Plastic Shrinkage and Cracks in Concrete

Er Balkrishna Kulkarni, Technical Manager – Technical Customer Solutions, UltraTech Cement Ltd, writes on Plastic Shrinkage and Cracking of Concrete, and the measures that can be taken to nullify the phenomena.

Plastic shrinkage cracking occurs when fresh concrete is subjected to a very rapid loss of moisture. It is caused by a combination of factors such as air and concrete temperature, relative humidity, and wind velocity at the surface of concrete. These can cause high rates of evaporation in either hot or cold weather. The plastic shrinkage cracks are due to a differential volume change in the plastic concrete. Therefore, the control measures require reduction in volume change between the surface measures and other portions of the concrete. Plastic shrinkage cracks can be deep; the width of a crack can vary from 0.1 to 0.3 mm.


Concrete cracks under tensile stresses; non-structural plastic cracks occur very often primarily due to a combined effect of restraint and contraction characteristics of concrete. Workmanship during placing and finishing concrete plays a prominent role in the phenomena. Concrete of different workability levels need to be treated differently during placing and vibration to achieve proper compaction level and homogeneity.


Early Age Shrinkage
Early-age shrinkage can develop strains in concrete of similar magnitude to those resulting from drying shrinkage. These develop at a time when the concrete has a very little tensile strength and hence cracking is likely. Similar cracking is also known as ‘plastic cracking’. The fact that cracking sometimes occurs at early ages in very benign environments (eg indoors where drying conditions and moisture loss are not evident and plastic cracking therefore unlikely) is evidence of the existence of early-age shrinkage.

Concrete maintains almost semi-fluid properties for few hours. Hydration reactions are very slow during the first 3–4 hours of a concrete’s life but accelerate over the following 8–12 hours when the concrete changes from a semi-fluid state into a more rigid one capable of cracking. The time period depends on many variables including the constituents of the concrete, the mix proportions and weather conditions.

In freshly consolidated concrete, water in the spaces between the cement and the aggregate particles exert a positive pore pressure on the mix. As the surface layer of the concrete dries out, this situation changes and the concrete moves from a saturated to a partially saturated state. This causes capillary tension in the pore water which creates a capillary suction that draws the solid particles closer together. Where the concrete is restrained from contracting in this manner, the shrinkage causes stresses to develop in the external layers. Where this stress exceeds the tensile capacity of the concrete, it inevitably cracks. The difference in saturation levels throughout the depth of the concrete provides more than adequate internal restraint to cause these surface or near-surface tension stresses to develop. It is principally this internal restraint that leads to early-age shrinkage cracking. This internal restraint mechanism supplemented by external restraint from formwork, subgrade friction, member geometry, etc can lead to early cracking. Early-age shrinkage cracks may be obvious immediately or become apparent at a later stage.

Immediate Cracking – This can be evident within a few hours due to the internal and external restraints producing stresses in the upper layers of the concrete that exceed the tensile capacity of the concrete.

Later Cracking – Early age shrinkage effects can leave a ‘weakness’ in the concrete that will provide a strain release point or crack propagation location in the concrete when drying conditions occur.

Why Plastic Shrinkage Cracks Occur
Plastic shrinkage cracks are caused by a rapid loss of water from the surface of concrete before it has set. The critical condition exists when the rate of evaporation of surface moisture exceeds the rate at which rising bleed water can replace it. Water receding below the concrete surface forms menisci between the fine particles of cement and aggregate causing a tensile force to develop in the surface layers.

If the concrete surface has started to set and has developed sufficient tensile strength to resist the tensile forces, cracks do not form. If the surface dries very rapidly, the concrete may still be plastic, and cracks do not develop at that time; but plastic cracks will surely form as soon as the concrete stiffens a little more.

Conditions that cause high evaporation rates from the concrete surface, and thereby increase the possibility of plastic shrinkage cracking, include:

• Wind velocity in excess of 5 mph
• Low relative humidity
• High ambient and/or concrete temperatures

Small changes in any one of these factors can significantly change the rate of evaporation. ACI 305-R-102 provides a chart to estimate the rate of evaporation and indicates when special precautions might be required. However, the chart isn’t infallible because many factors other than rate of evaporation are involved.

