Belgian Building Research Institute. Lightweight self-compacting concrete with Argex aggregates

Transcription

1 Belgian Building Research Institute Lightweight self-cmpacting cncrete with Argex aggregates

2 1. Intrductin and prject bjective: This dcument is a shrt verview f the research prject entitled Lightweight self-cmpacting cncrete with Argex aggregates. As part f this prject, a design methd was studied and develped fr several types f lightweight self-cmpacting cncrete. The fllwing principles prvided a starting pint fr the prject: Use f Argex aggregates and particularly grains intended fr structural applicatins ; A systematic apprach, based n a theretical mix design mdel, in rder t develp a versatile prcedure, which can be adjusted t the varius primary materials used. Designing a lightweight SCC presents a challenge at varius levels: Integrating prus, lightweight aggregates may have an impact n the water demand f any mixture, as well as cnsequences fr the rhelgy and effective E/C factr. Lightweight aggregates carry a risk f (inverted) segregatin: due t their lwer density, they may rise t the surface f the matrix/mixture. Using accurately measured quantities f lightweight aggregates, in rder t btain a specific density fr the cncrete, makes it necessary t limit the ttal grain size in the skeletn. Figure 1: 4/8 fractin Argex aggregates Figure 2: Sectin f lightweight SCC with Argex aggregates The prject fcused n a wide range f pssible types f lightweight SCC, in terms f the cmpressive strength class and dry density f the cncrete. Rughly speaking, the dry density (ven dried) is between kg/m³, while the cmpressive strength class ranges frm LC 25/28 t LC 50/55 (Table 1: Cmbinatins f cmpressive strength and ). Table 1: Cmbinatins f cmpressive strength and dry density D 1.6 (1400 < dry density 1600 kg/m³) D 1.8 (1600 < dry density 1800 kg/m³) D 2.0 (1800 < dry density 2,000 kg/m³) Cmpressive strength class LC 25/28 - LC 35/38 LC 25/28 - LC 40/44 LC 30/33 - LC 50/55 Lightweight Self-Cmpacting Cncrete Final Reprt Summary DE611XA339 update 2015Page 2/9

3 In additin t the resulting characteristics fr hardened cncrete, the prperties f fresh liquid cncrete are bviusly crucial. The prgramme f practical tests made it pssible t fine-tune r braden the knwn criteria fr typical SCC tests, such as flwability ( slump flw ), V-funnel flw time, L-bx and ther tests. 2. Apprach and basic principles The mdel adapted by Bruwers, based n the Chinese design mdel, served as a starting pint fr this prject. The basic principles f this methd are as fllws: The matrix is made cmpact by ptimising the granular skeletn. In rder t achieve this, Fuller s principle is applied, but the theretical curve is adjusted, s that finer sand can be used. The curve adapted by Andersen & Andreassen is mre suitable fr cncrete types with a relatively high quantity f sand and fillers, such as high resistance and selfcmpacting cncrete. In principle, a cmpact matrix prduces a mre cmpactable mixture and greatly imprves its liquid behaviur. Using fine sand may reduce the filler cntent, which is useful in ecnmic terms. Using this methd t design an extensive test prgramme made it pssible t make specific adjustments fr lightweight SCC, in which the integratin f large quantities f lightweight aggregates smetimes adds an extra prblem. The develpment f a new set f reference curves, adapted t each density class, facilitates the new mix design (see als Figure 3). Figure 3: Develpment f empirical reference curves based n results btained using theretical curves relative t grain sizes 3. Integratin f lightweight aggregates The main prblem, when using lightweight aggregates, is caused by the influence f slightly less prus aggregates n the water demand f the mixture. Certainly in their dry state, the aggregates can absrb water relatively quickly, as shwn fr 3 types f Argex grain in Figure 4. It becmes clear that the absrptin speed is very high at the start f this curve (fr the first 5 minutes), but gradually falls. The slutin is therefre t pre-misten the aggregates, which greatly reduces absrptin (Figure 5). Lightweight Self-Cmpacting Cncrete Final Reprt Summary DE611XA339 update 2015Page 3/9

