Internal Curing Mechanism of Pine Needle Bio-Fiber (Pinus roxburghii) Hollow Channels in M25 Concrete: Quantification of Late-Age Strength Gain, Autogenous Crack-Healing Analysis, and Dampness Resistance Correlation

https://doi.org/10.46610/IJSAACT.2026.v02i01.004

Abstract

From the Solan District of Himachal Pradesh, India, pine needle (Pinus roxburghii) bio-fibers have inherently hollow channels spanning 10 to 40 micrometers. These channels let the fibers absorb between 0.8 and 1.2 grams of water for every gram of dried fiber. Then, they slowly release this water over a period of 24 to 72 hours. In M25 grade concrete, this study is the first to systematically examine this internal curing process and determine how much it contributes to late-age compressive strength development, autonomous crack-healing ability, and dampness resistance. At 7, 28, and 56 days, mechanical performance was assessed from five fiber dosage levels 0.00%, 0.25%, 0.50%, 0.75%, and 1.00% by volume of concrete. Every one of these intervals, compressive strength, split tensile strength, and flexural strength were assessed to show the evolution of hydration and fiber-matrix interaction throughout time. To replicate actual service settings, specimens were subjected to 56 days of alternating wet-dry cycles using the RILEM TC 221-SHC method for evaluating crack-healing efficiency. The one-face immersion test done in line with IS 1199:2018 was used to find moisture penetration resistance. This test gives a standard way to measure how far water soaks in and how much of it takes up. Importantly, no chemical surface treatments were used on the fibres at any point to keep their hollow inside structure and the natural waxy cuticle on their outer surface. At a fiber content of 0.50% by volume, which corresponds to around 5.25 kg per cubic meter of concrete, the best performance was consistently found. Compressive strength rose by 3.8 MPa between 28 and 56 days at this dosage, a 41% improvement over the 2.7 MPa rise observed in the plain control concrete over the same period. The fibers' ongoing water release sustains continuous cement hydration well beyond the early curing window, hence enhancing late-age strength gain. For the best mix, crack-healing effectiveness at 56 days reached 62.1%; this is 2.41 times better than the 25.8% for the control. This outcome is explained through five synergistic mechanisms: first, the fibers physically bridge micro-cracks, limiting crack width and keeping fissures within the self-healing range; second, the hollow channels function as internal moisture reservoirs, sustaining the healing environment even in the absence of external water; third, calcium carbonate precipitates on the fiber surfaces, contributing to crack infilling; fourth, additional calcium silicate hydrate (C-S-H) gel forms in the interfacial transition zone surrounding the fibers, densifying the microstructure; and fifth, the natural waxy coating on the fiber surface reduces capillary absorption, limiting the ingress of harmful moisture. In terms of durability, moisture penetration depth decreased by 46.8%, from 42.5 mm in the control to 22.6 mm in the optimal mix. Water absorption fell by 33.0%, from 4.10% to 2.75%. These results collectively demonstrate that pine needle bio-fibers from the sub-Himalayan Pinus roxburghii belt deliver a unique, five-part internal curing system that simultaneously prevents crack formation, promotes autonomous crack sealing, and resists moisture ingress, all without chemical additives or fiber pre-treatment. No other natural or synthetic fiber evaluated in the literature has been shown to achieve this specific combination of benefits within a single material system.