Regional Sales Manager (South) – Parex Construction Chemicals
D Ahsok Kumar
Head of the Department, Civil Engineering Department, GKM College of Engineering and Technology, Chennai
Road network in India plays a vital role for transportation of goods, vehicular traffic etc. The pavement courses like subgrade, sub-base, base and surface courses should be able to withstand the load coming from the vehicular traffic to resist failures like rutting and fatigue cracking. This project attempts to study the behaviour of pavement sections with and without geogrid reinforcement in Accelerated Pavement Testing (APT) equipment. Two pavement sections were prepared one with no reinforcement and the other reinforced with Geogrid on top of the Subgrade. The pavement section consists of 3 layers namely Subgrade, Sub-base and Base courses and are casted in APT equipment at required density, load cells and strain gauges are placed at required depths to measure stresses and strains at that depth. The Subgrade soil used in pavement section has a low CBR value of 2.87%. After casting the pavement sections cyclic and static loads are applied through a hydraulic jack on a circular plate of 30 cm diameter. The results showed that
Flexible pavements are the pavements which have a low flexural strength. These pavements transfer the load through grain to grain contact of the particles and distributes over a wide area. So that the maximum load will be on top surface, which is in direct contact with the wheel and the load goes on decrease with depth. Generally the flexible pavement consists of four or three layers. In four layered system it consists of Surface course, Base course, Sub-base, Subgrade and in three layered system, there won’t be Sub-base course. The most common failures in flexible pavements are fatigue cracking and rutting. Fatigue cracking is due to horizontal tensile strain at the bottom of the surface course and rutting is due to the vertical compressive strain on top of Subgrade. Fatigue cracking can be rectified by using proper asphalt mix designs. Rutting can be rectified by using the Subgrade soil which has high strength, by proper compaction of the Subgrade and by preventing the outward movement of the aggregates in Sub-base and Base courses. But it is not possible always to obtain Subgrade material of high strength nearby that site, and it will be uneconomical to get the soil from long distances through transportation. At that time the soil present at those areas should be modified for improving its strength and for using it effectively as Subgrade material. There are several methods for improving the soil properties like chemical stabilisation, polymer product stabilisation etc. Among those stabilisations use of Geosynthetics will be more effective, i.e. it increases the subgrade soil strength and it also prevents the movement of aggregate in Sub-base or Base by interlocking.
Geogrid is a geosynthetic material which is very open and grid like configuration used to reinforce soils. These are commonly used to reinforce Sub-base or Subgrade soils below roads. When load is acted on soil, the soil particles tends to pull apart under tension, the geogrids are strong in tension and they will hold the particles without moving apart and the strength of the soil increases.
Accelerated Pavement Testing Equipment
In general it is not possible to test the actual pavement in site due to traffic flow and adverse climatic conditions. To overcome this accelerated pavement testing can be used for testing the pavement sections under controlled conditions and by applying the load similar to that of actual wheel load. The accelerated pavement testing equipment used in this project is rectangular tank of size 180cm ×160cm×120cm. It has a loading capacity upto 30 tonnes and the load is applied through a circular plate of 30 cm diameter and 1.5 cm thick at a frequency of 1.2Hz on top of pavement.
Materials and their Properties
The materials used for casting of pavement model are tested and their properties are given below
Before laying the subgrade course in the APT equipment, flyash of 10 cm height is compacted at the bottom of tank with MDD of 13.06 kn/ and OMC of 9% for filling purpose.
The Subgrade is the foundation of the pavement structure made up of in-situ material, selected soil, or stabilized soil. The soil used in this project as a Subgrade material is lateritic soil collected near puravankara, Bangalore. The soil contains more than 50% of clayey particles (from wet sieve analysis) and remaining soil is classified as poorly graded sand from sieve analysis. The thickness of the Subgrade in two pavement sections is 50cm, and the soil is compated in APT equipment to the required density to limit additional densification during loading. The tests like atterberg limits, compaction characteristics and CBR are carried on the subgrade soil and the results are shown in Table 1
From liquid limit the soil is classified as highly compressible. As per IRC the Subgrade soil should have a dry density of 1.75 gm/cc and CBR of 8% for roads having traffic of 450 commercial vehicles per day. But the CBR value obtained is less than the recommended value; therefore soil strength should be improved.
The material used as Sub-base in this project is collected from a quarry near Chennai. Thickness of the Sub-base course in this project is taken as 24cm considering traffic as more than 10 msa (As per IRC 37:2012). From the grain size analysis the material is classified as Grade V as per MORTH, The materials used as Sub-base are tested for Aggregate Impact test, Aggregate Abrasion test, Aggregate crushing test and specific gravity and the results are given in table
The base course serves as principle structural component of the flexible pavement, which distributes the imposed loads to the sub-base or subgrade. In this project WMM of 25cm thick is laid as Base course by considering traffic as greater than 2 msa (as per IRC 37:2012). The materials used for Base are tested for Aggregate Impact test, Aggregate Abrasion test, Aggregate crushing test and the results are given in Table 3
Experimental Programme and Test Results
After casting the pavement section in APT equipment cyclic and static loads are applied on the pavement section through a circular plate of 30 cm diameter and 1.5 cm thickness at a frequency of 1.2Hz. First a Cyclic loads of 4 kn which is half of the axle load is applied upto 1,00,000 cycles, similarly 6 kn, loads are applied and readings are recorded upto 10,000 cycles. The readings are automatically recorded in the DAS except surface deformations, which are noted manually.
