The plastic shrinkage cracking behaviour of restrained mortar overlays on a concrete substrate was studied with the aim of quantifying the influence of commercially available polypropylene pp. Fogging the air above the concrete and erecting windshades lessen the risk of plastic shrinkage cracking. The procedures used to evaluate cracking in concrete and the principal techniques for the repair of cracks are presented. Possible concrete changes if allowed by specification and approved by the engineer 1. Dissolved salts in the pore water generate osmotic suctions. The most important properties of plastic concrete are workability and. The phenomenon is defined as the shrinkage of young concrete which occurs due to rapid and excessive drying.
When the concrete is first mixed it is like bread dough. Plastic cracking of concrete is primarily attributable to desiccation by evaporation from unprotected surfaces. Cip 5 plastic shrinkage cracking what is plastic shrinkage cracking. Evaluation of plastic shrinkage cracking of selfconsolidating concrete. Plastic cracking occurs when the concrete you have just placed and finished is exposed to warm drying wind, and then develops cracking over the surface of the concrete. Plastic cracks occur before the concrete has hardened. Prevention of plastic settlement cracks in concrete the restraints that cause plastic settlement cracking are inherent in the construction and generally cannot be avoided. Pdf plastic shrinkage cracking in cementitious repairs. It has been reported that if the rate of surface evaporation exceeds about 0. When moist curing is interrupted, the development of strength continues for a short period and then stops after the concrete s internal relative humidity drops to about 80%. It is important to pay attention to changes in weather, especially when casting products outdoors. It is almost impossible to camouflage the plastic settlement cracking, the object being simply to ensure that the durability and the wear characteristics of the surface are not impaired. Also, as the creation of a void underneath the reinforcement mesh is also associated with plastic settlement cracking, any subsequent drying shrinkage may result in cracking through the full depth. These cracks are either irregular in shape and appear as tears in the concrete, or they are.
There are chemical admixtures that may help to reduce the amount of drying. They are usually parallel to each other on the order of 1 to 3 feet apart, relatively shallow, and. It may also lead to a void under the restraining element and where this is reinforcement it may affect the local bond. Plastic shrinkage cracks should be distinguished from other early or prehardening cracks caused by settlement of the concrete around reinforcing bars, formwork movement, early age thermal cracking, or differential settlement at a change from a thin to a deep section of concrete. Jul 31, 2007 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. At this time the concrete is plastic and has little strength. They are usually parallel to each other on the order of 1 to 3 feet apart, relatively shallow, and generally do not intersect the perimeter of the slab.
Surface cracking in plastic concrete can often be avoided through the use of proper mix designs and curing procedures. It is soft and can be worked or moulded into different shapes. The term plastic shrinkage cracking of concrete is self explanatory if it is broken down into its components. Causes, evaluation, and repair of cracks in concrete structures reported by aci committee 224 aci 224. Rapid drying of the surface of the plastic concrete causes it to shrink and crack, but the cracks are not always evident during finishing operations and may not be discovered until the next day. Cracks caused by excessive drying from exposed surfaces plastic shrinkage cracks can form in young concrete, within the first few hours after placing.
Cracks and crack control in concrete structures fritz leonhardt professor emeritus dr. This term refers to cracking which occurs while the concrete is still in a plastic state. Apr 01, 2018 how to form, pour, and stamp a concrete patio slab duration. Plastic shrinkage cracking in concrete influence of test. Plastic cracking of concrete during the hot, dry months of the year it is by no means rare to find cracks appearing on the surface of the concrete within a few hours of its placement. Sep 04, 2015 for example, as plastic settlement cracking may expose areas of the slab reinforcement mesh, the probability of concrete cancer is greatly increased.
Cracking in the overlay is monitored with time and characterized. The concrete surface dries out and shrinks at a time at which it has little strength and hence it cracks. Founded in 1904 and headquartered in farmington hills, michigan, usa, the american concrete institute is a leading authority and resource worldwide for the development, dissemination, and adoption of its consensusbased standards, technical resources, educational programs, and proven expertise for individuals and organizations involved in concrete design. Diagnosing 6 types of concrete cracks plastic shrinkage cracks probably the single most common reason for early cracks in concrete is plastic shrinkage.
Properties of concrete in plastic and hardened state. Plastic shrinkage cracking and bleeding of concrete prepared. Both plastic shrinkage and plastic settlement cracks occur within a few minutes or hours of concrete placing and finishing. Standard test method for evaluating plastic shrinkage. Influence of polypropylene fiber geometry on plastic. How to form, pour, and stamp a concrete patio slab duration. Statistical methods of experimental design and analysis. Hairline cracks may occur in as little as a few hours after a concrete pour, depending on the weather.
Hot, dry air and excessive water in the mix may both cause cracking. Water can move relatively freely and tends to move upwards towards the surface, a phenomenon known as bleeding. This restraint creates tensile stresses in the concrete slab which in turn may produce drying shrinkage cracking. To prevent plastic shrinkage cracking, all concrete operations should be matched to the conditions in which the concrete is being placed. This water takes up space and makes the slab a certain size. While the concrete is shrinking, many things such as reinforcement or the subgrade restrain the concrete.
