Salient changes in IRC : 37 (2012) & bituminous works of 5 th Revision of Specification for Road and Bridge Works (MORTH

Salient changes in IRC : 37 (2012) & bituminous works of 5th Revision of Specification for Road and Bridge Works (MORTH) Rajib Chattaraj* Abstract: It has always been a challenge to the highway engineers to build a durable road against many odds. In many places of West Bengal the soil is clayey soil which is unfriendly to highway construction, the annual rainfall in most part of the state is more than 1500 mm, in some parts of North Bengal the annual rainfall is as high as 3500 mm+, in summer the highest temperature is experienced more than 450C in some of the places, the width of the roads are mostly capacity constrained, featured with low height of embankment, frequently encroached in built-up areas, with poor drainage, with overloaded vehicles plying on the roads. All these have detrimental effects on durability of roads but unfortunately it is only the engineers who are blamed for early damage of the road. However, in recent times some initiative from Government's end has been taken to develop the roads of the state. For deciding the specification of works and execution – specially for bituminous works (because mostly the works are strengthening or widening and strengthening work, thus the main part of the work is bituminous), the judicious engineering decision for making durable roads is very important. In this paper, an overview has been presented on the recent changes in codal provisions related to bituminous works to achieve durable roads in the context of IRC : 37 (2012) and Ministry’s Specification for Road and Bridge Works. (5th Rev.-2013) Introduction: Generally, two types of road construction work are most common--widening and strengthening and only strengthening. Both type of works are carried out on existing road. The scope of construction of new roads is rarely available. For widening and strengthening projects, the design considerations are a bit complicated because for widening part, procedure laid down in IRC:37 has to be followed and for strengthening part, procedure laid down in IRC:81 (Benkelman beam deflection test) has to be followed. However, Benkelman beam Deflection test has to be replaced by Falling weight deflectometer test (IRC:115, 2014) which method is more appropriate to evaluate remaining life of the pavement in terms of Msa and in turn the required overlay thickness. * Superintending Engineer, NH wing, PW(Roads)D It is more important to take judicious decision to match the outcomes of two different procedures and finally to prescribe the composition which will fit simultaneously to the widening part as well as the existing width for strengthening part. Here, the judgment of the engineer who is familiar to the features of the road is very important rather than simply implementing the outcome of design. If the pavement composition of the existing road is featured with bituminous layer of less than 50 mm thickness and the bituminous surface is hungry with the bitumen being aged and oxidised has been turned to brittle state, it is better to sacrifice the bituminous layer. Fresh non bituminous layer should be laid on the existing width to compensate the deficiency of pavement composition of the road after removal of the damaged existing bituminous layer. Here, it has to be ensured that there is no impervious layer which is most likely bituminous layer underneath. If at all, it would be detrimental. It is one of the common causes of failure where without proper investigation of existing pavement composition (with impervious bituminous layers underneath), fresh non bituminous layer has been laid over existing road surface. On the contrary, if the layer thickness of the existing road is more than 50mm, the entire thickness of the bituminous layer may not be aged out or oxidized, only the top portion may be oxidised as evidenced with a few sign of distress, it would be wise not to remove the bituminous layer to put fresh overlay of non bituminous layer even if the design demands. Normally it should not be like that as the road which has deficiency in crust composition or thickness, does not have a sound bituminous layer on the top without any distress. Picking up of sound bituminous layer of more than 50mm thickness by earth moving machineries (like JCB type of machine) is also very difficult and there is every possibility that a part of bituminous layer remains which will prevent the free drainage of water. Had it been done by cold milling machine, it would have been alright. The