Durability Study of Rubberized Lightweight Concrete Using Nano-Silica

. Lightweight concrete (LWC) is used in the construction of partition and panel walls in framed structures because it reduces the dead load acting on the structure. Rubberized lightweight aggregate concrete is made by using waste rubber tyre chips as a partial replacement of coarse aggregate which increases the ductility, and toughness and minimizes the impact effect of concrete. The presence of rubber content decreases the compressive strength and bond strength, which affects the durability of concrete. Nowadays, nanomaterials' application has received much attention to enhance concrete properties. Due to the nano-filler effect and the pozzolanic reaction, the microstructure becomes more homogeneous and less porous, especially at the interfacial transition zone (ITZ), which leads to reduced permeability. Among the nano-materials, nano-silica has gained particular attention compared to conventional mineral addition due to its better performance in concrete. Incorporating a small number of nanoparticles in concrete can modify the nano-structure of cementitious materials, thus producing high strength and durability. This study is to investigate the durability properties of rubberized LWC containing nano-silica at dosages of 1%, 3%, and 5% by replacement of cement, respectively. Durability under a marine environment is to be analysed based on compressive strength, bond strength, and resistance to the penetration of chloride ions for 28 and 56 days.


Introduction
Construction of partitions and panel walls requires the use of lightweight concrete with excellent strength since it lessens the structure's dead load.The biggest problem facing industrialized and developing nations right now is waste materials like car tyres.Investigations towards recycling used tyres are being conducted to reduce environmental harm.Studies reveal that old tyres are made of materials that do not disintegrate in the environment and result in notable pollution.Although burning is a possible method for their disintegration, the gases released when tyres burn cause considerable pollution.Another method is the use of tyres in the concrete industry, according to experimental research.Recycled coarse aggregate can be used in place of natural coarse aggregate to generate green, sustainable concrete while also reducing the quantity of waste that is dumped in landfills.Additionally, this can cause the concrete's compressive strength to decrease, which an admixture addition might help to counteract.containing nanometer-sized particles.The particles' tiny size also results in a larger surface area.By filling in the tiny gaps and pores in the microstructure, Concrete, the most widely used cement composition in practical applications, was also altered by substituting different nanoparticles for the binder, such as titanium oxide, nano alumina, and nano-silica.Due to its higher reactivity and extremely small particle size, nano silica has an impact on the characteristics of concrete when compared to other fillers.
Numerous researchers were interested in the introduction of nano silica (NS) into concrete because of its potential to enhance cement composite qualities as well as the chemical similarity between NS and components of C-S-H.NS is a highly reactive pozzolan, similar to silica fume.These nano-crystals fit into the cement concrete's tiny pores, enhancing the permeability and toughness of the material.So, this research paper gives a comparative study durability properties of rubberized lightweight concrete with and without nano-silica.

Material properties
The specific qualities of the constituent materials, such as cement, water, coarse and fine aggregate, NS, and tyre shreds that were necessary for the casting of specimens for the experimental work are described below.

Cement
OPC 53 grade Chettinad cement is the type of cement utilized.Table 2.1 indicate the results of the preliminary test on cement.The properties such as standard consistency obtained was around 32%.The specific gravity of the cement obtained was 3.05 and the initial setting time was 40 minutes.

Fine aggregates
M sand is used for concrete mixes.The features of M sand are presented in Table 2.2.The water absorption obtained after the preliminary test was around 0.91%.The obtained value of fine aggregate properties such as specific gravity was 2.52 and the fineness modulus was 4.29.

Coarse aggregates
Table 2.3 shows the properties of coarse aggregate.Light-expanded clay aggregate (LECA) with a maximum particle size of 15 mm is used.The obtained value of coarse aggregate properties such as specific gravity was 0.80 and fineness modulus was 2.57.

Waste Tyre shreds
Tyre chips passing through a 16 mm sieve and retained on a 4.75 mm sieve are used in the concrete mix.Table 2.4 shows the properties of rubber shreds.The obtained value of rubber shreds properties such as specific gravity was 2.15 and the fineness modulus was 1.16.

Nano Silica
The particle size is 17 nanometres.The range of specific gravity is 2.2 to 2.

