EXPERIMENTAL STUDY ON THE PROPERTIES OF LIGHTWEIGHT HIGH PERFORMANCE CONCRETE

Michala Hubertová

Brno University of Technology, Faculty of Civil Engineering, Department of Technology of Buildings Materials and Components, Czech republic, hubertova.m@fce.vutbr.cz

Lias Vintirov, LSM k.s., 357 44 Vintirov, Czech Republic, hubertova@liapor.cz

Keywords: High Performance Concrete, Self Compacting Concrete, Workability, Lightweight Aggregate

Abstract:

Research work is aimed on development of Lightweight High Performance Concrete (LWHPC), especially of Lightweight Self Compacting Concrete (LWSCC) with the use of lightweight aggregates „Liapor“ manufactured in the Czech Republic.

The development of concrete is aimed to the so called high performance concrete in the last years. It is especially possible to follow pronounced applications of self compacting concrete and of high strength concrete and the beginning of lightweight concrete applications utilizing lightweight natural or artificial porous aggregates. It is necessary to mention that the development of new types of concrete is always confronted with the lack of adequate standards, proposed directions and necessary experience. For these reasons it is the aim of this work to verify and to apply the theory of high performance concrete and of light-weight self compacting concrete and further to verify the possibility of these materials pumpability . The paper describes the experience concerning the combination of these directions, especially the development of lightweight self compacting concrete under the condition to reach the most possible strength values with possible pumpability.

1. Lightweight High Performance Concrete

High performance concrete (HPC) is as the term itself indicates, concrete with higher utility properties. This concrete fulfils the special combination of properties and demands, which cannot be obtained by normally used concrete components, by the normal process of concrete mixing, placing and treatment. It means that special demands are laid on properties of this concrete in fresh and in hardened state. Easy placing and compaction of concrete without segregation, sedimentation and bleeding is deciding for fresh concrete and in the case of hardened concrete in addition to this demands, high strength demand is deciding and also durability, resistance against aggressive medium, better long-term mechanical properties, small shrinkage, homogeneous structure, surface of quality etc. The advantages of high performance concrete can be seen in more simple concreting by decreasing the degree of reinforcement, in general slimming down of structures and in this way in decreasing the load of connected structures, in significantly higher resistance owing to better microstructure of concrete (higher water-tightness, resistance against frost and abrasion, chemical resistance against chlorides, limited rate of carbonation and sulphatation etc.). This all means longer service life of concrete.

Concerning lightweight concrete, their advantages and disadvantages are well known. Applications in the field of nonstructural, heat-insulating, filling concrete are quite common but applications in the field of structural concrete still wait for advantage of broader utilization.

The demands for lightweight high performance concrete (hereafter LWHPC) are identical with demands for HPC, but in addition the demand arises for low volume mass, best up to 1800 kg/m³, under achieving the most possible strength values minimum at the level C30/37. The utilization of porous aggregates in concrete which should have high strength can seem as surprising, considering the importance of the aggregate strength for the strength values of high performance concrete. Nevertheless the volume mass decrease of concrete with the strength 40–60 N.mm^-2 under the value of 2000 kg/m³, better under 1800 kg/m³ can mean significant costs reduction considering the mass reduction of the total construction. It is known that LWHPC can be prepared with strength values up to 80 N/mm². It is of course important to become aware of the fact that this result can be achieved only by the application of adequate aggregate type [1]. This paper will further inform about physico-mechanical properties of concrete with the artificial aggregate Liapor.

Liapor (former Keramzit) is a very lightweight granulated material manufactured by expansion of natural clay. It is in its substance classified as a ceramic material. Technologically it is defined as lightweight, artificial, porous aggregate from expanded clay. It is characterized by granulated form with nearly spherical grains, with homogeneous porous structure and with sintered closed surface. Raw material for the production of Liapor is cypress clay-stone from roofs of the brown coal seams. This raw material is partially consolidated, has laminated structure and it is expansible by heat also without addition of other pores forming admixtures (coaly materials). The condition of expansion is only the adequate natural composition of the clay and its correct treatment.

