A great part of bridge population in many countries, including Poland, requires to be structurally strengthened. During the last two decades, bridge strengthening has become one of the most technical and economical problems in bridge engineering.

Three fundamental cases leading to the necessity of the bridge strengthening are presented and discussed, namely:

• bridges showing a bad technical condition – restoration of their load-carrying capacity predicted in the original design;

• bridges showing a generally good technical condition – upgrading of their load-carrying capacity in accordance to the new requirements concerning utilization of the structures (e.g. growth of the live load level);

• bridges showing a generally non-satisfying or even bad technical condition – their further utilization requires the strengthening corresponding to the higher level of live loads than the designed level.

The paper is mainly devoted to the strengthening of concrete bridges.

The methods applied for their strengthening are classified according to the original author’s criteria. The are classified into two fundamental groups, namely:

• the methods based on the redistribution of the internal forces in the structure (i.e. active methods);

• the methods not based on the redistribution of the internal forces in the structure (i.e. passive methods).

The methods applied are briefly characterized taking into account their cost and time of the required operations. Material solutions of the methods are presented in particular.

The newest methods, such as external prestressing and applications of the carbon fibre reinforced polymer (CFRP) products (i.e. strips and fabrics) are described in detail and exemplified by the case studies taken from the Polish bridge experience. The use of the above methods are well developed in Poland.

Some research and practical conclusions are formulated at the end of the paper.

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