Excerpt
CONTENTS
ABSTRACT
I. INTRODUCTION
II. DETERIORATION OF CONCRETE STRUCTURES
III. CONSTRUCTION SITES- EARLY DETERIORATION
IV. ENHANCEMENT OF CONCRETE REUSE
V. EMERGING TECHNOLOGIES
VI. BARRIERS AGAINST REUSE
VII. CONCLUSION
REFERENCES
ABSTRACT
Concrete is the most widely used construction material for infrastructure needs in the Asian region and in the world. Unfortunately, the concrete industry is one of the largest consumers of natural resources and energy, and is responsible for large emissions of carbon dioxide that is one of the greenhouse gases responsible for global warming. It is imperative that the concrete industry must be in an active role of balancing the infrastructure needs and the protection of environment.
This work presents a summary of some recent research closely associated with the sustainable development of concrete technology. The research projects include study and analysis of: - Causes of deterioration of concrete structures, problems at construction sites that causes early deterioration of concrete structures. In addition to above this book also presents some environmentally-friendly and sustainable concrete technology including the use of supplementary cementing materials (SCM), recycling concrete and other materials, enhancement of service life of concrete structures. Emerging technologies that have the potential to significantly contribute to sustainable concrete industry and barriers against reuse are presented at the end of book.
Keywords - Concrete reuse, Concrete durability, Supplementary cementing materials (SCM), Sustainable development, Sustainable concrete technology.
I. INTRODUCTION
Concrete is the most widely used construction material for new and replacement infrastructure. It is estimated that the concrete production in the world is expected to rise from about 10 billion tons in 1995 to almost 17 billion tons in 2012. Unfortunately, concrete industry is one of the largest consumers of natural resources and energy, and is responsible for large emissions of carbon dioxide that is one of the greenhouse gases responsible for global warming. The sustainable development is frequently defined as “the development that meets the needs of the present without compromising the ability of future generations to meet their own needs.” By this definition, sustainable development encompasses three general elements: environmental stewardship, social responsibility, and economic prosperity. The challenges faced by the concrete industry to meet both the current and future generation’s sustainable development include:
A) Populations will continue to increase. The world population in 2008 was about 6.7 billion and it was predicted that by 2050 the world’s population will reach 10 billion. Asia covers only 8.6% of the earth’s total surface area, but it contains 60% of the world’s population.
B) Infrastructure needs will grow in order to provide the basic needs of the increasing population.
C) Natural resources are limited. The mineral resources (limestone and aggregates) and non-renewal energy necessary for cement and concrete productions are becoming less abundant.
D) There is an urgent need to reduce and decrease the greenhouse gas emissions, mainly carbon dioxide (CO2) to reduce global warming.
Purpose of recycling in the concrete industry usually define the purposes of the research to be saving resources and energy, rational use of materials, and sustainable use of materials. It also includes zero-emissions and building a closed cycle of materials usage. These are all important keywords today. The current usage and research and development on the reuse of by-products related to concrete can be classified into the following three categories:
Category 1: Use of by-products, from non-construction industries to concrete.
Category 2: Use of by-products, from concrete to concrete
Category 3: Use of by-products, from concrete to other materials.
The ultimate purpose of recycling materials is to minimize the impact of human activities on the environment and the planet. From this viewpoint, the first priority of concrete engineers is to maximize the lifespan of concrete structures, at least concerning Category 2, because buildings and infrastructures must be used for a very long time and, generally speaking, reuse of concrete and/or recycling of concrete materials is not easy technically or economically. Also the waste from concrete structures should be reduced before considering how to reuse or recycle it. Furthermore, if the use of by-products from industries other than construction degrades the quality, especially durability, then one should carefully consider how to properly use those materials. Recently, it has become necessary to accept large volumes of many kinds of by-products for use in concrete, with reuse sometimes taking priority over concrete quality meet the sustainable development challenges which concrete industry is facing currently, the option we are left with are those techniques like concrete reuse in on-going and further construction for achieving sustainable and harmonious development.
II. DETERIORATION OF CONCRETE STRUCTURES
The actual possible causes of deterioration of concrete structures are listed after surveying some buildings in several districts. The photographs of basically distressed regions in the structures lead to identify the possible causes of the distress.
