The influence of temperature on the cracking of plastic concrete

. High early age concrete temperatures can lead to many problems such as an increased rate of cement hydration, as well as an increased rate of moisture loss from fresh concrete which can ultimately lead to the occurrence of plastic shrinkage cracking. Concrete is also batched and cast at various ambient temperatures which greatly influences the temperature development of the concrete after placement. There is a need to understand the influence of concrete temperature on the plastic cracking of concrete. This study investigated the temperature development over the thickness of a concrete slab when exposed to different initial concrete and ambient temperatures as well as the effect these factors have on plastic shrinkage cracking. This was achieved by experiments on concrete samples at varying temperatures while measuring the concrete temperature, pore water evaporation, shrinkage, settlement, setting times and plastic shrinkage cracking magnitude. These tests were conducted in a climate controlled chamber. It was concluded that exposure to higher initial concrete and ambient temperatures significantly increases the average temperature over the thickness of the concrete. The evaporation of pore water was higher when exposed to higher evaporation conditions. The plastic shrinkage, settlement and plastic shrinkage cracking were more severe in the presence of higher initial concrete and ambient temperatures even though the critical period and setting times were reduced due to an increase in the concrete temperature. Finally, the surface temperature of the concrete as tested in slabs up to 100 mm in thickness can be used as a good indication of the temperature development in the lower layers of the concrete.


Introduction
When concrete is placed, water from the surface of the newly placed concrete evaporates.The rate at which the water evaporates is dependent on the temperature of the concrete as well as the environmental conditions to which the concrete is exposed such as: air temperature, relative humidity and wind.Due to the difference in densities between the particles found in the concrete, free water within the concrete is forced upwards until reaching and assembling on the concrete surface.This process in which water is displaced to the concrete surface in known as bleeding.Once the rate of evaporation exceeds the rate at which bleed water rises to the surface of freshly placed concrete, the water within the concrete pores or capillaries start to evaporate which causes the concrete to shrink.This is called plastic shrinkage and occurs especially within the first few hours after the concrete is placed in conditions with high evaporation [1].
Equations for predicting the rate of evaporation of free water from concrete are available with Uno's equation being one of the most commonly used.This equation is dependent on the concrete temperature as well as environmental factors which include the ambient temperature, wind velocity and relative humidity [2].
Plastic shrinkage leads to the forming of early surface cracks known as plastic shrinkage cracks.During the plastic state, concrete has not reached sufficient tensile * Corresponding author: meyerm@sun.ac.za strength to resist these cracks.When the combination of concrete and ambient temperature leads to a high evaporation rate, the chance of the concrete surface drying prematurely and plastic shrinkage cracks forming increases [3].These cracks lead to durability and maintenance problems as they serve as pathways for corroding agents into the concrete and the steel within the concrete [4].
High early age concrete temperature can lead to many problems such as an increased rate of cement hydration as well as an increased rate of moisture loss from freshly placed concrete [5].There are several internal and external factors that may influence the temperature of concrete during its lifespan.Internal factors include the constituents used in the concrete as well as the influence of the constituents on the hydration reaction.The hydration process is an exothermic chemical reaction that influences the temperature of the concrete as it generates heat over time [6].The initial temperature of the constituents used can also affect the temperature of the concrete when cast.External factors include the air temperature, formwork type and temperature, solar radiation, evaporation cooling and construction methods [2,4].
Concrete can be vulnerable to many durability problems when cast in extreme climates.In South Africa concrete is often batched and cast in hot, dry and windy conditions.These conditions have a significant influence on the evaporation of free water and could result in surface cracks on the concrete.There is therefore a need to understand how these factors influence the temperature of concrete as well as how this then influence the plastic cracking of concrete.

Experimental framework
Experiments were conducted in order to investigate the influence of air temperature and concrete casting temperature on the concrete temperature development, pore water evaporation and plastic cracking of fresh concrete.The ambient and concrete casting temperatures were varied while the wind velocity and relative humidity were kept constant.Concrete samples were placed in a climate chamber in order to simulate these different climate conditions.
A climate chamber that controls the air temperature, wind speed and relative humidity was used to create the desired climate conditions for all the tests.The wind velocity and relative humidity were kept constant at 5 m/s and 40% respectively.Two different ambient and three different concrete casting temperatures were used.The ambient temperatures were characterised as moderate (25°C) and warm (35°C).The initial temperature of the concrete materials were 15, 25 and 35°C.The materials were placed in a fridge at 10°C to obtain an initial temperature of 15°C.For the tests performed with an initial concrete temperature of 25°C the constituents were placed in a climate controlled room with an ambient temperature of 25°C.An initial concrete temperature of 35°C was reached by placing the materials in an industrial oven at a temperature of 45°C.All moulds used during a test were placed in the same climate conditions to prevent major temperature loss at the start of the test.All the materials and the mould were placed in the climate conditions 24 hours prior to the start of the test.A total of six climate conditions were tested and are referred to as Experiments 1 to 6 as shown in Table 1 and each test ran for 6 hours.

