INVESTIGATIONS ON ROLLCRETE PAVEMENT LAYERS

Abstract

The hikes in petroleum prices in 197 3 and 1979 led many countries to make a shift to concrete roads as a viable and costeffective option. Rollcrete (RC) attracted attention for road construction, because of the existence of a large stock of suitable equipment for compaction of pavement courses and also because it allowed maintenance work to be done without closing the road to traffic. The concept of rollcrete, now rapidly evolving, probably had its beginning in extensive applications to dam construction in various countries. One of the earlier developments that led to the future use of RC for heavy duty pavements, occurred in 1970 when cement treated crushed limestone was selected for the pavement structure at the seaboard lumber terminal in North Vancouver, Canada. This pavement is a cement stabilized aggregate base with an asphalt wearing surface. The U.S. Corps of Engineers was the first to develop the use of RC pavement in the United States. A small test section was laid by Waterways Experiment Station (WES) on a street in 1975, but it was not until the mid - 1980's that RC pavement construction actually commenced in the United States. The first production project was a hardstand for tracked vehicles constructed by the Ft. Worth District at Ft. Hood, Texas in 1984. Later in 1985, an apron for heavy-load commercial aircraft was constructed at the Portland International Airport. This was followed by the construction of RC street pavements in Portland, Oregon. In 1986, commercial use of the RC pavement construction spread throughout the United States. II The first application of RC in Spain occurred around 1970 for a pavement with light traffic, but recent experiences include main roads carrying heavy to medium traffic loads. Norway and Sweden are gradually extending the applications of rollcrete for pavement construction. France has started to use RC operationally for lightly travelled roads. These countries have not yet fully mastered the technique of building roads with RC but are counting on its progressive introduction to eliminate the current difficulties in areas that receive heavy and low speed traffic such as coal storage yards, log sorting yards, port facility areas, heavy equipment parking areas, haul roads and dams, etc. The major use of RC around the world has been in the construction of dams. RC has, however, been receiving prominence recently as a pavement surfacing layer, in contrast to its use in earlier times as a sub-base under concrete or bituminous surfacings. In the earlier usages, lean cement concrete mixes with very low workability were laid and compacted by roller for serving as a sub-base. The RC pavements currently receiving attention, make use of pavement quality-concrete of zero slump and a vibratory roller and as such the RC pavement is an innovation in concrete paving. The speed of construction of RC pavement using conventional machinery, its higher strength because of low watercement ratio, and the economy arising out of these two factors makes a strong case for its adoption as RC pavement surfacing for III highways and airfields with improved surface evenness. For this reason, it is necessary to ensure that the laying and compaction operations of rollcrete are carried out with a paver and virbatory roller to achieve adequate rideability. Globally, significant experience on RC paving projects has been gained through adoption of paving trains and vibratory rollers. Due to dependence primarily on coal-based thermal power stations for producing electrical energy and due to high ash content of Indian coal, about 70 million tonnes of flyash are being produced every year as a waste material in the country. This waste material, because of its pozzolanic property, can be converted into wealth when used in large quantities in Civil Engineering Works. With further addition in capacity of the existing power plants and commissioning of some super-thermal power plants, the annual production of flyash is estimated to be about 100 to 120 million tonnes by the turn of the century. The already serious problem of flyash disposal will thus get worsened in the years to come unless appropriate measures for flyash utilization and proper disposal of the surplus are not taken. Large quantities of flyash can be consumed in building of roads and embankments, reclamation of low-lying land and refuse dumps, filling of mines, treatment of polluted waters and unsuitable soils for agriculture. These uses are well known and are adopted, whenever required and wherever applicable. RC pavement construction will lead to the use of a large volume of flyash. Construction of concrete pavements will become unavoidable in the near future which will offer considerable IV scope for the use of flyash in road projects. The utilization of flyash in RC pavement construction not only saves apart from saving energy but also reduces environmental pollution and costly disposal problems. RC represents a new concept (ACI Committee-207) in which concrete of no slump consistency is transported, placed and compacted by external vibration utilizing vibratory