- Carbonation induced corrosion of steel bars (Figure 1 –a, b)
- Mud in aggregate
- Chloride induced corrosion of steel bars (Figure 1-c, d)
- Efflorescence in bricks
- Drying shrinkage (Figure 1-e, f)
- Sulphate attack/Chemical attack
- Leakage through joints (Figure 1-g)
- Lack of reinforcement in structural members
- Heat of hydration
- Lack of cover thickness of structural members
- Thermal expansion (Figure 1-h)
- Leakage of water through the roof (Figure 1-i)
- Differential settlement
- Lack of maintenance (Figure 1-j)
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Figure 1: - Photographs of some typical types of deterioration observed in concrete structures [ Source: Pictures captured during site visits]
Due to the high humidity (60% ~100%) and high temperature (38-48 ºC in summer), a high rate of carbonation is expected in concrete structures. The use of low strength concrete as well as poor quality concrete works at the construction stage also accelerates the process of carbonation. Many concrete floors, beams, and columns are severely damaged due to the carbonation induced corrosion of steel bars, 10–15 years after construction. Generally, the depth of carbonation is expressed by the equation (1) D=k√ t where, D is the depth of carbonation in mm, k is carbonation coefficient in mm/year0.5, and t is time in years. Experimental investigations are necessary to find the value of the carbonation coefficient. In the coastal areas, concrete structures are damaged due to the combined action of chloride and carbonation-induced corrosion of steel bars in concrete. Fick’s Second Law of diffusion is commonly used to determine the chloride profile in concrete due to the diffusion of chloride into the concrete from the outside environment. The closed form solution for pure diffusion of chloride ions into concrete is expressed as:-
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where, C(x, t) is the chloride ion concentration at a depth x (mm) and time t (year), Co is the chloride ion concentration at the surface (here it is assumed to be equal to the chloride ion concentration at a mean sampling depth of 1 cm) as weight percentage of cement, D ac is the apparent diffusion co-efficient in mm2/year, and erf is the standard error function. Investigations are necessary to determine the diffusion coefficient of chloride into concrete exposed to the marine environment. Cracks in the walls are found due to drying shrinkage of the structure at an early age. A lime concrete coat is applied to the roofs of the buildings to reduce the heat flow in summer (Figure-1i). Unfortunately, the lime concrete soaks water in rainy seasons for long time and thus accelerates the deterioration of the roof slab. Storage of materials on roofs as well as water logging on roofs is also found to be causes of deterioration of roof slabs. Efflorescence is found in the partition wall due to the presence of salts in the brick. Generally, patch type repairs are carried out, but they are found to be ineffective after a short time (Figure-1j).
III. CONSTRUCTION SITES- EARLY DETERIORATION
The causes associated with the early deterioration of concrete structures during service are identified as:-
- Un-sieved aggregates (Figure 2a)
- Poor mixing/ mixture proportion (Figure 2g)
- Unwashed aggregates (Figure 2b)
- Problems associated with volumetric mix proportions
- Overly Wet sand (Figure 2c)
- Lack of cover concrete (Figure 2 h and i)
- Mud water for mixing (Figure 2d)
- Problems associated with formwork (leakage of mixing water)
- Rusted reinforcement (Figure 2 e, h and i)
- Placing of concrete from a large height by labors
- Excess water in mix (Figure 2f)
- Inappropriate compaction
- Higher w/cm
- Inappropriate curing
- Excess fine aggregate (sand)
- Brick efflorescence
- Excess coarse aggregate.
- Poor workmanship
Volumetric mix proportions are generally used for most construction except for the ready mix concrete industries. Generally, mixture proportions for concrete are set at 1:1.5:3 (strength range of concrete 3,500~4,000 psi, 24.5 MPa – 28 MPa) or 1:2:4 (strength range of concrete 2,500 – 3,000 psi, 17.5 MPa – 21 MPa) for most construction work. For concrete work 25 liters of water per bag of cement and at construction sites water is added until the mixture become workable without any measuring. For this reason, in actual construction the strength of concrete becomes 2500 – 4000 psi (17.5 MPa – 28 MPa). The use of a high W/C ratio makes concrete relatively porous and consequently easy paths for the ingress of harmful constituents is developed. The concrete cover is not maintained adequately due to the lack of knowledge of durability based design by civil engineers. More credit hours are necessary to teach students about the microstructures of concrete, the process of deterioration of concrete structures, durability based design, repair and maintenance of concrete structures, quality control at construction sites and life cycle management of concrete structures, etc. At many construction sites, unskilled workers are involved. It is necessary to create skilled workers through professional organizations.