As a general rule, if the rate of evaporation is greater than 1 kg i.e. 1 lit of water per square metre of concrete per hour (1Kg/m2/Hr), then precautions against premature drying and plastic shrinkage cracking should be taken. Alternatively, the rate of evaporation can be calculated from the following equation [ACI 305R-102]:

E = 5([T + 18]2.5 – r [T + 18]2.5)(V + 4) x 10-6

Where:

E = evaporation rate, kg/m2/h r = relative humidity/100

Ta = air temperature, °C

Tc = concrete (water surface) temperature, °C

V = wind velocity, Km/Hr

Concrete mixtures with an inherent reduced rate of bleeding or quantity of bleed water are susceptible to plastic shrinkage cracking even when evaporation rates are low. Factors that reduce the rate or quantity of bleeding include high cementitious materials content, high fines content, reduced water content, entrained air, high concrete temperature, and thinner sections. Concrete con- training silica fume requires particular attention to avoid surface drying during placement.

Plastic shrinkage cracking in hot weather conditions accelerate the evaporation of moisture from the surface. If the rate of evaporation is greater than the rate of bleeding (rate at which water rises to the surface), surface drying will occur, resulting in shrinkage of the concrete. When the shrinkage stresses exceed the tensile capacity of the concrete, cracking will occur. The likelihood of plastic shrinkage cracking is therefore greater whenever hot weather conditions increase evaporation or the concrete has a reduced bleeding rate. Plastic shrinkage cracks can be quite deep, as the plastic concrete has little capacity to resist shrinkage stresses, and cracks continue to widen and propagate until the shrinkage stresses are relieved. (Note: plastic shrinkage cracks generally do not extend to free edges as unrestrained contraction of the concrete is usually possible at these locations).

Codal provisions for Concrete Temperature during Placing

• According to BIS 7861:Part 1 Hot Weather Concreting - Any operation of concreting done at atmospheric temperatures above 40°C or any operation of concreting ( other than steam curing) where the temperature of concrete at time of its placement is expected to be beyond 40°C.
• AS 1379 places a 35°C limit on the maximum concrete temperature at the time of delivery.
• ACI 301-16 “Specifications for Structural Concrete” and ACI 305.1-14 “Specification for Hot Weather Concreting” limit the maximum concrete temperature to 95 °F (35 ºC) at the time of discharge. This limit is for general types of hot weather construction such as pavements, bridges, and buildings, not mass concrete.
• According to ACI 305.1-14, qualification of a concrete mixture with a maximum concrete temperature that exceeds 95°F (35 ºC) shall be supported by successful past field experience or preconstruction testing. The request for a maximum concrete temperature that exceeds 95°F (35 ºC) shall be submitted to the architect/engineer prior to concrete placement.
• According to Concrete Manual, 8th Edition, US Department of The Interior Bureau of Reclamation, Concrete Temperature while deposition is 80°F for concrete to be placed in hot arid climates & 90°F for most other concretes.
• As per MoRTH, total time taken from the addition of the water to the mix, until the completion of the surface finishing and texturing shall not exceed 120 minutes, when concrete temperature is less than 25°C and 90 minutes when the concrete temperature is between 25°C and 30°C.

Various codes limit on the maximum concrete temperature at the time of delivery, however, when the air temperature rises above 30°C, it is usually recommended that precautions be taken, particularly if there is also hot dry wind. This is firstly to ensure an acceptable concrete temperature at the point of delivery, and secondly to avoid problems with plastic shrinkage cracking and premature stiffening of the concrete.


Other influencing factors on early-age shrinkage & cracking
The following factors have a significant impact on the extent and magnitude of early-age shrinkage and associated cracking.

• Restraint Against Movement
  Restraint against movement is one of the prime reasons for cracking. Restraint does not change the early age shrinkage per se but increases the tensile stress on the concrete and the likelihood of the concrete cracking. The greater the restraint, the higher the risk of cracking. All concrete is subject to some form of external restraint and some level of internal restraint as it dries out.


• Compaction
  Well-compacted concrete limits the amount of contraction that is possible by eliminating voids and reducing the early-age shrinkage and potential for cracking. Good compaction increases the tensile strength of the concrete and hence its ability to resist tensile stress without cracking. Voids near the surface of the concrete are of particular concern as they significantly reduce the tensile capacity of the concrete in that location and can provide sufficient weakness for crack formation.


• Bleeding
  Inadequate bleed water on the surface of concrete will increase the early-age shrinkage and the propensity of the concrete to crack. The rate at which concrete bleeds is more important than the total amount of bleed water in predicting whether or not the concrete will dry, shrink and crack at an early age. While bleed water remains on the surface of the concrete there can be no suction created in the capillaries of the concrete and no danger of early-age shrinkage and cracking.


