Pete pointed me to several articles about surface effect, including some by Mike Sandford. But they all seemed to compare roads that had significant differences in roughness, very smooth vs. bumpy. None of them specifically mentioned asphalt vs. concrete. My two surfaces are both quite smooth, even though one is asphalt and one is concrete. Pete didn't mention the relative smoothness of his two courses. Also, Pete's asphalt readings in that temp range vary by 9 counts/km. Seems reasonable to expect that if he had more concrete readings they would vary by a similar amount. So are we looking at two values on the bottom, in the middle, or on the top of that range? I think his data shows that it is very likely his concrete constants are smaller than his asphalt constants, but it's dangerous to try quantify that based on two readings.
But I do plan to repeat my experiment on another road where the surface will be exactly the same for both courses.
There is little doubt that the difference I see in my two courses is real. I ran two separate experiments with 32 blind trials in each, and got almost exactly the same difference between the two courses in each test. The only question in my mind is what is causing this difference. I see several possibilities:
1) Course length - I measured my courses two times on different days using a scale to measure tape tension. The difference in the two measurements was 1cm. I think it is very unlikely my two courses differ in length by more than 2cm (about 1 count/km).
2) Surface effect - even though my two surfaces are about the same smoothness, the friction may be quite different and that could be as important as smoothness.
3) Tire effect - I'm using a mountain bike tire that is relatively low pressure and includes some tread.
4) Hill effect - Neville sees no overall hill effect. Pete sees a small overall hill effect which changed directions in his two experiments. I see a strong overall hill effect.
I see uphills giving a significantly smaller constant than downhill rides on moderate slopes. Pete sees uphill rides giving a significantly smaller constant on steep slopes. Neville sees the opposite on steep slopes.
I'm not sure we have all this hill stuff figured out yet.
5) Combination of one or more of the above. It's possible that the tread on my tire deforms differently depending on the friction of the road surface. It's possible there is a hill effect, but it's quite small, and my tire simply magnifies this effect.
I will have another set of courses that have a similar topology, but the same surface throughout. I also now have a higher pressure road tire. I plan to repeat this experiment with different combinations in order to find out what is causing the difference I'm seeing, so I can avoid that problem in the future.