Concreting method that produce high modulus of elasticity

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1 MATEC Web o Conerenes 11, 1 ( 14) DOI: 1.151/ mateon/ C Owned by the authors, published by EDP Sienes, 14 Conreting method that produe high modulus o elastiity H.S. Abdelgader 1 and A.S. Elbaden 1 1 University o Tripoli, Civil Engineering Department, Tripoli, Libya Abstrat. During the last deades, the onrete industry has been widely developing in many ways suh as the methods o pouring onrete in order to ahieve high quality onrete and low ost. Two-stage onrete is haraterised by a higher proportion o oarse aggregate thereore the variation in aggregate ontent inluenes signiiantly its mehanial properties. The mehanial harateristis o the two-stage onrete (TSC) in ailure onditions are dissimilar rom the ordinary ones. Behaviour o TSC in ompression has been well doumented, but there are little published data on its behaviour in tension and modulus o elastiity. This paper presents the results o experimental testing o one type o oarse aggregate and three dierent mix proportions o grout. It was ound that the modulus o elastiity and splitting tensile strength o two-stage onrete is equivalent or higher than that o onventional onrete at the same ompressive strength. 1 Introdution Two-stage onrete (TSC), also known as pre-plaed aggregate onrete, derives its name rom a unique plaement method. Unlike onventional onrete, it is made by irst plaing oarse aggregate in the ormwork and then injeting a ement grout to ill the voids between oarse aggregate partiles [1-4]. Mehanial properties o TSC are thus inluened by the properties o the oarse aggregate [4], the properties o the grout [,5], and the eetiveness o the grouting proess [6,7]. When plaed properly, TSC has beneiial properties suh as low drying shrinkage, high bonding strength, high modulus o elastiity, and exellent durability []. The method o TSC has proved partiularly useul in a number o appliations like underwater onstrution, onrete and masonry repair, situations where plaement by usual methods is extremely diiult, mass onrete where low heat o hydration is required, and tunnel and sluieway plugs to ontain water at high pressure where very low shrinkage is important [1,,8,9]. It is also useul in the manuature o high density onrete or atomi radiation shielding where steel and heavy metalli ores are used as aggregate []. TSC diers rom onventional onrete not only in the method o plaement but also in that it ontains a higher proportion o oarse aggregate. TSC ompressive strength is also dierent rom onventional onrete beause o the speii transmission o stresses that our in TSC. The losepaked oarse aggregate exhibits ontat areas in all diretions, whereas in normal onrete the aggregate is usually smaller in size and rather dispersed. Thus the ompressive stresses are mainly transmitted by the oarse aggregate [5,1,11]. The speii mehanism o stress distribution produes shear stresses and stress onentration in the ontat areas. The modulus o elastiity is partiularly important rom the design point o view in plain or reinored onrete, sine it an desribe the onrete mehanial behaviour. The elasti modulus and the ailure harateristis o TSC dier rom those o ordinary onrete. While the mehanial properties o TSC in ompression have been well doumented, there remains little published data on tensile strength and modulus o elastiity. The objetive o this investigation was to study the strength o TSC in ompression and tension and modulus o elastiity using dierent grout mixtures. Materials.1 Coarse Aggregate The oarse aggregate used in these experiments was sub angular basalt. A large sample was aquired, washed, and sieved to reate the gradation with a maximum size o 5 mm. The oarse aggregate had a bulk loose density 14 kg/m, dry speii gravity o.687, absorption o.4%, and void ratio o 47%. Beause o its low absorption and high apparent speii gravity, this basalt is onsidered a high quality oarse aggregate.. Cement Grout In this investigation, a simple mortar was used or the ement grout with admixtures (Superplastiizer) was used or all mixes and was added at the rate o % by weight o ement. The ine aggregate used in the manuature o grout was produed rom silia sand, whih is subangular in shape with 1 perent passing a No. 8 sieve. The sand has an absorption o 1.5 %. The partile size distribution o sand is divided into three rations:./1. mm (1%), 1./.5mm (67%), and.5 /. mm This is an Open Aess artile distributed under the terms o the Creative Commons Attribution Liense., whih permits unrestrited use, distribution, and reprodution in any medium, provided the original work is properly ited. Artile available at or