The pellets from pre-crushed and plasticized raw materials are heated in the rotary kiln where they expand at the temperature of 1100 – 1200°C. Rotary kiln of normal construction is used for burning the Liapor in the Czech Republic. The kiln dimensions are 3.4 x 57 m, the output 30 – 50 m³/hour. The time of material passage through the kiln necessary for the expansion should be 10 till 15 min., the total time of material passage through the whole kiln is 45 till 60 minutes. During the burning the surface of pellets sinters and the expansion of hollow pores takes place in material grains. The expanded granulated product passes afterwards through the cooler. A long slow period of cooling has to follow the burning. Internal tension is removed by cooling and in this way the strength of Liapor increases. The cooled Liapor is transported to the separator and it is sorted into individual fractions. You can find the properties of Liapor in Tables no. 1, 2, and 3.

Table 1 Principal properties of Liapor

PROPERTIES	VALUES
bulk weight	250 – 900 kg/m^3
volume weight	500 – 1900 kg/m^3
void space of bulk Liapor	40 – 50 %
void space if milled Liapor	55 – 65 %
heat conductivity coefficient l	from 0.09 W/m.K
cylinder strength	0.7 – 15 N/mm^2
absorption	it is not hydroscopic

Table 2 Absorption of Liapor

Time of Absorption	Absorption Capacity	Volume Absorption
after 30 min.	2 – 7 %	1 – 4 %
after 24 hours	7 – 19 %	6 – 8 %
after 48 hours	20 – 25 %	11 – 13 %
after 120 hours	22 – 30 %	13 – 16 %
after 180 days	30 – 45 %	18 – 24 %

Table 3 Next principal properties of Liapor

PROPERTIES	VALUES
frost resistance	porous non-capillary structure of the grain makes possible expansion of frozen water; therefore it is resistant to repeated freezing.
content of sulphur and chlorides	sulphur content is 0.2-0.5 percent by weight. Chlorides content is 0.005-0.01 percent by weight Þ suitable for reinforced and pre-stressed concrete
content of organic and foreign particles	loss by annealing (1000^oC) is zero

2. Lightweight Self Compacting Concrete

Lightweight self compacting concrete (hereafter LWSCC) is a new high performance material which combines the known advantage of lightweight concrete and self compacting concrete (SCC). LWSCC can be owing to its advantageous physical properties, its low volume mass and its relatively high strength in combination with excellent workability, low noise emission and lowering of labour consumption during concreting find a broad application scale in practice, especially in the production of precast elements and in the reconstruction of old buildings which should not be loaded additionally.

The main demands connected with the rheological properties of SCC, such as the broad extent of workability caused by high flowing property and the mobility under sufficient cohesion and resistance against segregation during transport and placing and also the resistance against blocking during concreting of close reinforced elements and the prolonged workability time, should be applied also on LWSCC.

Nevertheless it is necessary to respect certain facts during the design of concrete with lightweight aggregate Liapor. This facts don´t occur in the case of common concrete. The significant difference of lightweight concrete in comparison with common concrete is given by the water adsorption of lightweight aggregates. This absorption has significant influence on the behaviour of lightweight concrete during mixing, transport, pumping and placing. Besides the water absorption of lightweight aggregates under atmospheric pressure also the absorption under high pressure is of importance. LWSCC can be exposed to this elevated pressure during pumping. The additional water pressed into the pellets during pumping of concrete is in the phase of mixing and transport excessive and therefore efficient stabilizing agents should be applied. These agents prevent the segregation of fresh concrete. The partial adsorption of water can cause premature hardening of LWSCC till a total loss of self compacting properties. The lightweight aggregate has further a significant trend to segregation caused by the low volume mass and the tendency to flow on the surface of the cement sludge. It is advantageous in order to limit the water adsorption by the porous lightweight aggregate to pre-moisten the aggregate by a defined quantity of water. It is further necessary to respect the worse mobility and self compacting properties of fresh lightweight concrete. This is caused by the lower volume mass, which causes lower kinetic energy.