4 35 30 AM 4/ Water absrptin [% weight] AR 0/ AM 4/ ' 5' 30' 24 h. 2 d. 3 d. 6 d. 7 d. Time Figure 4: Absrptin curves fr Argex grains frm dry state Misture absrptin after 5' fr AM 4/8-650 and AM 4/ % AM 4/8-750 AM 4/ % Expn. (AM 4/8-750) Expn. (AM 4/8-650) Absrptin d humidité après 5' [%] 8% 6% 4% 2% 0% 0% 5% 10% 15% 20% 25% Misture cntent [%] Figure 5: Measurements f water absrptin by aggregates after 5 minutes starting frm a specific initial misture cntent When the misture cntent f the aggregates exceeds a certain threshld, such as 15%, it can be seen that any further absrptin is limited and shuld nt disturb the mixture anymre. Fr this study, the researchers therefre used pre-mistened aggregates with a misture cntent f apprx. 20%. Fr everyday use, we wuld prbably agree that a misture cntent f 15-25% is perfectly acceptable. Excessively mist aggregates and a high level f absrptin arund the grains can lead t the release f water. Anther new methd f cntrlling this absrptin invlves applying a cating arund the Argex grains, which greatly limits the absrptin f water (see Figures 6 and 7). Lightweight Self-Cmpacting Cncrete Final Reprt Summary DE611XA339 update 2015Page 4/9

5 Waterabsrptin [% weight] 25% Waterabsrptin [% weight] 25% 23% 23% 20% 18% misture = 12 % Reductin water absrptin : 66 % UNCOATED 20% 18% misture = 8.5 % Reductin water absrptin : 75 % UNCOATED 15% 15% 13% 13% 10% 10% 8% 8% 5% 5% 3% COATED 3% COATED 0% WA 5' (%) WA 30' (%) WA 1h (%) WA 2h (%) 0% WA 5' (%) WA 30' (%) WA 1h (%) WA 2h (%) Time Time Figure 6: AM 4/8-650 C cating - Measurements f water absrptin by aggregates ver a perid f time (in cmpliance with EN ) Figure 7: AR 0/4-700 C cating - Measurements f water absrptin by aggregates ver a perid f time (in cmpliance with EN ) A secnd prblem assciated with the use f lightweight aggregates is caused by the risk f (inverted) segregatin: lightweight aggregates have a tendency t rise t the surface. Generally, using an admixture t increase viscsity can reslve this prblem. In additin, this adixture can act as a buffer and cunteract the effects f any variatins in the misture cntent f the aggregates. Finally, it is als necessary t pay careful attentin t the dsage f lightweight aggregates. Bth the vlume and weight f the lightweight aggregates can be dsed. Due t the ptential risk f expansin (especially fr light 0/4 fractin sand), weight dsage is always advisable. Weight dsage mainly invlves regularly checking that the density and misture cntent f the mist aggregates is crrect. In fact, this value is used in the calculatin. Checking that the misture cntent is right makes it pssible t adjust the aggregate dsages and t assess whether the misture cntent is suitable fr the prductin f high quality SCC. 4. Labratry test prgramme The basic characteristics f the designed mixtures were mnitred, using tests cnducted in the labratry, bth in fresh and hardened state. Fr typical SCC tests cnducted n fresh SCC, the fllwing target values can be prpsed, which are nt very different frm the values fr a SCC with a nrmal density: Flwability r slump flw: ideally mm, a brader range f mm als being pssible, prvided the risk f insufficient fluidity r blckage is als taken int accunt; V-funnel flw time: 5-25 secnds, with high variatins, depending n the chsen admixture; L-bx rati: minimum 0.7 (assessed accrding t the field f applicatin); J-ring: blcking step belw 15 mm; Sieve stability test: difficult t use, a test n the hardened cncrete is recmmended. Lightweight Self-Cmpacting Cncrete Final Reprt Summary DE611XA339 update 2015Page 5/9

6 Average cmpressive strength 28d (f_cm) [N/mm²] 10/03/2015 Figure 8: Spreadability test (left) and L-bx rati (right) The cmpressive strength and dry density f the hardened cncrete represent basic characteristics and were therefre determined fr all labratry mixtures. Figure 7 shws the rati between the average cmpressive strength and measured dry density (ven dried). It als cmpares the results fr a lightweight SCC t the average values fr nrmal lightweight cncrete. Figure 10 shws the characteristic values fr cmpressive strength, accrding t dry density (ven dried) (based n 3-6 samples) Average LWAC Oven-dry density cncrete [kg/m³] Figure 9: Rati between the average cmpressive strength and dry density (ven dried) Lightweight Self-Cmpacting Cncrete Final Reprt Summary DE611XA339 update 2015Page 6/9

