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• Gap-graded Aggregate - Aggregate containing particles of both large and small sizes, in which particles of certain intermediate sizes are wholly or substantially absent.
• Gas Concrete - See Cellular concrete.
• GFRC (Glass Fiber Reinforced Concrete) - Concrete panels, usually architectural designs, reinforced with a high zirconia (16% minimum), alkali-resistant glass fiber. Optimum glass fiber content of 5% by weight. Lower fiber content results in lower early ultimate strengths, higher fiber content can produce composite compaction and consolidation difficulties.
• Gillmore Needle - A device used in determining time of setting of hydraulic cement, described in ASTM 0 266. Gradation The sizing of granular materials; for concrete materials, usually expressed in terms of cumulative percentages larger or smaller than each of a series of sieve openings or the percentages between certain ranges of sieve openings.
• Grout - A fluid mixture of (1) cement, sand, and water or (2) cement and water: the hardened equivalent of such mixtures.
• Gunite - A term sometimes used to designate dry-mix shotcrete.
• Heat of Hydration - The quantity of heat expressed in calories per gram, evolved upon complete hydration of portland cement at a given temperature.
• Holding Period - Period In the manufacture of concrete products, the period between completion of casting and the introduction of additional heat or the steam curing period.
• HRM (High Reactivity Metakaolin) - Refined form of an ASTM C618, Class N (natural) pozzolan. A high performance mineral admixture, similar in performance to silica fume, additionally comparable in cost. Pure white powdered in form will not effect the natural color or darken concrete as silica fume does. Suitable for high-performance color matching in architectural concrete. Dosage at 5% to 10% of cement by weight. No bleed water, better finishability, more creamy, cleanup is easier with slightly higher 28 day strengths and 25% - 35% less plasticizer is required than silica fume. Process: Produced by heating a purified kaolinite clay to a specific high temperature to alter the physical composition (calcined). Through a carefully controlled refining process, impurities are removed producing an almost 100% reactive, pure white, pozzolanic powder, very evenly distributed in particle size and results in a mineral admixture which is consistent in appearance and performance from lot to lot.
• Hydration - Formation of a compound by the union of water with some other substance. In concrete it is the chemical reaction between water and the cement. A concrete slab needs to completely hydrate prior to the application of paints, coatings, and flooring materials.
• Hydraulic Cement - A cement that is capable of setting and hardening under water due to interaction of water and the constituents of the cement (ASTM 219).
• Hydrogenesis - Another term for condensation. The term is especially applied to base and soil substrates under highway pavements, where the barometric pump causes the inhalation of humid air, which then condenses in those structures, causing an ever increasing moisture content and sometimes instability.
• Hydrologic Cycle - The Hydrologic Cycle consists of the evaporation of water from oceans and other bodies of open water; condensation to produce cloud formations; precipitation of rain, snow, sleet or hail upon land surfaces; dissipation of rain or melted solids by direct run-off into lakes and by seepage into the soil. Thereby producing a continuing endless source of water in the sub-grade.
• Impermeable - The ability of a material or product to reduce or eliminate gaseous transmissions through it's mass; measured as the rate of Water Vapor Transmission (WVT). Note: Not all materials that are waterproof are vaporproof; all materials that are vaporproof are inherently waterproof.
• Initial Set - A degree of stiffening of the cement and water mixture. This is a degree lees than final set and is generally stated as an empirical value, indicating the time in hours and minutes required for a cement paste to stiffen sufficiently to resist to an established degree the penetration of a weighted test needle. (Refer to ASTM C191 or C286 for weight and penetration data.)
• Initial stress - In prestressed concrete, the stresses occurring in the prestressed members before any losses occur.

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