Ready-Mix Concrete

Concrete class is written as C followed by a number, such as C25 or C30, denoting the characteristic 28-day compressive strength in MPa. In ordinary Tehran residential buildings, C25 and C30 dominate, with higher classes reserved for the lower columns of towers or aggressive exposure conditions. The class is the structural engineer's decision and appears on the drawings. Downgrading it to save money reduces the capacity of every member and is traceable by both the supervising engineer and the municipal inspector.

Slump measures workability. A slump of 7 to 12 cm suits conventional placing, while pumped concrete and heavily reinforced sections need 16 to 22 cm achieved with a superplasticiser, not with added water. Maximum aggregate size is normally 19 or 25 mm and must stay below one third of the slab thickness and three quarters of the clear bar spacing. In ordinary conditions the water-to-cement ratio should not exceed 0.50.

The single most damaging site practice is adding water to the truck mixer. Drivers add it to discharge faster and workers add it to spread more easily, and that alone can strip 20 to 30 per cent off the design strength. Next comes poor vibration: too little leaves voids and honeycombing, too much causes segregation. Third is leaving the fresh surface exposed to sun and wind without covering and watering during the first seven days.

For every significant pour, take cylinder or cube samples in the presence of the supervising engineer and file the 7 and 28-day results. Each truck ticket records the class, batching time and volume. The interval between batching and discharge should not exceed roughly 90 minutes; beyond that the mix no longer matches the approved design.