2 MATEC Web o Conerenes (%),.The ement used throughout the experiments was ordinary (Type I) Portland ement. Water was used diretly rom the tap and was slightly older (1C) than room temperature. The temperature in the lab when mixing and plaing grout was near 7C. For larity, the ement grout will be reerred to as mortar throughout this paper. Grout mixture proportioning and onrete speimen preparation The seletion o water-ement-sand ratios is more ritial in TSC beause the amounts o sand and water ontrol the pump-ability o grout, an essential requirement in the prodution o TSC []. Coarse aggregate was irst plaed in 15 x mm hard plasti ylindrial moulds. The void ontent o the preplaed aggregate was 47 % and the bulk density 14 kg/m. Five dierent proportions o water to ement (.4,.45,.5 and.55) were investigated with ement to sand ratios o 1/1.5, 1/1, and 1/.8. Grout preparation was aomplished by ombining ingredients with an eletri mixer or about three minutes to ahieve the desired grout uniormity and onsisteny. Trials were made with grout to ind the minimum waterto-ement (w/) ratio at whih the preplaed aggregate ould be eetively grouted. A w/ ratio o.45 was ound to be the minimum ratio suitable or grouting; it was not possible to penetrate all voids in the aggregate skeleton with a grout at a w/ ratio o.4. The displaement o aggregate was less notieable as the w/ ratio was inreased; at a w/ ratio o.55 there was no displaement. 4 Experimental program The experimental program onsisted o a series o unonined ompression tests, split tensile tests and modulus o elastiity tests on ylinders prepared rom dierent grout mixtures at ages o 8 days. To eetively ompare the unonined ompressive strength to tensile strength, it was determined that all tests should be onduted using 15 x mm ylindrial speimens prepared in the same manner. Exluding the speimens at a w/ ratio o.4, there were 81 onrete speimens tested. Three speimens rom eah one o the three grout mixtures were tested in unonined ompression, split tension and modulus o elastiity at 8 days. Ater one day o asting the onrete speimens were removed rom the ylinders and overed with moist burlap or seven days and stored in the laboratory limate. 5 Experimental test proedures Mortar ubes were prepared and tested in unonined ompression per ASTM C 94. Three ubes o eah mortar were tested at 8 days using a universal hydrauli testing mahine with 6 kn apaity. Unonined ompression tests on two-stage onrete ylinders were tested in aordane with ASTM C 9. Three speimens o eah onrete were tested at 8 days using a universal hydrauli testing mahine with kn apaity. Splitting tensile tests were also onduted on three speimens o eah onrete at 8 days aording to the proedures outlined in ASTM C 496. Splitting tensile tests were perormed using the same mahine as the mortar ubes. The splitting tensile strength is alulated using Equation 1 as ollows: s=p/ ld (1) Where: s=splitting tensile strength [MPa], p=maximum applied load [KN], l=speimen length [mm], d=speimen diameter [mm] 6 Investigation o strength o two-stage onrete 6.1 Compressive strength The unonined ompressive strength o TSC was measured at 8 days. In this way or eah o the TSC type three ylinders were subjeted to a ompressive test ater 8 days o uring. During the loading proedure, the vertial deormations were measured on three sides o the speimen with respet to axial ore inrement. On the basis o these results, stress and appropriate strain values were alulated. Some o the test results are presented in igure 1. At the same time 7 onrete ube samples ( w/ ratios, /s ratios and one type o oarse aggregate) o xx mm were tested or ompressive strength. The results presented in Table were alulated aording to an algorithm, whih was proposed earlier or designing o two-stage onrete [4]. Cylinders and the ubes have been ast and tested in the same onditions. STRESS [MPa] rushed aggregate w/ =.5, /s = STRAINS 1 - Fig. 1. Some results o ompressive tests o two-stage onrete ylinder samples igure shows the mean and individual ylinder strengths o three speimens per w/ ratio at ages o 8 days. It an be seen that mean ompressive strengths o 1.9 is attainable at 8 days with a w/ ratio o.45. igure demonstrates a strength redution as the w/ ratio inreases. Although there is some variation in strength measured per w/ ratio, the strength redution is approximately linear Table 1. This observation is onsistent with unonined ompressive strength measurements o TSC ube speimens ( x x mm), where Abdelgader [4] Table determined the ompressive strength o TSC at 8 days as: = ( w / ) +.5 ( / s ) () 1-p.

