Mechanical properties and flexural behaviour of green light weight concrete beams exposure to salty soils

This paper presents an experimental investigation on the mechanical properties, durability and flexural behaviour of green concrete beam when subjected to harsh environments by using recycled building materials. Concrete resistance to external aggressive environmental condition is one of the most significant characteristics to maintain the concrete durability. The fundamental goal of this study is to understanding the performance of green lightweight concrete beam of (10×10×40 cm) under two different severe saline conditions; Ground Water Condition (GWC) and Severe Saline Clay Soil Condition (SSC) that contain chlorides and sulfates at concentrations like to those existing in groundwater and soil of the southern and middle parts of Iraq. In this research, in addition to normal concrete; three basic categories of green lightweight concrete are used: structural, non-structural and moderate strength using recycled of clay bricks and themestone blocks as coarse Light Weight Concrete (LWA) by a replacement ratio of (0, 50 and 100) % volume of natural coarse aggregate. All green Light Weight Aggregate Concrete (LWAC) beams and related specimens for mechanical properties were subjected to GWC, SSC and Laboratory Air Condition (LAC) for till (3, 6 and 9 months) after twenty-eight days of curing by tap water. Compressive strength, splitting tensile strength and density were also studied. The results obtained offered that the GWC and SSC have impacted the performance of all concrete beam types negatively with exposure age. The decadence was more for non-structural green LWAC beam. It is found that the resistance of beams contains bricks aggregate to GWC and SSC was a better than the other types of the green LWAC, the percentage decreases in failure load were 15.47% and 13.31% at 9 months of exposure to GWC and SSC measured relative to concrete beam exposed to LAC, respectively. Under the effect of aggressive conditions till 3 months, the results clarified that the improvement rate in the mechanical properties of the green LWAC were increased with age in comparison with ordinary concrete. The percentage reduction in compressive, splitting tensile strength and density at 9 months were ranged between (8.70-17.05) %, (14.71-36.50) % and (0.11-3.98) %, respectively measured relative to the same specimens in LAC.