Mix design
The design was based on IS 456:2000 and IS 10262:2019.The mix proportion of rubberized lightweight aggregate concrete is indicated in Table 2.1.

Beam design
The beams were cast for finding the cover strength of the specimens using the pull-off test.
The beam is designed utilizing IS 456:2000.Beam having dimensions 150 x 150 x 700 mm is used for the design.The detailing of the beam is shown in Fig 2 .1.

Curing
Curing is the process of keeping newly placed concrete moist and at the appropriate temperature to allow it to properly hydrate and gain strength.Curing is critical for the longterm durability and strength of the concrete.Cubical and RCPT specimens were totally submerged in water after the casting, while beams were covered in gunny bag and the specimens were immersed in seawater as shown in Fig 2 .3.

Slump cone test
The concrete slump test is done to determine whether freshly produced concrete is still workable.The test is conducted using a metal mould that is shaped like a slump cone or Abrams cone.The workability is estimated according to IS:1199-1959.

Compressive strength test
Compressive strength refers to the ability to tolerate loads that tend to reduce size.The compressive strength is tested for each age of 7 days and 28 days.As per IS:516-1959, the compressive strength is estimated.

RCPT Test
The Rapid Chloride Permeability Test (RCPT) evaluates the level of resistance to chloride ion penetration.The relation between the concrete permeability v/s charges passed is given in Table 2.The bond strength or cover strength of the beam specimens from various levels of deterioration caused at 28 and 56 were studied using pull-off test.

Slump cone test result
The slump test result obtained indicates that all the mixes are medium workability mix.The slump cone test result obtained is shown in Table 3.1.

Compressive strength test result
The compression test result obtained after 7 days and 28 days of curing of specimens is shown in Table 3  Increase in compressive strength is observed for specimens having varying percentages of nano silica from 1-3%.The highest compressive strength observed for specimens with 3% nano silica from average compressive strength results in 28 days.

Rapid Chloride Penetration Test (RCPT)
The RCPT test is conducted on the samples are tested for a duration of 6 hours with readings taken at every 30 minutes.The combined charge pass for all the samples is shown in Table 3.

Pull off Test
The pull-off test is done on the beam specimen where the high-tension bolt is fixed into the concrete with the help of epoxy and pulled out after an epoxy curing period of 24 hrs with the help of a hydraulic pull-off tester.The results of the pull-off test are shown in Table 3.4.
The Fig. 3.3 shows the cover strength of specimens after 28 and 56 days.

Fig. 3.3 Cover strength after 28 and 56 days
The NS-3 specimen has the most cover strength after exposing the specimen to sea water for a period of 56 days as compared to control specimen.

Conclusion
The major findings from the study are listed below.
• The optimum NS percentage ratio for rubberized lightweight concrete is obtained as 3% nano silica • The strength gain in observed more in early ages from the compressive strength result.
• The cover strength of concrete is found to be decreasing as the period of sea water exposure increases.
• The pore refinement is found to increase as the age of the concrete increase due to the use of nano silica as a supplementary cementations material.• Sea water exposure leads to lower cover strength which may be due to the decrease in development of C-S-H gel progress over period • The pore refinement is found to be the most for NS-3 56 days sample with chloride permeability nearing low permeability.The chloride permeability is expected to reach low permeability with increase in time.

3 .
The Fig 2.4 shows the Rapid chloride penetration testing machine.

3 .
The combined current pass vs. time graph is shown in Fig 3.2.

Fig. 3 . 2
Fig. 3.2 Current pass vs.Time combined NS-3 56 days specimen has the least current passed, hence pore refinement is better around 56 days of age.

Table 2 . 1
Mix proportions for rubberized concrete with LECA as coarse aggregate

Table 2 . 2
Mix proportions for rubberized concrete with LECA as coarse aggregate with NS

Table 2 . 3
Concrete permeability v/s Charge passed

Table 3 . 2
.2.The relationship between different NS% and compressive strength at 7 & 28 days is shown in Fig 3.1.Concrete permeability v/s Charge passed

Table 3 . 3
Combined charge pass for all the sample