Generally, the rheology of con crete is measured by the familiar slump test astm cl43783, as well as a subjective observation of its ability to flow and consolidate. Silica fume concrete with an hrwr is more cohesive and sticky than conventional concrete. Guide to using silica fume in precastprestressed concrete. Cip 5 2014 national ready mixed concrete association.
Quality control of silica fume concrete bridge decks youtube. Benefits of using silica fume in concrete civil engineer. Occurs when some aggregates in concrete react with alkalis and water resulting in an expansive gel that causes cracking and deterioration from within the concrete. High watercement ration also directly supporting this issue. The concrete was to exhibit superior durability characteristics when exposed to freezethaw, as well as resistance to other deleterious effects such as scaling, cracking and corrosion. Tyler, skanska usa civil northeast unless batched, handled, and placed with tight controls and optimal conditions, silica fume decks can suffer wide spread cracking thru. This effect is caused primarily by the high surface area of the. Fict, dip m, chartered mcim, dms consultant, united kingdom keywords. When pozzolana is added to concrete, the strength of interface zone is increased due to secondary reaction of existing calcium hydroxide with pozzolana. Silica fume produces concrete that is stronger and more durable than conventional concrete.
The use of silica fume in concrete shows a tendency for plastic shrinkage2, drying shrinkage3and cracking resulting in water penetration and corrosion of reinforcing steel. Chapter two types and causes of concrete cracking in. Concrete has a coefficient of thermal expansion and contraction of about 10 x 106 per c 5. Experimental research on the effect of silica fume on tensile. The procedures used to evaluate cracking in concrete and the principal techniques for the repair of cracks are presented. The causes and prevention of cracking in concrete basf. Surface cracking caused by rapid moisture loss for. Cracking can occur in plastic concrete when rapid evaporation of bleedwater from the concrete surface causes the surface to dry out. Working with silicafume concrete norchem silica fume products. Silica fume can make highly cohesive, sticky concrete, with little bleeding capacity. Effect of silica fume additions on porosity of fly ash. Produced through a partnership program between the sfa and the federal highway administration.
Alkalisilica reaction in concrete alkalisilica reaction asr can cause serious expansion and cracking in concrete, resulting in major structural problems and sometimes necessitating demolition. As the name implies, these cracks are very small about 0. The higher proportion of silica fume may cause more cracks. Silica fume is a pozzolanic material that is used with portland cement to produce portland pozzolana cement. Cracking tendency and drying shrinkage of silica fume. Cracking tendency and drying shrinkage of silica fume concrete for bridge deck. Cracking tendency and drying shrinkage of silica fume concrete for. Silica fume microsilica, fly ash, ground granulated blast furnace ggbf slags, calcined clay metakaolin chlorides allowed by aci 318 for corrosion protection of reinforcing steel reinforced concrete exposed to chlorides 0.
This reaction causes the expansion of the altered aggregate by the formation of a soluble and viscous gel of sodium silicate na 2 sio 3 n h 2 o, also noted na 2 h 2 sio 4 n h 2 o, or nsh sodium. Concrete containing silica fume requires particular attention to avoid surface drying during placement due to its very low rate of bleeding. A comparision between plastic shrinkage of concrete containing. Discover solutions and technologies that can help control and prevent concrete cracking. Silica fume is very high in fineness and in silica content, and a very effective pozzolanic material depending on free lime content of cement use. Silica fume reduces the chloride binding capacity of the binder, leaving more chlorides to attack the steel, but again silica fume will also improve the resistance against chloride ingress. The bridges are located on interstate 35 in carter county, l.
Plastic shrinkage cracking is commonly associated with hot weather concreting, but it can also occur when the surface concrete dries and shrinks because of rapid moisture loss. Does use of silica fume mean engineers may design structures, in which chloride intrusion. Kim reduces the common cracking effect of silica fume concrete. Accomplish this by applying a sealer such as paint or a moisture barrier to the concrete surface. Adding silica fume to concrete affects the rheological flow properties of the mix. It is produced during the synthesis of silicon metal or ferrosilicon alloys, when the reduction of highpurity quartz at high temperature in an electric furnace gives off silicon oxide gas. Longterm strength losses of concrete containing silica fume are linked to the structural effects of drying.
This is a short introduction to asr for more information, see the understanding cement bookebook. Therefore, there is a higher risk of autogenous shrinkage in. Sulfate attack occurs when water containing dissolved sulfate penetrates into the concrete, reacts, expands and causes cracking and deterioration. The effect of silica fume only on concrete was give 61% increment in compressive strength, but with polymers together give about 72% increment, flexural strength increased from 3. Fogging the air above the concrete and erecting windshades lessen the risk of plasticshrinkage cracking. Cracking tendency and drying shrinkage of silica fume concrete for bridge deck applications authors.
If the silica fume concrete contains superplasticizers, both the shear resistance and the cohesiveness of the concrete will be reduced, and improved flow will result. For instance, it has higher compressive and flexural strength. Fumed silica is being used in various fields such as pharmaceuticals, paints, adhesives and sealants, plastics, rubber and ink except for concrete. What are the disadvantages of silica fume in concrete. The a z guide of silica fume why we use it in concrete, its advantages and disadvantage, its replacement ratio and its effect on fresh and hard concrete. Pdf effect of silica fume on workability and compressive. Causes of microcracking in concrete civil engineering. Silica fume, cstr74, properties of concrete, effects, cementitious materials, high. It has many advantages and its advantages outweigh its disadvantages. Because of its extreme fineness and high silica content, silica fume is a very effective pozzolanic material.
