Concrete Technology 2/6. Aalto University School of Engineering Department of Civil and Structural Engineering Building Materials Technology

Transcription

1 /6 Aalto University School of Engineering Department of Civil and Structural Engineering Building Materials Technology

2 Plasticizers (water reducing admixtures) 1 Lignosulphonates 2 Hydroxycarboxylic acids 3 Hydroxylated polymers 4 Sodium naphthalene sulphonate formaldehyde condensates 5 Sodium melamene sulphonate formaldehyde condensates 6 Polycarboxylate ethers Superplasicizer enables a remarkable increase on consistency of concrete mix or decrease in batch water amount.

3 Lignosulphonates -decomposition products of lignin and cellulose from paper industry -neutralization and precipitation of the products composition and purity of the admixture varies -sugar/sucrose content can be 1 3% Properties -impure admixtures increase the air content of the mix usually a small dosage of air-detraining agents is used -has a tendency to delay hydration of the binder accelerating admixture is added Volume of the contraction pores

4 -accelerating plasticizers are combinations of lignosulphonate and calcium chloride or calcium formate (33% of CaCl2 and 4 % of calcium lignosulphonate in the solution) -air-entraining lignosulphonate-based plasticizers increase air content of the mix by 2 3% to assure freeze-thaw durability of concrete this is not enough and additional amount of surfactants is added to the admixture Hydroxycarboxylic acids -are produced from pure raw materials of cellulose -if a small amount of CaCl2 is added plasticizer -if air-entraining agent is added accelerating air-entraining plasticizer -are not used in large dosages due to the retarding effect

5 Sodium naphthalene sulphonate formaldehyde condensates -decrease the surface tension of water in concrete increases air content addition of an air-detraining agent -mostly used as a superplasticizer Sodium melamene sulphonate formaldehyde condensates -no detrimental effects on setting time or air content -mostly used in superplasticizers

6 Domestic admixture regulations Plasticizer has to decrease water demand over 5 %, so that a 5 % water decrease causes an increase of > 6% in 7 day strength and an increase of > 10% in 28 day strength -increase in shrinkage must be below 25% -usually shrinkage is diminished Plasticizers increase compressive strength (for example when water demand is decreased by 30%) -100% increase in 2 day strength -30% increase in 28 day strength

7 Superplasticizers Chemically purer plasticizer group which enables even 30% decrease in water demand -Japan Germany Japan 1990 Highly refined admixtures possessing very small amount of impurities only few side effects Application -normal strength -high strength -higher strength improved workability normal consistency better than normal consistency

8 Classification - sodium melamene sulphonate formaldehyde condensates SMF - sodium naphthalene sulphonate formaldehyde condensates SNF - modified lignosulphonates MLS - miscellanious group (polyacrylamide, polyacrylic acids, polyglycerol) - polycarboxylate ethers PCE Effects on concrete Slump -upper range mm -large number of variables -type of superplasticizer, dosage, and dosage time

9 -water-cement ratio -type and amount of cement -temperature Setting -addition of % by cement weight retards slightly the beginning of setting time for SMF, SNF, or MLS superplasticizers -the larger the dosage the longer the delay of the setting time -preliminary tests Air content -usually a change takes place (with small dosages insignificant) -with large dosages air content decreases for SMF and SNF types and increases for MLS type of superplasticizer

10 Workability time -the plasticizing effect lasts usually only minutes, for PCE types it is slightly longer -with large dosages the workability time could be somewhat longer -SMF type looses workability time faster compared to SNF and MLS types -workability time increses slightly when cement dosage is large -cement properties have a large influence

11 Strength -if the water-cement ratio is equal superplasticized concrete possesses better strength values (SMF types even 25% better at the age of 28 days) -flexural nor tensile strength values are not affected that much -modulus of elasticity is nor changed Creep and shrinkage -similar as with comparison concretes having same water-cement ratio Durability properties -using air-entrained concrete durability properties are same if w/c is same -freeze-thaw durability tests

12 Effects of superplasticizers -high strength (10 20 svb -fluidized concrete (2 3 svb 1 2 svb) < 1 svb) -stiffening consistency faster -no bleeding (slump < 240 mm) -ready mix concrete truck + additional dosage -shrinkage and freeze-thaw durability same -fluidized concrete improves concrete surfaces Applications -precast unit factories large early strength and smaller noise level -high structures (decreased form pressures)

13 -concrete floors (faster working pace) -massive structures (minimizing binder amounts) -under water concreting (smaller segregation) -heavily reinforced structures -joint sealing of prefabricated units

