MEASURING SHRINKAGE OF HARDENING PASTE WITH A MINI- TSTM

Transcription

1 MEASURING SHRINKAGE OF HARDENING PASTE WITH A MINI- TSTM E.A.B. Koeners (1), G. Leegwater (2) an E. Schlangen (2) (1) Stevin II la, Fac. Civil Engineering, Delft University of Technology, The Netherlans (2) Microla, Fac. Civil Engineering, Delft University of Technology, The Netherlans Astract The start of the chemical reaction etween cement an water is the moment that concrete starts to evelop its properties. More precisely, it is the paste that starts to generate a loaearing microstructure that exhiits a certain numer of mechanical properties. One of the properties generate uring harening is the so-calle harening shrinkage. Quite often, this property is measure from harening concrete y linear measurements an relate to the paste eformation y using mathematical moels. The opposite way, y measuring the paste eformation an relate it to the eformation of concrete is another possiility that can e applie. In this paper, a new apparatus calle mini-tstm (Thermal Stress Testing Machine) for measuring the harening shrinkage of paste will e presente an iscusse [1]. The evice has een evelope to measure the harening eformations irectly from the paste. It provies more insight in the contracting forces that act on the paste locate in etween the aggregates. The mini-tstm is scale own from the full-scale TSTM machine which is commonly use for measuring the thermal stresses of harening concrete elements. With the mini-tstm, the contraction forces can e measure (loa controlle) as well as the eformations (eformation controlle) which are moilize y a paste uring harening. The mini-tstm is a tool that fits into the moisture controlle specimen chamer of the ESEM with enales the opportunity to sample the paste eformations uring harening as well. The paper provies the results of the shrinkage eformations an forces for four ifferent cements, measure from a harening paste with the mini-tstm. Three similar Blast furnace slag cements are teste which were prouce y ifferent mills, an one Portlan cement is use as reference cement. Results are compare with measurements from the full-scale TSTM. The results show significant ifferences etween shrinkage measurements from oth instruments. The results provie a clear picture of the main effects that play a role when relating shrinkage eformations from paste to concrete or visa versa. 1. INTRODUCTION One of the main issues that exhiit uring concrete harening is the so-calle early age cracking. The relevance of this cracking phenomenon is still increasing ue to the growing importance of the uraility in concrete esign. Early age cracking is inuce y oth temperature evelopment ue to hyration an y autogenous eformation. There are several

2 computer programs availale to calculate the stresses cause y temperature evelopment only. Autogenous eformations are not always taken into account in these programs, ut in orer to achieve accurate results, this shoul e one as well. In general, there is limite ata availale to verify these programs an the effect of autogenous eformation on early age cracking. Especially for last furnace slag cement, frequently use in the Netherlans, limite ata is availale. In orer to e ale to conuct more accurate cracking preictions an to prevent egraation of a structure, more knowlege is neee aout autogenous eformation. Besies the research on autogenous eformations of concrete in general, there is the amition to moel this particular concrete ehavior explicitly. If the autogenous ehavior of concrete can e calculate from the autogenous ehavior of its constituents (paste) irectly, it relates the response of the concrete system irectly to the source where the eformations originates from. In orer to e ale to evelop a concrete moel that is ase on its constituents, more knowlege is neee aout the material ehavior of these constituents. In this research, the focus is on the shrinkage ehavior of harening concrete with emphasis on the harening properties of the cement paste, which are consiere to e most relevance in this respect. Information aout the cement paste ehavior can e use as input ata for computer moels that are ale to represent concrete as a heterogeneous material [2]. During the evelopment of these moels the output has to e verifie with ata from actual concrete. Autogenous eformation of concrete is the eformation that is cause y hyration reaction of cement an water. The eformation properties are largely etermine y the composition of the cement an the concrete mixture. Intervention of this composition uring casting has no significant effect on this type of eformation. The mix esign is, therefore, a very important parameter with respect to the autogenous eformation. It etermines the riving force ehin the autogenous shrinkage process, which is calle chemical shrinkage. This is a result of the fact that the volume of the hyration proucts of the reacte cement are smaller, compare to its original constituents. The fineness of the cement, together with the water-cement ratio, largely etermines the rate at which the hyration reaction will procee, an with this, it etermines the magnitue of autogenous eformation as well. Especially for Blast furnace slag cements, significant ifferences have een oserve for cement paste with ifferent cement finenesses. One of the ojectives of this research is to etermine the autogenous eformation of Blast furnace slag cement with ifferent finenesses. Previous research has proven that the specific surface of cement is an important parameter with respect to autogenous eformations [3]. Besies this, the stress generation associating whit these eformations were investigate while uner full restraine. In orer to investigate these issues, tests are performe on similar cements prouce y ifferent mills an exhiiting ifferent specific surfaces, viz. Blaine values. The paste eformations were measure with a newly evelope mini-tstm apparatus an compare with the full-scale TSTM results. The mini-tstm is enaling the measurement of the harening eformations an the harening stress-generation irectly from the harening paste. Up till now, stress generation ue to autogenous eformation has mainly een measure from full-scale concrete or paste specimens [3]. In this research an attempt is mae to measure eformations for oth cement paste an concrete an examine the ifferences. The paper provies an overview of the experimental results.

