LIME IN ROAD CONSTRUCTION

IMPROVEMENT OF SUB GRADE STRENGTH WITH LIME TREATMENT
ABSTRACT
DLime treatment is quite useful for modification of engineering properties of the sub grade soils having clay contents of 7 percent or more, Plasticity Index more than 10 and more than 25 percent passing 75 micron sieve. More specifically , it is used to (1) increase the strength and (2) to decrease the their plasticity index. The increase in strength is used to justify decrease in structural section. The reduction in plasticity index is used to extend the life expectancy of the structure. Obviously, considerable cost saving can accrue from the permanent modification of either of the two engineering characteristics. 

INTRODUCTION:
Almost all civil engineering projects deal with soil, and the demand for improvement of soil properties is increasing day by day. Due to exponential increase in traffic volume, sub grade strength has become quite important in the design of highways/roads. In the paper attempt has been made to find out effect of lime treatment on engineering properties of the sub grade soils of Nawanshahar area. For this purpose, soil samples were taken from the sub grade soils at four different places. The objective of the study was to evaluate the CBR and Expansion Ratio of the original soil and the effect of lime on the CBR and Expansion Ratio value of different soils with different percentages of lime. 

PROPERTIES OF THE LIME USED IN SUB GRADE TREATMENT: 
e of the impervious nature of the surface course such as thin layers of premix carpet without proper sealing coat, cracks and potholes and undulations causing pooling, thus allowing the passage of surface water in to the road pavement matrix.

Hydrated Lime was used in this case.

LITERATURE REVIEW:
Sastry & Kumar studied the unconfined compressive strength and CBR strength of lime clay mixes and sand -clay -lime mixes prepared at optimum moisture content and compacted to maximum dry density for soil lime mixes. They observed that a minimum quantity of finer fraction binder soil(40%) is required to make the soil sound and sand-lime mixes workable. 

Uppal and Bhasin conducted a laboratory study on the effect of delayed compaction on strengths of soil-lime mixes and concluded that the strength of soil lime mixes decreases with and increase in the delayed time between mixing and compaction. In the case of delay between mixing and compaction, they suggested that compacted mass should be cured for longer period to compensate the loss in strength.

EXPERIMENTAL PROGRAMME:
Standard Proctor Test: Standard proctor test was carried out to find out the influence of 
different percentage of lime on the maximum dry density and optimum moisture content of the soil. Table 1 shows optimum moisture content and maximum dry density at different percentages of lime. It is observed from Table 1 that the maximum dry density decreases with increase in lime content. The fall in density and increase in optimum content is significant at lower percentage of lime than at higher percentage. 

In the lower ranges, the lime reacts quickly with the soil and brings about change in base exchange, aggregation and flocculation. These reactions result in the increase in the void ratio of the mix leading to a significant decrease in density of the mix and increase in the optimum moisture density of the mix and increase in the optimum moisture content. Addition of lime beyond that is mainly utilized for pozzolanic reactions. Thus in the higher range of lime contents, decrease density is not significant. 

The occurrence of pozzolanic reactions results in the formation of cementatious products causing the soil to be densely compacted and hence increase in optimum moisture content is not much significant at higher percentage of lime.

LIQUID LIMIT, PLASTIC LIMIT AND PLASTICITY INDEX:
Table 2 Shows that: Liquid Limit of the soil at lower range of lime becomes almost constant for increase of lime content. Increase in the plastic limit is significant at lower %age of lime content. As in lower range of lime content the agglomeration and flocculation process makes the soils coarser consequently more water is required to form threads. Plasticity indices are reduced with increase in lime content, as increase in Plastic Limit is much more than Liquid Limit.

CALIFORNIA BEARING RATIO TEST:
C.B.R. values for mixes stabilized with different percentage of lime were determined both in unsoaked and soaked conditions. For conducting this test, the stabilized soil specimens were prepared at their optimum moisture content and statically compacted to their maximum dry densities and test CBR values of unsoaked samples were conducted. For test under soaked condition, the specimens were moist cured for 3 days followed by soaking in water for 96 hours. The CBR values were then determined after draining the samples. In both unsoaked and soaked conditions, the specimens were covered with equal surcharge weights to simulate the effect of overlying pavement. CBR values for different percentages of lime are shown in Table 3. During soaking, an expansion measurement device was mounted on the mould to measure the expansion of the specimen. It is observed from Table 3 that CBR strength increases with lime content upto a certain value probably due to an increase in cohesion and angle of internal friction of the soil. The increase in cohesion can be attributed to formation of cementing products during pozzolanic reactions and increase in angle of internal friction may be due to the effect of aggregation resulting in greater interlocking and rough surfaces. It is also observed that rate of increase in CBR strength under soaked conditions is much higher as compared with unsoaked conditions is much higher as compared with unsoaked conditions probably due to long time pozzolanic reactions during curing period of about 7 days. 
 

