Fibre reinforced and ferrocement car park pavers by R Sri Ravindrarajah
Fibre-reinforced and Ferrocement Car-park PaversDr R. Sri RavindrarajahCentre for Built Infrastructure Research, University of Technology. Sydney, Australia1-IntroductionNormal weight concrete is accepted as the suitable construction material for roads,highways and airport pavements because of its ability to carry service loads and lowmaintenance cost. Rainwater cannot infiltrate into the subsurface because of the lowpermeability of the traditional concrete pavement. In addition, it is difficult for soil toexchange heat and moisture with atmosphere, and consequently the temperature andhumidity of the air are affected, causing the phenomenon of heat islands around large cities.The impermeability of pavement increases the storm-water runoff and overloads thedrainage system. This leads to flooding in many cities during heavy rainfall for a shortperiod of time. The intensity of flooding further increases when the existing drainagesystem is either blocked or inadequate.In order to reduce stormwater runoff from redeveloped properties, municipal councils inSydney, Australia adopted on-site stormwater detention (OSD) systems in the early 1990s. OSD systems consist of surface or underground storages with a control pit for waterdischarge. Although OSD has been adopted in some larger country towns and in Canberra,it was rejected as an option for the Central Business District of Sydney.As an alternative, porous pavement can be easily considered as suitable solution in urbancities to mitigate the storm-water runoff problems. The typical porous pavement systemscommonly consist of a matrix of concrete pavers with voids filled with sand, gravel or soil.These voids allow storm-water to infiltrate through the pavement into the soil. InSingapore, precast reinforced concrete pavers of different shapes and sizes, with opening,are used for a number of years for the construction of open car parks, in the public housingestates. This paper reports the results of a survey conducted in monitoring the condition ofthese precast reinforced concrete pavers after a number years of service.Ferrocement is known to have significant crack resistance and suitable to produce thinstructural elements [2-4]. Similarly, steel fibre reinforced slabs are also have significantresistance to cracking . In order to achieve improvement of the precast concrete car parkpavers, it is necessary to look for an alternative car park paver designs with improvedstrength, ductility and cracking resistance. An experimental investigation was carried outwith voided steel fibre reinforced concrete and ferrocement pavers. These pavers weretested under simple bending to determine their first crack and ultimate strengths, mid-spandeflection and cracking patterns. A comparative performance of these pavers with currentlyused reinforced concrete pavers is reported.
2-Survey of voided car park paversVoided reinforced concrete pavers are commonly used for the construction of open carparks in many public housing estates in Singapore. The public policy of Singapore requiresall open car parks with trees for the protection of public from sun radiation heat and tomaintain greenness of Singapore. The tropical environment helps the growth of the trees inSingapore. Conventional impervious pavements with either concrete or bitumen fail are notcompletely suitable to provide moisture saturation to the soil, which is needed for thegrowth of the trees and grass. The use of precast voided reinforced concrete pavers providecar park with grass surface appearance and aesthetically pleasant aeration patterns. Theopenings in the slabs allow percolation of surface water into subsoil, hence reducingheaving problems. However, the use of voided pavers appears to be less superior toconvention concrete or bituminous pavement from the strength point of view.2.1-Shapes of precast reinforced concrete aerated paversThe main factors affecting the choice of a particular pattern of the precast reinforcedconcrete pavers for car park construction are aesthetic and performance specificationrequirements. The available shapes of these pavers with openings were rectangular, squareor hexagonal as shown in Figure 1. The thickness of the pavers varied between 60 to 75mmand the shapes of the openings are rectangular, square or rounded. The void area content ofthese pavers varied between 16.6% and 44.3% of the total area. The largest and heaviestpaver had the dimensions of 735mm by 735mm by 75mm with 8 rectangular openings of245mm by 55mm (producing the void area content of 31.6% of the total area). The weightof this type of paver is 66.5kg. At least two adults are needed to carry a single paver.Fig. 1 -Shapes of precast reinforced concrete voided pavers
The most commonly used car park paver for open car park construction is rectangular inshape (Figure 1(A)). The typical dimensions of such paver are 735mm by 375mm by 65mmwith four 240mm by 50mm voids, providing the void area content of 38.7%. The weight ofa single paver is 26.8kg and can be easily lifted by an adult worker.Fig. 2 -Typical deterioration for reinforced concrete pavers in open car parks2.2-Field performance of reinforced concrete car park paversA satisfactory performance of car park pavers requires crack-free condition during itsdesigned service life period. A field survey conducted at a typical open car park inSingapore housing estate showed that a number of voided concrete pavers were crackedand deteriorated. The main cause for cracking and deterioration is associated with theunevenness of the base and structural weakness of the pavers. Unstable base led to tilting,settling and rocking of the pavers. Some possible causes for base instability are lack ofinterlocking between adjacent pavers, unsound base materials, uneven moisture distributionand presence of compressive soil at the joint locations. The concrete pavers are not capableof fully resisting the developed bending stresses. In addition, the steel reinforcing bars arenot sufficiently increasing the ductility of the concrete pavers.A conditional survey was conducted at a typical open car park in Singapore housing estateshowed that 473 out of 1500 concrete pavers were cracked and deteriorated. Cracking insome concrete pavers was very severe as shown in Figure 2 and endanger the car parkusers. If no maintenance is performed, further deterioration of these cracked pavers couldbe unavoidable with the increase of service time. In addition, labour shortage in Singaporeis not helping to maintain open car parks with concrete voided pavers without cracks.In order to improve of the service performance of concrete pavers, suitable for open carpark construction, it is necessary to look for alternative concrete pavement designs withimproved first-crack strength, ductility and cracking characteristics. In this studyperformance of voided rectangular steel fibre-reinforced mortar pavers and ferrocementpavers, similar to the concrete pavers used in Singapore was investigated and comparedwith those for the reinforced concrete pavers.
