Yes, the strength is significantly reduced, as the water bound in the concrete turns to steam, and
BLOWS PIECES OFF,
thereby reducing the cross section, and hence the strength.
If you wish to conduct an experiment, hold an oxyacetylene torch close to a piece of concrete, with your arm outstretched, wearing gloves and a face shield and other suitable PPE. You will likely be impressed with the results. I am hoping you are not also injured.
Interesting. Could this spalling occur after several launches where it's a progressive loss of moisture. Do you think there would be warning signs such as change of colour before spalling occurs? Some of the pad is not on grade and spans across two supports. Part of me wonders if compressive strength could be decreased short of spalling and fail the RC span.
What is the duration of the firing and what is the thrust of the rocket?
I worked in the facilities division of a rocket engine manufacturer. We had engines from 180,000# to 1,500,000# of thrust vertical downward and a firing duration from 30 seconds to 7 minutes. In most cases, we had slanted steel flame deflectors with a water film for cooling. There was always some spillage around the area and we used a thick heavy coating called "fondu fire"(spelling?) for protection of the surrounding surfaces. The test stand foundations were concrete.
We did not have a serious problem with any debris in the blast, but did have a 24" pipe wrench impact a observation window in a observation and control center that was about 200-300 yards away. - It only affected the outer 1" laminated pane one window and the remaining 4 panes behind that were fine.
Due to the desert climate (Southern California) the moisture content of the rock was extremely low because of the constant low humidity. All surface protected areas were concrete since asphalt would just have been fuel when in contact with the liquid oxygen or more exotic compounds.
Leonhardt - Possibly you could research a product similar to what we used on the for the protection of rock and surrounding concrete.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
Let's say 2,000,000 pounds of thrust and 10 seconds firing. I believe the flame trenches are all specialty fondu fire, or similar, but the pads are plain concrete. I'm most interested in the need for the deluge system and it's effect on the concrete.
Sounds like an interesting job you had. Were you structural at this position? I presume designing the stands and flame deflectors
2,000,000# and 10 seconds sounds like a launch of the recent rockets. I only saw a couple of firings.
On the ground in FLA you will likely have a good amount of moisture in the concrete. Our tests were on a rush 24/7/365, so the concrete never dried out and that is the reason for water cooled flame deflectors. We kept the flames away from the concrete because it never had a chance to dry out and Fondue Fire was used generously around the stands.
I was a structural engineer straight out of school. Since I had taken surveying(required class) in a 5 year CE curriculum, I used it my second day of work to observe the actual connection of the engine to the thrust measuring system on the rocket test stand. The new type engine was days old and scheduled to be fired 4 days. I used the transit, drew up a sketch, my supervisor looked at it for a couple of minutes and sent it off to be fabricated, installed and used a couple days later. - We referred to it as "cowboy engineering". Because we had about 12 test stands 10 miles from the office, we frequently instrumented the joints and mid-span gauges and accelerometers) on a 150' high steel test stand (with about 15,000 gallons of liquid hydrogen and liquid oxygen for determining the typical behavior as the tanks emptied quickly during a test. We did not design the original stands, but modified them as the rocket engines were modified and upgraded. Virtually everything was stainless steel (beams, columns and even grating and stairways) because of the cryogenics and corrosive properties of the fuels and oxidizers. The stands were built on the side of a mountain slope, so the rock and soil had to be protected from the temperature and erosion of the blast.
Good first job. The company paid night graduate courses at USC for 3 or 4 nights a weekly for 2 years including 50 miles daily of mileage, dinner and books. The continuing education was required to get more than the usual 10% or 15% annual increase.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
