Water Flow Through Slab Joseph Lstiburek, Ph.D., P.Eng Building Science Some Interesting Stuff on Slabs From under the slab From the edge of the slab From the slab itself presented by www.buildingscience.com Joseph Lstiburek 2 Mechanisms of Flow Liquid Bulk Hydrostatic Pressure Capillary Concentration Gradient Osmosis Concentration Gradient Vapor Diffusion Vapor Pressure Air Transport Air Pressure Joseph Lstiburek 3 Joseph Lstiburek 4 Lstiburek Some Intersting Stuff on Slabs 1 of 34
Joseph Lstiburek 5 Joseph Lstiburek 6 Joseph Lstiburek 7 Joseph Lstiburek 8 Lstiburek Some Intersting Stuff on Slabs 2 of 34
9/28/09 Joseph Lstiburek 9 Joseph Lstiburek 10 Joseph Lstiburek 11 Joseph Lstiburek 12 Lstiburek Some Intersting Stuff on Slabs 3 of 34 3
Fick s Law (Fourier s Law) direct functions Heat Flow Q = A u t delta T Vapor Flow M = A u t delta Pv A = surface area u = thermal conductance T = temperature u = permeance Pv= vapor pressure Joseph Lstiburek 13 Joseph Lstiburek 14 Surface Tension: Wettable Water attracted to surface more than self Water attracted to self more than surface Joseph Lstiburek 15 Joseph Lstiburek 16 Lstiburek Some Intersting Stuff on Slabs 4 of 34
Capillary Pressures Calculating capillary rise Result of surface tension = attraction to surfaces pressure varies with pore size e.g., height rise in a glass tube Surface tension up Gravity Down Joseph Lstiburek 17 Joseph Lstiburek 18 Capillary rise versus diameter Capillary Flow 100 90 80 70 Eg. : Crushed stone, air gaps large pores - no suction ( wicking ) capillary rise [inch] 60 50 40 30 20 10 0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 diameter [inch] Joseph Lstiburek 19 Joseph Lstiburek 20 Lstiburek Some Intersting Stuff on Slabs 5 of 34
Capillary Flow Capillary Flow- concrete sucks Example: Sand, siding laps Smaller pores - some wicking (inches to feet) Example: Clay or silt Wicking (dozens - hundreds of ft) Joseph Lstiburek 21 Joseph Lstiburek 22 Joseph Lstiburek 23 Lstiburek Some Intersting Stuff on Slabs 6 of 34
Joseph Lstiburek 25 Joseph Lstiburek 26 Joseph Lstiburek 27 Joseph Lstiburek 28 Lstiburek Some Intersting Stuff on Slabs 7 of 34
Joseph Lstiburek 29 Joseph Lstiburek 30 Joseph Lstiburek 31 Joseph Lstiburek 32 Lstiburek Some Intersting Stuff on Slabs 8 of 34
Joseph Lstiburek 33 Joseph Lstiburek 34 Joseph Lstiburek 35 Joseph Lstiburek 36 Lstiburek Some Intersting Stuff on Slabs 9 of 34
Joseph Lstiburek 37 Joseph Lstiburek 38 Joseph Lstiburek 39 Joseph Lstiburek 40 Lstiburek Some Intersting Stuff on Slabs 10 of 34
Capillarity + Salt = Osmosis Mineral salts carried in solution by capillary water When water evaporates from a surface the salts left behind form crystals in process called efflorescence When water evaporated beneath a surface the salts crystallize within the pore structure of the material in called subefflorescence The salt crystallization causes expansive forces that can exceed the cohesive strength of the material leading to spalling Joseph Lstiburek 41 Joseph Lstiburek 42 Diffusion + Capillarity + Osmosis = Problem Diffusion Vapor Pressure 3 to 5 psi Capillary Pressure 300 to 500 psi Osmosis Pressure 3,000 to 5,000 psi Joseph Lstiburek 43 Joseph Lstiburek 44 Lstiburek Some Intersting Stuff on Slabs 11 of 34
Joseph Lstiburek 45 Joseph Lstiburek 46 Joseph Lstiburek 47 Joseph Lstiburek 48 Lstiburek Some Intersting Stuff on Slabs 12 of 34
Joseph Lstiburek 49 Joseph Lstiburek 50 Joseph Lstiburek 51 Joseph Lstiburek 52 Lstiburek Some Intersting Stuff on Slabs 13 of 34
Joseph Lstiburek 53 Joseph Lstiburek 54 Sand Layer = Problem Desiccates slab Creates reservoir Prevents drainage Exacerbates capillary wicking Negates vapor barrier Joseph Lstiburek 55 Joseph Lstiburek 56 Lstiburek Some Intersting Stuff on Slabs 14 of 34
Joseph Lstiburek 57 Joseph Lstiburek 58 Joseph Lstiburek 59 Joseph Lstiburek 60 Lstiburek Some Intersting Stuff on Slabs 15 of 34
Joseph Lstiburek 61 Joseph Lstiburek 62 Joseph Lstiburek 63 Joseph Lstiburek 64 Lstiburek Some Intersting Stuff on Slabs 16 of 34