Plastic shrinkage cracks appear in the surface of fresh concrete soon after it is placed and while it is still plastic. Concrete slab drying shrinkage cracking can be the unfortunate end result of a combination of factors related to material choices, construction planning, placement. During this research, influence of coarse aggregate content on plastic shrinkage cracking of self compacting concrete scc was studied. Cracking in plastic concrete cracks that form in plastic concrete can be categorised as either plastic shrinkage cracking or plastic settlement cracking.
Plastic settlement cracks are socalled because they form while the concrete is still plastic, ie has not set. Plastic shrinkage cracks are socalled because they form while the concrete is still plastic, ie has not set. The causes of nonstructural cracking nonstructural cracking in concrete may be caused by any one or a combination of. One of the frequent occurrences of cracking of concrete is due to plastic shrinkage of plastic settlement. Cracks in concrete is it a design issue or just poor. Plastic shrinkage cracks may extend the full depth of elevated structural slabs. If the settlement of solids in the concrete can freely take place without hindrance there will be a reduction in depth and volume of the cast concrete but no cracking. Plastic shrinkage cracking of polypropylene fiberreinforced. Use the following approaches to help minimize or control this cracking. Plastic settlement cracks arise due to water migration to concrete exterior surface, causing decreases in volume and, consequently, settlement of the fresh concrete.
The cracking occurs when the concrete surface dries and shrinks so fast, that the induced tensile strains exceed the strain capacity of the very young concrete. Fresh concrete when placed in deep formwork such as that of column or wall has a tendency to settle or subside. After 22 the placement of the concrete, all solid particles are normally covered by a plane water film the. Plastic shrinkage cracking of polypropylene fiberreinforced concrete slabs parviz soroushian, fa1z mirza, and abdulrahman alhozaimy the effects of polypropylene fibers and construction operations on the plastic shrinkage cracking of concrete slabs were investi gated. Plastic shrinkage cracks in concrete and its prevention. Not all plants will experience problems with plastic shrinkage cracking, but all should know how to prevent it. Plastic shrinkage an overview sciencedirect topics.
Types of concrete cracks plastic shrinkage cracking cap. Types of concrete cracks plastic settlement cracking. Such cracks usually appear from one to six hours after concrete placement. The phenomenon is most common in hot, dry climates and can usually be attributed to settlement of the mix in the form or overrapid drying. Both of these types result from the bleeding and segregation process that occurs when fresh concrete is placed. The critical condition exists when the rate of evapora tion of surface moisture exceeds the rate at which ris ing bleed water can replace it.
Cracking due to plastic and autogenous shrinkageinvestigation of early age deformation of selfcompacting concreteexperimental study. Results indicate that while polypropylene fibers in general are effective in controlling plastic shrinkage cracking in concrete, a finer fiber is more effective than a coarser one, and a longer fiber is more effective than a shorter one. However, should the crack penetrate the full depth of the member water penetration problems may arise, or if the cracking is located in wet areas, future. If this settlement is restrained due to obstructions like steel bars or large aggregates it causes short horizontal cracks and the subsidence due to reduction in volume is termed as plastic settlement cracks. This causes high matric suctions to develop in the pore water in the voids adjacent to these surfaces. Surface cracking in plastic conc rete can often be avoided through the use of proper mix designs and curing procedures. Plastic shrinkage cracking occurs most commonly in concrete with a large surface area such as in floors and pavements. Consulting engineer stuttgart, frg the material presented in this paper is based on more than 30 years of research, observations and experience concerning causes, control, and consequences of cracking in concrete structures. Plastic shrinkage cracking in concrete barzin mobasher asuadot pavement materials conference, 2011. For example, as plastic settlement cracking may expose areas of the slab reinforcement mesh, the probability of concrete cancer is greatly increased.
To reduce plastic shrinkage cracking it is important to recognize ahead of time, before placement. The cracks may extend right through the depth of the slab. Plastic shrinkage cracking, capillary pressure, astm c 1579, nordtestmethod, coarse aggregate content. Fogging the air above the concrete and erecting windshades lessen the risk of plasticshrinkage cracking. Also, rapid drying of the slab will significantly increase the possibility of cracking. Capillary pressure monitoring in plastic concrete for. Plastic shrinkage cracking in cementitious repairs and overlays. The chemical reaction, which causes concrete to go from the liquid or plastic state to a solid state, requires water. Cip 1 00 concrete in practice cip 1 dusting concrete surfaces cip 2 scaling concrete surfaces cip 3 crazing concrete surfaces cip 4 cracking concrete surfaces cip 5 plastic shrinkage cracking cip 6 joints in concrete slabs on grade cip 7 cracks in concrete basement walls cip 8 discrepancies in yield cip 9 low concrete cylinder strength. Surface crusting is the most severe condition, resulting in brittle crusts that can be as thick as. Abrupt changes in section depth could be avoided at the design detailing stage but the main reduction of risk is through mix design and suitable cohesion of the concrete to. It mainly occurs in concrete members with large surface to volume ratio such as pavements and slabs. Preventing plastic surface cracking concrete construction. Plastic shrinkage may cause early age cracking on the young concrete surface before the setting.
