USE OF WOLLASTONITE MICRO FIBERS IN REHABILITATION OF PQC ROADS

Abstract

Self compacting concrete (SCC) is a concrete that has three specific rheological properties: flowability, passability and segregation resistance. These properties enable a concrete mix to flow, pass the obstructions as a whole unit, and compact fully under its own weight. SCC was primarily invented in Japan in the late 1980’s because of the need of such concrete, which could fill the spaces in heavily reinforced structures resisting earthquakes. Over the past several years, it has gained popularity due to its commercial benefits in terms of ease of placement, no requirement of external vibration and above all improved durability. Mix composition and fresh state properties of self compacting concrete are clearly different from that of ordinary concrete. The higher content of powder materials (cement, flyash, silica fume, fine aggregates passing 0.125 micron etc.) and lower content of coarse aggregates, improves the rheological properties, change the granular skeleton and microstructure of the self compacting concrete. As a result of which, the hardened properties of SCC and its durability properties are significantly affected. The ultimate aim of concrete mix designer is to produce good concrete. The good concrete has to be satisfactory in its fresh state and hardened state. The requirements in the fresh state are that the consistency of the mix be such that it can be compacted by the means desired without excessive effort, and also that the mix is cohesive enough for the methods of transporting and placing used so as not to produce segregation with a consequent lack of homogeneity of the finished product. The primary requirements of a good concrete in its hardened state are a satisfactory compressive strength and an adequate durability. India has drawn up an ambitious plan to achieve a network of 4.4 million kms by 2021 at a projected financial outlay of Rs. 30, 92, 636 million. The rate of traffic growth that stood traditionally at 7.5% is about 10.16% per annum for the past 10 years. Considering the magnitude of road construction, improvement and strengthening, the demand of asphalt has grown many folds against supply constraints. Simultaneously, the cement production has increased substantially and the demands of pavement quality concrete (rigid pavements) are fast growing due to inherent advantages over bituminous flexible pavements. They have much longer life, lower maintenance cost, are more fuel efficient due to the hard surface, good riding quality, increased load carrying capacity for heavy vehicles, permeability to water and speedy construction over flexible pavements. Considering the above advantages possessed by PQC and scarcity of asphalt and rising prices of asphalt, it has become prudent to consider PQC (rigid pavement), a iii far better alternatives to flexible pavement. The initial cost of PQC roads is about 25% to 30% more than that of bituminous roads which strongly suggests that efforts towards economizing the initial cost should be actively pursued. If we consider the life cycle cost, PQC has proved to be more economical over flexible pavements. One of the possible ways to offset the initial cost of PQC is to reduce the quantity of cement in the concrete. Therefore, there is an urgent need to economize the initial cost of pavement quality concrete (PQC) using low cost and abundantly available materials, capable of producing matching strength with better durability. All pavements deteriorate with time. The rate of deterioration of concrete pavement is comparatively much slower than the flexible pavement. Distresses in PQC (concrete pavements) are either structural or functional. Structural distresses primarily affect the pavement’s ability to carry traffic load. Functional distresses mainly affect the riding quality and safety of the traffic. Understanding the causes of pavement distress is essential for providing appropriate effective repair and developing maintenance strategies. Repair techniques can be broadly classified into two categories i.e. preventive techniques and corrective techniques. For concrete roads in operation, the cost of repair and lane closure is two important considerations in deciding the type of repair to be undertaken. Repairs are only intended only to ensure that concrete pavements perform till design service life. The strategies for repair of older pavements could thus be different than those of new pavements. Decision is based on a trade-off between the “cost” of repair and the “remaining” life of the pavement. Full depth repair is recommended, if weak concrete is identified or suspected or the pavement had multiple type of distresses such as cracking, ravelling, large pop-outs/potholes and compression failure as blow-ups etc. Repair of full depth transverse cracks always requires new dowel bars to be placed and one new joint constructed. The large cracked slab is thus replaced by two smaller slabs with lower curl and warping stresses. The total distressed and surrounding areas (to be repaired) are marked on the pavement in rectangular form with sides parallel and perpendicular to the centre line by ensuring not less than 50 mm cutting beyond unsound concrete. This rectangular marked simplify saw cutting and concrete removal. A great care is taken to achieve stable patches and provided adequate room in the pit for dowel hole drill rigs and compaction. Normally, newly cut joint faces are scrabbled with a chisel or sand blasted to create roughness for better bond between old and new concrete. A precise and effective way to do is, before concreting the bottom and sides of the pit shall be kept wet for few hours (not less than 4 hours). The condition of surface shall be Saturated Surface Dry (SSD) and cement: sand 1:1 slurry with water-cement ratio not more than 0.62 shall be employed to coat the sides and bottom of pit to ensure better keying for the repair work. The unfortunate part is most of the iv contractors seldom