CONCRETE QUALITY CONTROL DURING CONSTRUCTION

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By
1) KAUSHAL KISHORE
Materials Engineer
Roorkee
Kaushal Kishore is retired from IIT, Roorkee and now a Consulting Materials Engineer. He has over 50 years of experience in all types of Concrete Mix Design.

2) RAKESH SHARMA
Vice President Project
Supertech Limited
Rakesh Sharma is Vice President Project of M/s Supertech Limited. He has 28 years of experience in Construction.

I.T. Park, Doon Square Mall at Dehradun, Uttarakhand is being constructed by M/s Supertech Ltd. For this site M-30 grade of concrete suitable for pumped concrete is required. Aggregates for construction was stored at site. Its analysis report is given in Table 2.

For laboratory trial 5 brands of PPC cement bags were taken from local market. They were identified as cement brand, 1, 2, 3, 4 and 5. Two reputed brands of normal superplasticizers based on sulphonated naphthalene formaldehyde (SNF) were taken for trials. They were identified as SP (A) and SP (B) to find their compatibility with the given 5 brands of PPC cement identified as cement brand 1, 2, 3, 4 and 5.

COMPATIBILITY
The behavior of concrete in the presence of superplasticizers is related to the amount and type of sulfate added to the clinker. The rheological and setting Behavior are changed depending on whether the sulfate is added as anhydrite, hemihydrates, or gypsum. The difference are explained by the different rates of dissolution of these sulfates.

It is also important to note that all admixtures belonging to the same generic type may not behave similarly in concrete because of the variations in the molecular weight, cautions associated with them, chain length, etc. Similarly the same type of cement generically may be different in mineral, alkali and sulfate contents, and fineness.

As per IS: 456-2000 Admixture should not impair durability of concrete nor combine with the constituent to form harmful compounds nor increase the risk of corrosion of reinforcement. The workability, compressive strength and the slump loss of concrete with an without the use of admixtures shall be established during the trial mixes before use of admixtures. If two or more admixtures are used simultaneously in the same concrete mix, data should be obtained to assess their interaction and to ensure their compatibility.

The performance of concrete admixtures including superplasticizers depend upon the admixtures and concrete making materials and their compatibility. It is necessary to evaluate concrete admixtures for specific use with the concrete – making materials and mix proportions to be used on the work under actual field conditions.

Cement is composed of four major compounds, namely, C₃s, C₂s, C₃A and C₄AF. In additions, a number of minor oxide such as K₂O and Na₂O, MgO and SO₃ – which is contributed by gypsum, which is added in the final stages of cement manufacture as a set regulator. When there is less C₃A available, superplasticizer would tend to get absorbed in higher amount on C₃S and C₂S, resulting in a reduction in the strength development.

Table 1 : Superplasticing chemicals

Ref.: 1

MIX DESIGN DETAILS

1. As per IS: 456-2000, M-30 grade of concrete design for a mean target strength of

30 + 1.65 x 5 = 38.3 N/mm²

at 28 days age

2. Workability (a) Mix with NO admixture, only for comparison

Slump + 40 mm

(b) Mix with superplasticizer for pumped concrete slump +90 mm.

Pouring time with in 40 minutes.

3. From many laboratory trials, the following mix design identified for final trials.

M-30 grade of concrete quantity of materials per cu.m of concrete on the basis of saturated and surface dry aggregates.

1. Water =          180 kg/m³
2. Cement, PPC =          410 kg/m³
3. Sand =          750 kg/m³
4. 5 mm crushed aggregate =          500 kg/m³
5. 20 mm crushed aggregate =          540 kg/m³
6. Normal superplasticizer =          5.800 kg/m³

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Total =          2385.8 kg/m³

Note: With cement brand No. 3 and SP (B) 28 – days laboratory cubes average density found to be 2394 kg/m³.

4. All the 150 mm cube samples are the average of 3 cubes and they are with in + 15% of the average value.
5. For conclusion of Lab trials the cubes compressive strength is given in Table 3.
6. The mix design is carried out from references 2 and 3.
7. The above given design mix is used in all the Lab trial mixes. The same design mix along with cement brand 3 with superplasticizer (B) is followed in the construction. Site cube results are given in Table 4.

CONCLUSION
1. If we are producing concrete into superplasticizer, then we can achieved 11% more strength compare to normal concrete with respect to about 4% increments in cost per cubic meter. This is very important superplasticizer must be compatiable with the cement proposed to be used in the construction.

2. On the basis of laboratory experimental investigations and studies carried out in the present work, it is concluded that though the concrete mix proportion is the same in all the trial mixes, the workability, compressive strength and setting times of cement concrete are considerably different depending on the brand of the cement and brand of superplasticizer.

3. If superplasticizer is not compatiable with the cement lower strength will be obtained. This can be seen in Table 3.

4. The results given in this paper are our findings other researchers/sites should draw conclusions by testing with their own site materials, and not depend upon the test results of this paper.

5. In the mix design cement is 410 kg/m3. With the present set of materials this cannot be reduce, as per reasons given below:

(a) Water = 180 kg + 3.654 kg water in the superplasticizer total water = 183.654 kg/m³. Free W/c ratio = 183.654/410 = 0.45.
(b) For workability consideration water cannot be reduce. For durability requirements free W/c ratio also cannot be increase from 0.45.
(c) The dosage of superplasticizer cannot be increase due to retardation of setting time of concrete.

REFERENCES
1. R. Rixom and N. Mailvaganam, Chemical Admixture for Concrete, E & FN Spon, London 1999.
2. Kishore Kaushal, “Mix Design for Pumped Concrete”, Journal of Central Board of Irrigation and Power, Vol. 49. No.2, Apr. 1992, pp. 81-92.
3. Kishore Kaushal, “Mix Design for Pumped Concrete”, NBM & CW Sept. 2010, pp. 204-217.
4. IS: 456-2000, Plan and Reinforced Concrete – Code of Practice (Fourth Revision) BIS, New Delhi.
5. IS: 1489, Specifications for Portland Pozzolana Cement, BIS, New Delhi.
6. IS: 9103-1999, Specification for Admixtures for Concrete (First revision) BIS, New Delhi.

Table 2. Test report of site aggregates

Table 3 : Cube compressive strength (N/mm²) Laboratory results

Note:
1. Cement brand 2 and 3 found to be achieving the designed target strength of 38.3 N/mm2 with superplasticizer brand (B). As cement brand 3 is found to be available in bulk at site. Hence cement brand 3 and superplasticizer brand (B) is allowed in the construction.

2. Manufacturer of all the 5 brands of PP cement claims, cement conforms to IS: 1489 Manufacturer of 2 brands of superplasticizer claims, their product conforms to IS: 9103-1999.

Table 4 : Cube compressive strength construction site results with cement brand number 3 and superplasticizer brand SP (B). All the samples are of M-30 grade concrete of the same site.

REMARKS
1. Characteristic compressive strength compliance requirements IS: 456-2000 with amendments Table 11 (clause 16.1 and 16.3)

RESULT : The site concrete have the compliance of M-30 grade of concrete.
2. With the present sets of material further economy in cement cannot be obtained. See item 5 of conclusion of this paper.

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