3-Experimental investigationVoided rectangular concrete pavers (635mm by 310mm) were made with three differentmaterials, namely reinforced concrete, steel fibre-reinforced mortar and ferrocement.Ordinary Portland cement (OPC) was used in both mortar and concrete mixtures. The mixcomposition of mortar used for both fibre-reinforced mortar and ferrocement was 1:2:0.50(cement: fine sand: water), by weight. Dramic steel fibres, having the fibre length of 30mmand 0.50mm diameter (l/d = 60), were used in fibre mortar mix with 2% fibre volume.Fig. 3 -Reinforcement details for voided concrete and ferrocement paversFigure 3 shows the reinforcement details for the cast test specimens with reinforcedconcrete and ferrocement voided pavers. The ferrocement pavers had the welded wire meshat the top and bottom with 7.5mm thickness. Three 5mm diameter mild steel bars were usedwith lateral bars at the top and bottom of the ferrocement and steel fibre reinforced mortarpavers. The reinforced concrete pavers had three 10mm diameter mild steel bars placed atthe centre with lateral bars. The composition of the concrete mixture was 1:2:2:0.50(cement : fine sand : coarse aggregate : water), by weight. The maximum aggregate size ofthe coarse aggregate was 10mm.Fig. 4 -Test set-up for flexural testing of pavers
For each type of paver, six identical voided pavers were cast in wooden moulds. Fullcompaction during casting of the test pavers was achieved by using a vibrating table. Thepavers were demoulded after 24 hours and stored in water at 28oC for the next 13 days.Three pavers for each type were tested under mid-point line loading over a simply-supported span of 555mm using an Instron testing machine, as shown in Figure 4. The mid-span deflection under the loading point was continuously monitored using an LVDT underthe increasing applied load. The first-crack load, ultimate load and load-deflectioncharacteristics were determined. The failed test pavers under bending were examined toassess the cracking patterns.Fig. 5-Strength development for plain and steel fibre mortars with ageProperty Plain mortar Fibre mortar ConcreteCompressive strength (MPa) 34.2 34.1 42.5Tensile strength (MPa) 2.50 3.45 -Flexural strength (MPa) 5.51 16.0 5.80Modulus of elasticity (GPa) 21.0 21.1 27.1Toughness Index (I5) 1.0 5.9 -Toughness Index (I10) 1.0 12.8 -Toughness Index (I30) 1.0 41.5 -Table 4.1. Properties of plain mortar and fibre mortar at 28 days4.-Results and Discussion4.1-Properties of materialsFigure 5 shows the development of plain and steel fibre reinforced mortar with age undercontinuously water curing condition at 28oC. Table 4.1 shows the mechanical properties ofplain and steel fibre mortar at 28 days. The presence of steel fibres showed no significantimprovement in the compressive strength until 90 days. At the age of 28 days, bothcompressive strength and modulus of elasticity are not affected by the addition of steel010203040500 30 60 90 120 150 180Compressivestrength(MPa)Age (days)Plain2% Fibre
fibres. However, the results showed that both tensile and flexural strengths are significantlyincreased by the presence of 2% steel fibres. Fibre mortar had the flexural strength of16.7MPa is 2.6 times of that for the plain mortar. Toughness index of both plain and fibremortar was determined by testing (125mm by 25mm by 1000mm) specimens under simplebending. The details of the testings are reported elsewhere . Toughness indices of I5, I10and I30 were 5.9, 12.6 and 41.5, respectively. Therefore, the ductility of steel fibre mortarwas remarkably increased over the plain mortar.The compressive strength of concrete at 28 days was 42.5 MPa and the modulus ofelasticity was 27.1 GPa due to the presence of high modulus natural river granite coarseaggregates. The flexural strength of concrete was 5.81 MPa which is comparable with thatfor the plain mortar of 5.51 MPa.Paver type First crack momentN-mUltimate MomentN-mUltimate/First CrackMoment RatioPlain mortar 560 560 1.00Plain concrete 575 575 1.00Fibre mortar 640 1480 2.31Reinforced concrete 530 1470 2.77Ferrocement 625 1685 2.70Table 4.2. First crack and ultimate mid-span moments for different paver typesFig. 6-Moment-Deflection relationship for pavers under bending04008001200160020000 2 4 6 8 10 12Mid-spanmoment(N-m)Mid-span deflection (mm)ReinforcedSteel fibreFerrocement
