METHODS OF REDUCING EARLY-AGE SHRINKAGE

Transcription

1 METHODS OF REDUCING EARLY-AGE SHRINKAGE Erika E. Holt and Markku T. Leivo VTT Building Technology, Technical Research Centre of Finland Abstract Concrete shrinkage is typically measured after 1 day, though recent research has shown that volume changes occurring during the first day can significantly contribute to the total shrinkage. Early-age drying shrinkage was measured on concrete slabs in a test arrangement developed at the Technical Research Centre of Finland. The tests evaluated the factors influencing the concrete s shrinkage magnitude and likelihood of cracking. In many cases the shrinkage occurring immediately after placing (< 4 hours) was greater than standard long-term shrinkage due to environmental conditions. Construction practice needs to note these early-age risks and take precautions to prevent detrimental shrinkage. It was found that early shrinkage could be predicted based on merely the setting time and evaporation. Material parameters of normal strength concretes, such as aggregate, cement and admixtures, only influenced the set time and bleeding. Some environmental conditions, such as varying wind, temperature and humidity, were examined to determine how they effected the evaporation rates. In addition to taking curing precautions it was possible to use shrinkage reducing admixtures to lower the shrinkage. The key methods to reduce early-age drying shrinkage are presented in this paper, along with a way to estimate early-age drying shrinkage based on establish evaporation prediction methods. 1. Background A -year project has recently been completed at VTT Building Technology that investigated the factors influencing early-age concrete shrinkage. The project s aim was to recommend methods of reducing concrete shrinkage in typical floor-type concretes and publish guidelines for the concrete industry. Regular shrinkage tests begin measurements on a concrete beam at the time of demoulding, or about 4 hours, and continue for many months. In these cases the first 435

2 day volume changes are ignored and deemed insignificant. But the reactions occurring in the first 4 hours are also significant and should be included in the measure of total concrete shrinkage. In this case early-age shrinkage includes the plastic shrinkage phase while the concrete is still pliable, as well as the few hours immediately after setting since early-age reactions are still driving shrinkage. The early-age shrinkage tests conducted at VTT were done on slabs beginning immediately after concrete mixing. The test included measurements of shrinkage, settlement, internal capillary pressure, setting time and evaporation. Long-term tests were then continued on the same concrete slabs for comparison to standardized measurements. It was possible to measure material parameters as well as the influence of the surrounding environments on the magnitude of concrete shrinkage.. Materials All tests in this project used concrete that is typically made for floors. The concretes contained 3 kg/m 3 of Finnish rapid hardening cement (CEM IIA 4.5R), unless otherwise noted. Finnish natural granite aggregate was used, with a maximum size of 1 mm and a gradation with approximately 6% passing the.15 mm sieve. The water-tocement ratio was.63 with a 3% air content. Target tested values for standard properties included a slump of 6 to 1 mm, final setting time of approximately 5 hours, and compressive strength of 35 MPa at 8 days. In one test series shrinkage reducing admixtures (SRA) were used to determine if they were effective during early-ages. These chemicals have recently entered the markets and work by reducing the surface tension of the water so the forces pulling on the paste pore walls are reduced. It is understood in practice that SRAs do not eliminate shrinkage but reduce the ultimate shrinkage on the order of 5 to 5%. Their recommended dosage is from 1 to.5% by cement weight and two products, Peramin and Tetraguard, were tested in this project. In this series high strength concrete (6 MPa) was also tested, containing 5 kg/m 3 of rapid cement and having a w/c ratio of Test Program The early-age shrinkage testing arrangement shown in Figure 1 was developed to evaluate how concrete is behaving immediately after placing. [1,,3] The test is conducted on a fresh concrete slab with dimensions cm approximately 3 minutes after casting and traditional long-term tests continue on the same specimen. Similar arrangements have been developed at other facilities and VTT work is often in cooperation with these facilities. [4,5,6] Measurements on the early-age specimens include horizontal and vertical shrinkage, capillary pressure [7], temperature and evaporation. Vertical shrinkage, or settlement, primarily occurs immediately after placement as the concrete is bleeding. At approximately hours the settlement has 436

3 ceased and most of the continuing volume change during early-ages is shown as horizontal shrinkage. The horizontal shrinkage occurs as a result of the internal capillary pressure change. The shrinkage is measured by LVDTs that are attached to two plates embedded.5 cm into the concrete and suspended from above. These horizontal shrinkage measurements are the values presented in the graphical depictions of the following sections. Past research by VTT and others [8] has shown that shrinkage over 1 mm/m is risky since the likelihood of cracking increases. This amount can be achieved within the first hours after casting if severe environmental conditions exist. But in properly cared for and designed concrete, this limit may never be reached. The majority of early-age tests were conducted and slabs subsequently stored at C and 4% RH. In some cases wind was used to increase drying and induce excess evaporation from the specimens. This project specifically looked at the materials that were contributing to the early-age shrinkage, such as cement type and amount, environmental conditions, and admixture type and dosage. [9] Top View 3. cm 7. cm Measure of Horizontal Shrinkage 3. cm 7. cm Concrete Measure of Settlement Pressure Transducer Mould Plan View Sample on Balance 1. cm Figure 1. Early-age test arrangement schematic. 437

