Mix design of high strength concrete, special cases in mix design

Exercise 7

• The strength of high strength concretes is K70 – K100 (By50).

• Ultra high strength concrete (RPC aka Reactive Powder Concrete) contains rock powder as aggregate with steel dust and steel fibres which compact the concrete so that it can reach strengths of up to 800 MPa.

• Compare to: High Performance Concrete• Low water/cement ratio: below 0,35 even below

0,20

• High strength concretes are composed of same components as normal strength concretes.

• The desired strength gain and heat development affect the selection of cement and additives

• The used aggregate contains only small amounts of fines and silt. Due to the high amount of binder the aggregate grading does not have as big influence as in normal strength concrete.

• Manufacture requires the use of water reducing admixtures (superplasticizers)

• Requires careful curing

Mix design• A generalized systematic approach to the selection of

mix propotions of HPCs has not yet been developed• However some specific comments can be made:• Water content should be chosen on the basis of the

required w/c ratio (from strength considerations)• Excessive content of cementitious material should be

avoided (to control shrinkage)• Compatibility between Portland cement and the

superplasticizer• If air entrainment is to be used, mix proportions have to

be modified by trial and error

The course of the mix design process

• The guidelines of the mix design are drawn up for 100*100*100 mm cubes

• Binders:– Cement kcem 1

– Blast furnace slag kMK 1

– Fly ash kLT 0,3

– Silica ksil 2,5

• The mix design guidelines are for concretes of consistency 2-3 sVB (≈ S3/S2)

The course of the mix design

1. Define proportioning strengthAverage strength + effect of deviation

2. Define the amount of binder from figure

From the book ”Korkealujuuksisten betonien suhteitus”; Penttala V. et. al. (1990). Publication 108. Figure from page 40.

C = cementSi = Silica fumeLt = Fly ashMk = Blastfurnace slag

The course of the mix design

3. Calculate the amount of cement and additional binders

4. Define the amount of (super)plasticizer from figure

From the book ”Korkealujuuksisten betonien suhteitus”; Penttala V. et. al. (1990). Publication 108. Figure from page 40.

The course of the mix design

5. Define the water amount from figure6. Calculate the amount of aggregate with the

basic equation of concrete. The amount of air is assumed to be 10 dm3/m3

From the book ”Korkealujuuksisten betonien suhteitus”; Penttala V. et. al. (1990). Publication 108. Figure from page 39.

The course of the mix design

7. Combine the aggregate8. Define the components of the batch9. Make a trial batch

1.

Proportion a K100 concrete (at the age of 28 days) with CEMI as binder with 8 % silica fume.

1. Proportioning strength– Lets assume that the deviation is 10 MPa

Ks = 110MPa

2. Define the amount of binder from figure

Amount of binder from figure is 630 kg/m3

3. Calculate the amounts of cement and additional binders from the binder amount (in this case the amounts of cement and silica).

Binder amount (C+2,5 Si+0,3 Lt+Mk)∙ ∙C+2,5 0,08C = 630 ∙C(1+0,2) = 630 C = 525 kg/m3 and Si = 42 kg/m3

4. Define the amount of (super) plasticizer from figure

Amount of plasticizer is 3,3 % 0,033*(525+42) = 18,7 kg/m3

5. Define the water amount from figure

We get(W+Nt+I)/S = 0,25(W+Nt+I)/(C+2,5*Si) = 0,25(W+18,7+10)/(525+2,5*42)→ W = 0,25(630) – 18,7 – 10 → W = 128,8 kg/m3

6. Calculate the amount of aggregate with the basic equation of concrete

=1000

Thus,

= 1000

QAGG = (1000 – 128,8 – 18,7 – 10 – 169,4 – 19,1) * 2,68

QAGG = 654 dm3 = 1753 kg/m3

2.

Proportion concrete for which the reference strength for 150 mm cubes is 55 MPa at the age of 1 day. As binder use CEMI, 10 % silica fume and 30 % fly ash.

