Polyvinyl Alcohol is a water-soluble synthetic polymer popular in many industries, for cement and concrete mixes, adhesives, textile finishing, papermaking, ceramics, personal care products, and the printing industry. In the construction industry, water-soluble polymers are commonly used as polymeric additives in applications such as plastering, underwater concreting, and macro defect-free cement. Moreover, dissolved Polyvinyl Alcohol (PVA) has been used as an additive in concrete.
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Similar to PVA, Polymer Modified Concrete (PMC) is rarely used in structural applications because of the high cost of polymers and the composite’s limited mechanical properties, especially its strength. Similar observations also apply to mortars. In this blog, we will explore the mechanical properties of polyvinyl alcohol in regard with concrete with the help of various studies.
Materials and Mix Design
A few recent studies examined polymer-modified concrete and polymer-modified mortar. To conduct the research, a base mixture was added to the PVA in various dilution ratios. The production process includes incorporating the polymer, similar to other additives, directly during the paste mixing phase.
This produces a material capable of achieving maximum strength without any visible surface cracking. In the study, several factors were taken into account in the mix design, including the type of cement and aggregate, workability during concrete casting, environmental conditions, as well as the preservation or enhancement of concrete strength, stiffness, and durability.
The study used Portland cement, while inert lava of varying particle sizes, sourced from a local quarry, was used as the aggregate. As a result of polymer-added proportion, PVA occupies the voids formed during the hydration process and undergoes chemical reactions during the setting and hardening stages.
The key factors that characterize PVA’s physical properties are its molecular weight and alcoholysis level. However, it was also observed that a highly concentrated PVA solution can delay the hydration process by forming a semi-permeable film around the cement particles.
Polymer Modified Concrete and Polymer Modified Mortar
The PMC mixture was prepared by adding a small quantity of water-soluble PVA into the cement paste at various dilution ratios with Portland cement. In the initial phase of the new concrete, the compressive strength was abnormally low. A large air bubble inclusion was observed in a number of scanning electron microscope tests conducted. The inclusion was influenced by the mixture and the particle size of the aggregate.
It is likely that large-sized aggregates prevented the release of entrained air bubbles and were found to have played a role in decreasing the mechanical properties of the modified material in comparison with the unmodified one. To fix this, finer sand had been used and no big gravel fraction was present within the mix.
In addition, some samples were tested due to the minimum content of constituents different from clinker, which reduces possible chemical reactions between PVA, the cross-linker, and the cement paste to the main component, and thus easier to identify. Other factors that were considered included the percentage of PVA used; samples with higher PVA content had the poorest mechanical properties; thus, a lower PVA percentage is more appropriate. A few experiments were conducted in the labs of researchers with different samples of mortar cement with different quantities of aggregates and PVA content. The following is the experimental analysis conducted in this study.
Experimental Investigation
In the experimental investigation of the studies, the material used was PVA which is mixed into the concrete mix to check its enhancement in mechanical properties. In fact, the mechanical properties of polyvinyl alcohol has some effect on concrete characteristics. Standard concrete ingredients like cement, aggregate, and water were used with PVA. The experiment was carried out on both polymer-modified concrete and polymer-modified mortar.
Five cubic specimens of iron mold mixers were prepared, sized 150 mm and 100 mm depending on the maximum size of the aggregates. All the sands and gravel used in recent studies were weighted and oven-dried for 24 hours to eliminate moisture content inside their pores. A release agent was also treated on these materials before mixing.
The mixing process was carried out in a 25-liter mixer: wetting the mixer, and adding sand, gravel, and cement for 1 minute of mixing. Water was then added, and the mixture was mixed for an additional 5 minutes. First, the concrete was conveyed into a tank and then poured into moulds where it was vibrated and trowelled.
The surface was kept moist with wet rags. The slump test was carried out in conformity with the UNI EN 12350-2 and yielded the following results: plastic base concrete, S2 consistency; semi-fluid mixtures with PVA modification, S3 consistency, featuring an improved mixture fluidity due to PVA addition.
In the case of PVA-modified samples, the water used for dissolving the polymer and the cross-linking agent, if present, was subtracted from the total amount of water required to achieve the designed w/c. During mixing, the addition of PVA increased the fluidity and volume of the mixture, probably due to air encapsulation.
The specimens were demoulded after 1-2 days from casting and cured at 95% humidity and 25°C for 28 days to minimize shrinkage. After curing, the bulk density was measured by weighing the cured cubes.
Polyvinyl Alcohol Result on Concrete and Mortar
The various types of research on the effects of adding Polyvinyl alcohol to concrete and mortar show various results in terms of improvement in mechanical properties. Following are the collective observations of the experiments conducted on PVA with concrete and mortar.
1. Effect on Mechanical Properties
The addition of PVA in concrete materials enhances its mechanical properties up to a certain level. Particularly, mortar with 0.6% PVA shows the highest compressive strength, demonstrating a 12.1% increase compared to ordinary concrete. In the case of flexural strength, a PVA of 0.1% resulted in a 24.8% increase over the control. However, as we increase the PVA content beyond the optimal levels, the mechanical properties tend to decline due to poor workability and increased internal porosity which negatively affects the strength of the concrete and mortar.
2. Effect on Rheological Properties
It is evident through studies that the incorporation of PVA enhances the workability and fluidity of fresh concrete and mortar at lower concentrations than <2%. This positively results in better slump values. For example, a 0.2% PVA addition in concrete can decrease slump by 13.9%, and higher concentrations of PVA lead to reduced flow properties. To better understand the PVA effect on the rheological properties of mortar refer to the modified Bingham model.
3. Effect on Hydration Process
When PVA is added to the concrete or mortar, it affects the hydration process depending on the concentration of it. At a lower concentration of around 0.6%, PVA accelerates the hydration process, while a higher concentration (e.g., 2.0%) delays the hydration rates, resulting in reduced early strength of the concrete. Furthermore, the presence of PVA in the concrete affects the bonding strength positively, resulting in improving the overall durability when used appropriately.
4. Effect on Microstructural Changes
In various studies, Scanning Electron Microscopy (SEM) analysis shows addition of PVA makes a three-dimensional network within a cement matrix, which contributes to increased crack-filling capabilities and reduces porosity when used in moderation. Excessive amounts can cause film formation that coats the particles, obstructing hydration and possibly weakening the structure.
5. Effect on Water Retention and Density
The addition of PVA enhances the water retention characteristics which is beneficial for maintaining moisture content during the curing processes. However, higher contents can lead to expanded air voids and reduced density due to poor mixing characteristics.
With the above-mentioned results evident by many studies in various universities and labs, it is found that incorporating Polyvinyl Alcohol into concrete and mortar can significantly enhance mechanical properties like compressive and flexural strength when used at appropriate levels (around 0.6% for compressive strength and 1.0% for flexural strength).
In general, mechanical properties of polyvinyl alcohol have a positive effect on concrete mixture. However, adding PVA above optimal concentrations, workability and overall structural integrity may be adversely affected by increased porosity and delayed hydration effects. Hence, careful consideration of the dosage of PVA is important for maximizing its benefits in construction applications.
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