Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Plastic shrinkage cracking (PSC) is the cracking caused by the early age shrinkage of concrete
within the first few hours after the concrete has been cast. It results in unsightly surface
cracks that serve as pathways whereby corroding agents can penetrate the concrete which
shortens the expected service life of a structure. PSC is primarily a problem at large exposed
concrete surfaces for example bridge decks and slabs placed in environmental conditions
with high evaporation rates.
Most precautionary measures for PSC are externally applied and aimed to reduce the
water loss through evaporation. The addition of a low dosage of polymeric fibres to
conventional concrete is an internal preventative measure which has been shown to reduce
PSC. The mechanisms involved with PSC in conventional and low volume fibre reinforced
concrete (LV-FRC) are however not clearly understood. This lack of knowledge and guidance
leads to neglect and ineffective use of preventative measures. The objective of this study is
to provide the fundamental understanding of the phenomena of PSC. To achieve the
objective, an in depth background study and experiments were conducted on fresh
conventional concrete and LV-FRC. The three essential mechanisms required for PSC are: 1→ Capillary pressure build-up
between the particles of the concrete is the source of shrinkage. 2→ Air entry into a
concrete initiates cracking. 3→ Restraint of the concrete is required for crack forming. The experiments showed the following significant findings for conventional and
LV-FRC: PSC is only possible once all the bleeding water at the surface has evaporated and
once air entry has occurred. The critical period where the majority of the PSC occurs is
between the initial and final set of concrete. Any preventative measure for PSC is most
effective during this period. The bleeding characteristics of a mix have a significant influence
on PSC. Adding a low volume of polymeric fibres to concrete reduces PSC due to the added
resistance that fibres give to crack widening, which increases significantly from the start of
the critical period.
The fundamental knowledge gained from this study can be utilized to develop a
practical model for the design and prevention of PSC in conventional concrete and LV-FRC. / AFRIKAANSE OPSOMMING: Plastiese krimp krake (PSK) is die krake wat gevorm word a.g.v. die vroeë krimping van beton
binne die eerste paar ure nadat die beton gegiet is. Dit veroorsaak onooglike oppervlak
krake wat dien as kanale waardeur korrosie agente die beton kan binnedring om so die
dienstydperk van die struktuur te verkort. Dit is hoofsaaklik ʼn probleem by groot
blootgestelde beton oppervlaktes soos brug dekke en blaaie wat gegiet is in klimaat
kondisies met hoë verdamping tempo’s.
Meeste voorsorgmaatreëls vir PSK word ekstern aangewend en beperk die water
verlies as gevolg van verdamping. Die byvoeging van ʼn lae volume polimeriese vesels is ʼn
interne voorsorgmaatreël wat bekend is om PSK te verminder. Die meganismes betrokke ten
opsigte van PSK in gewone beton en lae volume vesel versterkte beton (LV-VVB) is vaag. Die
vaagheid en tekort aan riglyne lei tot nalatigheid en oneffektiewe aanwending van
voorsorgmaatreëls. Die doel van die studie is om die fundamentele kennis oor die fenomeen
van PSK te gee. Om die doel te bereik is ʼn indiepte agtergrond studie en eksperimente
uitgevoer op gewone beton en LV-VVB.
Die drie meganismes benodig vir PSK is: 1→ Kapillêre druk tussen die deeltjies van die
beton is die hoof bron van krimping. 2→ Lugindringing in die beton wat krake inisieer. 3→
Inklemming van die beton is noodsaaklik vir kraakvorming. Die eksperimente het die volgende noemenswaardige bevindinge opgelewer: PSK is
slegs moontlik indien al die bloeiwater van die beton oppervlakte verdamp het en indien lug
die beton ingedring het. Die kritiese periode waar die meerderheid van die PSK plaasvind is
tussen die aanvanklike en finale set van die beton. Enige voorsorgmaatreël vir PSK is mees
effektief gedurende die periode. Die bloei eienskappe van ʼn meng het ʼn noemenswaardige
effek op die PSK. Die byvoeging van ʼn lae volume polimeriese vesels tot beton verminder die
PSK deur die addisionele weerstand wat die vesels bied teen die toename in kraakwydte. Die
weerstand vergroot noemenswaardig vanaf die begin van die kritiese periode.
Die fundamentele kennis wat in die studie opgedoen is, kan gebruik word vir die
ontwikkeling van ʼn praktiese model vir die ontwerp en verhoed van PSK in gewone beton en
LV-VVB.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/71648 |
| Date | 12 1900 |
| Creators | Combrinck, Riaan |
| Contributors | Boshoff, W. P., Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 127 p. : ill. |
| Rights | Stellenbosch University |
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