3. Properties of fresh Lightweight self compacting concrete

It still doesn´t exist a Czech or European Standard which would exactly define the properties and the testing methods of self compacting concrete. The main Centers developing this concrete have elaborated different directions but these are not unified and introduced for broad utilization. The attempt to unify the European procedures for design and testing of self compacting concrete is the manual published by the EFNARC – organization, which works on European level in the CEN and summarizes mainly the knowledge of Japan and English specialists in the branch of concrete technology [2]. Some of undermentioned test procedures are used mostly for the description of fresh SCC properties. (See Table no.4).

Table 4 Testing procedures used to test fresh SCC properties

TEST NAME	Described property
Slump test	Mobility, flow
J – Ring	Resistance to blocking
L- Box	Mobility, resistance to blocking and segregation of aggregate coarse grains
Orimet	Flow velocity, mobility
Orimet + J - Ring	Flow velocity, mobility, resistance to blocking
V - Funnel	Mobility
U – Box	Mobility, segregation, bleeding
Fill – Box	Resistance to blocking
GTM – test	Resistance to segregation

The method of applied experimental laboratory work was the comparison of the formula using dry Liapor aggregate (dried in a drier at 110°C) with the water addition in the quantity 25 % of the aggregate weight, with the same formula using water-saturated aggregate (wetted 1 day in water). The aggregates were dried before wetting in a drier at 110°C too, in order to determine the water absorption of individual fractions. Table 5 shows the dependence of the necessary total water volume on the type of the applied aggregate. The quantity of water depends on the water absorption of individual aggregate fractions and it was in this case not affected by the initial moisture content of the aggregate.

Table 5 Saturation of lightweight aggregate Liapor (water absorbed after 24 hours under water)

The aggregate fractions	limit values of absorption [%]		statistical mean of absorption [%]
	min	max
4-8/650	31.8	40.5	35.9
4-8/450	23.3	34.1	24.1
4-8/350	15.3	23.6	18.5
0-4/550	50.8	59.1	55.6
1-4/750	43.8	50.4	46.6

All LWSCC formulae were designed with lightweight aggregates having the maximum fraction 8mm. The Liapor fraction 8–16 mm was not used in the formulae, because it is in the Czech Republic not available with strength values sufficient for the LWSCC production. Admixture on the base of secondary raw material – power plant fly ash from electric power plants - was used for the design [8]. Super-plasticizing additives based on polycarboxylates were used for the correction of LWSCC workability in the long term perspective of about 90 minutes and also a stabilizing agent was used which is destined for the stabilization of lightweight pumpable concrete. In the design of experimentally tested batches different combinations of Liapor aggregate were used, with different fractions and volume masses utilizing different grain size curves of the resulting aggregate mixture. We have designed and experimentally tested altogether 40 different formulae which differed not only by rheological properties of fresh concrete but also by properties of hardened concrete.

Portland cement CEM I 42.5 R was used for all tested batches. The grain size curves constructed according to EMPA I and FULLER were verified to determine the mixing proportion of individual Liapor aggregate fractions.

The procedure of components mixing was as follows. When using dried aggregate and additional water: after dosing of all aggregate fractions the in advance calculated dose of additional water was dosed into the mixer and it was mixed for the period of 20 seconds. Cement, powder admixtures and additives were added after wetting the aggregates and 70 % of effective water was added under simultaneous mixing. After 30 seconds the super-plasticizing agent with the rest of effective water was added. The mixture was further mixed for the period of at least 60 seconds in order to achieve the demanded homogenization and to secure the intensive effect of the super-plasticizer. When using pre-wetted aggregate: after dosing this aggregate into the mixer cement and all powdered admixtures were added and the further procedure was identical as mentioned above.

Some resulting properties of LWSCC are interesting. The test results comparison of LWSCC according to the formula starting with dried aggregate as input material and additional water, with the results of LWSCC according to the formula having as input material pre-wetted aggregate showed that the strength values and also the volume mass were in the case of the latter concrete higher by up to 20 %.

The following test procedures were applied during checking the rheological properties of LWSCC prepared following 40 different formulae: Slump test, Orimet + Ring test, L-Box, U-Box and V-Funnel tests. The results of all these tests you can find in Table no. 6.