Introduction
In recent structures applications, recycled wastes are majorly used as raw materials in concrete.However, succeeded use of wastes or materials by-products of industrial in concrete relies on the needful properties of the goal product [1].Lightweight Aggregate Concrete (LWAC) made from magnetic recycled or waste by-products is a thriving subject in building and construction materials.It has the advantages of a negative influence on an environment that supports the thinking in the manufacture of cement and decreases the natural resources consumption [2].In Iraq, building units of thermestone and clay bricks, are exceedingly used.During the process of building, many of these building units are break down due to of its brittle fracture; in addition to old building demolition which produced much of crushed them [3].In LWAC, the use of recycled clay bricks and thermestone as coarse Light Weight Aggregate (LWA) is very important because of economic and positive environmental effect which has been studied by many researchers [3][4][5][6][7].
The underground and hydraulic reinforced concrete structures such as tanks, pipes, tunnels, footings, piles, piers and etc. are constantly undergoing conditions of aggressive environmental during a period their serving.These environmental circumstances are acted by the offensive chloride and sulphate raid from underground water or surrounded soil.Which suggests to degeneration concrete cover and permeation of chloride and sulphate ions to inside the body of concrete.Depended to which, the factual strength of these structures reduces and corrosion damage of the embedded steel reinforcement bars with considerable distortion of steel and concrete increased [8,9].Rasheed et al.[10], studied the behaviour of durability and strength of reinforced ordinary strength concrete columns exposed to different aggressive sandy and clayey soils for two hundred and forty days (eight months) that had SO3 about 10.6% and 2.6%; respectively.It was found from the results which the RC columns strength decreases by about 12.5% for RC columns specimens entombed in the type of clayey soil, whilst the strength increased by about 11.7% for RC columns that entombed in the type of sandy soils, in comparison with control RC column.
The same researchers Rasheed et al [11] used the same study mentioned above, but using polymeric concrete instead of normal strength concrete under the same conditions and for a period of 150 days (5 months), it was found that using the polymer (SBR) has the negative impact on the strength capacity of RC columns after 5 months therefore is not recommended for using SBR in concrete members which are surrounded by the aggressive soils.
Moffatt and Thomas [12], presented the durability performance to 25 years of LWC in combination with supplementary cementing materials (silica fume and fly ash) and steel fibers subjected to a harsh marine environment.Concrete samples (915×305×305) mm with water to cement ratio ranged between (0.60-0.26).The chloride penetration depth was 90 mm for all the control samples without supplementary cementing materials.However, mixes with fly ash and silica fume within the range of water to cement ratio (0.60-0.40) performed very good resulting in penetration of chloride about 40mm for the same period.The results also indicate considerable increases in the chloride ion resistance penetration for ternary concrete with supplementary cementing materials.
Balam and others [13], evaluated the Sporosarcina pasteurii influence at cell concentrations of 10 6 cells ml -1 on water permeability, water absorption, rapid chloride permeability (RCP) and compressive strength, of LWAC.Leca aggregates were left to soak in a solution of urea-CaCl 2 with bacteria for six days to explore biological improvement of aggregate quality.Four types of LWAC were used under the 3 treatments of bacterially-treated of aggregates, bacteria inoculated in concrete mix water, and both the techniques employed simultaneously and with the no bacteria used in the either the aggregate or concrete mix solution as control.The results revealed reduction about 10% in the water absorption, 20% increase in the compressive strength, and 20% reduction in the chloride penetration in the specimens relative to same properties in the control ones.
Real and others [14], investigated the characterises the penetration of chloride into a large array of structural lightweight aggregate concrete (SLWAC), after a five-year subjected period to aggressive marine conditions.The chloride diffusion coefficients, the contents of surface chloride and the chloride profiles, were studied.The exposure conditions and the influence of the main composition parameters was assessed.Recent service life models were evaluated through the chloride profiles estimation.The results propose that LWAC can offer the same durability as normal weight concrete (NWAC).However, despite having the same coefficients of chloride diffusion, the content of surface chloride of LWAC tends to be greater than that of NWAC, resulting in higher penetration of chloride at a given depth.
Thomas and Bremner [15], studied the concretes containing lightweight aggregate (LWA) retrieved from the intertidal zone at marine exposure site.The LWAC performed equivalently to similar concrete of the same age produced with natural density retrieved from the same site two years ago.Partial replacement of Portland cement with slag significantly reduced chloride permeation and chloride diffusion coefficient.Concrete with LWA, silica fume and water to cement ratio of 0.33 showed better than the expected performance in terms of resistance to chloride ion penetration and this is expected to be partly attributed to the internal curing provided by LWA which reduces the effect of self-drying.
Due to practical advantages that LWC possesses, it has become an important material for structural engineering applications in over ground and underground members such as footings, piers, piles, ship building, frigs of offshore oil due to its low density and in the orchards, sea structures and garages underground parking because of its ability to absorb water which are as an example of underground LWC infrastructure.They are commonly exposed to offensive environmental surroundings during the operation period [16,17].There are few studies on the effect of salts on LWC mechanical properties by studying the effect of internal salts through materials components of concrete [18,19] or external ones [20,21]; so, more studies are needed.In the current research, the performance of green LWAC beam will be verified under different severe saline conditions using recycled bricks and themestone blocks as coarse LWA.

Experimental program
The experimental program consisted of flexural behavior testing of fifty of green LWAC beams of (10×10×40 cm) under two-point load exposure to different environmental conditions; Ground Water Condition (GWC) and Severe Saline Clay Soil Condition (SSC) in comparison with Laboratory Air Condition (LAC) for (0, 3, 6 and 9) months after twenty-eight days of curing by tap water.Density, splitting tensile strength and compressive strength testing of green LWAC specimens were also studied.All the details of materials, samples preparation, test setup and aggressive environmental conditions are set out in this section.

Materials and mix proportions
All beam samples and related specimens were casting by using Portland sulfate resistant cement known as (Kufa).The natural sand of zone 2 from (Al-Akaidur) region was used as fine aggregate.Also, natural gravel from (Al-Nebai) quarry were used with a max.aggregate size of 1.25 cm.Crushed of the clay bricks and thermestone were used as a coarse lightweight aggregate (LWA) in saturated surface condition with a same max.aggregate size used for natural gravel according to ASTM C330-14 [22] as shown in figure 1 [23] and ASTM C29-17 [24].Some of important chemical and physical properties for material used are listed in table 1.
By weight, the mix proportion used was 1: 1.46: 1.88 for cement, sand and natural coarse aggregates; respectively for normal concrete which was named M1as a control type for comparison.The W/C was 0.38 and the content of cement was 430 kg/m 3 .Green concrete produced by using LWA from crushed clay bricks and thermestone blocks by a replacement ratio of (50 and 100) % volume of natural coarse aggregate for each type.M2, M3, M4 and M5 mixes symbols of green concrete used 50% bricks, 100% bricks, 50% thermestone and 100% thermestone; respectively.All other components of mix proportion have kept constant in order to understand the effect course LWA type and amount on concrete properties in normal and aggressive conditions.