3 CMSS 1 Equation is illustrated in igure or a /s ratio o 1., and it an be seen that it under-predits strength o the ylindrial speimens tested in this program. The predited mean strength rom Equation is 87 to 9% o the measured mean strength. Dereasing the multipliative ator on the w/ ratio in Equation to aount or ylindrial speimens yields the ollowing relationship: = ( w / ) +.5 ( / s ) () Only /s = 1. or speimens investigated herein. Further researh is required to determine the suitability o Equation or a range o /s ratios. Another signiiant inding rom the ompressive strength data was the somewhat limited rate o strength development. This an be explained, in part, beause o the at that no ly ash or other pozzolans were inorporated in the ement grout. The observations show that, although the mehanism o stress transer is believed to be dierent rom onventional onrete, the mortar strength is a ontrolling ator in the strength o two-stage onrete. Table 1 summarizes the mean and range o ompressive strength o mortar and onrete at 8 days. Firstly, it an be seen that the range o measured ompressive strengths at 8 days is sometimes high or both mortar and onrete. Seondly, the ratio o mean onrete-to-mortar strengths is reasonably onsistent and alulates rom.5 to.51 or all mixtures at 8 days. This observation suggests that the ompressive strength o two-stage onrete an be onservatively estimated as one-hal o its mortar strength. I this ratio an be substantiated with more mixtures and other soures o oarse aggregate, this simple rule-o-thumb an be adopted in the design o two-stage onrete. Compressive Strength (MPa) Speimen Strength Mean Strength Eq. Eq w/ Fig.. Cylinder ompressive strength vs. water-ement ratio at age o 8 days. Table 1. Cylinder ompressive strength o onrete and mortar at 8 days. W/C Conrete `( 8 days) (MPa) Mortar `( 8 days) (MPa) Mean Range Mean Range onrete : mortar ratio Table. Cube ompressive strength or various two-stage onrete types. Water/Cement Sand/Cement Conrete `( 8 days) ratio, (W/C) ratio, (S/C) (MPa) / Jan / Tensile strength The splitting tensile strength o TSC was also measured at 8 days. Table shows the mean strengths o three speimens per w/ ratio at age o 8 days. The results indiate that a strength redution as the w/ ratio inreases. However, there appears to be little dierene in strength between speimens produed with a w/ ratio o.45 and those produed with a w/ ratio o.5. This was also observed with the ompressive strengths measured or the same grout mixtures. The atual values o tensile strength at w/ ratios o.45 and.5 measured rom.1 to. MPa, whih indiate satisatory results, espeially when one onsiders the minimum ost o onreting and that no vibration tools are used. Furthermore, exellent results an be expeted even when using a high w/ ratio o.55, where the mean tensile strength is nearly.7 MPa. Test observations show that ailure in splitting tension was restrited prinipally to the line o split and ours through the mortar and oarse aggregate. Visual assessments o the ailed speimens suggest that the perentage o ailed aggregate inreased in onrete with higher mortar strength (lower w/ ratio). Table. Mean tensile strength o onrete at 8 days W/C 8 days tensile strength (MPa) p.

