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Research Paper 2015

Literature Survey of Curing requirements for Precast Concrete

An online research paper to gather existing knowledge on practices and requirements for curing and their benefits relating

to precast concrete.

Research by Emma Mcfarlane, (2nd year engineering student, University of Canterbury)

Sponsored by Precast NZ Inc.

This paper can be downloaded from www.precastnz.org.nz TECHNICAL


Foreword (by Justin Bragg, Director Concretec New Zealand Ltd)

This paper was written to aid the New Zealand construction industry in understanding the benefits of curing precast concrete products and the effects of different curing practices. The motivation for the paper came from the apparent ‘gap’ between historic industry curing practices and the requirements of NZS3109, which is often quoted in specifications for curing concrete. In addition it was recognised NZS3109 is quite simplistic and prescriptive in the requirements for curing, and there was a belief a more ‘fitness for purpose’ and cost benefit approach to curing could be beneficial.

As expected, the research undertaken found multiple laboratory testing studies showing that extended curing of concrete after casting clearly aided in the development of strength and durability properties, which superficially suggests curing after casting is an essential part of all concrete casting processes. While curing improves the finished concrete, almost all concrete can be improved by better aggregates, optimisation of additives, water reduction to the level required for hydration amongst others. All processes to improve the finished concrete involve a cost. In most areas it is recognised that over specifying does not provide benefits that justify the cost, and in this respect curing is no different.

It was evident that the relationship between curing and concrete properties is complex, and is dependent of many factors including both concrete mix design and the environment. Specifying curing requirements based on any one theme of laboratory testing results does not lead to the most cost effective outcome. What was difficult to find, were research papers relating these types of test results to performance in the real world, or comments within the papers reviewed offering the same.

It is reasonable to conclude from this survey, that the curing requirements for pre-‐cast concrete after casting should recognise the particular environment (is the casting indoors or exposed to wind and temperature fluctuations?), the casting process (is the product heat cured or is it retained in an impervious mould?), the mix used (is it higher strength low W:C ratio to achieve quick turnover?), the end use (is it exposed to a particularly harsh environment or will it have a coating applied?). Some European concrete standards recognise differences. However, it is also clear appropriate curing of concrete is an essential step when casting conditions and concrete product end use dictate.

This paper and supporting research papers provide a significant resource for those wanting a better understanding of the chemistry of concrete curing and the common terminology used. Finally this research has raised many questions, the most important being how the testing results reviewed relate to real world performance which is potentially another area for research.

Precast NZ wishes to thank Emma McFarlane for her work that produced this paper, and the others who have contributed informed comment.


Contents 1. Summary ........................................................................................................................................ 6

2. Introduction ................................................................................................................................... 8

3. Objectives ...................................................................................................................................... 9

4. Glossary of key words .................................................................................................................. 10

5. Methods of Curing ....................................................................................................................... 10

6. Concrete Standards around the World ........................................................................................ 11

6.1 Australian Standards ............................................................................................................ 11

6.2 Canadian Standards ............................................................................................................. 11

6.3 British Standards .................................................................................................................. 12

6.4 European Code .................................................................................................................... 12

6.5 German Standards ............................................................................................................... 15

6.6 Japan’s Concept for Durability design ................................................................................. 15

7. Chemistry of Curing ..................................................................................................................... 16

7.1 Background .......................................................................................................................... 16

7.2 Microstructure of Concrete ................................................................................................. 16

7.3 Connectivity of Pores ........................................................................................................... 17

8. The Impact of Mix Design on results and conclusions ................................................................. 17

8.1 The required water/Cement ratio ....................................................................................... 17

8.2 Cement Content is related to the Concrete’s sensitivity ..................................................... 18

8.3 Analysis of research papers comparing different mix designs ............................................. 18

8.3.1 The impact that water-‐cement ratio has on the measured carbonation depth .......... 18

8.4 Conclusion ........................................................................................................................... 19

9. Aspects which impact the strength of concrete .......................................................................... 19

9.1 Analysis of Research Papers about Strength ....................................................................... 20

9.1.1 Influence of curing duration on tensile strength with respect to distance .................. 20

9.1.2 Effect of different curing methods and duration on Compressive Strength ................ 21

9.1.3 Compressive strength of Air and Water cured cubes for different durations ............. 22

9.1.4 Strength development for concrete cured using different methods ........................... 22

9.2 Conclusion ........................................................................................................................... 22

10. Durability ................................................................................................................................. 23

10.1 Background .......................................................................................................................... 23


10.2 Analysis of Research papers concerning Durability properties ........................................... 23

10.2.1 The effect of increasing the curing duration on the permeability of the sample ........ 23

10.2.2 Impact on carbonation depths .................................................................................... 25

10.2.3 Impact of different curing methods on the water absorption and sorptivity values of concrete ..................................................................................................................................... 26

10.2.4 The effect of different curing methods on the chloride ion diffusion ......................... 27

10.3 Conclusion ........................................................................................................................... 28

11. Casting Environment ................................................................................................................ 28

11.1 General Information ............................................................................................................ 28

11.2 The use of a nomograph to estimate evaporation rates ..................................................... 28

11.3 Conclusion ........................................................................................................................... 30

12. Exposure Environment ............................................................................................................ 30

12.1 How the environment impacts the life of the concrete element ........................................ 30

12.2 Research paper measuring the carbonation depth of concrete specimens exposed to different environments after different curing methods. ................................................................. 31

12.3 Conclusions regarding the impact of the environment ....... Error! Bookmark not defined.29

13. Accelerated Curing of concrete ............................................................................................... 31

13.1 Background information regarding New Zealand’s current standard ................................. 31

13.2 Analysis of Research papers comparing accelerated curing with standard curing .............. 32

13.2.1 The impact of accelerated curing on the compressive strength .................................. 32

13.2.2 Impact of heat curing on the durability properties of concrete .................................. 33

13.2.3 Impact of steam curing on standard concrete mixes ................................................... 33

13.3 Conclusion ........................................................................................................................... 34

14. The impact of admixtures on concrete performance when subjected to different curing methods ............................................................................................................................................... 34

14.1 Background information ...................................................................................................... 34

14.2 Analysis of research papers testing blended cement .......................................................... 35

14.2.1 Impact of curing method on the strength of concrete containing silica fume ............ 35

14.2.2 Impact of addition of silica fume on the durability of the concrete ............................ 36

14.2.3 Impact of steam curing on samples containing silica fume ......................................... 36

14.3 Conclusion ........................................................................................................................... 36

15. Surface treatments of Concrete .............................................................................................. 37

Bibliography ........................................................................................................................................ 37


Table 1: Minimum periods of curing & protection according to British Standards (Meeks & Carino [12]) ..................................................................................................................................................... 12 Table 2: CEB-‐FIP 1990 Exposure classes summarized (Meeks & Carino [11]) ..................................... 13 Table 3: CEB-‐FIP minimum duration of curing for exposure classes 2a, 2b, 4a and 5a (Meeks & Carino [11]) ..................................................................................................................................................... 13 Table 4: Rate of development of impermeability of concrete (Meeks & Carino [11]) ........................ 14 Table 5: ENV 206 minimum curing durations for exposure classes 2 and 5a (Meeks & Carino [11]) .. 14 Table 6: Strength development for Table 9 (Meeks & Carino [11]) ..................................................... 15 Table 7: Minimum curing time for concrete with all exposure classes with the exception of X0 and X1, which have no risk to corrosion (Grube & Dusseldorf [25]) .......................................................... 15 Table 8: Carbonation depths (mm) from Balayssac, Detriche and Grandet experiment [18] ........ Error! Bookmark not defined. Table 9: Summary of estimates 95% confidence intervals of average strength at 25mm (Carino & Meeks [20]) .......................................................................................................................................... 20 Table 10: Time for Capillaries to become Discontinuous (American Concrete Institute [1]) .............. 23 Table 11: Descriptions of the different curing methods tested (Zhang & Zong [18]) .......................... 25 Table 12: Carbonated depths mm (Balayssa, Detriche and Grandet [13]) .......................................... 26


1. Summary Curing is normally considered to refer to the actions taken following casting of concrete to control the environment and maintain sufficient moisture in the concrete to affect the rate and extent of cement hydration so that concrete develops its required design strength and durability properties to improve the strength and durability of the concrete. Typical methods used are: applying water, absorptive mats, wrapping in plastic or applying a surface curing compound.