Moulds and measurement setup
All the moulds used for testing are made of PVC and can be categorised as either a restraining of non-restraining mould.The restraining moulds were used to restrain the concrete in order to form cracks, while non-restraining moulds were used to measure the factors that influence the cracking behaviour.

Evaporation moulds
Moulds with inner dimensions of 200 x 200 x 100 mm were used to measure the rate of evaporation.This mould gives an exposed concrete surface area of 0.04 m 2 from which water can evaporate.The evaporation rate was determined by weighing the concrete filled moulds every 30 minutes with a scale measuring to the nearest 0.1 g.Three samples were used for each test and the average reported.

Temperature moulds
A similar mould as used for the evaporation tests were used to measure the concrete temperature.Two metal plates, one horizontal and one vertical were used to keep the temperature sensors in place during casting.The tips of the sensors where the temperature is measured ends up in the concrete once cast and far from the metal plate.The setup used to measure the concrete temperature is shown in Fig. 1.The temperature sensors, with a resolution of 0.1°C, were cast in the concrete.Five sensors were used throughout the depth of the concrete with a spacing of 20 mm.Three samples were tested for each test and an average reported.

Fig. 1.
Temperature mould with metal plates and sensors.

Settlement and shrinkage moulds
The plastic settlement and shrinkage of the concrete were measured using 300 x 300 x 100 mm moulds, similar to the mould and methods used by Slowik et al. [7].The mould used a top mounting bracket which was used to keep a linear variable displacement transducer (LVDT) in place.A non-spring loaded LVDT was connected to a metallic wire lattice that rested on the surface of the concrete and measured the settlement of the surface of the concrete.The mould also has two side mounting brackets used to keep the two spring loaded LVDT's in place and in a horizontal position to measure the horizontal movement or shrinkage of markers embedded in the concrete.

Plastic shrinkage cracking mould
The mould is based on the proposed mould from ASTM C1579 [8] with the exception of two additional steel bars at each end of the mould.These steel bars were added to increase the horizontal restraining of the concrete.The mould was similar to the ones used by Combrinck et al [9] and Meyer et al. [10].The crack development and area was determined by taking high resolution photos every 15 to 20 minutes of the concrete specimen.These photos were scaled and measured using MATLab software similar to the method used by Moeilich et al [11].In accordance with ASTM C1579 [8], the first 25% of the crack from the side of the mould was neglected as it is assumed that this part of the crack is accentuated due to the friction from the mould.

Setting time mould
Circular PVC moulds and baseplates were used to determine the initial and final setting time of the concrete during each test.Three similar moulds and samples were used during each test.The setting times were measured by means of a normal penetration resistance test in accordance with SANS 50193-3 [11] that makes use of a Vicat apparatus.

Concrete materials and mix proportions
The concrete mix was designed to reach 30 MPa at 28 days and had a slump of 60 mm.The material constituents, relative densities and mix proportions are summarised in Table 2. Locally available natural quarry sand, known as Malmesbury sand, with a fineness modulus of 2.6 was used as the fine aggregate.The coarse aggregate used was a Greywacke stone with a nominal stone size of 13 mm.

Test results and discussions
Table 3 illustrates a summary of the average results gained during testing.This includes the ambient temperature (AT), the average concrete temperature (CT) over the depth of the specimen at the start and end of each test, the average evaporation rate (ER) and the crack area (CA) for each test.The average of all 5 sensors in the concrete was used to determine the average CT.

