FORMS OF SILICA FUMES BASIC INFORMATION
What Are The Forms Of Silica Fumes?
Silica fume is available commercially in several forms in both North America and Europe:
• As-produced silica fume is silica fume collected in dedusting systems known as bag houses. In this form, the material is very fine and has a bulk density of about 200 to 300 kg/m3, compared with 1500 kg/m3 for Portland cement (Malhotra et al., 1987).
As-produced silica fume is available in bags or in bulk. Because of its extreme fineness, this form poses handling problems; in spite of this, the material can be and has been transported and handled like Portland cement.
• Compacted silica fume has a bulk density ranging from 500 to 700 kg/m3 and is considerably easier to handle than as-produced silica fume.
To produce the compacted form, the as-produced silica fume is placed in a silo, and compressed air is blown in from the bottom of the silo. This causes the particles to tumble, and in doing so they agglomerate.
The heavier agglomerates fall to the bottom of the silo and are removed at intervals. The air compaction of the asproduced silica fume is designed so the agglomerates produced are rather weak and quickly break down during concrete mixing.
Mechanical means have also been used to produce compacted silica fume.
• Water-based silica fume slurry overcomes the handling and transporting problems associated with as-produced silica fume; the slurry contains about 40 to 60% solid particles. Typically, these slurries have a density of about 1300 kg/m3.
Some slurries may contain chemical admixtures such as superplasticizers, water reducers, and retarders. One such product (known as Force 10,000®) has been successfully marketed in North America.
Thursday, April 5, 2012
EFFECT OF FLY ASH ON CONCRETE STRENGTH BASIC CIVIL ENGINEERING TUTORIALS
EFFECT OF FLY ASH ON CONCRETE STRENGTH BASIC INFORMATION
What Is The Effect Of Fly Ash On Concrete Strength?
The first difference among fly ashes is that some are cementitious even in the absence of Portland cement; these are the so-called ASTM Class C, or high-calcium, fly ashes, usually produced at power plants that burn subbituminous or lignitic coals.
In general, the rate of strength development in concretes tends to be only marginally affected by high-calcium fly ashes. Concrete incorporating high-calcium fly ashes can be made on an equal-weight or equal-volume replacement basis without any significant effect on strength at early ages.
Yuan and Cook (1983) examined the strength development of concretes with and without high-calcium fly ash (CaO = 30.3 wt%). The data from their research are shown in Figure 2.5 and Figure 2.6.
Using a simple replacement method of mixture proportioning (Table 2.6), they found the rate of strength development of fly-ash concrete to be comparable to that of the control concrete, with or without air entrainment.
Low-calcium fly ashes, the so-called ASTM Class F fly ashes, were the first to be examined for use in concrete. Most of what has been written on the behavior of fly-ash concrete examines concretes that use Class F ashes.
In addition, the ashes used in much of the early work came from older power plants and were coarse in particle size, contained unburned fuel, and were often relatively inactive pozzolans. Used in concrete and proportioned by simple replacement, these ashes showed exceptionally slow rates of strength development.
This led to the erroneous view that fly ash reduces strength at all ages. Gebler and Klieger (1986) evaluated the effect of ASTM Class F and Class C fly ashes from 10 different sources on the compressive strength development of concretes under different curing conditions, including effects of low temperature and moisture availability.
Their tests indicated that concrete containing fly ash had the potential to produce satisfactory compressive strength development. The influence of the class of fly ash on the long-term compressive strength of concrete was not significant.
In general, compressive strength development of concretes containing Class F fly ash was more susceptible to low curing temperature than concretes with Class C fly ash or the control concretes. Gebler and Klieger concluded that Class F fly-ash concretes required more initial moist curing for long-term, air-cured compressive strength development than did concretes containing Class C fly ashes or the control concretes.
What Is The Effect Of Fly Ash On Concrete Strength?
The first difference among fly ashes is that some are cementitious even in the absence of Portland cement; these are the so-called ASTM Class C, or high-calcium, fly ashes, usually produced at power plants that burn subbituminous or lignitic coals.
In general, the rate of strength development in concretes tends to be only marginally affected by high-calcium fly ashes. Concrete incorporating high-calcium fly ashes can be made on an equal-weight or equal-volume replacement basis without any significant effect on strength at early ages.
Yuan and Cook (1983) examined the strength development of concretes with and without high-calcium fly ash (CaO = 30.3 wt%). The data from their research are shown in Figure 2.5 and Figure 2.6.
Using a simple replacement method of mixture proportioning (Table 2.6), they found the rate of strength development of fly-ash concrete to be comparable to that of the control concrete, with or without air entrainment.
Low-calcium fly ashes, the so-called ASTM Class F fly ashes, were the first to be examined for use in concrete. Most of what has been written on the behavior of fly-ash concrete examines concretes that use Class F ashes.
In addition, the ashes used in much of the early work came from older power plants and were coarse in particle size, contained unburned fuel, and were often relatively inactive pozzolans. Used in concrete and proportioned by simple replacement, these ashes showed exceptionally slow rates of strength development.
This led to the erroneous view that fly ash reduces strength at all ages. Gebler and Klieger (1986) evaluated the effect of ASTM Class F and Class C fly ashes from 10 different sources on the compressive strength development of concretes under different curing conditions, including effects of low temperature and moisture availability.
Their tests indicated that concrete containing fly ash had the potential to produce satisfactory compressive strength development. The influence of the class of fly ash on the long-term compressive strength of concrete was not significant.
In general, compressive strength development of concretes containing Class F fly ash was more susceptible to low curing temperature than concretes with Class C fly ash or the control concretes. Gebler and Klieger concluded that Class F fly-ash concretes required more initial moist curing for long-term, air-cured compressive strength development than did concretes containing Class C fly ashes or the control concretes.
Subscribe to:
Comments (Atom)