3 2. MATERIALS AND SPECIFICATIONS For the testing program, four ifferent types of cement have een selecte. Emphasis is on three similar Blast furnace slag cements, which were prouce y ifference cement mills. These Blast furnace slag cements, i.e. CEM II/B 42,5, prouce y the ifferent mills turne out to have a large variation in their fineness (Blaine values). Two cements were selecte from the same prouction location (ENCI IJmuien, The Netherlans), where one of these has een grine finer than the other. The thir cement has a very low Blaine value an is from a prouction location in Italy. The fourth type of cement is a Portlan cement from Maastricht in the Netherlans, with a high Blaine value, an serves as a reference material. A etaile overview of the four selecte cements is liste elow, where first the type of the cement is note, after this the prouction location an finally the Blaine value. 42,5 N LH plus, IJmuien: 580m 2 /kg; 42,5 N LH, IJmuien: 480m 2 /kg; 42,5 N LH, Tirreno: 380m 2 /kg; CEM I 52,5 R, Maastricht 550m 2 /kg. From these cements in can e oserve that the Blast furnace slag cements, although they possess the same ientification, show significant variation in their fineness. The Blaine value of the Tirrento cement is almost 20% less than the same Blast furnace cement achieve from a prouction location in IJmuien in The Netherlans. The particle size istriutions of the cements have een measure as well an are presente in Figure 1. For the cement with the highest fineness (Blaine 580 m 2 /kg), also a full-scale TSTM test has een conucte. For this, regular aggregates (D max 16 mm) are applie Volume percentage [%] CEMI CEMIII/B [380m2/kg] Italy CEMIII/B [480m2/kg] IJmuien CEMIII/B [580m2/kg] IJmuien Particle size in [µm] Figure 1: Slive analysis of the four selecte cements.

4 3. TESTING PROGRAM Tests are conucte in orer to investigate the effect of the cement finenesses on the autogenous eformation of harening paste for ifferent water-cement ratios an measure with the mini-tstm apparatus. For these experiments, a water-cement ratio commonly use for normal concrete is aopte, viz. 0.44, an these results are compare with the lowest possile water-cement ratio, without using super-plasticizers, viz For these tests, only the Blast furnace slag cement with the highest Blaine value (580 m 2 /kg) has een teste. With the mini-tstm apparatus, it is possile to select either fixe or free ounary conitions. The first conition provies the opportunity to measure the strain evelopment uring harening of the paste an the secon ounary conition is enaling the measurement of the harening stresses. Therefore, for this research, the following ounary conitions have een impose to the harening paste: Restraine eformations (x = 0); Free eformations (σ = 0). During testing, the top of the paste specimens have een covere y an aluminum foil in orer to prevent rying (shrinkage). Excluing the effect of rying shrinkage from the measurement ensures to measure the autogenous shrinkage from the harening paste. The tests are conucte in a temperature controlle room at 24 C. In figure 2, the complete testing program that has een conucte within the scope of this research [1] has een provie. In this paper, not all results will e presente. CEM I 550 m 2 /kg wcr=0.35 restraint unrestraint 380 m 2 /kg wcr=0.35 restraint unrestraint 480 m 2 /kg wcr=0.35 restraint unrestraint 580 m 2 /kg wcr=0.35 restraint unrestraint wcr=0.44 restraint unrestraint Teste for 7ays Figure 2: Overview of the full testing program conucte within the scope of this research.

