Advanced Materials Research Vol. 893

Advanced Materials Research Vol. 893 (2014) pp 585-592 Online available since 2014/Feb/19 at www.scientific.net © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.893.585

Compressive and Tensile Strength of Two-Stage Concrete Abdullah F. Saud1,a, Hakim S. Abdelgader1,b and Ali S. El-Baden1,c 1

Tripoli University, Faculty of Engineering, Civil Engineering Department, Tripoli, Libya a

b

c

[email protected], [email protected], [email protected]

Keywords: Two Stage Concrete, Compressive Strength, Tensile Strength, Grout Mixture.

Abstract. An experimental investigation was conducted to evaluate the compressive, tensile strength and modulus of elasticity of two-stage concrete (TSC) at different water-to-cement ratios. The primary objectives were to measure the elastic modulus, compressive strength and splitting tensile strength of TSC and to determine if there is a quantifiable relationship between compressive and tensile strength. Behavior of TSC in compression has been well documented, but there are little published data on its behavior in tension and modulus of elasticity. This paper presents the experimental results of preplaced, crushed granite aggregate concreted with five different mortar mixture proportions. A total of 48 concrete cylinders were tested in unconfined compression modulus of elasticity and splitting tension at 28 and 90 days. It was found that the modulus of elasticity and splitting tensile strength of two-stage concrete is equivalent or higher than that of conventional concrete at the same compressive strength. Splitting tensile strength can be conservatively estimated using the ACI equation for conventional concrete. Introduction Two-stage concrete, also known as pre-placed aggregate concrete, derives its name from a unique placement method. Unlike conventional concrete, it is made by first placing coarse aggregate in the formwork and then injecting a cement grout to fill the voids between coarse aggregate particles [14]. Mechanical properties of two-stage concrete are thus influenced by the properties of the coarse aggregate [4], the properties of the grout [3,5], and the effectiveness of the grouting process [6,7]. When placed properly, two-stage concrete has beneficial properties such as low drying shrinkage, high bonding strength, high modulus of elasticity, and excellent durability [2]. The method of twostage concrete has proved particularly useful in a number of applications like underwater construction, concrete and masonry repair, situations where placement by usual methods is extremely difficult, mass concrete where low heat of hydration is required, and tunnel and sluiceway plugs to contain water at high pressure where very low shrinkage is important [1,2,8,9]. It is also useful in the manufacture of high density concrete for atomic radiation shielding where steel and heavy metallic ores are used as aggregate [2]. Two-stage concrete differs from conventional concrete not only in the method of placement but also in that it contains a higher proportion of coarse aggregate. One cubic meter of two-stage concrete contains one cubic meter of coarse aggregate (bulk volume) as compared to 0.67 to 0.75 cubic meters in conventional concrete [9]. It may be regarded as a ″skeleton concrete″ as the coarse aggregate effectively rest against one another and the remaining voids are filled with cement grout. Voids must be large enough to allow complete penetration of the cement grout; thus, a coarse gradation of coarse aggregate is required. A particle size of 37.5 mm is often the predominant size and particles smaller than 12.7 mm must be removed [2]. The resulting void content is normally in the range of 35 to 50% of total concrete volume [2,8,9]. Two-stage concrete compressive strength is also different from conventional concrete because of the specific transmission of stresses that occur in two-stage concrete. The close-packed coarse aggregate exhibits contact areas in all directions, whereas in normal concrete the aggregate is usually smaller in size and rather dispersed. Thus the compressive stresses are mainly transmitted by the coarse aggregate [5]. The specific mechanism of stress distribution produces shear stresses and stress concentration in the contact areas. These stresses are responsible All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 41.252.37.161-03/07/14,01:25:31)