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Figure 2:- Photographs depicting typical construction conditions contributing to the potential distress of structures during their service life. [ Source: self-captured pictures]
IV. ENHANCEMENT OF CONCRETE REUSE
4.1 Use of Supplementary Cementing Materials
Supplementary cementing material (SCM), such as fly ash, ground-granulated blast-furnace (GGBF) slag or silica fume, is one of the most sustainable construction materials because it recovers an industrial by-product through beneficial use when incorporate into concrete. It Avoids disposal of industrial by-products, reduces portland cement content in concrete, resulting in decreased emission of greenhouse gas and decreased use of natural raw materials, and increases structure service life by improving the durability of concrete. The current annual production of fly ash is on the order of 900 million tons worldwide, with major production occurring in China, India, and the U. S. The use rate and the way fly ash is batched in concrete vary from country to country as shown in Table 1. One of the major developments in the area of fly ash utilization in concrete has been the technology of high performance, high-volume fly ash concrete. Studies have shown that, when the water-cementitious materials ratio (w/cm) is maintained at 0.30 or less in the super plasticized concrete mixtures, up to 60 percent of Portland cement can be replaced by ASTM Class F or Class C fly ash to obtain excellent long-term mechanical and durability properties. Table-2 shows an example mixture proportion for a high-volume fly ash (HVFA) concrete. The compressive strengths of this HVFA mixture were 8, 55, and 80 MPa at 1, 28 and 182 days respectively. Extensive laboratory tests concluded that the Young’s modulus of elasticity, creep, drying shrinkage, and freezing and thawing characteristics of HVFA concrete are comparable to normal Portland cement concrete. The HVFA concrete also has high resistance to water permeation and chloride-ion penetration.
Another by-product that is useful for cement substitution is ground-granulated blast-furnace (GGBF) slag. Although the world production of this slag is approximately 100 million tons per year, only approximately 25 million tons of slag is processed into the granulated form that has the cementitious properties Because GGBF slag is derived as a by-product from the blast-furnaces manufacturing iron, its use has environmental benefits. The use of GGBF slag in concrete significantly reduces the risk of damages caused by alkali-silica reaction, provides higher resistance to chloride ingress, reduces the risk of reinforcement corrosion, and provides high resistance to attacks by sulfate and other chemicals. The use of GGBF slag in concrete has increased in recent years and this trend is expected to continue. Laboratory work by Lang and Geiseler on a German blast furnace slag cement (405 m2/kg specific surface area) containing 77.8 percent slag showed that excellent mechanical and durability characteristics were achieved in super-plasticized concrete mixtures with 455 kg/m3 cement content and 0.28 w/cm. The compressive strengths at ages 1, 2, 7, and 28 days were 13, 37, 58, and 91 MPa, respectively. The concrete also showed good resistance to carbonation, penetration of organic liquids, freezing and thawing cycles (without air entrainment), and salt scaling. Approximately 5 million tons of GGBF slags were used in concrete mixtures annually in Taiwan. Up to 55% of the Portland cement (ASTM Type V) had been replaced by GGBF slag in concrete mixtures where high sulfate resistance is required. In the moderate sulfate resistance applications, 45% of Portland cement (ASTM Type II) can be replaced by GGBF slag with excellent performance. Concrete containing 45-50% of GGBF slag was commonly used for concrete slurry wall constructions in Taiwan.
Silica fume is a by-product resulting from the reduction of high-purity quartz with coal or coke and wood chips in an electric arc furnace during the production of silicon metal or ferrosilicon alloys. The condensed silica fume contains between 85 and 98 percent silicon dioxide and consists of extremely fine spherical glassy particles (the average particle size is less than 0.1μm). Because of its extreme fineness and high silicon dioxide content, condensed silica fume is a very efficient pozzolanic material. The worldwide production of silica fume is estimated to be about 2 million tons. Because of limited availability and the current high price relative to portland cement and other pozzolans or slag, silica fume is being used primarily as a property enhancing material. In this role, silica fume has been used to provide concrete with very high compressive strength or with very high level of durability or both. It has been used to produce concretes with reduced permeability for applications such as parking structures and bridge decks and for repair of abrasion damaged hydraulic structures.
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Table 1- 2004 Coal ash production and use in concrete [Ref. 4]
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Table 2- Mixture proportion for a high volume fly ash concrete [Ref. 15].
One of the major barriers against the use of large quantities of fly ash and other in concrete is the current prescriptive-type of specifications and codes. The prescriptive-type of specifications generally place limits on the maximum percentage of the cement that can be replaced by the supplementary cementing materials. For example, ACI 318 Building Code limits the maximum percentage of fly ash or other pozzolans to not exceed 25% of the total cementitious materials by mass for concrete exposed to deicing chemicals.
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