5 4. TEST SETUP The starting point for the esign of the mini-tstm test setup for measuring autogenous eformations of harening paste is the mini tensile test setup that was alreay availale at the Microla (see Figure 3). It is a very small apparatus of which the total imensions are smaller then half the size of an A4 page. A special feature of this particular tensile test setup is that it fits insie the ESEM climate chamer, so that tests can e conucte while scanning the actual eformations of the microstructure with the electron microscope. One of the important ojectives for the esign of the mini-tstm was the aility to measure eformation of harening cement paste as well as to measure the stress evelopment ue to restraine eformations. To satisfy this ojective, the gloal esign of the full-scale TSTM set-up has een scale own to comply with the scale of the mini tensile test setup. In orer to achieve this, the concept ha to e scale own 20 times. This reuce the cross section of the ovetaile specimen from 150x150mm to 7.5 x 7.5 mm. Similar to the full-scale TSTM, the ovetaile outer ens of the specimen are require as well in orer to e ale fix the specimen while sujecte to tension. This aske for a specially esigne moul that shoul fit within the ounaries availale insie the tensile setup. The overall rawing of the esign can e seen in Figure 3. The length of the test specimen is 70 mm. Because in this test setup the tensile setup an the TSTM concept are comine, the total setup is calle the mini-tstm. In orer to make emouling possile, the moul is mae of separate assemling parts. The ovetaile contra parts of the moul at the outer ens of the specimen are mae of stainless steel. This prevents them from corrosion an provies sufficient strength an stiffness to the system. In etween the two ovetaile parts of the specimen the cement paste shoul e ale the move freely, without any external restraint. Therefore, within the mini-tstm luricants are use to reuce friction to a minimum. Because of the small size of the apparatus, it turne out to e possile to faricate parts of the moul as single units, mae of Nylon. This is a synthetic material with low surface friction. The parts of the moul that form the sies of the prism an the ottom of the prism are mae of this low friction material as well (see Figure 4, left). Figure 3: Overview of the mini-tensile test set-up.

6 Figure 4: Left: Top view of the mini-tstm apparatus. Right: the casting proceure. While esigning the moul, part of the challenge was to make it water tight on the one han, ut also allow eformations to evelop an minimizing the external friction on the other. The mile part inclues the ottom plate an the sie plate, an has to e ale to move freely from the other parts which are fixe to the parallel arms. This is achieve y using small riging springs that press the ottom plate against the steel ovetail heas of the moul, an comine with Vaseline luricant etween the contact areas. The same Vaseline is also use to connect the other parts, so that there is sufficient flexiility in the connections that allow for the eformations of the cement paste specimen to evelop. Since no super-plasticizers were use in the paste, a special mixing tool has een aopte for the mixing of the paste. This proceure turne out to e not in fully agreement with the stanar proceure for mixing cement pastes, explaine in ASTM C305. Therefore, the proceure escrie in this stanar is ajuste a little an mixing has procees as follows: The cement an the water are weighe; The paste is pre-mixe with a wooen spoon; Mix the paste for 60 secons using the improvise mixer (see Figure 5); Stop the mixer an scrape the walls an let the mix stan for 90 secons; Finish y mixing for another 60 secons. Casting is one with a surgical syringe with a capacity of 50 ml (Figure 4, right). The most important feature is the imension of the mouth of the syringe. It has to e small enough to cast within the small imensions of the setup, ut large enough to ensure a homogenous mix. In this research a syringe with an opening of 4mm is use. Because the specimen is cast insie the rather vulnerale limite space of the mini-tstm apparatus, it was not possile to use a viration tale to remove the entrappe air from the specimen. Instea of that, a milk frother, where the hea has een remove, was use as an alternative virating evice (see Figure 5, right). After viration the freshly cast specimen, the reunant paste was remove an the specimen was seale with an aluminum foil in orer to prevent it from rying.

7 Figure 5: Left: Specially prepare apparatus for mixing the paste. Mile: Mixing lae, Right: Milk frother use for virating the specimen. 5. RESULTS: PASTE MEASUREMENTS The test results, representing the free eformation of the harening paste, measure with the mini-tstm are presente in Figure 6. The tests are conucte on the four ifferent types of cements as iscusse efore, an with a w/c-ratio of The temperature turne out to range etween 20 an 23 C. From the Figure, it can e seen that CEM I shows the largest eformation an, even more important, shows the highest rate of the evelopment of the eformations. This was to e expecte since the hyration procees faster in a fine Portlan cement relative to the Blast furnace slag cement. The Portlan cement shows a steep rop in the early eginning of the hyration process whereas the Blast furnace slag cement shows a slight tenency to swell. The evelopment of shrinkage eformations of all the 42.5 cements procees much slower, an all show some expansion, followe y shrinkage. This can e expecte ecause of the slower hyration in time. The expansion can e relate to the ig volume of the early reaction proucts forme uring hyration. However the eformation of (580 m 2 /kg) is sustantial, at 40 hours it is more than half of the eformation of Portlan cement. It can also e oserve that (480 m 2 /kg) ehaves roughly the same as (580 m 2 /kg), with the only ifference that the evelopment of the eformation is slower. The expansion in the very early stage of harening is a little igger an the shrinkage after 50 hours is sustantially smaller. When consiering these shrinkage results as a function of the egree of hyration, a goo agreement etween oth these results woul e very likely. The ehavior of [380 m 2 /kg] oes not correspon well with the results of the other two last furnace slag cements. It is very likely that this is ue to the fact that this cement is from a ifferent prouction location in Italy an that this cement exhiits very ifferent material characteristics. In general, it must e state that the results are only measure for a seven ays perio. From the results presente in Figure 6, it can e oserve that a longer measuring perio woul e esirale in orer to achieve a etter overview of the shrinkage ifferences.