4 4. Results 4.1 Early-Age Combined with Long-Term Most specifications for shrinkage note the beginning of testing at one day. But the following example shows that the shrinkage of un-hardened concrete during the first 1 hours can also play a significant role in the long-term performance. If early-age shrinkage measurements are supplemented with standard long-term monitoring, it is seen that sometimes the early-age can be much greater than long-term measurements. The concrete precautions provided in the first 1 hours can be more important than the next 5 years of crack monitoring care! This is shown in Figure, where the early-age and longterm measurements are compiled for cases of first day wind or no-wind curing environments. In all cases the long-term curing is in a normal drying environment. The solid vertical line identifies the switch in age from early to long-term. The lower curve for wet first day curing represents ideal curing conditions where there was only autogenous shrinkage in the concrete. If the concrete is allowed to dry with no wind (middle curve), the shrinkage magnitude was equal for the first 4 hours compared to the next 3 months. When drying with wind was included over the concrete surface during the first day (upper curve), the early-age shrinkage was 7 times greater than the longterm shrinkage. It is obvious that the first day measurements need to be considered when evaluating shrinkage of concrete, especially in severe environmental conditions Wind Dry Wet Wind Dry Wet (hours) Time (days) Figure. Early-age together with long-term shrinkage measurements where long-term storage in basic dry environment. Wind at.5 m/s in extreme case during early stage. 438

5 4. Effect of Wind As seen from Figure, a slight wind drastically effected the amount of early-age shrinkage. The addition of wind accelerates evaporation of free water from the concrete surface. After the excess bleed water is depleted the evaporation will pull water out from within the internal concrete mass. This loss of water is directly responsible for increased shrinkage, which will ultimately cause cracking. Figure 3 shows the influence of increasing the wind speed over fresh concrete. As the wind speed was increased, more water evaporated from the concrete and greater shrinkage resulted. At approximately 6 hours the concrete was set enough to resist the shrinkage forces, though the evaporation could still be problematic m/s 5 m/s.5 m/s m/s Figure 3. Effect of increasing wind speed over fresh concrete Time (hours) 4.3 Effect of Curing If the concrete is bleeding then this extra water on the surface slows the removal of water from the concrete interior. Though too much bleed water is never desired due to the resulting change in concrete properties, in the case of early-age shrinkage it can be a benefit as it lengthens the time until shrinkage starts. As earlier mentioned, when the bleed water is removed by the surrounding environmental conditions there is a greater risk of cracking. The bleed water acts as self-curing to prevent the shrinkage and excess water curing can also combat the drying forces. Figure 4 demonstrates how much water needed to be added to a concrete surface to prevent shrinkage. [3] The water was sprayed on the fresh concrete surface at the age of 1 hour. In this case the wind speed was approximately 4 m/s and the test specimens were mortar. The water added to the mortar surface behaves like an extra blanket to protect the mass from losing its internal water. The more water that was added, the less shrinkage occurred. With 3 kg/m of water added, the amount of 439

6 shrinkage for the wind-exposed mortar (Figure 4) was of similar magnitudes to that of the concrete with no wind (Figure 3) water added 1 kg/m water added kg/m water added 3 kg/m water added Time (hours) Figure 4. Effect of adding water on the fresh mortar surface, with 4 m/s wind. [3] Care is needed when adding water to a concrete surface, as the water changes the surface properties of the concrete. The excess water alters the porosity of the concrete in the upper millimetres of the surface. For this reason chemical products are also available on the market that act as curing blankets. In these tests a curing agent was sprayed onto the concrete surface immediately after placing. The chemical acts as a barrier to evaporation of water from within the concrete mass. The amount of curing agent applied was altered to determine how much was needed in various environmental conditions. For normal drying conditions (4% RH, no wind) the amount of curing agent added was varied and is shown in Figure 5. In this case, approximately 1 g/m of non-diluted curing agent was sufficient to prevent shrinkage. When a slight wind of.5 m/s was added to the concrete s environment the amount of curing agent required to prevent shrinkage was greatly increased, from 1 g/m (no wind) to over 3 g/m. Again, at 6 to 8 hours the concrete was able to withstand the drying forces after setting and early-age shrinkage ended. 3 44

