Study Report on Concrete with replacement of Cement by GGBS

Study Report on Concrete with replacement of Cement by GGBS
Introduction:-

Use of GGBS replacement with Portland cement intended because of to have sulphate & chloride resistance, durable concrete with commercially economical. Also there are green environmental benefits by using waste recycled material as GGBS with replacement for OPC cement.

Concrete mix designs & finding
Concrete mix designs have been prepared for M40 grade & trials was done with different proportions ( 40 to 60%) of GGBS (JSW) by blending with OPC 53grade cement (Ultratech, ACC, Ambuja, JK Lakshmi) as per IS 10262 & IS 456. The raw materials which was used as VSI Coarse aggregate 10mm, 20mm, fine aggregate was used as VSI C. Sand (artificial sand) & SNF (sulphonated naphthalene) based admixture.

As per IS 455, IS456 we can replace GGBS with cement 25% to 70% & as per BS 6699 up to 70%.

So we tried with 40 to 60% GGBS replacement for OPC cement. The trials was carried out by using SNF based admixture name as Fosroc conplast SP 430 to have better workability & water reduction. We observed that there is only 10 to 15% water reductions is there in 1.0 to 1.2% SNF based admixture dosing. Workability was observed in term of slump 130mm to 180mm (Collapse) & retention up to 90 minutes.

Based on workability, cohesiveness & 28days strength results (52.89mpa, commercially economical, we concluded 476 kg/cum cementious mix with proportion cement to GGBS (60:40) is good for use. This controlled lab. trial was carried out in Dec 2013.

The ingredients of mix in kg/cum were as below during use-
Cement = 286 Kg/cum
GGBS = 190 Kg/cum
20mm = 610 Kg/cum
10mm = 402 Kg/cum
sand = 785 Kg/cum
w/c = 0.36
admixture = 4.76 to 5.712 Kg/cum

This mix was started using from 16th Jan 2013 for raft construction tower-2 in winter season. The ambient temp. was below 27 degree centigrade i.e. cold frosting weather was observed. The mix was pumped & poured smoothly. Slightly delay setting observed as compared to 100% OPC concrete due cold season, cold frost.

Setting Times
The initial setting time of concrete is dependent on the concrete’s constituents, curing conditions and its application use. Concrete with up to 30% GGBS will exhibit similar initial setting as concrete with Portland cement only. At replacement levels of 40 to 50% the initial set is likely to be extended by one to two hours and for concrete containing more than 50% GGBS setting time maybe extended past three hours.

We observed that the ambient temp. was below 27 degree centigrade. in June 2013 i.e. similar to cold frosting weather observed in winter. The mix was pumped & poured smoothly. Slightly delay setting observed as compared to 100% OPC concrete due cold season, cold frost. Initial set of all concretes is extended in cold conditions and the effect of cold temperatures on extending the initial setting time of GGBS concrete is more pronounced. This effect can be 10 to 12 hours depending on conditions, concrete constituents and the section size of the pour.After casting on finished surface we observed no thermal cracks, no shrinkage cracks observed due to less evaporation of moisture due to winter & less heat of hydration due to curing temp. below 25 degree centigrade & use of GGBS.

Longer setting times can have the advantage of allowing concrete to be worked for longer periods meaning time delays, including delays in transport, between mixing and using concrete are less critical. They also reduce the risk of cold joints in larger concrete pours. So we got the benefit of it during raft casting.

Bleeding
Concrete with up to 40% GGBS replacement does not exhibit different bleeding characteristics from that of concrete made with Portland cement. For higher percentages of GGBS there is a longer period of bleeding due to the increase in setting times of these mixes. It can be avoid by using high water reducing PC (Poly carboxylete) based admixture with viscosity boosting admixture (i.e.VMA)

Water Demand
We observed GGBS allows for water reduction of 3 to 5% as compare to OPC cement concrete without any loss in workability..

Placing, Compacting and Pumping
We observed GGBS makes concrete more fluid, making it easier to place into formwork and easier to compact by vibration. GGBS concrete remains workable for longer periods allowing more time for placing and vibrating. Pumping is also easier due to the better flow characteristics.

Concrete with 40% GGBS
Strength development

GGBS concrete has slightly slower strength development at early ages, but will have equal if not greater strength at 28 days compared to non GGBS concrete. At 7 days GGBS concretes will have 50 to 60% of its characteristic strength compared to 70 to 80% for Portland cement only concrete at the same time. At 28 days GGBS concrete will have fully developed its characteristic strength and will continue to develop Strength past 90 days. It is good practice to make 56 day cubes when using GGBS concrete at 50% and above should there be any concern over later strength development.

Winter season observation
We observed in winter season at 3 days 38 to 48% & at 7 days 50 to 65% of its characteristic strength compared to 70 to 80% for Portland cement only Concrete at the same time.

Summer season observation
In summer season due to more ambient & curing temperature same early strength gain was observed as 100% OPC concrete mix.
Also we observed at 28 days strength was developed its characteristic strength plus 5 to 10 mpa At 56 days we observed 28 days strength plus 2 to 3 mpa.

Monsoon/rainy season
In rainy season less early strength was observed as compare to winter & summer due to immeasurable rain moisture in c sand. Due to immeasurable moisture in c. Sand not possible to control designed water cement ratio. It is become very tricky in monsoon. Also in rainy season C. Sand having scarcity & quality issue. To get rid of it we stored plenty of C. Sand from selected C. Sand quarry at Nerul & Uran.
Change of C. Sand source has significance on strength.
Hence it is advisable to use secure side mix which will have more cementious. So tried & concluded based on strength result to be use 500 kg/cum cementious mix instead of 476 kg/cum further.