8 Deformation [ ] a c a c CEM I [550m2/kg] Maastricht [380m2/kg] Italy [480m2/kg] IJmuien [580m2/kg] IJmuien -2 Time [hours] Figure 6: Autogenous eformation of three Blast furnace cement pastes an one Portlan cement paste with ifferent finenesses an a water-cement ratio of The test results representing the harening stresses, associating with the harening eformations uner full restraint (Figure 6), for three Blast furnace slag cements an one Portlan cement paste, an measure with the mini-tstm, are presente in Figure 7. The tests are conucte for the four selecte cements as iscusse efore in section 2 for a watercement ratio of 0.35 an a temperature etween that range etween 20 an 23 C. The results inicate that the ifferences in stress evelopment etween the Blast furnace slag cement an the Portlan cement are quite significant Stress [N/mm2] Time [hours] c a a c CEMI [550 m2/kg] Maastricht [380 m2/kg] Italy [480 m2/kg] IJmuien [580 m2/kg] IJmuien Figure 7: Harening stresses of three Blast furnace cement pastes an one Portlan cement paste with ifferent finenesses an a water-cement ratio of 0.35.

9 From the figure, it can e seen that the Portlan cement CEM I cracks after 30 hours of harening. This is in goo agreement with literature results [3,4,5]. Although the specimens mae of Blast furnace slag cements uil up a sustantial level of tensile stresses, they turne out not to crack with the eformations fully restraine. In comparison with the Portlan cement specimen, the inclination of the stress evelopment for the last furnace slag cements is much slower, which can inicate a more moerate acceleration of the hyration process. The level of the stresses also suggests an influence of the stress relaxation. This has also een suggeste efore, y Lokhorst [5]. An influence of the stress relaxation in comination with the higher strength of the specimen al later ages might e the reason that the specimens mae of last furnace slag i not crack (see also [5]). It can also e oserve that (480 m 2 /kg) shows almost the same stress evelopment in comparison with the specimen mae of (580 m 2 /kg). The only ifference is the elaye stress uil up ue to the slower hyration, an, esies this, the stress after 72 hours is aout 25% lower. Due to the expansion at early ages (see Figure 6), the Blast furnace slag cements show a slight level of compression at the very early eginning of harening. For the IJmuien cements, the compression is a it igger with a lower Blaine value. Interesting is to see that the stress evelopment of the (480 m 2 /kg) is almost horizontally after 72 hours of harening. This inicates that no aitional stresses are uilt-up ue to the autogenous eformations as presente in Figure 6. Again the ehavior of the Italian (380 m 2 /kg) oes not coincie at all with the other last furnace slag cements. Finally, it shoul also e note that the vertical rops in the lines in Figure 7 are proaly ue to the slipping of specimen insie the ovetaile claws. 6. RESULTS: PASTE VERSUS CONCRETE In orer to compare the shrinkage eformation measurements an the stress evelopment of the mini-tstm, tests are conucte with the TSTM as well. The results provie insight in the relationship etween the internal restraint of concrete eformation (full-scale TSTM) an tests performe on pure pastes (with the mini-tstm). The results for the autogenous shrinkage eformations (ADTM) are provie in Figure 8 an the associating stress evelopment (TSTM) in Figure 9. The tests are conucte on a concrete with a water-cement ratio of 0.44 uner isothermal conitions at a temperature of 20 C. The eformation measurements for the ADTM specimens were installe after ceasing of the ormant perio. For these full-scale specimens this was after aout 7.2 hours, whereas the paste eformation measurements with the mini-tstm coul start irectly after casting. The results from oth test setups are compare for a cement type with a Blaine value of 580 m 2 /kg. In Figure 9, results are provie for two inepenent ADTM 1 tests, a mini-tstm testing results an a 10% results of the mini-tstm test. This latter result has een inclue in the figure, since in general, it is aopte that the autogenous eformation of concrete is aout 10 to 15% of the paste eformation [3]. This figure shows that this approach 1 Autogenous Deformation Testing Machine. Is commonly use as the ummy specimen for a TSTM test.

