Concrete is a composite material composed of coarse aggregates bonded together with fluid cement which hardens over time. The aggregates are mixed together with the dry cement and water, which form a fluid mass that is easily moulded into shape. The cement reacts chemically with the water and other ingredients to form a hard matrix which binds all the materials together into a durable stone-like material that has many uses.

 

Concrete is a mixture of:

Cement

• This is the binder in the mixture. It is made from limestone and looks like a grey powder.
• There are a few different types of cement, the one commonly used in concrete is called “Portland Cement”.
• It sets and become adhesive due to a chemical reaction between the dry ingredients and water. The chemical reaction results in mineral hydrates that are not very water-soluble and so are quite durable in water and safe from chemical attack. This allows setting in wet condition or underwater and further protects the hardened material from chemical attack.

 

Sand

• Small size hard particles. Size from 0,07 to 5 mm
• Must be free from salt and other impurities

 

Aggregates

• Big sized hard particles, like pebbles and gravel, from app 5 up to 40 mm, irregular in shape.
• Sand will fill in the voids in-between the aggregates

 

Water

• Clean, free from salt and organic material

 

Additives

• Speed up or slow down the curing
• Enable use in cold climates
• Makes the concrete more flexible

Typical Properties of Concrete

Benefits High compression strength High resistance to water and humidity High temperature resistance Good impact resistance Good non-skid properties

Limitations Low elasticity Low tensile strength Dusty May be difficult to keep clean Appearance

 

Tensile- and Compressive Strength

• The compressive strength is good (e.g. columns)
• The Tensile strength is poor (e.g. beams)
• Steel reinforcement bars are necessary for load bearing structures

 

Concrete for construction is never used alone

Concrete is almost always used together with steel reinforcement bars, more commonly known as rebars. Rebars improves the tensile strength of the structure.

• Rebars are bent and shaped and placed inside the structure mould before the concrete is poured.
• The size and amount of rebars, as well as their placing within the structure is calculated and determined by structural engineers.

 

Concrete is used as a construction material for:

• Dwelling and commercial buildings
• Public and governmental projects
• Industrial and production facilities
• Water treatment plants
• Offshore installations
• Bridges and highways
• Leisure facilities
• Infrastructure
• Power plants
• Mining
• Etc.

 

Curing of concrete

After pouring, the concrete must be left in the mould until it has set and become strong enough to carry its own weight.

 

Typical curing time

• Approximately 30 days at room temperature
• The mould (form) can be removed after 2 days
• Concrete need moisture during this curing period, so contractor has to provide measures to keep the concrete humid during the curing process
• If allowed to become dry before the estimated full curing time, the curing reaction is stopped permanently

 

Drying time

• After the full curing, the concrete needs to get through dry. This can take months

 

Testing concrete

During pouring of important structures, small separate cubes are also poured from the concrete mixture going into the structure.

• Cubes are crushed after 3, 7 and 28 days to measure the compressive strength of the concrete used in the structure
• Concrete compressive strength is generally based on the 28 days’ test results. There is a correlation between time and strength:
  • 3 days: 16 N/mm2
  • 7 days: 24 N/mm2
  • Full cure after 28 days: 30 N/mm2

 

The testing and correlation factor ensures early warning of potentially low strength concrete

 

Concrete quality

Factors influencing the concrete quality:

• High water content:
  • Gives less compression strength, more porosity, separation
• Low cement content:
  • Less compression strength
• Poor quality sand:
  • Less compression strength
• Poor quality aggregates:
  • Less compression strength

Concrete deterioration

Concrete can fail and require repair for many reasons:

• Honeycombing
• Spalling
• Stress cracking (makes access for water and chemicals)
• Shrinkage cracking (makes access for water and chemicals)
• Rebar corrosion
• Erosion
• Impact/abrasion damage (from tires with steel spikes)
• Chemical attack

 

High pH of a new fresh concrete

• A bi-product of the curing reaction is Calcium Hydroxide
• Calcium Hydroxide is water soluble and gives very high pH
• As freshly made concrete contain lots of water, the pH in new concrete will be high
• The pH of new / fresh concrete is in the range of 12 – 14
• Steel will not corrode in an a highly alkaline environment, which is why sound concrete protects its own reinforcement bars so well

 

What do we mean by concrete corrosion?

