Concrete

Question 1

Sulphate attack

Answer 1

Sulphates in soil (or seawater)

React with cement

As sulphates dry= concrete expands = cracks

Lab tests to determine how much sulphate content in cement or surrounding water

cracking together with white crystalline accumulations

Question 2

Calcium chloride

Answer 2

Present from inclusion as an accelerator / de-icing salt / marine aggregate

Calcium chloride was routinely added to concrete from the late 1800s until 1977, when its use as an admixture was effectively prevented

Free chloride ions react with passive film

Localised pitting corrosion

Concrete may look fine …,
Sometimes white deposits
Difficult to repair
Repair: sacrificial zinc annode in patch repair
Catholic protection(potential of metal made more negative)

Treatment of chloride affected concrete is very difficult owing to the possibility of creating new corrosion cells at the boundary of the repair and the existing concrete. Methods of electrochemical treatment are becoming well established, with cathodic protection now a recognised method. Other systems of chloride extraction and migrating corrosion inhibitors have met mixed reviews and their effectiveness needs to be demonstrated by better performance data over time (see Repair methods).

Question 3

HAC

Answer 3

Pop50-60s. Banned 76

Pre-cast

Thought to resist chemical attack… quick curing
Expensive tho

CONVERSION
= loses strength

Highly converted HAV= vulnerable to chemical attack

Choc brown & friable.

BAD

Question 4

Alkali silica reaction

Answer 4

Critical silica (from some types of aggregate) + high alkalinity + moisture

= GEL

= MAP CRACKING + SPALLING

Structural failure

Question 5

What is in concrete

Answer 5

1 part cement, 2 parts sand, and 3 parts aggregate (1:2:3)

Question 6

Issues with raac

Answer 6

Can collapse with little to no warning.

1. The bubbles allow water to enter which can rust and weaken the reinforcement.
2. Planks sag and bend. 20mm dip= significant
3. Poor design- steel reinforcement does not extend far enough to hold the weight of the panel
4. May be weekend if structural load changed (covered with a heavier material)

This is why it’s oven covered with another material(bitumen). But this can degrade too!!)

It has a reported lifespan of 30 years, however this can vary massively dependant on maintenance & the design and installation. cutting the reinforcement bars on-site, can dramatically reduce the end bearing capacity of the planks.

If the building is well maintained and there haven’t been different load weights, it could be ok.

Later RAAC planks are known to use galvanised steel or stainless steel reinforcing bars, and are of less concern provided the roof is kept watertight.

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