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As known, Aggregate quality the thing important to produce of the economical concrete. It will influence cement content in the concrete.
Cement : Pxxx Cement Grade-43
Fine Aggregate
However, a change in FM of 0.2 in fine aggregate may lead to a significant change of water demand for the same workability of concrete mixture. That is also will lead increased cement demand for keep strength (w/c).
2. Mica
Almost Fine aggregate from Sunkoshi River contains ‘mica’ with various content of mica. Some standard specification not clear yet how much mica content minimum will allow in fine aggregate. Some rumors says in German not allowed mica content in fine aggregate. So far, although not many research about effect mica in fine aggregate but of course mica in fine aggregate will effect to strength of concrete, that will contributed to cement demand in concrete.
The others parameter such as soundness, alkali reactivity are still in tolerance. In addition, sand equivalent, clay lump, washing loss, organic impurities may in tolerances because this fine aggregate is processed trough washing plant, except it not wash properly.
Coarse Aggregate
Table-2 below shown results of abrasion and elongation test for Sunkoshi coarse aggregate.
Table-2 Abrsion and Elongation of Coarse Agg. Sunkoshi
Kind of Agg. | Surface | Abrasion | Elongation Index |
Gravel | | TS.40% max | BS.25% max |
Max. 40 mm | Smooth | 36- 38% | 30.0% |
Max. 25 mm | Smooth | 36 -40% | 27.1% |
Refer to table-2, elongation index of aggregates is more than 25 % and the surface aggregates is smooth. A note, elongation index of gravel40mm higher than gravel 25mm. Higher elongation will guide to increase sand aggregate ratio (S/a).
Mix Proportion
Class Concrete Targer Strength W/c S/A Cement/m3
D1=40mm max 21.2 Mpa 55 40 327
D2=25mm max 21.2 Mpa 56 40 300
In General, the larger maximum size of aggregate used then the smaller value of sand aggregate ratio (s/a), it will smaller water demand in the same workability. That means, the larger maximum size of aggregate used is more economically (lower cement demand) than smaller size used.
But in this case, relation between aggregate maximum and S/A is conversely(see Table 3 and 4 above). For example: mix-proportion in Table-3, S/A for D1 (agg.40mm under) is equal D2 (agg.25mm under), the same water content 180 kg/cm3 at the same slump. In the same target strength cement content for D1 is bigger than D2. This reality probably cause by shape or elongated of aggregate.
Condition of mixer-pan or Truck Mixer for concrete mixing, variation of quality of aggregate, quality of cement and water, technician, making sample, curing etc., all of them will effect to variation quality of concrete.
1. Quality of sand specially variation in grading, FM and Mica Content in fine aggregate.
2. High elongation and smooth surface of coarse aggregate
3. When low quality of Cement used, alone or together with quality of aggregate will contributed to concrete stress, and leads to increase cement demand to reach the target strength.
Introduction
Generally concrete ingredients consist cement, fine aggregate, coarse aggregate and solvent water. Some additional additive or other materials for increase the concrete performance.
In The project of Construction Road Sindhuli-Nepalthok Section 3 NEPAL the source of material aggregate is explored from the riverbed of Sunkhosi river from borrow pit sta. 1+000, sta 18+000, 29+500, 30+800 and others.
As known, Aggregate quality the thing important to produce of the economical concrete. It will influence cement content in the concrete.
Following a case study of factors affected to high cement content in mix design.
- Quality of Concrete Ingredients
Water : Well water located at side of Sunkoshi river. This water is acceptable for water mixing for concrete.
Cement : Pxxx Cement Grade-43
Mortar strength test results: for period April 2010 up to Dec 2010 the mortar stress tendency lower than specification refer to (Indian Standard) IS-8112.
In this case, necessary to increase cement into the mix proportion due to low quality of cement.
Fine Aggregate
The quality and properties of aggregate most important to get durability of concrete.
The Sunkoshi fine aggregate two matters to be concerned that the grading related to fineness modulus, and Mica.
1. Grading and Fineness Modulus (FM)
Fineness Modulus (FM) is the sum of cumulative percentages retained on sieves starting from size 0.15mm to maximum size divided by 100. It is note that a fine grading has a lower FM.
Variation for FM of Sunkoshi fine aggregate (washing screen plant) is too many variation. In Similar borrow pit has minimum value of FM is 1.91 and maximum value is 2.97, with standard deviation 0.29.
The grading is very important such that a reasonable workability and minimum segregation are obtained. A workability mixture can produce a strong and economical concrete.
However, a change in FM of 0.2 in fine aggregate may lead to a significant change of water demand for the same workability of concrete mixture. That is also will lead increased cement demand for keep strength (w/c).
2. Mica
Almost Fine aggregate from Sunkoshi River contains ‘mica’ with various content of mica. Some standard specification not clear yet how much mica content minimum will allow in fine aggregate. Some rumors says in German not allowed mica content in fine aggregate. So far, although not many research about effect mica in fine aggregate but of course mica in fine aggregate will effect to strength of concrete, that will contributed to cement demand in concrete.
The others parameter such as soundness, alkali reactivity are still in tolerance. In addition, sand equivalent, clay lump, washing loss, organic impurities may in tolerances because this fine aggregate is processed trough washing plant, except it not wash properly.
Coarse Aggregate
Table-2 below shown results of abrasion and elongation test for Sunkoshi coarse aggregate.
Table-2 Abrsion and Elongation of Coarse Agg. Sunkoshi
Kind of Agg. | Surface | Abrasion | Elongation Index |
Gravel | | TS.40% max | BS.25% max |
Max. 40 mm | Smooth | 36- 38% | 30.0% |
Max. 25 mm | Smooth | 36 -40% | 27.1% |
Refer to table-2, elongation index of aggregates is more than 25 % and the surface aggregates is smooth. A note, elongation index of gravel40mm higher than gravel 25mm. Higher elongation will guide to increase sand aggregate ratio (S/a).
The stress at which the breaks developed depend largely on properties of the coarse aggregate; smooth gravel leads to breaking at lower stresses than rough and angular crushed rock, because mechanical bond is influenced by the surface properties and certain degree, by the shape (elongated) of the coarse aggregate. (Neville, properties of concrete 4th, 1995, p288)
Mix Proportion
The mixproportion of concrete for Nepalthok site are as follows;
Class Concrete Targer Strength W/c S/A Cement/m3
D1=40mm max 21.2 Mpa 55 40 327
D2=25mm max 21.2 Mpa 56 40 300
In General, the larger maximum size of aggregate used then the smaller value of sand aggregate ratio (s/a), it will smaller water demand in the same workability. That means, the larger maximum size of aggregate used is more economically (lower cement demand) than smaller size used.
But in this case, relation between aggregate maximum and S/A is conversely(see Table 3 and 4 above). For example: mix-proportion in Table-3, S/A for D1 (agg.40mm under) is equal D2 (agg.25mm under), the same water content 180 kg/cm3 at the same slump. In the same target strength cement content for D1 is bigger than D2. This reality probably cause by shape or elongated of aggregate.
- Concrete Production (Process)
Condition of mixer-pan or Truck Mixer for concrete mixing, variation of quality of aggregate, quality of cement and water, technician, making sample, curing etc., all of them will effect to variation quality of concrete.
- Conclusion
1. Quality of sand specially variation in grading, FM and Mica Content in fine aggregate.
2. High elongation and smooth surface of coarse aggregate
3. When low quality of Cement used, alone or together with quality of aggregate will contributed to concrete stress, and leads to increase cement demand to reach the target strength.
Nepal, Road Nepalthok-Shinduli Sec-III, 2010
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