Samples preparation and testing
Depending on ASTM C192-18 [25] , All beam samples and related specimens of study were casted with concrete and also cured as shown in figure 2. After casting and curing of specimen till 28 days in tap water; they are kept in three different environmental conditions; severe groundwater (GWC), saline soil (SSC) and laboratory air condition (LAC) for till (0, 3, 6 and 9 months).To simulate GWC; two sulfates types; 0.5% Na2SO4 and 1.0% MgSO4.7H2O and two chlorides types; 4.5% NaCl and 0.5% CaCl2.2H2O by weight of drinking water are used.SSC of 3.1% SO3 and 0.9% CL was used to bury the beams samples and related specimens.Soil was taken from a barren land in the middle of Iraq from a depth of two meters and placed in laboratory covered basins to place samples and samples in it.The control beam samples and related specimens were kept at air laboratory condition (LAC) that were tested at the same ages.figure 3 shows the samples and the specimens in the basin of GWC, SSC and LAC.The main goal of the research is testing of fifty beams of (10×10×40 cm) under two-point load as illustrated in figure 4. The reinforcement of beam (flexure and shear) was discarded so to understanding the pure behavior of flexural concrete beam under the effect of various aggressive environmental conditions by using various types of recycled materials as a course LWA.The beam samples symbols are identified according to concrete type (BM1) concrete contains natural Al-Nebai aggregate, (BM2) concrete contains 50% LWA bricks, (BM3) concrete contains 100% LWA bricks, (BM4) concrete contains 50% LWA thermestone and (BM5) concrete contains 100% LWA thermestone also considered the environmental conditions types subjected to it.The study also includes the fresh and hardened properties of green LWAC.The fresh properties of concrete include testing of density and slump depended on ASTM C642-13 [26] and ASTM C143-10 [27]; respectively.The air dry density of all concrete types was tested according to ASTM C567-11 [28].The (15 cm) cubes for compressive strength f cu were tested depended on BS1881-02 [29].Also, the cylinder of (10×20 cm) for splitting tensile strength fps was tested depended on ASTM C496-17 [25].All hardened concrete tests under different aggressive environmental conditions were performed for till (3, 6 and 9 months) after twenty-eight days of curing by tap water.

Test results and discussions
With a view to understand the behavior of different concrete beams exposure to different conditions, it is necessary first to understand the changes that will happen in some of the mechanical properties of these beams.The failure load of all beam sample with related properties of fresh and hardened concrete state for normal and green concrete at twenty eight days age; are listed in table 2. The highest slump of the green LWC, 8.2 cm, was for M2 containing 50% crushed bricks as coarse LWA; the lowest value of 6 cm was obtained for M5, incorporating 100% crushed thermestone; this is because of the nature of microstructure and surface texture of thermestone aggregate which inverted on the water needed for the mix in order to obtain a sufficient workability.Between all mixes, the slump of control mix of normal concrete M1 was 10 cm that is the higher value; this is explicated by the solid and smooth surfaces of the particles of gravel.These results are in agreement with the findings of Nasir et al. [3].As expected, based on the aggregate density used, the fresh density for LAWC is 1773 Kg/m 3 for M5 which are the lowest value whereas the M3 has the higher value of fresh density.The fabric nature of the thermestone aggregate is not absorbed water, it has cracks with wide open, that keep the water absorption speedily and miss it speedily and this behaviour be inverted later on the fresh densities' values.While, the brick aggregate has connected inner pores to each other and it is keeping the absorbed water for a certain time after lifted it from the soaking container which it work as a sponge.The limits of most codes for green LWC set the maximum value of dry density (air) for green LWC by 1900 kg/m 3 .M5 record the lowest value of dry density (air) with lowest value of the compressive strength (cube) by about 14.3 MPa which is less than the minimum specified for structural concrete.The lowest value of the compressive strength (cube) for M5 because of the lower strength of thermestone aggregate material compared with brick aggregate.While, M3 have the highest value of the compressive strength (cube) by about 25.9 MPa with dry density (air) equal to 1850 Kg/m 3 .Based on the results obtained for density and compressive strength and depended on ACI 213 R-14 [30], concrete is classified according to its application as presented in table 2. The splitting tensile strength of green LWC ranged between (1.4 -2.5 MPa).As it's known, the tensile splitting strength of concrete is directly proportional to the compressive strength as shown in table 2. The effect of different aggressive environmental conditions on behavior and properties of concrete beam samples are set out in the following subsections.

