Effect of IC/SAP on concrete properties

From previous discussions direct conclusions applicable to this section can be made. In particular, since SAP used in concept of internal curing:

(1) provoke certain changes in original composition of concrete, (2) absorb water/pore solutions and swell in fresh concrete,

(4) rivalry to hydration regulate the water balance (the action of which takes place irrespectively the concept of internal curing is executed or not), and

(5) act as a flow barrier,

impact of the internal curing methodology is expected to regard majority of concrete properties. It seems reasonable to divide particular field of importance into two, i.e. effect of internal curing (or, intuitively, SAP themselves) on autogenous shrinkage and another one, accounting for influence on all other properties.

Because nearly every addition of SAP with extra water leads to reduction of free autogenous shrinkage, it is most reported effect of IC and currently the best comprehended one in terms of controlling factors. It is seen that effectiveness of IC is governed by the amount and properties of SAP [Jen 02][Lur 06][Sch 12b], especially the particle size [Jen 02][Lur 06] and desorption kinetics [Gor 11][Sch 12c], and generally any factor contributing to distribution of sufficient amount of water at right time (e.g. curing fluid content). These particularities must be adjusted/fit while considering limited travel distance of curing fluid from the stretches of SAP, but precisely how much restricted, is still under debate [Øst 01][Iga 10][Lur 14][Wyrzykowski et al. 2012 Ibid. Lur 14][Zhu 11b]. As signalized earlier, it is simultaneously not excluded that certain physical changes in microstructure exerted by IC variables will decide about the extent of curing success too [Jen 01b][Lur 06][Est 09].

Additionally, the characteristics of original cement-based materials, i.e. before modification by internal curing, must be taken into account. In recent RILEM TC 225-SAP Round-robin Test [Mec 14], it has been clearly demonstrated that the same IC agent and amount of curing water will provide only somewhat similar effect when the raw materials properties of mix to be cured (and not the original proportions) are slightly changed. Similar finding was made by the author after comparing introduction of one type of SAP to two different matrices of mortar and finely grained UHPC. This obviously adds up to conclusion that evaluation of internal curing must remain material-orientated, as being established in this thesis.

As far as second group is considered, experience gained by different scientists pioneered by Jensen and Hansen [Jen 01b][Jen 02] through two conferences [RILEM pro52][RILEM pro74] and PhD theses [Moe 09][Est 09][Sol 11][Ass 13] to the state-of-the-art reports [TC 196-ICC][TC 225-SAP] allows to state a considerable list of properties undergoing simultaneous changes. These are widespread over all concrete life states. In particular, of

concern are properties in fresh state (workability, air content, density), ones ascribed to transition from fluid to solid (e.g. setting behaviour) as well as properties in the hardened state (mechanical, elastic, visco-elastic). Upon application of IC agent, especially ones without individual sorptivity taken into account, some changes in permeability and diffusivity, therefore important durability-related properties, may result too.

Unlike the influence of IC on concrete in its fresh state (typically leading to compensable changes of workability, some air content increase and decrease of density, e.g. [Dud 10a][Dud 10b]), one straightforward course of changes on particular property of hardened concrete is from principle hard to find. The fact is that positive impact of one factor can be easily offset by a rival negative one, the best examples being increased degree of hydration at simultaneously increased porosity and much higher moisture content. Furthermore, some parameters may be found of greater importance for one property than the other e.g. tensile strength being more dependent on microcracking and its elimination than compressive strength. In Table 2.7, the most important variables influencing hardened properties from the time of mixing has been collected; more detailed information can be found in current section’s references.

Table 2.7: Review of effects of internal curing on concrete properties, based on references from Section 2.5.3.

Potential change due

to IC or pure SAP Microstructural effect Condition required

Hydration kinetics / course alteration (early ages)

Time-shifted accomplishment of specific thresholds and related events, in accordance to

the change

Ability of SAP for invoking particular sorption course, or other, except for level of relative humidity (maintained as a rule of thumb high)

Increased degree of hydration (later ages)

Reduction of capillaries, lower porosity, denser paste phase

Sufficient quantity of water (criterion e.g. level of relative humidity) and hydration product deposition allowed also in the modified material

capillaries Reduction of effective

w/c

Strength increase-favourable changes in capillary pore structure

Pure addition of SAP or quantity of extra water not covering the absorption of IC agent used

Increased relative humidity

Existence of pores of high saturation influencing strength

Sealed conditions and/or low diffusion coefficient

Increase in gel porosity

Pore size distribution shifted towards smaller pore diameters

Internal curing takes effect while the SAP pores have been completely eliminated

Increased porosity with simultaneous emptying of SAP pores

Additional voids (especially if large SAP particles applied), lower gel-space ratio, but

perhaps increased fracture toughness; elimination of fine, irregular capillary pores

Usage of SAP of specific type and perhaps particle size

Table 2.7: Review of effects of internal curing on concrete properties, based on references from Section 2.5.3

(continued).

Potential change due to

IC or pure SAP Microstructural effect Condition required or speculated

Creation of porosity of

irregular shape Higher stress concentrations

Usage of robust SAP that show irregular shape in any state (collapsed, swollen) Diminished autogenous

shrinkage

No internal stresses due to shrinkage restraints, no cracks

Efficiency of curing (criterion, e.g. relative humidity or capillary pressure evolution)

Careful analysis of available experimental data and variables just mentioned is important for another reason. In fact, it may serve as valuable source for drawing general conclusions with regards to variations of internal curing effects or their extent. Many of the parameters collected in Table 2.8 will mostly apply to compressive strength, as this mechanical property has been preferably investigated. In general, however, it seems proper to assume that some negative impact may regard many mechanical properties including compressive and tensile strength as well as Young’s modulus. While the loss of strength is typically not high (if any), it should not be overestimated since the primarily goal of internal curing especially in materials of much improved mechanical properties compared to ordinary concretes is in fact mitigation of autogenous shrinkage and associated cracking. Important enough, due to complex nature of stress generation, some loss in elasticity, for instance, may be found favourable too.

Table 2.8: Parameters decisive for internal curing effect on strength, based on references from Section 2.5.3.

Parameter and its

specification (if applicable)

Influence on strength

Increasing SAP and IC water

dosages Higher loss of compressive strength, especially in early ages Increasing age of material

Decreasing relative difference in strength (if occurred); possible reasons: (1) hydrates growing inside original boundaries of the SAP particles, (2) carried moisture redistribution

Composition of unmodified material

In case of mortar with normal quartz aggregate, it is decisive about largest defect size, contrary to cement paste and likely finely grained UHPC/RPC Decreasing effective w/c of

material before modificiation

General higher sensitivity to changes due to IC; possible reasons: less capillaries to deposit new hydrates, material also becoming more brittle Curing conditions

No or some sensitivity to external curing conditions; loss in mechanical resistance, if occurred, resultant from e.g. stress gradients or microcracking caused by moisture gradients