I have to thank the Editor of the JSCE Concrete Committee Newsletters again for giving me the opportunity, for the second time, to express my opinions via the Readers Voice column. This time, let me share some information and opinions, as one of the experienced educators, researchers and engineers in the field. My opinions are directed to the articles on fly ash which had been published in a series in the Newsletter no. 17, 18, 19, 21 and 22 since April 2009 until July 2010.

I would like to first appreciate the attitude of JSCE in encouraging the use of fly ash in concrete industry. The use of fly ash in the industry surely helps conserve natural resources and reduce emission. We all know by literatures and experiences that being used in a proper amount in concrete, fly ash improves workability and longterm strength; reduces autogenous and drying shrinkage; reduces heat of hydration and permeability; increases resistance against chloride penetration and sodium sulfate attack as well as reduces the risk of alkali aggregate reaction of the concrete. On the other hand, its major disadvantage is the slow rate of strength development which leads to low early strength. Delayed setting times, low carbonation resistance, low magnesium sulfate resistance and difficulty in entraining air are some other disadvantages. It should be realized that there is no material which is good in all properties. It is the matter of wisely utilizing its merits. However, when it comes to real application, one of the most important problems becomes the inconsistency of the fly ash properties or quality. As it is a by-product, not the main industrial products, the control of its quality is normally, not at all, the priority in the production. This, I believe, is one of the main problems in many countries. If the control of quality of fly ash can not be done at the stage of production, other additional appropriate methods will be necessary to alleviate the problem. One possibility is to blend various lots or sources of fly ash to obtain blended fly ash with satisfactory consistent properties. Another possibility is to establish a rational mix design which can take into account property variation of fly ash. Certainly, both methods can be effectively applied in combination. Fly ash has been introduced to the concrete industries in Thailand since early 90’s. At the beginning stage of application, there were successes and failures. One of the major problems occurred when casting large slabs and pavements. Those structures mainly require strong surface to resist abrasion and also preferably require no appearance of cracking. Assuming that design had been reasonably conducted, two major mistakes in practice in Thailand were
1) excessive bleeding due to inappropriate mix proportioning and
2) insufficient curing.
The first one was usually caused by ignoring modification of mix proportion when fly ash was used. Many mixtures were prepared by just replacing a part of cement with fly ash by targeting only at the required slump and strength.

These mixtures, in many circumstances, have larger bleeding than the comparable mixtures without fly ash, especially when water to cement ratio is high (low strength) and unfortunately, this range of strength is generally the range used for the slab and pavement types of concrete structures (strength about 20 to 30 MPa). For curing, though there has been a recommendation on curing of concrete with fly ash in the standard of the Engineering Institute of Thailand, insufficient curing is still one of the prime problems for fly ash concrete practice. This reduces cracking resistance of fly ash concrete and hence makes fly ash concrete cracks easier than concrete without it. The improper curing practice has somehow distorted the understanding on cracking properties of fly ash concrete among practical engineers in Thailand. I noticed that the issue on cracking behavior of fly ash concrete had also been elaborated in the JSCE newsletter no.19. My own research group is now working hard to clarify this issue to the engineering society in Thailand. Since fly ash is a cement replacing material which is rather extreme in term of effect on concrete performances. It can either very much improve or on the other hand very much reduce the performances of the concrete. In order to cover the disadvantages of the fly ash, as one of the objectives, the concept of multi-binder system has been introduced to the Thai concrete industry since the past decade. Thailand is now moving toward the concept of practically using more than 2 types of binder in concrete. For example, the system of cement with fly ash and limestone powder is now gradually becoming popular especially in the precast concrete industry. Considering that cement is the first type of binder, the main advantage of introducing the third type of binder is to cover some major disadvantages of the second type. In this case, limestone powder can help increase the early strength and cracking resistance as well as reduce the sensitivity to curing of the fly ash concrete while the other merits of fly ash concrete are still kept with the concrete. With the aim to wisely utilize the merits of various types of binder in concrete by considering cost, performances and environment, it is believed that the use of multi-binder concrete will become a normal practice in concrete industry in Thailand in the near future.

The last issue which I would like to mention is on the recently published proposal to regulate coal combustion residuals - i.e. fly ash by the U.S. Environmental Protection Agency (EPA), which may result in requiring additional processes to handle the ash. Many are worrying that if officially regulated, the use of fly ash in concrete industry may be affected. I personally see this issue as both threat and opportunity. The possibility of cost increase for additional process to handle the fly ash may be considered as the threat. On the other hand, since the most effective way to utilize all fly ash, both in terms of volume and economic value at the moment, is to use it in cement and especially concrete industries, it is an opportunity to stimulate more use of fly ash in a higher value application. Finally, I still believe that the application of fly ash in cement and concrete industries is a kind of all-win application and hope to see more fly ash being used in the industries.

I wish all readers good luck.