The New Zealand Standards 3109 & 3101 have prescriptive requirements for curing. They require protection from premature drying, excessively hot or cold temperatures and mechanical injury for 3 days or 7 days depending on the exposure zone of the product in its end use. NZS3101 permits alternative curing methods providing a special study proves the alternative method provides equivalent durability performance.

Historic common practices built on experience in the precast industry often do not involve any of the curing processes noted above. This is probably because of the use of relatively high strength concrete, heat cured in factory conditions away from the impact of weather. This produces concrete which has overnight strengths of approximately 20-24 MPa, which is close to the 7 day strength of normally cured concrete. Exceptions to this are when curing is specifically requested by the engineer, and or when conditions after casting create excessive drying of the concrete (e.g., windy & low humidity).

This implies that the precast industry often does not conform to the construction code with respect to curing. Collective views from the industry are that since typical precast manufacturing methods (there is also scope in the code to alter the curing period for accelerated curing methods clause 7.8.5) will achieve the required concrete properties for many product applications, a fitness for purpose approach should be applied to curing methods as opposed to a blanket approach for all.

In view of that “gap” between code and typical practices, research has been undertaken via an online papers search to gain more knowledge about curing versus concrete performance, international practices, and ideally provide clearer guidelines on a fitness for purpose approach to curing as opposed to a blanket approach suggested by the code.

The University of Canterbury database and others such as science direct and other university’s resources yielded approximately 200 papers on the subject of curing. There was a research focus towards topics that tested mix designs which was similar to those used by the precast industry.

A number of national standards were reviewed, and some of these provided a more comprehensive guide to curing concrete than the NZ standard, including specifying either none or 1 days of curing when conditions allow. Interestingly, the European standards allowing this, define favourable conditions which would be typical for many NZ pre-casting environments. These concrete codes factoring in how fast the concrete develops impermeability and the exposed ambient conditions during and after curing are beneficial to the precast industry as concrete is generally cast in favourable controlled conditions gaining


high early strength with a low w/c ratio. However none were found to relate specifically to precast concrete curing and performance

Today with the advancement in technology there are many different types of concrete mixes. Curing specifications cannot cover all of these mixes but they should still reflect the varying hydration rates and permeability properties of different mixes, environment and casting conditions instead of stipulating an equal curing duration for all.

The environment in which casting takes place also has a dominant role in determining the evaporation rate of water and consequently how much extra water may need to be supplied to ensure enough hydration of the cement paste occurs. If the concrete is protected from environmental factors such as wind and low humidity, longer moist curing durations may not have a significant impact as there may be little or no net evaporation occurring in the first place.

Even though air cured samples typically have a lower compressive strength than moist cured samples, precast concrete is often higher strength than required and hence additional moist curing may have little benefit in increasing the strength, and other properties such as the mix design should be of more focus.

Insufficient curing predominantly only affects the outer surface of concrete up to 50 mm. This means that if the concrete element is thick the curing duration will not significantly affect the strength.

Having high strength concrete does not automatically indicate good durability, therefore research has been done to establish how large the difference is between permeability, carbonation depth and chloride ion resistance values when samples are subjected to different curing methods and durations. It was found that the longer the moist curing duration the better the durability attributes, however concrete elements which are kept dry and are either inside or exposed to mild environments may not need to have a high durability performance; therefore, these concrete elements may not gain significant performance benefits to justify the extra costs of increasing the curing duration.

Clear testing evidence was found for a significant reduction in concrete permeability, carbonation depths and chloride ion diffusion when moist curing is extended from 1 to 3 days. These results were impacted by W/C ratio’, and these trends were also continued for water absorption.

There were differences between the conclusions made by various research papers which could be attributed to the different specimens tested having different mix design. The strength of concrete is impacted by the water cement (w/c) ratio, which determines how much hydration is needed for the cement pores to become discontinuous and will therefore, impact how long curing is needed. The cement content also determines how sensitive concrete is to different curing methods as results showed that the higher the cement content the greater the difference in permeability between air and moist cured samples.


Lastly, the findings of the research suggests there is a need for New Zealand specifications, practices and standards to be reviewed in order to more closely reflect standards around the world and the complex nature of curing, and fitness for purpose in the intended design life environment. Curing products longer than required to reach the properties needed for the concrete element’s end may add unnecessary costs to the concrete products. Ideally there should be a move towards a standard which differentiates between different hydration rates, water/cement ratios; and casting and exposure environments. This will more closely align current industry practices, engineer’s requirements and future New Zealand standards.

2. Introduction Curing is defined as “the action taken to maintain moisture and temperature conditions in a freshly placed cementitious mixture to allow cement hydration and (if applicable) pozzolanic reactions to occur so that the potential properties of the mixture may develop” [1]. The New Zealand Standard for Concrete Construction NZS3109 [2] has relatively a prescriptive methodology around curing with little mention about pre-cast concrete. Section 7.8.4 specifics the length of curing and states that “When mean temperatures exceed 10°C, curing shall be continued for at least 7 days, except where otherwise noted on the drawings and specification. When temperatures are less than 10°C the length of curing period shall be nominated by the construction reviewer.” However there is a clause which states that “If the designers designate that concrete is in an A1 or A2 exposure zone, then curing may be reduced to 3 days.”

In order to look into the required methods and duration of curing two aspects of concrete performance must be addressed; the design strength and the necessary durability properties. The strength of concrete is relatively easy to test as the required strength is provided by the design. Precast concrete is usually cast using relatively high strength concrete and overnight reaches strengths of around 20-24 MPa. As the design strength is reached earlier longer curing durations may not be necessary, especially if durability is not a major issue.

On the other hand, the durability of concrete is harder to test as there are a range of properties such as permeability, water absorption, sorptivity, carbonation depth and chloride ion diffusion that need to be considered as well as the environment that the product is exposed to. These factors affect the risk of corrosion of the reinforcing occurring and hence it is in the 30 – 50 mm of the outermost layer of exposed concrete that the impact of curing is relevant to the durability of reinforced concrete. [3] [4] [5] [6]

The report is broken into different aspects which contribute to determining the required curing duration. Each section starts with an introduction giving general information; the next part has evidence from numerous research papers, while the last part has an overall conclusion relating the information back to the precast industry and the significance of it. The first half of the report (section 5-8) provides background into the main aspects of curing such as, the different methods, the chemistry involved and the impact of the design mix on


strength and durability. This section also includes information about concrete standards around the world in order to see how New Zealand standards fit with other countries. The second part of the report focuses more on summarizing conclusions from a range of research papers looking at the impact of different curing durations; it also contains detail around surface treatments and admixtures.

3. Objectives The objective of this literature research was to collate a database of papers with findings and conclusions in order to give pre-casters more confidence in their method of curing and ideally provide sufficient information to influence an alignment of engineer’s project specifications, standards and pre-casting practices. Overall, the objectives of the report include:

• Compiling numerous research papers about curing • Presenting a summary of the findings and conclusions of these papers • Relating the research back to the industry • Providing specifiers and pre-casters with more insight into the chemistry and methods

of curing.