Concrete temperature
Due to limited space the results of Experiments 1, 4 and 5 were selected at random for discussion in this section.Fig. 2 to Fig. 4 illustrates the average difference in temperature over the thickness of the concrete specimen for Experiments 4, 5 and 1 respectively.The results represents the average of the tested concrete specimens.The sensors in the concrete were numbered from the bottom to the top, with M1 being the sensor at the bottom and M5 the sensor at the surface of the concrete.Fig. 2 illustrates Experiment 4 with an initial concrete temperature of 35°C and ambient temperature of 25°C.It can be observed that at the beginning of the test there was a noticeable difference in temperature over the thickness of the concrete with the upper layers of the concrete being significantly colder than the lower layers.This difference was due to the fact that the upper layers in the concrete were more exposed to the ambient conditions compared to the lower parts.Thus, the top layers lose temperature at a faster rate during the mixing and transportation period prior to each test and result in lower starting temperatures.Fig. 3 illustrates Experiment 5 with an initial concrete and ambient temperature of 35°C.The average temperature of the concrete dropped to approximately 31.5°C after 180 minutes, where after it started to increase again and reached an average of 33°C at the end of the test.It was also noted that the difference in the temperature between the layers of the concrete becomes less pronounced as the test progress.The drop in temperature within the first 180 minutes after the concrete was cast, can be as a result of evaporative cooling.During this time there was a significant amount of bleed water on the surface of the concrete and when this evaporated it resulted in a drop in the temperature.Once the majority of the bleed water has evaporated and the surface starts to dry out, the effects of evaporative cooling was less which resulted in the temperature of the concrete to rise due to the ambient temperature being higher than the concrete temperature.It can also be seen that the surface temperature was initially low, but undergoes a more rapid increase near the end of the test when compared to the rest of the concrete layers.This only occurred during tests for which the concrete specimens were exposed to high ambient temperatures (Ta > 35°C).
Similar results were observed with a casting -and ambient temperature of 25°C, as shown in Fig. 4 during Experiment 1.These results also showed an initial drop in temperature during the first 180 minutes after placement, due to evaporative cooling, with an average decrease of approximately 1°C at 140 minutes after placement.The effect of evaporative cooling is highly dependent on the evaporation rate of the bleed water from the surface of the concrete, which was also confirmed when comparing the temperature profiles from Experiment 1 and 5.
The results illustrated in Fig. 2 to Fig. 4 can prove to be extremely important during the plastic state of concrete, especially the surface temperature.It was expected that the surface temperature will differ greatly from the lower layers of the concrete as it is more easily influenced by environmental factors.This hypothesis was proven incorrect as the results illustrate that the lower layers followed a similar trend to that of the surface temperature.It was shown that the surface temperature is a good representation of what happens to the temperature in the lower layers of the concrete.Although the surface shows a quicker initial increase or decrease in temperature, the same temperature profile translates to the deeper layers.Thus, it can be assumed that an increase or decrease in the surface temperature will result in a similar increase or decrease in the lower layers of the concrete.
It can also be seen that this translation of the surface temperature in the lower layers of the concrete comes with a delay.It can be assumed that the deeper the concrete layer, the longer the change in concrete temperature delay will be.
Fig. 5 and Fig. 6 shows the surface and core temperature of the concrete specimen at different ambient and casting temperatures.The sensors representing the surface and core temperature were located at 5 and 45 mm below the surface of the concrete respectively during each test.Fig. 5 shows the core and surface temperature with varying initial concrete temperature with a 25°C ambient temperature.The results from Experiments 1, 2 and 3 were used to represent the results in this figure.Exposure to a moderate ambient temperature caused a noticeable decrease in temperature for the warm concrete and increase in the cold concrete.6 was similar to the results in Fig. 5 with the concrete with the warmest casting temperature reaching the highest temperature at the end of the test, followed by the moderate and cold concrete.Again, the core temperature for all the samples was slightly higher compared to the surface temperatures.
Exposure to a warm ambient temperature caused noticeable increases in temperature for the moderate and cold concrete.A small decrease in the surface and core temperature was noticed for the concrete with a warm casting temperature.Fig. 5 and Fig. 6 illustrates the results when the ambient temperature was kept constant.Fig. 7 to Fig. 9 shows the change in the core and surface temperature of the concrete when the initial concrete temperature was kept constant and compared at varying ambient temperature.Fig. 7 shows the change in the core and surface temperature of the concrete specimens with a moderate initial concrete temperature (25°C) and exposed to varying ambient temperatures.These are the results from Experiments 1 and 4. The specimen exposed to the warm ambient temperature experienced an immediate rise in temperature that continued throughout the entire test.After 6 hours, the concrete temperature was on average 7.5°C warmer over the thickness of the concrete than at the start of the test.Ultimately, the core and surface temperature of the specimens exposed to a warm ambient temperature was similar with only a minor difference.The core and surface temperature of the concrete specimens with a warm casting temperature (35°C) at varying ambient temperatures is shown in Fig. 8.These results are from Experiments 2 and 5.There were noticeable differences between the core and surface temperatures at the start of each test.This was due to the fact that the surface temperature lost its temperature significantly faster compared to the core temperature during the transporting phase of the concrete specimens to the testing location.The results of the specimens exposed to warm ambient temperatures were discussed earlier in this subsection.For the moderate climate, it is clear that it experiences a decrease in temperature during the first 240 minutes.This decrease in temperature is mainly due to exposure to a colder ambient temperature as well as small effects due to evaporative cooling.At the end of the test the surface and core temperatures are similar with less than 1°C difference.The difference between the core and surface of the warm concrete was less, with a difference of less than 0.1°C after 6 hours.
When comparing the results between the two concrete specimens exposed to different air temperatures it can be seen that the decrease in the ambient temperature caused a decrease in the core and surface temperature of 9.8°C and 6.7°C respectively.The core of the concrete experienced a greater drop in temperature due to being 3°C warmer at the start of the test.Fig. 9 shows the core and surface temperature of the concrete specimens with a cold initial concrete temperature, as per Experiments 3 and 6.There is noticeable difference between the surface and core temperature at the start of the test due to the transportation of the concrete.Both specimens experienced an immediate increase in temperature when placed in the climate chamber.For the specimens exposed to the warmer ambient temperature this increase was more severe and resulted in a noticeably higher temperature at the end of the test compared to the specimen exposed to the moderate ambient temperature.

