By Kim Basham, Contributing Author
Plastic-shrinkage and drying-shrinkage cracks are the most common types of random cracking that occur in concrete roads and bridge decks. Each is related to moisture loss. Plastic-shrinkage cracking occurs when the concrete is still plastic, and dry-shrinkage cracking occurs after concrete hardening.
Concrete shrinks due to moisture loss in the plastic and hardened states. Tensile stresses form because of internal or external restraints, and when the tensile stresses exceed the tensile capacity of the concrete, cracking occurs.
Understanding the root causes is the first step to developing best practices to minimize the risk for these types of cracks.
Random surface cracking that occurs prior to concrete setting is caused by premature and excessive moisture loss from the surface of the freshly placed concrete. When the evaporation rate from the surface exceeds the bleed rate of the concrete, the volume of concrete along the surface shrinks.
As surface moisture evaporates, a complex series of menisci form creating negative capillary pressures that cause the volume of the surface paste to contract. Restrained tensile stresses form because of the moist concrete beneath the shrinking surface.
When the stresses exceed the tensile capacity of the plastic concrete, surface tears or random cracking occurs.
What is Bleeding?
Bleeding is the upward flow of mixing water within the freshly placed concrete caused by the settlement of the heavier particles including the cementitious materials, fine and coarse aggregates. Bleeding of concrete is characterized by the capacity and rate. Capacity is the total volume of water released, and rate is the volume of water released per time interval per surface area. The bleeding period also is an important property and dependent on many factors, including bleed capacity, bleed rate and setting characteristic of the concrete. In general, bleeding properties are a function of factors that include the amount of mix water, type and fineness of the cementitious materials, sand gradation and chemical admixtures (especially air entrainment and retardation). Bleeding properties set the characteristics of the concrete, such as the temperature of the placed concrete, depth of the section and suction of water by the subbase or formwork.
In general, the following reduce bleeding: mixes with lower water contents, entrained air, ultra-fines content of the concrete (i.e., fine cements, supplementary cementitious materials including fly ash, silica fume and fine sands), high placing temperatures and accelerators. Depending on the fineness, slag cements may increase or decrease bleeding. A recent ACI/ASCC survey shows Type IL Cements can significantly reduce bleeding due to the fineness of the cement, which increases the risk for plastic shrinkage cracking.
Plastic-shrinkage cracking occurs when the evaporation rate from the surface exceeds the bleed rate of the concrete. Therefore, the critical factors for determining the risk of plastic-shrinkage cracking are bleed rate of the plastic concrete and evaporation rate from the surface created by the ambient conditions.
Ambient conditions (i.e., air temperature, relative humidity, concrete temperature and wind velocity) can have an impact on the potential rate of evaporation. Wind velocity is the key factor affecting evaporation rates. An increase in the wind velocity increases the potential evaporation rate.
Plastic-shrinkage cracking is associated with hot weather concreting, but plastic-shrinkage cracking can occur during cold weather concreting too — or whenever the surface evaporation rate exceeds the bleed rate of the concrete.
This can occur during cold weather placements, especially when warm concrete meets cold temperatures, and steam rises from the surface.
Specifications require contractors to take precautions when the evaporation rate exceeds 0.20 lb/sqft/hr. This limiting value assumes that common concrete bleed rates are 0.20 lb/sqft/hr or more.
However, this is no longer correct, especially for air-entrained concrete where bleed rates are less than 0.20 lb/sqft/hr. In general, contractors should take precautions when the potential evaporation rate approaches or exceeds about 0.10 (or at the most 0.15) lb/sqft/hr.
To reduce the risk of plastic-shrinkage cracking, consider the following best practices: lower the concrete temperature with chilled water or ice, add microfibers to the concrete to increase the tensile capacity of plastic concrete, dampen the subgrade and forms to minimize mix water loss, erect temporary wind breaks, fog the concrete after placement and between finishing operations until final curing occurs to slow the rate of surface evaporation.