4. Glossary of key words Absorption Uptake of fluid into a porous, unsaturated material under the

action of capillary forces. Capillary A tube which has a thin hair like internal diameter.

Capillary action/Capillarity The ability of a liquid to flow in narrow spaces without the assistance of and in opposition to external forces like gravity.

Carbonation Occurs when CO2 from the air or water enters the concrete and reacts with the calcium in the cement paste reducing the pH of the concrete below 9 at which level the passive coating of the reinforcing which inhibits corrosion breaks down.

Corrosion A process in which something is destroyed by a chemical action a common example is when iron rusts.

Diffusion Flow of matter through a medium from a force created by a concentration gradient. The movement of a substance from a high concentration to a low concentration.

Hydration Chemical reaction in which chemical bonds form with water molecules and become hydrates/hydration products.

OPC Ordinary Portland Cement or GP. Permeability The ability of a fluid to move through a porous material under

an externally applied pressure. Plastic Shrinkage Cracking Cracks formed on the surface of concrete during the setting

process due to a high rate of surface drying. SCC Self-Compacting or Self-Consolidating Concrete. SCM Supplementary Cementing/Cementitious Materials.

Self-desiccation The process of taking up free water by hydration to such an extent that it dries from the inside out.

Sorptivity The rate of movement of a wetting fluid being absorbed into a porous material.

5. Methods of Curing In NZS 3109, the NZ standard for Concrete Construction section 7.8.1 it states that “From immediately after placement, concrete shall be protected from premature drying, excessively hot or cold temperatures and mechanical injury. The concrete shall be maintained with minimal moisture loss for the period necessary for hydration of the cement and hardening of the concrete” Curing of concrete takes place after all finishing processes are complete. Suitable methods of curing include (in order from most effective to least [7]):

• Maintaining water saturation of concrete through water immersion, ponding or continuous spraying.

• Covering exposed surfaces with absorptive mats or a layer of sand and maintaining continuously wet.

• Covering with sealed plastic film to prevent loss of water from the concrete. • Retaining the formwork in place. • Applying a membrane forming curing compound.


Maintaining the presence of mixing water can be achieved by ponding/immersion which is done on flat surfaces such as floors and pavements however; it is most commonly used for curing specimens in the laboratory. Wet coverings such as water saturated fabric can also be used to maintain moisture [8]. The prevention of water loss is achieved through plastic sheets which not only prevent evaporation of water, but also help control the temperature of the concrete element by either absorbing or reflecting sunlight or reducing heat loss. Membrane forming compounds act in a similar way to plastic sheets by creating a thin film on the surface to prevent evaporation and create a barrier; however, these compounds are not ideal for products that will have another material on top of them as they can interfere with the bonding properties of the two materials [8]. Additionally, keeping the forms wet can also prevent water loss.

Curing compounds can protect the concrete from loss of water, slow down the rate of evaporation, improve the strength and reduce the permeability. The end use of concrete needs to be considered when using curing compounds as some will prevent the adhesion of finishes such as paint or further concrete. [9]. Curing products need to be removed before application of a liquid hardener, water repellent like silane or siloxane, or a sealer like an epoxy. Some resin based curing compounds will normally oxidize and wear off; however, additional cleaning such as sand blasting may be required [10].

6. Concrete Standards around the World Concrete is a very old material, therefore it is no surprise that some standards date back many years. It is interesting to note that a 7 day curing period was first mentioned in American standards in 1909 with there being very little change since this. Generally, the standards are prescriptive; however, there is a desire to move away from this and have standards in which durability is specified by performance [11]. (The summaries of the following standards are referenced from Meeks and Carino [12])

6.1 Australian Standards Australian Standard 3600 [12] curing requirements are based mostly on the severity of the exposure condition, with exposure being classed into temperate, tropical and arid environments. For mild exposures at least 3 days moist curing is required, or if accelerated curing is used, curing to a compressive strength of 15MPa is required. For a more aggressive environment, at least 7 days curing under ambient conditions is required, or if accelerated curing is used, curing is required to have a compressive strength of 32 MPa for equivalent curing to be reached.

6.2 Canadian Standards

The Canadian standard CSA 23.194 states: “basic curing to consist of 3 day wet curing at a temperature equal or greater than 10°C. For durability purpose additional 4 days or time to reach 28-day compressive strength.” This is very similar to New Zealand standards, however


worded the opposite way with increasing curing if durability is needed instead of decreasing if it is not, though the duration remains the same between 3 to 7 days.

6.3 British Standards

British standards state that for ordinary portland cement, rapid-hardening portland cement, and sulphate-resisting portland cement, if the surface temperature is kept above 10°C and curing occurs under average ambient conditions after casting (relative humidity is greater than 50 %, but less than 80%, and somewhat protected from sun and wind) then a minimum curing period of 3 days is required. However, the formula in the table would produce shorter curing periods than this. Other concretes will require 4 days if the conditions are average and 7 days if poor. If conditions are good (R.H. is greater than 80%) then there are no special requirements.

Table 1: Minimum periods of curing & protection according to British Standards (Meeks & Carino [12])

Note the application of this British standard in New Zealand, especially Auckland with average humidity of 80%, and temperate climate, would imply often no special requirements for curing are required.

6.4 European Code

The Euro-International Committee for Concrete-International Federation for Prestressing (CEB-FIP) is a guide to aid drafters of national code. The tables below show the exposure classes and minimum curing durations.


Rate of development of impermeability

w/c ratio Class of cement

Very rapid 0.5-‐0.6 Rapid Hardening High Strength (RS)

<0.50 Rapid Hardening High Strength (RS) and Rapid Hardening (R)

Rapid 0.5-‐0.6 Rapid Hardening (R)

<0.50 Normal (N)

Table 2: CEB-‐FIP 1990 Exposure classes summarized (Meeks & Carino [12])

Table 3: CEB-‐FIP minimum duration of curing for exposure classes 2a, 2b, 4a and 5a (Meeks & Carino [12])


Medium 0.5-‐0.6 Normal (N)

<0.50 Slowly Hardening (SL)

Slow All other cases

The European Committee for Standardization, Prestandard ENV 206 gives the following provisions for curing which are shown in Table 5. The exposure classes are the same as what is given in the Table 2 while the strength development is given in Table 6

Table 5: ENV 206 minimum curing durations in days for exposure classes 2 and 5a (Meeks & Carino [12])

However the temperatures in the above table are lower than typcial temperatures used in the precast industry meaning that these curing durations could possibly be reduced if the trend in the table was continued.

Table 4: Rate of development of impermeability of concrete (Meeks & Carino [12])


Table 6: Strength development for Table 9 (Meeks & Carino [12])

The key points in European practice (EN 13670) include that curing can be provided by maintaining air humidity above 85% , or keeping the forms in place, or covering with waterproof sheets or covers, or applying water, or using curing compounds.

6.5 German Standards The German Institute for Standardization is unique because it is based on the ratio of the average 2 day and 28 day compressive strength at 20°C. This leads to the classification of rapid, average, slow and very slow strength development, which along with the surface temperature which will give the curing time as seen in table 7.

