems must be
applied to a suitable dry concrete substrate surface so that water vapour
diffusion or osmosis will not be a future issue/problem. As a result, resinbased systems take longer to install; in addition the resin materials are
considerably more expensive than cementitious coating systems.
Repair of degraded concrete within waste water treatment assets involves
the removal of all degraded concrete. In addition to the deterioration
mechanism of sulphuric acid attack, other deterioration mechanisms can
include chloride contamination and