Flexural behavior of beams
Fifty non RC beams were tested under two point load, to investigate the effect of different variables that are a considered in current study.The variables are concrete strength, concrete density and aggressive environmental conditions.The outer surface of the beam samples extracted from a basin of SSC were washed and cleaned before tested.The results obtained indicate that all concrete beam sample have increase in failure load at 3 months of exposure.Also, beam sample which casted using M3 (BM3) exposed to GWC and SSC exhibited a smaller decrease in failure load than other beam samples at 6 and 9 months.figures 5 show the failure load values of beam samples at all concrete and exposure types.The beams sample BM5-GWC and BM5-SCC show further reduction in failure load over other green LWAC beam samples especially after 3 months; however, the percentage of decrease in failure load at 9 months were about 30.92% and 22.37% for BM5-GWC and BM5-SCC, respectively in compared with the same samples at 28 days.
figures 6 show the percentage of development in the flexural load for the beam samples with age cured in the same cured condition.The results showed that all concrete beam sample gave flexural strength development at three months in GWC and SSC more than same beam samples in LAC except concrete beam samples with concrete M1 gave similar results in all conditions.It is also clear from the tested BM1 samples, the outward appearance do not include fundamental differences at 3 months; where simple voids appeared on the surface of the BM1-GW samples, while white spots appeared on the surface of the BM1-SSC samples after washing and drying.

Density of concrete
The results of density change against time for green LWAC and NWC cured in LAC and exposed to GWC and SSC are depicted in table 3. The concrete density continued increasing, that was noted for all concrete type specimens cured in LAC, probably referred to the water absorption continuously, softly reparating for the chemical shrinkage because of the hydration of cement.Similar conclusion was observed by Persson [31].while, the further increasing in density change of specimens M3 which contains LWA bricks perhaps because of the ettringite formation and the gypsum.Similar observation was mentioned by Al-Robayi [32] which works to restore hydration and continue the rehydration process with internal maturation.The density has increased with time in all concrete types exposed to aggressive environmental conditions; GWC and SSC till 6 months and the largest increase was at 3 months.Table 3.Air dry density of concrete.

Concrete type symbol
Hardened density kg/m The density values have decreased in all concrete types after 6 months in all exposure conditions and the largest decreases was in M1 and M5 concrete type.This smaller density decrease of concrete which contains LWA except M5 can be explained by the fact that the very small voids expanded over the surface of aggregate act as structural breaks in the continuity and are, simultaneously, a chance for the collecting of the ettringite produced, the ettringite forms in a fine voids and microcracks demands less power of surface energy than forming it in the bulk paste [33].Several researchers [33][34][35] have observed in the damaged concretes, which the crystals of ettringite are commonly present in voids, cracks and transition zone at the binder-aggregate interface, if concrete is subjected to expansion because of the ettringite, fact that leads to an additional stress on the matrix-aggregate interface and then to micro-cracks.Besides, the formation of ettringite in the micro-cracks, because of the pressure of expansion will develop these micro-cracks.These processes will lead to debonding of matrix/aggregates under the influence of low applied stresses, resulting rise to rapid failure.The presence of LWA will delay all these processes because of existence of the pores through it.So, reducing the negative impact of the sulfates on the concrete.

Splitting tensile strength of concrete
The splitting tensile strengths development of all concrete types cured at various exposure periods are presented in figure 7. Results proved that, generally, all concrete types offered a continuous increasing with age in the splitting tensile strength when subjected to LAC. also, the results concluded that the splitting tensile strength contains green LWA exposured to GWC and SSC exhibited noticeable strength increase at 3 months of exposure by ranges (1.76-10.34)%;after this period, splitting tensile strength was noted reducing as shown in figure 5.The increasing in concrete splitting tensile strength contains green LWA at 3 months maybe referred to filling up of pores with crystallization salts and products of reaction.This phenomenon has also been mentioned by many researchers such as [36].As well as, under the effect of aggressive exposure GWC and SSC; M3 demonstrated a better performance than M4 and M5; at 6 months (6 months) of subjected to aggressive solution, the percentage decrease in tensile strength of M3 was 3.03% and 3.64% in GWC and SSC; respectively relative to specimens in LAC.while, the percentage decrease in tensile strength of other green LWAC was ranged between (4.35-21.05%).This may be attributed to the effect of LWA percentage (100%) which decreases the serious problems associated with use of brick aggregate in green LWC.At 9 months, concrete containing natural aggregates showed greater decreases in tensile strength than those containing LWA as it appears in figure 8.This trend may be associated by the chemical reactions between products of hydration process and the aggressive salts which include formation of expansive products that can lead to a loss of certain tensile strength.