Evaporation
Fig. 10 illustrates the difference in cumulative evaporation per area (kg/m 2 ) for concrete specimens exposed to different initial concrete and ambient temperatures at the start of the test.Half of the specimens in Fig. 10 were exposed to moderate ambient conditions (25°C) and the other half to warm conditions (35°C) for the entire duration of the test.It is clear that the casting temperature of concrete significantly influences the amount of mass loss due to evaporation.The highest amount of mass loss due to evaporation was experienced by the concrete with the higher initial temperature at the start of the test during both warm and moderate conditions.During the moderate conditions, the warm concrete experienced an increase of 15% in evaporation after 6 hours compared to the moderate concrete and 25% when compared to the cold concrete.In warmer ambient conditions (35°C) the warm concrete experienced a 10% increase in evaporation compared to the moderate concrete after six hours and a 25% increase compared to the cold concrete.Thus, an increase of 10% in the casting temperature of concrete resulted in an increase of 10-15% in evaporation during both at warm (35°C) and moderate (25°C) ambient conditions.

Fig. 10. Difference in cumulative evaporation per area for varying ambient and casting conditions.
In Fig. 11 the mean evaporation rate (kg/m 2 /h) of these concrete specimens are shown as well as the mean temperature of the concrete.The results from Experiments 1 to 3 are shown where concrete specimens with a warm, moderate and cold casting temperature was exposed to a moderate ambient temperature.Fig. 11 show the rate of evaporation change when there is a change in concrete temperature while still in the plastic state.At the start of the test, there is a significant difference in the evaporation rate between the concrete specimens, due to the difference in casting temperatures, with the warm concrete having a rate twice as high as the moderate concrete.Similarly, the rate of evaporation is double that of the concrete specimen with the cold casting temperature.Thus, the initial evaporation rate is 50% higher with a 10°C increase in casting temperature.
As the concrete temperature increases for the cold concrete and decrease for the warm concrete, the evaporation rate for these specimens increase and decrease respectively.Ultimately the warm concrete still has the highest rate at the end of the test, due to the fact that it has a higher concrete temperature, but the difference between the specimens are smaller.

