a- What are the broad types of admixtures? b- How would you improve the bond of fresh concrete to hardened concrete?

 

Q#

a-      What are the broad types of admixtures?                                                                             

b-      How would you improve the bond of fresh concrete to hardened concrete?                                  

Solution#

 

Answer A

Classification:

·         Water reducers

·         Set-controlling chemicals

·         Air – entrains

·         Specialty admixtures

-Shrinkage reducing

-Viscosity modifiers

-Corrosion inhibitors

-Styrene – Butadiene latexes(SBR Latexes)

-Anti – freeze compounds

-Water proofing

-Alkali – Silica reaction mitigating admixtures

1.Water Reducers

For a given workability, the water demand is reduced, thus resulting in higher strength and durability.

·         For a given W/C and strength, workability can be increased.

·         For a given W/C, strength and workability, the quantity of cement can be reduced.

 

Normal water reducers (5-8) %

High range water reducers (15-25) %

 

2.Set-Controlling Chemicals

·         Set controllers are organic or inorganic chemicals that interfere with the hydration process of cement.

·         The rate of dissolution of cement compounds, that is necessary for hydration to occur, is either speeded up or slowed, depending on the chemical.

There are two types of set – controlling chemicals,

I. Accelerating chemicals

II. Retarding chemicals

 

3.Air-Entrains

Air – entraining agents are also surface – active chemicals. Unlike the water – reducing surfactants, the hydrocarbon chains does not have any polar groups, and is entirely hydrophobic.

The hydrophobic polar groups (- coo- , - So3 - , - NO3 - ) are similar to water reducers.

 

5.Viscosity Modifying Agents (VMAs)

To provide stability to extremely flow able concrete (which may be prone to segregation). To prevent the wash – out of concrete in under water application – in this case the VMA is also called “Anti – Washout Admixture”.

·         The dosage of VMA is generally 0.03 – 0.08 % by weight of cement.

·         Biopolymers and synthetics:

-Diutan gum

-Alginates

-Xanthan gum

-Hydroxy propyl methyl cellulose

-Hydroxy propyl starch

The bond of fresh concrete to hardened concrete

 After cleaning the surface of old concrete, adhesive are used, these adhesives include sand cement grouts, water cement grouts, epoxy resins.

By creating patches in old concreted bonding of old and new concrete is increased. Regular shaped patches are proved to be effective than irregular patches

 

for increasing the bond strength between fresh concrete and hardened concrete.

 

1. Providing reinforcement above than the hardened concrete to make bond strength

 

2. Using the some Epoxy resns and Epoxy Chemicals to increase the bond strength