Strength development of the concrete

Rapid Average Slow Very slow

r= ratio of the average values of the compressive strength s after 2

and 28 days (r=fcm2/fcm28)

r ≥ 0.50 r ≥ 0.30 r ≥ 0.15 r ≤ 0.15

Surface temperature T in

°C

Minimum curing time in days

T ≥ 25 1 2 2 3 25 > T ≥ 15 1 2 4 5 15 > T ≥ 10 2 4 7 10 10 > T ≥ 5 3 6 10 15

Table 7: Minimum curing time for concrete with all exposure classes with the exception of X0 and X1, which have no risk to corrosion (Grube & Dusseldorf (Grube & Kerkhoff, 2000))

6.6 Japan’s Concept for Durability design The Japan Society of Civil Engineers (JSCE) in 2002 provided a concept for durability design as part of their performance-based design concept. The JSCE states that the specification for durability is in four parts: structural performance verification, seismic performance verification; materials and construction, and maintenance. Verification of aspects that impact the durability of the concrete is relatively straight forward. Verification for carbonation includes being able to verify that the depth of carbonation is less than the critical depth to


initiate steel corrosion. To verify that corrosion of reinforcement due to the ingress of chloride ions will not occur, it must be proven that the chloride concentration at the location of the reinforcement is below the critical concentration. These require tests to be carried out; however, verifying cyclic freezing and chemical attack is assessed merely on the basis of if it occurs and how much deterioration it will cause, due to the fact that there is simply no quantitate evaluation as of yet. Some of these tests are cheap and reliable while others are not. Therefore, it must be decided if it is beneficial to base the curing duration solely on the ability to verify these parameters [14].

Conclusions

Most of the standards above provide a considerably more sophisticated approach to assess curing requirements than New Zealand’s NZS3109. Further, applying these standards to New Zealand conditions, especially Auckland with relatively high humidity and warm temperatures, yields curing times of just 1 or 2 days, or even no requirements. This is a contrast from the New Zealand standard which mostly prescribes 7 days.

7. Chemistry of Curing

7.1 Background Cement is made up of a range of phases (about 75% Tricalcium silicate and Dicalcium silicate) which undergo hydration when water is added to them. During this reaction, tricalcium silicate (2Ca3SiO5) reacts to release calcium ions (Ca+), hydroxide ions (OH-) and heat. (Note the release of the hydroxide ions increases the pH of the concrete to over 12, which provides the protection against corrosion of reinforcing steel.) The reaction will continue until saturated and when this happens the calcium hydroxide (3Ca(OH)2) will begin to crystallize as calcium silicate hydrate (3CaO.2SiO2

.4H2O) forms. The crystals grow thicker and interlock which make it harder for water to get to the unhydrated molecules. The speed of the reaction is now determined by how fast the water can diffuse through the calcium silicate hydrate coating, which in turn thickens over time [15]. Although all of these compounds contribute to the final product, it is calcium silicate that is responsible for the strength. Tricalcium silicate contributes to the early strength during the first 7 days, while dicalcium silicate reacts slower contributing to the strength later on. The curing process referred to in this paper is ensuring that there is enough water for the reaction to continue until the required properties are reached

7.2 Microstructure of Concrete Concrete has a complex microstructure made up of aggregate, Calcium Hydroxide, Calcium Silicate Hydrate; hydrated and unhydrated cement grains, and pores. Water that is not consumed in the hydration reactions will remain in the microstructure of the pore space [15]. These pores make the concrete weaker due to calcium silicate hydrate bonds not being able to form adequately enough to provide strength. Hardened cement paste can have two general types of pores; capillary and gel. The gel pores are spaces between the solid products of


hydration within the cement gel; while the capillary pores are the spaces between the cement gel formed during the hydration of the cement grains. When saturated these pores are normally filled with water that is strongly held to the solids, but when exposed to drying conditions the pores will empty due to evaporation [16].

7.3 Connectivity of Pores An important characteristic of cement paste is the connectivity of the capillary pores. When these pores become discontinuous (due to the increased formation of cement gel) there will be a reduction in the permeability of the concrete. Research by Powers and Brownyard (as cited by Carino & Meek [12]) concluded that pastes with a higher water-cement ratios need a higher degree of hydration before the capillaries become discontinued which can be seen in Figure 1. Therefore, the duration of curing should be measured as the time required for the capillary pores to become discontinuous as this will be when permeability is the lowest.

8. The Impact of Mix Design on results and conclusions

8.1 The required water/Cement ratio The rate of hydration, and hence the strength and durability of concrete is very dependent on the w/c ratio. The amount of water in the concrete system will help decide if extra water needs to be applied in order for adequate hydration to take place. The duration of curing is sensitive to the w/c ratio of the paste because a lower w/c ratio results in closer initial spacing of the cement particles, which requires less hydration to fill interparticle spaces with hydration products.

Higher strength concrete with water binder ratios below 0.45 may be vulnerable to self-desiccation [3], as all of the mix water is required for cementing reactions. Self-desiccation is the process of taking up free water by hydration to such an extent that there is not enough to

Figure 1: Degree of hydration at which capillary pores become discontinuous (Meeks & Carino [12])


cover the unhydrated particles [17], though, it is still not clear if additional curing will prevent self-desiccation of concrete with a low w/c ratio.

Neville (as cited in Newman & Choo [4]) stated that a sealed concrete with a w/c ratio below 0.5 will experience self-desiccation. As well as this, Powers (as cited in American Concrete Institute [1] ) demonstrated that concrete mixtures with a w/c ratio less than approximately 0.5 and sealed cannot develop their full potential hydration due to a lack of water; however, he also stated that not all mixtures may need to reach their full hydration in order to develop adequate properties to achieve the specified design life.

More research has been carried out by Powers and Brownyard (as cited in Milestone & Gorce [16]) to gain a deeper understanding into the required w/c ratio. Though, it is generally accepted that a w/c ratio of at least 0.25 is needed to fully hydrate the calcium silicates this does not provide enough space for the expanding hydration reaction to take place; so complete hydration rarely occurs when the w/c ratio is below 0.38. Furthermore, due to water being locked in the fine gel pores of the concrete’s microstructure and unable to be used for hydration; it was concluded that for complete and uninhibited hydration to occur a w/c ratio of approximately 0.42 is required.

8.2 Cement Content is related to the Concrete’s sensitivity One of the main difficulties when comparing the results from different research papers is the fact that the cement content influences the concrete’s sensitivity to of curing. Balayssac, Detriche and Grandet [18] carried out tests and found that concrete with a cement content of 300 kg/m3 had a 10% decrease in carbonation depth when the duration of curing was increased from 1 day to 3 days. While on the other hand, when the cement content was increased to 420 kg/m3 the depth decreased by 50% when curing was extended. This showed that as the content of cement increased so too did the concrete’s sensitivity to curing.

8.3 Analysis of research papers comparing different mix designs

8.3.1 The impact that water-‐cement ratio has on the measured carbonation depth The w/c ratio of the concrete impacts a range of durability parameters. The risk of damage in a freeze thaw environment is reduced if the w/c ratio is less than 0.45 and curing is carried out to obtain the benefit of using a low w/c ratio [6]. Additionally concrete with a lower w/c ratio tends to have a shallower carbonation depth [19]. In an experiment conducted by Balayyssac, Detriche & Grandet [18], they compared specimens with different strengths and w/c ratio and measured the carbonated depths every few months for 18 months. The results showed that a lower w/c ratio may be able to compensate for poor curing as the carbonation depth of the specimens is initially low. The specimens were stored at 20°C, with a relative humidity (R.H) of 60% and a CO2 content of 0.03%. Their results showed that a concrete with a w/c ratio of 0.65 and a 28 day compressive strength of 25 MPa which was moist cured for 28 days had a 9mm carbonated depth after 18 months; while, concrete with a w/c ratio of 0.48 and a compressive strength of 43.5 MPa cured for 1 day had a carbonated depth of 9.5 mm after 18 months. Therefore, as seen from the results shown in table 8 below the strength


and w/c ratio of the concrete had more impact on the depth of carbonation, than the curing duration as the difference between them was only 0.5 mm.