deficiency of the existing pavement as per design calculation, if at all should rather be compensated by bituminous overlay. Here pavement evaluation technique as per IRC: 115 (2014) using Falling weight deflectometer (FWD) is very useful and Benkelman beam deflection test should be replaced by FWD. Changes in IRC:37 (2012) Design Philosophy of IRC:37(2012): Guidelines for the design of flexible pavements has been revised for the third time in 2012 after it was first published in 1970 and then revised twice in 1984 and 2001. This time the word “Tentative” has been placed before “Guidelines for the design of flexible pavement” because during approval of this draft by flexible pavement committee of IRC, some members opposed the newer concept of cemented / stabilized base / sub-base course. Thus, a compromise had to be done by placing the word “Tentative” to leave up to the choice of pavement designer whether to go for newer options or not. Since 2001, pavement technology has gone through distinguished advancements mainly in the field of new materials and mixes like Warm Mix Asphalt, Foam Bitumen, Polymer, Crumbed Rubber Modified Bitumen, Higher and Improved Grade of Bitumen, Stone Matrix Asphalt, Cementitious Base and Sub-bases, Recycling etc. In the mean time traffic pattern has also changed with the increase of tandem, tridem and multi-axle vehicles manifold. IRC:37(2012) has been formulated to incorporate these changes. However, the design approach i.e., Mechanistic Empirical approach as was adopted in IRC:37 (2001) remained same with 80% reliability up to 30 msa. 80% reliability means the pavement is likely to last within its design life till the fatigue cracking in bituminous surface extend to 20% of pavement surface or rutting in the pavement reached the terminal rutting of 20mm whichever happen earlier. Thus, up to 30 msa, the design charts of pavement composition w.r.t. CBR value and cumulative number of standard axles (N value) remained unaltered from IRC:37 (2001) version to IRC:37( (2012) version. But for design traffic exceeding 30 million standard axles, 90% reliability approach has been considered in IRC:37 (2012) which means that there is a maximum of ten percent chance that 20% of the pavement area may display cracking or 20 mm rutting within the design period. But in this case, IRC:37 (2012) recommends to use VG-40 grade of bitumen instead of VG-30 grade bitumen. Thus, sometimes the thickness of bituminous layers corresponding to 50 msa traffic has come lesser than that of 30 msa in IRC:37 (2012). Here much greater importance has been put on bituminous mix design and is considered as an integral part of the pavement design. Though design requirement demanded improved material in form of VG-40 grade bitumen, it's availability is not very smooth. It is question of demand-supply relation. The more and more will be the demand of the VG-40 grade of bitumen more smooth the supply will be. Salient points of changes in IRC:37 (2012) Growth rate of commercial vehicles has been reduced from 7.5% to 5%, if specified data is not available or less than 5%. (Cl. 4.2.2) Instead of only sub-grade CBR, effective sub-grade CBR has o be considered. (with embankment soil of lower CBR, effective sub-grade CBR comes somewhat less than CBR of sub-grade only) (Cl. 5.2 and Fig. 5.1 Page : 11). This is an important concept incorporated in this version of the guideline by which not only the subgrade CBR has to be checked but also the CBR value of the soil below 500mm of the subgrade (which is supposed to be embankment) has to be checked. Design life of NH and SH is minimum 15 yrs., for Expressways and Urban Roads it is minimum 20 yrs. For any other category of roads the design life is recommended to be considered for 10 yrs to 15 years. (Cl. 4.3.2) Resilient Modulus of Bituminous mixes at different temperature and with different grade of bitumen and even with modified bitumen has been specified as per table 7.1. It is useful for finding out equivalent thickness using the formula E1H13/12(1 – m12) = E2H23/12(1 – m22) as given in Annex-III (Page : 65). [Table 7.1 (Page : 23) is much detailed than the earlier version]. However, there are some questions for the MR values of mixes with different binders as mentioned in table 7.1. No reference research back-up has also been mentioned against this table. Up to 30 msa, 80% design reliability and beyond 30 msa, 90% design reliability has been considered. (Cl. 6.2.2 and Cl. 6.3, Page : 13, 15) Beyond 30 msa, higher grade of bitumen i.e., VG-40 has been recommended both for DBM and BC layers (Cl. 10.1) and up to 30 msa VG-30 grade bitumen has been recommended. Cl. 6.2.2. [last paragraph (Page : 14) ] and Annex-I. (Page : 49) For traffic below 2 msa, IRC:SP:72 (2007) (Guidelines for design of flexible pavements for low volume rural roads) should be used. (Note (a) page 25, Cl.10.) Pavement thickness design for traffic between 2 and 30 msa is exactly same as IRC:37 (2001) [Note (b) page 25 Cl.