14 Air-entraining admixtures -introduced into concrete mix in a water solution protective pores mm -diameter of the additional air pores mm -commonly the air content in freeze-thaw durable concretes is 4 7 % Types of air-entraining agents -ordinary foam producing surfactants -inert polymer or rubber particles

15 Ordinary air-entraining agents -abietic and pimeric acid salts -fatty acid salts -alkyl-aryl sulphonates -alkyl sulphonates -phenol ethoxylates Effects in concrete -rheology -decreased segregation -a small plasticizing effect -improved chemical durability -improved water impermeability -decreased permeability reinforcement corrosion better workability, better cohesion decreased carbonation rate, decreased

16 Air pores diminish compressive strength: additional 1 % lowers compressive strength by 3 4 %. Air-entraining agents improve concrete consistency water demand can be decreased this compensates partially the strength loss due to additional air pores. Air-entrained pores should be stable enough so that they cause pores to be generated into hardened concrete and they should be evenly distributed in the concrete -pore diameter < 0.01 mm -spacing factor < 0.2 mm The distance to a protective pore should not exceed 0.2 mm nowhere in the cement paste.

17 If there are no additional salts in the pore water, the air content in the hardened concrete should exceed 4.5 % (max aggregate # 32 mm). The sufficient average value can be 5.5 %. If there are additional salts in the pore water, the air content should exceed 5.5 % (max aggregate # 32 mm). in this situation the sufficient average value should exceed 6.5 %. -during transport and compaction air content in the fresh concrete decreases by % -pumping decreases air content -air-entrainment lessens pumpability of the mix

18 The pore building capability of the air-entrainment admixture is most efficient when -the consistency of the mix is plastic or more fluid -there is an optimum amount of fines in the mix Maximum diameter of aggregates [mm] Air content of concrete [volume %] 40 4 ± ± ± 1.5

19 Inert polymer or rubber particles -hollow or solid easily deformable polymer or rubber spheres -dimension mm -volume demand only 1 2 dm 3 /m 3 No noticeable effect on the consistency and strength of the mix (air content of non-air-entrained concrete is dm 3 /m 3 ) These inert particles do not segregate during transport and compaction even if self-consolidating concrete is applied. Water demand of the mix is kg/m 3

20 Can be used freely with other admixtures Disadvantages -price -highest allowable temperature about 70 o C -dosing of the admixture can be done only manually -control can be performed only by special apparatus -dosage amount is so small that it is difficult to assure even distribution in concrete -inert particles are lost to ready mix truck mixer and concrete containers

21 Accelerators Escalates hydration reactions usually by means of catalysis (mainly reactions of C3S) -accelerating early strength development -accelerating setting time Admixtures that accelerate setting time influence the aluminate phases of cement (C3A and C4AF) Strength accelerators Increase of the form circulation rate Winter concreting Precast unit production Setting time accelerators Shotcreting Repair works Patch works

22 Accelerator types Chloride-based - calcium chloride CaCl2 Chlorideless -triethanol amine (HOCH2CH2)3N -calcium formate Ca(CHO2)2 -alkali carbonates Na2CO3, K2CO3, Li2CO3 -calcium nitrate Ca(NO3)2 -calcium tiosulphate CaS2O3 -sodium tiosulphate Na2S2O3 -sodium nitrate NaNO3 -formaldehyde HCHO

23 Cheapest and most applied accelerator: Calcium chloride CaCl2 Most abundantly used chlorideless acceleraror: Calcium formate Ca(CHO2)2 -dosage 1 5 % of the cement weight Calcium chloride CaCl2 Mechanism -functions as a catalyst to C3S -reacts thereafter with C3A -normal dosage 1 2 % of cement weight -enhances reinforcement corrosion not allowed in tensioned structures

24 -increases especially 1 2 day strength values, also 28 day strength values Accelerators do not have an effect on -workability -air content Enhanced effect in -low temperatures -when coarse binders are used

25 Effects caused by use of CaCl2 -larger shrinkage -increased risk for infiltration of lime on the surface -long term freeze-thaw durability is compromised -inferior durability against sulphates Strength loss -associated with heat treatment -on large dosages > 4 % By-products of accelerators -admixtures decreasing freezing point of pore water -used in concreting seams and joints -sodium nitrates and pot ash K2CO3

26 Accelerators shortening setting time -setting time can be adjusted to nearly instantaneous -used in stopping water leakages especially during shotcreting -usually strength increases fast to 10 MPa, thereafter strength increase follows that of the comparison concrete without admixtures -triethanol amine (HOCH2CH2)3 N -water glass x SiO2 y Na2O z H2O