Age (day) Concrete Properties

Curing 90 180 360 540

C = 300 kg/m3 w/c = 0.65 R28 = 25.1 MPa

1 d 6.5 11 13 15 3 d 4 6 9.5 13 28 d 3 5 6 9

C = 340 kg/m3 w/c = 0.61 R28 = 32.6 MPa

1 d 5.5 10 12 13 3 d 3.5 5 6 8 28 d 2.5 4 4.5 6

C = 380 kg/m3 w/c = 0.53 R28 = 37.8 MPa

1 d 4.5 9 10 11.5 3 d 3 5 5.5 7 28 d 2 3.5 4 5

C = 420 kg/m3 w/c = 0.48 R28 = 43.5 MPa

1 d 4 7.5 8.5 9.5 3 d 2.5 4 3.5 4 28 d 1.5 3 3 3.5

Table 8: Carbonation depths (mm) from Balayssac, Detriche and Grandet experiment [18]

8.4 Conclusion It is important to consider the w/c ratio and the cement content of the samples being tested when considering the impact of curing. For example, while high cement concrete shows significant benefits from extended curing, the relatively “poor” durability properties found with little or no curing can still exceed the same of well cured concrete of lower cement content. Precast concrete is typically made with a low w/c ratio which is beneficial since the lower the w/c ratio the lower the permeability and the shallower the carbonation.

9. Aspects which impact the strength of concrete The strength of concrete is impacted by the microstructure of the cement paste which is impacted by the w/c ratio and the rate of hydration. As previously stated pores make the concrete weaker as cement bonds are unable to form in these spaces. The porosity will increase as w/c ratio increases due to water “diluting” the cement paste and creating water filled pores. This ultimately means that the lower the w/c ratio the higher the strength [15]. The rate of hydration also impacts the strength because the chemical bonds within the concrete cured at a higher temperature will having a higher initial strength than concrete at a lower temperature [15]. In addition the rate of hydration slows down after the internal relative humidity drops below 80% ( [1] [12] [6]) and is negligible when the internal humidity is 30% [12].


When curing is terminated drying of the surface occurs and hydration stops as the moisture content decreases [12]. However, it takes time for this drying front to reach the interior of the concrete meaning some strength is gained after curing has stopped. Therefore, insufficient curing often has only a minor effect on the strength development of thick concrete elements because the core has higher moisture content for a longer period of time compared to the outer layers. It is suggested that the curing period should be long enough to ensure that the 28 day strength at the depth of the first layer of reinforcement is equal to the design strength [20].

The size and shape of the test cylinders being used in tests are also important given that the impact of insufficient curing on strength depends on the thickness of the concrete element. In most laboratory tests, the concrete specimens are small meaning that there is a large proportion of the total volume affected by the curing process [6]. Typically the concrete element being cured is significantly larger than the test cylinders, meaning that the impact of poor curing on strength decreases as the specimen size increases since the concrete core is further away from the exposed surface.

9.1 Analysis of Research Papers about Strength

9.1.1 Influence of curing duration on tensile strength with respect to distance Carino and Meeks [20] carried out tests using two mortar mixes with either a w/c ratio of 0.3 or 0.45 (they felt it would show similar trends to typical concrete) to find the influence that curing duration had on tensile strength with respect to the distance from the surface. The data (which is displayed in table 9) showed that one day of moist curing could be sufficient to ensure acceptable strength development at a depth of 25 mm from the sample’s exposed surface. This was surprising as it was believed that a shorter curing duration would result in rapid drying and a decrease in the rate of hydration. They concluded that an increase in strength due to drying may have compensated for a loss in strength due to incomplete hydration.

Water/cement ratio

Moist Curing, Days

Relative Humidity %

Average Strength, MPa

0.3 Continuous 4.88 ± 0.17 1 50 4.75 ± 0.24

70 4.70 ± 0.24 3 50 4.57 ± 0.50

70 5.22 ± 0.14 7 50 5.13 ± 0.27

70 4.61 ± 0.21 0.45 Continuous 2.94 ± 0.14

1 50 3.16 ± 0.23 70 3.24± 0.20

3 50 3.36 ± 0.13 70 3.52 ± 0.13

7 50 3.31 ± 0.17 70 3.33 ± 0.19


Table 9: Summary of estimates 95% confidence intervals of average strength at 25mm (Carino & Meeks [20])

9.1.2 Effect of different curing methods and duration on Compressive Strength Haque (as cited in the Concrete Institute of Australia [7]) studied the effect of different curing methods on the strength. The curing consisted of the concrete being in moulds for 1 day and then moist cured for 0, 7 or 28 days with different temperatures and humidity for 91 days until the compressive strength was tested. There was barely any difference in the compressive strengths after different curing durations for general purpose (GP) cement, except for the samples stored at a R.H. of less than 50%; while there was considerable difference seen with the GP+30% Fly Ash which is depicted in Figure 2

The final strength of the samples was very high as the plain concrete generally had a compressive strength greater than 60 MPa. The results from this study are more insightful than other papers because each sample was tested after 91 days regardless of the curing duration. This meant that the strength at the same age was being compared instead of comparing the initial overnight strength of an air dried sample with the 28 day strength of a continuously submerged sample

Figure 2: Effect of Curing on Compressive Strength (Concrete Institute of Australia [7])


9.1.3 Compressive strength of Air and Water cured cubes after 90 days Research carried out by Abalaka and Okoli [21] comparing 100mm concrete cubes cured in water and air found that air cured cubes had a 90-day compressive strength which was 9.53% less than that of water cured cubes at 90 days. The cubes were made from a commercial brand of OPC, with a cement content of 530 kg/m3 and a free w/c ratio of 0.3. Overall, an increase in the compressive strength was found to be related to an increase in the extent of hydration.

9.1.4 Strength development for concrete cured using different methods As previously mentioned, concrete which has been air cured does not get a substantial amount of water added to it to help it hydrate. Research was carried out by Goel et al. [22] to measure the increase in strength from 3 to 28 days of a concrete sample when subjected to different curing methods. The samples were100mm x 100mm and 150 x 150mm sizes. Figure 2 shows the compressive strength of concrete sample made with a concrete mix of M20 (20 Mpa and OPC cement (43 grade). The results in figure 2 below show only a small strength difference between the curing methods when tested after 3 or 7 days of curing. This difference increased significantly after 28 days of curing. This curing period is not commonly used in the industry due to cost and time constraints.

Figure 2a: The impact of Curing period on the measured Compressive Strength (Goel et al. [22])

9.2 Conclusion The effect of curing and its benefits decrease rapidly with distance from the concrete surface. The strength of concrete is impacted by the w/c ratio, the rate of hydration and the temperature. The lower the w/c ratio the higher the initial strength; however, without additional moist curing strength development may be limited due to incomplete hydration. Precast concrete generally has a lower w/c ratio meaning strength could be compromised if there is not enough water for hydration. Though air cured samples had the lowest compressive strength values, they still reached the required design strength. This may mean


that an increase in strength due to drying may be able to compensate for any loss of strength due to a reduction in hydration. The compressive strength for concrete is easily verified with tests, therefore regardless of the curing method or duration it is simple to determine whether the required strength has been reached.

10. Durability

10.1 Background Durability problems in concrete result from external agents penetrating the surface and reacting with the constituents of concrete and the reinforcing steel. For deterioration reactions to take place there needs to be a continuous supply of oxygen, water and aggressive ions. Durability is largely dependent on the concrete’s quality of microstructure of near surface concrete. Durability is controlled by the near surface region of concrete and its ability to protect against the ingress of harmful substances. It is largely dependent on the concrete’s permeability which is related to the discontinuity of the capillary pores.

Table 10 shows the time taken for the capillary pores to become discontinuous[12].

Table 10: Time for Capillaries to become Discontinuous (American Concrete Institute [1])

10.2 Analysis of Research papers concerning Durability properties

10.2.1 The effect of increasing the curing duration on the permeability of the sample Test carried out by Grube and Lawrence (as cited in the American Concrete Institute [1]) found a 50 % reduction in permeability achieved by extending the duration of moist curing from 1 day to 3 days, and a similar improvement by extending it to 7 days (both Figure 3 and Figure 4 show this trend).