STRENGTH BENEFIT INDEX:
CBR values were used to determine the corresponding strength benefit index (SBI). Table 4 shows the strength benefit indices for both unsoaked and soaked conditions. It is observed from Table 4 that SBI for soaked conditions are always higher than the unsoaked conditions. Further, gain in strength when lime is added to the soil.

EXPANSION RATIO:
The Expansion Ratio is defined as the difference between final dial gauge reading and initial dial gauge reading divided by initial height of the specimen. Table 5 shows the Expansion Ratio for different percentage of lime after 4 days of soaking period. From table 5, it is noticed that Expansion Ratio reduced from 0.25% & 0.22% to almost zero with 4% of lime for Banga Garhshankar & Nawan Shahar Phagwara road, Expansion Ratio also reduced to zero percent corresponding to 2% lime for Nawan Shahar-Garhshankar road and Nawan Shahar Rahon road. The reduction in the Expansion Ratio with increase in lime contents may be attributed to the reactions of aggregation and more attraction among clay particles caused by the base exchange phenomenon.


TABLES:

Lime As Modifiers In Asphaltic Layers
Asphaltic pavements generally fail due to: Stripping of the aggregates, Due to rutting, Cracking due to weathering. Asphaltic concrete can be modified in many ways to create high performance pavements. Hydrated lime is one of the modifiers that improves performance of asphaltic pavement to overcome the above shortcomings.

Hydrated lime is the most effective anti-stripping agent available, and is universally used to deal with serious stripping problems. Certain types of aggregates are particularly susceptible to stripping. When lime is added to hot mix, it reacts with aggregates, strengthening the bond between the bitumen and the stone, while it treats the aggregate, lime also reacts with the asphalt itself. Lime reacts with highly polar molecules that can otherwise react in the mix to form water-soluble soaps that promote stripping.

The hydrated lime is able to make an asphaltic mix stiffer and resistant to rutting. Hydrated lime significantly improves the performance of pavement in this respect. Lime is a chemically active filler. It reacts with the bitumen, removing undesirable components at the same time that its tiny particles disperse throughout the mix, making it more resistant to rutting and fatigue cracking. The addition of the lime will not, however, cause the mix to become more brittle at lower temperatures. At low temperatures the hydrated lime becomes less chemically active and behaves like any other inert filler.

Another benefit that results from the addition of hydrated lime to many asphalt cements is a reduction in the rate at which the asphalt oxidises and ages. This is a result of the chemical reactions that occur between the calcium hydroxide and the highly polar molecules in the bitumen. If left undisturbed in the mix, many of those polar molecules will react with the environment, breaking apart and contributing to a brittle pavement over time.

Hydrated lime combines with the polar molecules at the time that it is added to the asphalt and thus, they do not react with the environment. Consequently, the asphalt cement remains flexible and protected from brittle cracking for years longer than it would without the contribution of lime. hydrated lime also reduces asphalt cracking that can result from causes other than aging, such as fatigue and low temperatures. Synergistic benefits also accrue when lime is used in conjunction with polymer modifiers. In some situations lime and polymers used together can produce improvements greater than each of them used alone.

Generally 1 per cent hydrated lime by weight of the mix is used, and is added to the drum at the same time as the mineral filler. The hydrated lime comes in contact with the aggregate itself, directly improving the bond between the bitumen and the stone, while the balance enters the bitumen. This method is called "dry method" and is the simplest to implement.

Alternatively, lime is applied to damp aggregate in order to insure more complete coverage of the stone than is achieved using the dry method. Lime that does not adhere to the stone is dispersed throughout the mix where it will contribute to the other improvements that have been described and is called "dry on damp" method and is also relatively simple, but driving off the additional water required by the process uses additional fuel and may slow down plant production to some degree. Lime slurry (mixture of lime and water) that is also applied at a metered rate to the aggregate, insuring superior coverage of the stone surfaces. After the slurry is applied, the aggregate can either be fed directly into the plant or marinated in stockpile for some period of time, allowing the lime to react with the aggregate. Because the lime is bound to the stone, it is also the method that results in the least dispersion of the lime throughout the rest of the mix.

Daljeet Sidhu
E-mail : daljitsidhu@hotmail.com