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for fracture of some coarse aggregate and the tearing of other aggregate from the grout. Prior research of two-stage concrete in compression has shown that numerous cracks form around the surface [5,10,11]. Unlike conventional concrete, it exhibited extensive lateral expansion in the form of bulging prior to failure, and the failure, in general, was not sudden and explosive [10]. It was also noticed that a large proportion of failures took place through the coarse aggregate particles [11].While the mechanical properties of two-stage concrete in compression have been well documented, there remains little published data on tensile strength. Davis [12] commented that the bond strength, as determined by modulus of rupture, of conventional concrete repaired with preplaced aggregate concrete approaches the flexural strength of the weaker component of the composite. However, flexural strength data on preplaced aggregate concrete were not presented. Davis [13] presented two case histories of high-density concrete using preplaced aggregate where modulus of rupture was measured. Because of the point-to-point contact of the coarse aggregate and the subsequent transmission of stresses through the interlocked aggregate skeleton, the tensile strength is theorized to be higher than for conventional concrete. The objective of this investigation was to study the strength of two-stage concrete in compression and tension at 28 and 90 days using different grout mixtures. Materials Coarse Aggregate. In the manufacture of two-stage concrete the two basic constituents are the coarse aggregate and the grout. The coarse aggregate used in these experiments was sub-angular granite. A large sample was acquired, washed, and sieved to create the gradation with a maximum size of 50 mm. The coarse aggregate had a bulk dry specific gravity of 2.629, absorption of 0.51%, and Los Angeles abrasion of 24%. Because of its low absorption and high abrasion resistance, this granite is considered a high quality coarse aggregate. Cement Grout. Cement grouts are often a mixture of Portland cement, pozzolans such as fly ash, fine aggregate such as sand, water, and admixtures such as a grout fluidifier [2]. In this investigation, a simple mortar was used for the cement grout with no admixtures. The fine aggregate used in the manufacture of grout was produced from silica sand, which is subangular in shape with 100 percent passing a No. 8 sieve. The sand has an absorption of 0.63% and was mixed in a moist state at an average moisture content of 3.02%. The cement used throughout the experiments was ordinary (Type I) Portland cement. Water was used directly from the tap and was slightly colder (21C) than room temperature. The temperature in the lab when mixing and placing grout was near 27C. For clarity, the cement grout will be referred to as mortar throughout this paper. Grout Mixture Proportioning and Concrete Specimen Preparation The selection of water-cement-sand ratios is more critical in two-stage concrete because the amounts of sand and water control the pumpability of grout, an essential requirement in the production of two-stage concrete [3]. With a certain proportion of water, cement and sand it may be possible to cast conventional concrete, but it may not be at all possible to grout two-stage concrete at that proportion because there is no mechanical compaction. It is necessary to use trials to obtain a suitable mixture for each particular condition of grout ingredients and coarse aggregate gradation. Coarse aggregate was first placed in 150 x 300 mm hard plastic cylindrical molds. The void content of the preplaced aggregate ranged from 48 to 51% and the bulk density ranged from 1298 to 1394 kg/m3. Five different proportions of water to cement (0.42, 0.45, 0.50, 0.55 and 0.65) were investigated with a constant 1:1 ratio of cement to sand. Grout preparation was accomplished by combining ingredients with an electric mixer for about three minutes to achieve the desired grout uniformity and consistency. Trials were made with grout to find the minimum water-to-cement (w/c) ratio at which the preplaced aggregate could be effectively grouted. A w/c ratio of 0.45 was found to be the minimum ratio suitable for grouting; it was not possible to penetrate all voids in the

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aggregate skeleton with a grout at a w/c ratio of 0.42. As a result, the concrete specimen has a honeycombed structure with partial binding of the aggregate skeleton as shown in Fig. 1a. Due to the higher viscosity of this grout, some of the aggregate near the top of the cylinder was displaced, as shown in Fig. 1b. The displacement of aggregate was less noticeable as the w/c ratio was increased; at a w/c ratio of 0.65 there was no displacement. Grout with a w/c ratio of 0.50, on the other hand, filled all voids and created a smooth surface of the sides and ends of each cylinder. With the exception of the concrete specimens produced with a w/c ratio of 0.42, the bulk density of twostage concrete specimens ranged from 2275 to 2339 kg/m3.

a)

b)

Fig. 1: Concrete cylinders produced at a w/c = 0.42; a) hardened cylinders with forms stripped and b) placing mortar in preplaced aggregate cylinders. Investigation of Strength of Two Stage Concrete Compressive Strength. Unconfined compression tests on two-stage concrete cylinders were tested in accordance with ASTM C 39. The unconfined compressive strength of two-stage concrete was measured at 28 and 90 days. Fig. 2 and 3 show the mean and individual strengths of three specimens per w/c ratio at ages of 28 and 90 days, respectively. It can be seen that mean compressive strengths of 31.9 and 33.4 MPa are attainable at 28 and 90 days with a w/c ratio of 0.45. Both figures demonstrate a strength reduction as the w/c ratio increases. Although there is some variation in strength measured per w/c ratio, the strength reduction is approximately linear. This observation is consistent with unconfined compressive strength measurements of two-stage concrete cube specimens (300 x 300 x 300 mm), where Abdelgader [4] determined the compressive strength of two-stage concrete at 28 days as: f c = 62.08 − 71.00( w / c ) + 0.52(c / s )

(1)

Eq. 1 is illustrated in Fig. 2 for a c/s ratio of 1.0, and it can be seen that it under-predicts strength of the cylindrical specimens tested in this program. The predicted mean strength from Eq. 1 is 87 to 93% of the measured mean strength. Decreasing the multiplicative factor on the w/c ratio in Eq. 1 to account for cylindrical specimens yields the following relationship: f c = 62.08 − 68.00( w / c ) + 0.52(c / s )

(2)