10 Deformation [ ] a Dummy 1 Dummy 2 Mini-TSTM c Mini-TSTM 10% a c Time [hours] Figure 8: Autogenous eformation of a paste (mini-tstm) an concrete (ADTM) with a water-cement ratio of 0.44 an a cement fineness of 580 m 2 /kg. oes not lea to a goo comparison etween the concrete an paste eformations. The most striking ifference is appearing at the early eginning of the harening process, where the concrete shows a strong expansion an the paste oes not show any eformation yet. The explanation for this ifference in eformational ehaviour, which is the result of aing the aggregates to the paste, is not investigate in more etail in this research project Stress [N/mm2] TSTM 2 mini-tstm Time [hours] Figure 9: Autogenous stress evelpment of a paste (mini-tstm) an concrete (TSTM) with a water-cement ratio of 0.44 an a cement fineness of 580 m 2 /kg.

11 The stresses that associate with the autogenous eformation of concrete measure with the TSTM an the autogenous eformation of the paste measure with the mini-tstm are presente in Figure 9. The results show a significant ifference etween oth the stress evelopment of the paste an concrete. The results show that at first, oth specimens are in compression. After having reache its maximum compressive stress, the stress evelopment of the paste is much more rapi in comparison with the stresses uilt-up y the concrete specimen. The ifferent shrinkage ehaviour can e attriute to the aition of aggregates to the concrete mixture, which allows for more internal reistriution of the stresses an most likely also for the microcracks that evelop while uner tension. 7. CONCLUSIONS The results of the research presente in this paper are part of the MSc-thesis of Ms. G. Leegwater. The research was on the evelopment of a tool to measure the autogenous eformations irectly from the paste an on the aility to compare these results with eformation measurements measure irectly from a concrete. The results of this research project clearly show the complexity of the interaction etween paste an concrete shrinkage. The internal microstructure is significantly affecte when aggregates are ae to the mix. The mini-tstm seems to e a valuale tool to measure the paste eformations with the potential to visualise the microstructural eformations with the ESEM. Within the scope of this research tests have een conucte with the mini-tstm, on four ifferent types of cement. There are test performe on three last furnace slag cements with exhiits a ifferent specific surface. The Blaine values of the testes cements were 380, 480 an 580 m 2 /kg. Besies this, tests are performe on Portlan cement that serves as reference test. All tests are performe uner free an restraine eformations. Most of the tests are performe on cement paste mixes with a w/c-ratio of For the Blast furnace slag cement with the highest Blaine value, only some tests are performe with a w/c-ratio of From the tests conucte within the experimental stage of this research project, it can e oserve that a higher Blaine value leas to a larger shrinkage an a faster evelopment of the shrinkage after three ays. The test performe on Blast furnace slag cement with a Blaine value of 580m 2 /kg showe an autogenous eformation within the same orer of magnitue as CEM I. The only ifference was a more moerate evelopment of these eformations. Most striking is the oservation that the stresses in the Blast furnace slag specimen inuce y autogenous eformation o not lea to cracking, while the Portlan cement paste specimen i crack. A possile reason for this can e a ifferent relaxation ehavior of last furnace slag cement in comparison with the Portlan cement in comination with a slower hyration. REFERENCES [1] Leegwater, G. (2006), Autogenous Deformation in Cement Paste an the effects on Concrete Behaviour, MSc-Thesis, Delft University of Technology, Delft The Netherlans.

12 [2] Schlangen, E., Leegwater, G. an Koeners, E.A.B., (2006, This conference), Moelling of Autogenous Shrinkage of Concrete ase on Paste Measurements, Delft University of Technology, The Netherlans. [3] Koeners, E.A.B., (1997), Simulation of Volume Changes in Harening Cement-Base Materials, PhD-thesis, Delft University of Technology, Delft, The Netherlans. [4] Lee, K.M., Lee H.K., Lee, S.H., Kim, G.Y., Autogenous shrinkage of concrete containing granulate last furnace slag, Cement an Concrete Research, article in press (2006). [5] Tazawa, E., Miyazawa, S., (1997), Influence of constituents an composition on autogenous shrinkage of cementitious materials, Magazine of concrete research 49, No. 178, pp [6] Lokhorst, S.J., (1999) Deformational ehaviour of concrete influence y hyration relate changes of the microstructure, Delft University of Technology, Internal report.