7 Basic 7 g/m 15 g/m 75 g/m Time (hours) Figure 5. Effect on shrinkage when applying non-diluted chemical curing agent (ConFilm) to the fresh concrete surface, no wind. 4.4 Effect of Environment In addition to wind, the ambient temperature and relative humidity can effect the rate of evaporation from the concrete. This is demonstrated in Figures 6 and 7, where the standard conditions of C and 4% RH are altered. These tests also included wind of.5 m/s to provide more pronounced differences in the shrinkage magnitudes in the various climates. 5 3 o C 5 o C o C cold hot normal Time (hours) Figure 6. Effect of temperature on shrinkage, with.5 m/s wind at 4% RH. 441

8 At the higher temperature of 3 C the evaporation proceeded much faster and the shrinkage starts earlier but reaches its maximum by 6 hours. When the temperature was lowered to 5 C the shrinkage continued for a long time and surprisingly exceeded the amount occurring in the hot climate. This was due to the delayed setting time in the cold climate, as this mixture did not begin to set until after 11 hours. This is critical when considering winter concreting, if the exterior environment is even colder and the concrete is heated to above freezing. In cold weather the concrete still has great risks of shrinkage and cracking during the early ages. As the relative humidity changes the shrinkage magnitude is also altered, as seen in Figure 7. Compared to the standard extra dry test situation of 4% RH, when the humidity was increased to a less dry condition of 7% RH the shrinkage dropped about 15%. When the humidity was further increased to 1% or wet conditions the shrinkage was eliminated. This 1% RH curing represents ideal conditions where there is no moisture loss to the environment and any volume changes are attributed to autogenous behaviour. 5 extra dry dry wet 4% RH 7% RH 1% RH Time (hours) Figure 7. Effect of relative humidity on shrinkage, with.5 m/s wind at C. Shrinkage may occur early in the day on a construction site, and curing practices need to be implemented immediately after concrete placing. If waiting until the end of the day before taking protective measures, the concrete may already be suffering from early-age shrinkage cracking. As seen from these examples, it is critical to be aware of the surrounding environmental conditions during the first hours after concrete placing. 44

9 4.5 Effect of Materials When evaluating different materials in the concrete mixture design it became evident that there was little variation. The actual material changes did not have an effect, such as when changing cement amounts, aggregate size or gradation, or adding superplasticizers. The only material parameter that was effective at drastically altering the magnitude of early-age shrinkage was the addition of a shrinkage reducing admixture (SRA). There is little, if any, data available on the effect of SRAs on early-age shrinkage. In this test series they were tested at the same dosage of 1% as recommended for long-term shrinkage. The results are shown in Figure 8 for an average of different SRA chemicals, Tetraguard and Peramin. When adding the SRA the shrinkage occurred over the same time frame but at a slower rate for equivalent evaporation amounts. The shrinkage amount for both the basic and high strength (HS) mixtures was lowered by about 5%. This shows that the SRAs are beneficial in reducing shrinkage in the early stages of shrinkage in addition to tradition long-term practices. 1.5 HS 1..5 Basic HS + SRA Time (min) Figure 8. Effect of adding shrinkage reducing admixtures on shrinkage, no wind. HS = high strength. 5. Predicting Early-Age Shrinkage SRA It is possible to predict the evaporation rate expected from fresh concrete. Hopefully from this evaporation it would be possible to estimate early-age shrinkage magnitudes. Evaporation rate predictions have been well established by methods such as the 1954 ACI Nomograph [1] for water loss from a free surface. Paul Uno [8] has simplified the nomograph with Equation 1, where vapour pressures are substituted by temperatures in the range of 15 to 35 C. 443

10 E.5.5 = 5([ T + 18] r [ Ta + 18] )( V 6 c + 4) 1 (1) where: E = evaporation rate, kg/m /hr, T c = concrete (water surface) temperature, C, T a = air temperature, C, r = (RH %)/1, and V = wind velocity, kph. Examining the recent data collected by VTT and presented in many of the earlier figures, it is possible to compare how well the evaporation predicted by the above equation and ACI nomograph correspond to the actual measured evaporation rate. Figure 9 shows the correlation between the measured and estimated evaporations. When the VTT evaporation rate is taken from the start of drying at 1 hour, the correlation is very close to that of the predicted values. It is important to take the evaporation rate during the earlier hours since the concrete still has enough water on the "free surface" to be lost to the environment. Estimated Evaporation Rate (ACI)' R = Measured Evaporation Rate (VTT) Figure 9. Correlation of VTT measured evaporation rates to estimated evaporation rates (kg/m /hr). After estimating the evaporation, it would be beneficial to predict the shrinkage amounts. Most of the data collected by VTT for concretes made of rapid hardening cement in the recent shrinkage project is shown in Figure 1. The best correlation was found when using the total amount of evaporated water until setting time (about hours after initial set) at which time the concrete has nearly reached its maximum shrinkage amount. 444