Preventing plastic shrinkage cracking to prevent plastic shrinkage cracking, all concrete operations should be matched to the conditions in which the concrete is being placed. Plastic settlement cracks appear in freshly laidplaced cement concrete in reinforced structure. Astm c1579 standard test method for evaluating plastic. Evaluating cracking in concrete bluey technologies. What is plastic shrinkage cracking plastic shrinkage cracks appear in the surface of fresh concrete soon after it is placed and while it is still plastic. Most of this excess water begins to exit the concrete during placementcuring and causes the concrete to shrink. Plastic shrinkage cracking of selfcompacting concrete. As mentioned at the outset of this note, some forms of concrete cracks are inevitable. Unlike the others, these cracks, formed in the plastic state of concrete, cannot be controlled by provision of rebars at all, and can only be mitigated by workmanship on site. Causes of concrete cracks concrete repair specialists. It usually takes the form of wide diagonal cracks at intervals of 0. Internal restraint of the surface plastic shrinkage can thus cause cracking of concrete surfaces between the time of placement and the final setting of. In figure 1, the process of 21 capillary pressure buildup which may eventually lead to cracking is shown schematically. Mar 03, 2016 generally if plastic shrinkage cracking is less than 1mm in width, it will not require rectification as plastic shrinkage cracking rarely impairs on the strength of a concrete element.
This can lead to plastic shrinkage cracking if the rate of evaporation is higher than that of the bleeding water rising to the surface of the concrete. The plastic shrinkage cracks will appear in the first few hours of the slabs life. In these circumstances, the immediate reaction is to declaim the quality of cement, though the prime reason for the cracking could actually be found to lie elsewhere. Plastic shrinkage cracks are caused by a rapid loss of water from the surface of concrete before it has set. Mechanism of plastic settlement cracks in concrete. The plastic shrinkage cracking tendency was evaluated, using a modified nordtest method nt build 433, where the concrete sample was exposed to early drying conditions and where the restraining. When the concrete is still in its plastic state before hardening, it is full of water. In hardened concrete after the concrete has hardened, plastic settlement cracks may be chased out and filled using a suitable proprietary material. Plastic shrinkage cracking in concrete is mainly a physical process, in which chemical reactions between cement and water do not play a decisive role. Plastic shrinkage cracking in concrete barzin mobasher. Types of concrete cracks plastic settlement cracking cap.
Cip 4 cracking concrete surfaces cip 5 plastic shrinkage cracking cip 6 joints in concrete slabs on grade cip 7 cracks in concrete basement walls cip 8 discrepancies in yield cip 9 low concrete cylinder strength cip 10 strength of inplace concrete cip 11 curing inplace concrete. They are caused by rapid loss of water during curing or settlement in the concrete itself pca 2001, p. As plastic shrinkage cracking can dramatically reduce the durability of a concrete member and causes considerable repair costs annually, a comprehensive understanding of the mechanism of the. Keep the concrete sufficiently moist after set to lower early drying shrinkage c minimize thermal cracking due to temperature gradients i.
Technical report, chalmers university of technology, sweden, 2005. Mike day everything about concrete 1,059,524 views. Cracks repair in reinforced concrete structures case study. They are usually parallel to each other on the order of 1 to 3 feet apart, relatively shallow, and gen. When weather conditions may occur that are conducive to plastic shrinkage cracking.
Cracking caused by plastic shrinkage in concrete occurs most commonly on the exposed surfaces of freshly placed floors and slabs or other elements with large surface areas when they are subjected to a very rapid loss of moisture caused by low humidity and wind or high temperature or both. Expansive reactions steel reinforcement corrosion, alkalis or sulphate attack cracking. Since cracking because of plastic shrinkage is due to a differential volume change in the plastic concrete, successful control measures require a reduction in the relative volume change between the surface and other portions of the concrete. Cip 5 2014 national ready mixed concrete association. The plastic settlement cracks also occur on the surface before the concrete has set, when there is the relatively high amount of bleeding. Plastic shrinkage occurs in a freshly mixed concrete, with loss of water by evaporation from its surface, after placing and before hardening of the concrete. This chemical reaction, or hydration, continues to occur for days and weeks after you pour the concrete.
Plastic shrinkage cracks plastic shrinkage cracks are caused by a rapid loss of water from the surface of concrete before it. Plastic shrinkage cracking generally occurs on the exposed concrete surfaces which, due to more rapid drying, undergo greater plastic shrinkage movements when compared with the body of concrete. Plastic shrinkage cracking is more likely to occur when high evaporation rates cause the concrete surface to dry out before it has set. Firstly, plastic shrinkage cracking occurs in concrete, where concrete is a.
The study of plastic shrinkage cracking is complicated because the material properties that determine. This paper deals with various aspect of these types of cracking. Plastic shrinkage cracks occur when wind velocity, low relative humidity, high ambient temperature, or a combination of all three, cause water to evaporate from a concrete surface faster than it can be replaced by bleeding to the surface. Many factors affect plastic shrinkage cracking, in particular the evaporation of water from the surface of freshly placed concrete.