Cement and food will also expire, once the expired cement particles will be damped, resulting in hydration, making the bonding capacity decline. There will be safety hazards after use. But before the cement expires, there is a shelf-life first. The storage period of cement is three months before it is used. Because the storage time is long, the cement may absorb moisture, and some of the cement may be hydrated, caking or reducing its activity, so the validity period of general cement is three months. After cement has been used in concrete or mortar, concrete or mortar hardening, there is no problem of expiration, but "durability" problem.
Generally speaking, 28-day strength is the criterion for judging civil and industrial construction projects, and the maintenance time of general concrete stipulated in the code is 14 days, that is to say, after 14 days, the strength of concrete is still increasing, but the impact of external factors on the strength of concrete can be said to be very small. As long as the concrete or mortar is in a good environment, that is, no corrosion, no freeze-thaw, no high temperature (over 100 C) and other destructive factors on concrete/mortar exist, the strength of concrete/mortar will not decrease and can be used for a long time. This is where concrete / mortar is superior to organic materials and metal materials. Concrete / mortar coagulated by cement belongs to inorganic mineral material, and its stability is very good. There is no need to maintain concrete after many years, because as long as there is no fire, there is basically no problem. After the building reaches its useful life, the buildings should be assessed for safety. Those who meet safety requirements may continue to use them. If they do not meet the requirements, they will be demolished and rebuilt.
But the concrete/reinforced concrete structure applied in practical engineering is in natural environment, and it will inevitably encounter many destructive factors, such as the bridge and wharf in the north will be frozen in winter, the sea-crossing bridge and the coastal wharf will be corroded by sea water (chlorine can penetrate into concrete in sea water will lead to steel corrosion), and the soil in some areas will be corroded by sea water. Sulphate or acid rain in soil may corrode concrete and so on. In addition, concrete may also undergo internal self-expansion damage, such as alkali-aggregate reaction, delayed ettringite formation, and so on. If no measures are taken to prevent these destructive factors, the life of concrete structures may be as short as ten years or even several years.
In view of the damage factors existing in the environment, the maintenance, maintenance or reinforcement of existing concrete structures is an important means to prolong the service life of structures. If the concrete structure is unsafe, the function does not meet the requirements or the cost of maintenance and reinforcement is too high, demolition and reconstruction may be the best choice.