• Surface Drying
  Surfaces that dry in the first few hours will exhibit higher early-age shrinkage and are more prone to cracking. Early-age shrinkage will lead to cracking only if the surface of the concrete is allowed to dry in the early hours of its life. This is of concern in all environmental conditions and not merely in hot or drying weather.


• Very Early Protection
  Protection of concrete surfaces during the first few hours will reduce the early-age shrinkage and the chance of cracking. Protection may be provided by using wind breaks, fog spraying with water or applying an evaporative retarder (eg aliphatic alcohols). The latter is frequently the most practical method; they are sprayed onto the concrete surface immediately, it is screened and re-applied after each subsequent finishing operation such as trowelling and brooming. Recent studies have shown that this alone can stop a great deal of the cracking caused by early-age shrinkage. The aliphatic alcohols slow down the rate of evaporation from the concrete surface and hence delay the increase in the suction forces that promotes the cracking until the concrete has a higher tensile capacity and is able to resist the developed tensile stresses.


• Curing
  Concrete that is not cured will exhibit greater early-age shrinkage and be more prone to cracking.

It is vital to keep moisture in the concrete surface for longer than the few hours the surface protection method covers; hence it is important to initiate an adequate curing regime within a few hours of placing.

Preventive Measures
Wrt preventive measures to be taken, we have to give focus mainly on controlling the following three parameters:

• Concrete Temperature
• Wind Velocity
• Relative Humidity

To monitor these crucial parameters, which majorly governs Concrete Surface Evaporation Rate, we can use ACI 308 nomograms or equation given as below. The inputs required i.e. Wind Velocity & Relative Humidity, the values can be very precisely obtained from metrological URL or apps e.g. Accuweather, Google etc.

The concrete temperature can be obtained from Temperature Gun or Infrared Thermometer or Thermometer etc.
For determining Surface Evaporation Rate, along with above nomograms and formulae, we can use the ‘My Concrete App’ developed by BASF-Admixture division; it is free to download from the playstore. It is a real helping guide wrt concrete placing condition as it is giving real time data for respective locations. For the location mentioned, the app will fetch data i.e. air temperature, wind velocity and relative humidity.


Guidelines using this app,

1. Go to Calculations option & select Surface Evaporation Rate option
2. Enter the location
3. For given location, Air Temp , Rel Humidity & Wind Speed will be taken automatically for given location
4. Enter w/c value
5. Enter type of SCM used & its %
6. Enter Concrete Temp
7. Surface evaporation rate with % of critical value will be given with precautions to be taken

• For the proposed date and time of concreting, one will have to check the concrete surface evaporation rate and accordingly the decision has to be taken whether the atmospheric conditions are suitable or not.
• In general, wind velocity more than 8Km/hr, Concrete Temperature above 30deg and Relative Humidity below 50% will be boundary limits. Combination of any two parameters will definitely develop the favourable conditions for plastic shrinkage cracking. To overcome these adverse conditions, we can take the following precautions so that their impact will be minimized.
• Use of Water Spray Pump

To maintain the surface moisture level intact, spraying of water after 1 to 2 hrs depending upon the atmospheric conditions by these pumps will help in great extent. Spray pumps are available in various options as given below:

Hand-operated Spray Pump: Called as knapsack spray pump, the pressure given is operated by hands. It is basically used in gardening /farming and is available in agriculture shops.


Battery operated Spray Pump: These are battery operated spray pumps and very efficient to use.


Tractor operated Power Spray Pump: These pumps are attached to tractors and one of best and efficient curing can be done by these pumps. Also, the spray gets very high pressure and can distances up to 30 feet.

• Use of Wind Cutters
  Wind movement plays important role in plastic shrinkage cracks. We can determine the wind speed direction from the URL or apps and wind breakers or cutters will be constructed accordingly which will help in reducing/cutting the wind speed.
  
  Most of the times, atmospheric temperature & relative humidity is well within limit but wind speed is on much higher level can drastically increase the shrinkage cracks. For wind cutters, we can use shade net cloths, tarpaulins, or sarees. Wind speed above 8Km/Hr is generally considered as critical and wind cutters will drastically reduce wind speed.





• Covering/Shading the Aggregate Stockpiles
  Shading of aggregate is of utmost important. Rise in aggregate temperature will lead to rise of concrete temperature. When day temperature rises to its peak, aggregate temperature is found almost 10 to 15oC more than atmospheric temperature. For shading the aggregate stockpiles, we can either use existing shades or cover with tarpaulin.





