Moayyad Al-Nasra / International Journal of Engineering Research and Applications(IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.1058-10621058 | P a g eOptimizing the Use of Sodium Polyacrylate in Plain ConcreteMoayyad Al-NasraDepartment of Engineering Technology, West Virginia University Institute of Technology, Montgomery, WestVirginia, USAABSTRACTThis study focusses on the use of anoptimum amount of Sodium Polyacrylate, SP, tobe mixed in ordinary plain concrete. SP has twomajor effects on concrete. The first is to provideinternal water source that helps in the hydrationprocess, and curing and ultimately increases theconcrete strength. The second effect is providingadditional voids, due to drying process, in theconcrete mass. These voids affect the concretestrength negatively; at the same time improve theconcrete performance by improving the concreteworkability and placeability, reducing theconcrete susceptibility to freezing thawing cycles,reducing shrinkage, and improving the freshconcrete stability. The main focus of this studywill be on the concrete strength. Several batcheswere prepared to determine the most effectiveamount of SP to use in the concrete mix.Keywords- Concrete Durability, Hydration,Internal Curing, Sodium PolyacrylateI. IntroductionExcess amount of SP will leave theconcrete with large amounts of voids, which in turnreduces the concrete strength and durability. Smallamount of SP, on the other hand, will havenegligible effect on the concrete performance. Theamount of water added to the fresh concrete is oneof the most important key factors that affect theconcrete properties, including durability andstrength. The water is an essential ingredient neededfor the hydration process in the fresh concrete andfor the curing process in the hardened concrete at itsearly stages. Excessive amount of water added in thefresh concrete improves the concrete workability ingeneral, reduces the concrete strength, and increasesthe drying shrinkage of the hardened concrete.Different admixtures were used to reduce theamount of water demand in the fresh concretewithout jeopardizing the workability. Water reduceradmixtures were used extensively in the ready mixplants. The most common admixture used nowadaysis the high range water reducer, better known assuperplasticizer which is water reducer and at thesame time retarder.Jensen (2013) used superabsorbentpolymers in concrete. His study focused on thestrength and shrinkage of concrete. He concludedthat the shrinkage of concrete due to loss of water tothe surroundings is the cause of cracking both in theplastic and in the hardened stage. This type ofcracking can effectively mitigated by slowing downthe water loss. The superabsorbent polymers use inconcrete has the potential to reduce concretecracking. Jensen and Hensen (2001) studied theautogenous shrinkage phenomena in concrete. Theyconcluded that the autogenous shrinkage may leadto cracking and affect concrete strength anddurability, which is also, can be considered astechnological challenge of high performanceconcrete. Addition of superabsorbent polymer in theultra-high-performance concrete can be used tocontrol the autogenous shrinkage. They alsoconducted tests that show that the shrinkagereduction due to superabsorbent polymer is relatedto a corresponding increase in the internal relativehumidity of the cement paste. In addition, the use ofsuperabsorbent polymer in concrete resulted in areduction or elimination of stress buildup andrelated cracking during restrained hardening of thesehigh-performance cementitious systems (Jensen andHensen 2002).The use of superabsorbent polymer inconcrete is also useful in frequent freeing-thawingcycle environment, by providing the concrete frostprotection. The superabsorbent polymers particlesshrinks during the hydration process leaving voidsin the concrete similar to the voids created byadding air entrainment agent to the concrete. The airbubbles left in the concrete are critical to absorb thehydraulic pressure caused by the freezing of waterin the concrete mass. Water expands upon freezingabout ten percent in volume generating hydraulicpressure in the concrete that has the potential tocause the concrete to crack. Providing voids in theconcrete absorb the hydraulic pressure and provideadditional space for the water to expand. The samecan be said about the osmotic pressure in theconcrete. The osmotic pressure is usually generateddue to the difference in salt concentration in thewater. This difference in salt concentration can becreated by adding deicer to the concrete top surface,for the purpose of melting the ice on the concrete.Also these voids can be useful to absorb other kindsof internal pressures in concrete including alkalireactivity pressure.Snoeck et al (2012) studied the use ofsuperabsorbent polymers as a crack sealing andcrack healing mechanism in cementitious materials.Their research focused on the use of thesuperabsorbent polymer to seal concrete cracks. As
