👉The blue material you see in the formwork is a PVC waterstop or rubber waterstop, depending on type).

🔵 What it is: It’s a waterproofing strip placed at construction joints—especially where concrete is cast in stages (like footing,wall and slab).

🎯 Its purpose: Prevent water leakage through construction joints
Act as a barrier inside the concrete so water cannot pass through the joint

🎲Commonly used in:

⏺Water tanks
⏺Basements
⏺Retaining walls
⏺Foundations exposed to groundwater

🏗️ In your case:
From the image, it looks like it's placed along the joint between the base slab and the vertical wall formwork.

💫This means: The concrete will be poured in two stages

The blue strip will be embedded halfway in the first pour and halfway in the second
It ensures a watertight connection between the two pours

⚠️ Important notes:

⏺It must be properly aligned and fixed before pouring
⏺Any displacement can cause leakage later
Joints should be clean before the second pour.

@etconp