• Brooming or Texturing of Concrete Surface
  Brooming or texturing the slab concrete surface with initial set will help in arresting and reducing crack depth and widening.





• Retempering with help of Power Trowel Finish
  Giving power trowel finish within 2 to 4 hrs after concrete finishing will help in sealing of opened cracks (if any) and minimizing chances of further cracks opening.





• Controlling the Fines in Fine Aggregate
  Fines in Fine Aggregate especially passing through 150 microns play an important role in higher water demand in further cracking. This cause is witnessed more in using crushed rock sand produced without air or water classifier.





• Covering of concrete with plastic sheet or hessian cloth
  Covering freshly laid concrete with plastic sheet or hessian cloth will eliminate chances of rapid loss of moisture from the concrete surface.





• Control on w/c and use of cohesive mix
  Use of lean concrete mix with very high w/c will have greater chances that concrete mix will shrink as weak layer formed on concrete surface which is unable to resist tensile stresses in early age of concrete and more susceptible to drying shrinkage factor in later stage.





• Proper Concrete Mixing & Maintaining Concrete Mixing Time
  This issue more observed in 10/7 concrete mixer which is used in rural as well as urban area. Minimum mixing time should be at least 2-3 minutes considering 15-20 RPM. But in most of the cases concrete mixing is observed only for 30 seconds to maximum one minute. Improperly mixed concrete shows tendency towards segregation as bleeding.





• Use of adequate workability mix
  Adequate workable concrete will help to a great extent in reducing the shrinkage effect. Lean concrete tends to have more shrinkage as concrete film has lesser resistance towards tensile stresses due to capillary surface tension effect.





• Water sprinkling over aggregate to maintain aggregate temperature
  This is the most economical and a powerful tool as most of the aggregates are found lying in an open area without any cover. To reduce the temperature of aggregate, water spaying helps in lowering the concrete temperature. Utmost precaution needs to be taken for applying moisture correction.





• Late night concreting to control temperature of concrete
  For placing concrete, as norms says concrete temperature should not be more than 30deg C. Late night concrete is one of the best options to mitigate shrinkage cracks. This practice we see in PQC laying. Aggregate, water, and atmospheric temperature within control limits, will ultimately help in reducing and maintaining concrete temperature within limit.





• Use of ice to control temperature of concrete
  Ice is a powerful tool in maintaining or lowering the concrete temperature. As ice is easily available, lowering the temperature of water to be used for concrete will ultimately help in reducing the concrete temperature.





• Use of SRA (Shrinkage Reducing Admixture)
  SRA’s are now a days are easily available from renowned admixture players. SRA will helpful in minimising effect of shrinkage cracking whether from drying or self desiccation. This is because the interfacial area at which they must act (liquid vapor) increases radically in the course of drying. Indeed hardened cementious materials have large internal surface area, which when covered only by liquid films, provide a very large interfacial area where the SRAs will remain active. Working mechanism of these admixtures have been developed by examining the Helmholtz free energy. This is the energy made available by drying is used in two process: deformation and creation of interfacial area. SRAs increase the energy fraction used in creation of interfacial by reducing the surface tension of the liquid vapor interface.

References

• Drying shrinkage of Cement & Concrete - Cement Concrete & Aggregates Australia
• Early age shrinkage of concrete - Cement Concrete & Aggregates Australia
• Curing of Concrete - Cement Concrete & Aggregates Australia
• Avoiding Early Cracking - Cement Concrete & Aggregates Australia
• Hot weather concreting - Cement Concrete & Aggregates Australia
• BIS 7861(Part 1): 1975 Reviewed In:2021
• CIP-05: Plastic Shrinkage Cracking
• Plastic Settlement of Cracking - Cement Concrete & Aggregates Australia
• Tips on preventing plastic shrinkage cracking –Grasim Industries
• ACI 305R-20
• AS 1379-2007 (R2017)
• My Concrete App by BASF
  www.master builders solutios.asiapacific.basf.com (Master Builders Solutions Is Now part of Sika)
• www.accuweather.com
• Science and Technology of Concrete Admixtures, ISBN 9780081006931, Working mechanisms of shrinkage reducing admixtures by A.B. Eberhardt, R J Flatt, Sika Technology, Zurich, Switzerland, 20 November 2015 https://sciencedirect.com/science/article/pii/B9780081006931000138
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