Concrete Building path L1

Question 1

What are concrete components?

Answer 1

Water: cement: aggregates

Question 2

What should reinforcement coverage be?

Answer 2

50mm

Question 3

What are common concrete defects?

Answer 3

Blisters, cracking, crazing, curling, delamination, dusting, Efflorescence, pop outs scaling and spalling

Question 4

Describe Blisters

Answer 4

BLISTERS are bumps of varying sizes that appear on the surface when entrapped air rises through the plastic concrete and gets trapped under an already sealed surface. Three principal causes are
- Excess amount of en-trapped air in the mix,
- Insufficient or overuse of vibration
-Finishing still spongy concrete.
Avoid by using the correct ratios of cement, fines and air in the mixes, not overworking the concrete during placement, using proper finishing techniques and reducing evaporation over the slab with the use of a cover or a fog spray.

Question 5

Describe Cracking

Answer 5

CRACKING due to shrinkage, settlement and applied loads. Avoid by:
-Proper sub grade preparation and sub base materials to enable a well supported slab and
prevention of settlement cracks.
-Proper concrete mixes using the lowest amount of water required for workability, maximizing the coarse aggregate, avoiding calcium chloride admixtures.
-Preventing a rapid loss of surface moisture will all help prevent shrinkage cracks.
-The use of handmade joints will help control cracks that develop from shrinkage or settlement and applied loads should be avoided from finished concrete until it has had enough time to gain most of its strength.

Question 6

Describe Crazing

Answer 6

CRAZING is a “chicken wire” pattern of fine cracks on the concrete surface, but not considered to be structurally serious or an indication of future deterioration. Causes of crazing are related to rapid changes in temperature and loss of moisture during the curing process. Prevention of crazing includes protecting the surface from any rapid changes in temperature and moisture if possible.

Question 7

Describe Curling

Answer 7

CURLING is the rising of a slab’s corners or edges when there are differences in moisture content or temperature between the top and bottom of the slab. It is usually caused when the top dries out or cools and begins to shrink before a wetter and warmer bottom. Prevention of curling is achieved using low-shrink mixes, proper control joints, large amounts of reinforcement steel or thickened edges.

Question 8

Describe Delamination

Answer 8

DELAMINATION is similar to blistering where areas of surface mortar crack and separate from the underlying concrete. The cause of delamination is also similar to blistering in that bleed air and bleed water become trapped under a prematurely sealed surface. Prevention includes using proper finishing techniques to get as much of the bleed water and air out before the surface becomes too dense and seals them under the surface.

Question 9

Describe Dusting

Answer 9

DUSTING is the formation of a fine, powdery material easily rubbed off the concrete surface. The cause of dusting is related to working excess water on the surface during the finishing stage allowing the finest particles to rise to the top and subsequent wear results in dust forming on the surface. In this case, similar to blistering and delamination, proper finishing and protection of the surface are required to prevent dusting.

Question 10

Describe Efflorescence

Answer 10

EFFLORESCENCE is a salt deposit that occasionally develops on concrete surfaces. It is caused by a combination of factors involving soluble salts in the material that dissolves in the water added to the mix, then migrates to the surface through hydrostatic pressure, and after evaporation, leaves a deposit behind. Prevention can be difficult but the use of properly graded aggregates, adequate cement content and low water-cement ratio combined with a thorough curing process is the best assurance against efflorescence.

Question 11

Describe Pop outs, Scaling and Spalling

Answer 11

Surface defects such as pop outs, scaling and spalling have similar causes but vary in size and depth. Typically they are caused by the penetration of water under the surface and the resultant pressure exerted on the surrounding concrete from expansion due to freezing or chemical re-actions damages the concrete surface. Prevention includes proper design of mixes for intended use, proper sloping to shed water away from the slab and proper finishing and curing techniques. Topical and penetrating sealers will also help pre-vent these defects from ruining a perfectly placed concrete slab.

Question 12

How do you undertake a patch repair?

Answer 12

Method for minor concrete defects such as surface spalling. Damaged or defective concrete is to be hacked off down to sound substrate and patched up with appropriate repair mortars to protect the steel reinforcement from rusting. Two types of materials are commonly used for patching up by hand:

• Cementitious mortars such as cement mortar and polyester-modified mentitious mortar or
• Resin-based mortars such as epoxy resin mortar and polyester resin mortar

After all defective concrete has been hacked off, rusty reinforcement bars should be properly cleaned, and primed with suitable cement/epoxy based primer matching the mortar used for patching if the environment is particularly aggressive, before patching up. Only primers specially manufactured for the purpose can be used, otherwise, the bonding strength between concrete and steel bars will be impeded, totally nullifying the repair efforts.

Question 13

What is carbonation?

Answer 13

Concrete carbonation is the result of an electrochemical reaction between carbon dioxide, moisture and calcium hydroxide that is present in cement, producing calcium carbonate. Calcium carbonate lowers the alkalinity of concrete from pH12 – 13 to around pH9

Question 14

How would you recognise carbonation?

Answer 14

Carbonation may be recognized by discoloured zone in the surface of the concrete. The colour may vary from light grey and difficult to recognize to strong orange and easy to recognize.

