Cofeat(temple)

By Ir. Dr. Lim Char Ching,Senior Assistant Director, Forensic Engineering Unit,PWD Malaysia In the 1930s, when the boom in concrete construction potential durability of concrete, it cannot be a generally began, it was generally believed that concrete valid criterion for several reasons. For example, the structures typically designed for a design life of 50 compressive strength of a concrete cube or cylinder years or so would actually last much longer with constitutes the mean value of a property of an entire cross- little or no maintenance. However, that belief was section of the specimen, whereas concrete durability is squashed when reports of premature deterioration of governed primarily by the properties of the concrete cover concrete in tunnels, marine structures and bridge decks (covercrete). Concretes of the same strength grade may were made known. It has become a worldwide problem differ in their durability resistance against chloride today. The widespread premature deterioration and penetration, carbonation and sulphate attack. Now, it is durability issues affecting many concrete structures have generally accepted that concrete durability is largely eroded public confidence in the use of concrete as a governed by the resistance of concrete cover to the ingress construction material. It is now prudent to critically re- of aggressive media. So, the emphasis should be on the assess the provisions in design codes on concrete means of achieving a good quality concrete cover or ‘skin’ CODE REQUIREMENTS ON DURABILITY
Cracks versus Durability
Cracks are inherently present in concrete due to In the present design codes, durability requirements overstress, environmental effects and chemical reactions.
for concrete structures are largely based on the These cracks may range from wide and deep cracks on conventional method of specifying arbitrarily certain the concrete surface, down to microcracks at the limiting values e.g. concrete grade, minimum cement aggregate-cement paste interface in the concrete. Although content, maximum water-cement ratio, cover thickness, these cracks do not generally affect the structural capacity, and maximum structural crack width. This so-called they are rather harmful from the viewpoint of durability.
deemed-to-satisfy approach of specifying for concrete When the cracks are limited in number and size, they “durability” frequently yields concrete performance that are discontinuous and do not pose any direct effect on is not always satisfactory. Chloride-induced corrosion of the durability of concrete. However, with time, they have reinforcement continues to represent the single largest the potential of becoming continuous (or interconnected) cause of deterioration of concrete structures worldwide.
and enlarged under the influence of stress or due to Based on substantial data available from field performance, leaching. These interconnected cracks can serve as main exposure trials and laboratory tests on concrete, many conduits for transport of harmful external ions and gases researchers are convinced that the current code into the concrete. Concrete, thus, becomes vulnerable to requirements do not provide adequate resistance to the processes of deterioration, as it gradually loses its chlorides, even when properly implemented. In the light watertightness in the course of its service life.
of current research, the deemed-to-satisfy rule for concrete Shrinkage cracks are often limited through the use of “durability” in the present codes can be challenged.
a larger quantity of steel reinforcement, as permitted inmany codes. Mehta (1997) believed that this simply Strength versus Durability
transforms the wider surface cracks into many finer cracks In many codes and specifications, the compressive and microcracks in the concrete. The microcracks and strength of concrete is often used as a criterion for pores can form an interconnected pathway for ingress of durability. Though it may give some indication of the aggressive substances into the concrete.
B U L E T I N I N G E N I E U R 8
Table 1 : Interplay between Crack Width and Concrete Cover on Durability
its durability performance in a marineenvironment. A value of W /C not Concrete
DESIGNING CONCRETE TO LAST
concern of practising engineersworldwide. While there has been an understanding of the causes of, andsolutions to problems found in Concrete structures can be designedand built to last for many generations.
This noble aspiration can be achieved Crack Width Limits
by engineers, utilising state-of-the-art knowledge in It has been well established that corrosion of reinforcements in concrete is dependent on crack widthand cover thickness, amongst other factors. For durability, The Concept Of Service Life
many codes would specify 0.3 mm as limiting crack width The concept of service life prediction for concrete for reinforced concrete structures. For marine exposure, structures is now becoming an area of increasing interest the minimum concrete cover given in the codes may range for engineers. In this respect, durability of concrete plays from 40 mm to 60 mm. Assuming other factors being the an important role. Prediction models have been developed same, Table 1 illustrates the interplay between crack width to predict and quantify structural service life based on material resistance and environmental loads. The service Examples 1 and 2 illustrate a comparison of a single life of a structure is defined as the period of time after parameter (either crack width or cover) on concrete installation until such time when costly repair becomes durability. In both cases, the comparison is straight forward. Example 3 illustrates a comparison between both For concrete structures in marine environments, the crack width and cover on concrete durability. In this resistance of concrete to chloride-induced corrosion largely case, the assessment of concrete durability becomes controls its long-term durability performance. Concrete deterioration due to chloride-induced corrosion can be It should be pointed out that crack width and cover represented by a simple service life model consisting of thickness are inter-dependent. For a given stress level in an initiation stage and a propagation stage, shown in the steel bar and keeping other variables constant, crack width increases with cover thickness. Nowadays, high The initiation stage is the time from initial exposure tensile bars with yield strength of 460 MPa and above are until depassivation of steel in concrete. This is largely commonly used as reinforcements in concrete. In this dependent on the rate at which chlorides penetrate into case, crack width larger than expected may develop when concrete. The propagation stage is the time from the onset a large cover is adopted. Of course, 0.3 mm crack widthcan still be achieved with a large cover, provided the steelis designed with much lower stress than permitted. In this case, optimum use of high tensile bars as reinforcements in concrete is not achieved.
