Rehabilitation of Overstressed Rigid Pavements

    M. D. Apte, Maj (Retd) Chartered Engineer

    Due to shortage of resources like cement as well as finances, hardly any concrete roads were constructed in free India, though, in the long run they are cheaper than bituminous roads, was an established fact. Some internal city roads were concreted, but State Highways or National Highways were rarely, if at all concreted. The city roads were designed for traffic of 20 years hence with determined increase over the period. By the end of the designed life of the roads some repairs were needed and the usefulness of the roads hardly affected adversely. This was mainly because the proposed increase in traffic rarely materialized. Indian scene of road traffic development was improving too slowly. Available vehicles were not improved nor their tonnage increased to any appreciable measure. Shortage of money was keeping all the roads bituminous and hence had to have yearly maintenance of the wearing coats at least. Since early nineties, the situation changed in that due to open economy, the infrastructural development had to be world standard. Moreover, our resource position also had improved and rigid pavement roads on BoT and BOLT systems became a possibility. The traffic paid the toll charges to use these roads, which could be helping in repayment of the investment in construction of these roads that the private parties made.

    Rehabilitation of Overstressed

    This was the position of roads, but runways on airports had to be rigid due to the heavier loads operating thereon and heavy aircraft also had to land there. Comparatively, money shortage as an obstacle was not enough to deny concrete construction for these pavements. The traffic on these pavements was determined by the quantity of take-offs and landings of various aircraft that have used the runway. As long as the traffic was not excessive, by the end of the predetermined life of the runway/pavements, certain defects like, surface level becoming uneven, minor cracks becoming visible or drainage system functioning inefficiently etc., become obvious. This necessitated repairs to the pavement. If the predetermined utilization of the pavement is completed, the pavement condition may deteriorate to such an extent that it may have to be redone. Mainly the airports under use of Air Force finish their utilization before designed life since the use is not only more intensive, but also because frequently aircraft heavier than the designed loads are also being used. Relaying of pavements in such cases not only is very costly, but alternative arrangements for the Air Force operations have to be made for the duration of this repair.

    Rather, we can say that any rigid pavement needs very little maintenance during its full utilization or designed life, but by its end, heavy repairs and maintenance, not excluding replacement cannot be avoided. This is for normal use of the facility. The maintenance/repair needed during the life can be covered under Concrete Pavement Restoration (CPR) procedures like sub-seal, full-depth patching, partial-depth patching, load transfer restoration, diamond grinding and joint resealing. We can see that these procedures if undertaken can overcome the defects that may crop up during the designed life/utilization of the pavements. Problem really comes up when the pavements have to sustain loads as well as traffic heavier than the assumed ones during the design calculations. After opening of Indian market to foreign vehicles since last 10 years this situation is being faced by the highways as well. Population of vehicles in the country (as well as on the road) has suddenly increased exponentially. Heavier load carrying vehicles have reported on the scene. As more international airports are being opened in the sub-continent heavier and larger aircraft are using these (local) airport runways. All this has deteriorated the rigid pavements at those locations needing quite heavy repairs, the CPR being inadequate substitute. Basic reason of this premature heavy deterioration is the fact that these pavements are being subjected to loads and traffic conditions quite heavier than what was planned at the design stage. Quite heavy cracks are observed on the surface (many have gone deeper as well). The pavement has become weaker to support even the designed loads satisfactorily. The traffic cannot maintain its designed speed. Because of the deeper cracks water gets trapped in the pavement body to make the slab unstable slowly but surely. What we need is rehabilitation. Rehabilitation of rigid pavement means replacement of base as well as the slab of the pavement fully.

    If we start reconstructing toll roads with this system not only it will take longer time, but the collection of toll will suffer since for quite some time the road will remain closed for traffic. The damaged/deteriorated slab/s will have to be broken down completely and the material disposed off. The sub-base then will have to be properly compacted and levelled before further pavement is laid. Since the cracking and other damages are likely to be extensive the rehabilitation in this way will take quite long. The results in case of repairs to damaged runways in this manner will not be much different except the airport will remain non-operational for quite some time.

