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: – Cementk cem 1 – Blast furnace slagk MK 1 – Fly ashk LT 0,3 – Silicak sil 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 strength Average 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 = cement Si = Silica fume Lt = Fly ash Mk = 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 figure 6.Calculate the amount of aggregate with the basic equation of concrete. The amount of air is assumed to be 10 dm 3 /m 3

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 aggregate 8.Define the components of the batch 9.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  K s = 110MPa 2.Define the amount of binder from figure

Amount of binder from figure is 630 kg/m 3

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/m 3 and Si = 42 kg/m 3 4.Define the amount of (super) plasticizer from figure

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

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/m 3

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 cubes For 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 K s = 63 MPa.

The compressive strength at 28 days from figure: K s at the age of 28 days is 85 MPa Next we define the amount of binder

Amount of binder is 440 kg/m 3 The amount of binder from figure

The amount of binder (C + 2,5Si + 0,3Lt) = 440 Si = 0,1C Lt = 0,3C C (1 + 2,5*0,1 + 0,3*0,3) = 440 C = 328,4 kg/m 3 Si = 32,8 kg/m 3 Lt = 98,5 kg/m 3 Next we define the amount of plasticizer

Amount of plasticizer 2,8 % 0,028*(328,4 + 32,8 + 98,5) = 12,9 kg/m 3

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/m 3

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

Solution The compressive strength is below the proportioning strength (K s1 = 63) and in addition the concrete is too stiff. We´ll select K s28 = = 90 MPa Define the new amount of binder

New amount of binder is 460 kg/m 3

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

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/m 3

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 kg/m 3 – Amount of fines <0,25 mm: kg/m 3

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