7 Shrinkage [µm/m] Characteristic cmpressive strength f_ck 28d [N/mm²] 10/03/ Oven-dry density cncrete [kg/m³] Figure 10: Rati between characteristic cmpressive strength and dry density (ven dried) Fr the sake f cmpleteness, the shrinkage values and elasticity mdulus were als mnitred fr a limited number f mixtures. The highest shrinkage values were bserved n the lightest mixtures, which can be explained by tw factrs: Higher pwder cntent (bth cement and fillers); Lwer ttal aggregate cntent. The rder f magnitude f the shrinkage values fr lightweight SCC remains identical t that fr nrmal lightweight cncrete. Figure 11 cmpares the values measured t the shrinkage curves indicated fr lightweight cncrete in Eurcde 2 accrding t the relevant cmpressive strength classes D1.8 D1.6 D1.8 D2.0 C30/37 C35/45 C45/ D1.6 D Time [d] Figure 11: Cmparisn f shrinkage curves with the shrinkage curves in the Eurcdes fr lightweight cncrete Finally, the elasticity mdulus was als determined fr six mixtures and the results f each measurement are shwn in Table 2. The results are in line with expectatins, if they are cmpared t the Eurcde 2 values (calculatin based n cmpressive strength and density). Lightweight Self-Cmpacting Cncrete Final Reprt Summary DE611XA339 update 2015Page 7/9

8 Table 2: Mdule E measurements and cmparisn with Eurcde 2 curves D1.6 D1.8 D2.0 M Md E [N/mm²] Values EC2 [N/mm²] Difference cmpared t EC2 [%] M M M M M M Industrial tests As part f the final phase, the prpsed mix design methd was assessed, based n a number f industrial tests cnducted by prefabricated element manufacturers, which prduce larger vlumes f cncrete and cast larger cncrete elements. Fr each test, the fllwing diagram was respected: Mix design Preparatin f ne r mre labratry mixtures, using the primary materials prvided by the relevant manufacturer Optimised test n an industrial scale, with r withut adjustments, such as admixture dsage Assessment f btained elements and samples Figure 12: Lightweight SCC is ften used fr large wall panels. Figure 13: The prpsed mixes are used in the manufacturing prcess and cast using bins. Figure 13: It is imprtant t mnitr the prperties f fresh SCC (in this case, flwability r slump flw ). Figure 14: Preparatin f cubes, in rder t mnitr cmpressive strength and dry density Lightweight Self-Cmpacting Cncrete Final Reprt Summary DE611XA339 update 2015Page 8/9

9 These industrial tests made it pssible t fine-tune practical aspects f the design methd. They als shwed the extent, t which the theretical mix design can be used t rapidly btain a cnsistent mix that meets specific requirements (weight, cmpressive strength, fluidity, etc.). 6. Summary and cnclusins The prject entitled Lightweight self-cmpacting cncrete with Argex aggregates fcused n a design methd fr several types f lightweight self-cmpacting cncrete (LSCC). This study is based n the adapted Chinese methd, while an extensive test prgramme made it pssible t further ptimise the design methd and wrking prcedures. The design methd itself was integrated int sftware, which shuld make it pssible, at a later stage, t rapidly design new mixtures, using new primary materials. The resulting mixtures als shw the wide range f pssibilities created by lightweight selfcmpacting cncrete, based n mix types and characteristics f the hardened cncrete. The characteristics shwn in Table 3 can be prduced using the adjusted mixtures. Table 3: Cmbinatins f cmpressive strength and dry density D 1.6 (1400 < dry density 1600 kg/m³) D 1.8 (1600 < dry density 1800 kg/m³) D 2.0 (1800 < dry density 2,000 kg/m³) Cmpressive strength class LC 25/28 - LC 35/38 LC 25/28 - LC 40/44 LC 30/33 - LC 50/55 The prject was nt limited t labratry tests and included industrial tests, in rder t test the design prcess, which made it pssible t wrk with ther materials and under real wrking cnditins. In mst f the industrial tests, the abve criteria fr fresh cncrete culd be used, which required the same prperties as fr hardened cncrete and met the necessary criteria. The fllwing summary shws, nce again, the cnclusins based n tests cnducted n fresh cncrete: Slump flw: mm V-Funnel time: 5-25 secnds J-ring: blcking step belw 15 mm L-bx rati: minimum 0.7 (cmbined assessment!) Sieve stability test: less useable Lightweight Self-Cmpacting Cncrete Final Reprt Summary DE611XA339 update 2015Page 9/9