Plastic Shrinkage Cracking How to make sure it happens!

Causing Plastic Shrinkage Cracking  Specifications  Concrete Mix Design  Concrete Delivery  Placing the Concrete  Finishing the Concrete  Curing the Concrete

Specifications That Guarantee* Plastic Shrinkage Cracking  Focus only on strength, and ignore all other characteristics of the mix;  Require a minimum cement content, and a maximum water/cementitious ratio;  Require or allow a “2 bin” mix to be used;  Require only that an ASTM C33 sand and a size 57 or 67 coarse aggregate be used; (* = In real life you want to do the opposite!)

Specifications That Guarantee* Plastic Shrinkage Cracking  DO NOT EVEN MENTION EVAPORATION POTENTIAL OR ITS LIMITS  If “sufficiently foolish” to mention evaporation potential, then make sure it is measured only once sometime before the actual placement. (* = In real life you want to do the opposite!)

Effects of Mix Design on Plastic Shrinkage  In order to insure* plastic shrinkage cracking, make sure there is no bleed water;  Bleed water is a direct function of total surface area in the mix: More surface area →Less bleed water Less surface area → More bleed water  Gap graded mixes have more surface area. (* = In real life you want to do the opposite!)

Plastic Shrinkage Cracking 1.) Bleed water appears on concrete surface 2.) Rate of evaporation exceeds bleed rate 3.) Concrete surface dries…. 5.) Moist concrete resists shrinkage…. 4.) Concrete surface tries to shrink…. 6.) Stress develops in soft “plastic” concrete…. 7.) “Plastic” shrinkage cracks form

Grading Influences:  Total Surface Area of Mix Components

Equivalent Volume

Surface Area 290 in in 2

Surface Area of Cement A single pound of typical Type I-II cement has an approximate surface area of: 274,500 square inches 1,900 square feet

Surface Area of Cement The cement in a cubic yard of 6 ½ sack concrete has a total surface area of: 167,545,976 square inches 1,163,513 square feet 26.8 acres

Summary on Surface Area More surface area requires more paste to coat each particle; Larger particles have less surface area than smaller particles; The best way to maximize aggregate volume and minimize surface area is to use a uniform aggregate gradation – Make sure you use gap graded mixes* Too much cement is as harmful as too little! (* = In real life you want to do just the opposite!)

Impacts from Environmental Factors Concrete Mix Temperature Ambient Air Temperature Wind Speed Relative Humidity All of this is tied together in the standard Evaporation Potential Nomograph::

Effects of Air Temp on Evaporation Potential  The relationship of air temperature (Ta) to Evaporation Potential is: E ~ (T c 2.5 -rT a 2.5 ) x 10 6 Air Temperature Evaporation Potential

Effects of Relative Humidity on Evaporation Potential  The relationship of relative humidity to Evaporation Potential is: E ~ (T c 2.5 -rT a 2.5 ) x 10 6 Relative Humidity Evaporation Potential

Effects of Windspeed on Evaporation Potential  The relationship of wind speed to Evaporation Potential is: E ~ (1+0.4V) x 10 6 Wind speed Evaporation Potential

Evaporation Retarders  EVAPORATION RETARDERS ARE NOT CURING COMPOUNDS  Evaporation retardiers are applied to the surface of the freshly “struck off” concrete to prevent the excessive loss of bleed water while the concrete is initially setting;

Evaporation Retarders  Create a mono-molecular film essentially the same as a large water-proof soap bubble laying on the surface of the concrete;  The film is extremely fragile and will be completely destroyed if touched lightly by any finishing tools  It should remain untouched until the surface is ready for the final floating and finishing.

Factors to insure* Plastic Shrinkage Cracking  Mix Design Use smaller top sized aggregate to make it easier to finish the mix; Use gap graded “2 bin” mix; DO NOT use uniform gradation; Use extra cement to make sure compressive strength is achieved far earlier than specified; Oversand the mix to make the finishers happy. Make sure sand is extra dirty. (* = In real life do you want to do the opposite!)

Factors to insure* Plastic Shrinkage Cracking  Environmental Factors Make sure that concrete is placed under windy, dry conditions; Do not consider evaporation potential; Do not erect or consider use of wind breaks; Place concrete at convenient time of day, regardless of weather or wind; (* = In real life you want to do the opposite!)

Factors to insure* Plastic Shrinkage Cracking  Environmental Factors (continued) Do not use evaporation retarder. If evaporation retarder is used, make sure finishers get onto slab immediately after placement so film is destroyed; (* = In real life you want to do the opposite!)

CURING  A good curing system makes sure that the evaporation potential at the surface of the concrete is 0.0;  In normal climates, a single curing system is adequate;  In New Mexico, multiple curing systems should be considered mandatory.

Different Curing Systems  In order of most effective to least effective; Flooding with water; Covering with saturated burlap or burleen (and making sure burlap does not dry out); Covering with visqueen; Double coat of curing compound; Single Coat of curing compound.

To Insure* Plastic Shrinkage Cracking from Lack of Curing  Do not use any curing system;  If curing system is used, use only one method, and apply that as late as possible, just before leaving project for the evening;  If burlap is used, make sure it is dry or becomes dry soon, so moisture is actually wicked from the concrete; (* =In real life you want to do the opposite!)

To Insure* Plastic Shrinkage Cracking from Lack of Curing  Do not consider a multiple system, such as: Curing compound applied immediately after final finish; Applying a second coat of curing compound in a direction perpendicular to that of first coat; Covering curing compound with saturated burlap; Covering saturated burlap with visqueen, weighted down around edges. * = In real life you want to do the opposite!