4.2 –First crack and ultimate moment capacityTable 4.2 summarises the first crack and ultimate moment at the mid-span location underbending for plain mortar, plain concrete, steel fibre-reinforced mortar, reinforced concreteand ferrocement pavers. The first-crack moment is the lowest for the reinforced concreteslab and equal to 530 N-m which is similar to unreinforced plain concrete paver. Therefore,the inclusion of 10mm diameter steel rebar at the centre of paver had failed to increase thefirst crack moment due to rebar position at the neutral axis. Both ferrocement and fibremortar pavers showed improved first-crack moment compared to reinforced concrete paver.The first-crack moment for steel fibre reinforced mortar paver was 640 N-m compared to535 N-m for reinforced concrete paver. Similarly, ferrocement paver experience first-cracking at nearly the same moment when compared with that for the fibre reinforcedmortar paver. Therefore, both fibre reinforced mortar and ferrocement pavers resistedsignificant moment before developing the first crack than the reinforced concrete paver.The highest ultimate mid-span moment of 1685 N-m was recorded with ferrocement pavercompared to 1480 N-m for fibre mortar paver and 1470 N-m for the reinforced concretepaver. Ferrocement having increased crack resistance showed 2.70 times the first-crackmoment until the ultimate failure was occurred. The corresponding values for reinforcedconcrete and fibre reinforced pavers are 2.77 and 2.31, respectively. The presence of 10mmsteel rebars in reinforced concrete paver had contributed to the improved ultimate momentcapacity.Fig. 7-Cracking patterns in failed fibre reinforced mortar,reinforced concrete and ferrocement pavers4.3 –Ductility of the paversFigure 6 shows the typical moment-deflection characteristics at the mid-span location forsteel fibre mortar, ferrocement and reinforced concrete paver under simple bending. Theferrocement paver showed highest fracture toughness (area under moment-deflection curve)
compared to fibre-reinforced and reinforced concrete pavers. Even though the first crackingmoment is important, from the point of view of energy required to develop cracking in thepavers, the fracture toughness is equally important. Considering these two in addition todeflection control, it can be said that ferrocement paver provide an effective alternative tothe currently used reinforced concrete paver.4.4 –Failure pattern of the paversFigure 7 shows the crack patterns at failure for fibre reinforced, reinforced concrete andferrocement pavers under increasing flexural loading. The fibre reinforced and reinforcedconcrete pavers had a clean single major crack after sustaining significant deflection underflexure. However, ferrocement paver experienced multiple cracking and narrower crackwidth compared to other two types of pavers.Considering the performance of these three types of pavers, it can be said that theferrocement paver is the best alternative to reinforced concrete paver in controllingcracking and inducing fracture toughness. The fibre mortar paver showed comparableperformance with reinforced concrete paver. Considering the possibility of corrosion ofsteel fibres, this may not be a good alternative. In addition, steel fibres if exposed maycause harm to the car tyres and car park users.5. -Concluding RemarksVoided pavers made from cement composites, namely seel fibre-reinforced mortar,ferrocement and steel reinforced concrete pavers were tested under simple bending untilfailure. The voids in the pavers allow the infiltration of storm-water. Traditional use ofreinforced concrete paver with nominal steel rebars experienced cracking and deteriorationdue to the instability of the base of the pavers. Based on the current investigation,ferrocement pavers have superior performance from the load carrying capacity, crackresistance, deflection and cracking pattern. With proper adoption of production technology,it is possible to produce precast ferrocement car park pavers without any difficulties.6. References1-O’Loughlin, G., Beecham, S., Lees, S., Rose, L. and Nicholas, D. On-site stormwaterdetention systems in Sydney, Water Science and Technology, IAWQ, 32(1), 1997.2-Sri Ravindrarajah, R. and Tam, C. T. Dimensional stability of ferrocement, J. ofFerrocement, 13(1) 1983, 1-12.3-Sri Ravindrarajah, R. and Tam, C. T. Watertightness in ferrocement, Journal ofFerrocement, 14(1), 1984, 11-20.4-Paramasivam, P. and Sri Ravindrarajah, R., Effect of arrangement of reinforcement onferrocement properties, ACI Structural J., 85, Jan-Feb. 1988, 3-11.5-Sri Ravindrarajah, R., Behaviour of fibre-reinforced thin slabs under short and long-term loading, Proc. of an Int. Symp. on Ferrocement and Thin Reinforced cementComposites, 2006, Bangkok, Thailand