Vertical strain on top of the subgrade is an important parameter which is responsible for rutting of the pavement. Total 8 strain gauges of 120 O are used in various layers to determine the deformations. Two Strain gauges are fixed to the scales which are further attached to the side walls of the tank. One strain gauge is fixed at free end of the scale and the other strain gauge is fixed at a distance of 15 cm from strain gauge. Four strain gauges are fixed in the longer direction of the tank at the interface of subgrade and sub-base (60 cm height from bottom). Remaining four strain gauges are fixed in shorter direction of the tank in sub-base (at 78 cm height from bottom). The rutting of the pavement should not be more than 20 mm. The readings from strain gauges are converted into strain by using the following formula
where is Change in resistance
R is Resistance of strain gauge (120 ohms)
GF is Gauge factor (2)
The results of Vertical strain in pavement section under cyclic and static loads at various depths are shown in below figures
Vertical strain under cyclic loading
Cyclic loads of 4 kn and 6 kn are applied on the pavement and the response of strain at various depths with increase in number of cycles is shown in figure
From the results it is clear that the maximum strain in sub-base is and the maximum strain in subgrade is which goes on decreasing with increase in number of cycles.
Vertical strain under static loading
Static loads of 4Kn, 5Kn, 6Kn, 7Kn and 8Kn are applied on pavement section and the response of strain under static loads at various depths are shown in figure
From the results it is clear that the maximum strain in sub-base and subgrade under static loading is
Four load cells are placed in the pavement section at different levels to calculate stresses. One load cell is placed on top of the subgrade at 60 cm height from bottom. Two load cells are placed in the Sub-base at a height of 78 cm from bottom and one more load cell is placed in half of the base course at a height of 96 cm from bottom. In DAS the readings from load cells are recorded in terms of KG, these reading are converted into stress by dividing the load by load cell area.
Stress = Load/Area of Load cell
Vertical stress under cyclic loading
Cyclic loads of 4 kn and 6 kn are applied on the pavement and the response of stress at various depths with increase in number of cycles is shown in figure
Vertical stress under static loading
Static loads of 4,5,6,7 and 8 Kn are applied on pavement section and the response of strain under static loads at various depths are shown in figure
Deformation on the surface of pavement model is recorded by placing two dial gauges on pavement surfaces at distances of 10 cm from the circular plate. The deformation readings under static and cyclic loads are shown in figure
From the results it is clear that
A pavement model with three layers is casted in APT equipment and cyclic and static loads are applied to the pavement through a circular plate of 30 cm diameter. The stresses, strain and surface deformations at each layer is recorded by placing load cells, strain gauges and dial gauges at that depth. The results shown that
Two pavement sections are casted with and without geogrid reinforcement in APT equipment in three layers namely subgrade, sub-base and base. The subgrade material used is lateritic soil which has a CBR value of 2.8 % which is less than the recommended value by MORTH. After casting of pavement sections cyclic and static loads are applied. Half axle load of 40 kN is applied upto 100000 cycles and then the load is increased to 60 kN. Static loads ranging from 20 kN to 80 kN are applied. Stress, strain and surface deformations at each layer is recorded by placing load cells, strain gauges and dial gauges at that depth .
From the results it is clear that the provision of geogrid reinforcement on top of sub-base reduces the strain in sub-base and subgrade both under cyclic and static loads compared to that of control section. The stresses in reinforced pavement are decreased under static loads but there is no variation in stress under cyclic loading. Therefore the subgrade material which is having a CBR of 2.8% can be used effectively by using geogrid reinforcement. The location of geogrid reinforcement has to be changed and the results are to be compared
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Arivusudar Nagarajan was born in Krishnagiri, Tamil Nadu, India, in 1984. He received the B.E. degree in Civil engineering from the Adhiyamaan College of engineering, Hosur, India, in 2006, and the M.E.degree in Structural engineering from Government College of engineering, Salem, Tamil Nadu respectively.
He has a professional experience of 10 years with specialization in Cement, Concrete, Construction Chemicals, Marketing Management, Building Materials Industry. He his currently working as the Regional Sales Manager (South) – Parex Construction Chemicals.
Ashok Kumar Duraisamy was born in Neyveli, Tamil Nadu, India, in 1986. He received degree of M.Tech in Environmental engineering from Thiyagarajar College of Engineering, Madurai, and Tamil Nadu respectively. In 2008 he joined Techno print Chemicals, Hosur in R&D Department, His Presently working in GKM College of Engineering as HoD(ic).Professional experience with nearly 5 years of experience specialization in Bacterial Concreate, Solid waste management, Waste water Engineering.