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Only c/s = 1.0 for specimens investigated herein. Further research is required to determine the suitability of Eq. 2 for a range of c/s ratios. Another significant finding from the compressive strength data was the somewhat limited rate of strength development. This can be explained, in part, because of the fact that no fly ash or other pozzolans were incorporated in the cement grout. Comparing Fig. 2 and 3 reveals that the mean 90day strengths are higher than the mean 28-day strengths. The percent increase ranges from 5 to 17%, where concrete with a w/c ratio of 0.65 demonstrated the largest strength gain from 17.4 to 20.4 MPa. These observations show that, although the mechanism of stress transfer is believed to be different from conventional concrete, the mortar strength is a controlling factor in the strength of two-stage concrete. 40

S p e c im e n S t r e n g t h M e a n S tr e n g th Eq . 2 Eq .3

30

25

20

15

10 0 .4 0

0 .4 5

0 .5 0

0 .5 5

0 .6 0

0 .6 5

0 .7 0

w /c

Fig. 2: 28 day compressive strength vs. water-cement ratio 40 Spec imen Strength Mean Strength 35 Compressive Strength (MPa)

Compressive Strength (MPa)

35

30

25

20

15

10 0.40

0.45

0.50

0.55 w /c

0.60

0.65

Fig. 3: 90 day compressive strength vs. water -cement ratio

0.70


589

Variability in the data can be simply expressed by the range-to-mean percentage, which calculates from 12 to 31% for concrete at 28 days. However, the variability in compressive strength measurements at 90 days reduces to less than 12% for mortar and concrete. Secondly, the ratio of mean concrete-to-mortar strengths is reasonably consistent and calculates from 0.49 to 0.59 for all mixtures at both 28 and 90 days. This observation suggests that the compressive strength of twostage concrete can be conservatively estimated as one-half of its mortar strength. If this ratio can be substantiated with more mixtures and other sources of coarse aggregate, this simple rule-of-thumb can be adopted in the design of two-stage concrete. Tensile Strength. Splitting tensile tests were also conducted on three specimens of each concrete at 28 and 90 days according to the procedures outlined in ASTM C 496. The splitting tensile strength of two-stage concrete was also measured at 28 and 90 days. Fig. 6 and 7 show the mean and individual strengths of three specimens per w/c ratio at ages of 28 and 90 days, respectively. Both figures demonstrate a strength reduction as the w/c ratio increases. However, there appears to be little difference in strength between specimens produced with a w/c ratio of 0.45 and those produced with a w/c ratio of 0.50 (see Fig. 4 and 5). This was also observed with the compressive strengths measured for the same grout mixtures. More importantly, there appears to be almost no increase in tensile strength from 28 to 90 days for all mixtures. The actual values of tensile strength at w/c ratios of 0.45 and 0.50 measured from 3.1 to 3.3 MPa, which indicate satisfactory results, especially when one considers the minimum cost of concreting and that no vibration tools are used. Furthermore, excellent results can be expected even when using a high w/c ratio of 0.65, where the mean tensile strength is nearly 2.5 MPa. Failure in splitting tension was restricted principally to the line of split and occurs through the mortar and coarse aggregate. Visual assessments of the failed specimens suggest that the percentage of failed aggregate increased in concrete with higher mortar strength (lower w/c ratio).

4.0 SpecimenStrength

Splitting Tensile Strength (MPa)

Mean Strength 3.5

3.0

2.5

2.0

1.5 0.4

0.45

0.5

0.55

0.6

0.65

w /c

Fig. 4: 28 day tensile strength vs. water-cement ratio

0.7

590


4.0 Splitting Strength Averages

Splitting Strength (MPa)

3.5

3.0

2.5

2.0

1.5 0.4

0.45

0.5

0.55

0.6

0.65

0.7

w /c

Fig. 5: 90 day tensile strength vs. water-cement ratio Compressive–Tensile Strength Relationship. The results of this investigation show that the splitting tensile strength of two-stage concrete can be approximated well by the ACI equation for conventional concrete, as shown in Fig. 6. For conventional concrete, the predictive equation is given by: f s' = 0.56 f c'

(3)

Where: f'c and f's are in units of MPa. In this investigation, the splitting tensile strengths can be estimated using Eq. 4 as the following:

f s' = (0.55 − 0.58) f c'

(4)

which conforms closely to the ACI equation (Eq. 3). Eq. 3 is also valid for estimating 90-day splitting tensile strengths based on 90-day compressive strengths, as the data in Fig. 6 suggest. In this investigation, the factor in Eq. 3 ranges from 0.52 to 0.56 for 90-day strengths.