Other best practices include the temporary application of white plastic sheeting and the application of a spray-on evaporation retarder after placing and between finishing operations until the final cure is applied.
Use caution with accumulated water on the surface of the concrete from fogging. Similar to bleedwater and rainwater, do not finish accumulated fog water into the surface of the concrete.
Finishing water into the surface will raise the water to cementitious materials (w/cm) ratio, weakening the surface and increasing the risk of premature wear and deterioration, especially surface scaling associated with winter conditions.
Overworking a wet surface can damage the air-void system of the entrained air reducing the concrete freeze/thaw resistance.
Do not use a spray-on evaporation retarder as a finishing aid. Since most evaporation retarders are mixed with water, finishing an evaporation retarder into the surface (to restore paste mobility) is the same as finishing accumulated fog water into the surface.
Drying-shrinkage Cracks
Random cracking after hardening is caused by drying shrinkage of the concrete combined with concrete restraints.
Drying shrinkage is the reduction of concrete volume caused by the chemical and physical loss of water during the hardening process and subsequent drying of the concrete. This is sometimes referred to as long-term drying shrinkage cracks because it takes days and weeks before drying shrinkage cracks appear or become noticeable.
With regards to concrete shrinkage, a 100-foot long, unrestrained concrete slab will shrink or shorten from about .5 to .75 of an inch. The slab that is free to move relative to the base will simply shorten about .5 to .75 of an inch but the slab restrained by the base will crack due to the tensile stresses created by the concrete shrinkage and the base (or other) restraints.
Pavements are restrained by the base (granular material) and bridge decks are restrained by the substructure (beams, girders). For roadways and bridge decks, the concrete is restrained against shortening so tensile stresses form due to concrete shrinkage and restraints.
Contraction Joints and Reinforcing
For roadways, sawcut contraction joints control the location of the cracks by creating a weakened section so concrete cracks at the predetermined joint locations. For bridge decks, reinforcement controls the crack widths – not the location of the cracks.
Contraction joints control the location of cracks and reinforcing controls the width of the cracks. If joint spacing, sawcut depth and timing are correct, shrinkage cracks occur in the contraction joint (weakened or thinner concrete sections).
Timing and depth is critical so that the weakened sections are installed prior to the tensile stresses forming and exceeding the tensile capacity.
For bridge decks, the characteristics of the cracks are a function of the bridge geometry, restraints, shrinkage potential of the concrete and reinforcing details including rebar sizes, spacings and concrete cover.
In recent years, designers have specified maximum 28-day concrete shrinkages as determined by ASTM C 157 and strict curing requirements to minimize the amount and widths of cracks. ASTM C 157 is the most common laboratory method to establish the shrinkage potential of a concrete mix.
In general, concrete shrinkage can be reduced by using the minimum cementitious materials and water contents by incorporating water-reducing admixtures and optimizing the aggregate gradations. But there is a practical limit due to the availability of local materials. Other means to reduce the drying-shrinkage potential include adding fibers, shrinkage-reducing (SRA) and shrinkage-compensating (SCA) admixtures.
Proper curing also can reduce plastic shrinkage and drying-shrinkage cracking of concrete roads and bridge decks. Initial and intermediate curing protects the freshly placed concrete from premature and rapid moisture loss, which minimizes the risk of plastic-shrinkage cracking.
Initial curing starts when the concrete is discharged from the mixer. Intermediate curing takes place between initial and final curing. Final curing delays the onset of drying shrinkage so the concrete gains more tensile capacity to resist tensile stresses associated with shrinkage and restraints.
It takes a team effort involving the designer, specifier, concrete producer and contractor to minimize plastic shrinkage and drying-shrinkage cracks in concrete roads and bridge decks.
Minimizing cracks increases the durability and service life of the concrete. Best practices need to be a priority for all concrete projects. RB
Kim Basham, PhD, PE, FACI is the president and senior structural engineer at KB Engineering.
