Mitigating autogenous shrinkage of hardening concrete

Transcription

1 Mitigating autogenous srinkage of ardening concrete H.W.M. van der Ham, E.A.B. Koenders & K. van Breugel Delft University of Tecnology, Delft, Neterlands ABSTRACT: Te probability of early age cracking in ardening concrete can be affected by using additives tat reduce te total srinkage of a mixture wit empasis on te reduction of autogenous srinkage. In order to become more insigt in te effectiveness of srinkage reducing additives, an experimental programme is currently running at TU Delft. Experiments are performed for water/cement ratios varying from.3 to.5, for Portland cement as well as Blast Furnace Slag cement. All mixtures are tested isotermally at 2ºC. Te earlyage srinkage deformations are measured over a testing period of 9 days. Te development of compressive strengt, tensile splitting strengt and elastic modulus will be determined from respectively cubes and prisms tat arden under similar conditions. Results obtained from tis experimental programme provide te effectiveness of srinkage reducing additives and teir influence on development of te mecanical properties. Te autogenous srinkage, being a single contribution of total srinkage, will be addressed explicitly. 1 GENERAL Autogenous srinkage is one of te causes of cracking of ardening concrete elements. Autogenous srinkage occurs if less water is present in te mixture tan required to ydrate all binder material and tat, as a result of tis, capillary forces develop. During te ardening process of a so called low-waterbinder ratio concretes, te internal demand for water is iger tan te amount of water available. Tis is called internal drying and causes te concrete to srink ( autogenous srinkage ). Mitigating autogenous srinkage will reduce te probability of cracking of te ardening concrete elements. Super Absorbent Polymers can be used to mitigate autogenous srinkage by internal curing. To become more insigt in te effectiveness of tose Super Absorbent Polymers, an experimental programme (figure 1) is currently running at TU Delft. First two mixtures are compared, one mixture witout polymers and one mixture wit polymers. Preliminary results of tose tests are presented are presented in tis paper. 2 MIX DESIGN Two mixtures ave been tested, as presented in table 1. Firstly, a C68/85 concrete witout polymers and secondly, te same concrete wit polymer addition. First mixture as also been used in previously researc by Sule (23), Lokorst (21), and Koenders (1997). Te polymer dosage was 2 kg/m3. Te water-cement ratio was increased from.28 to.31 to compensate for te influence of te polymer on te workability. Table 1. Mix proportions (kg/1 l) Mixture 32REF water CEM III/B 42.5 LH HS CEM I 52.5 R sand -4mm aggregate 4-16 mm Addiment BV1 1 1 Addiment FM Silica fume slurry Polymer Powder 2