Plastic shrinkage and settlement
Fig. 12 shows the average total shrinkage of two concrete specimens at a moderate ambient temperature with varying casting temperatures.The results show that an increase in casting temperature of the concrete significantly increases the shrinkage of freshly cast concrete when exposed to a moderate ambient temperature.It can also be seen that the concrete started to shrink at approximately 70, 120 and 240 minutes after placement for the warm, moderate and cold concrete respectively.Thus, when exposed to the same ambient temperature, an increase in the casting temperature results in an increase in the total shrinkage and a shorter time for the concrete to start shrinking.Similar results were observed when the concrete specimens were exposed to a warm ambient temperature with the warmer concrete experiencing the most shrinkage followed by the moderate and cold concrete.Similar to the moderate conditions, the warm concrete also started to shrink first followed by the moderate and cold concrete.
Fig. 12 also show that a higher ambient temperature will result in a higher total shrinkage after 6 hours.Exposure to a warm ambient temperature resulted in an increase of 12, 18 and 53% for the warm, moderate and cold concrete casting temperature respectively when compared to the moderate ambient temperature.
It can also be seen that during Experiment 1 (A25C25), 3 (A25C15), 4 (A35C25) and 6 (A35C15) the concrete specimens experienced thermal expansion prior to the plastic shrinkage.This thermal expansion is due to the lower initial concrete temperature compared to the ambient temperature and the bigger the difference the more expansion occurred in the specimens.Expansion therefore only occurred if the concrete was colder than the ambient temperature and experienced a sudden rise in temperature, as no expansion occurred when hot concrete was exposed to a cold ambient temperature.This expansion was not expected in plastic concrete.The test setup was designed to measure shrinkage and not expansion and therefore the full effect of the expansion cannot be measured with the test setup used.13 shows the total settlement of the concrete specimens at a warm ambient temperature with varying casting temperatures.Within the first 30 minutes after the placement, all the specimens experienced pure settlement due to the effect of gravity.After this initial settlement, the moderate and cold concrete starts to stabilise and only starts to settle again at around 100 minutes.This is called secondary settlement and is due to the effects of evaporation that causes capillary pressure to build up in the concrete and cause a three dimensional shrinkage.The timing of the secondary settlement also coincides with the time the shrinkage starts, as shown in Fig. 12.

Plastic shrinkage cracking
Since plastic shrinkage cracking (PSC) is normally caused by a high evaporation rate, it would be expected that an increase in the evaporation rate will cause an increase in the severity of the PSC.Fig. 14 illustrates the crack development for the concrete specimens for all the experiments conducted for this test series.These results illustrate the influence of ambient temperature on the severity of PSC and the time it occurs.
Exposure to high casting and ambient temperatures resulted in larger crack areas compared to specimens exposed to lower temperatures.It is clear that the crack area increases with an increase in the casting temperature of fresh concrete.A higher casting temperature also resulted in the plastic shrinkage cracks to occur earlier.
The warm concrete also started to crack earlier than the moderate and cold concrete in both ambient conditions.
On average when exposed to the warm ambient temperature the crack area increased with between 19% and 20% for the warm and moderate concrete specimens when compared to the moderate ambient temperature conditions.For the cold specimens, the increase was higher and experienced an increase of over 60% after 6 hours.On average the cracking on the surface of the concrete started 20 minutes earlier for the warm and moderate concrete with an increase of 10°C in the ambient temperature.For the cold concrete, cracking started 60 minutes earlier at the warmer ambient temperature.
An increase in both the initial concrete and ambient temperature resulted in more severe cracks and also caused these cracks to develop earlier compared to colder conditions.

Setting times
Fig. 15 and Fig. 16 show the initial and final setting times that were measured, as well as the mean crack area for specimens exposed to warm and moderate ambient temperatures respectively.At higher concrete and ambient temperatures the hydration process accelerates which resulted in reduced setting times. .On average the initial setting time decreased by 15 minutes when the concrete temperature was increased by 10°C and exposed to moderate ambient temperature, while it decreased by 30 minutes when exposed to warm ambient temperatures.It can also be seen from Fig. 15 that an increase in the ambient temperature also resulted in a decreased initial and final setting times for the concrete specimens.An increase in the temperature can cause a decrease in the setting time of concrete and ultimately result in an increased tendency and severity for PSC.The results also confirm that the time of the PSC onset on the surface of the concrete is normally near or just after the initial setting time.For the warm concrete, the initial setting time occurred at the same time the first plastic shrinkage crack appeared on the surface of the concrete specimen.For the moderate and cold concrete, the initial setting time occurred 40 and 60 minutes before the first crack.It can be concluded that a decrease in concrete casting temperature results in a longer time period between the initial set and the first onset of PSC.
The results also showed that no cracks started after the final set was reached.It can also be seen that the time between the first crack and the final set increases with an increase in concrete casting temperature.
It is clear that the casting and ambient temperatures of fresh concrete greatly influence the setting times.A rise of 10°C in the ambient temperature resulted in the time between the first crack onset on the concrete surface and final set decreasing by an average of 12.5 minutes.A decrease in the time between initial set and the first crack was also noted during the warmer conditions with an average decrease of 27.5 minutes.

