**4. Investigation of curing process and strength development of concrete with maturity method**

In-place concrete strength evaluation is an important step in achieving the reliable performance of structures and construction scheduling [18, 19]. The level of maturity is in need of evaluation for deciding the forms' striping time, posttesting, protection

removal, and progressing with further construction plan. The minimum level of the strength required for concrete is required at different stages before further progress, which imposes careful monitoring of the strength evaluation through curing time. However, concrete under harsh environmental conditions normally experiences unexpected issues that decline the strength development. Significant expenses might be occurred if the concrete curing process is delayed under environmental effects to be assured that the concrete has reached the specified minimum strength.

Maturity in general is estimated by tracking the changes in strength development for fresh concrete under various temperatures over time intervals. As the cement hydrates, the strength increases; however, the amount of hydrated cement depends on many factors, specifically curing temperature. From the strength-maturity index curves of corresponding samples, the strength on-site could be predicted at any moment after concrete placement, which shows how far the hydration is processed. Usually, the concrete reaches the expected strength under warm weather condition, if the temperature is not abruptly increased. However, in cold temperature situation, the fluctuation in temperature during days and nights might stop the cement hydration process. For understanding, the reasonable evaluation of the strength of the placed concrete, maturity-meters shown in **Figure 5** are implemented at different stages of curing process.

The concrete strength is directly related to the curing time, humidity index, and temperature fluctuation. The maturity investigation is a method with which the early age concrete strength is monitored and the results subsequently could be implemented for in-site concrete placement [15, 17]. The main assumption with maturity concept is that the concrete is able to attain the same strength if the mixture reaches the same value of maturity index [18]. Another assumption with maturity evaluation is that the combination of temperature and time leads to the same strength for the considered concrete mixture. Therefore, strength development is a function of time and temperature. This function could be nonlinear or linear with respect to time and temperature. The instrument shown in **Figure 5** indicates how mature the concrete mix is which is represented as a number ultimately. The information is subsequently used for establishing maturity curves which are unique for any concrete mixture and related to compressive strength [19–21]. The schematic maturity curve is shown in **Figure 6**. The maturity curves are based on the time-temperature factor (TTF) which is compatible with the assumption that the maturity is a function of time and temperature.

In general, there are many common methods in obtaining the maturity level of the concrete. The general idea is based on the time-temperature factor which considers the proportional relation between combined temperature-time and compressive strength. The important issue related to this commonly used method shown in Eq. (1) is that the strength development in significantly higher or lower

**97**

*Compressive Behavior of Concrete under Environmental Effects*

temperatures is not accurate; hence, the careful interpretation of the results in

0 t

where M is the maturity index in °C h, T is the average temperature in °C over time interval of ∆t per hours, and T0 is the datum temperature usually taken as 0°C at which the hydration is stopped and concrete does not gain any strength. M index is usually referred to as the maturity index related to TTF factor [2]. To understand the assumption in this method, **Figure 7** is shown in which the area of temperature over time for a specific mixture under cold condition is M1, and under corresponding mild temperature condition is M2. If M1 and M2, which represents the maturity index of a mixture, are equal, then the compressive concrete strength is expected to be the same. It is highly important that this assumption is only useful if the concrete does not experience any harsh environmental condition (e.g., excessive cold or hot

To further investigate the effect of harsh environmental conditions on maturity and strength development of the concrete, the cylinder-shaped specimens are used for maturity evaluation of the four different concrete mixtures. Different temperatures and rates of humidity are considered for all the mixtures. **Figure 8** shows the maturity evaluation for all the concrete mixtures in which the compressive strength of the specimens are extracted at different time intervals. It is noted that the effect of significant humidity change, even though not considered in maturity concept formulation, is clearly observed. It is shown that environmental effects such as very dry conditions could tangibly change the strength development of the concrete; however, the change might not be as much as temperature variations. The lower water-cement ratio leads to significant gain in early stages of curing or lower TTF values, since all the cement materials actively participate in chemical hydration reactions. As the TTF increases or curing progresses, the rate of strength gain

(T − T0)t (1)

*DOI: http://dx.doi.org/10.5772/intechopen.85675*

extreme temperature is inevitable.

