In older style buildings, many balconies were constructed without a waterproof membrane. As a result, the soffit (underside) of the balcony is often affected by concrete spalling (concrete cancer). This is due to moisture seeping from the exposed top surface of the balcony migrating through the slab to the soffit where the moisture can become trapped by impermeable paint coatings. The trapped moisture often then reacts with the steel reinforcement embedded in the concrete causing it to corrode and the concrete to spall.
To alleviate this problem modern buildings have a waterproof membrane applied to the top surface of the balcony slab thus preventing moisture penetration.
The balconies in the photo have now been repaired and the top surface treated with a waterproof membrane and finished with new tiles to mitigate the risk of damage to the concrete.
