Questions from the Field is a selection of questions the NPCA Technical Services Department receives from calls, emails and comments on blogs or magazine articles on precast.org. If you have a technical question that needs an answer, contact us by calling (800) 366-7731 or visit the Technical Services web page.
Dheeraj writes: Is concrete with fly ash useful in curing?
NPCA Technical Services Department answers: Fly ash has pozzolanic properties and the reaction is generally slower than cement’s hydration reaction. Therefore, the use of fly ash can delay set, which may result in delaying finish operations. This also depends on the type of fly ash you use, as class F fly ash will likely increase setting time while class C may accelerate or reduce set time. The effect on set, also depends on the amount of fly ash replacement in the mix. Higher amounts can lead to significant delay of initial set.
The use of fly ash will generally lower the heat of hydration in comparison with mixes using only Portland cement. Fly ash in concrete may also lead to lower early age strengths. This varies depending on the type of fly ash used and replacement levels. However, fly ash in concrete has been shown to increase long-term strength.
To answer your question, fly ash is generally not used to enhance curing. It has many other benefits including lowering water demand, increasing workability and reducing bleeding and segregation. It also enhances concrete’s hardened properties by increasing long-term strength, lowering permeability and increasing durability. It will not require less water for curing. As with all curing, maintaining ideal temperature and relative humidity is key for pozzolanic and hydration reactions.
NPCA Technical Services Department answers: Concrete must be made with a workability, consistency and plasticity suitable for job conditions. The slump test is generally used to measure concrete consistency. Consistency is the ability of freshly made concrete to flow. When used with different batches of the same mix design, a change in slump indicates a change in consistency and in the characteristics of the materials, mixture proportions, water content, mixing, time of test or the testing itself.
The reality is if we measure the slump, the only thing we really know at this point is the slump. The slump of a concrete mix is influenced by everything. Changes in any of the following can affect the slump of the concrete:
• Content, proportions, chemistry, fineness, particle size distribution, moisture content and temperature of cementitious materials
• Content, proportions, size, texture, grading, cleanliness and moisture content of aggregates
• Dosage, type, combination, interaction, sequence of addition of chemical admixtures
• Air content
• Batching, mixing and delivery/placement methods
• Temperature of the concrete
• Sampling, slump-testing technique and the condition of the test equipment
• Amount of free water in the concrete
• Time since batching at the time of testing
Slump is indicated in the job specifications as a range or as a maximum value not to be exceeded. If slump is not specified, ACI 211.1 Table 6.3.1 has established recommended slumps based on various types of construction.
Daniel writes: A customer of mine had a question about the methods for installing a manhole into a hole. Most of their customers do not use spreader bars to install the manhole. I thought spreader bars were specified in ASTM, but I don’t know where. Do you have any idea where I should look for such a specification?
NPCA Technical Services Department answers: The use of a spreader bar for structure handling is not required. It is often the contractor who determines the method as long as the finished product meets the specs and expectations. Some precasters will furnish a spreader bar with delivery of their product.
Within the proposed manhole installation standard 8.2.2 there is a small mention of spreader bars only if “safe lifting angles cannot be achieved.”
Bryan writes: What will be the earliest time to remove the precast form from the Portland precast cement wall?
NPCA Technical Services Department answers: The minimum stripping time is a function of concrete strength. The method used to determine stripping time involves comparing the actual strength gained to the required strength for stripping the structure. The required stripping strength is sometimes specified in the contract documents. In precast, the stripping strength in usually listed in the plant specific quality control manual.
The earliest time to remove formwork is when the concrete has achieved enough strength to support its own weight and the weight of anticipated loads. ACI 547, “Guide to Formwork for Concrete’ states “when forms are stripped, there should be no excessive deflection or distortion and no evidence of damage to the concrete due to either removal of support or to the stripping operation. If forms are removed before the specified curing is completed, measures should be taken to continue the curing and provide adequate thermal protection for the concrete.”
So unfortunately, we can’t give you an amount of hours. Various concrete mixes will have different rates of set. You just need to cure cylinders in the same environment as the structure and periodically break one until the desired strength has been reached. The time required to reach that strength can then be used as a basis for further stripping for that particular mix design.