Figure 3: The effect of curing on reducing the oxygen permeability of a concrete surface (American Concrete Institute [1])


Dinku and Reinhardt [5] measured the gas permeability of the concrete cover of samples submerged in water for different durations. A range of samples were tested with w/c ratios from 0.46 to 0.65 and cement contents from 260 to 369 kg/m3. Samples cured at a temperature of 20°C and a relative humidity of 100% for either 1, 3 or 7 days. After this they were stored in a control room at a temperature of 20°C and a relative humidity of 65% until testing. The results showed that increasing the duration of curing from 1 day to 3 days or 7 days reduces the gas permeability coefficient by at least half. It was concluded that for normal OPC concrete a good quality concrete has a gas permeability coefficient of less than 0.7 × 10-17 m2 while a poor quality concrete has a coefficient higher than 2.5 × 10-17 m2.

Zhang and Zong [24] tested permeability of two types of OPC concrete with a cement content of 420 kg/m3 and a w/c ratio of either 0.4 or 0.45. The specimens were subjected to different curing methods which are described in Table 11.

Figure 4: The influence of curing on the water permeability of concrete (American Concrete Institute [1])


Method of curing and storage Curing a Specimens were submerged in water at a

temperature of 20 ± 3°C after demoulding until testing.

Curing b Specimens were cured in air condition at a temperature of 20 ± 3°C and a relative humidity of 90 ± 5% until testing.

Curing c Specimens were cured in air condition at a temperature of 20 ± 3°C and relative humidity of 60 ± 5% until testing.

Curing d Specimens were submerged in water for 7 days after demoulding and subsequently placed in air conditions with a temperature of 20 ± 3°C and relative humidity of 90 ± 5% until testing.

Curing e Specimens were submerged in water for 7 days after demoulding and subsequently placed in air conditions with a temperature of 20 ± 3°C and relative humidity of 60 ± 5% until testing.

. Table 11: Descriptions of the different curing methods tested (Zhang & Zong [24])

The results showed a large difference in permeability between the two w/c ratios regardless of the curing method which further demonstrates that permeability is greatly affected by the w/c ratio which may be the reason as to why there are dissimilarities between research papers. The concrete sample cured in air (20 ± 3°C, RH 60 ± 5%) with a w/c ratio of 0.45 had the highest value, which is seen in Figure 5.Air cured samples stored under high humidity had similar results to samples which were water cured for 7 days. This suggests that a humid environment was able to provide additional hydration.

10.2.2 Impact on carbonation depths due to cement content & curing. Balayssac, Detriche and Grandet [18] carried out research into durability by measuring the carbonation depths of four different concretes (different cement contents and W:C) stored at 20°C and a relative humidity of 60% for 18months. The tests found that as the cement content increased the carbonated depths decreased, irrespective of the curing time. However, it was also found that carbonation depth decreased rapidly when curing time was increased

Figure 5: Permeability coefficient measurements after different Curing methods (Zhang & Zong [24])


from 1 day to 3 days. For a concrete with a cement content of 300 kg/m3 there was a 10% decrease in carbonation depths when the curing time was increased to 3 days. When the cement content was 420 kg/m3 the depth decreased by 50%, albeit the depths were much lower than the 300kg/m3 concrete. Table 12 shows the carbonated depths for different concrete compositions.

10.2.3 Impact of different curing methods on the water absorption and sorptivity values of concrete

Zhang and Zong [24] carried out an experimental study on the influence of curing methods on water absorption for the same mixes and curing methods used for Table 11. Water absorption was measured as an increase in mass as a percentage of dry mass. It was found that samples air cured at a high relative humidity (90%) showed the lowest water absorption properties (curing b) along with the sample which was continuous submerged in water until testing (curing a) . The sample which was submerged for 7 days and then placed in a high humidity environment (curing d) had an absorption value similar to the lowest ones. The sample cured at a low humidity (60 ± 5%) had the highest value. There was little difference for samples with different w/c ratios, with the surface water absorption being only slightly higher in samples with a w/c ratio of 0.45. Figure 6 shows the surface and internal water absorption of samples with a w/c ratio of 0.4 & 0.45.

Table 12: Carbonated depths mm (Balayssa, Detriche and Grandet [18])


10.2.4 The effect of different curing methods on the chloride ion diffusion Zhang and Zong [24] also carried out tests regarding the chloride ion diffusion coefficient using the same curing methods used for Table 11.The results from the chloride ion test again found that the air cured, type c (20 ± 3°C, RH 60 ± 5%) sample had the highest value; however, it should be noted that the difference between this value and the specimens submerged in water for 7 days was not large. Chloride diffusion occurs via capillary absorption when a surface of concrete is exposed to a salt solution. As seen in Figure 6 the samples with the lower w/c ratio of 0.4 had a lower chloride ion diffusion coefficient.

Figure 6a: Chloride ion diffusion coefficient after different curing methods (Zhang & Zong [24])

Bioubakhsh [23] found that moist curing achieves the lowest chloride penetration in concrete regardless of the w/c ratio. Concretes which had no curing after demoulding showed the poorest performance, and concrete which was moist cured for two days performed better than uncured concrete. In addition, the effect of poor curing on the diffusivity of concrete containing materials such as fly ash and slag was more severe [23].The samples tested were subjected to cycles of wetting and drying.


10.3 Conclusions The above samples of research shows a clear relationship between moist curing duration and durability parameters of permeability, water absorption, carbonation depth and chloride ion diffusion. It also shows a strong correlation between these factors and w/c ratios, with the lower w/c ratios usually exhibiting better results, i.e. more durable. What was not provided was the relationship between these outcomes and durability required in the real world.

11. Casting Environment

11.1 General Information The environment in which casting takes place is important in determining the product’s final microstructure and finish. The environment can greatly impact the rate of hydration if it is not properly managed. There are four main environmental aspects which impact the overall evaporation rate of water off the exposed concrete surface. These include:

Air temperature, Relative humidity, Concrete temperature, and Wind velocity.

The concrete environment needs to be managed to ensure excessive moisture loss does not occur. If the exportation rate of bleed water off the concrete is greater than 1kg/m2/h there is a very high likelihood of plastic shrinkage cracking occurring [1]

It is assumed that concrete cast in a humid and moderate environment requires less or no curing as moisture is partially provided by the surrounding environment and no net evaporation will take place, meaning that there may be an adequate amount of moisture for hydration to take place. [12].

Daily changes in temperature need to be considered as rapid temperature changes of more than 20 °C would be enough to cause cracking, spalling and delamination [25]. Overall, concrete needs to be cast in an environment that does not cause it to dry out before the necessary hydration has occurred. As previously stated, once the relative humidity drops below 80% hydration at the surface will cease and refinement of the pore structure will stop meaning that there will be little improvement in the concrete properties which impact strength and durability.

11.2 The use of a nomograph to estimate evaporation rates There are many methods to approximate the evaporation rate of water all of which require the same measurements of temperature, humidity and wind speed. The National Ready Mixed Concrete Association of America reformatted these different methods to produce the nomograph below (Figure 7). The nomograph is most commonly used to estimate the risk of plastic shrinkage cracking [1]. Drying occurs when the evaporation rate exceeds the bleeding rate; as different concrete mixes have different bleeding rates the nomograph can only be used to give an estimate the evaporation rate and is only helpful in forecasting the need for protection and curing.


It is assumed that when evaporation rates are less than 0.5 kg/m2/hr no curing is required as long as conditions stay roughly the same (until the concrete has developed 70 % of its specified strength) provided there is enough mixing water to maintain the proper moisture needed for hydration [26]. Precast concrete is generally cast in a controlled factory setting, meaning that when it is cast indoors it is in an ideal environmental condition protected from wind and sun resulting in typically low evaporation rates and lower temperature variations.

