Aggressive phenomena typical of marine environments

The marine environment is particularly aggressive towards reinforced cement conglomerates. The aggressive mechanisms and resulting degenerative pathologies are the result of interaction between chemical or electrochemical type abrasive actions due to the presence of sulphates, magnesium and chloride compounds, with physical actions, connected to the dissolution of binders, with the osmotic pressure of the crystallization/re-crystallization processes of hygroscopic salts, with the mechanical actions: abrasion, cavitation, erosion, etc, caused by the wave-like motion, and so on.

Permanent immersion Vs. alternating immersion

Naturally, significant differences exist between the various use and exposure conditions, according to whether the reinforced cement conglomerate structures are: partially/alternatively immersed instead of being completely/permanently immersed. In fact, in the first case, as you can see from the analysis of the electrochemical phenomena causing erosion, in completely immersed structures, the reduced availability of free oxygen extremely reduces the speed of the processes. The situation is completely different for partially or alternatively immersed structures, due to the particular relevance of the mechanical actions connected to the wave-like motion, alternating the saturation/drying conditions, due to the complex run-off mechanisms with osmotic pressured determined by salt crystallization and so on.

Distance from the shore

The distance from the shore of the manufactured parts has a direct impact on the erosive phenomena to which the structures are subjected. The wavy motion is more accentuated near the shore, and therefore also the "mechanical" effects deriving from its action.

Sea winds

Furthermore, the degenerative mechanisms due to the aggressive action of the wind should not be underestimated, among other things, a considerable distance from the shore. In fact, the sea breeze transports solid and highly abrasive particles and salt sprays in the air from the waves, transformed into aerosols. These salts deposit on the surfaces of the concrete, stabilising in the pores, causing crystals that gradually grow and determine stress statuses capable of causing cracks. Furthermore, the aerosols mostly contain the aggressive agents of seawater, with all the resulting interactive complications.

Fouling

It is a type of attack that particularly interests the zones directly wet by the sea, determined by coverage of the structures by deposits formed by animal and plant organisms, overall referred to as "fouling". The phenomenon has high quantitative values: in the Adriatic, for example, the deposits can vary between 80 and 90 kg/year/metre squared. The extremely complex aggressive action is connected to the production of organic acids, through the metabolism of certain macro and micro-organisms composing fouling, which neutralise the alkalinity of the conglomerate, passivating the reinforcements and causing salt precipitation in the capillary pores.

Chemical aggression

Chemical aggression of seawater, in direct or indirect contact with concrete, is mainly due to magnesium sulphate (MgSO4) which reacts with free calcium hydroxide in the hydrated cement (Ca(OH)2) to form calcium sulphate and precipitate magnesium hydroxide, reacting with the calcium alluminate hydrate, to form a calcium sulfoalluminate expanse, with a disintegrating effect. The chemical/electro-chemical attack also occurs through the reaction of carbon dioxide (CO2) with calcium hydroxide (carbonatation).

Notes for renewal

• chemical processes: acid attack, sulphate attack, chloride action
• physical/mechanical processes: cracks, erosion, frost-defrost cycles, re-crystallization processes
• biological processes: action of lichens, seaweed, fungi and fouling.
• corrosion: the aggressive processes mentioned lead to or are however accompanied in most cases by the corrosion of the  reinforcement (an extremely significant “pathology” particularly for reflexes that influence structural "safety").

From the description of the degenerative phenomena examined, it is possible to deduce that resistance to abrasion in a marine atmosphere, by a reinforced concrete structure, on par with other conditions, increases as the quantity of free calcium hydroxide decreases by hydrolysis of the cement hydration process, as well as reducing both intrinsic and structural permeability of the conglomerate. The most significant degradation phenomena are also connected or linked to the loss of alkalinity of the conglomerate and the corrosion processes on the reinforcement.

Reconstruction materials

The choice of most suitable cement system for reconstruction and/or coating is of fundamental importance. The morphology of the degradation, especially in dimensional terms, orientates towards the first selection indications. The thickness of the reconstruction, for example, determines the choice between a grouting system (elevated sections) and a thixotropic section (for finer thicknesses), also orientating towards the maximum diameter of the aggregate. For the remaining properties, you need to refer to the causes and the mechanisms causing the degradation and the essential chemical, physical and mechanical performance mentioned below:

• Impermeability.
• Resistance against chloride penetration.
• Resistance against sulphate attack.
• Electrical resistance.
• Reduced development of calcium hydroxide (high super pozzolanic capacity).
• Bonding with supports.
• High mix cohesion (leach-proof capacity).
• Dimensional stability (shrinkage control function).
• Correct flexible module.
• Fibre-reinforced, composite structure with a more favourable isotropy.

The properties and performance identified is particularly favourable in the products listed below, based on the technology of the fibre-reinforced cement composites, functionally integrated with ultra-fine reactive fillers (micro-silicates) with very high pozzolanic capacity. GROUT CR, one of the main products in this context, is an anti-leach, composite cement mortar with controlled shrinkage and a glue-like consistency, with a high strength cement base, super-pozzolanic fillers, silicate mineral micro-fibres, selected aggregates, shrink-proof agents and special additives for the construction, repair and protective coating of hydraulic works, marine and sub-marine structures, manufactured parts in the presence of chemical-physical abrasives, leach water, marine, industrial and mountain atmospheres. The same properties are also found in the very high performance mortar REPAR TIX HG.