adopt such techniques as it demands high meticulousness and requires good nos. of skill labourers; therefore considered a lengthy process to them. Moreover, low and medium slump concrete mix could not evenly distributed across the cut portion and achieve desired level of compaction particularly in the corners, along the patch perimeter and around the dowel bars. As a result of which poor bonding at the interface of old and new concrete exhibits and leaves undulations in the surface. Thus, the repaired slab doesn't lasts for a long period of time, indeed the spot became more sensitive zone. In such dire circumstances, a concrete like SCC is a boon, which could easily fill the entire cut portions of the slabs by virtue of its high flowability and passability properties and thus imparts excellent bonding action at the interface of old concrete pavements & new concrete. Taking note of shortcomings of normal concrete mix and advantage of SCC mix in mind for old concrete pavement rehabilitation, the present research work is directed towards the use of wollastonite microfiber in SCC which is found abundantly in Rajasthan, Tamil Nadu, Uttarakhand and Chattisgarh in contact metamorphic zones, in some schists, gneisses and limestone inclusions in volcanic rocks for repairing of old concrete pavements. The wollastonite microfiber used in the present research work was obtained from Udaipur, Rajasthan. Chemically it comprises of 45.60% CaO and 48% SiO2 which was confirmed by the presence of these oxides in large magnitude from the XRD study. Many researchers have worked on concrete by part replacing cement by flyash and microsilica and their influence and performance were judged against the standard of concrete containing only Portland cement. It has been reported that they are promising materials for production of high performance concrete and save energy and conserve resources. Flyash and microsilica are well-established pozzolans; a concrete mix containing flyash is cohesive and has a reduced bleeding capacity. The mix can be suitable for pumping and for slip forming; finishing operations of flyash concrete are made easier. This is linked to the shape of the flyash particles. Flyash is a by-product of burning pulverised coal to generate electric power and variability of the material is very large, even the same power station will produce flyash with varying properties if the coal used is non-uniform in the short or long term. The flyash used in the present investigation was obtained from National Thermal Power Corporation, Ghaziabad, Uttar Pradesh. On the other hand, microsilica is a by-product of the manufacture of silicon and ferrosilicon alloys from high-purity quartz and coal in a submerged-arc electric furnace. Microsilica in the form of glass (amorphous) is highly reactive, and the smallness of the particles speeds up the reaction with calcium hydroxide produced by the hydration of Portland cement to produce additional calcium silicate hydrates. The microfine particles of microsilica v can enter the space between the particles of cement, and thus improve packing. The microsilica used in the present study was procured from a factory at Ludhiana. Considering the benefits offered by flyash and microsilica in high performance concrete, these cementitious materials were chosen for admixing jointly with wollastonite microfiber or alone in cement for observing performance of the self-compacting concrete for a set target flexural strength of 4.5 MPa at 28 days moist curing. Initially, physical and chemical properties of the three cementitious materials were investigated including Portland cement in order to appreciate better their influences in PQC mix as well as SCC. The wollastonite microfiber used in the present research work was acicular in structure and possessed needle like structures under SEM images. It has average length of 0.03mm and mean size of 1.83 microns revealed from SEM study and particle size analysis. Whereas, the mean sizes of cement, flyash and microsilica are 20.05, 25.69 and 0.145 microns respectively. This analysis clearly suggested that microsilica has finest particles followed by wollastonite microfiber, flyash and cement respectively. Highest specific surface area is reported for microsilica (18000 m2/kg) followed by wollastonite microfiber (827m2/kg), flyash (380m2/kg) and Portland cement (298m2/kg) respectively. The pastes were investigated for consistency, setting times, soundness and compatibility for various mixture proportions. XRD analysis and SEM images were analysed to determine the presence of different oxides and hydrated products respectively. Compatibility test was conducted using naphthalene formaldehyde sulphonate (NFS) and poly carboxylate ether (PCE) based superplasticizer. Then, wollastonite microfiber admixed cement mortars were prepared by part replacing cement with different amount of wollastonite (10%, 20% & 30%). Similarly, flyash admixed mortars were prepared by part replacing cement with different amount of flyash (10%, 20% & 30%). Another separate twelve mix proportions were prepared keeping flyash content constant @ 10%, 20% & 30% for different percentage microsilica content starting from 2.5% at an equal interval upto 10%. Similar, twelve mix proportions were prepared for wollastonite microfiber & microsilica admixed mortars by maintaining wollastonite content constant @ 10%, 20% & 30%. Lastly, fourteen admixed mortars were prepared containing all the three cementitious materials jointly in different quantities. Compressive strength, flexural strength, splitting tensile strength and modulus of elasticity of hardened mortar of the aforementioned mixes were ascertained to correlate better with the hardened concrete parameters. Further, efforts have also been made to evaluate the durability properties of these mixes by conducting rate of water absorption & sorptivity and abrasion resistance. Admixing of WMF improves flexural strength & modulus of elasticity of SCC significantly. This vi improvement is