INTRODUCTION TO CONCRETE

What is reinforced concrete ?

Reinforced concrete is an assembly of concrete structured by steel bars. These two components of reinforced concrete have their interest and advantages. Concrete is composed of cement, aggregates and water. Cement being a hydraulic binder, it hardens in contact with water in a process called hydration. It is assimilated to dicalcium silicate with the chemical formula Ca₂SiO₄. We can therefore express the formation of concrete by the following chemical reaction :

$$2 Ca_{2}SiO_{4}(s) + 4 H_{2}O(l) \longrightarrow Ca_{3}Si_{2}O_{7}, 3H_{2}O(l) + Ca(OH)_{2}(s)$$

The calcium hydroxide Ca(OH)₂ produced is a strong base. It makes the concrete basic as its water's pH is between 12 and 14.

How is concrete aging ?

Over time, concrete undergoes an aging process known as carbonation. Carbon dioxyde CO₂ present in the atmosphere diffuses in concrete throught its pores and gets dissolved because of the water. During carbonatation, carbon dioxyde will dissolve calcium hydroxyde which we can write now :

$$Ca(OH)_{2}(s) \longrightarrow Ca^{2+}(aq) + 2HO^{-}(aq)$$

The phenomenon of carbonatation makes carbon dioxyde and calcium hydroxyde react to form calcium carbonate and water. The other chemical entities are spectator ions. Therefore the chemical equation of carbonatation is the following :

$$Ca^{2+}(aq) + 2HO-(aq) + CO_{2}(aq) \longrightarrow CaCO_{3}(s) + H_{2}O(l)$$

Carbonatation makes the concrete acidic.

How does the aging of concrete affect its properties ?

In order to evaluate the strenght of concrete, we used Young's modulus expressed by the formula above :

$$E = \frac{\sigma}{\varepsilon}$$

We acidified a block of concrete with hydrochloric acid to test its durability as well as a normal block of concrete.

Our basic block of concrete had a Young's modulus of 24MPa and our acidic block of concrete one of 19MPa. Our hypothesis was confirmed, aged concrete is less resistant of around 20% than fresh concrete. The danger is real.