4 MATEC Web o Conerenes 6. Compressive-tensile strength relationship The results o this investigation show that the splitting tensile strength o TSC an be approximated well by the ACI equation or onventional onrete, as shown in igure. For onventional onrete, the preditive equation is given by: s =.56 (4) where and s are in units o MPa. In this investigation, the splitting tensile strengths an be estimated as: s = ( ) (5) whih onorms losely to the ACI equation (Equation 4). Equation 4 is also valid or estimating 9-day splitting tensile strengths based on 9-day ompressive strengths, as the data in igure suggest. In this investigation, the ator in Equation 4 ranges rom.5 to.56 or 9-day strengths. igure inludes ompressive and tensile strength results rom a onurrent investigation by Abdelgader [1]. In that investigation, 15 x mm onrete ylinders were produed with a similar mortar mixture (w/ ratios o.45,.5,.55 and.6 at a /s ratio o 1.). The oarse aggregate soure was a rushed dolomiti limestone rom Rageat, loated 45 km south o Tripoli, Libya. Partiles were subangular in shape and the gradation was uniorm, with almost all partiles passing a 5 mm sieve and retained on a 7.5 mm sieve. The physial harateristis o that limestone and this granite are quite similar, but the results show a higher splitting tensile strength or two-stage onrete produed with limestone. The splitting tensile strengths an be estimated as s = (.6.68 ) (6) The data rom both studies show that the tensile strength o TSC is at least as high as that o onventional onrete, and in at it an be higher depending on the seletion and properties o the oarse aggregate. No auses were apparent or the relatively higher tensile strength in twostage onrete. However, the greater mehanial interloking among partiles in two-stage onrete ould be responsible or the higher tensile strength sine ators like aggregate gradation are dierent rom onventional onrete. These observations warrant a muh deeper investigation into the inluene o oarse aggregate properties on two-stage onrete behavior in tension. Splitting Tensile Strength (MPa) Days Strength.4 Eq.4. 8 Days Strength (Abdelgader[1]) Compressive Strength (MPa) Fig.. Mean splitting tensile strength vs. mean unonined ompreeeive strength. 6.4 Stress-Strain relations The obtained results are statistially analysed to estimate equations or the ompressive strain-stress relations o various two-stage onrete types. The statistial analysis is perormed by use o the MINITAB program [1]. The Minitab Regress Pakage its an equation to the experimental data by the least square method. The alulation an be divided into the ollowing steps: assumption o the mathematial relation, estimation o the equation oeiients, testing o the assumed equation and hoie o the best model. The statistial analysis o the TSC test data is perormed in two suessive phases. First, equations or stress-strain relations or type o oarse aggregate and dierent grout proportions are estimated. In this step the data onsist o 7 ( 9) stressstrain sets o experimental results obtained rom ylinder ompressive tests. Ater various trials the ollowing simple relation or all onrete types is: σˆ = aε1 + bε1 + ε [MPa] (7) 1 In the above ormulae ˆ σ stands or the estimated onrete stresses, and ε1 = ε 1, where ε denotes the onrete strain. The onstants a, b, and in Eq. (7) or eah onrete type are statistially obtained. In Table 4 some results (6 out o 18 test data sets) or /s = 1/1.5 and various w/ ratios are presented. Equation (7) gives reliable results in the ollowing range o strains: ε.1 (8) Table 4. Constants a, b, o equation (7) or oarse aggregate type and /s=1/1.15 Aggregate type w/ a b Correlation oeiient Crushed Typial statistially obtained stress-strain urves or twostage onrete in dierent grout proportions and type o oarse aggregate are presented in igure 4. STRESS [MPa] rushed aggregate /s = 1/1.5 w/ =.45 w/ =.5 w/ = STRAINS 1 Fig. 4. Stress-strain relations or various TSC (Eq.7). 1-p.4