Field strengths of 14,000 psi have been achieved with this highly reactive pozzolan. It is easy to cause cracks in mortar and concrete and need concrete maintenance. Tests were performed on some 40 mixes made using silica fume as a replacement for. Any factor which delays setting increases the possibility of plastic shrinkage cracking. The ability of high c3a cement to complex with chlorides results in the formation of insoluble compound, able to reduce the mobility of free chloride ion to the reinforcementconcrete surface. Despite low initial permeability, silica fume is not without limitations. Properties of the fumed silica and silica fume have been shown in. Causes, evaluation, and repair of cracks in concrete structures reported by aci committee 224 aci 224. Silica fume concrete is important in limiting the amount of chloride ion penetrating the overlay. Hairline cracks are commonly observed in freshly placed concrete and their occurrence is owed to the phenomenon of plastic shrinkage. Introductions strength development of high strength concrete in the body of large mass concrete structures is effectively influenced by temperature rising due to hydration heat. Silica fume will increase the autogenous shrinkage of the cement slurry, and the amount of inclusion will exceed 5%, which may increase the risk of cracking. The durability of this type of concrete is superior to conventional concrete.
Effect of steam curing on concrete piles with silica fume. The results of this project also showed that the severity of the cracking is. Thawing and freezing is the most common weatherrelated physical distress. The effect of silica fume on the properties of concrete.
Silica fume concrete transportation research board. Hairline crack in concrete causes, repair and prevention. Silica fume on the properties of concrete as defined in concrete society report 74, cementitious materials des king hon. Silica fume sf is the byproduct of the manufacture of silicon for the computer industry. Silica fume reduces the permeability of the concrete. Settlement cracking increases with larger bar sizes, higher concrete slump, and smaller concrete cover dakhil et al. The objective of this program was to establish a mix for the production of superior quality concrete using silica fume and type 10 cement at lower quantities. The alkalisilica reaction asr, more commonly known as concrete cancer, is a swelling reaction that occurs over time in concrete between the highly alkaline cement paste and the reactive noncrystalline silica found in many common aggregates, given sufficient moisture this reaction causes the expansion of the altered aggregate by the formation of a soluble and viscous gel of sodium. Silica fume is a substance that is used to improve the strength of concrete. Therefore, limiting the content of water with the used of waterreducing admixtures could be done.
The gas reacts with oxygen and condenses into silica fume, a fine white powder substance composed mostly of silicon dioxide. Silica fume is a byproduct in the carbothermic reduction of highpurity quartz with carbonaceous materials like coal, coke, woodchips, in electric arc furnaces in the production of silicon and ferrosilicon alloys. Silica fume increases the drying shrinkage of mortar and concrete 10, 11. However the pozzolanic reactions may counterbalance the uptake of calcium hydroxide, according. One disadvantage is low early strength development than portland ceme.
This increased densification of the concrete often creates an illusion of waterproofing capability. Silica fume concrete is composed of cement, silica fume, fine aggregate, coarse aggregate, and water. The use of silica fume in concrete has become widespread all over the world in the production of highstrength and durable concretes. Silica fume, cstr74, properties of concrete, effects, cementitious materials, high performance concrete. Concrete containing silica fume requires particular attention to avoid surface drying. The combustion process creates silicon dioxide molecules, which condense to form particles. It is a very finely divided material, many hundreds of times finer than cement. What are the main effects of adding micro silica silica.
Types and causes of cracking in concrete structures. Normally, cement content of high strength concrete is high, therefore temperature. Determine what causes concrete to crack and distinguish amongst the various types of cracks. Factors that influence the rate of evaporation of bleed water either separately or collectively include. Once again, this cohesiveness is due to its fine particle content. Finishing must be done immediately after screeding 95 71 curing compound and polyethylene sheeting should be applied to cure silica fume concrete as soon as finishing is completed if drying shrinkage cracking is to.
Notes accompany each slide with extensive comments and references. Not allowing water access to the material and maintaining the concretes internal relative humidity below 80% will stop further gel growth and alkalisilica reaction. These properties indicated silica fume concrete would have a number of advantages when used for the repair of concrete structures. What is the effect of silica fume on healing of concrete. Fresh and hardened properties of silica fume concrete is superior to conventional concrete. The alkalisilica reaction asr, more commonly known as concrete cancer, is a swelling reaction that occurs over time in concrete between the highly alkaline cement paste and the reactive noncrystalline amorphous silica found in many common aggregates, given sufficient moisture. Effective practices for control of concrete cracking. It reacts with lime and water in a similar way to fly ash, also known as pulversided fuel ash pfa, to produce a binder.
Abstract in september 1999, bridges a and b of contract tboi003511l0044 were overlaid with a silica fume concrete surface. With little or no bleed water on the surface, silica fume concrete is prone to plastic shrinkage cracking on hot, windy days. This presentation covers all aspects of using silica fume in concrete and can be viewed directly in your browser. The cracks are most likely to occur when the vertical settlement is restrained by horizontal reinforcing bars. Development of cracking patterns in modified cement matrix. Weathering processes can initiate cracking in concrete structures.
Silica fume and pozzolana can be included in concrete to strengthen interface zone. This download a contains several papers, technical articles and two instructional videos on finishing, and curing, silica fume concrete an integral step in the producing crack free structures. Compared to class f fly ash, class c fly ash reportedly causes more shrinkage. Two distinct crack progressions promoted by weather are thermally induced cracking from cyclic heating and freezing and scaling because of the interaction between the concrete surfaces and deicing chemicals. Improving concrete with silica fume requires using.
Further increase of additional silica fume causes corresponding increase of total porosity of paste specimens. In the propagation period, the electrical resistivity of the concrete and oxygen diffusion are the governing factors for corrosion rate. The a z guide of silica fume why we use it in concrete. Cracking concrete due to external seasonal temperature variation.