2 3 SUPER ABSORBEND POLYMERS Super absorbent polymers (figure 2) and extra water are added to te concrete mixture separately, in order to mitigate autogenous deformations. Te polymers are considered to absorb all te extra added mixing water wic will release during cement ydration. Te release of te extra water during ardening will affect te internal drying and, as a result of tis, te autogenous deformations as well. Mixing procedure: - prepare mixer; - mix sand and gravel for 1 minute; - add dry polymers and mix for 2 minutes; - add cement and mix for 1 minute; - add water and additives and mix for 1.5 min. In order to acieve a omogenous distribution of te polymers and avoid clusters of polymers in te mixture, te polymers are added to te sand-gravel mixture in a dry configuration. 4 TESTING PROGRAM AND METHODS Te early-age srinkage tests were carried out using tree ADTM (Autogenous Deformation Testing Macine) apparatus. It measures te deformation of a specimen tat is free to deform. Figure 5a gives an overview of its functioning. Tree insulated moulds are used (figure 5b); two of 1 mm long and one of 85mm long. All tree are 15 mm wide and 1 mm ig. Te walls of te mould contain plastic tubes tat are connected to cryostats units. Te temperature of te specimen is measured by means of tree termocouples, one in te middle and two in te ends of te specimen. Te cryostats control te water flow troug te tubes to maintain te required temperature regime in te specimen. All tests are performed under isotermal conditions, at 2ºC. Directly after casting, all moulds are covered wit plastic foil and covered wit an insulated plate. Two small steel bars were embedded in te tree specimens, 75 mm apart from eac oter. Tese bars protrude troug te two oles in te long side faces of te mould, wile not making contact wit te mould itself. Tus, te ends of te steel bars could move freely over a certain deformation range. As soon as te fres concrete starts to develop some strengt and stiffness, te LVDT s (linear voltage displacement transducers) positioned along te long side faces of te mould were activated and attaced to te small steel bars tat protrude troug te mould (figure 5c). From tat moment on (t-zero [8]), te deformation of te specimen over a 75 mm measuring lengt was registered, bot at te front and rear of te specimen. 5 TEST RESULTS 5.1 Adiabatic temperature development Wit respect to te adiabatic temperature development of te two tested mixtures (figure 7), sligt differences are expected in terms of te degrees of ydration. It is likely tat a prolonged ydration process will be observed due to te addition of te release of extra water from te SAP. 5.2 Sort term srinkage ADTM-tests Autogenous deformations are measured from tree sealed specimens. During testing, similar conditions were applied, i.e. temperature of 2 C and a relative umidity of 5%. Te measured autogenous deformations are presented in figure 6 and represent te mean values of te tree specimens. Variation of te results, wic turned out to be 2% for te concrete witout SAP and 23% for te mixture wit SAP, is calculated by dividing te standard deviation by te mean value. Tis ig variation of te concrete wit SAP is due to te mean value fluctuates around zero. Batc 1 Cubes Tensile and compressive strengt Mixture 24 l Batc 3 ADTM_1 ADTM_2 Autogenous deformations ADTM_3 Batc 2 Prisms Modulus of elasticity + creep Figure 1. Testing program

3 Figure 2. From left to rigt two polymers, one unsaturated and one saturated [4], 25 grams of unsaturated polymers and 63grams of water absorbed by 25 gram polymers Figure 3. Prisms Cubes Figure 5b. Scematically drawing of ADTM-setup Lokorst (21) Figure 4. Long term srinkage measurements Figure 5c. Scematically drawing of LVDT placement Lokorst (21) Figure 5a. ADTM1, 2 and 3

4 T [degrees Celsius] strain [1-3] Figure 6. Figure Measured autogenous deformations REF _corrected -65% 32REF Adiabatic temperature development Te deformations of te mixture are corrected for te expansion in te first 2 rs (figure 6) because tis will lead to relative small compressive stresses wic will relax, compared to te tensile stresses due to te srinkage after te expansion. Te reduction of srinkage between te reference concrete and te corrected deformations of te mixture turned out to be about 65% (figure 6). Note tat tis will not result in 65% lower stresses due to autogenous srinkage. In order to determine te efficiency of te polymers in terms of mecanical properties, information is needed about te development of tese properties of te concrete, like development of compressive strengt, tensile strengt, modulus of elasticity and relaxation. 5.3 Long term srinkage ADTM-tests Because te extra water added to te mix to obtain a similar workability as te reference concrete it is to be expected tat te ydration process will proceed for a longer period. Tis migt result in an ongoing refinement of te pore structure and, ence, a possible iger srinkage. It is terefore tat te srinkage of te mixture is measured for a longer period of at least 9 days (figure 4). Te results up to now are presented in figure 8. After correcting te expansion in te first 2 ours, te difference is still 5%. 5.4 Tensile strengt For te tensile strengt, even bigger reductions (- 14%) are observed (measured from cubes, figure 3) due to te addition of SAP, wit a maximum reduction of almost 2% after 14 days of ardening (figure 9 middle). 5.5 Modulus of elasticity Reductions in measured modulus of elasticity (measured from prisms, figure 3) are less (-6%) compared to differences in compressive strengt and tensile strengt. Te results of te measurements are provided in figure 9 (rigt) strain [1-3] % REF32 REF32 long term srinkage long term srinkage corrected corrected long term srinkage time after casting [days] Figure 8. Measured and corrected long term srinkage