Joseph Lstiburek 65 Joseph Lstiburek 66 Joseph Lstiburek 67 Joseph Lstiburek 68 Lstiburek Some Intersting Stuff on Slabs 17 of 34
Joseph Lstiburek 69 Joseph Lstiburek 70 Joseph Lstiburek 71 Joseph Lstiburek 72 Lstiburek Some Intersting Stuff on Slabs 18 of 34
Joseph Lstiburek 73 Joseph Lstiburek 74 Joseph Lstiburek 75 Joseph Lstiburek 76 Lstiburek Some Intersting Stuff on Slabs 19 of 34
Sand Layer = Never Needed It Control of cracking by soaking up excess mix water (use less water) Control of slab curl by soaking up excess mix water (use less water) Protects poly by reducing punctures (punctures don t matter if air flow doesn t occur - use concrete as air barrier) Allows finishing to occur faster by soaking up excess mix water (use less water) Joseph Lstiburek 77 Joseph Lstiburek 78 Joseph Lstiburek 79 Joseph Lstiburek 80 Lstiburek Some Intersting Stuff on Slabs 20 of 34
Less Water in Concrete, No Sand Layer, Polyethylene Wrapped Around Slab Edge and Polyethylene Capillary Break Under All Plates Continuous Wet Cure For 72 hours -When You Need It You Don t Always Need It - Pick Your Spots Water-to-cement ratio of 0.45 Mid range water reducer (Polyheed, Daracem, Mira) Fly ash is recommended - actually required in many regions for sulfate resistance and corrosion resistance ( Type F up to 30 percent may be used) Good things happen Lower permeability Increased sulfate resistance Reduced shrinkage and cracking Reduced curl Increased corrosion resistance Strength can be used as a surrogate for w/c ratio (4,000 psi) for field verification Necessary for dry windy hot months out west - June through October for CA, AZ, NV Dam the slab - pond water on top of slab Use burlap and keep it wet - can be covered with polyethylene Nothing is better than a continuous wet cure - nothing, nothing, nothing Curing compounds don t work - the good ones were banned Consider also using control joints or polypropylene mesh to control shrinkage cracking To get low water-to-cement ratio, don t just add cement - just adding cement increases shrinkage cracking Use combination of mid-range water reducer, increased cement content and fly ash Fly ash replaces cement and doesn t react immediately so that sufficient water is available to finish slab - finishing slabs becomes very difficult with w/c ratios below 0.45 - the fly ash kicks in later Joseph Lstiburek 81 Joseph Lstiburek 82 Joseph Lstiburek 83 Joseph Lstiburek 84 Lstiburek Some Intersting Stuff on Slabs 21 of 34
Joseph Lstiburek 85 Joseph Lstiburek 86 Joseph Lstiburek 87 Joseph Lstiburek 88 Lstiburek Some Intersting Stuff on Slabs 22 of 34
Joseph Lstiburek 89 Joseph Lstiburek 90 Joseph Lstiburek 91 Joseph Lstiburek 92 Lstiburek Some Intersting Stuff on Slabs 23 of 34
Joseph Lstiburek 93 Joseph Lstiburek 94 Joseph Lstiburek 95 Joseph Lstiburek 96 Lstiburek Some Intersting Stuff on Slabs 24 of 34
Joseph Lstiburek 97 Joseph Lstiburek 98 Joseph Lstiburek 99 Joseph Lstiburek 100 Lstiburek Some Intersting Stuff on Slabs 25 of 34
Joseph Lstiburek 101 Joseph Lstiburek 102 Joseph Lstiburek 103 Joseph Lstiburek 104 Lstiburek Some Intersting Stuff on Slabs 26 of 34
Joseph Lstiburek 105 Joseph Lstiburek 106 Joseph Lstiburek 107 Joseph Lstiburek 108 Lstiburek Some Intersting Stuff on Slabs 27 of 34
Joseph Lstiburek 109 Joseph Lstiburek 110 Joseph Lstiburek 111 Joseph Lstiburek 112 Lstiburek Some Intersting Stuff on Slabs 28 of 34
Joseph Lstiburek 113 Joseph Lstiburek 114 Joseph Lstiburek 115 Joseph Lstiburek 116 Lstiburek Some Intersting Stuff on Slabs 29 of 34
Joseph Lstiburek 117 Joseph Lstiburek 118 Joseph Lstiburek 119 Joseph Lstiburek 120 Lstiburek Some Intersting Stuff on Slabs 30 of 34
Joseph Lstiburek 121 Joseph Lstiburek 122 Joseph Lstiburek 123 Joseph Lstiburek 124 Lstiburek Some Intersting Stuff on Slabs 31 of 34
Joseph Lstiburek 125 Joseph Lstiburek 126 Joseph Lstiburek 127 Joseph Lstiburek 128 Lstiburek Some Intersting Stuff on Slabs 32 of 34
Joseph Lstiburek 129 Joseph Lstiburek 130 Joseph Lstiburek 131 Joseph Lstiburek 132 Lstiburek Some Intersting Stuff on Slabs 33 of 34
Joseph Lstiburek 133 Lstiburek Some Intersting Stuff on Slabs 34 of 34