Coastal and Marine Concrete Construction
Corrosion prevention with high performance concrete (HPC)
Embedded steel and other metals of reinforced concrete in coastal and marine areas are subject to corrosive environments where the presence of chloride ions, as well as oxygen is one of the main reasons for the deterioration of concrete. Deterioration usually starts when salt and moisture enter the concrete.
To prevent corrosion of concrete structures in marine environments, it is suggested to use high performance concrete (HPC) in splash zone, which should be of high stength, high durability, high resistance to chloride ion penetration, and good workability. In addition to selected aggregates, high-quality admixtures and cement compatible superplasticizers should be added to prepare high performance concrete.
Silica fume is used in high performance concrete to protect the embedded reinforcement. It has high pozzolanic activity and fine particles, can reduce the bleeding and increase the cohesiveness of fresh concrete, improve the strength, reduce permeability, and provide good corrosion resistance which is especially suitable for hydraulic structures and spraying, grouting and repairing projects.
Wharf project case study
Below is a case study of a wharf project using HPC. The in-situ transverse beams, precast longitudinal beams, trench beams of the wharf are all made of C45 concrete mixed with 10% silica fume.
The high performance concrete was mixed with forced action mixers that are of high mixing efficiency and provide good homogeneity. Silica fume was put in together with the aggregate and first mixed for 20 seconds to let the silica fume be dispersed well. And then water, superplasticizer, as well as other materials were added and mixed about 40 seconds longer than the conventional concrete to achieve good results. It should be paid special attention for any project that the workers on site must wear dust masks during construction to prevent the inhalation of silica fume from causing damage to their organs.
The concrete should be poured in a high slump condition. Generally, for manual operation, the slump should be above 12cm. In the project, the slump was about 20cm with a concrete pump. Vibrators were used to remove the bubble of the concrete. And when the concrete was poured onto the top surface, to ensure the compactness of the concrete, secondary vibration was conducted.
Concrete plastering is a difficult point in construction. After the high performance concrete is poured, vibrated and compacted, laitance should be removed and the plastering work should be completed quickly. The second plastering work should be applied when the surface of concrete is shrinking.
Protective measures should be taken immediately after plastering to prevent moisture evaporation that causes plastic shrinkage cracks. Operations in marine areas is difficult and inconvenient, so in order to reduce labor intensity and improve the quality of curing, additionally, concrete curing compounds can be used after the initial setting of the concrete to ensure the quality.