Due to more cementious fine in 500 kg/cum mix we observed more pumpable & strength as compare to 475 kg/cum mix.

The ingredients of mix in kg/m3 were as below during use
Cement = 300 kg/cum
GGBS = 200 kg/cum
20mm = 657 kg/cum
10mm = 421 kg/cum
sand = 754 kg/cum
w/c = 0.35
admixture = 5.0 to 6 kg/cum

Admixture role & limitations
By using 500 kg/cum cementious we can pump this mix up to highest floor of our project i.e. 37th floor. But there will be different admixture 10th floor to above. The reason behind change of admixture from 10th floor to have more workability, pumpabity, controlled W/C ratio with optimum dosage of admixture.

SNF(Sulphonated naphthalene formaldehyde) based admixture having limitation up to 1.5% dosage (Ref. As per manufacturers spec.). Also it has less water reduction 5 to 10% only.
So in secure manner we will have 1.4% dosing which we can do.
For upper floor it may exceed this limitation which results in setting time of implication of concrete. It may never set or set even after 24 hrs based on practical situation, temp.etc at that time.
So it is advisable to use polycarboxylate based mid range admixture for upper floor concrete. It has 10 to 20% water deduction capability. So in dosing will be as compare to SNF admixture.

Special concrete for Aluminium Farmwork
In case of mivan / S-form shuttering we should use flowable concrete (Flow 600mm to 700mm) to have proper placing, flowability, accessibility in congested reinforcement at column & beam junction. If concrete does not get placed in congested reinforcement section which may result in cavity, honey comb. Generally it is happen in conventional concrete due to less flow or lack of workability.
To get rid of defect like risk of cavity, honey comb, generally self compacting concrete are using at many site in India which having too much cost as compare to conventional concrete.

Economical Substitute for SCC, BASF has been developed concept as “SDC concrete”.
In SDC concrete they proposed 525 kg/cum cementious (38% cement, 62% GGBS), 48% c. Sand, w/C=0.32, Admixture (High rage PC- BASF Master sky glenium 8645) 1% dosing.

Trial was done in BASF lab with our raw materials & result found good.
To confirm practically, batching plant trials was done at site. The observation was made as 650 mm flow with 1.2% instead of 1%. Hence it concluded 1.2% dosing required to have flow more than 600 mm. V funnel test was carried out as per relevant BS standard.

Simultaneously we proposed 500 kg/cum cementious with (cement 40%, GGBS 60%), C. Sand 46%, W/C=0.30, Admixture dosing 1.2%.
The flow was measured as 650mm, v funnel test was found pass, slump retention up to 90 minutes.
Based on cube result, the strength result of 500 kg/cum with ratio of cement with GGBS (40:60) found more than 525 kg/cum mix due to less water cement ratio, less percentage of C. Sand.

In case of SNF & Mid range pc based admixture we can’t produce flowable concrete as like SCC & SDC.
In Rheology of SSD concrete have main role of BASF -master sky glenium 8645 admixture. Because this is of high water reducing type admixture which having water reduction 30 to 40%. Also BASF added VMA percentage in it.
Due to greater water reduction we are able to achieve very good strength result & flow even after 60% of GGBS.

VMA which is added in admixture which playing very vital role to make concrete viscous to avoid segregation, bleeding due to more flow. Practical our:- After no. of trial we instructed to BASF to have practical pour on site.

The practical pour of 10 cum was done in slab on date 08.07.14 & concrete found free flow without vibration, homogeneous, cohesive.
There is no setting cause observed of 60% GGBS as compare to 40% GGBS replacement.
Strength result of SDC concrete was found more than conventional concrete which pour in same slab.

The ingredients of SDC mix in kg/m3 were as below -
Cement = 200 kg/cum
GGBS = 300 kg/cum
20mm = 518 kg/cum
10mm = 481 kg/cum
sand = 864 kg/cum
w/c = 0.3
admixture = 5.70 to 6 kg/cum

Comparison of Cube results of SDC trial pour-

SDC 500=(40:60), W/C=0.3
Strength (N/mm2)
at 3 days = 20
at 7 Days = 34.60
at 28 Days = 49.98

Normal 500(60:40), W/C =0.35
Strength (N/mm2)
at 3 days = 15.63
at 7 Days = 24.22
at 28 Days = 45.4

Also we extracted core from beams & slab of respective poured concrete to ascertain actual insitu compressive strength of concrete. The same core has been tested at third party lab & results of core samples were as follows-
  1. Sample nos.1 (outer Beam no.104) = 43.70 mpa
  2. Sample no.2 (Inner Beam no. 150) = 36.56 mpa
  3. Sample no.3 ( Slab) = 46.84 mpa
The cost comparison with our regular M40 mixes & proposed mixes (mid range PC admixture) are as follows
SDC 500=(40:60), W/C=0.3, BASF master sky glenium-1.2% = ₹ 3726 per cum.
Normal 500(60:40), W/C =0.35, Fosroc SNF-1.2% = ₹ 3876 per cum.
Normal 500(60:40), W/C =0.35, Fosroc Mid range PC-1.2% = ₹ 3935 per cum.
 
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