Conclusion
The main objective of this study was to investigate and provide a better understanding of the temperature development in fresh concrete when exposed to different environmental conditions as well as the effects these conditions have on the occurrence and magnitude of plastic shrinkage cracking.The following significant conclusions can be drawn from this study: • The temperature of fresh concrete is greatly dependent on the ambient conditions, with the air temperature being the bigger influencing factor during the plastic state of concrete.The surface temperature of fresh concrete is more susceptible to change due to the ambient conditions compared to the core of the concrete.The effects of evaporative cooling are also more prominent on the surface of the fresh concrete.• Concrete with a higher initial temperature resulted in a higher average as well as ultimate concrete temperature over the thickness of the concrete after 6 hours.• The surface temperature of fresh concrete can be used as a good representation of the temperature development in the lower layers of the concrete.Although the surface is more susceptible to temperature changes due to the ambient temperature, similar changes will also occur in the lower layers.These temperature changes in the lower layers appear with a delay when compared to the surface and are dependent on the depth of the concrete.The deeper, the longer the temperature change delay.• An increase in the initial concrete and ambient temperature results in an increase in the total mass loss due to evaporation experienced by a concrete specimen.
• An increase in either the ambient or initial concrete temperature will result in higher shrinkage during the plastic state of concrete.The shrinkage is proportional to the mass loss due to evaporation which means that the difference in shrinkage between the specimens are proportional to the difference in ambient and initial concrete temperature.• Plastic shrinkage cracking is more severe when exposed to higher ambient and or initial concrete temperature.• In practice concrete should be kept as cold as possible when cast to minimise the occurrence and magnitude of plastic shrinkage cracking.
The more temperature the concrete has, the more evaporation will occur, resulting in more plastic shrinkage and ultimately in cracking.

Fig. 2 .
Fig. 2. Temperature variation over the thickness of concrete specimens with concrete temperature of 35 o C and ambient temperature of 25°C.

Fig. 3 .
Fig. 3. Temperature variation over the thickness of concrete specimens with concrete temperature of 35°C and ambient temperature of 35°C.

Fig. 4 .
Fig. 4. Temperature variation over the thickness of concrete specimens with concrete temperature of 25 o C and ambient temperature of 25 o C.

Fig. 5 .
Fig. 5. Surface and core temperature of concrete with varying concrete casting temperatures at an ambient temperature of 25 o C.

Fig. 6 .
Fig. 6.Surface and core temperature of concrete with varying concrete casting temperatures at an ambient temperature of 35 o C.

Fig. 6
Fig.6illustrates the core and surface temperature with varying initial concrete temperatures at a warm ambient temperature (35°C).The results from Experiments 4, 5 and 6 were used to represent the results in this figure.The results shown in Fig.6was similar to the results in Fig.5with the concrete with the warmest casting temperature reaching the highest temperature at the end of the test, followed by the moderate and cold concrete.Again, the core temperature for all the samples was slightly higher compared to the surface temperatures.Exposure to a warm ambient temperature caused noticeable increases in temperature for the moderate and cold concrete.A small decrease in the surface and core temperature was noticed for the concrete with a warm casting temperature.Fig.5and Fig.6illustrates the results

Fig. 7 .
Fig. 7. Surface and core temperature of concrete with moderate casting temperature (25 o C) at varying ambient temperatures.

Fig. 8 .
Fig. 8. Surface and core temperature of concrete with warm casting temperature (35 o C) at varying ambient temperatures.

Fig. 9 .
Fig. 9. Surface and core temperature of concrete with cold casting temperature (15 o C) at both 25 o C and 35 o C ambient temperatures.

Fig. 12 .
Fig. 12. Difference in total shrinkage for varying ambient and casting temperatures.

Fig.
Fig.13shows the total settlement of the concrete specimens at a warm ambient temperature with varying casting temperatures.Within the first 30 minutes after the placement, all the specimens experienced pure settlement due to the effect of gravity.After this initial settlement, the moderate and cold concrete starts to stabilise and only starts to settle again at around 100 minutes.This is called secondary settlement and is due to the effects of evaporation that causes capillary pressure to build up in the concrete and cause a three dimensional shrinkage.The timing of the secondary settlement also coincides with the time the shrinkage starts, as shown in Fig.12.

Fig. 13 .
Fig. 13.Total settlement of specimens at a warm ambient temperature (25°C) with varying casting temperatures.

Fig. 14 .
Fig. 14.Average crack area over time for all tested specimens.

Fig. 15 .
Fig. 15.Mean crack area over time for specimens exposed to warm ambient temperature with varying casting temperatures as well as initial and final set times.

Fig. 16 .
Fig. 16.Mean crack area over time for specimens exposed to moderate ambient temperature with varying casting temperatures as well as the initial and final set times.