continuous with days of moist curing. The improvement in flexural strength is even better on admixing of microsilica in the range of 5% -7.5% by weight of cement. In contrary, flyash admixed SCC improves compressive and splitting tensile strength irrespective of replacement levels and days of moist curing. Irrespective of amount and types of admixtures used, modulus of elasticity (E) was found to be lower in value in comparison to normal concrete. Higher abrasion resistance was offered by mortar containing WMF in presence of microsilica regardless of their content. From the statistical analysis, it is learnt that good correlation exhibits between abrasion resistance and compressive strength of mortar regardless of types of cementitious content. To arrive at the SCC mix, the pre-requisite was to initially establish design mix in accordance with IRC: 44-2008 "Tentative Guidelines for Cement Concrete Mix Design for Pavements" published by Indian Roads Congress (IRC). The normal PQC mix was first derived and subsequently modification was made by incorporating cementitious materials to achieve ideal SCC mix. Several trial mixes were carried out in the laboratory by examining on the rheological properties of SCC in its fresh state. Tests like flowability, passability, segregation resistance through Abram’s cone, V-funnel, J ring & probe test were conducted in the laboratory to judge the rheological properties of SCC and results were compared with European Federation for Specialist Construction Chemicals and Concrete Systems (EFNARC) specifications. Flyash addition improves the flow ability and passability of SCC mixes irrespective of part replacement level with or without microsilica. Segregation resistance of SCC mix improves considerably with an increase in flyash content. However, inclusion of flyash alone into the normal SCC was not possible to achieve ideal SCC mix on account of this, it is necessary to incorporate cohesivity inducing admixture like microsilica. Higher contents of flyash is possible for achieving ideal SCC provided the ratio of flyash to microsilica be maintained in the range of 4:1 to 3:1. Alike fly ash, inclusion of WMF at higher dosage affect the rheological properties of SCC mix considerably. The maximum part replaceable level of WMF in presence of microsilica and VMA is 20% by weight of cement. For achieving good flowability and passability of the SCC, about 5 -7.5% microsilica inclusion would be essential. Admixing of WMF modifies the gel pores and density of the SCC mix significantly. Part substitution of cement by both WMF and flyash together in presence of microsilica doesn’t impart significant impact on the rheological properties of SCC. Effect of WMF in reducing the flow is more prominent than flyash. The right amount to add WMF and flyash together in presence of 5% microsilica is 15% for achieving ideal SCC mix. vii The results from the tests conducted, showed that WMF with silica fume could act as a cement replacer in providing an economical SCC mix having good rheological, mechanical and durability properties. Overall there was an increment in flexural strength of all the WMF-silica fume admixed mixes by a value of 3.5%, whereas flyash-silica fume and combined mixes had a decrement of 17% and 8.5%, respectively. Modulus of elasticity had decreased for all the mixes, such that it had decreased by 17.3%, 44.3% and 45.3% for WMF-silica fume, flyashsilica fume and combined mixes, respectively. Talking about durability, the permeability of these mixes decreased by 82%, 74% and 61.5% respectively for WMF-silica fume, flyash-silica fume and combined mixes, respectively. Importantly, drying shrinkage reduced by 49% and 36.6% for WMF-silica fume and combined mixes, whereas it showed an increment of 1.25% for flyash-silica fume mixes. Therefore, it was concluded that, WMF-silica fume mixes have an overall advantageous effect on the properties of SCC. The best WMF-silica fume mix named, CWS6 was chosen for further testing in the rehabilitation program. A prototype was prepared in the laboratory, having its design done for axle load distribution specified in IRC 58-2011. A 100 mm thick DLC was laid on properly compacted subgrade layer and above it, 300 mm thick PQC was laid having a contraction joint between two panels of size 1.8 × 1.8 sq.m. each. After 28 days, the pavement was tested for deflections, obtained through plate loading at three locations i.e. corner, edge and interior. The deflections were recorded from the predetermined locations i.e. near both ends of the dowel bars in the two panels, approximately 300 mm from the panel division. This was done for three cases, normal PQC, normal PQC rehabilitated and WMF-silica fume reinforced SCC rehabilitated pavements. The load transfer efficiency was ascertained for all the cases. Afterwards the validation of finite element model of the prototype, designed on ABAQUS, was done by comparing the deflections at edge location for all the three cases. The model was validated for non-rehabilitated pavement and subsequently for WMF-silica fume reinforced SCC rehabilitated pavement. The validated model was used for determining the flexural stresses in the PQC under a specified set of loadings taken on the basis of axle load distribution on the road, as specified in IRC 58-2011. From the modeling, the flexural stresses so obtained yielded fatigue life for the left two pavements, namely, non rehabilitated pavement and WMF-silica fume reinforced SCC rehabilitated pavement. The fatigue life of pavement rehabilitated with WMF admixed SCC (CWS6) was higher than that of normal concrete rehabilitated pavement. Finally, cost comparisons between rehabilitated PQC without admixtures and pavement rehabilitated with admixed selfviii compacting concretes were also made and found that wollastonite microfiber inclusion would not only enhance the life of pavement but also would be able to offset the cost of pavement rehabilitation considerably in comparison to conventional rehabilitation technique