1. INTRODUCTION 

The need for connecting elements cast at different times in reinforced concrete (r. c.) structures frequently arises out of unplanned situations where dowel/starter cast-in reinforcing bars (rebars) or couplers were missed out. However, the need can also be in planned construction activities to optimize and speed up the workflow. Moreover, the requirement for such connections is of high relevance in strengthening/retrofitting of building and civil structures. 



In such situations, there aren’t many alternatives to the use of injection mortar or grout to establish post-installed rebar connections. They serve the function of anchoring, a connecting rebar to a previously cast r. c. member. Injection mortars are used in structural applications where the design load needs to be transferred safely.

At first sight, injection mortar or grout might look like similar technologies – still talking about chemical products surrounding a rebar, installed in a drilled hole in concrete, isn’t it? However, these two technologies are far from being similar for several reasons.

You may benchmark and compare the two technologies from different points of view, such as qualification and code suitability, safety, productivity and reliability. Spoiler alert: Injection mortar outperforms grouts in every aspect to establish concrete-to-concrete connections to strengthen or extend existing structures. Key roles are played by the appropriate design methods and selection of injection mortar.

Let’s deep dive into the comparison based on the regulatory framework for now.

2. BASIC DIFFERENCE BETWEEN INJECTION MORTARS AND GROUTS

Before we start, it makes sense to clarify a few points. Talking about grouts, we are referring to pre-dosed bicomponents products that need to be mixed (manually or using a machine) by an operator. The two components are usually contained in cans and after mixing, the grout is poured into the hole/gap using the "dip-and-stick" technique. Grouts are generally used to fill gaps or joints between non-structural elements such as brick or stone masonry units, tiles, etc. (See Fig. 1a and 1b). 

Talking about injection mortar systems, we refer again to bi-component products (Resin and Hardener) in predosed cartridges (such as Hilti HIT-RE 500 V4, HIT-HY 200-A/R, HIT-RE 100 HC, HIT-HY 170, HIT-FP 700 R). The mixing is carried out by a mixer that is a part of the dispenser that then discards the dosage of the injection mortar into the borehole by the required number of triggers (by an operator) as per the product’s IFU (Instructions For Use). An automatic dispenser can also be smartly used to inject accurate dosage of adhesive blend into the borehole that leads to reduced wastage, better productivity and easy handling (See Fig. 2). 

Note: Injection of adhesive mortars in long boreholes with minimal air voids is done using the help of piston plug and extension hose attached to dispensers (See Fig. 2a & 2b) that helps ensuring that load-carrying capacity is not compromised. 



An automatic dispenser is usually paired along with a volume calculator to manually pre-set the calculated required dosage of the injection mortar which then is injected into the borehole at the pull of a trigger in a single go (See Fig. 3)



The key differences between Injection mortar and grout are summarized in the table below: 

3. THE IMPORTANCE OF QUALIFICATION PROCESS IN PLACE FOR INJECTINO MORTARS 

When it comes to the design of safety-relevant applications, there are two important tasks to be accomplished: first of all, you need to define the design considerations such as acting loads. Secondly, you need to run a proper design to check if the resistance is higher than, or at least equal to, the demand. In order to be compliant with the international standards used for the design of the rest of the structure, the suitability of the entire system of postinstalled rebars including the material (injection mortar, rebars) and installation method employed must be proven comparable in performance to the cast-in rebar system (in terms of load/displacement behavior under different influencing parameters) by an authorized independent body (TAB/ICC-ES).

Only such proven post-installed rebar systems (injection mortar, rebars) may be designed according to the established design standards (Eurocode/ACI based). In addition, and if required, you need to ensure that loading conditions such as seismic actions and fire exposure are covered.

Robust qualification criteria to be used for assessing, designing and installing post-installed rebars for structural connections exist only when injection mortars are used, and structural usage qualification is not for grouts. Table 2 summarizes the main assessment criteria for post-installed rebar using injection systems and grouts.

4. ONLY MORTARS CAN BE USED FOR STRUCTURAL CONNECTIONS 

Imagine designing a concrete-to-concrete connection, for instance, a slab-to-wall. You have calculated and designed the overall structure and reinforcement according to Eurocode/ACI. You find a grout product that has been somehow tested for repairing a concrete member or for a filling purpose, but the system has not been assessed for connecting a new structural concrete member to an existing one. Perhaps you might also find some information about its bond strength in the product’s technical data sheet. But, without exact resistance values taken from a third-party assessment (such as TAB/ICC-ES), how would you know that the design or installation you have made with grouts will withstand the forces that will be transferred over time? 

The answer is straight forward: you cannot know it. We cannot simply run a design with such a grout system if the performance has not been determined specifically for post-installed rebar applications where concrete members are structurally connected. We might be comparing apples with oranges here. 

Since Hilti offers a wide range of qualified solutions for structural connections, we want to make it easy for our users to navigate through and select the best solution which is value engineered for their application conditions. We do this by offering our SPEC2SITE solutions (with injection mortar) that help ensuring: 

Productivity – For an Engineer/Architect/Specifier, our solutions make the specifications higher performing and value-engineered. For a Contractor, our solutions make the jobsite practices become faster and simpler.
Safety – Our approved solution systems help ensuring proper installation, even in complex jobsites, to fulfill the design specifications. Hence, more piece of mind with getting what one specifies and safer construction practices.  
Sustainability – More sustainable solutions with less material usage and CO2 reduction with design optimization. 

5. DESIGN USING PROFIS ENGINEERING SOFTWARE 

To start with your fully code-compliant and safe concrete-to-concrete connection designs, use Hilti’s proprietary software PROFIS ENGINEERING for quick, efficient and reliable solutions. PROFIS helps you in getting code compliant solutions according several national and regional, such as Eurocodes and ACI. You can calculate rebar embedment for a range of applications including lap splices, end anchorages and shear-friction (overlay) applications with qualified Hilti injection systems according to the state-of-the-art design codes and design standards.

In case you need more support to understand which qualification and design method is most suited to your application requirement, please leave a comment on this article or post your question in our Engineering Centre/ASK HILTI or refer to Hilti’s Handbook on Concrete-to-Concrete connections.