Moayyad Al-Nasra / International Journal of Engineering Research and Applications(IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.1058-10621059 | P a g econcrete cracks due to its low tensile strength,harmful unfriendly chemicals may migrate intothese cracks. The durability of concrete isendangered if no proper treatment or manual repairis applied. The first stage focused on hindering thefluid flow by swelling of superabsorbent polymersafter they are exposed to a humid environment. Thesealing capacity was measured by means of waterpermeability tests and through visualization ofpermeability tests by neutron radiography. They alsoconcluded that the use of superabsorbent polymersis beneficiary to seal cracks and thus allowing arecovery in water-tightness as a decrease inpermeability is noticed. The second stage focusedon healing of small cracks in by the use of fiberreinforced cementitious materials that have theability to restore the mechanical properties. Thesemechanical properties were analyzed by four-point-bending tests and the crack closure wasmicroscopically monitored. Cracks close throughthe combination of further hydration of unhydratedcement particles, precipitation of calcium carbonateand activation of the pozzolanic reaction of fly ash.Also they concluded that the desorption ofsuperabsorbent polymers triggers healing in thevicinity of crack faces and cracks up to 130 μm wereable to close completely in wet/dry cycles due to theprecipitation of calcium carbonate.The process of curing involves maintainingsatisfactory moisture content and temperature afterconcrete is placed in order to hydrate the cementparticles and produce the desired hardened concreteproperties. Proper curing can improve strength,durability, abrasion resistance, resistance to freeze-thaw cycles, deicer scaling resistance and reduceconcrete shrinkage. Traditionally, concrete has beencured externally either through the use of watercuring or sealed curing. Curing either suppliesadditional moisture from the original mixing wateror minimizes moisture loss from the concrete. Watermay be ponded directly on the concrete surface ormay use other methods like wet burlap bags orfogging near the surface of the concrete to preventevaporation of water from the fresh concrete. Sealedcuring is accomplished by applying some sort ofsealant to the surface of concrete in order to preventmoisture loss. Internal curing can be divided intotwo categories. The first category is internal watercuring in which an internal curing agent stores waterduring mixing which is gradually released ashydration processes. The second category is internalsealing which is very similar to external sealedcuring in that its goal is to prevent the loss ofmoisture from the concrete (RILEM, 2007).II. Sodium PolyacrylateSodium polyacrylate, also known aswaterlock, is a sodium salt of polyacrylic acid withthe chemical formula [-CH2-CH(COONa)-]n andbroad application in consumer products. It has theability to absorb as much as 200 to 300 times itsmass in water. Sodium polyacrylate is anionicpolyelectrolytes with negatively charged carboxylicgroups in the main chain. Figure 1, shows thechemical composition of the sodium polyacrylate.Figure 1: Sodium Polyacrylate chemical compound.Sodium polyacrylate is a chemical polymerthat is widely used in a variety of consumer productsfor its ability to absorb several hundred times itsmass in water. Sodium polyacrylate is made up ofmultiple chains of acrylate compounds that possessa positive anionic charge, which attracts water-basedmolecules to combine with it, making sodiumpolyacrylate a super-absorbent compound. Sodiumpolyacrylate is used extensively in the agriculturalindustry and is infused in the soil of many pottedplants to help them retain moisture, behaving as atype of water reservoir. Florists commonly usesodium polyacrylate to help keep flowers fresh.Figure 2 and Figure 3 show demonstration of theabsorption capability of the sodium polyacrylate.For fast absorption a slight increase of sodiumpolyacrylate can be used for demonstrationpurposes.Figure 2: 1.5 gram of sodium polyacrylate to bemixed with 200 ml of water
Moayyad Al-Nasra / International Journal of Engineering Research and Applications(IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.1058-10621060 | P a g eFigure 3: The mix turned into gel.III. Flow TestMini slum cone is used to measure theworkability of the cement past with sodiumpolyacrylates, SP. Figure 4 shows the mini slumcone used in this study. The height of the cone is 3inches, the top diameter is 1.5 inches, and thebottom diameter is 3.5 inches. Mixing SP with waterforms gel like material. This in turn kept the mixintact, and plastic. Visual comparisons between themix without any admixture (control sample) and theone with SP, one can see smooth texture, easier tomanage. Additional observation is that the mix withadmixture showed better flow ability over surfaces,compared with the mix without admixture. Thisobservation can be explored further, focusing ofconcrete place ability, especially using chutes orpumps. This property if proven to be the case, willimprove the concrete resistance to segregation, andimproves concrete pumping property. Figure 5shows a typical measurement taken of cement pastewith SP as admixture.Figure 4: Mini slump cone used to measure cementpaste flow ability.Figure 5: Typical flow ability measurement ofsodium polyacrlyate cement paste.Table 1 shows the results of cement pasteflow ability