Table 6 Statistical measured values of LWSCC rheologic properties of 40th formulations set

TEST METHOD	Entity	RecommeNded RANGE		CONSISTENCY
		min	max	after mixing	after 60 min.	after 90 min.
Slump test (slump flow)	mm	650	800	750	710	675
Slump test (T50cm)	s	2	5	4.7	6.4	7.6
J-Ring	mm	0	10	0	6	11
Orimet	s	1	5	7	9.2	10.5
L-Box	h2/h1	0.8	1	1	0.95	0.93
U-Box	mm	blížící se 0		0	3.2	11
V-Funnel (T)	s	optimum 10s		8.9	12.5	15
V-Funnel (T5min)	s			10.6	13.6	16.3

We used limit values data obtained from literature (see Table no. 6 –recommended range) [2] for the verification of the utilization suitability of methods used in present time for testing the fresh self compacting concrete consistency. The endeavour was to design formulae in the way that they fulfill the demanded criteria. Following the obtained results we can come to the conclusion that the methods which we have applied are in principle appropriate for the determination of LWSCC consistency. It is only necessary to correct the criteria of tests (the time intervals of outflow), because the lightweight self compacting concrete proves to be slower than normal self compacting concrete as already mentioned above in the introduction. The spacing between individual reinforcement bars in the case of J-Ring and L-Box tests proved to be optimal when it was the triple of the aggregate maximum grain size. Both these corrections are caused by the volume mass, which is in the case of concrete containing lightweight aggregate smaller and the lightweight concrete doesn´t have sufficient internal kinetic energy in comparison with concrete containing natural aggregates and they are slightly slower and they flow difficultly through the closely situated reinforcement bars. In the case of Orimet tests and Slump test in the time T50 (see Figures 1, 2) would be adequate to increase the criteria to 0–10 s and in the case of V-Funnel test to increase the optimum to 20 s. The table shows that the workability after 90 minutes slightly surpasses the proposed criteria in the case of some formulae, because the values increased only insignificantly. In spite of substantially lower volume mass the mixed concrete fulfilled the basic demand concerning the homogeneity and the uniform compacting in the whole cross section (see Figures 3, 4).

We have found during the experimental work that when the weight dosing of lightweight aggregate Liapor takes place, the individual formulae are not reproducible concerning the demand to achieve the once already verified properties including the workability of the specific formula. It is necessary in the case of lightweight aggregate utilization to pay elevated attention to the determination real volume mass of the grain as it is necessary in the case of natural dense aggregate, because the differences from declared parameters can be more significant and they can influence the real composition of the lightweight concrete. The manufacturer states the declared difference of volume mass of lightweight aggregate up to ±15 %. For instance if the volume mass of the aggregate is 1200 kg/m³ and the dose of this fraction is 100 kg/m³ of concrete, it can practically mean that with the mentioned difference of ±15 % the range of dosing will be from 85 to 115 kg/m³. Volume dosing should be applied instead of mass dosing if it is not possible to determine during the production the real volume mass of lightweight aggregate.

4. Properties of hardened Lightweight self compacting concrete

The volume mass of the lightweight aggregate should be the higher the higher strength values we want to achieve. The increase of strength can be achieved by the addition of natural aggregates. The substitution of fine fraction 0-1D/650 of Liapor aggregate by natural aggregate fraction 0–1 mm doesn´t help neither to increase significantly the strength nor to improve the other physico-mechanical properties. More advantageous is to use natural aggregate fraction 0–4 mm. This enables to increase the strength, to improve the surface resistance against water and against chemical deicing agents and also to improve the frost resistance. The use of natural aggregate fraction 4–8 mm proves not to be advantageous. The addition of ultra-fine admixtures (micro-silica, meta-kaolin) to lightweight concrete with the maximal grain size of 8 mm increases the strength and improves the surface resistance against water and chemical deicing agents. We can conclude from it that the strength of this fine grained concrete is formed mainly by the cement matrix containing fine fractions. The lightweight self compacting concrete with the Liapor aggregate has good frost resistance (after 100 cycles the frost resistance coefficient is in the range between 90 till 98%), but this concrete doesn´t have resistance against water and chemical deicing agents. Concrete containing lightweight Liapor aggregate shows good thermal properties (thermal conductivity coefficient is λ = 0.29 W/mK) This properties deteriorate by the addition of natural aggregate (λ = 0.33 W/m.K till λ = 0.69 W/m.K ).

