Studies on Utilization of Controlled Low Strength Materials in Pavement Layers

. Road networks play a essential role in transportation systems at the national, state, and local scenario. Ongoing efforts involve construction of new roads and the enhancement of existing ones to improve the overall efficiency of the transportation system. However, highway construction often results in environmental degradation. A more eco-friendly alternative known as CLSM relies significantly on industrial waste in its production process. CLSM, also known as flowable fill, is a self-compacted cementitious material that exhibits properties between concrete and soil. This paper focuses on evaluating the suitability of CLSM mixes in pavement layers and is prepared by combining ground granulated blast furnace slag, cement, and fine aggregates such as crumb rubber and copper slag sand. Flowability and marsh cone tests were conducted to assess the workability of the mixes. California Bearing Ratio (CBR) test, Unconfined Compressive Strength (UCS) to determine the mechanical properties of hardened CLSM. CBR values obtained for CLSM mixes (UCS range 0.28 – 2.42 MPa) was observed to be more than that of conventional pavement subgrade materials. Adoption of such CLSM mixes for pavement subgrade will lead to sustainable road construction.


Introduction
Controlled Low Strength Material (CLSM) is a self-compacting cement-based material typically utilized as a substitute for compacted fill in various applications.It comprises of industrial by-products, typically a mixture of fly ash, cement, water, and either fine or coarse aggregates.Instead of fly-ash various other industrial by products like Ground Granulated Blast Furnace Slag, crumb rubber, cement kiln dust, incernator ash, crushed limestone, calcium sulpho-aluminate, calcium carbide residue etc.The percentage of waste materials used in CLSM mix is shown in Fig 1 .Advantages of CLSM includes readily available, easy to deliver, safety during construction, easy to place, strong and durable, can be excavated, allows all-weather construction, makes use of industrial by-products, and does not require compaction or tamping.*Corresponding author: geethanjalis851@gmail.comDisadvantages of CLSM includes difficulty in excavation for high strength mixes, requires confinement during initial setting and the mix should be placed in lifts i.e, the first layer should harden before placing the next layers [1].Controlled Low Strength Material as various applications shown in Fig 2 .However based on the literature survey carried out it was found that CLSM mixes is not being extensively used in pavement applications.Hence the experimental work is carried out in order to determine whether CLSM mixes can be utilized in various pavement layers specifically subgrade and subbase.

Experimental Investigation
A test-driven investigation was conducted to determine the flow characteristics [4], Unconfined Compressive Strength (UCS) and California Bearing Ratio (CBR) tests were conducted.Ground Granulated Blast Furnace Slag (GGBS) and crumb rubber was utilized in first mixture and is referred to as GC series.In second mixture GGBS and copper slag sand was utilized and is referred to as GS series.The flow was fixed to be in a range of 200-300mm for CLSM to be called self-flowing.Subsequently, trial mixes were created to determine the amount of water needed for each mix to attain the indicated flow value.Tables 1 & 2 gives the proportion of CLSM mixes, for GC mix the ratio of cement, GGBS and crumb rubber is 2:1 and for GS mix the ratio of cement, GGBS and copper slag sand is 1:1 and quantity of cement was varied by 7% & 8% along with it water content is varied in order to produce mixes that flow easily into voids without the requirement of compaction.Total of 48 samples were cast for UCS and CBR and the samples [5] were tested after 28 days of aircuring.

Materials
Ordinary Portland Cement (OPC) 53 grade was utilized and the specific gravity of cement was found out to be 3.12 it imparts strength and provides cohesion.Ground Granulated Blast Furnace Slag was procured from Gogga Minerals and Chemicals in Hospet, Karnataka, having a specific gravity of 2.58 helps to improve the flowability of CLSM mixture.Crumb rubber and copper slag sand obtained from local industry was utilized as fine aggregates, and the material passing through 4.75mm IS sieve were considered.The specific gravity of copper slag sand was found out to be 2.68.

Specimens
Acrylic moulds of diameter 40mm and height of 80mm were considered in order to produce UCS samples [6].The moulds were greased and the mix was poured into the moulds after 6 to 8 hours the specimens were de-moulded and kept for air curing for 28days.The CBR specimens were casted by placing the spacer disc and pouring the mix which was air cured for 28days and was placed in water for 96 hours in order to simulate the worst moisture condition at site.

Results and discussion
Average flow value is shown in Fig 3, marsh cone and UCS values of GS and GC series are tabulated in Table 3 the flow of GS ranged between 200-385 mm and GC ranged between 200-320 mm [7][8][9][10].The flow increased with increased percentage of water content and the value is in between 200-300mm hence CLSM mix satisfies self-flowing and self levelling.
The marsh time for GS mix is in between 15.87-101 sec and for GC mix it is 0 seconds due to larger particle size of crumb rubber [11][12][13][14][15][16].UCS value of GC mix at 28 days is in the range of 0.28-0.48MPa and for GS mix it is in the range of 1.96-2.42Mpa [17][18][19].The value of CLSM 0.34 Mpa (50 psi) is considered to be suitable for future excavation works GC mix can be utilized for works where future excavation needs to be carried out except for two mixes (at 7% & 8% cement with water content of 44%).The GS mix is having a value greater than 1.38 Mpa which is the value required for the material to be considered as Cement Treated Base Material (CTB) [20].The CBR results is tabulated in Table 4.The CBR value for GC and GS mix ranged between 73.43-206.54%.The CBR value required as per code for selected subgrade material should be more than 8% in pavements for 450 cvpd [21].However for subgrade the CBR value must be greater than 8-30% and for subbase it should be within 20-50% hence both mixes satisfies this material requirement.

Conclusion
The following inferences were made based on the conducted experimental work: 1.The need for a greater quantity of water arose from the necessity to achieve the desired flowability values.It should be noted that, in comparison to GS mix, a larger amount of water was necessary for GC mix due to the inherent hydrophobic properties of crumb Flowability vs water content rubber particles.This increased water requirement was essential to ensure an even and efficient mixing process.2. The Unconfined Compressive Strength for GS mix is higher than that of GC mix, which is greater than 1.38 MPa.Hence the GS mix is suitable to be utilized as pavement material.3. The California Bearing Ratio value for GC mix is 3 times more than that of the requirement and for GS mix its 6 times more than the requirement.Hence the mixes can be utilized for pavement subgrade and subbase layers.

Fig 3 .
Fig 3. Variation of flow with water

Table 1 .
Mix design for GC series

Table 2 .
Mix design for GS series

Table 3 .
Flow, marsh cone and UCS results *Segregation and bleeding was observed