Civil Engineering Reference
In-Depth Information
placing and compaction without segregation. In the case of high-volume
replacement of Portland cement by finely ground natural pozzolans, the
concrete mixtures exhibit better cohesiveness and workability compared
to ordinary concrete. Other advantages include improved pumpability,
compactibility and finishability. On the other hand, high-volume incorporation
of pozzolanic materials into concrete mixtures makes use of superplasticizers,
which are necessary in obtaining high slumps due to the low water content
of HVPC mixtures. The type and dosage of superplasticizer vary depending
on the nature and particulate characteristics of the pozzolan, as well as
the compatibility between the superplasticizer and cementitious materials.
Selecting a superplasticizer for use in HVPC mixtures is critical to obtaining
a fresh concrete with satisfactory slump and workability.
7.6.2 Air content
In published literature, it has been reported that HVFa concrete requires
a somewhat higher dosage of air-entraining agent compared to the control
concrete mixture without fly ash. For high-volume fly ash concrete mixtures,
relatively high carbon content in fly ash (>6%) may cause difficulties in air
entrainment (Malhotra and Ramezanianpour, 1994).
The air content of non-air-entrained HVNP concrete is somewhat lower
than the control concrete mixture for the same water-to-cementitious
materials content (w/cm) (Uzal et al., 2007). This is actually true of all
types pozzolan incorporated into concrete mixtures. Similarly to HVFa
concrete, HVNP concrete might require a higher dosage of air-entraining
agent for a certain level of air content compared to a plain concrete
mixture.
7.6.3 Setting time
HVNP concrete mixtures exhibit quite different setting times compared
with HVFa concrete. HVFa concrete mixtures typically show comparable
initial setting time to the control concrete made with the same water content
and the same ratio of water to cement + fly ash by weight w/(c + f), and a
setting time approximately 3 hours longer that the control (Malhotra and
Ramezanianpour, 1994). However, published data on HVNP systems has
demonstrated that HVNP replacement may cause accelerated initial and final
set in paste and concrete specimens when compared to the control specimens
without replacement (Uzal and Turanli, 2003, 2012; Turanli et al., 2004,
2005; Uzal et al., 2007). Table 7.5 shows the setting times of HVNP cement
pastes and concrete mixtures in comparison to reference mixtures without
natural pozzolan. It is believed that the accelerated setting times of HVNP
cementitious systems are related to self-desiccation of the hydrating system