5 CMSS 1 In the seond step the statistial analysis is used to obtain the two-stage onrete stress-strain relations as untion o water-ement ( ω = w/ ) and ement-sand ( ς = /s ) ratios. Ater a omplex analysis the untions a= a( ω, ς ), b= b( ω, ς ) and = ( ω, ς ) in Eq. (7) are assumed as ollows: or rushed aggregate: a b ω, ς = ω ς, ω, ς = ω ς, ω, ς = ω 7.8 ς, Some estimation results or onrete stresses aording to Eq. (7) by use o untions (9) is presented in Table 5. The values o the orrelation oeiient reveal that using the untion (, ) a ω ς, b ( ω, ς ), and (, ) (9) ω ς the test results and their estimators are more dispersed (ompare Table 4 and 5). Table 5. Correlation onstants alulated aording toeq.(9) Aggregate type Crushed w/ a( ω, ς ) b( ω, ς ) ( ω, ς) Correlation oeiient The test observations and the statistial analysis have proved that the linear part o the stress-strain urve in two-stage onrete may reah 85% o the ultimate ylinder strength (see Fig. 4). This may result rom a very good ontat between the aggregate partiles. In onsequene the applied load is distributed mainly by the stones. During the laboratory tests it was observed that the skeleton o the two-stage onrete arried the load until the whole speimen ollapsed. It was the reason why the ailure o the two-stage onrete was not sudden and explosive but rather gradual. The ailure speimen showed extensive lateral expansion in the orm o bulging. Thus the speii stress-strain harateristis o the two-stage onrete an be explained by a dominant inluene o the rigid stone skeleton. It should be pointed out that the stress-strain relations o traditional onrete result rom transer o stress through the mortaraggregate uniorm matrix. In the two-stage onrete there are no suh uniorm relations beause the mehanial properties o two-stage onrete are mainly determined by the harateristis o the stone aggregate. Thereore, the subsequent rature o the two-stage onrete takes plae through the stone aggregate partiles [1]. In the TSC the ompressive stresses in the aggregate and in the grout annot be equal. The ompressive stress is inluened by the shear stresses ourring not only in the vertial setion but also in the horizontal ontat suraes. The produed shear stress aused the tearing o o the aggregate grains rom grout. It was also observed that some miroraking or interaial raks were randomly distributed even beore the speimens were subjeted to load testing. It seems that these raks were aused by bleeding, settlements o grout, or by shrinkage stresses indued by the drying proess. The raks ormed within the grout may be onsidered to be isolated or disontinuous. 6.5 Modulus o elastiity The experimental data analysis and the statistially obtained stress-strain relations allow or ormulating the relationship between the modulus o elastiity and the ompressive strength o the two-stage onrete. The ompressive strength (Table ) is alulated aording to the algorithm or designing the two-stage onrete proposed by Abdelgader, [4]: = β + β1 β g (1) Where: g stands or ompressive strength o grout [MPa], β, β 1 and β are onstants obtained rom regression analysis and are given in Table 6 p. 5 [4]. The modulus o elastiity o the TSC is mainly aeted by the physial properties o the oarse aggregate. The inluene o the ontent o grout in the onrete is rather meaningless. It has been observed that the same ators that aet the ompressive strength also alter the elasti modulus o the TSC. The elasti modulus is obtained rom the analysis o the stress-strain urves or eah type o stone aggregate and the mix proportions. The initial tangent modulus o elastiity o the TSC is ound with reerene to the tangent line drawn to the stress-strain urve at the starting point (ompare some examples presented in igure 5). STRESS [MPa] Fig. 5. Graphial presentation o modulus o elastiity o TSC. The modulus o elastiity o two-stage onrete E ε, determined as a untion o strains ts STRAINS 1 1 rushed aggregate /s = 1/1.5 ε1 = ε 1 ), an be alulated in GParom ε ( Equation (11) as ollows: d ˆ σ Ets ( ε1) = = a( ω, ς ) ε1 b( ω, ς ) ε1 + ( ω, ς ) (11) dε 1 w/ =.45 w/ =.5 w/ =.55 1-p.5