**Figure 6.**

M = ∑

*The schematic maturity-meter curve showing the concrete maturity.*

temperature) during initial stages of the curing process.

decreases until the concrete reaches the ultimate strength conditions.

**Figure 8** shows the combined effect of temperature and time on compressive strength by using the maturity index. It is noted that temperature drop significantly affect the hydration process, which leads to less ultimate strength over the same period of the time. It is noted that if the harsh environmental condition such as

**Figure 5.** *The maturity-meter device used for concrete strength evaluation.*

*Compressive Strength of Concrete*

removal, and progressing with further construction plan. The minimum level of the strength required for concrete is required at different stages before further progress, which imposes careful monitoring of the strength evaluation through curing time. However, concrete under harsh environmental conditions normally experiences unexpected issues that decline the strength development. Significant expenses might be occurred if the concrete curing process is delayed under environmental effects to

Maturity in general is estimated by tracking the changes in strength development for fresh concrete under various temperatures over time intervals. As the cement hydrates, the strength increases; however, the amount of hydrated cement depends on many factors, specifically curing temperature. From the strength-maturity index curves of corresponding samples, the strength on-site could be predicted at any moment after concrete placement, which shows how far the hydration is processed. Usually, the concrete reaches the expected strength under warm weather condition, if the temperature is not abruptly increased. However, in cold temperature situation, the fluctuation in temperature during days and nights might stop the cement hydration process. For understanding, the reasonable evaluation of the strength of the placed concrete, maturity-meters

The concrete strength is directly related to the curing time, humidity index, and temperature fluctuation. The maturity investigation is a method with which the early age concrete strength is monitored and the results subsequently could be implemented for in-site concrete placement [15, 17]. The main assumption with maturity concept is that the concrete is able to attain the same strength if the mixture reaches the same value of maturity index [18]. Another assumption with maturity evaluation is that the combination of temperature and time leads to the same strength for the considered concrete mixture. Therefore, strength development is a function of time and temperature. This function could be nonlinear or linear with respect to time and temperature. The instrument shown in **Figure 5** indicates how mature the concrete mix is which is represented as a number ultimately. The information is subsequently used for establishing maturity curves which are unique for any concrete mixture and related to compressive strength [19–21]. The schematic maturity curve is shown in **Figure 6**. The maturity curves are based on the time-temperature factor (TTF) which is compatible

be assured that the concrete has reached the specified minimum strength.

shown in **Figure 5** are implemented at different stages of curing process.

with the assumption that the maturity is a function of time and temperature.

In general, there are many common methods in obtaining the maturity level of the concrete. The general idea is based on the time-temperature factor which considers the proportional relation between combined temperature-time and compressive strength. The important issue related to this commonly used method shown in Eq. (1) is that the strength development in significantly higher or lower

**96**

**Figure 5.**

*The maturity-meter device used for concrete strength evaluation.*

**Figure 6.** *The schematic maturity-meter curve showing the concrete maturity.*

temperatures is not accurate; hence, the careful interpretation of the results in extreme temperature is inevitable.

$$\mathbf{M} = \sum\_{\mathbf{0}}^{\mathbf{t}} (\mathbf{T} - \mathbf{T}\_0) \Delta \mathbf{t} \tag{1}$$

where M is the maturity index in °C h, T is the average temperature in °C over time interval of ∆t per hours, and T0 is the datum temperature usually taken as 0°C at which the hydration is stopped and concrete does not gain any strength. M index is usually referred to as the maturity index related to TTF factor [2]. To understand the assumption in this method, **Figure 7** is shown in which the area of temperature over time for a specific mixture under cold condition is M1, and under corresponding mild temperature condition is M2. If M1 and M2, which represents the maturity index of a mixture, are equal, then the compressive concrete strength is expected to be the same. It is highly important that this assumption is only useful if the concrete does not experience any harsh environmental condition (e.g., excessive cold or hot temperature) during initial stages of the curing process.