When using the nomograph the air temperature should be taken 1.2 to 1.8 m above the surface in the shade. The relative humidity should be measured in the shade on the upwind side and 1.2-1.8 m above the evaporating surface. Finally the wind speed should be measured at a height of 0.5 m [1].

As mentioned previously the nomograph can only give an approximate value. Al-Fadhala and Hover (as cited by the American Concrete Institute [1] ) carried out tests to investigate the accuracy of the nomograph. They found that values taken off the nomograph were within ±25

Figure 7: Nomograph to predict evaporation rate (Taylor [6])


% of the actual evaporation rate up to 1.0 kg/m2/h, while at higher evaporation rates the nomograph overestimated the rate by 50 %. Another flaw with using the nomograph is that it computes the water loss from concrete only when the concrete surface is covered with bleed water. Therefore, while the nomograph is fairly accurate during the short time the concrete is covered in water, the actual rate of water loss from the concrete surface decreased to 50% of the nomograph result 3 hours after batching and diminished to 10 % after eight hours. If the nomograph does overestimate the evaporation rate then theoretically most precast conditions would cause low evaporation rates and require no additional water curing as the true evaporation rate is lower than what is reported by the nomograph.

11.3 Conclusion The environment in which concrete is cast impacts the hydration and ultimately determines the evaporation rate and hence drying of the concrete element. Concrete which is cast in low humidity and strong wind will have a greater evaporation rate and require more attention to ensure that plastic shrinkage cracking and adverse durability properties do not result. If concrete is in a protected environment and has sufficient moisture present (e.g high humidity) for complete hydration to take place, additional curing may not be needed due to there being little or no net evaporation.

12. Exposure Environment

12.1 How the environment impacts the life of the concrete element The environmental conditions which concrete is exposed to greatly impacts the lifespan and dictate how important durability properties such as low permeability are for the concrete element to obtain the required design life. Assuming good engineering design follows a fitness for purpose approach, an assessment matching required concrete properties including the curing process should match the intended design life & use.

As stated in section 10 the permeability of concrete is very relevant to durability as it determines how easy it will be for external compounds to enter the medium. In section 6.6 a range of tests are mentioned to assess the overall durability of the concrete element, however a few of these did not require a test and instead it needed to be decided if the mechanism impacting durability would be relevant. For example the ability of the concrete to withstand a chemical attack or freezing and thawing may not be of concern.

NZS 3101 [27] gives a range of cover depths for reinforced concrete in order to ensure that the carbonation depth is not deeper than the cover depth within the specified life of the element. When carbonation causes the pH to drop to below 9 at the reinforcement, the passive film on the re-bars would deteriorate and corrosion may commence. As previously mentioned in section 10.2.2 the speed at which this occurs depends on the porosity and moisture content of the concrete. Unreinforced concrete is at no risk of damage caused by carbonation because the changes in pH will not cause corrosion. Permanently dry concrete has a higher rate of carbonation due to the pores not being filled with water to obstruct the ingress of carbon dioxide, but corrosion will not occur if humidity is low [11].


12.2 Research paper measuring the carbonation depth of concrete specimens exposed to different environments after different curing methods.

Ewertson and Petersson [19] conducted an experimental investigation on the effects of rainwater and exposure conditions on concrete specimens being tested in Sweden. The area has a mean temperature of 6°C and annual humidity anywhere between 65-90%. The concrete tested had a w/c ratio of 0.5 or 0.35 and was cured by water, plastic foil, or air. The specimens were then placed in either the laboratory, outside exposed to rain, or outside protected from rain. The durability of the concrete was then measured in terms of the carbonation depth with the results shown in table 13. Overall, it was concluded from the results that rainwater was often able to cure the defects caused by insufficient curing directly after casting. For concrete structures exposed to rain, the carbonation depth after 50 years was small meaning that it may not be of major concern. However, when the structure is protected from rain the carbonation rate increases due to limited hydration as a result of insufficient curing; therefore, they concluded that in order to limit the carbonation to a depth of 25 mm the curing time after casting for concrete which is protected from rain must exceed 45 hours (at 20°C).

Exposure and Curing Conditions Top Surface (not against mould) Surface against mould Time until form removal Time until form removal 0 day 0 day 1 day 3 days w/c = 0.35 w/c = 0.50 w/c = 0.50 w/c =0.50

Exposed to rain

Water 1.0 1.5 1.0 1.0 Plastic Foil 1.0 2.0 0.50 1.0 Air 1.0 2.5 1.5 1.0

Protected from rain

Water 1.5 3.0 2.5 2.5 Plastic Foil 1.5 4.0 1.5 2.0 Air 3.0 7.5 6.0 3.5

In laboratory Water 1.0 5.0 3.5 3.5 Plastic Foil 3.5 6.0 5.0 5.0 Air 5.0 12.0 9.5 6.0

Table 13: Carbonation depth mm after two years from Ewertson and Petersson experiment [19]

`

Accelerated Curing of concrete

12.3 Background information regarding New Zealand’s current standard

The reactions that take place during the hardening of concrete are sensitive to temperature. Methods which increase the temperature of concrete such as heating the forms and steam curing are used by the precast industry to cause the cement to hydrate faster, ultimately leading to quicker demoulding times and faster production times. Overall, when concrete is


heated hydration occurs at a faster rate which enhances the early-age strength; but is believed to decrease the long-term strength and durability if the concrete is not subsequently water cured [3] . NZS 3101: Part 2 states “Accelerated curing generally has a detrimental effect on durability, this is more significant for SCM concretes. Thus seven days water curing is still recommended after the completion of the accelerated curing cycle. (Clause C3.6) [27]. Investigations in both New Zealand and Canada have been carried out to determine whether additional water curing is required after accelerated curing in order for concrete to have the required strength and durability.

12.4 Analysis of Research papers comparing accelerated curing with standard curing

12.4.1 The impact of accelerated curing on the compressive strength Scott et al. [3] carried out an investigation into the effects of heat treatment on the properties of GP cement using a 50MPa self-compacting concrete mix with a w/c ratio of 0.4. It was stated that there were some limitations with the tests as the mix tested was not a true representation of what is used in the precast industry. The samples were placed in an oven at a temperature of 50°C for 18 hours. After this time half of these samples were placed in a curing tank while the other half was placed under ambient laboratory conditions. A control set of samples was also produced which did not undergo heat treatment, but still went in the curing tank after demoulding. After one day the specimens which did not get heated had a compressive strength of 18MPa, while the specimens which had been cured at elevated temperatures had a compressive strength of 42MPa. At 90 days however, the heat treated concrete which did not go into the curing tank had a strength which was 15MPa lower than the ones which had been placed in the curing tank.

After 90 days the strength of the heat treated and ambient temperature cured specimens that were both moist cured were similar, indicating the heat treatment does not impact long term strength for similar curing process. These tests did not compare heat treated and ambient temperature cured specimens that were both air cured.


Figure 8: Compressive Strength development [3]

12.4.2 Impact of heat curing on the durability properties of concrete The trends stated above continued when Scott et al. [3] carried out oxygen permeability tests on the specimens. The oxygen permeability coefficients after 3 days were similar for all of the curing processes; however, this value decreased more with time for specimens that were wet cured due to the improvement of the pore structure as a result of further hydration.

In a different set of tests carried out by Lee [17] using a higher temperature of 80°C it was concluded that there was very little advantage in additional wet curing except for specimens containing microsilica; which had a 40 % reduction in chloride resistance. It was concluded that though there was a difference in chloride-ion coefficient it was not enough to impact the durability of the concrete under normal conditions and that these differences could be managed by increasing the amount of cover specified in the standards.

