Comparative shrinkage properties of pavement materials including recycled concrete aggregates with and without cement stabilisation

Semugaza, Gustave (2016-03)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: The construction of pavement base/subbase layers necessitates quality materials such as natural aggregates. The use of locally available materials offers numerous advantages, including the reduction in the need for quarrying and transporting, which reduces the cost of construction. Unfortunately, these local materials don’t always perform adequately for structural purposes, and require stabilisation. Cement is considered as a binder that can treat various types of materials and provide good results. Although cement stabilisation increases the material strength properties, cement-stabilised materials are prone to shrinkage, which is recognised as the major source of different forms of cracking, identified as the most severe distress for pavements with Cement stabilised layers (CSL). Owing to friction from the layer below, high tensile stresses are induced in CSL, and cracking results when these stresses exceed the tensile strength of the material. The use of low cement content has been considered as basic measures to mitigate shrinkage, but it is not necessarily the case for all materials. In addition, the use of polymer cement additives has been considered for reducing the shrinkage in pavement layers. The Super-Absorbent Polymers (SAP) can reduce the shrinkage due to their high capacity of retaining large quantity of water. Both these factors were investigated, with the addition of SAP to Hornfels. The shortage of natural materials and strict laws on opening new borrow pits and landfills have made the recycling of wastes one of the highest requirements in many countries. A number of countries adopted the use of recycled Construction and Demolition Waste (CDW) as unbound base/subbase materials for pavements construction. Recycled CDW include Recycled Concrete and Masonry (RCM) and Recycled Concrete Aggregates (RCA). However, most of previous research has only considered cement stabilisation for natural materials. The consideration of cement stabilisation for these materials, which present self-cementing properties due to their nature, is essential. To evaluate the effect of self-cementing properties on the material shrinkage potential, this research compared the shrinkage properties of three materials, which include G4 hornfels, Recycled Concrete Aggregates (RCA), and New Concrete (NC). The G4 hornfels material served as the base material, and the NC material helped to assess the degree of self-cementation in the RCA material. Considering the shrinkage properties of the three materials, the results revealed that the cracking potential of the RCA material was very low, so that it could be used in place of G4 hornfels for base/subbase layers construction. In addition, the results revealed that the latent hydration (due to self-cementation) decreased the pivot-point of optimum percentage cement in the mix, when compared to that of the normal G4 material. The RCA and NC materials tended to reach their maximum shrinkage values at 2.5% cement content. The evaluation of shrinkage crack-patterns due to shrinkage results indicated that all cement-stabilised materials (all three material types) are classified as materials inducing unacceptable crack-patterns (very severe cracks). For non-stabilised materials, only the NC material is classified as a material inducing unacceptable crack-patterns. The non-stabilised RCA material is classified as a material inducing medium crack-pattern (severe cracks), while the G4 hornfels material is classified as a material inducing acceptable crack-pattern (no cracks). Referring to these crack-patterns, the consideration of methods for mitigating shrinkage cracks is necessary, if these materials are used in pavement layers. Nonetheless, it is evident that specifically RCA, although it has latent self-cementing properties, will not be subject to the same potential for cracking as NC.

