Page 139 - New Trends in Eco efficient and Recycled Concrete

Waste rubber aggregates 113

On the other hand, possible drawbacks of using rubber sand instead of natural
aggregates in mortar or concrete mixes could be the decrease of workability, the
decrease of mechanical properties, the increase of drying shrinkage and water
absorption chloride ion penetration depth.
In conclusion, it is safe to say that crumb rubber can be used to realise light-
weight mortars or concrete and might have a potential market for pre-cast items
with good sound absorption. Interesting applications could be as shock absorber in
highway constructions or as shock-wave absorber in buildings. It can be used as
construction material with elevated thermal insulation, sound insulation and when
acoustic anti-vibration properties are required or as construction material with high
resistance to impact loads.
Rubberised concrete can be also used in foundation pads for machinery and in
railway stations where vibration damping is required.

4.8 Conclusions

The key findings from the experimental studies regarding the use of crumb rubber
as a partial substitute for fine aggregates in cement-based mortars or concrete can
be summarised as follows:

The addition of rubber admixture to cement-based concrete or mortars significantly reduce
their mechanical strength. This reduction increased as the content of rubber particles
increases. There are some possibilities to mitigate this reduction, such as by replacing a
suitable part of cement with silica fume or pre-treating rubber particles with NaOH aque-
ous solution or in water-soaking. Another possibility could be mixing rubber particles
with latex before adding them to the matrix.
The addition of rubber sand in mortars or concrete mixes reduces workability and
increases bleeding and setting time.
The addition of rubber sand in mortars or concrete mixes reduces fresh and dry density.
This reduction increases by increasing rubber sand content.
The addition of rubber sand in concrete, up to 50%, enhances impact energy and impact
load behaviour.
The addition of rubber sand increases concrete ductility and strain capacity.
The addition of rubber sand in mortars or concrete increases shrinkage.
Rubber sand increases the concrete abrasion resistance; however, appropriate rubber sand
content and suitable w/c ratio have to be used.
The substitution of natural aggregates with rubber sand increases the freeze/thaw resis-
tance of concrete. Freeze/thaw resistance increases as the rubber sand content increases.
The addition of rubber sand in concrete mixes increases the matrix porosity and its capac-
ity of water absorption. These properties also depend on the dimensions of rubber parti-
cles and on the amount of aggregate replaced.
The substitution of natural aggregates with rubber sand increases the concrete thermal
insulation, sound absorption, energy absorption and electrical resistance. These properties
increase by increasing rubber sand content. On the other hand, the addition of rubber sand
reduces the concrete fire resistance.