5 Compr. strengt [Mpa] Figure 9. 32REF differences age [days] % -5% -1% -15% -2% -25% Differences Tens. strengt [Mpa] age [days] % -5% -1% -15% -2% -25% Differences Development concrete properties and influence of SAP Young's modulus [Mpa] age [days] 1% 5% % -5% -1% -15% Differences 6 CALCULATED STRESSES DUE TO MEASURED AUTOGENOUS DEFORMATIONS From te experiments, substantial differences are observed in te development of te visco-elastic properties of te ardening concrete (figure 9) containing SAP compared to te ref. concrete (table 2). Table 2. Average effect SAP on properties and probability of failure P f compared to reference mixture effect on P f sort term srinkage (2 weeks) - 65% positive long term srinkage (5 weeks) - 5% positive compressive strengt - 12% negative tensile strengt - 14% negative modulus of elasticity - 6% positive To calculate te actual stresses due to te autogenous deformations, a trend line is fitted into te test results of te modulus of elasticity, bot for te mixture wit and witout polymers. Te development of te modulus of elasticity can be described by: E = 3592 Ln( t) (1) for te mixture witout polymers (32REF), and Modulus of elasticity [Mpa] 45 y = 3592Ln(x) E = 157,3 Ln( t) (2) for te mixture wit polymers () in wic t is te time after casting in rs (figure 1). Measured autogenous deformations of bot mixtures are modeled by polynomial formulations, as presented in figure 11. To correct for te starting time of te measurements (17 ours after casting, see figure 6), te polynomial formulations are sifted for 17 ours. Te adiabatic degree of ydration is calculated from te adiabatic temperature development by: α ( t) = ( T ( t) T ( ) ) a a Q max ρ cc C in wic: T a = adiabatic temperature [K] ρ = volumetric density of concrete [kg/m 3 ] c c = specific eat of concrete [kj/kg/k] C = amount of cement [kg] Q max = maximum eat production [kj/kg] strain [1-3].1 y = -9E-15x 6 + 9E-12x 5-4E-9x 4 + 7E-7x 3-7E-.5 5x 2 +.3x (3) 3 15 y = 157.3Ln(x) Figure Measured and modelled modulus of elasticity -.15 y = -1E-13x 5 + 2E-1x 4-8E-8x 3 + 2E-5x 2 -.2x -.2 Figure 11. Measured and modelled autogenous deformations of bot mixtures

6 [MPa] tensile strengt tensile strengt 32REF stresses 32REF witout relaxation stresses 32REF wit relaxation -2 stresses wit relaxation stresses witout relaxation Figure 12. Calculated stress development due to measured autogenous deformations Because te experiments are performed at isotermal conditions of 2 C, te degree of ydration in te specimen will be lower compared to te adiabatic degree of ydration. To calculate te process degree of ydration, te eat production in te specimen can be calculated by: E ( T ) T T Q p; j+ 1 = Q a; j+ 1 e (4) In wic: E a = apparently activation energy [kj/mol] R = universal gas constant [R=8,31J/mol/K] T a;j and T p;j are te temperatures [K] Q p;j+1 = produced process eat in timestep j = produced adiabatic eat in timestep j Q p;j+1 A p; j a:j p; j R Ta; j Tp; j Te calculation of te stress development is based on elastic and time-dependent deformational beavior of concrete. Te elastic stress increments are calculated according to: σ e ( τ) = ε a ( τ) E( τ) (5) Were: σ = elastic stress increment [MPa] τ = age at loading [rs] ε a = increment of autogenous def. [-] Degree of Hydration 8% 6% 4% 2% % adiabatic isotermal Figure 13. Adiabatic and isotermal degree of ydration E = modulus of elasticity [MPa] Te superposition principle is used to take into account te stress istory [5]: σ stress/(.75*tenisle_strengt-ratio) [-] j ( t) = ψ( t, τ) σ ( τ) i= e (6) Wit for te relaxation factor: (7) α ( t) ω * α ( ) τi Ψ( τ ) i, t, α τ,i, = exp d n α ( ) ( t) τ ( ) i t τi α τi And α = degree of ydration [-] τ = age at loading [rs] ω = water/cement ratio [-] d =.3 (slow) and.4 (rapid cement) [-] n =.3 [-] Te results of te calculations wit and witout relaxation are sown in figure 12. Te reference concrete witout Super Absorbent Polymers will reac a tensile stress of about 2 MPa, wile te concrete wit Super Absorbent Polymers develops almost no 32REF witout relaxation 32REF wit relaxation wit relaxation witout relaxation Figure 14. Calculated stress/(.75*strengt)-ratio

