Early Age Properties of High-Performance Concrete

. High Performance Concrete is used to not only minimize the environmental footprint of the cement and concrete industry, but also to enable the design and rapid construction of taller and more slender structures. Despite the significant change in concrete property requirements, the properties of concrete as used during design of concrete structures are mostly still based on 28-day water cured compressive strength, stiffness, long term drying shrinkage and creep. The relevance of these measured properties on the actual behaviour of the structure should be questioned as high strength concrete is known to have low water/cement ratios resulting in significant early age shrinkage, which is currently not taken into account by designers. This paper is based on a study where the early age properties of concrete was investigated, with specific focus on the first 24 hours after casting. The effect of factors such as low water/cement ratio, admixtures, ambient and material temperatures, shutter type and cast volume on properties such as setting time, early age shrinkage, strength development and stiffness was studied and observed trends discussed in this paper.


Introduction
Research on high strength concrete started in the 1970's where researchers first concentrated on the development of concrete mixes that can yield strengths in excess of 80 MPa.The definition of high strength changed significantly since then and most probably will keep on changing in the near future as researchers keep on looking for materials and methods to produce higher strength concretes.The unfortunate fact is that the ultra-high-strength-concretes are not yet used in day to day construction.The reason is most probably because the mixes were developed for very high strength in laboratories and manufactured from aggregates with a maximum size of 500 micron.The volume of cementitious materials in the mixes result in high predicted shrinkage and it is not really a "green" product.
Kearsley and Mostert showed in 2010 that by using high strength concrete the cost of structures can be reduced when normal strength concrete is replaced by high strength precast concrete [1].Since then work has been done to determine the early age properties of high strength concrete.The biggest concerns of many researchers about the use of high strength concrete include the early age shrinkage as well as the influence of admixtures on setting time as well as strength and stiffness gain in the first 24 hours.These trends have now been studied and some experimental results are discussed in this paper.
The main purpose of this work is to connect results from the standard tests to the claimed high shrinkage for high performance concrete [2].The main concern is that researchers from different countries use different times from cast to start measuring the shrinkage.The specimen sizes and shape is different and the direction of measure varies.Measurement of autogenous shrinkage should start at the moment when the material is able to resist tensile stresses, which can be defined as the moment that the concrete changes from a liquid to a solid, often referred to as time-zero [3].
It is known that measured properties are affected by the choice of time-zero.It seems as if the size and shape of the moulds as well as the material used for shuttering has a significant effect on the early age properties.Does this mean that shutter type and size of cast should be taken into account when time-zero is chosen?In this paper these factors affecting early age properties are discussed.

Experimental setup 2.1 Mix compositions
The mix compositions as displayed in Table 1 were used where the solid materials per cubic meter was kept at a constant volume and admixture was added to keep the workability constant.Even this low dosage of admixture does have an influence in the first 24 hours.The relative density of the cement (CEM I 52.5N) was 3.15 while the aggregate used was a crushed dolomite rock with a maximum aggregate size of 5 mm and a relative density of 2.86.The admixture was a Poly Carboxylic Ether known as a High Range Water Reducing Agent (HRWRA).The HRWRA was added as a percentage of the weight of the cement.

Table 1. Mix compositions.
The aim of this experiment was to determine the setting time, heat of hydration, shrinkage, strength and stiffness development of all mixes while keeping the mix material and water at a constant temperature (25 °C).The mixing pan of the mixer was also kept in the temperature controlled room before mixing.The strength testing started 6 hours after casting, every 2 hours up to 24 hours.The mix volume for all the initial mixes were kept the same and all mixes were also mixed for the same time.