Concrete can corrode (deteriorate) like any other type of material

• Mechanism:
  • A change in the chemistry
  • The pH value drops from 12 to 8
  • At this pH the rebars are no longer protected against corrosion
• For submerged conditions in salt water:
  • Aggressive chlorides will increase the corrosion of the rebars

 

How rebars are damaged

1. Carbonisation: Carbon dioxide (CO2) from the air reacts with Calcium Hydroxide in the concrete, forming Calcium Carbonate
2. pH decreases, the pH will become lower, leading to an increased danger of rebar corrosion
3. Corrosion starts, Sulphur dioxide (SO2) from the air will react and convert Calcium Carbonate to gypsum, which is water soluble. This increases the possibilities for further Carbonising
4. The corrosion products are very voluminous. When steel corrodes, its volume increases 4 to 6 times
5. An enormous pressure will be exerted to the surroundings by the corroding rebars
6. The concrete will crack, loosen and fall off

 

Reinforcing steel (rebars) need to have an adequate covering of concrete:

• Minimum 50 mm for flooring
• Minimum 35 mm for vertical surfaces

Concrete repair

1. Chip away the damaged concrete (not allowed in some countries)
2. Remove carbonated concrete (test pH with indicator paper strips)
3. Shape the repair area and renew rebar if necessary
4. Preparing the rebar by blast cleaning or other means
5. Add corrosion protection to the rebars (e.g. alkali-resistant anti-corrosive paint, like epoxy primer)
6. Fill in the repair area with new concrete
7. Apply Carbonation-inhibitive paint

Why painting concrete?

• Protect structures against atmospheric impact (CO2 / SO2 / pollution)
• Protect concrete against exposure to chemicals (spillage, bund walls, warehouses, garages, etc.)
• Protect concrete subjected to mechanical stresses, such as traffic, high loads, impact and abrasion
• Waterproofing
• Cosmetic

 

When can we paint the concrete?

• With physically drying and 2-pack paint systems, the concrete should have below 7% moisture content. For flooring lower than 4%
• The moisture can be measured by using a concrete moisture meter
• A simple test for concrete floors is to take a square piece of clear plastic sheet and tape this thoroughly around the edges to the floor. Leave it in place until the next day and observe whether any moisture has accumulated as condensation under the clear plastic sheet. If paint had been applied, that moisture would accumulate under the paint film and would cause paint failure.
• Remember to determine the relevant climate, with winter conditions we have to be very careful when painting outdoor, due to possibility of freezing temperatures

 

Concrete – surface preparation before painting

• The surface preparation is a critical factor to achieve a long lasting performance
• The pre-treatment is often even more important than the application of paint
• Always start the pre-treatment procedure by removing loose contaminants
• Apply a penetrating primer coat, essential for a good result!

 

Concrete floors - Laitance

• Laitance is a major cause of failure in floor painting. It consists of a weak, friable layer on the surface of concrete which appears after curing.
• It is made from cement and fine aggregates that rise to the surface when too much water is added. It may also be due to rain damage during placing, or from power floating or over trowelling. Subsequent poor curing of the surface will also contribute towards the formation of laitance.
• Surface preparation is the most important step of painting a concrete floor. Removing surface contaminants, such as laitance, is therefore vital to a long- term successful installation.
• Laitance is always present on new concrete and must be removed. However, surface laitance is not to be confused with a poor quality concrete or screed that needs addressing in other ways.
• Laitance comes in varying degrees of thickness, from a fine dust to several millimetres or more, depending on contributing factors.
• If laitance is left untreated, the application of subsequent materials, (such as paint), will have a high risk of failing.
• Removing any existing laitance is vital to ensure that subsequent treatments can successfully adhere to the concrete substrate. There are several ways to remove laitance and the method chosen should depend on the working environment and purpose of the floor.
  • Shot blasting is the fastest and most efficient form of laitance removal, especially for large areas. Shot blasting machines are available in varying sizes and with vacuum attachments, making them ideal for use on most surfaces, no matter the size. Using a shot blaster will allow up to 1000sq m of flooring to be prepared in just one day and, as shot blasting is a dry process, work may continue in other areas of the room while the surface preparation process is taking place.
  • Using a mechanical planer is often used to remove greater thicknesses of laitance. Also referred to as concrete planing, the machines carry rows of rotating cutters tipped with tungsten to provide an excellent removal of laitance.
  • Scrabbling, grinding and abrading are also recommended for removing laitance. Handheld grinding machines, designed for precision, control and safe operation are recommended for use in smaller areas and edge detail.
  • Low Pressure Water Cleaning (LPWC): <34 MPa (340 bar/5000 p.s.i.) can work well to remove laitance and it makes imperfections in the surface visible. No dust is produced, but the floor needs to dry before paint application can start

 

Unless it has been removed by previous surface preparation techniques, laitance may still be present on old concrete floors.