Compressive strength
The compressive strength (cube) values for all types of green LWA and normal concrete at a different age of subjecting in GWC, SSC and LAC shown in figure 9.All concrete cubes immersed in LAC with the progress of age exhibit a continuous increasing in compressive strength as shown in figure 10.This increase in compressive strength is due to the continuity of the hydration process that forms a new hydration product within the concrete mass [37].The green LWAC especially M3 have an increase in development strength more than normal concrete due to internal curing obviously till 3 months; this is in agreement with other researchers [38].figure 10 illustrate that green LWAC specimens exposed to GWC and SSC; respectively displayed a continuous gain in the compressive strength till to 3 months of immersion; the increase ranged between 0.63% in M5 to 5.48% in M3.A slight reduction in strength of normal concrete M1 was observed at 3 months by about 4.09% and 3.32% exposed to GWC and SSC; respectively when comparison with reference specimen in LAC while M2 gave slight improvement in compressive strength by about 2.13% and 0.8% at the same comparison.All strength of concrete mixes were reduced at 6 and 8 months as shown in figure 11; this reducing is because of offensive of sulfate ions that grant rise to the expansive compound's formation like calcium aluminate hydrate and ettringite.Also, the retrogradation of strength is because of filter of salts deposited in the concrete voids [39].It can clearly notice in figure 11 which the rate of deterioration of the compressive strength of green LWAC at 3 to 6 months ages is much less than the rate of deterioration in ordinary concrete.Because in ordinary concrete, formed ettringite and calcium aluminate hydrate by the interaction of salt ions in groundwater and soil, while causing great pressure in ordinary concrete because there is no place for it through the tissue structure 13.At 9 months, the percentage decrease of compressive strength in green LWAC except M5 ranges between (8.70-13.28)%,while it ranged in ordinary concrete between (11.51-14.53)%for samples immersed in GWC and SSC compared to those in LAC.While in M5 were 17.05% and 15.91%; the researchers believe that the reason for the largest decrease is because of the poor compressive strength of non-structural concrete M5 and the fact that thermestone aggregate is characterized by closed pores that do not allow the passage of hydration products or the products of reactions of cement oxides with salt ions present in groundwater and soil through it that led to deterioration the strength.Clearly, the effect of GWC and SSC on the compressive strength is somewhat similar to that on density and splitting tensile strength.From figure 12 the following equation may be proposed to fit the relation between compressive-splitting tensile strength and density-compressive strength for LWC.

Figure 12.
Relationship between f cu -f sp and f cu -density for LW.

Conclusions
Depended on the observations made in current work, the following conclusions were reached: The better behaviour under impact of aggressive environmental conditions is registered for structural LWAC beam samples that used crushed bricks as a course LWA.All concrete types are affected by aggressive environmental conditions, the defect degree is depended on the concrete strength, the sulphate's concentration and the age of exposure.The effect of immersion to aggressive environmental conditions becomes obvious after 6 months of immersion.The GWC has the more influence than SSC on beam samples.The strength capacity of beams contains bricks aggregate to GWC and SSC was best in compared with the other types of green LWAC, the percentage decreases in failure load were 15.47% and 13.31% at 9 months of exposure to GWC and SSC in comparison with concrete beam sample exposed to LAC, respectively.The percentage reducing in density, splitting tensile and compressive strengths at 9 months were ranged between (0.11-3.98) %, (14.71-36.50)% and (8.70-17.05)%,respectively measured relative to the same specimens in LAC.

References [1]
Sandanayake M, Bouras Y, Haigh R and Vrcelj Z 2020 Current sustainable trends of using waste materials in concrete-a decade review Sustain.

Figure 1 .
Figure 1.Types of coarse aggregate use.All materials was successed in laboratory tests according to the Iraqi limits standard IQS No.45-84[23] and ASTM C29-17[24].Some of important chemical and physical properties for material used are listed in table 1.

Figure 2 .
Figure 2. Casting and curing of concrete beam samples and related specimens.

Figure 3 .
Figure 3. Aggressive environmental conditions used in study.

Figure 5 .
Figure 5. Failure load of beam with age for all mixes in different exposure conditions .

Figure 6 .
Figure 6.Failure load value with age in different exposure conditions for the same beam samples.

Figure 7 .
Figure 7. Development of splitting tensile strength with age for all concrete and exposure type.

Figure 8 .
Figure 8. Percentage % of increase or decrease in f ps for all concrete and exposure types.

Figure 9 .
Figure 9. Values of compressive strength with age for all mixes for all concrete and exposure types.

11 Figure 10 .
Figure 10.Development of compressive strength with age for all mixes in in different exposure conditions.

Figure 11 .
Figure 11.Percentage % of increase or decrease in f cu in all concrete and exposure type. Fig.14-a

Table 1 .
Some of physical and chemical properties for material.

Table 2 .
Concrete properties in the fresh and hardened state at 28 days.
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