Tests conducted in the first 24 hours.
Early age shrinkage measurements were taken on specimen cast in 300 mm high and 105 mm diameter plastic moulds (see Figure 1).The readings were taken with the mould in the upright position.The moulds were lined with a sheet of Teflon and oiled to allow for the minimal friction when the material in the mould wanted to move.When measuring this way and starting the measurements immediately after casting, settlement and absorption of water into the dry cement and aggregate particles affect the shrinkage measured, but values should be representative of vertical length change in elements such as columns.For each mix, three different specimens were cast and the core temperature was recorded for more than 48 hours using a thermocouple at a point as close as possible to the centre of the casting.Different volumes of concrete were chosen to get the correct temperature for each type of specimen tested (Only one size shown in Figure 2).The most used test in concrete is the compressive strength test and it is also the easiest to conduct even if the concrete has a very low strength.The type of specimen used for ease of demoulding without damage was 100 mm cubes cast in steel moulds for the specimens tested up to 10 hours and plastic moulds for more mature specimens.The reason for this was that compressed air is used to demould the plastic moulds and that can damage the concrete when still weak.
During the first 24 hours after casting E-value testing was also conducted at the times when tensile testing was conducted.This test was however not done according to any standard specification, but these cylinders were loaded until failure and the deformation was measured continuously over the middle two-thirds of the specimen.
The direct tensile strength test was conducted for the same mixes at 8, 10 and 12 hours after casting.This test was conducted using a shear box apparatus base (normally used in soil mechanics) with a custom made mould producing a concrete specimen in the shape of an "I" with the dimensions of the portion of the specimen in direct tension 100 mm long, 100 mm deep and 70 mm wide.The overall setup of the apparatus can be seen in Figure 3 and the mould in Figure 4.
The influence of the HRWRA was monitored on all tests and the influence, where visible, is discussed.3 Test results and discussion.
In Figure 5 the measured shrinkage of the four mixes with the different water/cement ratio's shown in Table 1 can be seen.This result confirms is that the mix with the lowest w/c ratio, (mix 0.25) shows the highest strain, at almost 650 micro-strain in 48 hours, while mix 0.45 had the lowest shrinkage after 48 hours, at about 370 micro-strain.
What is also important to notice is that there is a definite change in slope (bend point) in the results of all mixes.This change in slope does however not correlate with the maximum temperature point in the heat of hydration graph for the same mixes that can be seen in Figure 6.The bend points in the shrinkage graph do however move forward in time as the w/c ratio reduces and so does the maximum point on the heart of hydration graphs.This behaviour is caused by the admixture that was used in the lower w/c ratio mixes to keep the workability constant.As proved by other researchers [4], increased admixture dosage retarded setting time and reduced maximum temperature caused by hydration.This explains why both the early age shrinkage slope change and the maximum heat of hydration temperature occurred longer after casting for mixes with lower water/cement ratios and higher HRWRA contents.When the graphs for autogenous shrinkage is drawn using the casting time as time zero, no clear trends can be observed as indicated in Figure 5.The second highest shrinkage was measured for mix 0.55 and this is completely contradicting to the norm claimed by other researchers like Holt [1] and Soliman [6].This is a concern that requires more research.
In Table 2 is a summary of the major turning points in the two graphs shown in Figure 5 and Figure 6, as well as the final setting time of the specific mixes as determined by the ASTM C403 (2016) method for concrete [5].
The trend observed for early age shrinkage results when the final setting time is used as time-zero can be seen in Figure 7.The early age shrinkage results of the mixes when timezero is taken as the time when the change in slope of the shrinkage graph occurred is shown in Figure 8.The early age shrinkage results as summarized in Figure 9, where it can clearly be seen that there is not a linear relationship between total early age shrinkage and w/c ratio when shrinkage is measured from casting.Most of the volume change occured within the first 24 hours after casting, with only limited increase in shrinkage during the second 24-hour interval.If time-zero is taken into account as either the final set time or the time where the rate of shrinkage changed (bend in Figure 5), the observed trends change.After setting the difference in early age shrinkage between the mixes is limited and the w/c ratio 0.55 showed the smallest volume change, regardless of both the chosen zero time and the use of 24 hour or 48 hour results.These results confirm that early age settlement of mixtures with high water to cement ratios could result in significant volume changes before the concrete sets.These volume changes would however not result in internal stresses or strains as the material cannot be defined as a solid yet.If the setting time is seen as the moment when the concrete changes into a solid, setting time could be defined as the moment concrete starts having a compressive strength.To determine the actual time after casting where concrete starts gaining strength, cube strengths were recorded from 6 hours after casting up to 24 hours after casting for the same 4 mixes.