7 tensile stresses during te first 144 ours. After 144 ours, no extra tensile stresses will develop due to more autogenous srinkage because tose extra stresses will relax immediately. For te calculations wit relaxation, bot calculations are performed using equal relaxation. For te concrete containing Super Absorbent Polymers, bigger relaxation is expected compared to te reference concrete, because of te bigger pores in te microstructure. Te total pore volume will not cange, but te amount of small pores will decrease Mőnnig (25). To determine te influence of Super Absorbent Polymers on creep and relaxation of ardening concrete, creep experiments under a constant compressive load will be performed soon. Tis effect is not taken into account yet. Stress/strengt-ratios are calculated for bot mixtures for te first 144 ours after casting. From experiments, a cracking criterion is proposed by Lokorst (21) to be stress >.75*tensile strengt. If relaxation is taken into account, a maximum stress/strengt-ratio due to only autogenous srinkage is observed to be almost.5 for te reference concrete witout Super Absorbent Polymers. For te concrete wic contains Super Absorbent Polymers, Te maximum stress/strengt-ratio in te first 144 ours is almost. Tis positive effect of te SAP migt be less at iger ages, due to faster srinkage of te specimen wat contains SAP, compared to te specimen witout SAP (compare figure 6 and 8). REFERENCES Bjøntegaard, Ø. and Hammer, T.A. 26. RILEM TC 195- DTD: Motive and Tecnical content. Volume canges of Hardening Concrete: Testing and Mitigation. RILEM Proceedings PRO 52: Breugel K. van Relaxation of young concrete, Delft University of Tecnology. Breugel, K. van Simulation of ydration and formation of structure in ardening cement-based materials (second edition). Delft University of tecnology. Jensen, O.H., Hansen, P.F. 21. Water-entrained cementbased materials, I. Principles and teoretical background, Cement and Concrete Researc 31: Koenders, E.A.B Simulation of volume canges in ardening cement-based materials. Delft University of Tecnology Lokorst, S.J. 21. Deformational beavior of concrete influenced by ydration related canges of te microstructure, Delft University of Tecnology Mőnnig. 25. Water saturated super-absorbent polymers used in ig strengt concrete, Otto-Graf Journal Vol. 16: Sule, M.S. 23. Effect of Reinforcement on Early-Age Cracking in Hig Strengt Concrete. Delft University of Tecnology 7 CONCLUSIONS - Adding 2 kg/1l Super Absorbent Polymers to te tested mixture results in a reduction of autogenous srinkage after 336 ours of 65%. After 8 ours, tis is reduced to 5%. - Adding 2 kg/1l Super Absorbent Polymers to te tested mixture results in an average reduction of te compressive strengt of 12%, te tensile strengt of 14% and te modulus of elasticity of 6%. - Adding 2 kg/1l Super Absorbent Polymers to te tested mixture, more relaxation is expected. - Adding 2 kg/1l Super Absorbent Polymers results to lower stresses in te ardening concrete (2 MPa after 144 ours), wic results in a lower stress/strengt-ratio due to only autogenous deformations from.5 to.4 after 144 ours. At iger ages, te effect of SAP migt be less, due to a faster srinkage after 144 ours of te mixture containing SAP compared to te mixture witout SAP.