AFRIKAANSE OPSOMMING: Die konstruksie van plaveisel kroon- en stutlae vereis hoë gehalte materiale soos natuurlike aggregate. Die gebruik van materiale wat plaaslik beskikbaar is bied verskeie voordele, insluitend die afname in die nodigheid vir steengroewe en vervoer, wat die koste van konstruksie verminder. Ongelukkig reageer die plaaslike materiaal nie altyd voldoende vir strukturele doeleindes nie en benodig stabilisering. Sement word gesien as ‘n bindmiddel wat alle tipe materiaal kan behandel en goeie resultate kan lewer. Alhoewel sementstabilisering die materiaaleienskap versterk, is sementgestabiliseerde materiale geneig om te krimp, wat erken word as die grootste bron van verskillende vorms van krake, geïdentifiseer as die ergste probleem vir plaveisels met sement-gestabiliseerde lae (SGL). As gevolg van wrywing vanaf die onderste laag word hoë trekspanning geïnduseer in SGL en krake volg wanneer hierdie spanning die trekspanning van die materiaal oorskry. Die gebruik van lae sementinhoud was oorweeg as basiese maatreël om inkrimping te beperk, maar dit is nie noodwendig die geval vis al die materiale nie. Die bykomende gebruik van polimeersement bymiddels was oorweeg om sodoende die krimping in plaveisellae te verminder. Die super-absorberende polimere (SAP) kan die krimping verminder, a.g.v. hul hoë kapasiteit om groot hoeveelhede water te behou. Albei hierdie faktore; die byvoeging van polimeersement, sowel as die byvoeging van SAP na Hornfels, was ondersoek. Die tekort aan natuurlike materiale en streng wetgewing oor die opening van nuwe leengroewe en stortingsterreine het die herwinning van afval een van die hoogste vereistes in menige lande gemaak. ʼn Aantal lande het die gebruik van Herwinde Konstruksie en Slopingsafval (RCM) as ongebonde kroon- en stutlae materiale vir plaveiselkonstruksie aangeneem. Herwinde Konstruksie en Slopingsafval (CDW) sluit herwinde beton en messelwerk en herwinde beton aggregate in. Vorige navorsing het egter slegs sementstabilisering vir natuurlike materiaal oorweeg. Die oorweging van sementstabilisering vir hierdie materiale, wat selfsementering eienskappe a.g.v. hul aard aanbied, is noodsaaklik. Hierdie navorsing het die krimp-eienskappe van drie materiale vergelyk, om sodoende die uitwerking van self-sementeringseienskappe op die materiaalkrimpingspotensiaal te evalueer, insluitend G4 Hornfels, Herwinde Beton Aggregate, en Nuwe Beton (NC). Die G4 Hornfels materiaal het as die basis-materiaal gedien en die Nuwe Betonmateriaal het gehelp om die graad van selfsementering in die Herwinde Beton Aggregate materiaal te assesseer. Wanneer die krimpingseienskappe van die drie materiale in ag geneem word, het die uitslae onthul dat die kraakpotensiaal van die Herwinde Beton Aggregate baie laag was, sodat dit in plaas van die G4 Hornfels gebruik kon word vir kroon- en stutlae konstruksie. Die resultate het ook onthul dat, in vergelyking met die normale G4 materiaal, die latente hidrering (a.g.v. self-sementasie) die spilpunt van optimale persentasie sement in die mengsel verminder het. Die materiale was geneig om hul maksimum krimpingswaardes by 2.5% sementinhoud te bereik. Die evaluering van krimping-kraakpatrone a.g.v. krimping resultate het aangedui dat alle sementgestabiliseerde materiale (al drie materiaaltipes) geklassifiseer is as materiale wat onaanvaarbare kraakpatrone (baie ernstige krake) veroorsaak. Vir ongestabiliseerde materiaal word slegs die Nuwe Betonmateriaal geklassifiseer as ‘n materiaalinduserende onaanvaarbare kraakpatroon. Die ongestabiliseerde Herwinde Beton Aggregate word geklassifiseer as ‘n materiaalinduserende medium kraakpatroon (ernstige krake), terwyl die G4 Hornfels materiaal geklassifiseer word as ‘n materiaal wat aanvaarbare kraakpatrone induseer (geen krake). Met verwysing na hierde kraakpatrone, is die oorweging van metodes om die krimp van krake te vermindes nodig wanneer hierdie materiaal in plaveisellae gebruik word. Nietemin is dit duidelik dat spesifiek Herwinde Beton Aggregate, alhoewel dit latent self-sementeringseienskappe het, nie onderworpe sal wees aan dieselfde potensiaal vir krimping nie.

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