Solution

The guidelines of the mix design are drawn up for 100*100*100 mm cubes.For 100 mm cubes the compressive strengths are about 5 % greater than for 150 cubesFor 100 mm cubes the compressive strengths should thus be 55*1,05 = 58 MPa The deviation is assumed to be 5 MPa, in which case Ks = 63 MPa.

The compressive strength at 28 days from figure:

Ks at the age of 28 days is 85 MPa

Next we define the amount of binder

Amount of binder is 440 kg/m3

The amount of binder from figure

The amount of binder (C + 2,5Si + 0,3Lt) = 440Si = 0,1C Lt = 0,3CC (1 + 2,5*0,1 + 0,3*0,3) = 440C = 328,4 kg/m3 Si = 32,8 kg/m3 Lt = 98,5 kg/m3

Next we define the amount of plasticizer

Amount of plasticizer 2,8 % 0,028*(328,4 + 32,8 + 98,5) = 12,9 kg/m3

Define the water amount from figure

We get:(W+Nt+I)/S = 0,39(W+12,9+10)/(440) = 0,39→ W = 0,39(440) – 12,9 – 10 → W = 148,7 kg/m3

Calculate the amount of aggregate with the basic equation of concrete

= 1000

Thus,

= 1000

QAGG = (1000 – 148,7 – 12,9 – 10 – 105,9 – 14,9 – 42,8) * 2,68QAGG = 664,8 *2,68 = 1782 kg/m3

3.

How would you change the mix design if the measured consistency of the concrete was 4 sVB and the 1st day strength was 58 MPa?

Solution

• The compressive strength is below the proportioning strength (Ks1 = 63) and in addition the concrete is too stiff .

• We´ll select Ks28 = 85 + 5 = 90 MPa • Define the new amount of binder

New amount of binder is 460 kg/m3

Binder amount (C + 2,5Si + 0,3Lt) = 460Si = 0,1C Lt = 0,3CC (1 + 2,5*0,1 + 0,3*0,3) = 460C = 343,3 kg/m3 Si = 34,3 kg/m3 Lt = 103,0 kg/m3

New amount of plasticizer

New plasticizer amount is 3,0 % but because the previous concrete mix was too stiff, we´ll raise the amount of plasticizer to 3,2 %0,032*(480,6) = 15,4 kg/m3

New water amount from figure:

(W+Nt+I)/S = 0,35(W+15,4+10)/(460) = 0,35→ W = 0,35(460) – 15,4 – 10 → W = 135,6 kg/m3

4.

Which matters should be taken into consideration when proportioning pumpable concrete? How about concrete with high wear resistance (kulutuskestävyys)?

Pumpable concrete• The grading of the aggregate should be

continuous– Bleeding is usually a result of non-continuous

grading or coarse sand which causes discontinuity between the finest material.

• Sufficient amount of fine material (cement, additional binders, filler), using of crushed aggregate increases the needed amount of fines. – Cement 240 - 400 kg/m3

– Amount of fines <0,25 mm: 350 - 500 kg/m3

• Consistency S4 – S2 (S1)• Use of plasticizers improves pumpability• Air entraining agents may hinder it• The amount of air is decreased during

pumping• The maximum size of aggregate– 1/3 of the size of the distributing pipe

High wear resistance • The hardened cement paste is the weakest

link, the aggregate the most durable • The aim is to use as much aggregate as

possible• Lower the amount of material passing the

sieves 0,125mm and 4mm • Avoid too plastic concrete compositions• Use of plasticizers• Sufficient strength

Other special cases • water-tightness– Strength, water/cement ratio– Stiff concrete composition– Plasticizers– Larger amount of cement and a sufficient amount

of fines – Smaller maximum size of aggregate– Placing of concrete and curing have a great

influence– Preferably no heat treatment

Other special cases

• Chemical durability– To be discussed in more detail in upcoming

calculation exercises– By50– Choosing the right binder– Higher strength, low water/cement ratio->

compact/tight concrete– Superplasticizers– Silica fume

• Long time of transport– Sufficient consistency, use of a retarder– Plasticizers– Sufficient amount of fines to prevent bleeding– The amount of air is reduced during transport

• Lightweight aggregate concretes – Absorbing water – Amount of cement– Strength

• Low heat evolution