Evaluative Study for Electromagnetic Density Gauge

Coarse as well as fine aggregates along with binder content and air voids are the prime constituents of hot mix asphalt (HMA) pavement. In real practice, HMA is spread as distinct layers. For newly built roads enhanced performance and prevention of hot mix asphalt (HMA) failure is assured by considering several factors. The amount of in place air voids in asphalt mixture is one of the decisive factors affecting the HMA performance. In place air voids greater than 3 percent is the requisition to avoid premature rutting right through the life of pavement [1]. Asphalt content and compaction effort along with additional compaction of traffic are the prime factors that control air voids. Moreover, density of asphalt pavement has a direct impact on its durability. Lower density for example, increases deterioration rate of pavement and has possibility for oxidation to occur [2], [3], water damage [4]– [6], raveling and cracking in asphalt pavement [7]. Since asphalt density brunt the air voids directly along with its impact on pavement durability thus enhancing the importance of density determination techniques. Moreover, accurate and rapid measurement of road density has always been central focus of researchers across the world as measurement of density for newly built asphalt road is carried out by contractor as a part of quality control while state or local agencies do it for quality assurance program.

Unlike many countries of the world, that has been using updated methods, and many modified equipment for asphalt density determination, Pakistan is still using traditional core method for density determination. Core method of density determination is carried out in accordance with American association of state highway and transportation officials (AASHTO) procedure AAHTO T 166 [8]. This method of density determination is associated with disturbing the road integrity by drilling out the cores thereby associated with risk of localized failure to occur despite cores being refilled. Moreover, this process involves labor and a full assembly for density determination thus not being time efficient. Other problems encountered while obtaining density from Core method may include time restrictions to make the core sample air dried before measuring dry weight, the method being tiresome and time taking activity involving labors [9]. Drying in oven at elevated temperature may result in distortion in core sample and thereby disturbing the actual results [9].

Over the ensuring years timely compaction results determination became the prime criteria for the paving industries. Many countries of the world are using non-destructive devices for density determination of newly laid asphalt pavements since late fifties and early sixties. Nuclear density gauges rectify the problem of road destruction as in case of core methods. Therefore, this equipment caught the eyes of paving industry across the globe. Nuclear gauges used for density measurement works on emission of gamma rays into the pavement and measuring the density of asphalt by the rays scattered back to the gauge [10]. Since such devices emits low level radiations for the density determination therefore associated with health hazard. Earlier such devices were quite heavy and operator use a scaler so that he can measure the density from a safe distance [11]. Over the years, manufacturers have refined the equipment but still owning and operating the instrument needs the state radioactive material license. Although asphalt density readings may vary from core values when measured using nuclear gauges along with safety concerns, such gauges were used because of speedy and non-destructive measurement of asphalt density. Many of the researchers have concluded that nuclear gauges are not as accurate as core method. One such research suggested that five different units of nuclear gauges did not result in the same results thereby pointing out the erroneousness of nuclear gauges. In addition nuclear density testing variability differed not only from gauge to gauge but also from location to location within one gauge as well [12].

One research conducted to check the reliability of nuclear density gauge considering different factors resulted nuclear gauges not to be accurate enough to be used as a replacement to core method. Researchers suggested that core method is more accurate than nuclear density gauge keeping repeatability in mind. More over research suggested to check the effect of environmental conditions involving ambient temperature, moisture content present on testing surface and mix design including aggregate gradation [13]. Nuclear density gauges delivered much to the paving industry but still there was a need for an instrument that was fast, non-destructive, being free from the restriction of licensing and radiations problem and more operator friendly [14]. Last decade has been the promising one for the paving industry as industry witnessed the evolution of non-nuclear devices for density determination which were non-destructive as well. Operation of NNDG is based upon sending and receiving electromagnetic waves thereby overcoming the issues related to safety as in case on nuclear devices and destruction of pavement as for core method [15], [16] This research work is the result of inspiration by the work of many researchers who have done admirable job until now to check the accuracy of many non-nuclear devices. The first of these devices called pavement quality indicator (PQI), was made commercially available by transtech system Inc.in 1998 and the second of these devices called pavetracker was made commercially available in 2000 [17]. One research conducted as pooled fund study on evaluation of non-nuclear density gauge concluded that density obtained by using PQI-300 was statistically different from core density in 54 percent of the projects featuring the equipment incompatibility to determine the density accurately [16]. A research done in 2006 involved three non-nuclear devices e.g. Trans tech PQI model 300 and 301 and troxler pave tracker model 2701b and their results were compared with nuclear method of density determination. The results suggested that all three non-nuclear gauges consistently read density values lower than that of nuclear gauge.

Analysis of variance resulted that several factors affected the density readings. Factors included lab air voids, specific gravity and pavement layer thickness [18]. Other research introduced effect of many factor on non-nuclear devices e.g. paints, gradation, orientation of gauge and presence of water. Research concluded that orientation of the gauge has significant impact, but it is mixture dependent property. Moisture presence has also significant impact on density reading. Presence of paint resulted in higher density values [19]. One of the research done on PQI suggests and recommends the device to be cost efficient and a better alternative to nuclear density gauges as it does not require any type of intense safety regulations and licensing procedure [20]. One research conducted on PQI-380 as non-nuclear asphalt density measuring technique stated device to be low cost and time efficient. Moreover number of cores drilled out from newly developed road can be reduced with the use of PQI-380 [11]. Pavement quality indicator (PQI) is said to be an accurate equipment for asphalt density measurement. It is user friendly and causes less back pain being lighter. PQI can be store and transported everywhere without any radioactive material license. It is fast and has good repeatability and has low standard deviation between the test [14]. Standardization of any devices is done by comparing it with already existing standard practice. PQI is said to be a standard equipment for asphalt density measurement until its results are comparable to AASHTO T 166 [21]

Incentive for this research work is primarily influenced by the fact that road infrastructure is on an emerging corner across the Pakistan, making it essential to have better quality assurance equipment for road construction. For this purpose, taxila institute of transportation engineering step forward making it possible to have PQI-380, a non-nuclear asphalt density determination equipment. This research work compares the results from PQI-380 with that of cores taken from the same location along with taking some local factors that may affect the accuracy of the instrument.