To further investigate the effect of harsh environmental conditions on maturity and strength development of the concrete, the cylinder-shaped specimens are used for maturity evaluation of the four different concrete mixtures. Different temperatures and rates of humidity are considered for all the mixtures. **Figure 8** shows the maturity evaluation for all the concrete mixtures in which the compressive strength of the specimens are extracted at different time intervals. It is noted that the effect of significant humidity change, even though not considered in maturity concept formulation, is clearly observed. It is shown that environmental effects such as very dry conditions could tangibly change the strength development of the concrete; however, the change might not be as much as temperature variations. The lower water-cement ratio leads to significant gain in early stages of curing or lower TTF values, since all the cement materials actively participate in chemical hydration reactions. As the TTF increases or curing progresses, the rate of strength gain decreases until the concrete reaches the ultimate strength conditions.

**Figure 8** shows the combined effect of temperature and time on compressive strength by using the maturity index. It is noted that temperature drop significantly affect the hydration process, which leads to less ultimate strength over the same period of the time. It is noted that if the harsh environmental condition such as

**Figure 7.** *The schematic representation of the maturity index assumption.*

freezing temperature occurs, the hydration is stopped and the concrete would not be able to reach the ultimate expected strength even the sufficient amount of curing time is provided. In another words, the concrete would not develop the expected ultimate strength regardless of the curing time interval. This phenomena is important since the maturity formulation typically shows that by increasing the curing time, the effect of lower temperature could be compensated, which only is valid under specific environmental conditions (e.g., temperature over zero and sufficient humidity). From **Figure 8**, it is concluded that for all the cylinder samples, as the temperatures decrease significantly during the curing process, the bounding between cementious materials is not fully generated leading to lower ultimate strength even in large period of curing.

**99**

curing ages.

**Figure 8.**

*Compressive Behavior of Concrete under Environmental Effects*

The strength of a given mixture placed and cured with the prescribed procedures is estimated from the temperature and time combinations. This method assumes that the only contributing factors to the strength development are time and temperature, while from the results of humidity rate study, it is shown that a drop of 50% in humidity index values would lead to average of 10% drop in compressive

*Maturity evaluation of different cementious materials. (a) Temperature 23°C and RH 100%. (b) Temperature* 

*23°C and RH 50%. (c) Temperature 10°C and RH 50%. (d) Temperature 5°C and RH 50%.*

In general, there are limitations of the maturity method implementation in

First, the implementation of this method is under the assumption that the placed concrete at the site has similar conditions to the concrete made in the laboratory. Any changes in batching accuracy, air content, and used materials could lead

Second, the areas that the maturity is evaluated within a specimen should not be only allocated to specific point. The hydration process within a large piece of

Third, the maturity method should be revised and carefully interpreted under extreme environmental conditions, which leads to incorrect estimation at early

Fourth, the size of the concrete piece usually is larger than the samples, which means that the hydration process produces more heat and higher temperatures inside. This large temperature difference might affect the compressive strength

**5. Durability of the concrete under cyclic environmental conditions**

The durability of cement mix is defined as the ability for resistance against weathering action, abrasion, and chemical reactions, or generally the processes that

*DOI: http://dx.doi.org/10.5772/intechopen.85675*

strength regardless of cement type.

estimating the concrete strength.

to different maturity curves and strength estimations.

concrete shows distinctive behavior aspects in outer and inner parts.

development, which is not considered in laboratory samples.

*Compressive Behavior of Concrete under Environmental Effects DOI: http://dx.doi.org/10.5772/intechopen.85675*

**Figure 8.**

*Compressive Strength of Concrete*

**98**

**Figure 7.**

strength even in large period of curing.

