PNRC 13 - Pulverized Fuel Ash in Concrete
Table of Contents
Introduction
This article would introduce the content of PNRC 13 – “Pulverized Fuel Ash in Concrete“.
In addition, the feature of PFA concrete and the importance for concrete curing would be introduced.
Source of PFA concrete?
Pulverised Fuel Ash (PFA) is the by-product resulting from the burning of pulverised coal in coal-fired electricity power statio.
Features of PFA concrete and its Pros and Cons
Feature (Pros) - Spherical Particle Shape - Reduction in water content
PFA is a fine material with spherical particle shape. When added to concrete it produces a more cohesive mix.
In addition, since fly ash particles are spherical and in the same size range as portland cement, a reduction in the amount of water needed for mixing and placing concrete can be obtained.
The reduction of water content reduces the rate of bleeding within concrete. Bleed water that collects at the surface of concrete results in a localised increase in the water/cement ratio, reducing the strength and durability
Feature (Pros) - Slow Hydration Reaction - Reduce thermal gradient
Fly ashes produce little or no decrease in the heat of hydration. (compared to plain portland cement).
At ambient temperatures, fly ash reactions are slow and only occur a few days after the hydration of the cement starts. This reduces the thermal gradient, and therefore the chance of thermal cracking is reduced, which improved long term performance in terms of strength and durability.
Economic and environmental benefits (Pros)
PFA is a by-product, with limited uses. Its major potential uses are as fill, as a raw material in cement manufacture and as a partial replacement for cement in concrete. The direct economic benefits deriving from the use of PFA as fill are small compared with its use as a cement replacement in concrete.
More importantly, the problem of disposal of ash as a waste material would cease to exist.
Other pros - resistance to alkali-silica reaction
More details refer to APP 74 – Alkali-Aggregate Reaction in Reinforced Concrete Structures
Slow rate of gain of strength (Cons)
The rate of gain of strength of PFA concrete is slower than PC concrete. This may affects the minimum striking time for formwork and falsework if the content of PFA is too high.
A experiment from GEO report No. 48 has shown the result as below. The 7-day relative strength percentage for Grade 45 Mixes with 0%, 25% and 55% is around 76%, 68% and 55% respectively.
The strength developement for OPC and PFA with different content is as below.
Result
1. Early strength of PFA concrete is lower.
2. 28-day strength for OPC and PFA concrete is identical.
2. Long term strength of PFA is higher than OPC.
Sensitive to water content and initial curing (Cons)
PFA concrete is more sensitive to water content and initial curing than conventional concrete, so that extra care must be taken in supervision of its production and curing, but the benefits available are worth the extra effort.
The effect of duration of curing for OPC and PFA concrete is as below.
It has shown that, without any curing, the strength of both OPC and PFA cannot reach 100%.
The codified requirements for concrete curing
Reason for Curing
Curing and protection should start immediately after the compaction of the concrete to protect it from:
(a) premature drying out (i.e. loss of moisture), particularly by solar radiation and wind;
(b) rain and flowing water (protection should be provided);
(c) rapid cooling during the first few days after placing;
Curing for high strength concrete
Early mist
For high strength concretes, to limit plastic shrinkage cracking, early mist curing starting two hours after placing of the concrete should be considered.
Adiabatic curing tests
The CoP recommended that for all concrete mixes of grade greater than C60, adiabatic curing tests should be carried out and a maximum temperature rise of 40°C imposed.
In theory, the temperature rise of concrete should be measured by an adiabatic curing test, in which heat transfer between the concrete specimen and the surroundings is cut off entirely.
To conduct an adiabatic curing test, since it is physically not possible to provide perfect heat insulation, it is necessary to control the temperature of the surrounding to be the same as that of the concrete specimen so that there is no temperature difference and no heat transfer between the concrete specimen and the surroundings.
Minimum periods of curing and protection
Surfaces should normally be cured and protected for a period not less than below table (min. 3 days):
Common methods of curing
cover the surface with damp (slightly wet) hessian
continuous or frequent applications of water to the surface
spray the surface with curing compound
Restriction on the use of PFA concrete by PNRC 13
PFA should not be used as a partial cement replacement in concrete in addition to blended cement.
The PFA content should not exceed 35% by mass of the cementitious content (OPC plus PFA) of the concrete. It should, however, be noted that 25% PFA replacement is usually only used in normal construction.
The AP/RSE should specify the PFA content and extent of its use in the structural submissions. He should satisfy himself and the registered contractor should make sure that the concrete supplier has adequate quality control measures to ensure that the finished concrete complies with the specifications and statutory requirements (QC Plan by RC).
When the PFA replacement exceeds 25%, the AP/RSE should also satisfy himself that there will not be any adverse effect on the structure due to removal of formwork, creep and long-term deflection etc. (Proof for the effect of PFA over 25%)
Specfication related to the use of PFA concrete
As per ARUP specification. the peak in-situ temperature of concrete could be relaxed from 75ºC to 85ºC if the High Strength Concrete contain a minimum of 25% PFA.
Introduction to different concrete admixture
Water Reducing Admixtures
Minimize the water demand in a concrete mix. Increase slump value but remain the w/c content and the concrete strength.
In other words, remain slump value but reduce water demand and application (i.e. increase the strength concrete)
Plasticizers: reduce the water demand up to 10%
Mid-range plasticizers: reduce the water demand up to 15%
Superplasticizers: reduce the water demand up to 30%
Superplasticizers generally do not affect the setting time. At the most, they may retard initial set about 15 minutes. There are Non-retarded superplasticizing admixture and Retarding superplasticizing admixture in BD’s Central Data Bank.
Retarding Admixtures (慢乾劑)
Retarding admixtures slow down the rate of hydration of cement in its initial stage and increase the initial setting time of concrete
Preventing cold joint formation by exposing the aggregate as shown below –
Accelerating Admixtures (Accelerator)
Accelerating admixtures are used to reduce the initial setting time of concrete. It also improves the strength of concrete in its early stage by increasing the rate of hydration.
Earlier hardening of concrete is useful in several situations such as early removal of formwork, less period of curing, emergency repair works, for constructions in low temperature regions (by faster hydration process).
BD Recommendation on Concrete Mixture
Application of a concrete admixture shall be strictly in accordance with the manufacturer’s specifications/recommendations.
Buildings Department Central Data Bank
A Central Data Bank (CDB) which contains lists of building materials, components and construction systems that have been accepted by individual government departments, including lists for Concrete Admixtures.
The CDB aims to provide a reference to parties in the building industry only. The user who makes use of or relies on any information in the CDB shall take full responsibility for the application and performance of the building material, component or construction system.
Details for CDB refer to ADM 20 – Central Data Bank.
THIS DOCUMENT IS FOR REFERENCE PURPOSES ONLY.