591

Fig. 6: Mean splitting tensile strength vs. mean unconfined compressive strength Fig. 6 includes compressive and tensile strength results from a concurrent investigation by Abdelgader [14]. In that investigation, 150 x 300 mm concrete cylinders were produced with a similar mortar mixture (w/c ratios of 0.45, 0.50, 0.55 and 0.60 at a c/s ratio of 1.0). The coarse aggregate source was a crushed dolomite limestone from Abou-Arogub located 45 km south of Tripoli, Libya. Particles were sub-angular in shape and the gradation was uniform, with almost all particles passing a 50 mm sieve and retained on a 37.5 mm sieve. The physical characteristics of that limestone and this granite are quite similar, but the results show a higher splitting tensile strength for two-stage concrete produced with limestone. The splitting tensile strengths can be estimated using Eq. 5 as the following:

f s' = (0.63 − 0.68) f c'

(5)

The data from both studies show that the tensile strength of two-stage concrete is at least as high as that of conventional concrete, and in fact it can be higher depending on the selection and properties of the coarse aggregate. No causes were apparent for the relatively higher tensile strength in two-stage concrete. However, the greater mechanical interlocking among particles in two-stage concrete could be responsible for the higher tensile strength since factors like aggregate gradation are different from conventional concrete. These observations warrant a much deeper investigation into the influence of coarse aggregate properties on two-stage concrete behavior in tension.

Conclusions The following conclusions can be drawn from this study:

• A mortar mixture with a water-to-cement (w/c) ratio of 0.45 to 0.50 and a cement-to-sand ratio of 1.0 optimizes compressive and tensile strength of two-stage concrete. Mortar mixed with a w/c ratio below 0.45 is too viscous and does not fully penetrate the voids between coarse aggregate particles, thus creating a honeycombing effect in the hardened concrete. Even when partially bound, however, two-stage concrete provides strengths equivalent to that of fully bound concrete with mortar at a w/c ratio of 0.45.

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• Compressive strength of two-stage concrete cylinders can be conservatively estimated as 50% of the mortar cube strength. • When mixed with a simple mortar, the mean compressive strength of two-stage concrete increases by 5 to 17% between 28 and 90 days. In this investigation, a maximum compressive strength of 34.2 MPa at 90 days was achieved. Long-term strength gain may have been limited by the fact that no fly ash or other pozzolans were incorporated in the grout. • The splitting tensile strength of two-stage concrete was found to be similar to that predicted by the ACI equation for splitting tensile strength of conventional concrete. In some cases, the measured tensile strength of two-stage concrete is in fact higher than that predicted by the ACI equation. However, there was no observable increase in tensile strength between 28 and 90 days. References [1] J.C. King, Handbook of heavy construction -concrete by intrusion grouting, McGraw-Hill, New York, 1959. [2] ACI Committee 304, Guide for the use of preplaced aggregate concrete for structural and mass concrete applications, ACI 304.1 R-92, 1997, pp.21-24. [3] H.S. Abdelgader, Effect of quantity of sand on the compressive strength of two-stage concrete, Magazine of Concrete Research, 48 (1996) 353-360. [4] H.S. Abdelgader, How to design Concrete Produced by a Two-Stage Concreting Method, Cement and Concrete Research, 3, 29(1999) 331-337. [5] H.S. Abdelgader. and J. Górski, Influence of Grout Proportions on Modulus of Elasticity of Two-Stage Concrete, Magazine of Concrete Research, 4(2002) 251-255. [6] N. Iwasaki, Predictions of grouting process in pre-packed concrete by green’s function, Proceeding of Japanese Society of Civil Engineering, 1985, pp. 41- 50. [7] S., J. Swaddiwudhipong, Zhang and S.L. Lee, Viscometric characterisation of cement grout for prediction of pre-packed concrete construction, Magazine of Concrete Research, 4(2002) 365-376 [8] J.C. King and A.L. Wilson, If it’s still standing, it can be repaired, Journal of Concrete Construction, 3(1988) 643-650. [9] E.R. Colle, Preplaced aggregate concrete repairs 63-year-old railroad bridge, Concr. Rep. Dig., The Aberdeen Group, 1992. [10] A.S.M. Abdul-Awad, Failure mechanism of pre-packed concrete, ASCE J. Struct. Eng., 3(1988) 727-732. [11] H.S. Abdelgader and J. Górski, Stress-strain relations and modulus of elasticity of two-stage concrete, ASCE J. Mat. Civ. Eng., 4(2003) 251- 255. [12] R.E. Davis, Prepakt method of concrete repair, ACI J. Am Concr. Inst, 2 (1960) 155-172. [13] H.E. Davis, High-density concrete for shielding atomic energy plants, J. Am. Concr. Inst., 11 (1958) 965-977. [14] H.S. Abdelgader and A. A. Elgalhud, Effect of grout proportion on strength of two-stage concrete, Structural Concrete, 3 (2008) 163-170.

Advanced Materials and Engineering Materials III 10.4028/www.scientific.net/AMR.893

Compressive and Tensile Strength of Two-Stage Concrete 10.4028/www.scientific.net/AMR.893.585