11 As noted in the previous section, material parameters had little effect on the shrinkage amount besides altering the bleeding and setting times. It appeared that as long as the amount of bleeding was consistent then the concrete shrinkage amount could be predicted. The shrinkage was merely correlated to the amount of evaporation that occurred until two hours after setting. At this age the concrete had gained enough strength to withstand the stresses causing shrinkage. The trend in Figure 1 is very general and encompasses only normal strength concrete with a variety of mixture proportions, but it still holds true that evaporation drives shrinkage R = Evaporation (kg/m ) Figure 1. Early-age shrinkage dependence on evaporation (amount hours after setting) for VTT data with normal concrete. The linear trend in Figure 1 can be given by Equation, where the total evaporation amount is taken at hours after initial set time: Early-Age Drying Shrinkage =( Evaporation ).5 () Note that the majority of tests shown in Figure 1 are done with no wind ( C and 4% RH), so many of the evaporation amounts are clustered around 1. kg/m. The data in Figure 1 also includes only normal concretes. It is likely that high strength concretes would not fall on the same line. The bleeding amount of the concretes is also unaccounted for in Figure 1 and Equation and the prediction method could be elaborated on to better define the bleeding factor. 445

12 6. Summary Taking care of concrete during the first hours is extremely important. The shrinkage occurring in the first day can exceed the standard long-term measurements and should not be ignored when describing the total shrinkage of a concrete element. To prepare for early-age shrinkage it is necessary to be aware of curing methods and environmental conditions to ensure the concrete is sufficiently protected to prevent shrinkage. More evaporation occurring in the early stages results in more shrinkage and thus cracking, as was shown in Figure 1. This shrinkage and resulting early-age cracks will only provide additional paths for more harm, either as chemicals and water infiltrate, or for later-age cracks to lengthen along the early paths. Methods, such as water or chemical compound curing, used during the early stages to ensure reduced shrinkage will definitely pay-off in the long term. Adjustments to material parameters will likely only effect the setting time and amount of bleed water which acts as self-curing on the concrete surface. The only material adjustment guaranteeing a reduction in early-age shrinkage under similar curing environments is the use of a shrinkage reducing admixture. It was found that early-age drying shrinkage could be predicted based on the setting time and evaporation from the concrete. The evaporation could be estimated from wellestablished methods and the shrinkage of normal concretes is easily predicted if the bleed water is not excessive and providing self-curing. Acknowledgements This work has been partially funded by VTT, TEKES and members of the Finnish cement and concrete industry: Lohja Rudus Oy, Finnsementti Oy, Semtu Oy and Masterbuilders Oy. Partial funding was also provided by a Fulbright grant from the United States government. References 1. Holt, E. and Leivo, M., Autogenous Shrinkage at Very Early Ages, in Autogenous Shrinkage of Concrete, Proceedings of the International Workshop, Hiroshima, June 1998 (E & FN Spon, London 1999) Leivo, M., and Holt, E., Autogenous Volume Changes at Early Ages, in Self- Desiccation and Its Importance in Concrete, Lund, Sweden, June 1997 (Lund University, Report TVBM-375) Kronlöf, A., Leivo, M., and Sipari, P., Experimental Study on the Basic Phenomena of Shrinkage and Cracking of Fresh Mortar, Cement and Concrete Research 5 (8) (1995)

13 4. Hammer, T.A., Test Methods For Linear Measurements of Autogenous Shrinkage Before Setting, in Autogenous Shrinkage of Concrete, Proceedings of the International Workshop, Hiroshima, June 1998 (E & FN Spon, London 1999) Tazawa, E., and Miyazawa, S., Influences of Cement and Admixtures on Autogenous Shrinkage of Cement Paste, Cement and Concrete Research, 5 () (1995) Radocea, A., A Model of Plastic Shrinkage, Magazine of Concrete Research, 46 (167) (1994) Radocea, A., A Study on the Mechanisms of Plastic Shrinkage of Cement-Based Materials, (CTH Göteborg, Sweden, 199) 8. Uno, P., Plastic Shrinkage Cracking and Evaporation Formulas, ACI Materials Journal, 95 (4) (1998) Holt, E. and Leivo, M., Reducing Early-Age Concrete Shrinkage With The Use of Shrinkage Reducing Admixtures, Proceedings Nordic Concrete Research, Reykjavik, Iceland, 1999, ACI 35R-96, Hot Weather Concreting, in Manual of Concrete Practice, Part, (Farmington Hills, American Concrete Institute, 1996). 447