Παρασκευή, 3 Δεκεμβρίου 2010

Waterproofing of basement joints with PVC waterstops






PVC waterstops are flexible profiles of various types and widths used for the waterproofing of below-ground expansion and construction joints. They are extruded from special types of thermoplastic PVC. PVC waterstops are used for the waterproofing both of water-retaining: water tanks and towers, dams, culverts, water treatment plants etc. and waterexcluding structures: basements, tunnels, foundations etc. PVC waterstops have to be embedded into the concrete usually in two different castings. Once in place they form a reliable, waterproof diaphragm to prevent water passage even under considerable hydrostatic pressures. There are so big differences among qualities of raw materials used that you have to make sure that you compare apples with apples. Furthermore you can find formulae compatible with asphalt or not. Although there are endless types of PVC waterstops, the most customary ones types are the ribbed types.

These products are placed:
 
- either centrally (most regular use)
- or externally (e.g. slabs-on-ground/ base-seal).

When used in practically non-working joints (e.g. construction or control joints) the ribbed profiles are plain, without a center bulb. These types of PVC waterstops can accommodate only little transverse movements. When used in working (e.g. expansion joints), PVC waterstops have a center bulb to accommodate lateral, transverse and shear movements. Obviously the larger the center bulb the large movement it can accommodate.  

These types of PVC waterstops are the most versatile ones and can be used for every sort of joints whether working or non – working. Ribbed type PVC waterstops provide a better sealing effect than other, smooth types. The path the water has to traverse to reach the opposite side is much longer and more difficult.

PVC WATERSTOPS CHOICE CRITERIA
 
PVC waterstops type, thickness and width are usually chosen according the following criteria. (Choice should be made only by experienced civil engineers):

* type of placement (internal or external)
* head of hydrostatic pressure
* reinforcement position
* type of joint
* thickness of building elements (walls, slabs etc.)
* movement type and size to be accommodated
* compatibility with bitumen
* compatibility with potable water
* granulometry of concrete
* chemical resistance (for fluids other than water) etc.
I never use widths less than 20cm and thickness less than 3 – 3,5mm.

THE TWO FACTORS THAT NEED OUR FULL ATTENTION
 
PVC waterstops are difficult to install properly. This, for me, is beyond dispute. Also PVC waterstops in order to guarantee a reliable waterproofing they need to be spliced correctly. Another thorny issue! These indisputable difficulties made inroads for self – expanding (bentonite etc.) waterstops into the market. Most of below-ground control and construction joints are now waterproofed with bentonite waterstops.
Please check the entry:
WATERPROOFING WITH BENTONITE WATERSTOPS: PROS AND CONS”.

SPLICING CONSIDERATIONS
 
For PVC waterstops to perform efficiently, continuity is of utmost importance. The majority of leaks come from improperly spliced joints.
Whenever possible use factory welded intersections which come in a big variety. On-site splicing is possible if heat welding equipment is available: a thermostatically controlled welding blade and an adjustable jig. The two pieces should reach a temperature of about 200oC and then pressed firmly to achieve a perfectly sealed welding.

APPLICATION OF PVC WATERSTOPS
 
And here we come to the quintessence of PVC waterstops technology: waterproofing through an ideal application procedure!

1. First of all: Deformed or damaged waterstops should be discarded! Check if you can cure the situation by proper stretching or heat treatment.
2. Waterstops should be installed symmetrically.
3. Proper fastening should hinder displacement during concrete casting. Many waterstops have preinstalled cyclical rings which help the easy and right fixing of profile by fastening it to the reinforcement bars through simple wire.

Take care: Totally prevent misalignment! There are also techniques to form these rings in situ.

4. In no case the center bulb should be encapsulated into the concrete mass when used in expansion joints! The center bulb should stay in the void space and accommodate every kind of movement.
5. Try to secure right embedment of waterstop in the concrete. Gently vibrate the adjacent concrete mass so as to ensure total contact with the profile. Only then sealing function could be taken for granted! Honeycombs, voids and segregation should be avoided at all costs and concrete should totally and safely embrace the ribs.
6. Obvious things shouldn’t be neglected! * Thoroughly clean joint area with appropriate means before concrete casting! Remove any loose materials that could inhibit an excellent contact between concrete and profile.

* Never lap profiles! Use the right splicing technique.
7. To avoid cavities in horizontal placements, install profiles looking upwards with an angle of 10o - 15o.
8. Heavier profiles should be used in certain circumstances to prevent displacement.
9. Split type waterstops: For the first concrete casting do nail split legs to the bulkhead between end of profile and the last rib. For the second casting (when forms are stripped) hold legs together by use of rings every 25-30cm placed again between profile edge and last rib.
10. Take care to discard badly spliced profiles (misaligned, purely bonded, burnt etc.).
11. Base seal waterstops should be fixed with appropriate fixings either to well compacted sub grade or to eventual formwork. Do align waterstop’s centerlign with the joint. For vertical applications these profiles will be nailed in the margin area directly on the formwork.

Hopefully we will see more details in a future entry.
 
Chris Strogilis
Civ. Eng. – MBA – DipM