rollers. It differs from the conventional concrete principally in its required consistency. For effective consolidation, RC must be dry enough to support the weight of the vibratory roller but wet enough to permit adequate distribution of the paste binder throughout the mass. The RC concept results in rapid consolidation in a short period of time with minimal labour and equipment when compared to the conventional PCC pavement. Properties of hardened RC are the same as those of conventional concrete of the same water-cement ratio. Rollcrete is an outgrowth of the traditional cement treated aggregate base as used in highway construction. The improved significant properties of RC incorporating high volume of flyash content are: Improved strength characteristics Enhanced durability (Freeze-thaw, Wetting-drying) Improved surface texture (rideability) Low co-efficient of permeability However, the following aspects of the use of RC need to be further investigated before recommending it for general usage : Mix Design and related strength characteristics Durability vis-a-vis volume fraction of flyash Rideability Pavement Thickness Design procedures Economy and maintenance costs vis-a-vis conventional pavement construction techniques. There is an urgent need to evolve a new concept of composite pavement construction comprising RC topped with PCC pavement as a resurfacing layer to improve the rideability, durability, surface texture and strength characteristics of RC pavement, vis-a-vis conventional concrete pavements. The replacement of cement by flyash in concrete pavements is a well known practice. The pozzolanic replacement is known to be effective in achieving improvements in durability and workability. In the present investigation, the following aspects related to RC have been studied in the laboratory : Design of RC mixes with varying flyash replacement (0, 10, 20, 30 and 40 %) Workability Compressive strength of RC comparable to PCC. Microstructural behaviour Load carrying capacity of RC pavement slabs (10 numbers) at critical loading conditions i.e. Central, Edge and Corner. To predict the theoretical behaviour of RC pavement slabs under the critical loading conditions, the following VI computational methods were adopted : - Westergaard theory Meyerhof theory Finite Element method using plate elements The experimental and the theoretical investigations showed that : The optimum flyash replacement from strength considerations was 20 - 30 %, which also led to reduced co-efficient of permeability due to pore-blocking in RC (9 x 10-11 m/s at 40 % flyash content and 100 x 10-11 m/s at 0 % flyash content). SEM studies of rollcrete showed the formation of a major CS- H gel phase and rods of ettringite in bundled form resulting in a microstructure which is non-crystalline and is characterized by the presence of a number of pore spaces. At a higher cement replacement of 40% with flyash, there is an evidence of distinctly visible interface between the cement paste and flyash particles. These lead to a reduced co-efficient of permeability thereby increasing the durability. The low permeability of RC with flyash, however, is a desirable feature from durability point of view. These observations are also corroborated by the reduced load carried by RC slabs due to presence of interconnected compaction voids and porous/undeveloped interfaces, vis-a- VII vis PCC slabs. The reduction being 30-40% at critical corner loading condition for flyash replacements of 20-30%. However, these reduced loads are still greater than the design wheel load of 51 kN. In case, of PCC specimen, a crystalline structure is formed and under normal compaction, the pore-spaces are found to be negligible and there is a complete absence of interfaces between the cement paste and aggregate particles. The structural behaviour of the RC slabs showed that: (a) Maximum deflections and strains occur under corner loading condition (5.85 mm, 275 microns) whereas the minimum occurrs under the central loading condition (0.89 mm, 80 microns). The variations of deflections/strains were non-linear with increasing loads. (b) The failure loads yielded a load factor of 1.4 to 1.8 (20-30% cement replacement), suggesting an average design load factor of 1.5 as laid down in IRC: 58-1981 and IS:456-1978 Standards. (c) The average crack width at first crack was 0.25 mm; the shrinkage, cracks were restrained due to presence of flyash and lower water content. The experimental results were found to agree within 10-15 % of the values based on Westergaard and Meyerhof theories. The FEM results were found to be within +/- 15% of the experimental results; indicating a 85 - 90% confidence VIII level for analysis of layered pavement system of RC and PCC slabs. The economic analysis showed that 20-30% flyash replacement of cement can be adopted successfully for future paving jobs. This may, in turn, accomplish performance improvement in rideability, durability and strength characteristics. The potential utility of RC and its composite pavements would soon gain wide acceptance in highway construction as a more efficient and effective construction technique. IX