-10]. For traffic above 30 msa, new design charts have been provided (Plate 1 – Plate 24). But it is better to follow 2012 version of IRC:37 for pavement composition, because some pavement composition like premix carpet etc. have been dropped. There is no provision of Premix Carpet and Seal Coat. For traffic 2 msa to 5 msa, provision of SDBC has been recommended and beyond 5 msa, only BC has been recommended as the top wearing course. (Plate 1-24) IRC:37 (2012) has provided 5 (five) different options of choosing base and sub-base with introduction of cementitious base and cementitious sub-base and foamed bitumen/bitumen emulsion treated Reclaimed Asphalt Pavement (RAP) material. This has given the scope of reduction of thickness of bituminous layer by providing newer alternatives of base and sub-base, different from the conventional idea of granular base and sub-bases as well as recycling of natural resources. (Cl. 10, Page : 24). This is one of the major new incorporations in the latest version. In eleven annexes, many important topics like drainage layer design, chemically stabilized base and sub-base, cementitious base and sub-base, mix-design with RAP material, choice of grade of bitumen, choice of bituminous wearing courses etc. have been discussed in details. The aspects of Bituminous Mix Design has been discussed in details and described as an integral part of pavement design. In the choice of wearing courses, Mastic Asphalt should have also been incorporated, however it is not there. The equations for rutting and fatigue has been provided and explained in this version. From these equations, allowable horizontal tensile strain in bituminous layers, allowable vertical compressive strain on sub-grade, allowable tensile strain in cementitious layer or on any other critical locations can be calculated. The actual strains whatever are coming on different critical locations after provision of different composition and thickness as designed as per Plate-1 to Plate-24, can be cross-checked with the help of a software – IITPAVE. If the actual strains are found to be more than the allowable strains, the thickness of the pavement has to be modified accordingly. Thus the design of pavement now requires much better and detailed understanding than it was before. The design of pavement for very high volume traffic (300 msa) has been provided in Annex-II (design example). (Page : 62) In Indian Context, comparatively newer concept of Perpetual Pavement (with very long life – 50 yrs. or more) has been incorporated in this version (Cl. 8 and design example in Annex-II) (Page : 23 and 63) Actual determination of vehicle damage factor (VDF) is very important in view of overloading. As code can not be formulated for illegal activity like overloading, the default values of VDF has been retained unchanged in this version of code (Table 4.2). But for medium to large projects (say for Rs.10 crores and more), axle load survey should be must for actual determination of VDF which is often omitted during preparation of DPR, otherwise the early damage of road due to overloading can not be taken into account. As per a recent study on VDF on a State Highway, the VDF after proper axle load survey was found to be as high as 22. The present guideline has recommended for actual determination of VDF by proper axle load survey, however, in absence of that general values are tabled, quite naturally which did not take care of the illegal activity like overloading. Lane-distribution factor for Two-lane single carriageway roads, has been modified to 50% from 75% (earlier version) but the higher VDF out of either direction has been recommended for design. (Cl. 4.5.1-ii, page-8). A newer concept about placement of upper and lower GSB layers has been introduced. As per Cl. No.