    One useful method of concrete rehabilitation in such situation is ‘Rubblization’. Rubblization effectively reduces the slab (along with its distresses like cracking, faulting, pumping, spalling etc.) to a crushed base. This base prevents reflective cracking by eliminating slab action, presents a sound base for Hot Mix Asphalt (HMA) layer overlay, and increases the service life of the pavement. A survey of rehabilitated pavements under this method has revealed that this rubblization technique typically extends the pavement service life by more than 20 years, at which stage a simple HMA overlay will be sufficient to enhance its life further.

    Rehabilitation of Overstressed
    An initial key element of this rehabilitation system is removal of any excess water from beneath the concrete slab, and to ensure free drainage for any water that enters the pavement after the slab is fractured. This can be accomplished by installing longitudinal side drains. These drains must have the capacity to carry the water being discharged from the sub-grade, any permeable base and/or the fractured slab. It is suggested that the trench depth should be deep enough to allow the top of the pipe to be located a minimum of 50mm below the bottom of the existing base material, and that the permeable fill material extends to the upper portion of the rubblized concrete. Most important, the pavement cross slope and shoulder slope must be adequate to convey the water away from the traffic lanes. The proposed HMA layer will be successful only when the moisture below the flexible pavement has been properly taken care of.

    The rubblizing a concrete pavement needs complete destruction of the concrete slab and all slab action before applying the HMA overlay. Rubblization breaks the concrete-to-steel bond in jointed reinforcement concrete pavements (JRCP) and continuously reinforced concrete pavements (CRCP).

    Rubblization reduces the existing concrete pavement to a base material with a high degree of particle-to-particle interlock, which exceeds the structural coefficient of an in-place crushed aggregate base course. Moreover, this base eliminates the inherent distresses of the slab like cracking, spalling etc.

    Since the rubblized material consists of broken angular concrete pieces having a high degree of interlock and are densely compacted, it provides for a very strong structural base course for the HMA. To prevent reflective as well as inherent cracks there are two machines available for rubblization: RMI PB-4 is a high frequency low amplitude resonant breaker. It can fracture the pavement into pieces ranging from 3mm at top to 150mm at the bottom of the pavement in distress. Another is an equipment known as Antigo multi-head breaker (MHB). This machine uses a series of falling weights that impact for breaking and is capable of breaking a 3.5 M lane in one pass. Normal rubblization rate of both these machines is about 1600 Meters per day. After rubblization of the pavement, the concrete material is compacted before placing the HMA overlay.

    The rigid pavements that are deteriorated prematurely due to heavier loads can be brought into effective use quickly by providing flexible layer. But for that the already broken slabs/rigid pavements need further and near complete breaking (to provide base for the flexible pavement). Rehabilitation of a broken rigid pavement needs its further breaking!

    The design procedures for the overlay thickness for this procedure have been outlined in the new Asphalt Institute MS-17. Although good results have been achieved using these design procedures, a minimum of 125 mm of HMA overlay is recommended. In many cases, the imposed traffic on the reconstructed pavement will need a design thickness between 150mm and 225mm to the overlay.

    This rubblising technique is becoming very popular in USA. From 1994 to 1998 the states in Northeast, South and Midwest have rubblized concrete pavement and placed HMA overlay over more than 20 mil square yards of highways. An additional 20 mil square yards of pavement were rubblized in 1999-2000 followed with HMA overlays (mainly Superpave). Several states have accomplished more than 30 rubblizing followed by HMA overlay laying projects during this period.

    Since this rehabilitation method has been in use successfully in USA it is prudent for us to see if any advantages can be gained by us by using this simple method for rehabilitation of overstressed rigid pavements in the country.

    NBM&CW October 2018

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