Those are amazing photos, which are also doing the rounds of airport engineers at the moment. They will no doubt find themselves into various technical presentations in the future.
Once again PPRUNE has taught me something, about pavers and jet blast. But first let me give some background to the use of concrete block pavers on airports.
There is a lot of block paving out there, used at many UK airports, Hong Kong, Cairns, Perth (Australia), Trondheim, Ben Gurion, etc. To illustrate their use, this is a quote from the literature:
“[have been used on] aircraft parking stands at Christchurch International Airport (International Stand 27 and Stand 34/35). The blocks have been laid on a 20mm thick layer of bedding sand on a cement stabilised basecourse layer with the blocks being sealed. To date the pavements are performing well under loading from Code E derivative aircraft.”
The use of concrete block pavers on airports is codified in several official standards or manuals, such as FAA ITEM P-502 Interlocking Concrete Pavement Construction For Airport Pavement. There is a very good manual on “Airfield Pavement Design With Concrete Pavers” written by Roy McQueen, John Knapton, John Emery (not me), and David Smith - Interlocking Concrete Pavement Institute – Updated May 2012. These authors are highly respected internationally.
However, there are certain situations where pavers are not recommended.
The British Ministry of Defense, British Aerospace, PLC, and the U.S. Air Force conducted tests on pavers exposed to high velocity jet exhaust. Although, the results of these experiments were favourable, concrete pavers are not recommended in airport locations subject to full power or reverse thrust, such as runways or apron areas where aircraft "power-back" operations are conducted. The FAA standard also says “The use of concrete pavers is not recommended for areas subjected to full power or reverse thrust (e.g. runways or apron areas where aircraft "power-back" operations are conducted).”
That pretty well suggests what happened at Pakistan's Sialkot International Airport. I do not know the technical details of the pavement, but I can see from the photos that the blocks are interlocking (as they should be) and they look to be the correct 80mm (3.125 ins) thick. There is bedding sand present, but possibly thicker than the required ¾ inch to 1 inch (20 mm to 25 mm); I cannot tell if the bedding sand meets the high quality standard needed. There is also the use of a geotextile under the sand which is recommended for certain bases (such as cement stabilised).
But the jointing sand looks a little odd, and even slightly missing in sections of undisturbed apron. The jointing sand is necessary to provide interlocking among the individual paver units. Aircraft loads are transmitted to surrounding pavers by shear forces through the joint sand, enabling the pavers and bedding sand to function as a distinct structural layer to allow distribution of loads in a manner similar to a hot mix asphalt layer. The sand should have been sealed (using something like an elastomeric urethane) to prevent its loss from jet blast. The joints are difficult to see in the photos but I am not certain that the jointing sand is always present, effective and sealed.
Based on past performance at airports, properly constructed concrete block pavers would be considered for:
- static parking positions with "tug-in/tug-out" or low engine speed "power-in/power-out" operations (excluding reverse engine thrust).
- low speed taxiways and taxi lanes (my own rule of thumb is an upper speed limit of 40 kph on concrete block pavers).
and this accident at Sialkot International Airport shows no reason to change that. It looks to me like the pavement was being used "outside the design envelope".