*The schematic representation of the maturity index assumption.*

freezing temperature occurs, the hydration is stopped and the concrete would not be able to reach the ultimate expected strength even the sufficient amount of curing time is provided. In another words, the concrete would not develop the expected ultimate strength regardless of the curing time interval. This phenomena is important since the maturity formulation typically shows that by increasing the curing time, the effect of lower temperature could be compensated, which only is valid under specific environmental conditions (e.g., temperature over zero and sufficient humidity). From **Figure 8**, it is concluded that for all the cylinder samples, as the temperatures decrease significantly during the curing process, the bounding between cementious materials is not fully generated leading to lower ultimate

*Maturity evaluation of different cementious materials. (a) Temperature 23°C and RH 100%. (b) Temperature 23°C and RH 50%. (c) Temperature 10°C and RH 50%. (d) Temperature 5°C and RH 50%.*

The strength of a given mixture placed and cured with the prescribed procedures is estimated from the temperature and time combinations. This method assumes that the only contributing factors to the strength development are time and temperature, while from the results of humidity rate study, it is shown that a drop of 50% in humidity index values would lead to average of 10% drop in compressive strength regardless of cement type.

In general, there are limitations of the maturity method implementation in estimating the concrete strength.

First, the implementation of this method is under the assumption that the placed concrete at the site has similar conditions to the concrete made in the laboratory. Any changes in batching accuracy, air content, and used materials could lead to different maturity curves and strength estimations.

Second, the areas that the maturity is evaluated within a specimen should not be only allocated to specific point. The hydration process within a large piece of concrete shows distinctive behavior aspects in outer and inner parts.

Third, the maturity method should be revised and carefully interpreted under extreme environmental conditions, which leads to incorrect estimation at early curing ages.

Fourth, the size of the concrete piece usually is larger than the samples, which means that the hydration process produces more heat and higher temperatures inside. This large temperature difference might affect the compressive strength development, which is not considered in laboratory samples.

### **5. Durability of the concrete under cyclic environmental conditions**

The durability of cement mix is defined as the ability for resistance against weathering action, abrasion, and chemical reactions, or generally the processes that deteriorate and affect the strength and stiffness. The durable concrete maintains the original quality and serviceability under environmental effects. Concrete is assumed durable if


The durability of the concrete is highly depended on the cement content, water-cement ratio, curing process, cover, sufficient compaction, and appropriate mix design. In general, the outer causes of durability loss are: extreme weathering condition, abrupt temperature change, high humidity, chemical attacks, and ettringite formation. The inner causes are volume changes due to thermal properties of the aggregates and high water content, chemical reactions of ingredients, and steel reinforcement corrosion.

For durability assessment of the concrete, ASTM C666 [17] provides an equation which is shown in Eq. (2). If up to 300 cycles, significant cracks are not existed, then the concrete maintains its durability under environmental changes.

$$DF = \text{PN} / \text{M} \tag{2}$$

**101**

**Figure 11.**

*(c) Quickrete after 34 cycles. (d) Dayton after 300 cycles.*

*Compressive Behavior of Concrete under Environmental Effects*

cals, extreme weathering, and concrete mix design.

test the specimens are considered, and the durability index is evaluated accordingly. **Figure 10** shows some of the specimens after significant number of freeze-thaw cycles. From this experiment, it is shown that the durability of the concrete is highly dependent on the cement matrix, water-cement ratio, exposure to harmful chemi-

The durability factor is obtained for each cement type at 30 each cycles, and the results are summarized in **Figure 11**. It is concluded that cement type and watercement ratio has significant effect on the resistance and durability of the concrete. The high water-cement ratio would lead to higher permeability, which will be filled

The water volume is expanded upon freezing and it is transformed to ice, which eventually causes microcrack propagation in concrete samples. Microcracks

*Durability of the cement under freeze-thaw cycles. (a) BASF after 300 cycles. (b) Five star after 300 cycles.* 

*DOI: http://dx.doi.org/10.5772/intechopen.85675*

with water in the next cycles.

