Preventing Plastic Surface Cracking
Some Midwest ready-mix producers report there have been more problems associated with plastic shrinkage in concrete this past spring. Dave Polosa, owner of Polosa Cement Contracting, Tinley Park, Ill., says that he has experienced shrinkage on his jobsites eight times so far this season. His company installs decorative concrete primarily in residential settings with most of his work involving applications of color hardeners or integrally colored concrete slabs. He notes that jobsites with southwestern exposures are the most prone to problems because that's the direction that most winds come from in his area.
The ACI nomograph was first developed by Delmar Bloem in July 1960 and published in a paper for the National Ready Mixed Concrete Association/National Sand and Gravel Association. The nomograph was based on a more complicated table developed by Carl Menzel in 1954, with some formulas about evaporation dating back to 1802. It was used to interpret the amount of evaporation taking place on water surfaces under different weather conditions. The surface type didn't matter; it could be on a lake, dirt, wood, fresh concrete, or even hardened concrete—wherever water vapor passed through a surface. But in its present form the nomograph is used to help predict if weather conditions can cause plastic cracking in freshly placed concrete.
Small portable weather instruments, such as the one shown here made by Kestrel, make it possible to know wind, relativity humidity, and ambient temperature conditions on your jobsite. Knowing this, you can predict whether plastic cracking will be a problem.
Credit: Joe Nasvik
To use the graph several pieces of information are needed: relative humidity, ambient temperature, the temperature of the concrete, and wind velocity. However, getting good jobsite weather information isn't that easy. Today, contractors often get their weather information via the Internet. In a metro area, that information might be taken from a location near the jobsite or from an airport located miles away. That information may also be up to the minute or recorded several hours ago. Furthermore, if you happen to be placing concrete several stories above ground on a deck, the wind speeds in particular might be much higher than those on the ground.
If you knew ahead of time that job-site weather conditions could cause surface cracking problems, there would be the opportunity to cancel a placement for that day or at least take steps to minimize the problem.
In addition to weather conditions, wide temperature differentials between the ground, concrete, and surface concrete temperatures can cause surface crusting and cracking problems. Under the right conditions the top surface of a slab dries out or sets faster than the matrix of the slab. Concrete with color added—particularly darker colors—is particularly susceptible to this condition.
Plastic shrinkage cracking
“Plastic cracking,” “surface cracking,” and “surface crusting” are all terms that describe the same phenomena. ACI 116 defines plastic shrinkage cracking as “cracking that occurs in the surface of fresh concrete soon after it is placed and while it is still plastic.” Surface crusting is the most severe condition, resulting in brittle crusts that can be as thick as ½ inch. Plastic shrinkage results when weather conditions cause water to evaporate rapidly from the surface of freshly placed concrete. The water evaporates faster than it can be replaced by bleed water making its way to the concrete's surface. The bleeding rate of most concrete mixes is usually less than 0.2 pounds/square foot/hour. Plastic shrinkage occurs when water evaporates from the surface at a higher rate than the bleeding rate. Some concrete mixes, such as those containing pozzolans, are more susceptible to surface cracking because their bleeding rates are slower. After initial set occurs, the bleed rate nearly stops so curing membranes are needed to ensure that water is available at the surface for good hydration.
Jobsite weather instruments
Information about jobsite weather conditions that cause plastic shrinking can be gathered using small portable weather instruments. With these tools project mangers and foremen can make decisions even before concrete arrives. Christy Munding, assistant brand manger for Kestrel, Boothwyn, Penn., says her company realized the need to market their handheld weather instruments to the construction industry as a result of data collected by Luke Snell, director of concrete industrial management at Arizona State University, Tempe, Ariz. Weighing only a few ounces, the instruments easily fit into a shirt pocket. Depending on the model purchased, a wide range of information can be obtained including wind speed, relative humidity, ambient temperature, barometric pressure, and elevation. ACI 308 prescribes how to gather the information. For instance, wind speed and relative humidity should be read 20 inches above the slab after concrete is placed on the windward side of the fresh concrete.
For predictive purposes, Kestrel provides a computer software disk with information and formulas provided by Snell. Users simply provide the information asked and the program will tell you if conditions are present that can cause surface shrinkage. You also can use the nomograph to plot the information.
Infrared (IR) thermometers are small and portable, and take instant temperature readings of what you point at and “shoot.” Most of them include lasers to help locate the area you selected. Increasingly, these instruments are being used by testing companies, ready-mix producers, and contractors. One advantage they have is that it's easy to record surface temperatures, including ground and concrete. They work by measuring the infrared radiation that all objects emit. The intensity of the radiation depends on the temperature of the object's surface. Hotter objects emit more IR energy than colder ones. But the reading on the instrument also is dependent on the emissivity of the object's surface. This measurement is defined as how much less than perfect a material radiates when compared to a black body. More expensive IR thermometers provide adjustments for emissivity. Lesser expensive units have fixed ratings, most being in the 0.95 range. Be aware of the emissivity rating of the instrument that you buy. The emissivity number for concrete is 0.95, making possible the purchase of lower priced versions.
Temperature readings of objects and materials in the sunlight are different than when they are shaded because of the radiation energy from the sun. More accurate temperatures of the material itself are taken when in the shade.
When conditions aren't right
When work involves placing concrete, bull floating, and adding a broom finish, the risk due to plastic shrinkage is minimal because little time is required to place and finish. There also is little manipulation to the surface. But be sure to place curing membranes quickly when evaporation rates are high. The risk for slabs that receive trowel finishes—either hand or machine applied—is much higher because the surface can be distorted under the weight of manpower and machines. It also can lead to very risky outcomes when a decorative stamped finish is desired. The worst condition of all for a decorative concrete contractor happens when they try to stamp impressions on surface crusted concrete.
There are several things you can do when jobsite weather information points toward rapid surface drying conditions. The key is to have enough lead time to make decisions and to adequately prepare. When evaporation rates are excessively high, or above 0.2 pounds/square foot/hour, the safest decision is to cancel plans to place concrete. If you decide to place concrete under less than desirable conditions, several things make a difference, including using evaporative retarders, windscreens, fogging the area of the placement, and using accelerating admixtures. The most common method for controlling surface evaporation is to spray evaporative retarders on the concrete just after it is struck off and immediately after each finishing operation.