Strength and stiffness development
The results for the cubes tested can be seen in Figure 10.When analysing the compressive strength gain during the 2 hour intervals between the cubes test, a similar pattern is visible than for the shrinkage and the maximum temperature measurements where the gain is higher in the beginning for the high water/cement ratio mixes in comparison to the low w/c ratio mixes where the admixture content was higher.It is clear from the low w/c ratio mixes that the rate of strength gain changes with time.The low w/c ratio mixes remained dormant for longer, also increased strength more rapidly once strength gain started.could be that the admixture working was overcome and that the normal behaviour expected from the w/c ratio took over causing a higher strength gain.These results indicate that the onset of strength gain was significantly retarded by the increase in HRWRA in the low w/c ratio mixes.It is possible that the retardation of setting and strength gain caused by the HRWRA is beneficial for high strength concrete as it now seems that the largest part of the early age shrinkage takes place before the concrete acts as a solid.This should reduce internal stresses and thus minimize the risk of early age cracking.More research should be conducted the prove this concept.In Figure 10 markers were added to show that for all mixes the maximum temperature (in °C) is reached at the time corresponding to the rapid increase in cube strength.This trend indicates that it would be possible to define time-zero as the time where the concrete turns into a solid based on setting time, heat of hydration or compressive strength development results.
In Figure 11 and Figure 12 the difference between the split cylinder strength and the direct tensile strength can be seen.The direct tensile strength as tested gave a higher strength than the split cylinder strength, which is known as an indirect tensile strength, at 8 and 10 hours.At 12 hours however the split cylinder strength is now higher than the direct tensile strength.A possible reason for this result is that the 8 and 10 hour tests were performed before the setting time of the concrete and for the 12 hour test only mix 0.25 passed setting time.
Figure 11 and Figure 12 show the results of the indirect and direct tensile strength tests done at time slots where setting was expected.It is clear that the admixture also played a role here.The mix with w/c 0.55 was the strongest from the beginning for at least the first 12 hours after casting.The results tend to follow the same patern than the cubes excluding the 0.35 mix which seems to have different rates of strength development in tension and in compression.It can be seen that the trend is very simmular than that observed for compressive strength and time-zero could also be taken as the time when tensile strengthstarts developing.These results seem to indicate that the excessive early age shrinkage of high strength concrete can be negated by using HRWRAs that retards setting.Further testing is required to confirm this hypothesis.
Cylindrical specimens were cast to firstly conduct Evalue tests but thereafter the cylinders were loaded until failure occurred.A typical set of these results can be found in Table 3.The results are from the mix with a w/c ratio of 0.35.The 8 hour tests were unfortunately too soft to get any usable results on the same scale than the 10, 12 and 24 hour strengths.
The stress-strain results of these tests can be seen in Figure 13 at different times after casting.Interesting to note how the maximum stress point moved closer to the 0.0035 strain value used in calculations where concrete fail in compression [7] The stiffness of the concrete changes rapidly and more than 80% of the long term concrete stiffness developed in the first 24 hours.Table 3. Cylinder strength and E-values at different ages.easily result in internal stresses exceeding the early age strengths, thus resulting in cracks forming.The incremental stress that would develop in a constrained element as a result of early age shrinkage can be calculated as the product of shrinkage increment at a given time and stiffness or E-value at that moment in time.

Effect of sample size and mould type
The volume of the cubes and the tensile test specimen were about 1 litre while the volume in concrete of the cylinders used for the shrinkage was about 2.6 litres.To determine the effect of specimen size on setting and strength development 1 litre and 3 litre containers were used to measure the temperature development curves as well.The temperature difference between the 3 litre specimens and the 1 litre specimens ranges from 1.9 °C to 5.2 °C for the four mixes used as can be seen in Figure 14.This temperature difference can have a significant influence on setting time for the cylinders cast to measure the autogenous shrinkage as well as on the strength gain over 24 hours.
In Figure 15 the heat of hydration temperature developing in the first 48 hours after casting is compared for concrete from one batch cast into different 100mm cube moulds, including steel, single plastic and double plastic moulds.It clearly can be seen that the temperature in the concrete is significantly affected even for castings as small as 1 litre.Based on these results the time it takes for concrete to set and change from a liquid to a solid could be a function of not only the size of the casting, but also the thermal properties of the mould used to contain the fresh concrete.The actual point in time from when internal stresses in concrete can develop, or the time-zero value of a concrete mixture, could thus not be a constant value but a variable that can be significantly affected by the size of the cast, the surrounding temperature and the mould material used.In Table 4 the strengths of the concrete cubes used for temperature readings can be seen.The cubes show the unexpected influence of mould type on the strength development after 1, 7 and 28 days.The 7 and 28 day cubes were all removed from their moulds after 24 hours and cured for the remainder of the time to testing in a 25°C water bath.It is interesting to note that the use of plastic moulds resulted in a marginally higher early age strength, which can probably be attributed to the higher internal temperature resulting from the heat retained due to the plastic insulating the specimen.After 28 days this effect was reversed, resulting in the cubes that were cast in steel being 15% stronger than the cubes cast in plastic moulds.

Conclusions
Early age properties of high strength concrete are, and most probably will always be difficult to determine.This problem will be exasperated if consensus between researchers cannot be reached on what material the moulds used must be made of, or what volume of concrete must be used for certain tests.
The purpose of this research was to highlight aspects that should be considered when measuring properties

Fig. 9 .
Fig. 9. Effect of w/c ratio and setting time on shrinkage.

Fig. 13 .
Fig. 13.Stiffness development.The E-value of the high strength concrete shows a rapid increase during the first 24 hours meaning that strains that occur due to early age volume change could

Table 4 .
Concrete densities and cube strength development over 28 days for different moulds.
MATEC Web of Conferences 364, 05013 (2022) https://doi.org/10.1051/matecconf/202236405013ICCRRR 2022 applicable to structures where high strength concrete is used.These aspects include:  Admixture plays a major role in the early age properties of low w/c concrete mixes. The volume of concrete in the specimen can change the properties of HSC. The material of the mould in use can change the properties of HSC  The development of early age strength and stiffness should be compared to early age volume change to ensure that early age cracking does not take place in confined elements.