*Significant number of freeze-thaw cycles.*

**Figure 10.**

where DF is the durability factor of the specimen and P is relative dynamic modulus of elasticity. M is the specified number of cycles at which the exposure is terminated. N is the minimum of the cycles' number at which P reaches the specified minimum value for discontinuing the test, and the specified number of cycles at which the exposure is to be terminated. The general prepared specimens for durability investigation are shown in **Figure 9**.

For further investigations, the behavior of the concrete samples is monitored for cycles of freeze-thaw. For each type of cement, two specimens are made to sufficiently validate the results of the test. ASTM 666 procedures [17] to build and

**Figure 9.** *Durability investigation of the concrete, and freeze-thaw test.*

### *Compressive Behavior of Concrete under Environmental Effects DOI: http://dx.doi.org/10.5772/intechopen.85675*

test the specimens are considered, and the durability index is evaluated accordingly. **Figure 10** shows some of the specimens after significant number of freeze-thaw cycles. From this experiment, it is shown that the durability of the concrete is highly dependent on the cement matrix, water-cement ratio, exposure to harmful chemicals, extreme weathering, and concrete mix design.

The durability factor is obtained for each cement type at 30 each cycles, and the results are summarized in **Figure 11**. It is concluded that cement type and watercement ratio has significant effect on the resistance and durability of the concrete. The high water-cement ratio would lead to higher permeability, which will be filled with water in the next cycles.

The water volume is expanded upon freezing and it is transformed to ice, which eventually causes microcrack propagation in concrete samples. Microcracks

**Figure 10.**

*Compressive Strength of Concrete*

ration is limited and controlled.

the components together.

steel reinforcement corrosion.

durability investigation are shown in **Figure 9**.

*Durability investigation of the concrete, and freeze-thaw test.*

assumed durable if

permeable.

deteriorate and affect the strength and stiffness. The durable concrete maintains the original quality and serviceability under environmental effects. Concrete is

2.Minimum impurities such as chlorides, slit, and sulfates are existed.

3.The aggregates are clean and well graded; therefore, the concrete is less

The durability of the concrete is highly depended on the cement content, water-cement ratio, curing process, cover, sufficient compaction, and appropriate mix design. In general, the outer causes of durability loss are: extreme weathering condition, abrupt temperature change, high humidity, chemical attacks, and ettringite formation. The inner causes are volume changes due to thermal properties of the aggregates and high water content, chemical reactions of ingredients, and

which is shown in Eq. (2). If up to 300 cycles, significant cracks are not existed,

then the concrete maintains its durability under environmental changes.

1.Under extreme environmental effects (e.g., cycles of freeze-thaw), the deterio-

4.The cement matrix is well structured and dense which appropriately bonds all

For durability assessment of the concrete, ASTM C666 [17] provides an equation

*DF* = *PN*/*M* (2)

where DF is the durability factor of the specimen and P is relative dynamic modulus of elasticity. M is the specified number of cycles at which the exposure is terminated. N is the minimum of the cycles' number at which P reaches the specified minimum value for discontinuing the test, and the specified number of cycles at which the exposure is to be terminated. The general prepared specimens for

For further investigations, the behavior of the concrete samples is monitored for cycles of freeze-thaw. For each type of cement, two specimens are made to sufficiently validate the results of the test. ASTM 666 procedures [17] to build and

**100**

**Figure 9.**

*Significant number of freeze-thaw cycles.*

#### **Figure 11.**

*Durability of the cement under freeze-thaw cycles. (a) BASF after 300 cycles. (b) Five star after 300 cycles. (c) Quickrete after 34 cycles. (d) Dayton after 300 cycles.*

adversely change the concrete resistance against further freeze-thaw cycles leading to major cracks and abrupt loss of strength. It is recommended that lower watercement ratio mix designs to be used under freeze-thaw possibility to limit the durability loss and maintain compressive strength.
