Dale Bentz, Phillip Halleck, Abraham Grader, and John Roberts RILEM Conference- Volume Changes of Hardening Concrete: Testing and Mitigation August 2006

Outline Need for internal curing –Blended cements “Undercuring” with internal curing Microtomography observations of water movement during internal curing –Quantitative analysis of 3-D images Mixture proportioning for internal curing

What is internal curing (IC)? Answer: As being considered by ACI-308, “internal curing refers to the process by which the hydration of cement occurs because of the availability of additional internal water that is not part of the mixing water.” For many years, we have been curing concrete from the outside in, internal curing is for curing from the inside out. Internal water is generally supplied via internal reservoirs, such as saturated lightweight fine aggregates, superabsorbent polymers, or saturated wood fibers.

Why do we need IC? Answer: Particularly in HPC, it is not easily possible to provide curing water from the top surface (for example) at the rate that is required to satisfy the ongoing chemical shrinkage, due to the extremely low permeabilities that are often achieved in the concrete as the capillary pores depercolate. Capillary pore percolation/depercolation first noted by Powers, Copeland and Mann (PCA-1959).

How does IC work? Answer: IC distributes the extra curing water (uniformly) throughout the entire 3-D concrete microstructure so that it is more readily available to maintain saturation of the cement paste during hydration, avoiding self-desiccation (in the paste) and reducing autogenous shrinkage. Because the autogenous stresses are inversely proportional to the diameter of the pores being emptied, for IC to do its job, the individual pores in the internal reservoirs should be much larger than the typical sizes of the capillary pores (micrometers) in hydrating cement paste and should also be well connected (percolated).

Cement paste Water reservoir

Blended Cements Internal curing can be particularly important in high-performance (low w/cm) blended cement systems –Increased chemical shrinkage of pozzolanic and slag reactions Cement: 0.06 to 0.07 mL/g cement Silica fume: 0.22 mL/g cement Slag: ~ 0.18 mL/g cement Fly ash (Type F): ~ 0.12 to 0.16 mL/g cement –Possible earlier depercolation of capillary pores and reduced permeability limiting water transport distances within the hydrating blended cement paste microstructure

Autogenous Deformation Results IC added via fine LWA to increase total “w/c” from 0.30 to 0.38 or 0.40 Note – chemical shrinkage of pozzolanic reaction of silica fume with CH is ~0.22 g water/g silica fume or about 3.2 times that of cement

Autogenous Deformation Results IC added via fine LWA to increase total “w/c” from 0.30 to 0.38 Note – chemical shrinkage of slag hydraulic reactions is ~0.18 g water/g slag or about 2.6 times that of cement

“Undercuring” with Internal Curing Hydrating cement paste is a complex and dynamic porous media and as such, internal curing mixture proportions that supply only part of the total needed water (demand) can potentially exhibit some interesting results as illustrated in the schematic on the following slide

Empty and Full Pores Saturated curing Sealed curing RH = 98 % RH = 93 % Sufficient Internal curing IC Reservoir Cement paste RH = 97 % Insufficient Internal curing Cement paste RH = 90 % IC Reservoir Better hydration Only pores in reservoirs empty Some increase in hydration Pores in both reservoirs and paste empty Cement paste Less hydration Largest pores in paste empty

Four-Dimensional X-ray Microtomography X-ray microtomography allows direct observation of the 3-D microstructure of cement-based materials –Example: Visible Cement Data Set In October 2005, experiments were conducted at Pennsylvania State University to monitor three- dimensional water movement during internal curing of a high-performance mortar over the course of two days (time is the 4 th dimension)

Mixture Proportions w/c = 0.35 Blend of four sands (Ferraris) to improve particle packing LWA added in saturated surface dry (SSD) condition SSD specific gravity of 1.7 Commercial cement – no particles larger than 30 μm diameter Hydration conducted at 30 o C maintained by circulating fluid from a temperature controlled bath

After mixing1 d hydration2 d hydration Subtraction: 1 d – after mixing Aqua indicates drying Red indicates wetting All images are 13 mm by 13 mm

Three-dimensional subtracted image of 1 d hydration – initial microstructure showing water-filled pores that have emptied during internal curing (4.6 mm on a side) 2-D image with water evacuated regions (pores) overlaid on original microstructure (4.6 mm by 4.6 mm) Four-Dimensional X-ray Microtomography

Quantitative Analysis Four-dimensional image sets analyzed to estimate volume of water moving from LWA to cement paste during first 2 d of hydration Analysis based on changes in greylevel histogram with time Results compared to conventional measures of hydration including chemical shrinkage, non-evaporable water content, and heat release

Preprocessing of 3-D Image Data Median filter applied to remove noise and sharpen greylevel histogram

Temporal Analysis of Greylevel Histograms Change in “empty” pores with time quantified

Tomography Water Movement vs. Hydration Measures Good ”quantitative” agreement between estimated water movement volume and other measures of hydration

Four-Dimensional X-ray Microtomography Empty porosity within LWA from analysis of 3-D microtomography data sets scales “exactly” with measured chemical shrinkage of the cement for first 36 h of curing

Mixture Proportioning for Internal Curing Questions to Consider When Using IC How much water (or LWA) do I need to supply for internal curing? How far can the water travel from the surfaces of the internal reservoirs? How are the internal reservoirs distributed within the 3-D concrete microstructure? Answers May be found at the NIST internal curing web site:

How much water (or LWA) do I need to supply for internal curing? Answer: Equation for mixture proportioning (Menu selection #1) M LWA =mass of (dry) LWA needed per unit volume of concrete C f =cement factor (content) for concrete mixture CS =(measured via ASTM C or computed) chemical shrinkage of cement α max =maximum expected degree of hydration of cement, [(w/c)/0.36] or 1 S =degree of saturation of LWA (0 to 1] when added to mixture ø LWA = (measured) absorption of lightweight aggregate (use desorption measured at 93 % RH (potassium nitrate saturated salt solution) via ASTM C 1498–04a)

How far can the water travel from the surfaces of the LWA? Answer: Equation balancing water needed (hydration) vs. water available (flow) (Menu selection #2) “Reasonable” estimates --- early hydration mm middle hydration mm late hydration mm or less “worst case” mm (250 μm) Early and middle hydration estimates in agreement with x-ray absorption-based observations on mortars during curing

How are the internal reservoirs distributed within the 3-D concrete microstructure? Answer: Simulation using NIST Hard Core/Soft Shell (HCSS) Computer Model (Menu selections #3 and #4) Returns a table of “protected paste fraction” as a function of distance from LWA surface Yellow – Saturated LWA Red – Normal weight sand Blues – Pastes within various distances of an LWA 10 mm by 10 mm Mortar from μCT experiment 97 % of paste within 2 mm of LWA

Summary Internal curing especially critical in high performance blended cement systems Too little internal curing can actually result in a lower internal RH than in a system with no internal curing X-ray microtomography can be used to “observe” water movement during internal curing in four dimensions Internet tools exist to assist in mixture proportioning for internal curing