The influence of both parameters, surface crack width (W ) and cover thickness (C), should be considered in totality, i.e., one dependent on the other. This is in contrastwith the durability requirements given in many designcodes, in which surface crack width and cover provisionsare recommended to be independent of the other.
Lim et al (2000) proposed the crack width-to-cover ratio (W /C) as an indicator for assessing durability performance of a cracked reinforced concrete in a marineenvironment. He concluded that it is desirable to minimiseW /C of a cracked reinforced concrete in order to enhance Figure 1 : Service Life Prediction Model B U L E T I N I N G E N I E U R 10
of steel corrosion until a specified acceptable “deterioration type of blended cement concrete may be suitable for limit” in concrete has been reached, e.g. first visible one application but not for the other. Selective usage of cracking on the concrete surface or first spalling.
Many engineers have associated service life of concrete structures primarily with the initiation stage, to maintain Performance-based Specification
structural safety and serviceability, acceptable appearance, Present specifications for concrete works are largely and without having to incur unforeseen high costs for method-based, in that they describe how works should be repair. Currently, some concrete structures around the carried out on site, e.g., placing and curing of fresh world have been designed using this concept, e.g. the Great concrete. The acceptance criteria for concrete at site are Belt Link Bridge in Denmark with 100 years of service solely based on tests carried out on “specially” prepared life! It is foreseen that within the next few years, specimens of fresh concrete. The specification “assumes” reliability-based service life designs may be incorporated that the workmanship in preparing the test specimens in today’s ordinary design procedures for concrete would be the same as that in the works. This assumption is seldom true. Furthermore, the test specimens aresubjected to a controlled curing regime and environmental Blended Cement Technology
condition, compared with the concrete placed at site. It is The term ‘blended cement’ is used to include both obvious that the quality of hardened concrete in the the products of blending of one or more mineral structure is generally different from the test specimens.
admixtures with an Ordinary Portland Cement (OPC). The The method-based specification has been proven to be process of blending can be achieved either by inter- inadequate in addressing durability issues affecting grinding the mineral admixture(s) with cement clinker, concrete structures. The specification cannot guarantee or by blending the mineral admixture(s) with OPC.
a satisfactory “performance” of hardened concrete in the The wide range of binder systems, namely, Portland cements and blended cements available in many parts A concrete structure designed for a specific service of the world provides opportunities for the best concrete life should be complemented with a performance-based to be chosen for a particular environment. Fly ash, slag specification. This is to ensure that the hardened concrete and silica fume are the three commonly available mineral “performs” in an environment for which it is expected to admixtures used in blended cements. With proper meet the service life requirement. The specification should dosage, these blends can be effective in enhancing the focus on tests to be carried out on in-situ hardened performance of concrete in high chloride and sulphate concrete. The test results should become the basis for conditions. They have been found to improve the accepting or rejecting the concrete at site. Improved resistance of concrete to chloride penetration and to performance-based specifications are being developed in reduce corrosion rate of steel reinforcement.
many countries. However, suitable short-term Improvements of the order of three to five times are not “performance” tests necessary for such specifications are uncommon. Blended cements also showed great potential in reducing expansion and loss of strength of concreteexposed to sulphate condition.
The use of blended cement concrete has been recognised by most national standards and codes ofpractice on concrete structures. In general, fly ash andslag are used in blended cements for durability and long-term engineering properties. Silica fume is used whenboth early age engineering properties and durability areneeded. The common dosages of these mineraladmixtures vary between 20 and 40% for fly ash, between35 and 80% for slag and five and 10% for silica fume.
The optimum dosages are obviously dependent uponspecific technical requirements and cost considerationrelevant to a particular application.
In the past, the lack of knowledge in blended cement technology had resulted in structures with prematuredeterioration. Today, it is possible to tailor-makeimproved quality concrete using blended cementtechnology for many applications. Blended cementconcrete has proven worldwide to enhance workabilityof fresh concrete and durability properties of hardenedconcrete. However, blended cement concretes also havetheir disadvantages and limitations too. A particular B U L E T I N I N G E N I E U R 11
CONCLUSION
improved quality concrete for most applications. A goodknowledge of blended cement technology is necessary to Concrete structures exposed to marine environments ensure its full potential in being utilised for making durable have been found to suffer mainly from corrosion of the reinforcing steel. It is necessary for engineers tounderstand the importance and mechanisms of chloride- REFERENCES
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