12.4.3 Impact of steam curing on standard concrete mixes The Canadian Precast/Prestressed Concrete Institute in conjunction with the Canadian National Research Council [28] conducted tests to look at whether additional wet curing for 3, 5 or 7 days was needed after having stream cured for 16 hours (at a temperature of 60-70°C) as per their standards. What was interesting about this study was that the samples were obtained from nine Canadian precast concrete plants, meaning that the mixes tested were commonly used in the industry. Two plants produced C-1 samples (Compressive strength > 35 MPa at 28 day) while 7 produced C-XL samples (compressive strength > 50MPa at 56 day). The C-XL mixes had either silica fume content between 5-8%, blast furnace slag content of 25% and/or fly ash contents of 8-19%. The results found that there was statistically no difference in compressive strength between the samples that were air cured after heating and moist cured for 7 days after heating. The results from the rapid chloride test


also showed no significant difference. They concluded that accelerated curing for 16 hours followed by air drying for 72 hours produced concrete cylinders that meet the performance criteria stated in CSA A23.1.

12.5 Conclusion Hydration reactions are temperature sensitive, with higher temperatures causing the cement to hydrate quicker which may impact the overall microstructure. Concrete which had been heated had a higher initial strength, but without additional moist curing has lower 28 day strength. The compressive strengths recorded were still above the stated mix design strength indicating that the concrete properties that contribute to the strength are not majorly impacted by accelerated curing. The strength difference for moist curing reduced over longer periods. There were however, differences in the permeability and carbonation depth results that could impact the concrete element if placed in a severe environment, though this could be managed by increasing the amount of cover on the reinforcement. On the other hand, these differences may not be of major concern if durability is not a main design factor.

13. The impact of admixtures on concrete performance when subjected to different curing methods

13.1 Background information Admixtures are used to modify the properties of concrete to either before or after setting. Different types of admixtures include:

• Water reducing admixtures which lower the w/c ratio resulting in a higher strength concrete without increasing the cement content while providing a workable mix.

• Retarding admixtures which slow down the hardening rate of concrete making it workable for a longer period especially in hot weather. These work mainly as water reducers.

• Accelerating admixtures which increase the early age strength and are useful in colder weather.

• Admixtures which alter the properties of the hardened concrete, typically to improve durability.

Admixtures fall into two main categories chemical and mineral. Mineral admixtures include fly ash, silica fume and slag. These are normally added in large amounts and are used to improve workability, reduce the likelihood of thermal cracking and reduce cement content. Alternatively, chemical admixtures are used in smaller amounts to reduce the w/c ratio, control the setting time and for the entrainment of air [29].

The use of admixtures are important when considering the required curing duration because blended cement hydrates at a different rate to ordinary cement, meaning that the optimum curing duration may be longer or shorter than expected [30]. The reactions that these admixtures undergo with the cement are also temperature sensitive; therefore, results may


differ when cured under accelerated temperatures The conclusions made concerning ordinary plain concrete may or may not be true for concrete containing admixtures such as silica fume.

13.2 Analysis of research papers testing blended cement

13.2.1 Impact of curing method on the strength of concrete containing silica fume There have been many debates on whether silica fume increases the strength of mortar and cement paste. The ACI committee 234 concluded that silica fume does not improve the 7- day compressive strength when cured at 10°C. However, when cured at temperatures higher than 20°C the 7-day compressive strength improves significantly. Toutanji and Bayasi [31] found that under steam curing and moist curing compressive strength was practically unaffected by increasing the silica fume content beyond 10%, except for the moist cured samples containing 30% silica fume. Figure 9 shows the strength of samples subjected to different curing methods. Moist curing occurred at 100% humidity, with a temperature of 23°C for 7 days, steam curing for 3 days at a temperature of 80°C, and air curing at a temperature of 23°C at a low humidity of 10-30%. The difference between air and moist curing decreases with increasing silica fume content; up to 20%. However, at 30% content the difference increases significantly due to shrinkage cracking.

Figure 9: Effect of various curing procedures on the compressive strength of silica fume concrete (Toutanji & Bayasi [31])

Yazdani [32] carried out an investigation into the impact of different curing methods on concrete with silica fume. Results showed that all steam cured specimens reached the 28 day


target strength (41.37 MPa) regardless of the curing duration (with a maximum duration of 24 hours. as stated by international standards). Though these samples continued to gain strength, they still had a lower compressive strength than moist cured specimens. Moist cured samples had a greater surface resistivity and less shrinkage. It was not surprising that the longer the steam curing duration the greater the shrinkage. However no shrinkage cracking was observed during these tests.

The main strength contribution of silica fume to the concrete’s strength (subjected to temperature of 20°C) occurs in the first 3-28 days. Mohammad Shamainn Khan and Michael Ayers (as cited in Reddy [30]) found that concrete with silica fume required 3 days moist curing to reach a specified strength, whereas it took 3.75 days of curing for plain concrete to reach the same strength, and 6.5 days for concrete containing 15% fly ash. This indicates that silica fume can help decrease the required curing duration

13.2.2 Impact of addition of silica fume on the durability of the concrete The addition of silica fume into the concrete mix can impact the durability of it. Byfors and Skjolsvold (as cited in King [33]) found that specimens containing silica fume carbonate had a greater carbonation depth which would reduce durability. However, from a positive perspective Whiting and Detwiler (as cited in King [33]) found that the use of silica fume reduced the chloride diffusivity by a factor of 3 when compared to plain concrete.

13.2.3 Impact of steam curing on samples containing silica fume In tests carried out by Gesoglu [34] two curing methods were used, standard water curing and steam curing. Concrete samples with different micro-silica contents plus an ordinary plain concrete sample were demoulded 1 day after casting and then stored in water at a temperature of 20 ± 2°C. Steam curing was carried out with a maximum temperature of 70°C for 17 hours and then stored in water. The intention of this investigation was to see if the addition of micro silica reduced the effect of accelerated curing on the concrete. Results found that the steam cured concrete had a higher compressive strength at an early age, but a lower strength at 7 days onward. An addition of 15% micro-silica reduces the sorptivity by almost 43 and 53% for water and steam curing conditions respectively. Chloride ion permeability resistance of steam cured concrete is higher than water-cured concrete. Again, the addition of silica fume decreased this value irrespective of the curing process.

13.3 Conclusion Admixtures are used in the precast industry to modify the properties of hardened concrete. They cause the cement to hydrate at a different rate than ordinary portland cement due to a finer pore structure. This means that the conclusions made in the previous section may not be necessarily accurate for blended cement, however the differences between curing methods decreases as silica fume content increases (up to 20%). Overall, if significant hydration of the cement has been reached the use of admixtures may be able to compensate for any reduction in strength and durability caused by a shorter curing duration.


14. Surface treatments of Concrete Concrete is an intrinsically porous material and surface treatment products can be used to prevent the ingress of potentially harmful agents [35]. As previously mentioned durability of concrete is impacted by the presence of water which could cause carbonation, freeze and thaw damage and the transportation of chloride ions. In order to know what type of product to apply, the environment and risk factors must be assessed to see what type of protection is needed.

There are two types of sealers; film forming and penetrating. Both of these sealers reduce the permeability of concrete. Film forming sealers form on the surface (e.g. paint) and create a barrier sealing the water out of concrete’s internal structure, whereas penetrating sealers work by coating the pores and repelling the water. The efficiency of these sealers depends on the composition of the product, the pores structure and the moisture content. Surface treatments act as a second layer for concrete, and will therefore, wear off at a rate which is influenced by the exposure conditions and the penetrating depth; for example, silane and siloxane treatments need to be reapplied every 10-20 years [37].

Overall, there are a range of surface treatments available on the market that can reduce the deterioration of concrete. Interestingly, these products were not referenced in any of the papers or standards with respect to durability or requirements for curing.

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