Caused by Carbon Dioxide (CO2) in the atmosphere reducing the alkalinity of concrete. This in turn results in reinforcing steel becoming susceptible to attack by water and oxygen, causing corrosion of the steel and eventually spalling of the concrete.

Question 15

What are causes of decay in concrete?

Answer 15

weathering leading to erosion,
water absorption leading to rusting of reinforcement ,
insufficient concrete depth – corrosion of reinforcements,
de-icing salts – chloride corrosion
Sulphates from vehicle exhausts

Question 16

How do you test for carbonation?

Answer 16

Concrete Carbonation is tested with a chemical indicator; the most commonly used indicator is a solution of phenolphthalein in alcohol and/or water.
Phenolphthalein solution applied to fresh strongly alkaline concrete will turn pink. If the alkalinity has been lost the concrete will not turn pink. In practice it is essential that the phenolphthalein solution is applied to freshly exposed concrete. This can be carried out in 2 ways, either:
1. By breaking a piece of concrete from the main area and spraying the underlying surface
immediately.
2. By drilling a hole into the concrete either at a given depth or in small increments.

The hole must be flushed out with de-ionised water immediately after the drilling to avoid contamination; this is then followed straight after with a spray of phenolphthalein. There should be a clear mark indicating the difference between pink and ‘natural’ concrete colour. The degree of carbonation can then be measured in millimetres.

Question 17

What does HAC mean?

Answer 17

High Alumina Cement

Question 18

What is HAC how does if differ from Portland cement?

Answer 18

HAC differs from Portland cement, being composed calcium aluminates rather than calcium silicates. Its rapid strength development made HAC popular from 1950 to 1970. However, mineralogical ´conversion´ sometimes caused reductions in concrete strength and increased vulnerability to chemical attack.

HAC concrete was effectively banned for use in new structural concrete in the UK following a few well publicised collapses in the 1970s. Time and experience have shown that the primary causes of these collapses were poor construction details or chemical attack, rather than problems with the concrete itself. Most HAC concrete in the UK went into precast beams. Up to 50,000 buildings with similar beams continue to remain successfully in service today in the UK. The beams can be found in public and industrial buildings such as schools, flats and business units

Question 19

How do you test for HAC?

Answer 19

If the presence of HAC is suspected, confirmation requires chemical or laboratory testing of samples.

In order to identify HAC concrete which may be potentially at risk and to distinguish it from concrete manufactured with ordinary Portland cement, a rapid chemical test was devised by the Building Research Station.

Question 20

What are the charaterisitcs of HAC?

Answer 20

• High alumina cement has low pH
• It has high refractoriness
• It has high durability in sulfuric acid
• It hardens rapidly
• It is less reactant than alumina or hydraulic lime
• It acts as a bonding material when added in refractory castables because it forms ceramic bond at high temperatures
• It has high resistance to chemical corrosion. So, it is widely used also in construction of water pipes, sewage pipes, factory drains, coastal constructions and in factory chimneys.

Question 21

How do you undertake a repair to steel reinforcement?

Answer 21

Step 1
* Break beyond the delimited area until reaching sound concrete and steel.
* With a jackhammer, saw-cut 2 cm deep in 90° angle the perimeter of breaking, in order to avoid feather edges.
* All weak, damaged and easily removable concrete should be chipped away. If the re-bars are only partially exposed after all unsound concrete is removed, it may not be necessary to remove additional concrete to expose the full circumference of the reinforcement. When the exposed reinforcement steel has a loose wrap, corrosion or is not well bonded to the surrounding concrete, the concrete removal should continue until a clear space of 15 to 25 mm is created behind the reinforcing steel.

Step 2
* Clean steel reinforcement with mechanical wire-brushing or by sandblasting.
* In case the reinforcing bars are corroded and have lost more than 25% of their diameter, they have to be cut and removed.

Step 3
* In such event, removed steel bars should be replaced with new steel bars of the same diameter either by respecting the overlap length or by welding 100 mm length to the existing steel.

Step 4
* Remove dust by air blowing and wash with clean water to achieve a Surface Saturated Dry (SSD) substrate.
Protection of steel reinforcement against corrosion:
After all necessary preparation, steel reinforcement should be protected against corrosion with a highly alkaline product weberep 370 PF or a zinc rich product that delays the aggressive elements attack steel, like weberep 501 ZRP

Question 22

what is a sump test and what is a compression test?

Answer 22

The SLUMP test shows the WORKABILITY of concrete. Workability measures how easy the concrete is to place, handle and compact.
The COMPRESSION test shows the best possible strength concrete can reach in perfect conditions. The compression test measures concrete strength in the hardened state. Testing should always be done carefully. Wrong test results can be costly.

Question 23

what are the main properties of concrete?

Answer 23

WORKABILITY
COHESIVENESS
STRENGTH and
DURABILITY

Question 24

Disadvantages of concrete?

Answer 24

the tensile strength of concrete is relatively low.
Concrete is less ductile.
Concrete may contains soluble salts. Soluble salts cause efflorescence.