6 MATEC Web o Conerenes Modulus o Elastiity (GPa) ε = the onrete modulus ts Assuming that 1 estimated as: Ets = ( ω, ς ) (GPa) (1) E is Equation (1) indiates that untion (, ) ω ς, alulated aording to Eq. 9, determines the elasti modulus o the two-stage onrete. It should be mentioned that the values o the elasti modulus obtained or % o the ylinder ultimate strength are almost the same. In this way the initial linear eature o the strainstress relation o the TSC has been proved. It is possible to elaborate an alternative version o the elasti modulus ormulae. Making use o the regression analysis o the experimental data expressions desribing the relationship between the elasti modulus and the ompressive strength o the TSC the ollowing assumption an be made: E ts = (GPa) (1) Where is the ube ompressive strengths (Eq. 1 and Table ). The limit value o the ube ompressive strengths in equation (1) is: MPa MPa (14) Equation (1) relation is graphially presented in igure 6. It should be notied that the obtained values o the modulus o elastiity are rather sattered igure 6. This an be explained by the random alloation o oarse aggregate in the orms. Nevertheless, Eq. (1) allows or the approximate determination o the modulus o Cube Compressive strength(mpa) elastiity or the examined rushed aggregate. It should be pointed out that the obtained results deviate at most ± 1% rom the alulated mean values igure 6. Fig. 6. Relation between modulus o elastiity and ompressive strength o TSC. 7 Conlusions The ollowing onlusions an be drawn rom this study: 1. A grout mixture with a water-to-ement (w/) ratio o.45 to.55 and a ement-to-sand (/s) ratio o 1./1.5, 1./1. and 1./.8 optimizes ompressive and tensile strength o TSC. Mortar mixed with a w/ ratio below.4 is too visous and does not ully penetrate the voids between oarse aggregate partiles, thus reating a honeyombing eet in the hardened onrete.. Compressive strength o TSC ylinders an be onservatively estimated as 5% o the mortar ube strength.. The splitting tensile strength o TSC was ound to be similar to that predited by the ACI equation or splitting tensile strength o onventional onrete. In some ases, the measured tensile strength o TSC is in at higher than that predited by the ACI equation. 4. The stress-strain relationships or dierent grout mixes (water/ement ratios and ement/sand ratios) do not show a big dierene. The initial stress-strain urves an be estimated by linear relations. This may result rom the stresses distributed mainly by the partiles o stone aggregate (skeleton o stones). The speii way o stress transmission may also ontribute to the initiation and propagation o raks. 5. The modulus o elastiity as a untion o ompressive strength o the TSC is elaborated. The modulus values or speii type o aggregate is desribed by linear onstant untions. The obtained equations allow engineers to design the TSC aording to the algorithm presented in [4]. Reerenes 1. King, J.C., Handbook o Heavy Constrution - Conrete by Intrusion Grouting, MGraw-Hill, New York, pp.11-4 (1959). ACI Committee 4, Guide or The Use o Preplaed Aggregate Conrete or Strutural and Mass Conrete Appliations, ACI 4.1 R-9., pp.1-4 (1997). Abdelgader, H.S., Eet o Quantity o Sand on The Compressive Strength o Two-Stage Conrete, Magazine o Conrete Researh, 48, pp. 5-6 (1996) 4. Abdelgader, H.S., How to design Conrete Produed by a Two-Stage Conreting Method, Cement and Conrete Researh, 9, pp. 1-7 (1999). 5. Abdelgader, H.S. and Górski, J, Inluene o Grout Proportions on Modulus o Elastiity o Two-Stage Conrete, Magazine o Conrete Researh, 4, pp (). 6. Iwasaki, N., Preditions o Grouting Proess in Pre-paked Conrete by Green s Funtion, Proeeding o Japanese Soiety o Civil Engineering, pp (1985). 7. Swaddiwudhipong, Zhang, S., J. and Lee, S.L., Visometri Charaterisation o Cement Grout or Predition o Pre-paked Conrete Constrution, Magazine o Conrete Researh, 5, pp (). 1-p.6

7 CMSS 1 8. King, J.C. and Wilson, A.L., I it s still Standing, It Can be Repaired, Journal o Conrete Constrution,, pp (1988) 9. Colle, E.R., Preplaed Aggregate Conrete Repairs 6-Year-Old Railroad Bridge, Conr. Rep. Dig., The Aberdeen Group.(199). 1. Abdul Awad, A.S.M., Failure Mehanism o Prepaked Conrete, ASCE J. Strut. Eng.,, pp (1988). 11. Abdelgader, H.S., and Górski, J., Stress-Strain Relations and Modulus o Elastiity o Two-Stage Conrete, ASCE J. Mat. Civ. Eng., 4, pp () 1. Abdelgader, H.S., and Elgalhud A. A., Eet o Grout Proportion on Strength o Two-Stage Conrete, Strutural Conrete,, pp (8). 1. SCHAEFER, R.L., ANDERSON, R.B., The students edition o MINITAB statistial sotware adapted or eduation. Addison Weslley Publishing Company, In. (1989). 1-p.7