- 7.2.1.3 the lower GSB layer should act as a separation / filter layer to prevent intrusion of Sub-Grade soil into the pavement, thus should be close graded and the upper GSB should act as drainage layer to drain away any water that may enter through surface cracks, thus, should be open graded. It is made even clearer in the design example in page 63 of the 2012 version of the code. In general, it is usual practice that upper the pavement layers, closer and denser they are. But in case of GSB, the coarser layer should come on upper layer, however, it may be denser than the lower closer graded GSB. While in IRC:37(2012), it is recommended to do the GSB in two layers, in 5th. Revision, it is said no layer of GSB should be done in thickness lower than 150mm. In many combinations of pavement design composition as per IRC:37(2012), GSB thickness is coming as 200mm only. Thus, in that case GSB can not be done in two layers. Here provision as laid down in cl.11.4 of IRC:37(2012) has to be followed which states about the requirement of the permeability of the filter layer and also preventing the entry of the soil particles in the drainage layer. The concept of bottom rich bituminous layer has also been introduced. Since the critical location of tensile strain is located at the bottom of bituminous layer [reference Fig. 10.1 page : 25], lower DBM layer is compacted to an air void of 3% after rolling (instead of generally 7% to 8 % for upper DBM and BC layer after rolling) with volume of bitumen close to 13% (Bitumen content of bottom DBM layer may be 0.5 to 0.6% higher than the optimum bitumen content) and thereby making the lower DBM layer more ductile to withstand greater tensile strain. As this layer is located below the BC layer and top DBM layer, around 80 to 100 mm below the top surface of pavement, by effect of higher temperature and heavy traffic, chances of rutting or plastic deformation in bitumen rich bottom DBM layer is remote. Therefore, it is called bottom rich bituminous layer. [reference Cl. 10.1 page 25 and also design example vide page 63] A new concept of 90 percentile CBR is recommended in the new guideline as per Annex-IV page : 67. The CBR values of the sub-grade soil vary along a highway alignment even on a homogeneous section. Therefore, sub-grade CBR likely to be applicable for a considerable length of stretch based on statistical calculation is more rational approach than dealing with varied CBR values from section to section. The method of determination of 90 percentile CBR has been described in Fig. IV.1 page 67. This guideline recommends 90 percentile CBR which is a bit on conservative side (i.e., the CBR value comes a bit lesser) than Asphalt Institute’s recommendation of 87.5 percentile sub-grade modules for design traffic greater than one msa. 3. Guidelines for construction of Bituminous Layer as per 5th.Revision of Specification for Roads & Bridge (MORT&H) The bituminous mixes generally we construct, can be broadly classified in two ways:- i)Open Graded Mix like Bituminous Macadam, Premix Carpet, Mixed Seal Surfacing etc. ii) Dense Graded Mix like Dense Bituminous Macadam, Bituminous Concrete etc. Semi Dense Bituminous Mix has the same design air void criterion as the BC, it may need a little higher bitumen because of semi-dense gradation. This mix has been deleted from the Morth specifications since BC grading-2 serves the purpose of thin wearing course. 3.1 Open Graded Mixes There is no definite guideline for the design of Open Graded Bituminous Mixes, as per provision of codes of practice prevailing [Specification for Roads and Bridges of MORT&H, 5th. Revision which has been published very recently, June 2013] except for grading and bitumen content. But we often have to construct Bituminous Macadam for which a better improvised design may be done. If we compare the compaction of Soil and Asphalt Mix, there in a fundamental difference – in case of soil, generally the aim is to compact the soil mass to its maximum possible density. But in case of asphaltic mix it is not so, we keep a designed air void which means compacting not to the maximum possible density. This designed air void is in between 3 to 5%. It along with two other volumetric properties called void in mineral aggregate and voids filled by bitumen serve a very important role for stability and durability of the bituminous layer. This provision is applicable for dense graded mixes. But for B.M., air void is nowhere near to 3 - 5%, it is around 12 – 13%, if at all measured. Thus, for BM, the better approach for good design is to compact the BM layer to its maximum possible density. It can be achieved not only by a few extra passes of rollers but more importantly by varying the gradation of the mineral aggregates keeping within the upper and lower boundary of the specified gradation curve. That way compaction of BM Mix can be compared with soil. As such there is no scope of variation of bitumen content of BM Mix because as per IRC : 27 (2009) : Specifications for Bituminous Macadam First Revision (Also 5th Revision of Ministry’s Specification just published) the bitumen content for Grade – 1 of BM is 3.3% and that of Grade – 2 is 3.4%. In 5th. Revision, specifications like Bitumen Penetration Macadam, Built Up Spray Grout, Semi Dense Bituminous Concrete have been dropped. However, specifications like Open-graded premix surfacing (Premix carpet), Mixed seal surfacing, Seal coat, Slury seal, fog spray, Micro-surfacing etc. items have been retained. In IRC:37(2012), for the range of traffic between 2 to 5msa, SDBC has been recommended as the wearing course but in 5th. Revision , SDBC has not been incorporated. On the contrary, there is no provision of premix carpet and seal coat in IRC:37(2012) for traffic above 2msa. Wherever applicable, MSS is preferred than PC & SC(Cl.VI-4,page-72). So there is a bit of lack of co-ordination in between IRC guideline and Ministry’s specification. 3.2 Dense Graded Mixes As per IRC: 111 (2009): Specifications for Dense Graded Bituminous Mixes and 5th.revision of Ministry’s Specification Only the minimum bitumen content is specified for different mixes and different grading instead of the range. For DBM grade-1, it is 4% and for DBM grade-2, it is 4.5%. For BC grade-1, it is 5.2 % (against the range 5 to 6 % in 4th. revision) and for BC grade-2, it is 5.4 % (against the range 5 to 7 % in 4th. revision) The range of air void has been changed to 3 to 5% from earlier range 3 to 6 % in 4th. Revision. The main basis of mix design is to determine optimum bitumen content corresponding to 4% air void (being mean of 3 &5 %) and that bitumen content should satisfy all the other criteria. Combined flakiness and elongation indices have been increased to 35% maximum from the previous version’s range 30% maximum. It has been recommended for BM also. Maximum stability values and the ranges of flow values have been provided for viscosity grade paving bitumen and also for modified bitumen separately for hot and cold climate. Specified value of Marshall Quotient has also been incorporated. However, stability criteria for VG-30 and VG-40 grade of paving bitumen can not be same which has not been addressed in 5th. Revision. The clause of Modified binder in 4th. Revision has been deleted in 5th.revision. But in Appendix-1- List of IRC publications referred, IRC:SP:53(2002) – Guideline on use of Modified Bitumen in Road Construction (second revision) has been mentioned, though the 3rd. revision of this guideline (IRC:SP:53) has been already published in 2010. In IRC:SP:53(2010), the limit of the required characteristics of all varieties of the modified bitumen including CRMB and NRMB have been made unified. It is like making same specification for two different kinds of thing. Thus, when it is the question of modified bitumen (PMB) or CRMB, it better to follow IRC:SP:53(2002), where the limit of different characteristics of PMB, CRMB & NRMB have been mentioned separately, not the 3rd. revision(2010) of the same which is also an industry practice. IRC:SP:53(2010) has also recommended the minimum bitumen content for the modified bitumen for different grades of bituminous mixes which seems to be on the higher side. The design bitumen content as determined by Marshall Method of mix design should be the guiding. On foot-note of Table 500-10 and 500-17 of the 5th. revision for DBM and BC respectively, it has been stated that the minimum bitumen content specified corresponds to specific gravity of aggregate being 2.7. In case aggregate have specific gravity more than 2.7, the minimum bitumen content can be reduced proportionately. On the other hand, in low temperature zone, it has been recommended to increase the bitumen content. There may be confusion in correctly interpreting the foot-note. This says the minimum bitumen content should be reduced if the aggregate specific gravity is more than 2.7, it obviously means the design bitumen content or the optimum bitumen content can not be reduced. The design bitumen content obtained by proper bituminous mix design should not be changed. References : Specification of Bridge and Road works, fifth revision, MoRTH (2013) IRC:37(2012):Tentative Guidelines for design of flexible pavements IRC:37(2001) : Guidelines for design of flexible pavements Gratitude :Prof. B.B.Pandey, Ex-Professor and Head of the department of Civil Engineering Department who is instrumental behind drafting IRC:37(2012) has been kind enough to go through this paper and correct it. Sincere thanks with regards is conveyed to him. PAGE 10 of NUMPAGES 10