Figure 4 shows the graphical results of compression strength for some selected formulae in comparison with the price of raw materials for this concrete (in EUR in Czech republic).

Table 7 Dependence of resistance to pressure on volume weight of LWSCC (using entirely lightweight aggregate Liapor)

The volume mass class	D 1,2-1,4	D 1,4-1,6	D 1,6-1,8
The compression strength class	LC 12/13	LC 16/18	LC 25/28

Table 8 Dependence of resistance to pressure on volume weight of LWSCC (using lightweight aggregate Liapor, natural aggregate and ultra-fine admixtures)

The volume mass class	D 1,2-1,4	D 1,4-1,6	D 1,6-2,0
The compression strength class	LC 16/18 – LC 20/22	LC 20/22 – LC 30/33	LC 30/33 – LC 40/44

Figure 4 Compression strength (after 7 and 28 day) of some formulations in comparison with a price of raw materials [EUR/m³] (price in Czech republic)

In short, we can conclude following the obtained experience that it is more advantageous to mix LWSCC with pre-wetted Liapor aggregate, Technically the aggregate can be pre-wetted in two ways, either by wetting it down in water for at least one day or by spraying in the pile for at least two days and in this case the aggregate is soaked to the water content of about 20 % [3]. It is of course very difficult to apply this method in practice especially in the case of tower concrete mixing plants. It is naturally necessary in this case to appraise very sensitively the quantity of additional water with respect to the volume mass of Liapor, the ambient temperature, period between mixing and storing etc. The aggregate should be dosed volumetrically, following the real volume mass in order to achieve the declared properties of fresh and also hardened concrete. We have come, to the conclusion after verification of six mostly used methods for the measurement of rheological properties that these methods are suitable for the testing of lightweight self compacting concrete, it is only necessary to correct in individual methods the time criteria of fresh concrete outflow times.

The use of lightweight aggregate into LWSCC without the addition of natural dense aggregate enables to reach the strength up to the class LC16/18, D1.3 till D1.6. The use of lightweight aggregate Liapor in combination with natural aggregate enables to achieve strength values of the class LC 25/28 D1.6 till D1.8. The use of lightweight and natural aggregates combination with addition of ultra-fine admixtures of quality we can achieve strength classes up to LC 40/44 D1.8 till D2.0.

Acknowledgements:

This outcome has been achieved with the financial support of the Ministry of Industry and Trade of the Czech Reublic, MPO FI-IM5/016 „Development of light-weight high performance concrete for monolithic constructions and for precast elements” and with the financial support of project GA 103/07/076.

References:

[1] Aïtcin, P.C., High Performance Concrete (Lehký vysokohodnotný beton), Prague Czech republic 2005, ISBN: 80-86769-39-9(in Czech).

[2] EFNARC, Specification and Guidelines for Self compacting concrete. Surrey United Kindom 2002. ISBN 0-9539733-4-4.

[3] Tomis, V., Readymix concrete with Liapor aggregate – handbook of technology (Transport betony z Liaporu – příručka technologa), 1. Edition, Company Lias Vintířov, LSM k.s., 2001 (in Czech).

[4] Hubertová,M., Hela, R., Lightweight High Performance Concrete (Lehké vysokohodnotné betony), in: Beton TKS no.5/05, Praha 2005. ISSN 1213-3116 (in Czech).

[5] Jones, T.R.: Metakaolin as a pozzolanic addition to concrete. Chapter fifteen of book: Bensted, J., Barnes,P. Structure and Performance of Cements. Second Edition. Str.372 – 398. ISBN 0-419-23330-X

[6] http://www.liapor.cz

[7] Hubertova, M. Proposal problems of Lightweight self compacting concrete. Conference proceedings Technologie, provádění a kontrola betonových konstrukcí 2005. Praha 2005. ISBN 80-903501-5-1.

[8] Fridrichova, M., Kulísek, K., Vehovská, L.: Use of fluidized ashes in cement industry, IX. Conference – Ekologie a nové stavební hmoty a výrobky, 1^st ed. Telč, VUSTAH, 2005, vol. 1, p. 116-120.