test. The measurement in this tableshows the amount of slum in inches. The tableshows a comparison between a control sample,cement paste without any admixture, and a sampleof cement paste with SP used as admixture. Thewater cement ratio is kept constant though out theentire test of a value of 0.5. One can observe fromthe data collected in Table 1 that the slump readingsof the sample with admixture are higher at thebeginning, and then reduced with time. The two setsof reading seem to converge with time, especiallyafter the initial setting of the cement.Table 1: Mini-slump test resultsTestNo.TimeMin.PlainCementPaste(in)SPCementpaste(in)1 5 1.57 2.252 20 1.72 1.483 35 1.38 1.384 50 1.25 1.255 65 1.12 1.15IV. Concrete Strength TestsSeveral samples were prepared todetermine the most appropriate amount of SP thatcan be added to the mix design in order to maximizethe concrete compressive strength. The originalcontrol mix design was not changed, where thewater cement ratio, W/C, is kept 0.5, and the mixproportion is taken as 1:1.6:3, cement to sand togravel. After several trials, a ratio of SP to cementof 0.11 percent is found to be the most effectiveratio as far as the concrete compressive strength isconcern. Twelve 6 inch X 12 inch cylinders wereprepared for each set to be tested by compression.Six cylinders were tested, three of each set, at 7days, 14 days, 21 days and 28 days. Figure 6 showsthe results of these tests. It can be observed fromFigure 6 that there is a slight increase in the concrete
Moayyad Al-Nasra / International Journal of Engineering Research and Applications(IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.1058-10621061 | P a g ecompressive strength of the mix with SP, comparedto the plain concrete. This increase is not substantialand can fall within the standard deviation of the test.Even though the compressive strength improvementis not quite noticeable, but other properties ofconcrete can be improved by using this type ofadmixture. Figure 7 shows the different texture ofconcrete with and without SP. The concrete with SPis slightly lighter in color and shows smalluniformly distributed voids in the concrete mass.These voids used to be filled by gel, and acted asinternal curing tool.Figure 7: Compressive strength of concrete, withand without SP.Figure 7: Concrete cylinders side by side afterfailure. The one on the left is the control concretesample, and the one on the right is with SP asadmixture.Flexural strength of concrete is alsoexplored by preparing a total of eight 4 inch x 4 inchbeams. The beams were tested at 7, 14, 21, and 28days also. Figure 8 shows a typical mode of failureof a tested concrete beam. The behavior of thebeams under test was almost identical. All fail withthe same failure mechanism. Figure 9 shows theeffect of the use of SP as admixture on the concretebeams modulus of rupture. There is a slight andinsignificant increase in modulus of rupture by usingSP-admixture in concrete. This increase is almostnegligible and falls within the standard of deviation.Figure 8: Typical beam failure, by flexure.Figure 9: Effect of SP on the modulus of rupture ofconcrete beamsV. ConclusionThe use of sodium polyacrylate in concretemay increase the concrete strength due to internalcuring process. This increase is relatively small evenat the optimum amount of sodium polyacrylate usedin the concrete. This may become advantageous inthe absence of concrete curing. Excessive amount of
Moayyad Al-Nasra / International Journal of Engineering Research and Applications(IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.1058-10621062 | P a g esodium polyacrylate used in concrete has asubstantial negative effect on the concrete strength.Other important properties can be improved usingthis type of admixture especially concrete stability,where the gel provides cushioning to the largeaggregates which is in turn reduces the possibilitiesof concrete segregation. Frost resistance is the otherimportant property that can be improved by usingthis admixture, by absorbing the hydraulic pressuregenerated by water expansion. Other internalpressures can also be absorbed by these voidsincluding the osmotic pressure and the alkalireactivity pressure. These voids seem to be verywell distributed in the concrete mass.References1. Jensen, M, “Use of superabsorbentpolymers in concrete,” ConcreteInternational, Vol. 35, No. 1, January 1,2013, pages 48-52.2. Jensen, O., and Hensen, “Autogenousdeformation and RH-Change inprospective,” Cement and ConcreteResearch, Vol. 31, No. 12, Dec. 2001,pages 1859-1865.3. Jensen, O., and Hensen, “Water-Entrainedcement-based materials: Experimentalobservations,” Cement and ConcreteResearch, Vol. 32, No. 6, June 2002, pages973-978.4. Snoeck, Didier, Van Tittelboom, Kim, DeBelie, Nele, Steuperaert, Stijin and PeterDubruel, Peter, “The use of superabsorbentpolymers as a crack sealing and crackhealing mechanism in cementitiousmaterials,” 3rd International conference onConcrete Repair, Rehabilitation andRetrofitting (ICCRRR -2012), Cape Town,South Africa, pages 152 – 157,ISBN:9780415899529.5. RILEM. Internal Curing of Concrete -State-of-the-Art Report of RILEMTechnical Committee 196-ICC. State of theArt Report, RILEM Publications SARL,2007.