Repair and structural reinforcement of pillars in marine environment

Descripción sistema

Single-family house located on the Mallorcan coast a few metres from the sea with terraces supported by reinforced concrete pillars. The pillars are degraded due to the attack by sulphates and chlorides which has led to the oxidation of the reinforcement and the fisuration of the concrete that covered them, putting their load-bearing capacity at risk.

Reason for the intervention

Due to the aggressiveness of the marine environment, the 6 concrete pillars that support the terraces of the villa are extremely deteriorated, with loss of concrete and totally rusted reinforcements that have been left exposed.
The loss of section of the pillars jeopardises their load-bearing capacity and requires urgent intervention to restore their integrity and carry out structural reinforcement.

Challenges and constraints

“Repair and reinforcement should be carried out using techniques and materials that ensure high durability in an aggressive marine environment.

Molins Solution

“Removal of degraded concrete and preparation of the substrate.
“Release of reinforcement and protection with BETOPRIM cementitious primer.
“Repair with PROPAM® REPAR TECHNO SR sulphate resistant structural repair mortar.
“Reinforcement of the columns with PROPAM® CARBOCOMP carbon fibre reinforcement system by wrapping the columns with PROPAM® CARBOCOMP TEXTIL embedded in BETOPOX® CARBO epoxy resin.
“Application of BETOPOX® 93 epoxy resin sprinkled with fresh silica aggregate.
“Application of a coat of PROPAM® REPAR TECHNO SR mortar as a final finish for the pillars.

Execution

“1. Preparation of the substrate
Since the main reason for the degradation of the columns is chloride attack (see technical note), it is particularly important to carry out an exhaustive preparation of the substrate and the reinforcement.
For this purpose, all the degraded, spalled and cracked concrete was removed by mechanical means, until the rusted reinforcement was completely exposed. The rust was removed and the reinforcement was cleaned with a steel spike brush.

“2. Protection of reinforcement
To ensure the durability of the repair and to protect the reinforcement against the marine environment, BETOPRIM cementitious reinforcement protection primer was applied with a short haired brush in 2 coats, ensuring complete coverage of the reinforcement.

“3. Restoration of the removed concrete
Before proceeding with the reinforcement of the columns with carbon fibre, it was necessary to restore the removed concrete. The choice of repair mortar was conditioned by the aggressiveness of the marine environment, so the sulphate resistant structural repair mortar class R4 according to UNE-EN 1504-3, PROPAM® REPAR TECHNO SR, was chosen.
This repair mortar allows applications of up to 50 mm thick in a single layer, so it was applied, after wetting the concrete of the support to saturation, with the help of a trowel and trowel, leaving the edges of the pillars rounded to facilitate the application of the carbon fibre.

“Carbon fibre reinforcement
The PROPAM® CARBOCOMP SYSTEM was used for the structural reinforcement of the columns. Specifically, the PROPAM® CARBOCOMP TEXTIL unidirectional carbon fibre fabric was used, specially designed for the reinforcement of elements subjected to compression by confinement, as is the case of the columns.
On the pillars repaired with PROPAM® REPAR TECHNO SR and 7 days after its application, the BETOPOX® CARBO epoxy resin was first applied with the help of a roller and with an approximate consumption of 350 g/m2. Immediately after application and while the resin was still fresh, the pillars were wrapped with PROPAM® CARBOCOMP TEXTIL carbon fibre by pressing with a stiff roller in the direction of the fibres to ensure that they were fully impregnated with the resin and any possible air pockets were removed. The overlaps between different fabric sheets were 20 cm.
As a final coating, a new layer of BETOPOX® CARBO resin was applied with an approximate consumption of 250 g/m2.

” 5. Application of epoxy bonding bridge and aggregate dusting
To enable the pillars to be finished with a cementitious mortar it is necessary to increase the surface roughness of the carbon fibre reinforcement. For this purpose, the low viscosity epoxy bonding bridge BETOPOX® 93 was applied and sprinkled with clean silica sand while it was still fresh.

6. Application of finishing coat with sulphate resistant repair mortar
Finally, to leave the conventional finished appearance of the pillars, a final coat of PROPAM® REPAR TECHNO SR sulphate resistant repair mortar was applied over the rough surface created by dusting sand over the carbon fibre.

Technical note

The aggressiveness of the marine environment for reinforced concrete structures
Seawater contains salts in solution (35 g/l on average) which are mainly sodium chloride and sulphates.
The atmosphere of areas with a marine environment contains high concentrations of chloride and sulphate ions which, transported by the high humidity, penetrate into the pores of the building materials. In the following, we explain the attacks they cause to reinforced concrete structures.

Chloride attack
It is characteristic of environments close to the sea (the strip of the first 5 km of coastline is considered as class IIIa environment: General marine exposure class area) or of cold climatic zones where de-icing salts (sodium or potassium chloride) are used.

Chlorides diffuse through the pores or cracks in the concrete until they reach the reinforcement. The chloride attack is punctual (pitting) and causes the corrosion of the iron of the reinforcement, which can affect the entire section of the reinforcement. Furthermore, because chloride salts are hygroscopic, they retain moisture in the concrete. The rust formed has a volume 2.5 times greater than that of the iron that initially made up the reinforcement, and this causes the concrete to crack and fracture. The breakage of the concrete, in addition to the loss of resistant section in load-bearing elements, facilitates the continuation of the oxidation process and therefore degradation.

To minimise this degradation, the EHE-08 (Spanish Structural Concrete Instruction) establishes minimum cement content and water/cement ratio in order to manufacture concrete that is as non-porous as possible.

Sulphate attack
The sulphates in contact with the tricalcium aluminate (C3A) contained in the cement and with the water, react giving rise to salts of large volume that expand in the concrete causing it to crack and break. These salts, known as ettringite, have a volume 8 times greater than that of the starting products.

The only way to avoid this reaction in soils or environments with the presence of sulphates is to use cements with a low aluminate content (sulphate-resistant) such as the cement with which PROPAM® REPAR TECHNO SR is formulated.