The first six installments in this series dealt with contracting issues. Now we change gears to focus on the physics of the slab on grade. For starters, the following true or false quiz should give you a fair sense of your current technical level. The context is slabs on grade, and there are no tricks. Take your time and use references if you like.
1. [ T ] [ F ] Putting the poly below a 2-inch sand layer will reduce curling.
2. [ T ] [ F ] The firmer the base, the less the shrinkage cracking.
3. [ T ] [ F ] The slump test is an effective concrete quality control tool.
4. [ T ] [ F ] The higher the shrinkage, the more the shrinkage cracking.
5. [ T ] [ F ] Overlapping the dowels at a joint intersection will lock it up.
6. [ T ] [ F ] Compressive strength is a key factor affecting performance.
7. [ T ] [ F ] Exceeding the time limit is a valid reason for rejecting a truck.
8. [ T ] [ F ] Ramping at construction joints usually is caused by curling.
9. [ T ] [ F ] A good wet screed crew should average at least a FL 20.
10. [ T ] [ F ] A slab's live load capacity is determined mainly by its thickness.
11. [ T ] [ F ] Proper curing is essential to minimizing edge curl.
12. [ T ] [ F ] FF and FL have about the same influence on user satisfaction.
13. [ T ] [ F ] In a warehouse slab, if the joint spacing is tight enough, aggregate interlock will provide the required cross-joint shear transfer.
14. [ T ] [ F ] A skilled trowel mechanic can make a slab delaminate wherever he wants.
15. [ T ] [ F ] Water content is the main factor affecting the shrinkage of a mix design.
16. [ T ] [ F ] Suppliers know how much their concrete shrinks, curls, and cracks.
17. [ T ] [ F ] Floor flatness can be controlled using a 10-foot straightedge.
18. [ T ] [ F ] To force crack formation, sawcut control joint depths must equal at least 25% of the thickness.
19. [ T ] [ F ] Slabs placed on plastic sheeting curl more than those placed on porous bases.
20. [ T ] [ F ] The main reason to add cement is to increase compressive strength.
21. [ T ] [ F ] Meshes lighter than 4 gage can be positioned properly.
22. [ T ] [ F ] The lower the slump, the less the shrinkage cracking.
23. [ T ] [ F ] Properly positioned rebar prevents cracking.
24. [ T ] [ F ] The Portland Cement Association, U.S. Army Corps of Engineers, and Wire Reinforcement Institute thickness design methods make sense.
25. [ T ] [ F ] Improved form accuracy is critical to higher FF numbers.
26. [ T ] [ F ] Low breaks are a valid reason for demanding removal and replacement.
27. [ T ] [ F ] Excessively curled joints are avoidable defects caused by the installer.
28. [ T ] [ F ] The engineer can compute a slab's tendency to develop shrinkage cracks.
29. [ T ] [ F ] As the live load increases, so must the thickness.
30. [ T ] [ F ] The onsite addition of mix water should be discouraged.
31. [ T ] [ F ] Exceeding the slump limit is a valid reason for rejecting a truck.
32. [ T ] [ F ] The higher the compressive strength, the less the shrinkage cracking.
33. [ T ] [ F ] The higher the flexural strength, the less the shrinkage cracking.
34. [ T ] [ F ] A skilled trowel mechanic can make a slab crack wherever he wants.
35. [ T ] [ F ] Division 9 floor flatness tests make sense.
36. [ T ] [ F ] Unplanned cracks are avoidable defects caused by the installer.
37. [ T ] [ F ] Delamination usually occurs when the trowel mechanic mistakenly starts to work the surface prematurely.
38. [ T ] [ F ] Multiple rescrapings and refillings after machine floating are essential to the production of higher FL numbers.
39. [ T ] [ F ] A slab gets hard enough to finish because the cement is hydrating.
40. [ T ] [ F ] Applying water to the surface during finishing degrades the quality of the floor.
Allen Face is inventor of the Face Floor Profilograph, Dipstick, F-Meter, D-Meter, and Screed Rail. He is also an ACI Fellow and a long-time member of ACI Committees 302, 360, and 117.